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Macromolecules 1985, 18, 1091-1095
1091
Nature of the Water-Epoxy Interaction
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L. W. Jelinski,* J. J. Dumais, A. L. Cholli,t T. S. Ellis,$and F. E. Karaszt
AT&T Bell Laboratories, Murray Hill, New Jersey 07974. Received September 6, 1984
ABSTRACT Absorption of small amounts (1-3 wt %) of water by epoxy resins effects a substantial plasticizing
action and degradation of their mechanical properties. We present results that provide new insight into the
molecular details of the interaction of epoxy resins with water. These results show (1)that the water in epoxy
resins is impeded in its movement, with the molecules hopping from site to site with an approximate residence
s, (2) that there is no "free" water, (3) that there is no evidence for tightly bound
time per site of 7 X
water, and (4) that it is unlikely that the water disrupts the hydrogen-bond network in the epoxy resin. These
results are consistent with the notion that water acts simply as a plasticizer for these hydroxyl-containing
systems, just as it does for polymers which contain no exchangeable protons.
Water is known to plasticize certain amorphous,1t2semi~rystalline,~
and cross-linked polymer^.^^ In particular
absorption of small amounts (1-3% by weight) of water
causes a large reduction in the glass transition temperature
(T,)of epoxy resin^,^^^ often considered to reduce the
temperature threshold of utility and general mechanical
integrity of this class of materials by a considerable margin.
This feature of water plasticization in this and other
systems has led to some speculation with regard to the
nature of the interaction of water molecules with polar
polymeric substrates. Here we present results which not
only clarify this situation but improve the fundamental
understanding of the molecular details concerning the
interaction of water with these materials.
Some early spectroscopic and calorimetric data have
been interpreted to indicate a highly specific "binding"
between the water molecules and the polar groups of the
epoxy resin,'i8 proposing that the water disrupts the hydrogen-bonding n e t ~ o r k . However,
~
recent work4Js12
shows that the plasticization of epoxy resins by water can
be explained simply by treating the T,depression as arising
from the expected composition dependence of miscible
polymer-diluent systems. The latter treatment has the
virtue that it successfully predicts the depression of Tgfor
polymer systems that contain polar groups such as OH
residues,12as well as for those that are considered relatively
inert.
In a preliminary communication on the interaction of
water (D,O) with epoxy resins,13 quadrupole echo deuterium NMR spectroscopy was used to show (1)that although the absorbed water is very mobile, it is not identical
with bulk water in this respect, (2) that the water is distributed homogeneously throughout the sample, (3) that
approximately one water molecule is entrained for every
six OH/OD sites on the polymer backbone, and (4)that
the water exchanges with the OH/OD groups on the
polymer backbone, but the time scale of this exchange was
not determined. The communication also established that
solid-state deuterium NMR spectroscopy shows particular
promise for the further investigation of plasticization
phenomena in these and other systems.
We report here an extension of the early study.14 The
present work is designed to clarify and further establish
the molecular details of the interaction between water and
epoxy resins. In particular, the experiments described here
are intended to (1)obtain in residence time for the mobile
water molecules, (2) examine the effect of water plasticization on the backbone mobility of the epoxy resin, (3)
determine whether any fraction of the water can be considered to be tightly bound to the epoxy resin, and (4)
Current address: BOC Corp., Murray Hill, NJ 07974.
* University of Massachusetts, Polymer Science and Engineering
Department, Amherst, MA 01003.
0024-9297/85/2218-1091$01.50/0
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establish upper limits on the rate of exchange between the
water molecules and the OH/OD groups on the epoxy
backbone.
Materials and Methods
Epoxy Resins. The epoxy samples used in this study are based
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on the diglycidyl ether of bisphenol A. The sample we designate
as EX4 was prepared as a stoichiometric cure of Epon 825 (Shell
Co.) (epoxy equivalent weight (eew) 175; theoretical eew of pure
monomer is 170) with m-phenylenediamine. The calorimetrically
determined onset T , of this completely dry, fully cured sample
is 424.2 K and the midpoint is 432.0 K (heating rate 20K/min).
The depression of Tgwith water plasticization4 shows no discontinuity in the depressed Tgup to a water content of approximately 3.5% w/w.
