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Chap 13 & 14Polymer Structures
Characteristics, Applications and
Processing of Polymers
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Ourrelationship with Polymers !
rubberProteins
plastic
Wool/ Cotton / other synthetic fibres
woodDNAstyrofoam
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How do polymers differ from metals & ceramics?
Entropy drives structure
primarily amorphous
Mechanical behavior strongly
dependent on temperature
Easy forming processes CHEAP!14/15 - 1
POLMER MOLECULAR STRUCTURE
Polymermer mer mer
H H H H H H H H H H H H H H H H H HC C C C C C C C C C C C C C C C C CH H H H H H H Cl H Cl H Cl H H HCH 3 CH 3 CH 3
Polyethylene (PE) Polyvinyl chloride (PVC) Polypropylene (PP)Adaptedfrom Figs. 14.1, 14.2, Callister6e.
Zig-zag structure easily kinked
H
C
~109
14/15 - 2
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What are Polymers?
Long molecules consisting of
repeating units connected by
covalent bonds
High molecular weightsLiquid/Gases ~ 100 g/mol
Waxy solids ~ 1000 g/mol
High polymers ~ 10,000 to 1 million g/mol
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Example of a polymer
- C C C C C C C C C C C C
H H H H H H H H H H H H
Polyethylene
H H H H H H H H H H H H
Repeating structural units mer
A single mer monomer
A way of representing thepolymer
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Mer units of some other polymers
C C
H H
C C
H H
H Cl
Poly-vinyl-chloride
H CH
Polypropylene
3
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Inorganic polymers
Do other elements of Group IV form polymers?
Polydimethylsilane
Here are some Group IV polymers.
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Inorganic polymersContd.
Do non-group IV element form polymers?
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Synthesis (polymerization) ofpolymersone way of doing it
Initiators
Act 1
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Synthesis (polymerization) ofpolymersone way of doing it
PropagationAct 2
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Synthesis (polymerization) ofpolymersone way of doing it
TerminationAct 3
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An incomplete story ofpolymerization
C = C
H H
R + C CR
H H
H H H H
C C
H H
R
H H
C = C
H H
H H
+
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Molecular weight of polymers
lymer
Molecular weight
Amountof
p
In a polymer, molecules have
varying sizes
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Number average molecular weightof polymers
ction
Molecular weight
Numberf
r n i i
x iM i
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Example calculate the numberaverage molecular weight
Molecular wt range x
(g/mol)
5000-10,000 0.05
Mean wt. (M) x M
7500 375
10,000 15,000 0.16
15,000 20,000 0.22
20,000 25,000 0.27
25,000 30,000 0.20
30,000 - 35,000 0.08
35, 000 40, 000 0.02
12500 2000
17500 3850
22 500 6075
27 500 5500
32 500 2600
37 500 750
Number avgd mol wt = x M = 21, 150
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Weight average molecular weight ofpolymers
ction
Molecular weight
Weight
fr
M = w Mw i i
wi
M i
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Example calculate the weightaverage molecular weight
Molecular wt range w
(g/mol)
5000-10,000 0.02
Mean M w M
7 500 150
10,000 15,000 0.1
15,000 20,000 0.18
20,000 25,000 0.29
25,000 30,000 0.26
30,000 - 35,000 0.13
35, 000 40, 000 0.02
12 500 1250
17 500 3150
22 500 6525
27 500 7150
32 500 4225
37 500 750
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Degree of polymerization
(Average number of mer units in the chain.)
Number averaged Weight averaged
n = M / mnn
M n No. avg molecular wt
m mer molecular wt
M wt. avg. molecular wt.w
n = M / m
w
w
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Example calculate the numberaverage molecular weight
Molecular wt range x
(g/mol)
5000-10,000 0.05
10,000 15,000 0.16
15,000 20,000 0.22
20,000 25,000 0.27
25,000 30,000 0.20
30,000 - 35,000 0.08
35, 000 40, 000 0.02
number-average degreeof polymerization
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Polymers.
