Characteristics, Applications, and

Processing of Polymers

PRESENTED BY:
LENRIE PONDEVIDA
DANIEL DELA CRUZ
RENIEL LANETE
Stress-Strain Behavior

 On the basis of stress–strain behavior, polymers fall within

three general classifica- tions (Figure 15.1): brittle (curve
A), plastic (curve B) , and highly elastic (curve C).
 Polymers are neither as strong nor as stiff
as metals. However, their high flexibilities,
low densities, and resistance to corrosion
make them the materials of choice for
many applications.
 The mechanical properties of polymers are
sensitive to changes in temperature and
strain rate. With either rising temperature
or decreasing strain rate, modulus of elas-
ticity diminishes, tensile strength
decreases, and ductility increases.
Viscoelastic Deformation
 Viscoelastic mechanical behavior, intermediate between
totally elastic and totally viscous, is displayed by a number
of polymeric materials.
 This behavior is characterized by the relaxation modulus, a
time-dependent modulus of elasticity.
 The magnitude of the relaxation modulus is very sensitive
to temperature. Glassy, leathery, rubbery, and viscous flow
regions may be identified on a plot of logarithm of
relaxation modulus versus temperature (Figure 15.7).
 The logarithm of relaxation modulus versus temperature
behavior depends on molecular configuration—degree of
crystallinity, presence of crosslinking, and so on (Figure
15).
Fracture of Polymers

 Fracture strengths of polymeric materials are low

relative to those of metals and ceramics.
 Both brittle and ductile fracture modes are possible.
 Some thermoplastic materials experience a ductile-to-
brittle transition with a lower- ing of temperature, an
increase in strain rate, and/or an alteration of specimen
thick- ness or geometry.
 In some thermoplastics, the crack-formation process
may be preceded by crazing; crazes are regions of
localized deformation and microvoids (Figure 15.9).
 Crazing can lead to an increase in ductility and
toughness of the material.
Deformation of semicrystalline polymer
 During the elastic deformation of a semicrystalline polymer having
a spherulitic struc- ture that is stressed in tension, the molecules in
amorphous regions elongate in the stress direction (Figure 15.12).
 The tensile plastic deformation of spherulitic polymers occurs in several stages
as both amorphous tie chains and chain-folded block segments (which
separate from the ribbon-like lamellae) become oriented with the tensile axis
(Figure 15.13).
 Also, during deformation the shapes of
spherulites are altered (for moderate
deformations); relatively large degrees
of deformation lead to a complete
destruction of the spherulites and
formation of highly aligned structures.
Factors That Influence the Mechanical
Properties of Semicrystalline Polymers
 The mechanical behavior of a polymer is influenced by both in-service and
structural/ processing factors.
 Increasing the temperature and/or diminishing the strain rate leads to
reductions in tensile modulus and tensile strength and an enhancement of
ductility.
 Other factors affect the mechanical properties:

- Molecular weight-Tensile modulus is relatively insensitive to molecular

weight. However, tensile strength increases with increasing Mn (Equation 15.3).
 Degree of crystallinity—Both tensile
modulus and strength increase with
increas- ing percent crystallinity.
 Predeformation by drawing—Stiffness and
strength are enhanced by perma- nently
deforming the polymer in tension.
 Heat-treating—Heat-treating undrawn and
semicrystalline polymers leads to
increases in stiffness and strength and a
decrease in ductility.
Crystallization
 Large elastic extensions are possible for elastomeric
materials that are amorphous and lightly crosslinked.
 Deformation corresponds to the unkinking and
uncoiling of chains in response to an applied tensile
stress.
 Crosslinking is often achieved during a vulcanization
process; increased crosslinking enhances the
modulus of elasticity and the tensile strength of the
elastomer.
 Many elastomers are copolymers, whereas silicone
elastomers are really inorganic materials.
Melting
 • The melting of crystalline regions of a
polymer corresponds to the
transformation of a solid material having
an ordered structure of aligned
molecular chains into a viscous liquid in
which the structure is highly random.
The Glass Transition
 The glass transition occurs in
amorphous regions of polymers.
 Upon cooling, this phenomenon
corresponds to the gradual
transformation from a liquid into a
rubbery material, and finally into a rigid
solid. With decreasing temperature there
is a reduction in the motion of large
segments of molecular chains.
Melting and Glass Transition
Temperatures
 Melting and glass transition temperatures may be determined
from plots of specific volume versus temperature (Figure 15.18).
 These parameter are important relative
temperature range over which a particular
polymers may be used and processed.
Factors That Influence Melting and Glass
Transition Temperature
 The magnitudes of Tm and Tg increase
with increasing chain stiffness; stiffness
is enhanced by the presence of chain
double bonds and side groups that are
either bulky or polar.
Temperature
 • At low molecular weights Tm and Tg
increase with increasing Ṁ.
Polymers Types
 • One way of classifying polymeric materials is according to their
end use. According to this scheme, the several types include
plastics, fibers, coatings, adhesives, films, foams, and advanced
materials.
 • Plastic materials are perhaps the most widely used group of
polymers and include the following: polyethylene, polypropylene,
poly(vinyl chloride), polystyrene, and the fluorocarbons, epoxies,
phenolics, and polyesters.
 • Many polymeric materials may be spun into fibers, which are
used primarily in textiles. Mechanical, thermal, and chemical
characteristics of these materials are especially critical.
 • Three advanced polymeric materials were discussed: ultra-high-
molecular- weight polyethylene, liquid crystal polymers, and
thermoplastic elastomers. These materials have unusual properties
and are used in a host of high-technology applications.
Polymerization
 • Synthesis of high-molecular-weight
polymers is attained by polymerization, of
which there are two types: addition and
condensation. For addition polymerization,
monomer units are attached one at a time
in chain- like fashion to form a linear
molecule. Condensation polymerization
involves stepwise intermolecular chemical
reac- tions that may include more than a
single molecular species.
Polymer Additives
 • The properties of polymers may be further modified
by using additives; these include fillers, plasticizers,
stabilizers, colorants, and flame retardants. Fillers are
added to improve the strength, abrasion resistance,
toughness, and or thermal/dimensional stability of
polymers. Flexibility, ductility, and toughness are
enhanced by the addition of plasticizers. Stabilizers
counteract deteriorative processes due to exposure
to light and gase- ous species in the atmosphere.
Colorants are used to impart specific colors to
polymers. The flammability resistance of polymers is
enhanced by the incorporation of flame retardants.
Forming Techniques for Plastics
 • Fabrication of plastic polymers is
usually accomplished by shaping the
material in molten form at an elevated
temperature, using at least one of
several different molding techniques.
Compression (Figure 15.23), transfer,
injection (Figure 15.24), and blow.
Extrusion (Figure 15.25) and casting are
also possible.
Compression Molding- The appropriate amount of thoroughly mixed
polymer and necessary additives are placed between male and female
mold members
Transfer, injection Molding- the polymer analogue of die
casting for metals-is the most widely used technique for
fabricating thermoplastic materials.
Blow Extrusion- the polymer analogue of die casting for
metals-is the most widely used technique for fabricating
thermoplastic materials.
Fabrication of Fibers and Films
 • Some fibers are spun from a viscous
melt or solution, after which they are
plastically elongated during a drawing
operation, which improves the
mechanical strength. • Films are formed
by extrusion and blowing (Figure 15.26)
or by calendering.