Sample NX2 was made according to literature methods15using
Epon 825 and nadic methyl anhydride (Aldrich Chemical Co.).
This sample has an approximate stoichiometry of 1.00.75 epoxide
to anhydride groups. An IR spectrum shows no evidence of
residual anhydride or epoxide moieties, although a small number
of OH groups are present in this sample. The onset Tgis 386 K
and the midpoint is 392.6 K (heating rate of 20°/min).
Poly(N-vinylpyrrolidone)(M, 360 000) was obtained from
Aldrich Chemical Co. This material was dried in a vacuum oven
at 180 "C for 24 h prior to use.
Figure 1 shows the chemical structures of the materials used
in this study.
Exchange with Deuterium. The epoxy samples were exchanged with DzO in one of two ways. In the first method, an
ampule of D 2 0 was placed in a glass tube containing the epoxy.
After evacuation and sealing, the ampule was broken by shaking
and the whole assembly was placed in an oven at 160 "C. This
process was repeated 5 times, each time with a fresh ampule of
DzO, to ensure complete exchange. After the final cycle a given
amount of D,O was introduced into the system and. the final
weight percent DzO was determined gravimetrically. The samples
plasticized in this manner contained between 1and 3 wt % D,O,
with the exception of NX2, which absorbed less than 0.5 wt %
D20.
In the second method, 2 X 2 X 2 mm cubes of the dried epoxy
samples were soaked in D 2 0 a t room temperature for a period
of 1 week. At the end of the soaking period, the samples were
blotted dry and allowed to equilibrate in a 100% relative humidity
D 2 0 atmosphere16 for 2 days prior to NMR measurement. This
method produced samples containing approximately 2 wt % DzO.
Samples prepared by either method produced equivalent NMR
results.
Nondeuterated epoxy resins were plasticized with Me$O-d6
by soaking the resin in this material for 48 h. The samples were
blotted dry prior to NMR measurement. They contained 5% by
weight Me2SO-d6.
The poly(N-vinylpyrrolidone)was plasticized with D 2 0 by
exposing the dried polymer to DzO in the 100% relative humidity
chamber for 24 h. This produced a sample containing -40 wt
% DzO. Several hours of drying under a slight vacuum a t room
temperature reduced this to 21% by weight of DzO.
Exchanged and Dried Samples. Some of the samples that
had been previously exchanged with DzO were dried to provide
reference samples. The dried samples were produced by heating
0 1985 American Chemical Society
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Macromolecules, Vol. 18, No. 6,1985
1092 Jelinski et al.
(a)
,O,
CHZ-CH-CH2-0-
(C)
0 f"b
0 -C-
0
/I
0 -0-CH2-CH-CH2
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cHgo
.**-N-CHz-CH-CH2-OI
CH3
b
(d)
N 1w
OH
N1
0
(e)
4 CH-CH,+,
I
O
m
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Figure 1. Chemical formulaa of materials used in this study: (a)
diglycidyl ether of bisphenol A; (b) m-phenylenediamine;(c) epoxy
resin EX4; (d) nadic methyl anhydride; (e) poly(N-vinylpyrrolidone).
the deuterium-exchanged resin to 170 "C in vacuo for 2 days in
the presence of phosphorus pentoxide. These sampleswere sealed
under dry nitrogen until the NMR experiments were performed.
NMR Measurements. Each NMR measurement was performed on a fresh sample (approximately 100 mg) of the appropriate polymer unless noted otherwise. NMR experiments were
performed by using a home-built solid-state deuterium NMR
spectrometer operating at 55.26 MHz for deuterium. The spectrometer" and its sample temperature controlls have been described previously. The quadrupole echo pulse s e q u e n ~ e 'was
~*~
used for most experiments. Relaxation data were obtained by
using an inversion-recovery pulse sequenceprior to the quadrupole
echo sequence. Tl values were measured from the null points in
the inversion-recovery spectra. The 90"pulse width was 3.2 ps
and data were obtained in quadrature by using 2K points per
channel. For most experiments the digitization rate was 100
ns/point (10 MHz).
