Advanced Polyimide Materials For Some Applications

Progress in Polymer Science 37 (2012) 907974
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Progress in Polymer Science

journal homepage: www.elsevier.com/locate/ppolysci
Advanced polyimide materials: Syntheses, physical properties

and applications
Der-Jang Liaw a, , Kung-Li Wang b , Ying-Chi Huang a , Kueir-Rarn Lee c ,
Juin-Yih Lai c , Chang-Sik Ha d
a
b
c
d
Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan
R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan University, Chung-Li 32023, Taiwan
Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e
i n f o
Article history:
Received 25 May 2011
Received in revised form 16 February 2012
Accepted 24 February 2012
Available online 1 March 2012
Keywords:
Polyimide
Synthesis
Physical properties
Applications
a b s t r a c t
Polyimides rank among the most heat-resistant polymers and are widely used in high
temperature plastics, adhesives, dielectrics, photoresists, nonlinear optical materials,
membrane materials for separation, and LangmuirBlodgett (LB) lms, among others. Additionally, polyimides are used in a diverse range of applications, including the elds of
aerospace, defense, and opto-electronics; they are also used in liquid crystal alignments,
composites, electroluminescent devices, electrochromic materials, polymer electrolyte fuel
cells, polymer memories, ber optics, etc. Polyimides derived from monomers with noncoplanar (kink, spiro, and cardo structures), cyclic aliphatic, bulky, uorinated, hetero,
carbazole, perylene, chiral, non-linear optical and unsymmetrical structures have been
described. The syntheses of various monomers, including diamines and dianhydrides that
have been used to make novel polyimides with unique properties, are reported in this
review. Polyimides, with tailored functional groups and dendritic structures have allowed
researchers to tune the properties and applications of this important family of hightemperature polymers. The synthesis, physical properties and applications of advanced
polyimide materials are described.
2012 Elsevier Ltd. All rights reserved.
Contents
1.
2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.
Monomer synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1.
Noncoplanar structures (kink, spiro, and cardo structures) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2.
Alicyclic units in main chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.3.
Fluorinated monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.4.
Miscellaneous structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.
Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1.
General polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2.
Other approaches to prepare polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3.
Dendritic and hyperbranched polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Corresponding author.
E-mail addresses: liawdj@mail.ntust.edu.tw, liawdj@ntu.edu.tw (D.-J. Liaw), klwang@ntut.edu.tw (K.-L. Wang).
0079-6700/$ see front matter 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.progpolymsci.2012.02.005
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3.
4.
5.
D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974
Physical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.
Solubility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.
Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.
Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.
Optical and electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.
Photoresists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.
Liquid crystal alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.
Gas separation membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.
LB lm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.
Electroluminescent polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.
Polyelectrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.
Electrochromic polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8.
Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.9.
Polymer memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.
Fiber reinforced composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
Marston Bogert rst produced aromatic polyimides in
1908 [1]. In 1955, high molecular weight aromatic polyimides were synthesized by a two-stage polycondensation
of pyromellitic dianhydride with diamines [2]. Since then,
interest in this class of polymers has been growing steadily
because of their thermo-oxidative stability, unique electrical properties, high radiation and solvent resistance, and
high mechanical strength. However, these polymers often
have low solubility in common solvents and have high softening temperatures, thus making their processing either
difcult or too expensive to be commercially viable. The
most common technique used to fabricate polyimides uses
a soluble poly(amic acid) as a precursor. Films are cast, and
then they are thermally dehydrated to produce the nal
imide form. Nevertheless, this process has other problems,
such as inefcient cyclizations. In addition, it is difcult to
remove water and prevent the formation of microvoids in
the nal material.
Fully aromatic polyimides have rigid chains and strong
interchain interactions, which result in the polymers
having poor solubility and non-melting characteristics.
These characteristics are a result of the highly symmetrical and highly polar groups. Strong interactions
originate from intra- and interchain charge transfer complex (CTC) formation and electronic polarization. The
CTC formation and electronic polarization are supported
by the strong electron acceptor characteristics of imides
and the electron donor characteristics of amine segments.
Polyimides have been reviewed from different perspectives [35]. For example, Hasegawa [6] and Hrdlovic [7]
examined the charge transfer (CT) interactions in fully aromatic polyimides; Negi et al. [8] reported on photosensitive
polyimides, and Ding [9] reported on chiral polyimides and
polyimides generated from isomeric diamines and dianhydrides. However, currently there are very limited reviews
on the design and synthesis of organo-soluble polyimides
and their applications.
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In the last few decades, many studies have been conducted to modify the molecular interactions of polyimides
to allow processing by conventional techniques, such as
melt processing or solvent casting, while maintaining the
thermo-oxidative stability of the polymer. These studies
have involved three major structural modications: incorporation of thermally stable but exible or unsymmetrical
linkages in the backbone, introduction of large polar or
non-polar pendant substituents to the polymer chain,
and disruption of symmetry and recurrence of regularity
through copolymerization. For instance, the incorporation
of exible linkages, such as O, CH2 , SO2 and hexauoroisopropylidene groups into the backbone, introduces
kinks in the main chain that decrease the rigidity of the
polymer backbone and inhibit close packing of the chains,
which reduces the interchain interactions and leads to
enhanced solubility. Ultem (polyetherimide, PEI) (General Electric Co.) is a good example of a polymer that is
readily processed, has desirable mechanical properties and
retains decent thermal properties.
Ding had concluded general rules for the relation
between glass transition temperature and structure of isomeric polyimides based on the experimental results [9].
Tg for polyimides of comparable molecular weight (or
inherent viscosity) based on isomeric dianhydride with
a given diamine increase in the order 4,4 - < 3,4 - < 3,3 dianhydride. The behavior is attributed to the suppressed
rotation around the bond between the phthalimide and
the bridge atom in the 3-substituted phthalimide unit. The
Tg difference between the isomeric polyimides generally
depends on the rigidity of the polymer chains. However,
the Tg of the polyimides derived from isomeric bis(ether
anhydride)s based on 3,3 -dianhydride is higher than that
on 4,4 -dianhydride (for a given diamine); because of the
exible structure of bis(ether anhydride), the difference of
the glass transition temperatures between two isomeric
polymers is not so large. On the other hand, the polyimides
from p,p -diamines (and a given dianhydride) usually have
the higher Tg s than those from m,m -diamines. This may be
due to the substitution, which impedes the chain mobility
but, at the same time, the bent chains decrease the packing of the macromolecular chain, hence tend to decrease
interactions between the macromolecular chains.
For general polyimides, bulky substituents can cause a
signicant increase in both Tg and the thermo-oxidative
stability as well as increase the solubility of the polyimide, especially for polyimides with unsymmetrical or
exible groups in the backbone. Bulky and asymmetric substituents decrease the crystallinity and packing efciency
by distorting the backbone symmetry and restricting its
segmental mobility. The extent of these effects depends
on the number, size, and polarity of the substituents.
These strategies generally suffer from a trade-off between
the thermal properties and the solubility of a polyimide
because the same structural features that enhance one
characteristic will decrease the other. Therefore, to create more easily processed materials, a balance between
these properties must be maintained without sacricing
the inherent high temperature resistance characteristics of
these polyimides.
Introducing aromatic structures and non-aromatic but
thermally stable cardo, and spiro, uorine-containing
structures or multicyclic structures (such as adamantine),
into the polymer backbone is a promising method of modifying the properties of the polyimide. These modications
can affect the color, dielectric constant (k), degradation
temperature, glass transition temperature (Tg ), and LCD
alignment properties of polyimides. The introduction of
certain specialized groups or species into the polyimides,
which include both amine and anhydride components, can
generate polyimides with special functionalities. In this
report, we review and integrate the relevant literatures on
polyimides from the viewpoint of monomer and polymer
design, and we focus on polyimides with special functionalities and applications.
2. Synthesis
Polyimides are a class of thermally stable polymers; this
thermal stability is a consequence of their stiff aromatic
backbones. The chemistry of polyimides is itself a vast
eld, and it includes a large variety of available monomers
and different synthetic methodologies. However, there has
been considerable debate about the various reaction mechanisms involved in the different synthetic methods. The
properties of polyimides can be dramatically altered by
minor variations in structure. Subtle changes in the structures of the dianhydride and/or diamine components will
have signicant effects on the properties of the nal polyimide. The important fundamentals regarding the selection
of monomers for polyimide synthesis and the basics for
understanding these structureproperty relationships are
discussed below.
2.1. Monomer synthesis
The applicability of polyimides is frequently limited
by their infusible and insoluble nature. To overcome
this drawback, extensive structural modications have
been attempted, such as the incorporation of exible or
unsymmetrical linkages in the backbone [1,10] and the
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introduction of kink, spiro, bulky, cardo or pendant substituents [8,9] that do not sacrice the thermal stability of
the polyimide. That is, the selection of the monomers plays
a key role in tailoring the basic traits of polyimides. Many
papers [1] have reviewed the synthesis and properties
of modied polyimides. Herein, we will introduce certain
moieties such as noncoplanar components, which possess
special thermal stability characteristics and increase solubility. These noncoplanar moieties include both in amine
and anhydride components, which we expect will help
expand the application of polyimides.
2.1.1. Noncoplanar structures (kink, spiro, and cardo
structures)
2.1.1.1. Kink. The kink structure is a crank and twisted
noncoplanar structure. The introduction of kink structures into polymer chains might prevent chains alignment
and disrupt the formation of efcient charge transfercomplexes (CTC). The kink structure can also result in
the formation of colorless polymer lms with high transmittance [1]. Some commercial diamines contain kink
structures, which improve the solubility and decrease the
crystallinity of the obtained polyimides; these diamines
are shown in Scheme 1. The incorporation of substituted
methylene and propylidene linkages can result in kink
structures. However, the chain exibility of the propylidene groups can decrease the thermal stability of the
polyimide [10].
Some of the diamines and dianhydrides with kink
structures that have been synthesized for the preparation of polyimides are shown in Scheme 2 [1015]. Liaw
et al. reported the synthesis of a kink bisphenol, bis(4hydroxyphenyl)diphenylmethane (BHPP), which was used
in the synthesis of diamines and dianhydrides [10,14,15].
In general, kink bisphenols can be prepared by the acidcatalyzed condensation of ketones with excess phenol in
the presence of hydrogen chloride. However, the reaction
of benzophenone and phenol under the same conditions
does not afford the desired phenol BHPP (Scheme 3). This
result can be explained by the steric hindrance of the
bulky phenyl substituents. BHPP was successfully prepared by reuxing dichlorodiphenylmethane and phenol
(molar ratio 1:2) in xylene (Scheme 3). Because the phenyl
substituents are bulkier than the methyl or triuoromethyl
substituents on isopropylidene, polyimides with a kink
diphenylmethylene linkage exhibit good solubility and
amorphous structure and have a higher thermal stability
than polyimides with an isopropylidene linkage [10,14,15].
In general, bis(ether dianhydride)s are prepared
through the nucleophilic nitro displacement reaction of
a bisphenol with 4-nitrophthalonitrile to form bis(ether
nitrile)s. The nitriles are hydrolyzed to obtain the corresponding bis(ether diacid)s. Finally, cyclodehydration
generates the bis(ether dianhydride)s, as shown in
Scheme 4.
Liaw et al. synthesized reactive polyimides using a novel
functional dianhydride (BHTDA) with a hydroxyl group
[16] (Scheme 5); this polyimide was further modied for
application on photoresist by Sekiguchi et al. [17].
Cheng and Jian [12] prepared a new noncoplanar
heterocyclic diamine with an unsymmetrical kink,
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Scheme 1. Some commercial diamines with kink structures.
Scheme 2. Some structures of novel kink diamines and dianhydrides.
1,2-dihydro-2-(4-aminophenyl)-4-[4-(3-phenyl-4aminophenoxy) phenyl]-(2H) phthalazin-1-one, from

a readily available, bisphenol-like unsymmetrical phthalazinone. The glass transition temperatures of the obtained
polyimides were in the range of 315340 C, and the
temperatures for 5% weight loss in nitrogen were in the
range of 487512 C. The polyimides derived from this
kink diamine exhibit good solubility.
2.1.1.2. Spiro. A spiro structure (called a spiro center)
consists of two rings connected orthogonally through
a particular tetrahedral bonding atom. Usually, a carbon atom serves as the spirocenter. Spiro diamines and
dianhydrides have been synthesized in different ways,

and their structures are shown in Scheme 6 [1834]. The
spirobiuorene monomer consists of two identical uorene
moieties connected through a common tetracoordinated
carbon atom [1821]. The monomers (diamines and dianhydrides) that contain spirocenters are summarized in
Scheme 6. The resulting polyimides containing spiro structures are expected to have a polymer backbone that is
periodically twisted at 90 angles at each spirocenter. This
structural feature restricts the close packing of the polymer
chains and reduces the probability of interchain interactions. Because of the less favorable molecular packing and
lower crystallinity, the polymer should be more soluble
Scheme 3. Synthesis of BHPP.
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Scheme 4. Preparation of bis(ether anhydride)s.
and have a signicantly increased Tg and thermal stability.

In addition, the new polyimides derived from 2,7-bisamino-2 ,7 -di-tert-butyl-9,9 -spirobiuorene (Scheme 6,
XIV) exhibit high oxygen permeability (P(O2 ) 18121
barrier) and desirable O2 /N2 gas separation properties
(P(O2 )/P(N2 ) 2.29) [18].
Scheme 5. Functional dianhydride (BHTDA) with a hydroxyl group.
Shu et al. synthesized polyimides derived from several

spiro diamines and dianhydrides, as shown in Scheme 6
(XVI, XVII, XIX). These polyimides have excellent solubility, good optical transparency, and high thermal stability
characteristics, which can be attributed to the presence
of spiro-fused orthogonal biuorene segments along the
polymer chain [1921]. Hsiao et al. reported the synthesis of spirobichromans that contain either a diamine
(XV) [22] or a dianhydride (XX) [23]. Polyimides derived
from these monomers are soluble in various solvents and
can be cast into transparent, exible, and tough lms.
Kumar et al. synthesized a novel diamine containing
bis(arylenedioxy)spiro-cyclotriphosphazene (XVIII) with
two spirocenters (phosphorus atoms) [25]. The polyimides
derived from the spiro-diamines give rise to colorless or
light-yellow lms due to the bulky bis-spiro-substituted
pendants. The polyimides also show good thermal stability and are noteworthy for their high char yield in
air. Han et al. [24,25] reported that polyimides from a
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Scheme 6. Chemical structures of novel spiro diamines and dianhydrides.
novel spirodilactone dianhydride (XXI) showed a high Tg

(above 400 C). Furthermore, polyimides containing the
spirodilactone unit (XXI) are capable of crosslinking via
lactamization, which enhances their thermal properties
[25]. Some novel dianhydrides containing aliphatic spiro
units have been prepared by Shiraishi et al. [2033]. Polyimides containing aliphatic spiro structures derived from
XXII [26,27] and XXIII and XXIV [28,29] showed excellent solubility and formed colorless lms due to their
unsymmetrical spiro and aliphatic structures. A series of
copolyimides was prepared from two alicyclic dianhydride
isomers, a spiro compound XXII and a non-spiro compound XXII, with p-phenylenediamine by the conventional
two-step procedure. By increasing the fraction of XXII

in the backbones, the copolyimides showed better lm
formation, enhanced solubility, increased glass transition
temperatures and birefringences and decreased average
refractive indices. These properties are most likely the
result of the unsymmetrical spiroalicyclic structure of XXII
[28].
2.1.1.3. Cardo and alicyclic. Cardo means hinge or
loop in Latin. Therefore, polymers that contain loop
shaped moieties in their main chains are called cardo
polymers. The cardo structure is very similar to the
spiro structure but has only one ring attached to a cardo
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Scheme 7. Chemical structures of novel cardo monomers [23,3034,36,78,79,448].
center, while two rings are attached to a spirocenter.