Configuration & Confirmation
Topology
Copolymers
Crystals
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Isomers
Structural Isomers
(different atomic connectivity)Stereo-isomers (different spatial
Isomers of propanol
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Stereoregular PolymersStereoregular Polymers
atacticatacticisotacticisotactic
s ndiotactics ndiotactic
POLYVINYL CHLORIDE
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Atactic PolypropyleneAtactic Polypropylene
random stereochemistry of methyl groups attachedrandom stereochemistry of methyl groups attached
to main chain (stereorandom)to main chain (stereorandom)
properties not very useful for fibers etc.properties not very useful for fibers etc.
formed by freeformed by free--radical polymerizationradical polymerization
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Isotactic PolypropyleneIsotactic Polypropylene
stereoregular polymer; all methyl groups onstereoregular polymer; all methyl groups on
same side of main chainsame side of main chain
useful propertiesuseful propertiesprepared by coordination polymerization underprepared by coordination polymerization under
ZieglerZiegler--Natta conditionsNatta conditions
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Syndiotactic PolypropyleneSyndiotactic Polypropylene
stereoregular polymer; methyl groups alternatestereoregular polymer; methyl groups alternate
sideside--toto--side on main chainside on main chain
useful propertiesuseful propertiesprepared by coordination polymerization underprepared by coordination polymerization under
ZieglerZiegler--Natta conditionsNatta conditions
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Polymer Chain Shape
Various confirmations of a molecule
Rotation
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Polymer Chain shape of poly-ethylene
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Confirmation of moleculescontd.
End-to-end distanceMolecular shape can appear
quite random & entangled
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Configuration of polymers - Isomers
C C
H H
C C
H H
Head-to-tail
A mixture of the two
H R H R
H R
C C
H H
C C
H H
R H
H R
C C
R H
C C
H R
C C
H R
C C
H R
H HH H
R H
C C C
H R
C C
H HH H
R H
C C C
Head-to-head
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Stereo-isomers(same connectivity of atoms but differentarrangement)
C C
H H
C C
H H
Iso-tactic configuration
Atactic configuration (random)
H R H R
H R
C C
H R
C C
H H
H H
H R
C C
H R
C C
H H
C C
H R
C C
H R
H HH H
H R
C C C
H H
C C
H RH R
H H
C C C
Syndiotactic configuration
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Stereo-isomers(same connectivity of atoms but differentarrangement)
Isotactic Syndiotactic
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Linear, Branched & Networktopologies
LinearBranched Network
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Linear,,, & cross-linkedPolymers
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Copolymers
B
Monomers
A polymer may be formed by polymerization
Nylon copolymer
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Different types of Copolymers
Alternating copolymer
Random copolymer
Block copolymers
Determined by
a) polymerization process
b) Relative proportion of two monomers
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Self-assembling of copolymers
Crystalline?
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Polymer crystallinity
They are usually never completely crystalline OR completely amorphous
near po ymers are eas y crys a ze .
Branched polymers are not so easily crystallized.
Network polymers are usually amorphous.
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Spatial arrangement of chains in
Polymer crystals .
crystallineregion
Folded polymer chains platellete
p eru teamorphousregion
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Spherulites
crystalline
amorphousregion
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Mechanical Properties
Modulus of elasticity ( = E),
Yield Strength (y) ,
Tensile Stren th (TS)
Characteristics, Applications and Processing of
PolymersIntroduction
300
600
900
1200
Stress(MPa)
Factors influencing themechanical propertiesStrain Rate,
Temperature,
Chemical nature of the
Environment,
0 0.04 0.08 0.12 0.16Strain
0
6Al-4V Titanium Alloy
Stress-Strain characteristics
of 6Al-4V Titanium Alloy
( )
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Fundamental Concepts (I)
MODULUS of ELASTICITY ( = E)The Proportionality Constant E (slope) in Stress-strain curve .
Significance: Greater E denotes:
Stiffer the materialSmaller the elastic strain for a given stress
Example: E for W (407 GPa) > Mg (45 GPa)
YIELD STRENGTH (y)Stress corresponding to 0.002 (0.005) strain in stress-strain curve
TENSILE STRENGTH (TS)
TS is the stress at the max. on the engineering stress-strain curve.