Results
General Description of N M R Spectra. Figure 2
shows a typical quadrupole echo solid-state deuterium
NMR spectrum of the exchanged epoxy resin EX4 when
it contains 2 w t % DzO (a) and when this same sample is
dried (b). We have previously shown that the sharp central
line arises from the sorbed DzO, at this peak disappears
when the sample is heated or dried.13 The outer, broad
part of the NMR spectrum is attributed to OH residues
that have undergone exchange with DzO and have become
OD groups on the polymer backbone. The quadrupolar
splitting between the outer peaks (125 kHz) indicates that
the OD groups are not undergoing any large-amplitude
motions on the NMR time scale.
The line width of the central sharp peak is somewhat
broader than that of free water. This broadening may be
attributed to contributions from sample inhomogeneities.
In addition, the line width decreases as the weight percent
of DzO increases.
Because sample EX4 contains OD residues on the
backbone (i.e., OH groups that have exchanged with DzO)
which produce a static-like pattern, such a broad pattern
would obscure the signal from a small amount of tightly
bound water, if it were present. Two experiments were
designed to circumvent this problem. In the first, sample
NX2 (which contains very few OH groups) was exchanged
with D20. It absorbed less than 0.5 wt % DzO, and this
- 200
- 100
0
IO0
200
kHz
Figure 2. Quadrupole echo deuterium NMR spectra of epoxy
resin EX4 (a) containing 2 w t % D20;
and (b) the EX4 sample
in a after it has been dried. The spectra were obtained at 20 "C
and 55.26 MHz for deuterium.
Figure 3. Inversion-recovery quadrupole echo NMR spectra for
epoxy resin EX4 containing 2 w t % D,O. From top to bottom,
the inversion-recovery delay times are 5000, 500, 200, 100, 50,
10,1,0.5,and 0.1 ms. The enlargement on the right shows data
obtained at inversion-recovery delay times of 9 (top), 7, and 3
ms. AU data were obtained at 20 O C and 55.26 MHz for deuterium.
level of deuterium was not detected in the NMR experiment. A model polymer, poly(N-vinylpyrrolidone), was
used in the second experiment. Poly(N-vinylpyrrolidone)
is a hydrophilic polymer that does not contain exchangeable OH groups. The deuterium NMR spectrum of a
sample of poly(N-vinylpyrrolidone)containing 21 wt %
DzO shows no evidence for bound or static-like water. At
this content of deuterium, a broad component arising from
bound water would be clearly observed.
Relaxation Data. Typical inversion-recovery relaxation data for an EX4 resin containing 2 wt % D20 are
shown in Figure 3. It is clear that the OD groups and the
signal due to DzO relax at significantly different rates.
Relaxation data for EX4 (both "wet" and dried) at
various temperatures are reported in Table I and are
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Nature of Water-Epoxy Interaction 1093
Macromolecules, Vol. 18, No. 6, 1985
Table I
Deuterium NMR Relaxation Data” for Epoxy-D20 and
Related Systems
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measurement temp,
sample
EX4-backbone OD
dry EX4
wet EX4b
DZO
*g
-
measurement
“C
temp, O C
T I ,ms
20
60
20
65
139
99
116
71
203
44
63
36
in EX4
in poly(vinylpyrro1idone)
neat
plus Fe(AC)z
20
20
20
20
12c
12
362
362
in EX4d
20
20
73
306
MezSO-d6
neat
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“All data were obtained at 55.26 MHz for *H.*Sample contained 2% DzO by weight. This value obtained by using a quadrupole-echo inversion-recovery pulse sequence. TI of 12 ms also
obtained by using standard high-resolutioninversion-recovery sequence. d Sample contained 5% MezSO-dsby weight.
compared to the relaxation times of pure and paramagnetically doped DzO. Several points are apparent from
inspection of the data in Table I.
First, the Tl of the mobile water in sample EX4 is
substantially different from that of pure DzO. It is unlikely
that this difference is due to small amounts of paramagnetic impurities, as paramagnetic doping of D20 (Table
I) does not change its relaxation time.
Second, the relaxation times for the static component
of the wet and dried EX4 samples are significantly different when measured at 20 OC. The glass transition
temperatures for the wet and dried EX4 sample are different, which could account for the differences in the T1
of the static component. The relaxation times for the
static-like component of these two samples become similar
when measured at a constant AT below Tg(Le., at 20 OC
for the wet EX4 and at 60 “C for the dried sample).