A series of polymers containing bis(phenyl)uorenes or
bis(phenyl)phtarides as cardo moieties have been synthesized, and their physical properties have been reported.
Liaws group and others have reported many aromatic
and alicyclic cardo diamines and dianhydrides, as summarized in Scheme 7. The phenyluorene-based cardo
polymers have good thermal stability, solubility, transparency, and refraction indices, among other favorable
properties. Thus, polyimides that have a cyclic cardo
group, such as cyclododecylidene, adamantane, or tricycle decane, exhibit high solubility and have outstanding
thermal properties. The high solubility enables the preparation of an ultrathin active layer, which can act as an
asymmetric or composite membrane. In addition, alicyclic cardo-containing polyimides exhibit a lighter color
than the corresponding aromatic cardo-containing polyimides.
2.1.1.4. Other noncoplanar structures. Harris et al. [35,36]
reported the incorporation of 2,2 -disubstituted biphenylylene (Scheme 8) in a para-linked polymer chain. The
substitution at the 2- and 2 -positions of the biphenyl moiety forces the rings to adopt a noncoplanar conformation.
The resulting twist in the backbones of the polymers prepared from these monomers hinders chain packing and
thus reduces the crystallinity and intermolecular interactions, enhancing solubility. Liaw et al. [37] also synthesized
and studied 2,2 -dimethyl biphenylylene-containing noncoplanar ether diamines, which have exible aryl ethers
and bulky 2,2 -disubstituted moieties. The aryl ether
linkages in the main aromatic chains (XLVIIIXLIX) significantly decrease the energy required for internal rotations,
This decrease not only results in lower glass transition
and crystalline melting temperatures but also leads to
signicantly improved solubility and other process characteristics of the polymers without greatly sacricing thermal
stability.
Kakimoto et al. [38,39] reported the synthesis of
crank and twisted noncoplanar structure diamines (LII
and LIII) and dianhydrides (LIV and LV) (Scheme 9).
The two diamines (LII and LIII) were successfully
synthesized using, respectively, biphenyl-2,2 -diol and
2,2 -dihydroxy-l,l -binaphthyl as starting materials. After
p-uoronitrobenzene was reacted with bisphenols, the
intermediate product was subjected to catalytic reduction to obtain the nal product. The two ether-containing
dianhydrides (LIV and LV) were rst synthesized by a
nitro substitution reaction of 4-nitrophthalonitrile with
the bisphenols to obtain a tetranitrile. The synthesis of
aromatic ether-containing tetranitriles by nucleophilic displacement of activated aromatic nitro groups with aryloxy
anions was also reported by Takekoshi et al. [40] in 1980.
The tetranitrile compounds were hydrolyzed with aqueous potassium hydroxide to produce the corresponding
tetracarboxylic acid. This compound was then converted to
914
Scheme 8. Monomers with noncoplanar structure.
the aromatic ether that contains the tetracarboxylic dianhydride moiety [38,41].
Mikroyannidis [42] developed the synthesis of rigid-rod
noncoplanar and twisted diamines using pyrylium salts, as
shown in Scheme 10.
2.1.1.5. Unsymmetrical monomers. The introduction of
geometrically or molecularly unsymmetrical diamines
(LVILXI) [12,4347] and dianhydrides (LXIILXVI)
[4852] into the main chain has led to new polyimides
with improved solubility, melt processability and other
desirable properties. The advantages of this method are
two-fold: (1) close chain packing and intermolecular

interactions of the resulting polyimide are restricted,
resulting in a relatively high solubility, and (2) the main
chain rigidity of the polyimides can be maintained, allowing the polyimides to have a high Tg and other excellent
thermal properties. Some unsymmetrical diamines and
dianhydrides are shown in Scheme 11.
Polyimides based on an unsymmetrical dianhydride
(LXIV) (aBPDA) were prepared, and their properties were
compared with those of corresponding symmetrical dianhydrides (sBPDA) by Hergenrother et al. [51]. Polyimides
derived from aBPDA had higher Tg values, a higher optical
915
Scheme 9. Crank and twisted noncoplannar structure diamines or dianhydrides.
transparency, and generally lower tensile properties (in

thin lms) than polyimides from sBPDA.
Liaw et al. [53,54] synthesized a series of novel
triphenylamine-containing diamines and polyimides, as
shown in Scheme 12. The triphenyl-containing polyimides
showed good solubility and excellent thermal and electrochromic properties.
Liaw et al. synthesized an unsymmetrical 2,2 dinaphthylbiphenyl-4,4 -diamine (4) (Scheme 13). First,
the biphenyl was nitrated with nitric acid and sulfuric
acid at 0 C to give 4,4 -dinitrobiphenyl (1). The dinitro
compound (1) was then iodinated using Marvels reaction
condition, which involves electrophilic aromatic substitution by an iodine cation, to give the 2,2 -diiodo-4,4 dinitrobiphenyl (2) compound. Next, the diiodo-dinitro
compound (2) was treated with 1-naphthylboronic acid in a
Suzuki coupling reaction to afford the 2,2 -dinaphthyl-4,4 dinitrobiphenyl (3). Finally, the nitrobiphenyl group was
converted to the diamine compound (4) with ammonium
formate and 10% Pd/C in a DMF solution at 80 C.
Normally, bisphenols containing cardo structure are

prepared from ketones and phenols [55]. In addition, Liaw
et al. [55] reported a synthetic route of bisphenols that,
according to a procedure by Schmidt, involves the rearrangement of a dihydrochloride, as shown in Scheme 14.
Unsymmetrical ether-containing diamines can be
prepared from the novel bisphenols via nucleophilic substitution and reduction, as shown in Scheme 15.
2.1.2. Alicyclic units in main chains
In recent years, polyimides with high optical transparencies and low dielectric constants have been in
demand for optoelectronic and microelectronic applications. Alicyclic polyimides are candidates for applications
in optoelectronics and interlayer dielectrics due to their
higher transparency and low dielectric constants when
compared to aromatic polyimides [5661]. These properties result from their low molecular density, low
polarity, and low probability of undergoing inter- or
intramolecular charge transfer [6267]. Numerous fully
Scheme 10. Synthesis of rigid-rod noncoplanar and twisted diamines using pyrylium salts.
916
Scheme 11. Chemical structures of unsymmetric monomers.
alicyclic polyimides [68,69] and partially alicyclic polyimides [33,48,67,70] have been studied. These alicyclic
polyimides all show excellent transparency and good solubility and thermal properties.
The incorporation of adamantane (tricycle[3.3.3.1.1]
decane), a rigid alicyclic compound composed of three
cyclohexane rings in chair conformations, can enhance
the thermal stability and optical properties of a polyimide without sacricing the its high transparency,
solubility, low dielectric constant, and low coefcient
of thermal expansion [68,69]. Typical examples of the
alicyclic monomers including diamines and dianhydrides

are shown in Scheme 16 [71,72] (LXVII). In addition, some
alicyclic dianhydrides have been reported and used as
alignment lms for color LCD applications. The alicyclic
monomers are summarized in Scheme 16.
Schenk et al. [73] prepared cis-cyclobutane-l,2,3,4tetracarboxylic dianhydride by irradiating a solution
of maleic anhydride in dioxane with a high pressure
mercury lamp. Suzuki et al. reported the synthesis of substituted 1,2,3,4-cyclobutanetetracarboxylic dianhydride
(CBDA) and performed the light-induced dimerization of a
Scheme 12. Synthesis triphenylamine-containing diamines [53,54].
Scheme 13. Synthesis of biphenyl dinathalene diamine.
Scheme 14. Synthesis route for bisphenol through rearrangement from dihydrochloride.
917
918
Scheme 15. Unsymmetrical ether-containing diamines.
919
Scheme 16. Chemical structures of alicyclic monomers.
Scheme 17. Preparation of cis-cyclobutane-l,2,3,4-tetracarboxylic dianhydrides.
maleic anhydride compound from 10 to 50 C in the range

of 300600 nm wavelength (Scheme 17). The R1, R2, R3,
and R4 groups can be either hydrogen atoms, alkyl groups
(110C), phenyl groups, or halogens [74].
The l,2,3,4-cyclopentanetetracarboxylic acid [7577]
was obtained by oxidizing a DielsAlder adduct of
cyclopentadiene and maleic anhydride with nitric acid,
followed by neutralization with an aqueous solution containing sodium hydroxide or ammonia to afford the
l,2,3,4-tetrasodium salt of cyclopentanetetracarboxylic
acid or the l,2,3,4-tetra-ammonium salt of cyclopentanetetracarboxylic acid, respectively. The 1,2,4-tricarboxyl-3methylcarboxyl cyclopentane dianhydride was obtained
by dehydrating the cyclopentanetetracarboxylic acid
(Scheme 18).
2.1.3. Fluorinated monomers
Because of the need for high integration and high
signal-propagation in miniaturized electronic devices and
components, speed is of utmost importance in the microelectronics industry. A reduced dielectric constant in insulation materials allows higher signal-propagation speeds.
Fluorine-containing polymers are of special interest
because of their low dielectric constants, high optical transparency, low refractive indices and remarkably low water
absorption. In addition, the 6F (1,3-ditriuoromethyl2-isopropyl) groups in the polymer backbone enhance
polymer solubility (a characteristic known as the uorine
effect) without reducing the thermal stability. The bulky
triuoromethyl group (CF3 ) also serves to increase the
free volume of the polymer, thereby improving gas permeability and electrical insulating properties.
McGrath et al. [36] reported a uorinated diamine
monomer based on triuoroacetophenone; this monomer
was synthesized via a straightforward, high yielding two-step procedure. Triuoroacetophenone was
reacted with 4-nitrophenyl phenyl ether to yield
the 3F-dinitro compound, which was subsequently
reduced to afford the uorinated diamine, 1,1-bis[4-(4aminophenoxy)phenyl]-l-phenyl-2,2,2-triuoro-ethane
(3FEDAM) (Scheme 19). Yangs group also reported the
synthesis of a 9F uorinated diamine [78] and dianhydride
[79] using similar synthetic procedures (Scheme 20). The
triaryluoroethane derivative, 3 ,5 -bis(triuoromethyl)2,2,2-tiuoroacetophenone (9FAP), was obtained by a
Grignard reaction between anhydrous lithium triuoroacetic acid, 1-bromo-3,5-bis(triuoromethyl)benzene
920
Scheme 18. Preparation of 1,2,4-tricarboxyl-3-methylcarboxyl cyclopentane dianhydride [7577].
Scheme 19. Chemical structures of XLII (3FDAM) and XLIII (3FEDAM).
and magnesium. The dinitro and tetramethyl intermediates were subsequently synthesized by a coupling reaction
of 9FAP with 4-nitrophenyl phenyl ether and o-xylene,
respectively, in the presence of trifuoromethanesulfonic
acid, which acted as a catalyst. The diamine was derived
via reduction of the dinitro group, and the dianhydride was
derived via oxidation of the methyl group and subsequent
dehydration reaction.
Liaw and Yang recently reported a number of monomers
and polyimides that contain triuoromethyl ether linkages
[64,8086]. The CF3 -containing diamines were prepared
via a conventional two-step procedure. The intermediate dinitro compound was synthesized by a nucleophilic
halogen displacement reaction of 2-halogen (F or Cl)-5nitrobenzotriuoride with a bisphenol in the presence
of potassium carbonate in NMP, DMF or DMAc. The
diamine monomers were obtained in good yields upon catalytic reduction of the dinitro compounds with hydrazine
hydrate (or hydrogen gas) and Pd/C catalysts in ethanol
under reux. Banerjee and his group [87,88] have also
reported the synthesis of diamines containing ether linkages and different rigid units with pendant triuoromethyl
in the same procedures. Some of the uorine-containing
diamines are summarized in Scheme 21.
Another approach to obtaining triuoromethylcontaining
diamines
is
through
nucleophilic
displacement of activated uorine atoms of intermediate triuoromethyl-containing diuoro compounds by
4-aminophenol. These reactions are performed in the
presence of excess potassium carbonate, which acts as a
base, in NMP with the concomitant azeotropic removal of
water formed in the acidbase reaction between phenol
and base, under well-established reaction conditions, as

shown in Scheme 22 [88].
Yoon et al. [89,90] reported the synthesis of 3,5bis(triuoromethyl)phenyl containing diamines, as shown
in Scheme 23. The resulting polyimides derived from the
diamines exhibit good thermal stability and have desirable adhesive properties, low dielectric constants, and low
refractive indices and birefringence properties.
The uorine-containing dianhydrides, 6FPPMDA [91]
and 12FPPMDA [92], which contain multi-uorine groups,
were prepared as shown in Schemes 24 and 25. The compound 6FBB was prepared from 3,5-bis(triuoromethyl)
bromobenzene via a Grignard reaction with trimethylborate. 6FBB was then reacted with B4MB (or 2B4MB) to
obtain 6FP4MB (or 12F4MB) in a Suzuki cross-coupling
reaction. The 3,5-bis(triuoromethyl)phenyl-containing
dianhydrides were successfully prepared by oxidizing
6FP4MB and 12F4MB, followed by cyclodehydration. The
polyimides based on the two diamines had high Tg values, low dielectric constants and low coefcients of thermal
expansion (CTE) [91,92].
Some triaryluoroethane-containing diamines and
dianhydrides with kink structures were introduced in Section 2.1.1.1.
2.1.4. Miscellaneous structures
Apart from the popular aromatic, noncoplanar,
aliphatic, acyclic and uorinated monomers described
above, specialized monomers for specic applications
are also an interesting area of research. Silicon atoms,
carbazoles, heterocyclic rings, perylene, and chiral functionalities have also been incorporated into polyimide
Scheme 20. Triaryluoroethanes-containing dianhydride (XLIV) and diamine (XLV).
921
Scheme 21. Diamines containing triuoromethyl with different linkages [87].
monomers to obtain polymers with properties that are

desirable for specic applications.
2.1.4.1. Silicon-containing monomers. Silicon-containing
aromatic polyimides have attracted considerable scientic
and technological interest because of their potential
applications in optoelectronic materials. Silicon, when
placed next to aromatic groups, provides conjugation and thus supports the transport of electrons along
macromolecular chains [93,94]. Polyhedral oligomeric
silsesquioxane (POSS) is a cube-octameric molecule
with an inner inorganic and oxygen framework that is
externally covered by organic substituents. The incorporation of POSS into polyimides could lead to the development
of high performance materials by combining the properties of inorganic and organic components. Applications of
polyimides with POSS side chains and end groups have
been reported. Recently, Wei et al. prepared polyimide
nanocomposites with tethered polyhedral oligomeric
silsesquioxane in their side chains, as shown in Scheme 26
[9597]. As also shown in Scheme 26, Kakimoto et al.
synthesized linear polyimides derived from a doubledecker-shaped silsesquinoxane dianhydride (DDSQDA)
[98], which was prepared from a double-decker-shaped
Scheme 22. Synthesis of monomer, LXXV [88].
922
Scheme 23. Synthesis of 3,5-bis(triuoromethyl)phenyl containing diamines [89,90].
silsesquinoxane (DDSQ) [99] containing two reactive

hydrosilane groups [100]. The POSS-polyimides exhibit
good thermal stability and mechanical properties, low
water absorption, good alkali resistance and low dielectric
constants. The double-decker-shaped poly(silsesquioxane)

(DDPSQ) showed excellent transparency and thermal stability. The thermo-optic coefcient of the DDPSQ is 3 or 4
times larger than that of conventional polymers [101].
Scheme 24. Synthesis of 3,5-bis(triuoromethyl)phenyl containing diamines [91].
923
Scheme 25. Synthesis of 12FPPMDA [92].
2.1.4.2. Heterocycle-containing monomers. The synthesis of

new heteroaromatic monomers and corresponding polyimides that have both good processability and thermal
stability would be of great interest. Heterocyclic ring-based
polyimides are considered to be a unique class of hightemperature polymers. Polyimides with heterocyclic units
incorporated into their backbones offer certain advantages,
such as higher Tg values, tensile strength and modulus, over
polyimides without heterocyclic units. The great variety
of known heterocycles has opened the possibility of formulating polyimides with completely different properties
[102].
1. The introduction of highly condensed heterocyclic fragments permits an increase in the thermal stability and
heat resistance of the polyimides.
2. Heterocycles containing bulky side groups have been

introduced to make polyimides soluble in organic solvents.
3. A majority of the aromatic heterocycles is more resistant to hydrolysis and nucleophilic attack than imide
rings, and this has made it possible to use high molecular weight heterocyclic monomers, which reduce the
content of imide rings per unit molecular weight of the
polymer and increase the hydrolytic stability of the polyimides.
Heterocycles such as phenylquinoxalines [103],
benzoxazoles, benzothiazoles [104,105], oxadiazoles
[106108], and triazoles [109,110], among others, have
been incorporated into the backbones of polyimides.
New heteroaromatic diamine and dianhydride
monomers are expected to play important roles in
Scheme 26. Double-decker-shaped poly(silsesquioxane) (DDPSQ).
924
Scheme 27. Synthesis of novel heterocyclic polyimides containing the azo-naphthol pendant group via DielsAlder-ene cycloaddition reactions from an
azo-ester.
the synthesis of advanced polyimides. Mallakpour et al.