DUCTILITY (% EL)% elongation or % reduction in area
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Strain
Schematic stress-strain diagram showing
linear elastic deformation
Schematic stress-strain diagram
showing elastic and plastic deformation
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eering
ress
TS
Typical engineering stress-strain behaviour to fracture point F
strain
engi
s
Typical response of a metal
Fundamental Concepts (II)
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Fundamental Concepts (II)
Stress-strain on an atomic scale
Elastic StrainAlteration of inter-atomic spacing and bonds,
E proportional to inter-atomic bonding forces
Modulus of elasticityproportional to the slope rodrdF )/(
in force-separation curve
Modulus of Elasticity
Ceramic Materials > Metal > Polymer
Polymer: 7MPa-4GPa , Metal: 48-410GPaExplains atomic bonding
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Force Vs Interatomic separation for weakly and strongly bonded atoms
Fundamental Concepts (III)
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Micro-structure of Polymer
Polymer = many mers
Covalentchain configurations and strength:
Branched Cross-Linked NetworkLinear
secondarybonding
Direction of increasing strength
Stress Strain Behaviour : Brittle and Plastic
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Stress-Strain Behaviour : Brittle and Plastic
Brittle PolymerFractures in Elastic Region
Plastic MaterialsInitially Elastic
followed by Yielding
and a region of plasticdeformation,
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TENSILE RESPONSE: ELASTOMER CASE
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20
40
60
(MPa)
x
x
x
elastomer
plastic failure
brittle failure
TENSILE RESPONSE: ELASTOMER CASE
ElastomersTotally elastic
Large recoverable Strains atlow stress levelRubber like elasticity,
initial: amorphous chains arekinked, heavily cross-linked.
na : c a nsare straight,
stillcross-linked
0
0 2 4 6
8
Deformation
is reversible!
Comparison of the responses to other polymers:--brittle response (aligned, cross linked & networked case)
--plastic response (semi-crystalline case)
Determination of Yield and Tensile strength in Plastic
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Determination of Yield and Tensile strength in Plastic
Polymer
Yield StrengthMaximum on the
stress-strain curve just
beyond the linear
region
Tensile Strength (TS)Fracture oint
TS may be greater
than or less than s
0 0.15 0.3 0.45 0.6Strain
0
25
50
75
100
Stress(MP
a)
Nylon
The Effect of Temperature on the Stress-Strain
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The Effect of Temperature on the Stress Strain
Characteristics of Polymer (PMMA)
Decreasing T...--increases E
--increases TS
--decreases %EL
Increasing
strain rate...--same effects
as decreasing T.
With Increase in T, the materials becomes
more ductile.
Macroscopic Deformation
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Stages ofDeformationUpper and lower
yield points and near
horizontal region.
Neck formation
Semicrystalline Polymers
Chain orientation
parallel to theelongation direction
Neck Extension
Resistance todeformation Chain
orientation
phenomenon results
neck extension
Important Features
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Important Features
Stress-Strain Curve1. Brittle Failure 2. Ductile Failure with neck formation 3. ductile
failure with cold drawing and orientational hardening 4. rubbery
behaviour
Yield stress Strain softenin draw stress and orientational hardenin
Glass Transition and melting
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Glass Transition and melting
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Viscoelastic Deformation
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Mechanical Behaviour-- Low T (glassy) --Elastic
-- Intermediate T (>Tg) (rubbery) --Combination of elastic
and viscous behaviour(Viscoelasticity)
-- High T (viscous/liquid) --Viscous
Amorphous Polymer
Dependence on Rate and Time period of loading-- Hookes law -- independent of loading rate
-- Newtons law -- Strain rate dependence