Figure 4 shows a plot of the T1data for the backbone
OD group. Data for both the dry (triangles) and wet
(circles) EX4 samples are shown. The abscissa corresponds
to the difference in temperature between Tgand the
measurement temperature. Data from both the wet and
dry samples fall on this line, suggesting that the T1differences between the wet and dry epoxy samples (Table
I) are due primarily to plasticization effects.
60
I
I
I
1
00
100
120
140
p
-
(
measurement temperature 1
Figure 4. Plot of TIdata for the OD backbone group in epoxy
sample EX4 vs. the difference between T and the temperature
of measurement. The triangles represent t i e dry samples; circles
represent the sample containing 2 wt % D,O.
by a single exponential, consistent with there being only
one species contributing to this signal. Finally, to eliminate
the possibility that the signal from free water is suppressed
by the quadrupole echo NMR experiment, the T1of this
sharp component was measured by using a standard inversion-recovery Bloch decay experiment. This experiment also produces a 12-ms T1for the sharp component
(Table I).
Calorimetric studies down to -80 OC on differently
prepared samples have also failed to reveal thermal phenomena associated with free water. Taken together, the
data from the low-temperatureexperiments, the relaxation
data, and the line width data support the conclusion that
there is no isotropically free water in this epoxy-water
system. This obviates the presence of the water in macroscopic voids, a situation which has been seen to occur
in a different epoxy systema8Instead, the water appears
to be distributed homogeneously throughout the system.
These data also show that the motion of the water is
impeded. When the standard relationship between correlation time and relaxation times is used,21the 12-ms T1
translates into a correlation time of 7 X
s. (The
correlation time of free water is 2 X lo-” s at this temperature.) This correlation time can be interpreted as the
residence time of the water as it hops from site to site. It
is likely that the OD or residual ND groups act as the
impeding sites.
Question of Tightly Bound Water. Although it is
clear that essentially all of the water is very mobile, we are
unable to directly eliminate the possibility that there is
a small amount of bound water. This situation arises
because the signals from the OD of the epoxy resin backbone would obscure any static water, if it were present.
However, the following experiments and considerations
indicate that it is highly unlikely that there is a significant
amount of bound water. (We define “bound” water as
water with a residence time of
s or longer, or water
whose only motion consists of a flip about the molecular
symmetry axis.)
Firstly, the depression of Tgin the EX4 system by water
exhibits a monotonic decrease from 0% moisture content:
If tightly bound water does not exercise the same plasti-
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Discussion
Question of “Free”Water. Several lines of evidence
argue against the presence of “free” water. (We define
“free” to mean water that is isotropically mobile and thus
has the same relaxation time as pure water.)
First, in our previous work13we showed that the water
in resin EX4 did not freeze at temperatures down to -20
“C. This experiment is not unequivocal, however, because
the signal for the water resonance did broaden, and a small
amount (less than 10%) of free water could have been
obscured by the OD signal from the polymer backbone.
In contrast to the above experiment, relaxation data
provide a clear indication that there is no free water in this
system. The spin-lattice relaxation time of pure D20
under our conditions is 362 ms, whereas the corresponding
relaxation time of the sharp signal in the spectrum of EX4
is 12 ms (Table I). This finding provides strong evidence
that the motion of the water molecules in the epoxy resin
is impeded. In addition, the relaxation data are described
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1094 Jelinski et al.
cizing action as normal “dissolved” water, then some discontinuity may be anticipated. However, evaluation of the
depression of Tgis obviously determined at the respective
temperature of the transition and not at ambient conditions. We know from our experiments here that Tl times
of water molecules in the epoxy matrix vary significantly
with temperature and hence the water molecules have
different mobilities at different temperatures. This in fact
forms the very basis of the great utility of the deuterium
NMR technique used here since molecular interactions are
determined at the temperature in question and are not
inferred from property measurements at a temperature far
removed from the point of interest.
Further evidence against the presence of bound water
a t ambient conditions comes from the fact that the composite line shape of Figure 2a can be simulated satisfactorily by assuming that there are only two components-a
static pattern and a Lorentzian line.I3
Thirdly, evidence comes from experiments on a model
polymer, poly(N-vinylpyrrolidone). This material is known
to be exceptionally hydrophilic, yet it contains no exchangeable NH or OH groups. When plasticized with DzO,
the deuterium NMR spectrum of this polymer shows only
a single line and no evidence of bound water.