[111] reported the synthesis of novel heterocyclic polyimides containing azo-naphthol pendant groups. This
polymer was accessed via a DielsAlder-ene cycloaddition reaction from an azo-ester derivative of isoeugenol
and bistriazolinedione at room temperature, as shown
in Scheme 27. The reactions are exothermic, fast, and
provide novel heterocyclic polyimides in high yield. The
polyimides contain OH and NH, as well as C O functional
groups. They can readily undergo hydrogen bonding to
form physical networks that contribute to the mechanical
properties of the polymers.
In general, the heteroaromatic structures in the main
chain of a polymer are expected to impart certain
properties to it. Pyridine would provide excellent thermal stability; good electronic, electron-transporting, and
electron afnity characteristics; and more resistance to
oxidation because of its molecular symmetry and aromaticity as well as the polarizability that result from
the nitrogen atom in the pyridine ring. Thus, new heteroaromatic diamines, dianhydrides or other monomers
containing pyridine units should be able to contribute to
the chemical stability and mechanical properties of the
resulting polymers at elevated temperatures and to exhibit
unique properties. Consequently, our group [37,112117]
and other researchers [116,117] have focused on adopting
monomers containing a pyridine nucleus for the synthesis of novel pyridine-containing polymers that have good
thermostability and processability (Scheme 28). Liaw and
Wang et al. [112115] reported that polymers containing
pyridine groups also exhibit interesting uorescent phenomena upon protonation by protonic acids. In the absence
of acid, the polymer has UVvis absorption bands in the
wavelength () region of 240400 nm. The absorption is
red-shifted to the 390500 nm region upon protonation.
The protonated polymer exhibits strong orange uorescence (around 600 nm) in a THF solution. It is proposed
that the different absorption spectra of deprotonated and
protonated poly(pyridine-imide)s result from a different
electronic distribution in the molecule that occurs when
the pyridine is protonated. Furthermore, the uorescent
intensity of the protonated polymer was inuenced by the
concentration of the acids [37].
The dinitro compounds containing pyridine heterocycles and pendant chromophore groups were synthesized
with the modied Chichibabin reaction (Scheme 29), which
is a facile method for the preparation of substituted
pyridines [112115]. The condensation of chromophorecontaining aldehydes with 4-nitroacetophenone in the
presence of ammonium acetate affords dinitro compounds
in one step. The coplanar conformation and polar nitro
group resulted in poor solubility of the dinitro compounds. Reduction of the dinitro derivatives in ethanol
with hydrazine monohydrate in the presence of a catalytic
amount of palladium on activated carbon at 90 C produced
new diamine compounds [118].
2.1.4.3. Carbazole-containing monomers. Carbazole is a
conjugated moiety that has interesting optical and
electronic properties, such as photoconductivity and photorefractivity [119]. In the eld of electroluminescence,
carbazole derivatives are often used as functional moieties
for hole transporting and in light-emitting layers because
of their high charge mobility, thermal stability, and blue
electroluminescence. The electroluminescence is a result of
the large band gap that arises from the improved planarity
that the bridging nitrogen atom gives the biphenyl unit
[120]. From a structural point of view, carbazole is a planar structure, whereas diphenylamine is a kink structure.
The thermal stability of the polymer with incorporated
925
Scheme 28. Synthesis of pyridine-containing dinitro derived by Chichibabin.
carbazolyl units is therefore improved. In addition, carbazole can be readily functionalized at the (3,6) [121,122]
(2,7) [123] or N-positions [124126] and then covalently
linked into polymeric systems. Carbazole can be incorporated both in the main-chain as a building block and in the
side-chain as a pendant group [127131]. It is thus worthwhile to explore the feasibility of new carbazole-based
aromatic diamines as starting monomers for the preparation of high-performance polyimide systems with novel
optoelectronic properties [132].
When the carbazole moiety is incorporated into the
polyimide backbone, it imparts the polymer with a higher
thermal stability, increased solubility, extended glassy
state and moderately high oxidation potential [133]. In
addition, carbazole is an excellent candidate for nonlinear optically (NLO)-active 2D chromophores, owing
to its isoelectronic structures between positions 3 and
6 and its second-order nonlinear optical characteristics and photoconductive properties [134]. Hsiue et al.
reported new highly thermally stable polyimides from
diamine monomers containing novel lambda-shaped twodimensional carbazole chromophores [135], as shown
in Scheme 30. The polyimides derived from carbazolecontaining monomers show good thermal properties and
unique electronic characteristics.
2.1.4.4. Perylene containing monomers. Perylene imides
represent a class of n-type semiconductors that exhibit
a relatively high electron afnity among large-band-gap
materials. The perylene-based polymers are of broad interest owing to their wide range of potential applications,
including electron-transporting components in optical
switching [136,137], electroluminescent [138140], solar
energy conversion [141144], and liquid crystal color
display [145] devices. Derivatization of perylene with
highly uorescent uorene derivatives in the polymer
molecules leads to added advantages in photophysical and photochemical properties [145]. However, the
actual application of perylene-containing polyimides
has been hampered by their poor solubility and low
Scheme 29. Synthesis of pyridine-containing diamines.
926
Scheme 30. Polyimides containing novel lambda-shaped two-dimensional carbarzole chromophore.
solid-state uorescence quantum efciency. These undesirable aspects are associated with the rigid perylene group [146]. Most of the perylene bisimides
are insoluble; therefore, lm preparation of the perylene bisimides requires vapor deposition or dispersion
in other polymer matrices, which substantially limits their applications [147,148]. Recently, a number of
authors [149151] have synthesized perylene-containing

polyimides that avoid these problems with structural
modications. The two most commonly used approaches
to preparing organo-soluble pyrene-containing polyimides are shown in Scheme 31. One approach uses
exible aliphatic diamines and pyrene-containing dianhydrides to form polyimides. The other approach uses
927
Scheme 31. Two most used approaches to preparing organo-soluble pyrene-containing polyimides.
pyrene-containing dianhydrides and other dianhydrides

to form copolyimides. Icil et al. [152] synthesized
a photostable polymer based on perylene-3,4,9,10tetracarboxylic acid-bis-(N,N -dodecylpolyimide). Ghassemi and Hay [153] reported red pigmentary polyimides prepared from N,N -diamino-3,4,9,10-perylene
tetracarboxylic acid bisimide. Wang and coworkers
[154,155] prepared perylene-containing polyimides and
investigated their xerographic electrical and voltage
dependent uorescence properties. Recently, perylenediimides have also been incorporated into conjugated
oligomers and polymers as energy- and electron-acceptors
[156].
2.1.4.5. Proton-conducting monomers for polyelectrolyte fuel
cells (PEFCs). Polyelectrolyte fuel cells (PEFCs) have been
identied as promising power sources for transportation vehicles and for other applications that require
clean, quiet and portable power. The most important
component of a PEFC is the proton-conducting membrane of the polyelectrolyte itself. Proton-conducting
polymers have attracted much attention in the past few
decades due to their important applications in fuel cell
systems. Faure and Merciers group rst synthesized various sulfonated copolyimides from naphthalene-1,4,5,8tetracaboxylic dianhydride (NTDA), 2,2 -bendizine sulfonic
acid (BDSA, a widely used sulfonated diamine), and common nonsulfonated diamine monomers. However, the
proton conductivity of these membranes is rather low
(<102 S cm1 at 100% relative humidity) due to the low
ion exchange capacity (IEC) [157160]. The well-studied
proton-conducting polymers that have been used in practical applications are sulfonated peruoropolymers, such as
DuPonts Naon membrane and Dows membrane, due to
their high proton conductivity, high mechanical strength,
and excellent thermal and chemical stability. However,
there are some shortcomings that could seriously limit the
applications of these polymers: they are expensive, have
a low conductivity at low humidity or high temperatures,
and high methanol permeability. Thus, the development
of alternative materials is strongly desired. One major
approach has been the attachment of sulfonic acid groups
to highly stable aromatic polymers. As such, polyimides are
one of the most ideal materials.
The introduction of sulfonic acid groups is achieved
either by direct sulfonation of the parent polyimides or by
928
Scheme 32. Sulfonated diamines derived from sulfonation reaction [161163].
polymerization of sulfonated monomers. Okamoto et al.

reported the synthesis of many new sulfonated diamine
monomers, as shown in Scheme 32 [161163]. The BAPFDS,
ODADS, and o-BAPBDS monomers were synthesized by
direct sulfonation of the corresponding parent diamines.
By conducting the sulfonation at different temperatures,
the sulfonic acid could be directed to specic positions.
For instance, the sulfonation reaction occurred only at
the 2,7-positions of the uorenylidene ring because these
two positions are more reactive than the others [161].
However, because the protonated amino group of ODA
is a strong electron-withdrawing group, the sulfonation
reaction occurred mainly in the position that was meta

to the amino group [162]. The ODADS-based polyimides
display much better stability in water than those derived
from the widely used sulfonated diamine (BDSA) because
ODADS-based polyimide membranes have a more exible
structure than the corresponding BDSA-based ones. The
ODADS-based polyimides retain their mechanical properties and high proton conductivity after being soaked in
water at 80 C for 200 h [161,162]. The homopolyimide
NTDA-o-BAPBDS membrane (Scheme 33) is soluble in
water at room temperature; however, when nonsulfonated
diamine moieties are incorporated by copolymerization,
929
Scheme 33. Homopolyimide NTDA-o-BAPBDS.
the polymer is signicantly less soluble in water but retains

its mechanical strength after being soaked in distilled water
at 80 C for 401000 h [163].
Okamoto et al. [164] have also reported the synthesis of a novel sulfonated diamine monomer, 3(2 ,4 -diaminophenoxy)propane sulfonic acid (DAPPS), as
shown in Scheme 34. A sulfonated polyimide (SPI) was
prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) and DAPPS. The SPI membrane displayed
proton conductivity of 0.120.35 S cm1 in water at temperatures ranging from 35 to 90 C. The proton conductivity
was found to be similar to or higher than that of Naon
117 and other sulfonated hydrocarbon polymers. The SPI
membrane displayed good stability in water at 80 C and
was thermally stable up to 240 C. It showed reasonable
mechanical strength with a modulus of 1.3 GPa at 90 C and
90% relative humidity (RH). Its methanol permeability (PM )
was 0.57 106 cm2 /s at 30 C with 8.6 wt% methanol in

the feed. These properties suggest that the SPI membrane
could have potential applications in direct methanol fuel
cells.
Okamoto et al. synthesized novel diamines bearing
sulfonated aromatic pendant groups, namely, 3,5-diamino3 -sulfo-4 -(4-sulfophenoxy) benzophenone (DASSPB) and
3,5-diamino-3 -sulfo-4 -(2,4-disulfophenoxy) benzophenone (DASDSPB), as shown in Scheme 35 [165]. Novel
side-chain-type sulfonated copolyimides (SPIs) have been
synthesized from these two diamines, 1,4,5,8-naphthalene
tetracarboxylic dianhydride (NTDA) and nonsulfonated
diamines, such as 4,4 -bis(3-aminophenoxy) phenyl sulfone (BAPPS), in sequenced and random approaches. Tough
and transparent membranes of SPIs with ion exchange
capacity of 1.52.9 mequiv./g were prepared. These membranes showed good solubility and high thermal stability
930
Scheme 34. Synthesis of sulfonated diamine monomer (DAPPS) [164].
up to 300 C. The membranes showed isotropic swelling

in water, unlike the main chain-type and sulfoalkoxybased side-chain-type SPIs. At low relative RH, the novel
SPI membranes showed much higher conductivity than
membranes from sulfoalkoxy-based SPIs. The membrane

of sequenced NTDA-DASDSPB/BAPPS (1/1) displayed reasonably high proton conductivities of 0.05 and 0.30 S cm1
at 120 C at an RH of 50 and 100%, respectively.
Scheme 35. Synthesis of diamine monomers with sulfonic acids at side chain [165].
931
Scheme 36. Synthesis of sulfonated diamine isomers (BSPB) [166].
Sulfonated diamine isomeric monomers bearing sulfopropoxy groups (BSPB), as shown in Scheme 36, were
also prepared by Okamotos group [166]. The diamine was
obtained by rearrangement reactions in hydrochloric acid.
The sulfonated polyimides derived from NTDA and BSPB
monomers show high proton conductivities and better stability than common sulfonated polyimides in which the
sulfonic acid groups are directly bonded to the polymer
backbone. The high proton conductivity and stability are
likely due to the microphase-separated structure of the
membrane, which can be observed by TEM, and the strong
basicity of BSPB diamine moieties that result from the
electron-donating effect of the propoxy groups.
2.1.4.6. Non-linear optical monomers. Waveguide materials based on polymers with nonlinear optical (NLO)
properties have attracted extensive attention because of
their potential applications in frequency doubling for data
storage, electro-optic modulation for optical telecommunications and optical interconnects, and integrated optics.
Inorganic crystals have been used as nonlinear optical
materials for several decades. However, the crystals are difcult to grow, are expensive, and are difcult to incorporate
into electronic devices. A number of organic chromophores
exhibit extremely high and fast nonlinearities that often
rival or surpass the performance of inorganic crystals [167].
Though some important issues, including the transparency
to efciency trade-off, centrosymmetric arrangement, and
phase matching, continue to challenge material scientists
and engineers, highly stable NLO polymers have been prepared by grafting NLO-active chromophores onto aromatic
polyimide backbones [168170].
In the development of NLO polymers for electrooptic device applications, stabilization of the electrically
induced dipole alignment is an important consideration.
Two approaches to minimizing the randomization have
been proposed. One is to introduce crosslinking, and the
other is to utilize high glass-transition temperature (Tg )
polymers. Wang et al. [171] synthesized a series of NLO
polyimides by grafting zwitterionic chromophores. The
poling and electro-optical (EO) studies revealed a strong
dependence of the EO coefcient on the polymer chain
mobility or the glass transition temperature. A thermally
crosslinkable group was introduced into the NLO polymers