A final line of reasoning comes from examination of the
crystal structures of inorganic molecules that contain water
of crystallization. Gypsum, or CaS04.2H20,provides a
relevant example. Although the water in CaS04.2Hz0
forms part of the structure in the solid state, NMR experiments show that it is actually undergoing two-site flips
about the c p axis.22 The NMR pattern for water that is
involved in two-site flips consists of a tentlike pattern that
is approximately 120 kHz in breadth. This line shape
would not be obscured by the signals from the OD groups,
and hence we can rule out the presence of bound, but
flipping, water molecules.
These experiments and lines of reasoning suggest that
it is highly unlikely that the epoxy-water system contains
tightly bound water.
Question of D 2 0 Disruption to the HydrogenBonding Network. In this section we obtain an upper
limit for the OD/DzO exchange rate and show that the
water molecules are hopping from site to site at least 6
orders of magnitude faster than this.
The two-component nature of the spectrum of EX4
plasticized with water (Figure 2a) indicates that the
OD/DzO exchange rate is slow compared to the frequency
separation between the OD signal and the D20 signal. If
the exchange rate were faster than this separation in frequency units (ca. 100 kHz), we would not observe these
two components. Instead, the signals would be averaged
together, producing only a sharp signal in the center of the
spectrum. This observation sets an upper limit on the
exchange rate of lo5 s-1.23
This upper limit can be lowered to ca. lo3 if we can
rule out the possibility of intermediate exchange. The T,
data indicate that the OD and DzO are not exchanging a t
an intermediate rate on the NMR time scale. If intermediate exchange were the case, we would expect that the
hopping water molecules could communicate their efficient
relaxation to the OD groups. We would predict that the
relaxation time of the OD groups would be shorter in the
EX4 sample that contains water than in the one that is
dry. T I measurements performed a t fixed temperatures
below the respective Tgof the dry and wet samples (Table
I) show that the static components have identical relaxation times. This finding provides strong evidence that the
OD/D20 exchange rate is much slower than 100 kHz, al-
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Macromolecules, Vol. 18, No. 6, 1985
lowing us to rule out the possibility of exchange at an
intermediate rate.
We therefore have an upper limit for chemical exchange
of lo3 s-,, and a hop rate of the water molecules of lo9 s-,.
Because the water molecules hop from site to site at least
6 orders of magnitude faster than they undergo chemical
exchange, it is unlikely that the water molecules serve to
disrupt the hydrogen-bonding network in the epoxy resin.
These conclusions are supported by water relaxation
measurements on poly(N-vinylpyrrolidone). The T1 of
water is the same in this system as it is in the epoxy-water
system (Table I). As poly(N-vinylpyrrolidone) does not
have the possibility of forming a hydrogen-bonded network, the similarity in relaxation times suggests that the
water is acting merely as a plasticizer in both of these
systems.
Summary
The results presented here provide new insight into the
molecular details of the interaction of epoxy resins with
water. In particular, these results establish (1) that the
water is impeded in its movement, with the molecules
hopping from site to site with an approximate residence
time per site of 7 X 10-los, (2) that there is no free water,
(3) that there is no evidence for tightly bound water, and
(4) that it is unlikely that the water disrupts the hydrogen-bond network in the epoxy resin.
These results are consistent with the notion that water
is simply a plasticizer for these systems, just as it is for
polymers that contain no exchangeable protons. However,
these results clearly demonstrate that the water does interact with specific sites or traps on the epoxy backbone,
insomuch as the water molecules hop from site to site,
rather than behaving as a gas in an inert matrix. However,
these results do not connect the hopping water molecule
with specific hydrogen-exchange mechanisms.
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Acknowledgment. T.S.E. and F.E.K. acknowledge
support for part of this work from AFOSR 84-0100 and
CUMIRP.
Registry No. Epon 825, 25068-38-6; water, 7732-18-5.
References and Notes
Sung, Y. K.; Gregonis, D. E.; Russell, G. A.; Andrade, J. D.
Polymer 1978, 19, 1362.
Tan, Y. Y.; Challa, G. Polymer 1976, 17, 739.