to achieve a high thermal stability of the poled NLO polymers [170]. Lee et al. reported novel T-shaped (Scheme 37)
[172], Y-shaped (Scheme 38) and -shaped (Scheme 30)
[135] chromophore-containing [173] NLO polyimides with
high thermal stability that could be used for second harmonic generation. Finally, Shu et al. synthesized a novel
main-chain-shaped NLO polyimide, as shown in Scheme 38
[173].
High-Tg aromatic polyimides with pendant dendronized NLO chromophores functionalized on a cardo
bisphenol linkage backbone were synthesized and characterized by Jen et al. [170]. The polyimide with dendronized
chromophores can achieve high poling efciency to afford
a very large electro-optical (E-O) coefciency (71 pm/V
at 1.3 m). NLO polyimides containing side chain chromophores were synthesized from functional polyimides
and were followed by a Mitsunobu reaction with diol
chromophores [104,174176] or by a post-azo-coupling
reaction [177]. The syntheses of novel NLO diamines containing azo groups, shown in Scheme 39, and the NLO
properties of polyimides derived from the diamines have
been reported [105,178180].
Compare to the NLO polyimides derived from functional polyimides following the Mitsunobu reaction with
diol chromophores (Scheme 40(A)) [104,174176], the
two-step process has the following disadvantages: (1) the
di(hydroxyalkyl) chromophore must be transformed into
a dialkylamino chromophore under Mitsunobu conditions,
the resulting reaction mixture is difcult to separate, and
the pure materials are obtained in fairly low yields; (2)
functionalization of precursor polyimides with a hydroxyalkyl chromophore results in random functionalization
and cannot be completely functionalized; and (3) harsh
conditions are required to prepare the chromophore
monomer; which imposes some limitations on the chromophore structure. Therefore, Samyn et al. [181] have
developed a one-step synthesis of NLO polyimides by reacting di(hydroxyalkyl) chromophores and diimides under
Mitsunobu conditions (Scheme 40(B)).
2.1.4.7. Others (pendant polyimides using mellitic acid
dianhydride). Illingsworth et al. have investigated the
932
Scheme 37. Synthesis of T-shaped NLO dianhydride [172].
Scheme 38. Synthesis of Y-shaped NLO dianhydride [173].
Scheme 39. NLO diamines containing azo groups [105,178180]. (A) Two-step hydroxyl-hydroxy Mitsunobu condition reaction and (B) one-step imidehydroxy Mitsunobu condition reaction.
933
Scheme 40. NLO polyimide via (A) two-step and (B) one-step Mitsunobu condition [181].
preparation of Zr-containing pendant copolyimides using

mellitic acid dianhydride (co-PIs). Methods are being
investigated to increase the Zr concentration and atomic
oxygen resistance of these compounds while retaining
other desirable lm properties. The immediate objectives
that must be met to achieve this goal are as follows: (1)
addressing the increased tendency of copolyamic acids (coPAAs) made from MADA diacids (Scheme 41) to undergo
gelation during polymerization and upon addition of N,N dicyclohexylcarbodiimide (DCC) during the Zr-appending
reaction; and (2) increasing the number of layers in multilayer lms that can be applied prior to crack formation.
The highest number of layers that have been incorporated
is ten (Scheme 41) [182]. The excellent solvent resistance
displayed by these pendant polymers also bodes well for
engineered materials applications.
The Toray Company (Japan) [183] synthesized ester
and amide groups containing dianhydride with bisphenol
and diamine compounds, respectively, in the presence of
trimellitic anhydride chloride (Scheme 42).
2.2. Polymerization
2.2.1. General polymerization
The literature on the formation reactions and properties of polyimides is vast. Imai has collected the methods
for preparing polyimides in a book review in Japanese
[184]. In general, polyimides are prepared from a dianhydride and a diamine. The most developed synthetic
method for polyimides is a two-step method. The rst
step involves a very fast, exothermic, stepwise polymerization at a relatively low temperature to form a poly(amic
acid) from a dianhydride and a diamine. Subsequently, the
poly(amic acid) is converted into the corresponding polyimide through an intramolecular cyclization (imidization)
that releases water condensate. Either chemical or thermal imidizations can be used to convert poly(amic acid)s to
polyimides. The solubility of polyimides prepared by different imidization methods differs to some extent. In general,
polyimides derived from chemical imidization are more
soluble than those from thermal imidization. However, the
934
Scheme 41. Structures of dianhydrides, diamines, and pendent group, and structure of polyimide products.
thermal properties of polyimides from thermal imidization, including Tg and decomposition temperatures, are
superior to those of polyimides from chemical cyclization. The difference can be ascribed to morphological
changes in the polymers, such as increased ordering from
molecular aggregation of the polymer chain segments
that occurs during thermal imidization [185]. A series of
pyridine-containing polyimides [37] has been prepared by
the conventional two-step polymerization method from
commercial dianhydrides and novel pyridine-containing
diamines (described in Section 2.1.4.2) as shown in
Scheme 43.
2.2.2. Other approaches to prepare polyimides
Polyimides can be prepared from diamines and dianhydrides, but they can also be synthesized from the following
reactions: (1) diisocynates and dianhydrides [186188];
(2) diamines and dithioanhydrides [185,189,190]; (3)
diamines and bis(maleimide)s (Michael addition reaction) [191]; (4) bisdiene and bidienophiles (DielsAlder
reaction); (5) silylated diamines and dianhydrides; and
(6) di(hydroxyalkyl) compounds and diimide compounds

(Mitsunobu reaction).
2.2.2.1. From diisocyanates and dianhydrides (Scheme 44).
The polymerization of polyimides can be accomplished
using diisocyanates in the place of diamines. It has
been reported that phthalic anhydride reacts with
aromatic or aliphatic isocyanates to form N-aryl- or Nalkylphthalimides. Based on these reactions, homo- or
co-polyimides have been successfully synthesized from
diisocyanates and dianhydrides [3,186,192194]. The polymerization involves hydrolysis of the diisocyanate to form
a diamine and then a polyamic acid, which is converted
subsequently to a polyimide by general imidization methods [187]. This route is advantageous because the reaction
is less sensitive to moisture and diisocyanates are generally more soluble in organic solvents than are diamines.
One disadvantage of this method is that there are few
diisocyanates to choose from, compared to the numerous
diamines available from synthetic or commercial sources.
However, the fast reaction between diisocyanates and
Scheme 42. Dianhydrides containing ester or amide groups [183].
935
Scheme 43. Synthesized poly(pyridine-imide)s prepared by the conventional two-step polymerization method.
tetracarboxylic acids makes this a viable route for the

preparation of polyimides [187]. Yeganeh et al. reported the
application of microwave radiation in the direct synthesis
of aromatic polyimides from the reaction of aromatic diisocyanates and dianhydrides [188]. The experimental results
showed that the polyimides obtained via this method had
superior inherent viscosities and higher yields when compared to polyimides obtained via the conventional solution
method.
2.2.2.2. From diamines and dithioanhydrides (Scheme 45).
In addition to dianhydrides, dithioanhydrides can be used
to prepare polyimides. Dithioanhydrides can be prepared
by reacting the corresponding aromatic tetracarboxylic
acids with sodium sulde. Imai et al. used dithioanhydrides with diamines to prepare the precursor poly(amide
thiocarboxylic acid)s. Polyimides were then obtained after
removing the hydrogen sulde by heating, as shown
in Scheme 45 [189]. The poly(amic thiocarboxylic acid)
cannot be removed during the reaction due to its high

reactivity [190]. Liou et al. compared the preparation
of polyimides generated from the one-step diamine and
dithioanhydride reaction to those prepared by the traditional two-step method [185]. The inherent viscosities
of polyimides derived from dithioanhydrides were comparable to those prepared by the traditional two-step
method.
2.2.2.3. From diamines and bis(maleimides) (Michael addition reaction) (Scheme 30). The Michael addition of
diamines to bis(maleimides) is another method of preparing polyimides. In contrast to the above methods, the imide
ring of polyimides is not formed during the polymerization but arises from the maleimide structure. Bella et al.
reported a polyimide synthesis via the Michael addition
reaction as shown in Scheme 46 [191]. This approach is
a general strategy for achieving thin lms from insoluble
or reactive functional polyimides.
Scheme 44. Synthesis of polyimides from diisocynates and dianhydrides.
936
Scheme 45. Synthesis of polyimides from diamine and dithioanhydride.
2.2.2.4. From bisdiene and bidienophiles (DielsAlder reaction) (Scheme 47). The DielsAlder reaction is a thermally
driven [4+2] cycloaddition reaction between a dienophile
and a conjugated 1,3-diene. The DielsAlder reaction provides a simple, efcient, and clean procedure to generate
new bonds by inter- or intramolecular coupling, and it
represents one of the most useful synthetic methods in
organic chemistry. In this reaction, a dienophile is added
to a conjugated diene to give a cyclic product called an
adduct. The furan ring is one of the most important heterocyclic dienes used in DielsAlder reactions [195]. Chi
et al. [196] prepared polyimides at 80 C in the presence
of NaI in DMSO by the in situ DielsAlder polymerization of 1,4-bis[4-(methyloxy)phenyloxy]-2,3,6,7-tetrakis(bromomethyl)benzene (MPBB) with four arylenebismaleimides (AMIs), as shown in Scheme 47. DSC and
wide-angle X-ray diffractometry studies indicated that all
APIs appeared completely amorphous, and UVvis spectroscopy conrmed that DPAI and APIs were transparent
at wavelengths longer than 375 nm.
2.2.2.5. From silylated diamines and dianhydrides

(Scheme 48). The rst synthesis of aromatic polyimides
using silylated diamines was disclosed by Boldebuck
and Klebe [197] in the patent literature in 1967. The use
of silylated amines has several disadvantages, such as
the need to synthesize and purify activated monomers,
which are difcult to isolate because of their sensitivity
to moisture. Silylated amines are also more expensive
than diamines. Kaneda et al. [198] circumvented these
problems by using in situ silylated diamines that were
generated by adding chloro(trimethyl)silane (CTMS) or
other silylating agents to the diamine solutions. In situ
silylation of aromatic diamines with CTMS in the presence
of a base, such as pyridine, has proved to be a facile
and convenient method to obtain high molecular weight
polyimides [199]. When sterically hindered amines or
amines with strong electron-withdrawing groups are
used, silylation can improve the low reactivity of the
diamines [199].
Scheme 46. Synthesis of polyimides from diamine and bis(maleimide).
937
Scheme 47. Synthesis polyimide by DielsAlder reaction.
2.2.2.6. Di(hydroxyalkyl)
compounds
and
diimide
compounds (Mitsunobu reaction). The reaction of
di(hydroxyalkyl) compounds and diimides under Mitsunobu conditions gives rise to the direct formation of
polyimides in a single step. These reaction conditions
offer an alternative and convenient method for the
design of NLO-functionalized polyimides [181,200,201]
(Scheme 40(B)).
2.2.3. Dendritic and hyperbranched polyimides
Dendritic and hyperbranched polyimides are a new type
of polymer that has unique properties, such as the presence
of multi-functional end groups and good solubility. Jikei
et al. [202] reviewed dendritic aromatic polyimides, including dendrimers and hyperbranched polyimides. In addition
to conventional stepwise reactions for dendrimer synthesis, an orthogonal/double-stage convergent approach
and dendrimer syntheses with unprotected building
blocks are described as new synthetic strategies for
dendritic polyamides. Besides the self-polycondensation
of AB2 -type monomers, hyperbranched polyimides have
been formed in new polymerization systems with AB4 ,
AB8 , A2 + B3 , and A + BB2 monomers. As examples, the
general procedures of dendritic polyimides including dendrimers and hyperbranched polyimides are shown in
Scheme 49.
Scheme 48. Synthesis of polyimides from silylated diamines and dianhydrides.
938
Scheme 49. (A) Preparation of dendritic poly(ether imide)s by stepwise reaction and (B) preparation of hyperbranched polyimides via poly(amic acid)
precursors.
3. Physical properties
Because polyimides possess many desirable properties,
this class of materials has found applications in many
technologies, ranging from microelectronics to high temperature adhesives to high-performance membranes. For
quite some time, there have been active R&D programs
focused on synthesizing new polyimides and/or modifying
existing materials. There are many reports that address the

aspects and new developments in polyimides. Many factors affect the properties of polyimides. Generally speaking,
amorphous polyimides show better solubility but have
lower mechanical and thermal properties than polymers
that exhibit crystalline morphology. Herein, the reported
physical properties of polyimides are summarized based
on their chemical structures.
939
3.1. Solubility
3.2. Thermal properties
The solubility of polyimides depends strongly on the

chemical structure of the polymer. The two key factors in designing soluble and processable polyimides are
(1) reducing the rigidity or regularity of the backbone,
and (2) minimizing the density of imide rings along the
backbone. Progress has been made in addressing these
issues by using uorine-containing dianhydrides, such
as 4,4 -(hexauoroisopropylidene) diphthalic anhydride
(6FDA), or by incorporating hinges, such as oxygen
atoms, into the diamine (e.g., oxydianiline). In addition,
aliphatic side chains have been incorporated into diamines
to reduce the interaction between polyimide chains and
increase the solubility [203]. A large number of structural
modications have been attempted in previous decades,
including incorporating thermally stable, exible, or nonsymmetrical linkages in the backbone and introducing
bulky substituents [8,9]. Polyimides containing bulky,
propeller-shaped triphenylamine units along the polymer
backbone are amorphous and exhibit good solubility in
many aprotic solvents, excellent lm-forming capabilities,
and high thermal stabilities [132]. The incorporation of
pendant cardo groups, such as cyclododecylidene, adamantane, tricyclo[5.2.1.0] decane and triphenylamine, into
the backbone of polyimides improves their solubility,
processability and thermal stability. Furthermore, the tertbutylcyclohexylidene group, which can be considered an
alicyclic cardo group, has been incorporated into polymer backbones to improve the polymers processability
[64].
Most of the polyimides derived from rigid dianhydrides, such as PMDA, BPDA and BTDA, show less
solubility in organic solvents. Polyimides derived from
diamines containing exible ether, isopropylidene,
bulky pendant, and noncoplanar bisphenylene groups
exhibit excellent solubility in organic solvents, including N-methyl-2-pyrrolidinone (NMP), N,N-dimethyl
acetamide (DMAc), pyridine, cyclohexanone and
tetrahydrofuran (THF). In particular, the presence of
noncoplanar, unsymmetrical, kink, spiro and cardo structures in the diamine or dianhydride moiety improves
the solubility of the polymer without sacricing its
thermal or mechanical properties. The noncoplanar
monomers cause twists in the polymer backbones and
hinder chain packing; these modications reduce crystallinity and intermolecular interactions and enhance
solubility.
Polyimides prepared via thermal imidization have
lower solubility than those prepared via chemical imidization. The lower solubility might be a result of a tight packing
or partial intermolecular crosslinks that form during the
thermal imidization procedure [117]. Nevertheless, these
polyimides often have excellent thermal and mechanical
properties.
The polyimide synthesized from pyromellitic dianhydride and 4,4 -oxydianiline is not soluble in organic
solvents (Scheme 50(A)). However, the polyimide can
soluble in organic solvents only if the dianhydride was
changed to be 4,4 -biphthalic anhydride (Scheme 50(B))
[204].
Thermogravimetric (TG) analysis reveals good thermal

stability for aromatic polyimides. In general, polyimides
are stable up to a temperature of 440 C in a nitrogen
atmosphere. Polyimides containing heteroaromatic units,
noncoplanar or rigid aromatic units show high heat resistances and high glass transition temperatures. However,
polyimides that contain exible linkages, such as ether
units, show lower glass transition temperatures because of
their relatively exible polymer backbones. Pyridine rings
increase the symmetry and aromaticity of the polymer and
increase the thermal and chemical stability. In addition,
pyridine rings help the polymer retain its mechanical properties at elevated temperatures [37,112117].
3.3. Mechanical properties
The mechanical properties of polyimides are inuenced
by many factors, such as the chemical structure, viscosity,
molecular weight, preparation procedure, heating history,
sample preparation and the method of property determination. Therefore, there is no clear rule can be discerned
for the mechanical properties of the polyimides. Thus it is
likely that differences in mechanical properties are concealed by large experimental uncertainties. In general,
polyimides exhibit modulus values of 1.53.0 GPa and tensile strengths of 70100 MPa. However, the elongation at
breakage ranges from 2 to 15%, depending on the chemical structure. Polyimides containing exible linkage units,
such as ether linkages and isopropylidene [4], in the main
chain exhibit more elongation. In addition, noncoplannar, asymmetrical and amorphous polyimides also usually
show higher elongation. It is a general rule but not absolute:
the polymers with high mechanical modulus show lower
elongation.
Yokota and coworkers reported the dynamic tensile properties of asymmetric polyimides derived from
an asymmetric dianhydride [205207] or diamine [208]
using dynamic mechanical analysis. Yokota group discussed dynamic tensile properties of two Kapton-type
polyimides [208]. A symmetrical (PI(PMDA/ODA)) and an
asymmetrical polyimides (PI(PMDA/p-ODA)) derived from
symmetrical 4,4 -diaminidiphenyl ether (ODA) and asymmetrical 2-phenyl-4,4 -diaminidiphenyl ether (p-ODA)
diamines, respectively, with pyromellitic dianhydrides
(PMDA). The symmetrical polyimide and the asymmetrical
polyimide lms showed very tough but different dynamic
mechanical properties above Tg . The symmetrical polyimides showed higher glass transition temperatures and
without obvious drop in the E , however, the asymmetrical polyimide showed a signicant drop in the E from 109
to 107 Pa above the Tg , which results from the rotational
inexibility of the ether linkage of p-ODA and restricted the
conformational change of the PI(PMDA/p-ODA) backbone
structure.
3.4. Optical and electrical properties
Polyimides generally exhibit brown coloration due
to charge transfer (CT) between the diamine donor
940
Scheme 50. (A) Synthesis of insoluble polyimide derived from 4,4 -oxydianiline and (B) synthesis of soluble polyimide derived from 4,4 -biphthalic
anhydride.
moieties and dianhydride acceptor moieties. The color of

the polyimide can be changed by incorporating strong
electron-withdrawing CF3 groups in the dianhydride moiety or by using aliphatic diamines and/or dianhydrides.
These modications reduce CT interactions. Less colored
or colorless polyimides can be obtained by using dianhydrides with lower electron-acceptor capabilities and
diamines with lower electron-donating capabilities. When
these monomers are used, the intra- and intermolecular
CT interactions are weakened. Soluble and colorless polyimides can be obtained by using alicyclic dianhydrides
or diamine monomers. However, the polymers are less
stable at high temperature because they have less-stable
aliphatic segments [209]. Polyimides that contain adamantane (tricycle[3.3.3.1.1] decane), a rigid alicyclic compound
composed of three cyclohexane rings in chair conformations, produce light-colored lms with high transmittance
in the visible region and good thermal stability [68,69].
The incorporation of bulky CF3 groups into polyimides is
another approach to obtain light-color polyimide lms.

Polyimide backbones containing CF3 units have increased
solubility and optical transparency, in addition to lower
dielectric constants. These polymer characteristics have
been attributed to the low polarizability of the CF bond
and increases in the free volume of polymer.
The optical and electrical properties of polyimides can
be tailored by incorporating chromophores such as triphenylamine, carbazole, perylene groups, etc. Carbazole is
a conjugated unit that has desirable photoconductivity
and photorefractivity properties. In the eld of electroluminescence, carbazole derivatives are often used as
materials for hole-transport and in light-emitting layers because of their high charge mobility and thermal
stability. Carbazole-containing polymers exhibit blue electroluminescence. Because the nitrogen atom improves the
planarity of the biphenyl unit, the carbazole unit has a large
band gap, and this leads to the observed electroluminescence [132].
Polyimides that contain electron-donor and electronacceptor units enhance the CT interactions in the polymer,
and these polymers exhibit interesting memory switching
behavior [210,211]. The application on memory materials
based on polyimides will be described in Section 4.9.
4. Applications
Aromatic polyimides have excellent thermal stabilities and good mechanical properties, and they have been
widely used in photoresists, liquid crystal alignments,
gas separation membranes, composites, LB lms, blending
applications, vapor phase depositions, electroluminescent
devices, polyelectrolytes, fuel cells, electrochromic materials, nanomaterials and polymer memory materials. The
applications of polyimides are discussed in detail in the
following section.
4.1. Photoresists
Photosensitive polyimides (PSPIs) are widely used in
interconnects, multichip modules, protection layers, optical interconnects and resists because of their excellent
thermal and chemical stabilities, low dissipation factors,
and reasonably low dielectric constants. Because polyimides are insoluble in most common solvents, they are
usually processed in the form of their precursor poly(amic
acid)s and are then thermally converted to their corresponding imide structures. Kerwin and Goldrick rst
reported on the use of polyimides as photoresists, which
include poly(amic acid). Sodium dichromate is used as
a photoreactive additive [212]. The application of these
materials in electronic devices has been difcult because
of the instability of the polymer solution and the persistent
contamination of residual chromic ions. The rst report
of a material applicable to microelectronics devices was
made by Rubner et al. [213,214]. They described negativetype photosensitive polyimide precursors in which the
poly(amic acid) side chain carboxyl groups were esteried
with photoreactive methacryloyl groups. In the application
of polyimides to electronic devices, the formation of holes
and/or bonding pads is usually accomplished by an etching process in which a photoresist is used as an etch mask.
When a photosensitive polyimide is utilized, the patterning process is simplied, and the pattern can be formed
directly without the use of photoresists [215]. Recently,
researchers have reported that polyimides can be functionalized by incorporating new functional monomers in the
polymer reaction [60,216,217] or by chemical modication of functional polyimides [218]. These new-generation
photosensitive polyimides have improved resolution, solubility, stability and mechanical properties.
Yamashita et al. have developed a convenient fabrication method that utilizes a PSPI resin to produce a
corecladding structure for a self-written waveguide [219].
The PSPI resin shows a photo-bleaching effect in which the
refractive index of the exposed portion becomes large compared to that of the unexposed portion [220]. By using the
PSPI resin, an all-solid corecladding structure was realized
through exposure and thermosetting processes. This allsolid self-written waveguide has some additional features
941
that make it practical in the low-cost fabrication of optical

devices. The resin acts as a glue for the optical components and can be used as plastic packaging for the optical
modules.
Photosensitive polyimide insulation layers have been
introduced for fabricating superconducting integrated circuits. PI Research & Development Company and the
National Institute of Advanced Industrial Science and
Technology (AIST) have proposed a new ne-resolution
photosensitive polyimide, synthesized using aliphatic
materials, as a KrF photoresist [221]. This photosensitive
polyimide is synthesized via a single-step polycondensation reaction, and the polyimide has the attractive feature
of not requiring thermal curing; these properties might
allow for a low-temperature process to produce the polymer. Because the photosensitive polyimide is used as the
insulation layer instead of a conventional inorganic insulation lm, there is no need for an etching process, which
simplies the fabrication process.
In the microelectronics industry, polyimides are used
as passivation or insulation materials. Photosensitive
polyimides (PSPIs) have signicantly enhanced the development of microelectronic devices because they eliminate
the need for a photoresist. Negi et al. have reviewed the synthesis, characterization and applications of photosensitive
polyimides [8]; some recent PSPI will be discussed here.
Two common photoresists exist: positive acting resists
[222230] and negative acting resists [231235]. Positive
acting resists exhibit enhanced solubility after exposure
to radiation, whereas resists that become insoluble after
exposure to radiation are termed negative resists. In
general, positive type PSPIs are a combination of hydroxylcontaining polyimides and a photosensitive compound
that contains diazonaphthoquinone (DNQ) groups. However, it is important that the PSPIs are colorless and transparent. Therefore, diamines and/or dianhydrides containing hexauoropropane groups are used to prepare PSPIs.
Ishii et al. [222224] reported two positive type polyimides based on soluble block copolyimides (Bco-PI)s
that have excellent transparency characteristics. These
polyimides have hydroxyl groups and utilize diazonaphthoquinone as a photoreactive compound. The structures
of the (Bco-PI)s are shown in Scheme 51. The hydroxyl
groups in the polyimide backbone allow Bco-PIs to
be alkaline. The ester of 2,3,4-trihydroxybenzophenone
with 1,2-naphthoquinonediazide-5-sulfonic acid p-cresol
ester (PC5) or 1,2-naphthoquinonediazide-5-sulfonic acid
(NT200) as the photoreactive compound were added to the
Bco-PI. The polyimide lms have low dielectric constants
because of the aliphatic cyclic dianhydrides and/or uorocontaining diamines. The structures and reactions of PC5
and NT200 under exposing and developing conditions are
shown in Scheme 52.
Hsu et al. [225,229] and Ueda et al. [226,230] also
reported novel positive working and photosensitive polyimides that can be developed in aqueous base. These
polymers included 2025 wt% of DNQ or DNQ derivatives
as photosensitive compounds.
Polyimides that contain an o-nitrobenzyl carboxylate or phenoxide groups have been shown to
undergo a photorearrangement reaction upon UV
942
Scheme 51. Structure of Bco-PI.
Scheme 52. Structures and reactions of photoreactive compounds under exposing and developing.
irradiation. These photochemical reactions generate an o-nitrosobenzaldehyde and a carboxylic acid

group [236] or a phenol group [237]. This transformation makes the polyimides soluble in an aqueous
base. Shin et al. synthesized a novel positive-working
photosensitive polyimide [237], poly[1,4-phenyleneoxy1,4-phenylene-2,2 -di(2-nitrobenzyloxy)-benzophenone3,3 ,4,4 -tetracarboxdiimide]

(OPI-Nb),
that
can
be developed with an aqueous base. The polymer was synthesized using o-nitrobenzylation of a
Scheme 53. Synthetic route to photosensitive OPI-Nb.
943
Scheme 54. Photochemical reaction of OPI-Nb.
polyimide,
poly(1,4-phenyleneoxy-1,4-phenylene-2,2 dihydroxybenzophenone-3,3 ,4,4 -tetracarboxdiimide)
(OPI), as shown in Scheme 53. The aromatic OPI was
completely soluble in dilute aqueous NaOH and tetramethylammonium hydroxide (TMAH), whereas OPI-Nb
did not swell in these solutions. In the micropatterning
process, OPI-Nb showed a line-width resolution of 0.4-lm
and a sensitivity of 5.4 J/cm2 when the thin lm was
irradiated with 365-nm light and developed with a 2.38%
aqueous TMAH solution at room temperature for 90 s. The
photosensitivity of OPI-Nb is poor in comparison to that of
other commercial systems. The low sensitivity might arise
from absorption of UV light by the aromatic backbone in
the 240270 nm range and absorption in the 350370 nm
range by the intramolecular charge transfer transition of
OPI-Nb (Scheme 54). This phenomenon is referred to as a
matrix effect and has been observed in many PSPIs.
Negative type PSPIs usually contain side-chain
methacryloyl or acryloyl crosslinking groups and also
have a photosensitizer. Yin et al. reported [231] a negative
photoinitiator-free PSPI that incorporated the photosensitive 4,4-bis[(4-amino)thiophenyl] benzophenone (BATPB)
into its backbone and methacryloyl or acryloyl groups into
its side chains. Upon UV irradiation, the BAPTB structure
in the polyimide chain undergoes photolysis to produce
several types of radicals that can initiate polymerization of
the methacryloyl or acryloyl groups to form the crosslinked
system shown in Scheme 55.
As shown in Scheme 56, Tomoi et al. [233] prepared
polyimides with pendant carboxyl groups, which were
blocked with photopolymerizable acrylamides or acrylates
through ionic bonding. The ionic-bonded photosensitive
polyimide lms contain Michlers ketone (MK) as a photosensitizer and ethylene glycol dimethacrylate (EGDMA) as
an external multifunctional crosslinker. These lms exhibit
negative-tone behavior upon near-UV irradiation after they
have been developed with a 10% aqueous NaOH solution
at 25 C. The length of alkylene groups attached to the
nitrogen in aminoalkyl acrylates might affect the sensitivity of ionic bonded negative PSPIs. The more hydrophobic
the substituents, the more sensitive the PSPI is. When
acrylamides are employed as pendant photopolymerizable
groups in PSPIs, the resulting patterns are better than from
PSPIs that employ acrylates (Scheme 56).
A negative hyperbranched PSPI was prepared by Yin
et al. [234]. This polymer was based on a novel triamine
(TAPOB) and 6FDA. The photosensitive cinnamate groups
were incorporated at the periphery of the polymer by
derivatizing the terminal phenol groups with cinnamoyl
chloride. The fully imidized hyperbranched polyimide was
obtained via end group modication of an anhydrideterminated hyperbranched poly(amic acid) precursor. In
the photolithography process, 5 wt% of Michlers ketone
relative to the hyperbranched PSPI was used as the photosensitizer.
A group of fully imidized, soluble polyimides based
on 3,3 ,4,4 -benzophenone tetracarboxylic dianhydride
(BTDA) and ortho-alkylsubstituted diamines has been
shown to be highly sensitive negative-resist materials.
Such polyimides are sensitive to 365 nm (i-line) radiation without added sensitizers, and the obtained images
do not suffer from thickness loss at high temperatures
[235]. The photo-crosslinking of polyimides containing
benzophenone units and alkyl moieties, such as the polyimide synthesized from DAI and BTDA, is caused by a
recombination of radicals. These radicals are generated
when the excited triplet state of benzophenone abstracts a
hydrogen from the alkyl moiety as shown in Scheme 57.
Two types of PSPIs have been developing. In general,
positive type PSPIs are a combination of hydroxylcontaining polyimides and a photosensitive compound;
negative type of PSPIs should contain photosensitizers and
crosslinkers. Novel photosensitive groups and compounds
with high sensitivity undergo a chemical reaction upon UV
irradiation are important for development of positive type
PSPIs. Novel crosslinkers and crosslinking approaches are
944
Scheme 55. The crosslinking reactions in negative photoresist [231].
Scheme 56. Ionic bonded negative PSPI.
945
Scheme 57. Photocrosslinking of a benzophenone unit and alkyl moiety.
necessary for negative type PSPIs. Different polyimide or

copolyimide structures carrying out novel chemical reactions for positive or negative under photo-radiation are
candidates for application in PSPIs. It is also important to
consider colorless and transparent of the PSPIs.
4.2. Liquid crystal alignment

Liquid crystal display devices are composed of a liquid crystal, a color lter and a liquid crystal alignment
layer. The liquid crystal alignment layer must uniformly
align the liquid crystal molecules. The rubbing processes
rub an organic-polymer-coated substrate with a rotating
drum that is covered with a rayon velvet fabric cloth. Different mechanisms have been proposed for the alignment of
liquid crystals on rubbed polymer surfaces. One theory suggests that the mechanical rubbing creates microgrooves or
scratches on the polymer surfaces. The LC aligns along the
grooves to minimize the energy of elastic distortion [238].
Alternatively, the rubbing process aligns surface polymer
chains, which in turn, align the liquid crystals through
intermolecular interaction [239]. The rubbing method is
simple, convenient and inexpensive. However, the process generates dust, has electrostatic problems, and it
is difcult to control the rubbing strength and uniformity after the production of TFT-LCDs has begun [240].
The rubbing process mechanism has been studied and
some alternative methods have been proposed, including
photoalignment [241245] and the use of microgrooved
surfaces and LangmuirBlodgett membranes [246,247].
Several processes to produce microgrooved structures
have been proposed, including reactive ion-etching on
glass surfaces with chromium masks [248,249], pattern
formation by laser-induced periodic structures on a polymer surface (LIPSS) [250], holographic light exposure or
exposure of photocurable polymer lms to UV through
masks with a grating pattern [251]. The photoalignment
technique involves the generation of an anisotropic distribution of alignment material molecules by using the
molecules dependence on the polarization direction of
absorbed light [241245,252]. There are three main types
of materials that are used as photoalignment layers. They
can be categorized according to the photochemical reaction responsible for the photoalignment: (i) azo-containing
polymers (photoalignment by reversible cistrans isomerization), (ii) crosslinkable materials (photoalignment by
photo-dimerization), and (iii) polyimides (photoalignment
by photodegradation) [253,254].
A vertical alignment method has been used to improve

the alignment of liquid crystals (LCs) with negative dielectric anisotropies. These LCs achieve faster response times
and have higher contrast ratios than do twisted nematic
liquid crystal displays [255]. In the negative dielectric
anisotropy LC display, a polyimide layer was used as
the liquid crystal alignment layer, and after a rubbing
process, the liquid crystals were vertically aligned in
the eld-off state at a pretilt angle above 89 . Yi et al.
synthesized a series of poly(amic acid)s from CBDA,
DDA and functional diamines, which have side chain
groups of different exibility (Scheme 58). The authors
researched the effects that the polyimide side chain
structure has on the pretilt angle of liquid crystal cells
[56]. For the full-color, TFT-LCD organo-soluble polyimide alignment, unsymmetrical bulky structures and high
voltage-resistances are required. In addition, non-polar and
non-conjugated polyimides are also necessary (Scheme 58)
[256].
Ueda et al. [257] reported novel siloxane-containing liquid crystalline (LC) polyimides with methyl, chloro, and
uoro substituents on mesogenic units (Scheme 59). These
polyimides were developed from siloxane-containing
diamines with pyromellitic dianhydride (PMDA) or
3,3 ,4,4 -tetracarboxybiphenyl dianhydride (BPDA), and
their thermotropic LC behavior was examined. Among
these polyimides, the chloro and uoro substituents
effectively form LC phases, particularly when the substituents are substituted away from the center of
the mesogenic unit. The isotropization temperature
is signicantly affected, but the crystalLC transition
temperatures are signicantly decreased. The methyl
substituent, however, tends to interrupt liquid crystallization. Thus, the uoro-substituted polyimide derived
from BPDA exhibits the lowest crystallineLC transition temperature (Tcrlc = 134 C) among all polyimides
and maintains its liquid crystal form up to 238 C. Xray diffraction measurements of the mesogenic phases
of brous polyimides were found to form SmA and
SmC as high- and low-temperature mesophases, respectively.
Many photoalignment approaches are developed as
described above, such as microgrooved structure, pattern
formation by laser-induced periodic structures, photocurable polymer through masks with a grating pattern, etc. As
a liquid crystal alignment layer to uniformly align the liquid
crystal molecules, more efcient and novel photoalignment techniques or photoalignment reactions should be
developing in the future.
946
Scheme 58. Synthesis of the (polyamic acid)s and the polyimides from CBDA and DDA [256].
4.3. Gas separation membrane