Jin, X.; Ellis, T. S.; Karasz, F. E. J. Polym. Sei., Polym. Phys.
Ed., in press.
Ellis, T. S.; Karasz, F. E. Polymer 1984, 25, 664.
McKague, E. L.; Reynolds, J. D.; Halkias, J. E. J . Appl. Polym.
Sei. 1978, 22, 1643.
Browning, C. E. Polym. Eng. Sei. 1978, 18, 16. Kong, E. S. W.;
Adamson, M. J. Polymer 1983, 24, 171.
Moy, P.; Karasz, F. E. Polym. Eng. Sci. 1980, 20, 315.
Maxwell, I. D.; Pethrick, R. A. J. Appl. Polym. Sci. 1983,28,
2363. Kong, E. S. W. Proc. Org. Coat. Appl. Polym. Sci. 1983,
48, 727.
Banks, L.; Ellis, B. Polym. Bull. 1979, I , 377.
ten Brinke, G.; Karasz, F. E.; Ellis, T. S. Macromolecules 1983,
16, 244.
Ellis, T. S.; ten Brinke, G.; Karasz, F. E. J . Appl. Polym. Sei.
1983, 28, 23.
Ellis, T. S.; Karasz, F. E. Proc. Org. Coat. Appl. Polym. Sei.
1983, 48, 721.
Jelinski, L. W.; Dumais, J. J.; Stark, R. E.; Ellis, T. S.; Karasz,
F. E. Macromolecules 1983, 16, 1019.
A preliminary account of some of these experiments has been
published in preprint form: Jelinski, L. W.; Dumais, J. J.;
Cholli, A. L. Polym. Prepr. (Am. Chem. Soc., Diu.Poly. Chem.)
1984,25, 348.
Antoon, M. K.; Koenig, J. Lo;
Serafini, T. J.Polym. Sci., Polym. Phys. Ed. 1981,19, 1667.
Young, J. F. J . Appl. Chem. (London) 1967, 17, 241.
Jelinski, L. W.; Dumais, J. J.; Engel, A. K. Macromolecules
1983, 16, 492.
zyxwvutsrqpo
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1095
Macromolecules 1985, 18, 1095-1100
(18) Cholli, A. L.; Dumais, J. J.; Engel, A. K.; Jelinski, L. W.
(22) Spiess, H. W.; Mahnke, H. 2.Naturforsch 1972, 27A, 1536.
Macromolecules 1984, 17, 2399.
~ ~ *discussed
~~
the situation of magnetization
(23) E i s e n ~ t a d thas
transfer between spin
nuclei in the presence and absence
(19) Jeffrey, K. R.; Bloom, M.; Valic, M. I.; Higgs, T. P. Chem.
of exchange. Such a treatment would be premature in the
Phys. Lett. 1976, 42, 390.
V. Chem. Phvs.
Dresent case. as we observe sinele-exDonentialrelaxation within
(201 Blinc. R.: Rutar. V.: Selieer. J.: Slak. J.: Smolii.
zyxwvutsrqpo
Lett.’1977, 48, 576:
the uncertainty of the data.
(21) Mehrine. M. “Hieh Resolution NMR in Solids”:. SDrinaer(24) Eisenstadt. M. J. Mam. Reson. 1980. 38. 507.
Ver1ag:-New Yo&, 1983.
(25) Eisenstadt; M. J. Main. Reson. 1980; 39;263.
,
I
I
-
I
.