DuPont (USA) and Ube Industries (Japan) have been
pioneers in the commercial application of polyimides in
separation processes. DuPont began developing membranes for industrial separations in 1962 and initially
sought to separate helium from natural gas (Scheme 60).
In 1981, Ube Industries developed and launched the production of Upilex-R, a polyimide material that exhibits high
thermal resistance and is stable to various organic solvents and vapors, hydrogen sulde, and ammonia vapor
[1]. An important objective in the development of new gas
separation polymer membranes is to combine a high gas
permeability with a high selectivity [258]. Over the past
947
Scheme 59. Novel siloxane-containing liquid crystalline (LC) polyimides [257].
two decades, there has been increasing interest in polyimides as membrane materials for gas separation purposes
[1,259265].
Many new polyimide structures have been proposed
for gas separation applications, including hyperbranched
polyimides [266269], indan structures [270272],
brominated polyimides [273276], noncoplanar structures [18,277,278], and polyimides with bulky groups
[258,279,280], among others, to improve the performance
(separation factors) of these applications. Mixed matrix
composite membranes (MMCMs) [281289] and carbon
molecular sieve membranes (CMSMs) [290299] have
also been examined for gas separation in recent years. The
MMCMs fabricated by encapsulating zeolites or molecular sieves into polymer matrices have been recognized
as a promising alternative to conventional membranes
[300304].
Carbonization of polymeric membranes has been studied in the past few years as a method to improve the
permeation properties and thermal resistance of polymeric
membranes. Carbonization offers a promising alternative
to both inorganic and polymeric membranes. CMSMs are
rigid and highly porous materials, and the selectivity phenomenon of CMSMs involves a size-sieving mechanism.
CMSMs possess a distribution of small, selective pores,
which are similar in size to diffusing gas molecules (36 A)
[305307]. CMSMs are responsible for the high permselectivity of small gas pairs, such as O2 /N2 , H2 /N2 , CO2 /N2 and
He/N2 . Thus, the high gas separating-ability is dependent
on an effective size-sieving mechanism in these materials. Koresh and Soffer [308] proposed the rst modication
methods of CMSM. They showed that the permeability
of CMSMs increased in oxidized membranes, whereas
lower permeability was observed in sintered membranes.
Carbonization of polymer precursors has been adopted
as a useful method for preparing CMSMs. Many studies
have reported that CMSMs with tailored microstructures
could be obtained by controlling the pyrolysis conditions
[305,306,309314] or the post-/pre-treatment conditions
[308,315320]. A number of researchers, including Koros
[299], Haraya [312,319], Kusakabe [320322], Centeno
[323,324], Okamoto [315], Tsotsis [309], Chung [325], and
Lee [326], have described the preparation and characterization of CMSMs by carbonization of polyimides.
One way of achieving good gas transport performance is
to fabricate an asymmetric polyimide membrane that consists of a defect-free (surface defects of less than 1 m)
skin layer and has a high gas permeability and good gas
selectivity [327]. Koros et al. [305] reported the fabrication
of defect-free polyimide hollow ber membranes using
a dry-jet, wet quench process. In addition, many other
groups [299,305,328333] have reported the preparation
Scheme 60. Polyimide for gas separations.
948
of defect-free polyimide hollow ber membranes. These

groups have also characterized and fabricated other polyimide hollow ber membranes for gas separations.
Many new polyimide structures have been proposed for
gas separation applications as describe above. The high gas
separating-ability is dependent on an effective size-sieving
mechanism in the polymers. The important factors affecting the separation efciency are the physical space and
chemical selective. Therefore, the polymer morphology and
chemical structures are important for the performance of
the separation membrane. Of course, the thermal properties of the membranes are also important issue for the
membrane application.
A novel combination of modied Stber solgel techniques has been used to prepare silsesquioxane nanoparticles. An aqueous synthesis technique for polyimide
resins has been used to prepare polyimidesilsesquioxane
nanocomposites. Silica nanoparticles are strongly associated with the polyimide and appear to be attached to the
surface of the polymer [334].
Chen et al. [335] prepared a series of new
polymersilica hybrid materials through the intrachain coupling of 3-aminopropyl trimethoxysilane
(APrTEOS) and interchain hydrogen bonding with
-glycidyloxypropyltrimethoxysilanes
(GOTMS)
(Scheme 61). The prepared hybrid lms were homogeneous and thermally stable. The thermal properties of the
organo-soluble polyimides were signicantly enhanced
by hybridizing 6.307.99 wt% of silica. It was found that
the intrachain chemical bonding could effectively enhance
the glass transition temperature or CTE in comparison to
interchain interactions. The prepared hybrid lms had low
dielectric constants, tunable refractive indices and a high
optical transparency, which suggests that these lms could
have potential applications in optoelectronic devices.
4.4. LB lm
The ultrathin mono- and multilayer lms of polyimides
have been successfully prepared using LangmuirBlodgett
(LB) techniques in conjunction with a precursor method.
Electrically insulating ultrathin LB lms of polyimides
(0.4 nm) have been successfully prepared on solid substrates [336,337]. The polyimide LB lm between two metal
electrodes showed excellent electrical insulating properties, had a resistance higher than 1015 cm and had an
electrical breakdown strength higher than 107 V/cm.
Electrostatic
phenomena
that
occur
at
the
metal/polymer interface are interesting to the elds
of organic and molecular electronics. Many investigations
have been performed to clarify the contact potential
formed at the metal/polymer interface. Various models,
such as the surface states model, the molecular-ion state
model and the local (intrinsic) model have been proposed
[338340].
Polyimide LB lms were found to work well as tunneling barriers in tunnel junctions. Furthermore, the
Nb/Au/PI/Pb-Bi junctions had an IV curve that is characteristic of a superconductorinsulatorsuperconductor (SIS)
transition. Iwamoto and coworkers [341] reported the fabrication of this Josephson junction as shown in Fig. 1. The
Fig. 1. Josephson junction (A) top view and (B) side view.
Josephson junction was very effective in the detection of

microwaves.
Sakai and coworkers [342,343] showed reproducible
memory switching effects between a high-impedance state
(OFF state) and a high-conductance state (ON state) in a
metal/polyimide or squarylium dye (SQ) LB lms/metal
device. This device also utilized a noble-metal base electrode. The device fabrication process is shown in Fig. 2.
The switching elements have also been applied in scanning
tunneling microscope (STM) probes.
Iwamoto et al. reported the preparation of polyimide
LB lms and their groups investigations of interfacial
phenomena, such as surface potential, spatial distribution of charge, potential changes with photo-irradiation,
and charge storage phenomena. The presence of electronacceptor and donor states was revealed, and excessive
electronic charges were discovered to transfer from metals
to the LB lms. It was found that a high density of electronic states (1025 1026 m3 ) and a very high electric eld
(108 109 V/m) exist in the interfacial space charge layer
within a range of 10 nm [344]. Thus, it is obvious that the
interfacial space charge makes a signicant contribution to
the device operation.
Kakimoto and Imai et al. [345] reported polyimide
LB multilayer lms that have different chemical structures. These lms were constructed from polyimides that
have a triphenylamine unit as an electron donor (D), a
tetraphenylporphyrin as a sensitizer (S), and an aromatic
polyimide as an electron acceptor (A) (Scheme 62). Two
types of photodiodes with E/D/S/A and E/A/S/D systems
were prepared on a semi-transparent gold electrode (E).
The dependence of the number of D, S, and A layers on
the photocurrent behavior was examined as a way to study
949
Scheme 61. Prepared series of new polymersilica hybrid materials through the intrachain bonding by -glycidyloxypropyltrimethoxysilanes (GOTMS)
[335].
the electron transfer in each layer. In the photodiode system, the hole mobility in the D layer was high, whereas
the electron mobility in the S and A layers was relatively
low.
LangmuirBlodgett techniques can be used in many
applications due to the ordered structures and layer controllable properties. Doubtlessly, more applications using
LB techniques to afford ordered and controlled layer structures will be reported in the near future.
4.5. Electroluminescent polyimides
In recent years, research in organic electroluminescence (EL) and polymeric light emitting diodes (PLEDs)
has intensied. Currently, -conjugated polymers, such as
poly(p-phenylene vinylene), poly(thiophene) and poly(pphenylene), are the most studied electroactive polymers
for light-emitting components in PLEDs, and they have

been demonstrated in large-area display applications
[346348]. Although aromatic polyimides have been
extensively studied as high-performance polymers for
their unique properties, few attempts have been made
to use them for the light-emitting layer in EL devices
[349351]. Their applications have been limited to transporting layers [352354] or as dye doped matrices in EL
devices [355357].
Kakimoto et al. fabricated a PLED device using a
polyimide-based LangmuirBlodgett lm with indium tin
oxide (ITO) as an anode and MgAg as a cathode [350].
The device emits orange-red light at a forward bias voltage above 7 V. The EL intensity depended linearly on
the applied current density, which indicates that the
light emission results from the recombination of injected
charges in the polyimide layer.
Fig. 2. Fabrication process of switching elements. (A) The resist is patterned with an undercut prole. (B) The base-electrode metal is deposited. (C) The
excess metal is lifted off when the resist is removed, leaving behind a patterned deposit on the substrate surface. (D) The LB lm is deposited. (E) The
top-electrode metal is deposited and patterned using a conventional wet etching after the resist process.
950
Scheme 62. Structures using triphenylamine unit as an electron donor (D), tetraphenylporphyrin as a sensitizer (S), and aromatic polyimide as an electron
acceptor (A).
Maltsev et al. also reported the fabrication and operational characteristics of polyimide electroluminescent
devices [356,358]. The photoluminescence of anthracenecontaining aromatic polyimides (APIs) has a strong and
pronounced exciplex character that arises from interchain donoracceptor interactions between the excited
anthracene groups and the diimide fragments. The LEDs
with uni-layer sandwich structures (ITO/API/Mg:Ag) have a

maximum brightness greater than 100 cd/m2 at 15 V [359].
The aromatic polyimides shown in Scheme 63 [356,358]
contain sulfur atoms in the polymer backbone and were
studied as electron-hole transporting and light-emitting
materials. These polyimides were used in combination
with tris(8-hydroxyquinoline)-aluminium in multilayer
Scheme 63. Molecular structures of APIs [356,358].
organic electroluminescent devices. When operating under

a forward bias with ITO as the positive electrode, bright
emission was observed in the visible range. A maximum
luminance of 15,000 cd/m2 was reached at 14 V.
Chan et al. [351] described the fabrication of singlelayered light-emitting devices with polyimides functionalized with [Ru(tpy)2]2+ complexes (Scheme 64). Red light
was emitted, and a maximum luminance of 120 cd/m2 was
observed.
Yang et al. [360] synthesized a novel diphenyluorenebased cardo copolyimide containing perylene (PFB5)
(Scheme 65). This polymer was synthesized by through
a polycondensation of a diamine, 4,4-(9H-uoren-9ylidene)bisphenylamine, with perylene dianhydride and
another dianhydride. The reaction was carried out in mcresol with an isoquinoline catalyst at 200 C. Because of
the bulky diphenyluorene units in the backbone, PFB5
has a high thermal stability and good solubility in common solvents. The high solubility of PFB5 in low boiling
solvents allows direct spin coating of the polymer lms,
which exhibit intense photo- and electroluminescence (EL)
in the visible range. This non-conjugated polymer could be
used in emitting and electronhole transporting layers in
polymer electroluminescent devices (PLEDs).
Moreau and coworkers [361] described the electroluminescence characteristics of a phenyleneethynylene-based
polyimide that combines a C2 chiral structure with a conned chromophore (Scheme 66). The C2 symmetry of the
chiral unit induces secondary structures that reduce intraand interchain interactions and -stacking. A monolayer
electroluminescent polymeric diode exhibits high performance (1500 Cd/m2 at 10.5 V with a turn-on voltage of
5.5 V).
Electroluminescent devices using polyimides as active
layers in the PLED were reported. In general, the polyimides
are prepared by incorporating chromophores with high
efciency. The organo-soluble and high thermal properties
are required for the developing of active-layer polymers in
PLED. The relationship between solubility, thermal properties and chemical structures should be considered in the
designing of polyimide-based PLED.
4.6. Polyelectrolytes
The idea of using an organic cation exchange membrane as a solid electrolyte in electrochemical cells was rst
described in a fuel cell in 1959. At present, the polymer
electrolyte fuel cell (PEFC) is the most promising alternative of all the fuel cell systems in terms of its mode of
operation and applications. The Naon series have been
almost the only advanced membranes that have been used
in practical systems. Unfortunately, these membranes have
some demerits, including a high cost, low conductivity at
low humidity or high temperatures, and high methanol
permeability. Rikukawa et al. presented an overview on
proton-conducting polymer electrolyte membranes based
on hydrocarbon polymers for fuel cell applications [362].
The peruorinated ionomer membranes are highly proton conductive (102 S cm1 in its fully hydrated state)
and chemically and physically stable at moderate temperatures. However, these desirable properties deteriorate
951
above their glass transition temperature (Tg ) of ca. 110 C

[363,364]. High gas permeability, high cost, and environmental inadaptability of the uorinated materials are also
serious drawbacks for their practical applications in fuel
cells.
In the past decade, some proton conductive materials have been proposed as alternative membranes
[365368]. These materials include a variety of nonuorinated hydrocarbon proton-conducting materials [369] that
are inexpensive and perform well. Several research groups
have proposed that sulfonated polyimide (SPI) membranes
used for PEFCs have a higher proton conductivity than peruorinated materials [362,370,371].
Mercier and coworkers rst synthesized a series
of sulfonated copolyimides from naphthalene-1,4,5,8tetracaboxylic dianhydride (NTDA), 2,2 -bendizine sulfonic acid (BDSA) and common nonsulfonated diamine
monomers [160,372] (Scheme 67). The proton conductivity of these membranes is rather low (<102 S cm1 at
100% relative humidity) due to a low ion exchange capacity
(IEC), which is essential for maintaining hydrolysis stability. These sulfonated copolyimide membranes have been
practically tested in fuel cell systems and exhibit fairly good
performance.
Litts group has also used BDSA as a sulfonated diamine
monomer in the preparation of various random and
sequenced copolyimides [373]. They reported that some
copolyimide membranes containing bulky and/or angled
comonomers had higher conductivities than Naon at all
humidities. However, the poor hydrolytic stability of their
membranes remains a problem.
Okamotos group synthesized a series of main-chain
type, side-chain type, branched/crosslinked (B/C) and sulfonated polyimides (SPIs) from sulfonated diamines [161]
(Scheme 32). The main-chain type copolyimide containing
uorine disulfonic acid and the side-chain type polyimide
containing either propane sulfonic acids or sulfopropoxy
groups displayed similar or higher proton conductivities
than commercial Naon 117 [163166,168,369,374,375]
(Schemes 3336, 68 and 69). The branched/crosslinked
structure SPIs also exhibited high mechanical strengths
and fuel cell performance comparable to that of Naon
112 in a single H2 /O2 PEFC system. At 120 C, the B/C SPIs
membranes showed conductivity values of approximately
0.020.3 S cm1 at 50100% RH [369,376], and they exhibited fuel cell performance comparable to that of Naon
112 in a single H2 /O2 PEFC system [376]. In addition,
Watanabes group has also reported the synthesis and
characterization of sulfonated polyimides (SPIs). The uorenyl containing SPIs exhibited proton conductivity up to
1.67 S cm1 (at 120 C and 100% RH) when the bulky uorenyl groups were incorporated at a level of 3060 mol%
into the polymer [377]. Branching was introduced with
melamine and cross-linking was introduced by electron
beam irradiation; these modications produced a positive effect on the oxidative stability and mechanical
strength while maintaining many of the desirable conductivity properties [378]. The introduction of 30 mol%
of bis(triuoromethyl)biphenylene groups into sulfonated
copolyimides resulted in a balance of oxidative stability, proton conductivity and mechanical properties. Direct
952
Scheme 64. Structure of the [Ru(tpy)2]2+ .
Scheme 65. Novel diphenyluorene-based cardo copolyimide containing perylene (PFB5) [360].
methanol fuel cell (DMFC) experiments revealed that the

methanol crossover through the FSPIH-30 membrane was
only 30% of that of Naon 112. The lower methanol
crossover would be a great advantage over the peruorinated ionomers when taking the efciency of fuel cells
into account [379]. The introduction of aliphatic groups
in the main chain and/or side chains could signicantly
improve the hydrolytic stability of sulfonated polyimides,
which could possibly make these polymers better than
Naon 112 [380,381]. Side-chain sulfonated polyimide
membranes have been evaluated as electrolyte membranes in DMFCs. The performance was compared with that
of Naon 112 under various operation conditions. The
methanol crossover in the cell based on SPI was a half of
that of Naon 112 and resulted in improved cell efciency.

The advantage of using SPI over Naon 112 became even
clearer when the DMFC was operated at a higher temperature (Tcell ) or a higher concentration of methanol (CMeOH )
[382].
Liu et al. [383] prepared a series of SPIs that contain ether and ketone linkages through a copolymerization
method, as shown in Scheme 70. At 80 C, the proton conductivities of several samples, including SPI-KK-X
and SPI-K-X, were higher than 0.10 S cm1 ; these values
are comparable to that of Naon . Methanol permeabilities of the obtained polymer electrolyte membranes
(PEMs) were in the range of 1.432.03 107 cm2 /s, which
is several times lower than that of Naon 117. It is
Scheme 66. Phenyleneethynylene-based polyimide which combines the C2 chiral structure with a conned chromophore [361].
Scheme 67. Structures of BDSA/NTDA/mAPI and BDSA/NTDA/ATB [160].
Scheme 68. Synthesis of BAPSBPS-based polyimides [168].
953
954
Scheme 69. Copolyimides bearing pendant sulfonic acid groups [375].
interesting to note that the SPI-KK-X series, which has

a more rigid phenylketonephenylketonephenyl moiety, had a lower dimensional swelling ratio and a lower
methanol permeability in comparison to the corresponding SPI-K-X series at the same ion exchange capacity
(IEC).
Sotzing et al. [384] described the template polymerization of EDOT with sulfonated poly(amic acid) (SPAA)
that resulted in a stable conducting polymer aqueous dispersion, PEDOT-SPAA, with a particle size of ca. 63 nm.
In lms of PEDOT-SPAA, the sulfonated poly(amic acid)
template undergoes imidization within 10 min at temperatures greater than 150 C and results in a PEDOT-sulfonated
poly(imide) (PEDOT-SPI) with a 10-fold conductivity
enhancement. This material is highly thermally stable as
compared to PEDOT-PSS (Scheme 71). Thermal stability
is necessary for many processing applications of conducting polymers. Isothermal TGA experiments were run
at 300 C for PEDOT-PSS and PEDOT-SPAA and revealed
that PEDOT-SPAA had a smaller slope for degradation.
Annealing the lms at 300 C for 10 min caused the conductivity of the PEDOT-PSS lms to be unmeasurable
(<1 105 S cm1 ), while those of PEDOT-SPAA increased
6-fold. Secondary doping of the PEDOT-SPAA system
with additives commonly used for PEDOT-PSS was also
investigated.
Commercial peruorinated polymer electrolyte membranes have been extensively used as polymer electrolytes
for fuel cells; however, they are expensive. Several research
groups have proposed that sulfonated polyimide (SPI) or
copolyimide membranes used for PEFCs have higher proton
conductivity than peruorinated materials. The polyimides
have great variety with regard to chemical structure and
can be modied chemically at very low cost. However,
the poor hydrolytic stability of their membranes remains a
problem. Therefore, low-cost new proton-conducting polymer electrolytes with long-term stability and mechanical
properties are necessary for the developing of polyelectrolytes.
4.7. Electrochromic polyimides
Electrochromism occurs when color can be alternated
by applying a potential [385387]. This interesting property has led to many technological applications, such
as smart windows, automatic antiglazing mirrors, largescale electrochromic screens, and chameleon materials
[388392]. The electrochromic behavior of polymers is
based on the redox behavior of the polymers.
Most hole-transport layer (HTL) materials are based
on ternary aromatic amines, such as N,N -diphenylN,N -bis(3-methylphenyl)(1,1 -biphenyl)-4,4 -diamine
Scheme 70. SPIs containing ether and ketone linkages through a copolymerization method [383].
955
Scheme 71. PEDOT-SPI derived from PEDOT-SPAA [384].
Scheme 72. Triphenylamine-containing monomers A1, TPD and a-TPD.
(TPD)
and
N,N -bis(1-naphthyl)-N,N -diphenyl-1,1 biphenyl-4,4 -diamine (a-NPD); these are shown in
Scheme 72 and are known for their high hole mobility
[357,393398]. Recently, Liaws group has reported many
triarylenylamine-based electrochromic polyimides from
novel diamines (A1), as shown in Scheme 73.
Electrochromic polyimides with high molecular
weights based on triphenylamine derivatives are
synthesized from the diamine and various aromatic
dianhydride via a direct polycondensation, as shown in the

Table 1 [54].
In Table 2, polyimide Ib shows a better solubility prole
and shorter switching and bleaching times but a lower thermal stability than polyimide P1 and P2 are results of the
pendant 2-phenyl-2-isopropyl groups. Polyimide Ib and
P1 lms changed from their original pale green-yellowish
color to green and then to Prussian blue. This color change
correlates with the oxidized form. However, the polyimide
Scheme 73. Copolyimides with propeller-shaped triarylamine unit prepared from the diamine and various aromatic dianhydride via direct polycondensation [54].
956
Table 1
Properties of electrochromic polyimides [54].
O
H2 N
CH3
C
CH3
NH2
1. DMAc
O Ar O
CH3
C
CH3
Ar
CH3
C
CH3
O
CH3
C
CH3
Ib
N
n
Ic
O
S
O
CF3
C
CF3
Ar
2. -H2O
Ia
Color
Switching voltage
Tg ( C)
Td10 in N2 ( C)
Td10 in air ( C)
First: 0.93 V
Second: 1.32 V
241
519
424
First: 0.97 V
Second: 1.30 V
266
528
466
First: 0.94 V
Second: 1.32 V
267
504
446
Table 2
The triphenylamine containing polyimide synthesized by Liaws and Lious group [54,447]. (For interpretation of the references to color in this table, the
reader is referred to the web version of the article.)
O
N
Ar
Code
Ib
Ar
Switching (oxidation)
voltage
P1
CH3
C
CH3
1st
2nd
Switching time (s)

Bleaching time (s)
CH3
C
CH3
P2
0.93 V
1.32 V
0.78 V
1.13 V
1.25 V
4.5
1.9
20
3
a
a
0 V (Yellow)
0 V (Yellow)
0 V (Yellow)
1.2 V (Green)
0.98 V (Green)
1.3 V (Blue)
Color changing
1.5 V (Blue)
Tg ( C)
Td10 in N2 ( C)
Td10 in air ( C)
Solubility in organic
solvent
NMP
DMAc
DMF
DMSO
m-Cresol
THF
CHCl3
1.35 V (Blue)
241
519
424
264
610
600
295
608
611
S
S
S
+h
+h
S
S
S
S
P
+h
+h
P
S
+h
+h
I
+h
+h
I
I
S: soluble at room temperature; +h: soluble on heating to 70 C; P: partially soluble on heating 70 C; I: insoluble.
a
Not reported.
Fig. 3. Electrochromic behavior of polyimide Ib lm (in CH3 CN with 0.1 M

TBAP as the supporting electrolyte).
P2 lm only changed from pale green-yellowish to Prussian blue color because the polyimide P2 had only one
triarylenylamine group in a unit.
The polyimide from diphenyl-3,3 ,4,4 -tetracarboxylic
dianhydride and pyromellitic dianhydride is insoluble.
However, the solubility can be improved by copolymerization to obtain a strong and tough lm (Table 1) [54].
The electrochromism of these polymer thin lms was
examined by an optically transparent thin-layer electrode
coupled with UVvis spectroscopy. The electrode preparations and solution conditions were identical to those used
in cyclic voltammetry. The typical electrochromic spectra
of polyimide Ib is shown in Fig. 3. The color switching
time was estimated by applying a potential step, and the
absorbance proles were followed (Fig. 4). The switching
time was dened as the time required for reaching 90% of
the full change in absorbance after switching the potential.
All theoretical calculations in this study were performed
using the quantum mechanical package of Gaussian 03
[399,400]. The equilibrium structure of basic unit M3 was
determined using DFT with the B3LYP functional and 631G(d) basis set. The atomic charge was determined using
Mulliken population analysis.
Fig. 4. (A) Potential step absorptometry and (B) current consumption of

polyimide Ib (in CH3 CN with 0.1 M TBAP as the supporting electrolyte) by
the application of a potential step 01.20 V.
957
The sketch map of the polyimide Ib structure was determined by DFT(B3LYP/6-31G(d)). The main atomic charge
difference was located on the 1N, 9C, 29C, and 32N atoms
(Scheme 74). In the rst oxidation (loss of the rst electron),
the 1N, 9C, 29C and 32N atoms contribute 3.6%, 2.0%, 3.8%
and 4.3% of an electron, respectively. For the second oxidation (loss of the second electron), the 1N, 9C, 29C and 32N
atoms contribute 2.1%, 1.5%, 1.5% and 1.6% electron, respectively. The electron density contour of the ground state and
rst oxidation state are plotted by Gauss View and is shown
in Scheme 74. This plot suggests that the nitrogen lone pairs
have a strong coupling with the electrons. The electron
density distribution of the rst oxidation state is broader
than that of the ground state, but the main distribution was
located on the N,N,N ,N -tetraphenyl phenylene diamine in
both cases.
A new oxidation mechanism based on molecular orbital
theory was proposed by Liaws group. In other words, all of
the atoms in the HOMO (rst oxidation) or SOMO (second
oxidation) contributed to the oxidation of the molecule;
the oxidation was not dictated only by the nitrogen atoms
(Scheme 74).
Polyimide-based polymers are great candidates for
the electrochromic materials due to their unique thermal properties. Triphenylamine-containing polyimides are
major structure developed for electrochromic polymers
in the literatures. Electrochromic polyimides containing different electrochromophores and exhibits longterm stability should be developed for electrochromic
application.
4.8. Nanomaterials
Recently, carbon nanotubes (CNTs) have attracted
much attention because their nanocomposite material is
expected to enhance mechanical properties and improve
load transfer and tear resistance. In addition, they may
be able to achieve certain levels of electric conductivity through a percolation network for charge mitigation
and electromagnetic shielding [401413]. The combination of carbon nanotubes and polyimides is expected
to play an important role in the development of novel
high-performance nanocomposites [414]. There are two
common processing techniques for fabricating the composites. One is to mix CNTs with the resin matrix in the melt
state to form composites. The other technique involves
dispersing the CNTs into a polymer solution, performing
solution casting, and removing the solvent to obtain the
composite.
Park and coworkers have reported a process that
effectively disperses single wall carbon nanotube (SWNT)
bundles into an aromatic polyimide matrix at a nanoscale
level [407]. The resultant SWNTpolyimide nanocomposite
lms are electrically conductive (antistatic) and optically transparent. A sharp increase in conductivity was
observed between 0.02 and 0.1 vol.% of SWNT (Fig. 5); during this process, the nanocomposite was converted from a
capacitor to a conductor. Incorporation of 0.1 vol.% SWNT
increased the conductivity by 10 orders of magnitude,
which surpasses the antistatic criterion of thin lms for
space applications (1 108 S cm1 ). The polyimide lm
958
Scheme 74. Electrochromic mechanism and electronic density contours of polyimide Ib structure.
Fig. 5. Volume conductivity of SWNTCP2 nanocomposites [407].

Copyright 2002, Elsevier Ltd.
that contained 1.0 vol.% SWNT still transmitted 32% of visible light at 500 nm, whereas the lm that was prepared by
direct mixing transmitted less than 1%. Dynamic mechanical testing data showed that the modulus increased up
to 60% at 1.0 vol.% SWNT loading and that the polyimide
thermal stability was enhanced in the presence of SWNT.
Connell et al. [408] reported the synthesis of an
alkoxysilane terminated poly(amic acid) polymer. The
SWNTs were added to a pre-made poly(amic acid) solution, which is in contrast to the previously mentioned
in situ method. At a loading of 0.05 wt% SWNT, the percolation threshold was reached, as evident by the sharp
drop in the surface resistivity of the material. The surface resistivity (1.7 108 /square) and volume resistivity
(1.7 109 cm) results indicate that the SWNTpolyimide
composite is conductive. However, the SWNTs in the
polyimide had a negligible effect on the Tg and tensile properties of the polymer [408]. Increasing the ionic
strength of the polyimide matrix by adding an inorganic
salt (CuSO4 ) resulted in sufcient SWNT network formation to afford conductivity. The addition of 0.014 wt% CuSO4
to a composite containing 0.03 wt% SWNTs resulted in
a lm that exhibited 4 orders of magnitude reduction
in surface and volume resistivity [415]. Polyimide lms