I
Exposure of Hydroxyl Groups in Phenol-Acetaldehyde Oligomers,
As Investigated by Photo-CIDNP lH NMR and Infrared
Spectroscopy
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Lucia Zetta,*2aAntonio De Marco,2aGiovanni Casiraghi,2bMara
and Robert KapteinZc
Zstituto di Chimica delle Macromolecole del C.N.R., 1-20133 Milano, Italy, Zstituto di
Chimica Organica dell’Uniuersitd, 43100 Parma, Italy, and Physical Chemistry Laboratory,
University of Groningen, 9747 AG-Groningen, The Netherlands. Received J u n e 18, 1984
ABSTRACT: A number of oligomers related to phenol-acetaldehyde novolac resins possessing different
stereoregularity are investigated by conventional and photochemically induced dynamic nuclear polarization
(photo-CIDNP) lH NMR and IR spectroscopy. Complete assignment of NMR spectra is achieved by double-resonance techniques, spin-echo multiplet selection, use of shift reagents, and comparison with spectra
of two monomeric model compounds. For each oligomer, the spectral parameters can be directly related to
the tacticityof the molecule (Le., to the sequence of meso and racemic units). The photo-CIDNP technique
provides information on the accessibility of the phenolic moieties. In both monomers access to the OH’S is
hindered. From the dimer to the pentamer, a gradual reduction of the exposure of the OH groups from the
e x t e n d rings is observed, whereas those from the inner rings are always accessible, irrespective of the number
of units. This indicates the absence of the isodromic intramolecular H bonds which were observed in the
X-ray structure. In the IR experiments the OH. .OH absorption band shifts to lower frequencies and broadens
by increasing the molecular weight. Combined with the photo-CIDNP results, this suggests a progressive
strengthening of the H bonding of the outer functional groups. Two empirical rules are proposed, which correlate
the intensity of the aromatic signals with the number of internal phenolic units.
Introduction
All-ortho alkylidene-bridged oligophenols (novolacs)
have been the subject of several investigation^.^-^ IR
studies have shown that in carbon tetrachloride these
molecules adopt well-defined conformations, involving
OH---OH intra- and intermolecular hydrogen bonds.”’
The X-ray structure analysis of a number of oligomers
indicates that the conformation in the crystal is mainly
determined by an isodromic, intramolecular H-bonding
system, formed by the phenolic functional groups.6 The
H-bond stabilization defines two domains, one highly polar,
containing all OH groups, the other strongly hydrophobic,
in which the aromatic rings form a lipophilic matrix.
The present communication is focused on the aromatic
resonances of a number of oligophenols (number of internal
rings 0 < n < 3) having different steric regularity. The
accessibility of the phenolic groups is investigated by
photochemically induced dynamic nuclear polarization
(photo-CIDNP) and IR spectroscopy, in order to obtain
some evidence on the molecular structure, in particular on
the stabilizing role of the hydrogen bonds.
Materials and Methods
The two monomers and the oligomers in this study were
obtained as described elsewherea6 The methyl ester of
3-N-(carboxymethyl)lumiflavinwas the kind gift of Dr. F.
Muller (Wageningen). C2HC13was from Merck Isotopes.
The conventional NMR spectra were recorded with a
Bruker HX-270 spectrometer, controlled by an Aspect 2000
computer. Chemical shifts are quoted in ppm from internal tetramethylsilane (Me,Si). Typically 64 scans were
0024-9297/85/2218-1095$01.50/0
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accumulated for each spectrum. The spin-echo experimenta shown in Figure 6 were performed according to the
pulse sequence (90°-tl-1800-tl-f900-t2-900-acquisition),
where tl is 1 / 2 J and t 2 allows for partial relaxation.8
Photo-CIDNP spectra were obtained with a Bruker
HX-360 spectrometer, also equipped with an Aspect 2000
computer. The sample concentrations ranged between 6
and 1 2 mM in C2HC13. Difference spectra were obtained
by f sking “light” and “dark” free induction decays (see
text) and subtracting the spectra after Fourier transformation. A Spectra Physics Model 171 argon ion laser was
employed as the light source. A 0.6-s light pulse (4 W,
multiline) was used, with a 0.05s delay before the 90° rf
pulse. Four scans were accumulated for each spectrum.
All spectra were resolution enhanced via the Gaussian
convol~tion.~
The line at 7.23 ppm in the photo-CIDNP
difference spectra results from incomplete cancellation of
the CHC13 signal.
IR spectra were obtained with a Perkin-Elmer 457
spectrometer, the sample concentrations varying between
0.2 and 2.0 mM in C2HC13. The cell paths ranged between
75 and 500 p.
Results and Discussion
Resonance Assignment. The phenol-aldehyde oligomers of this study are listed in Chart I. The monomers
I and I’ are models for inner and outer rings in the oligomers, respectively. Figure 1 shows the aromatic region
of the lH NMR spectra of I and 1’, after resolution enhancement. The axial symmetry of compound I prevents
the observation of J couplings across more than three
0 1985 American Chemical Society