containing SWNTs throughout the bulk of the lm had volume resistivities sufcient for electrostatic charge (ESC)
mitigation, but the optical properties (solar absorptivity,
and transparency, %T) were negatively affected. It was
found that lms coated with SWNTs (spray-coating) exhibited surface resistivities of 1 107 to 1 106 /square
and had a high degree of exibility and robustness, as
evidenced by their retention of surface resistivity after
harsh manipulation [416]. Polyimides prepared from side
chain aliphatic diamines and various aromatic dianhydrides were successful in breaking up SWNT agglomerates
and resulted in homogeneous SWNT suspensions in
DMAc. Increased electrical conductivity was observed
in the nanocomposite lms. However, electrical percolation occurred at higher loadings than those typically
expected for SWNTpolyimide nanocomposites. Modulus values of the lms increased slightly with higher
SWNT loading. Electrospun bers were prepared from the
same SWNTpolyimide suspensions used for preparing
the lms. High resolution SEM images showed that the
SWNTs were captured in the interior of the ber and
might have some directionality parallel to the ber axis
[417].
Sun et al. [418] reported the fabrication of functionalized CNTs using amine-terminated polyimides with
pendant hydroxyl groups. The resulting polyimidefunctionalized CNTs were found to be soluble in the same
solvents as the parent polyimide. A signicant advantage
with this approach is that the functionalized nanotube
samples can be used directly to prepare polyimide-CNTs
with relatively higher nanotube contents.
Bin et al. [419] prepared polyimidecarbon nanotube
composites by performing an in situ polymerization in the
presence of multi-walled carbon nanotubes (MWNT). The
percolation threshold for the electric conductivity of the
resultant PIMWNT composites was about 0.15 vol.%. The
electrical conductivity has been increased by more than
11 orders of magnitude to 104 S cm1 at the percolation threshold and was further increased to 101 S cm1
when the MWNT concentration was raised to 3.7%
in volume.
Nakashima and coworkers reported [414] the synthesis of completely aromatic polyimides that contain the
triethylammonium salts of disulfonic acids (Scheme 75).
The resulting polyimides are highly capable of solubilizing a large number of individual SWNTs in organic
solutions. The major driving force for the solubilization
of SWNTs is interactions between the condensed
aromatic moieties on the polyimide and the surface of
SWNTs. Higher concentrations of SWNTs in polyimide
solutions form gels that are composed of individually
dissolved SWNTs.
959
Ando et al. [420] prepared novel nano ZnO/hyperbranched polyimide hybrid (Scheme 76) lms via the in situ
solgel polymerization technique. The lms, which originated from colorless, uorinated HBPI and homogeneously
dispersed ZnO nanoparticles, exhibited good optical transparency. Furthermore, two types of model compounds with
and without ZnO and a hyperbranched polyimide (HBPI)
lm blended with ZnO microparticles were prepared. These
materials were used to clarify the uorescence mechanism in pristine HBPI and in situ hybrid lms. Efcient
energy transfer from the ZnO nanoparticles to the aromatic
HBPI main chains was observed in the in situ hybrid lms,
whereas energy transfer occurred only from the locally
excited states to the charge-transfer state in the HBPI lm.
These ndings demonstrate that the peripheral termini
of HBPI are covalently bonded to ZnO particles via the
mono-ethanolamine (MEA) functionality; this functionality operates as an effective pathway for energy transfer and
results in an intense uorescent emission.
Liou et al. [421] prepared polyimidenanocrystalline
titania hybrid optical lms with a relatively high titanium content and thickness from soluble polyimides
containing hydroxyl groups (Scheme 77). Two series of
newly soluble polyimides were synthesized from the
hydroxy-substituted diamines with various commercial
tetracarboxylic dianhydrides. The hydroxyl groups on the
backbone of the polyimides provided organicinorganic
bonding and controlled the mole ratio of titanium butoxide
to hydroxyl groups. This resulted in homogeneous hybrid
solutions. Flexible hybrid lms could be obtained, and an
analysis revealed that the lms had relatively good surface
planarity, thermal dimensional stability, tunable refractive indices, and a high optical transparency. A three-layer
antireection coating based on the hybrid lms was prepared and showed a reectance of less than 0.5% in the
visible range; these characteristics suggest that these lms
could be used in optical applications.
The combination of polyimides and other organic/
inorganic compounds is expected to play an important role
in the development of novel high-performance nanocomposites to apply in many elds.
4.9. Polymer memory
Polymer memories have simple structures, good scalability, low cost potential, 3D stacking capability, and a large
capacity for data storage. Recently, several types of polymer
memory, including nonvolatile ash memory [422430],
write once read many times (WORM) memory [431436],
dynamic random access memory (DRAM) [437,438], and
static random access memory (SRAM) [439] have been
reported. A polymer memory stores information in a manner entirely different from that of silicon devices. Rather
Scheme 75. Polyimides containing disulfonic acid neutralizated [414].
960
Scheme 76. Novel nano ZnO/hyperbranched polyimide hybrid [420].
than encoding 0 and 1 from the number of charges

stored in a cell, a polymer memory stores data on the basis
of high and low conductivity responses to an applied voltage. Polymer memory cells may be stacked to produce a 3-D
architecture, which could translate into memory devices
that have several times the storage capacity of conventional
semiconductor ash memory devices. Polyimides could be
potentially used for these practical applications due to their
high mechanical strength and high thermal stability.
A dynamic random access memory (DRAM) device
based on a functional polyimide that contains both
electron-donor (D) and electron-acceptor (A) moieties
within a single macromolecule has been reported [437].
The polyimide device exhibits an ON/OFF current ratio up
to 105 . Both the ON and OFF states were stable under a
constant voltage stress of 1 V and survived up to 108 read
cycles at 1 V. The molecular structures of the polyimide and
single-layer memory device are shown in Scheme 78.
The mechanism of eld-induced conductivity is similar
to that of photoinduced charge-transfer (CT) in photoconductive polyimides. The incorporation of donors enhances
the photocurrent in the polyimide by several orders of magnitude and arises from improved CT complex formation
in the polyimide backbone. More comprehensive reviews
on the topic of polymer electronic memories are available
[210,211].
Wang and Kangs groups [435,436] reported the synthesis and memory behaviors of a series of functional
aromatic polyimides (OXTA-PI) containing triphenylamine
and 1,3,4-oxadiazole moieties (Scheme 79) and polyimides
(AZTA-PI) containing triphenylamine-substituted triazole
moieties (Scheme 80). Resistive switching devices based

on the sandwich structure of indium-tin oxide/polymer/Al
were fabricated, and their memory behavior was tested.
The devices exhibit two conductivity states and can be
switched from an initial low-conductivity (OFF) state to a
high-conductivity (ON) state at threshold voltages of 1.8 V
and 2.5 V, respectively, under both positive and negative
electrical sweeps, with an ON/OFF state current ratio on
the order of 105 at 1 V. The devices are able to remain
in the ON state even when the power is turned off or if
the device is subjected to a reverse bias. The nonvolatile
and nonrewritable natures of the ON state indicate that the
devices are WORM memory.
Liu et al. [439] synthesized a functional polyimide,
P(BPPO)-PI, that contains oxadiazole moieties (electron
donors) and phthalimide moieties (electron acceptors), as
shown in Scheme 81. A switching device with the polyimide exhibits two accessible conductivity states, and the
device can be switched from a low-conductivity (OFF) state
to a high-conductivity (ON) state when swept positively or
negatively. This device has an ON/OFF current ratio on the
order of 104 . The device exhibits ON state remanence,
in which the ON state persists for a period of about 4 min
after the power is removed. The ON state can be electrically
sustained either by a refreshing voltage pulse of 1 V or
by a continuous bias of 1 V. The remanent but volatile
nature of the ON state and the ability to write, read, and
sustain the electrical states with bias are characteristic features observed in static random access memory (SRAM).
The mechanisms associated with the memory effect were
elucidated from molecular simulation results and changes
961
Scheme 77. Polyimidenanocrystallinetitania hybrid synthesized from the hydroxy-substituted diamines with various commercial tetracarboxylic dianhydrides [421].
in the photoelectronic spectrum of a P(BPPO)-PI lm that

occurred when the device was switched between the ON
and OFF states.
Ueda and Chens [430] group have successfully synthesized two new sulfur-containing polyimides, APTT-6FDA
and 3SDA-6FDA, as shown in Scheme 82, for memory
device applications. The memory devices showed nonvolatile memory characteristics with two low turn-on
threshold voltages at 1.5 and 2.5 V, respectively. These
devices could be repeatedly written, read, and erased. The
ON/OFF current ratios of the devices were all around 104
in ambient atmosphere. The different turn-on threshold

voltages apparently resulted from two different low-lying
HOMO energy levels. A theoretical analysis suggests that
a charge-transfer mechanism can be used to explain the
memory characteristics of the studied polyimides. The
higher dipole moment of the sulfur-containing polyimides,
compared to the triphenylamine-based polyimide, provides a more stable CT complex that can be used in a ash
memory device.
A more stable charge transfer might improve polymer
memory devices [210,211]. The memory behaviors of the
Scheme 78. Molecular structure and schematic diagram of the single layer memory device [437].
962
Scheme 79. Aromatic polyimides (OXTA-PI)s containing triphenylamine and 1,3,4-oxadiazole moieties [435].
polyimides depend on the donoracceptor interactions of

the polyimide structures.
1. If the donoracceptor interaction of a polyimide has an
unstable ON state, then the memory can be used as
DRAM [437,438].
2. If the stabilized donoracceptor interaction but erasable,
then the memory can be used as ash-rewritable memory [422430].
3. If a very stable charge-transfer complex and nonerasable

ON state exists, then the memory can be used as
WORM [431436].
4. If the donoracceptor state is a temporary interaction
due to a coupled conformational change in the ON-state,
then the memory can be used as SRAM [439].
Polyimides are potential candidates for new generation
memory materials because of their high mechanical,
Scheme 80. Aromatic polyimides containing triphenylamine-substituted triazole moieties [436].
963
Scheme 81. Synthesis of functional polyimide, P(BPPO)-PI, containing oxadiazole moieties (electron donors) and phthalimide moieties (electron acceptors)
[439].
thermal properties and exibility as exible devices. The

polyimide is suitable for developing polymer memory
due to it contains electro-donor diamine moiety and
electro-acceptor imide moiety. The DA interaction in the
polyimides plays an important role in the memory effects.
4.10. Fiber reinforced composites
Aromatic polyimides are well-known as high performance polymers due to their excellent thermal stability,
mechanical and electronic properties; therefore, they are
suitable to be used as a matrix resin in ber reinforced composites. Usually, composites of carbon bers
and thermosetting polyimides are fabricated by routing
an poly(amic acid) solution, because solubility of more

than 30% is required to produce a prepreg [9,12]. In
this route, water generated as a by-product of imidization in the fabricating process might result in voids in
the composites. Therefore, polyimides prepared through a
polycondensation reaction require extremely severe processing conditions for molding. For moldable materials and
matrices of carbon ber composites, many addition-type
polyimides (i.e. imide oligomers terminated with reactive
groups) have been developed [440442]. However, there
are drawbacks for the polyimide matrices such as brittle or
relatively low Tg [443,444]. Yokota et al. developed imide
oligomers containing asymmetric structures as shown in
Scheme 83 and they had low melting temperatures and
Scheme 82. Synthesis of two new sulfur-containing polyimides, APTT-6FDA and 3SDA-6FDA [430].
964
Scheme 83. Synthesis of the additive imide oligomers by Yokota and coworkers [440].
low viscosity. The cured polymers showed high Tg , high

heat resistance, high thermo-oxidative stability and good
fracture toughness. The polyimide containing uorenylidene groups, which have bulky and rigid cardo structures,
are good solubility and high heat resistance of polymers.
Polyimide/carbon ber composites were fabricated from
the imide solution prepreg in two steps. First, the laidup prepregs were heated at 300 C for 30 min on a hot
plate to remove the solvent. The prepregs were cured at
370 C for 1 h under pressure using a hot press. The resultant composites had good quality with high Tg up to 347 C
and the optical micrograph of the composite showed no
voids.
The polyimides containing special structures reviewed
in Section 2.1.1 such as cardo, spiro and adamantane
structures are also good candidates for using in the ber
reinforced composites. Some of the synthesis approaches
reviewed in Section 2.2.2 are also useful to avoid generating voids due to the by-product water of imidization
in the fabricating process in the composites. More breakthrough innovation using addition reaction rather than
condensation reaction to avoid voids in the composites
will enlarge the polyimide applications in ber reinforced
composites.
5. Conclusion and outlook

In this review, we have integrated the synthesis and
applications of various functional polyimides. Since high
molecular weight products were rstly synthesized by a
two-stage polycondensation of pyromellitic dianhydride
with diamines in 1955, the interest in this class of polymers
has been growing steadily. Polyimides possess a number
of valuable and unique physico-mechanical, electrical and
chemical properties. It is possible to use polyimides for
prolonged periods of time at temperatures up to 200 C,
whereas short-term applications are possible at temperatures up to 480 C. In fact, polyimides exhibit excellent
physico-mechanical properties in a broad temperature
range and have exceptionally high radiation resistance and
superior semiconductor properties. These characteristics
allow polyimides to dominate the applications in many
elds. This review is devoted to the description of the
synthesis, properties and applications of polyimides. The
unique advantages and specic functions of polyimides for
different purposes and applications have been critically
reviewed.
Companies of all sizes are involved in various aspects
of polyimide research, and many academic laboratories
have multidisciplinary efforts dedicated to expanding the

applications of these polymers. With their unique features,
polyimides will continue to nd new industrial applications in the future. Further work will be required to develop
polyimide materials with sufcient solubility proles and
processable properties, as well as to further optimize their
performance. Novel polyimides with various functions are
possible and can be explored using new monomers, polymer design principles, and modications, such as using
Suzuki couplings and alkylations. Polyimides can be used
in novel applications in a variety of elds, and the excellent
properties of polyimides will be developed and reported in
the future.
KYOCERA Chemical Corporation developed a low temperature curing type polyimide precursor (KEMITITE
CT4112), which can be cured at a lower curing temperature (180 C) compared to general polyimide resins [445].
The polyimides are used widely as dielectric layers, insulation layers, as substrates for exible printed circuits, and
other applications. Therefore, the low curing temperature
property of KEMITITE CT4112 will be very important in
future electronic developments, such as the development
of organic eld effect transistors (OFETs) [446].
Acknowledgments
The authors are deeply indebted to Prof. En-Tang Kang
(National University of Singapore) and Dr. Anu Stella
Mathews (Pusan National University) for their variable
comments. We also thank our students who collected the
literatures for this review. Finally, the authors thank the
National Science Council and the Ministry of Education, The
Republic of China for the nancial support of this work.
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Advanced Polyimide Materials For Some Applications

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What are some applications of polyimides mentioned in the introduction?

What are some applications of polyimides mentioned in the introduction?

What monomers have been used to synthesize polyimides with unique properties?

What monomers have been used to synthesize polyimides with unique properties?

Progress in Polymer Science 37 (2012) 907974

Contents lists available at SciVerse ScienceDirect

Progress in Polymer Science

Advanced polyimide materials: Syntheses, physical properties

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

Scheme 1. Some commercial diamines with kink structures.

Scheme 2. Some structures of novel kink diamines and dianhydrides.

1,2-dihydro-2-(4-aminophenyl)-4-[4-(3-phenyl-4aminophenoxy) phenyl]-(2H) phthalazin-1-one, from

dianhydrides have been synthesized in different ways,

Scheme 3. Synthesis of BHPP.

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

Scheme 4. Preparation of bis(ether anhydride)s.

and have a signicantly increased Tg and thermal stability.

Scheme 5. Functional dianhydride (BHTDA) with a hydroxyl group.

Shu et al. synthesized polyimides derived from several

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

Scheme 6. Chemical structures of novel spiro diamines and dianhydrides.

novel spirodilactone dianhydride (XXI) showed a high Tg

two-step procedure. By increasing the fraction of XXII

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

Scheme 7. Chemical structures of novel cardo monomers [23,3034,36,78,79,448].

center, while two rings are attached to a spirocenter.

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

Scheme 8. Monomers with noncoplanar structure.

two-fold: (1) close chain packing and intermolecular

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

Scheme 9. Crank and twisted noncoplannar structure diamines or dianhydrides.

transparency, and generally lower tensile properties (in

Normally, bisphenols containing cardo structure are

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

Scheme 11. Chemical structures of unsymmetric monomers.

alicyclic monomers including diamines and dianhydrides

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

Scheme 12. Synthesis triphenylamine-containing diamines [53,54].

Scheme 13. Synthesis of biphenyl dinathalene diamine.

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

Scheme 15. Unsymmetrical ether-containing diamines.

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

Scheme 16. Chemical structures of alicyclic monomers.

Scheme 17. Preparation of cis-cyclobutane-l,2,3,4-tetracarboxylic dianhydrides.

maleic anhydride compound from 10 to 50 C in the range

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

Scheme 18. Preparation of 1,2,4-tricarboxyl-3-methylcarboxyl cyclopentane dianhydride [7577].

Scheme 19. Chemical structures of XLII (3FDAM) and XLIII (3FEDAM).

and base, under well-established reaction conditions, as

Scheme 20. Triaryluoroethanes-containing dianhydride (XLIV) and diamine (XLV).

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

Scheme 21. Diamines containing triuoromethyl with different linkages [87].

monomers to obtain polymers with properties that are

Scheme 22. Synthesis of monomer, LXXV [88].

D.-J. Liaw et al. / Progress in Polymer Science 37 (2012) 907974

Scheme 23. Synthesis of 3,5-bis(triuoromethyl)phenyl containing diamines [89,90].

silsesquinoxane (DDSQ) [99] containing two reactive