Viscoelastic Case-- Low T and High Strain-rate -- Elastic
-- High T and Low Strain Rate -- Viscous
Rate and Time period dependent behaviour
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Elastic CaseTotal Deformation
Recovery
Viscous Case
Amorphous Polymer
Deformation
delayed
No complete
Recovery
Viscoelastic Case
Elastic and
Time dependent
strain
Elastic
Viscoelastic Viscous
Viscoelastic Relaxation ModulusVi l ti P l
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Stress Relaxation MeasurementsStress decreases with time
(Molecular Relaxation)
Relaxation Modulus Er(t)
Er(t) = (t) /o
Viscoelastic Polymer
(t) , T me epen ent Stress
o , Strain LevelSpecific Polymer Cases:
(t) = (0)exp (-t/)
T = elapsed time, = relaxation time
Time and T dependency
Decrease of Er(t) with time
Smaller Er(t) as T is increase
Viscoelastic Relaxation Modulus : T dependency
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Stress RelaxationMeasurements
Stress decreases with time
(Molecular Relaxation)
Amorphous Polystyrene, t1 = 10s
time
strain
tensile test
o
t( )
105 rigid solid
e axat on o u us r t
Er(t) = (t) /o
(t) , Time dependent Stress
o , Strain Level
Time and T dependency
Decrease of Er(t) with time
Smaller Er(t) as T is increase
Decrease in Er(t) => Easy Deformation
103
101
10-1
10-3
60 100 140 180
viscous liquid(large relax)
transitionregion
T(C)Tg
in MPa
*Interatomicbonding
concept
*Viscosity
Regions of Viscoelastic Behaviour and deformation
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Leathery (Tg) regionDeformation:
Time Dependent, Not total Recovery
Rubbery Plateau
Amorphous Polystyrene
Sample Tg(C) values:
PE (low Mw)
PE (high Mw)
PVC
PS
PC
-110
- 90
+ 87
+100
+150
Both Elastic and Viscous components
Easy deformation
Rubbery and Viscous Flow
Deformation: Not total Recovery
Independent Vibrational and
Rotational motion of chains: Viscous
Relaxation Modulus Vs T plots for polymers with
diff t C fi ti P l t
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different Configurations: Polystyrene
CrosslinkedAtactic (B)
Atactic : Random position of the R group
Isotactic : R group in the same side of the polymer backbone
,
Decomposition
Plateau in
Rubbery region
CrystallineIsotactic(A)
Er(10) is high
Decreases at Tm
E = K(RT/M)
Viscoelastic Creep Viscoelastic Creep
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Viscoelastic Creep-- Constant Stress
Level,
-- Time dependent
deformation
-- Isothermal
condition
Creep Modulus
Ec(t) = o/(t)
Increase in Ec(t):
-- Increase in T
-- Increase in degree
in Crystallinity
MECHANISMS OF DEFORMATION AND FOR STRENGTHENING OF
POLYMERS
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Molecular weight, Mw: Mass of a mole of chains.
smaller Mw larger Mw
% Crystallinity: % of material that is crystalline.
MOLECULAR WEIGHT & CRYSTALLINITY
crystallineregion
amorphousregion
Adapted from Fig. 14.11, Callister 6e.
(Fig. 14.11 is from H.W. Hayden, W.G. Moffatt,
and J. Wulff, The Structure and Properties of
Materials, Vol. III, Mechanical Behavior, John
Wiley and Sons, Inc., 1965.) Fringed-micelle model
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DEFORMATION OF SEMICRYSTALLINE POLYMERS
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Mechanism of Elastic Deformation-- Elongation of Chain Molecules by bending and stretching of bonds
-- Displacement of adjacent molecules (resisted by secondary and van der
walls bonds)
Mechanism of Plastic Deformation
-- Amorphous region slip past each other and alignment. (ribbon extended)
-- Tilting of the Lamellae
-- Crystalline block segments separate from the lamellae
-- Orientation of blocks and tie chains
STAGES OF DEFORMATION OF SEMICRYSTALLINE
POLYMERS
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, .
FACTORS INFLUENCING THE MECHANICAL PROPERTIES
OF SEMICRYSTALLINE POLYMERS
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Intermolecular forces resulting from large number of van der Walls interchain bonds
Molecular Weight-- Seems No effect on Tensile Modulus
-- !!Increase of TS With Mw TS = TS- (A/Mn)
-- Increased chain entanglement TS= TS at infinite Mw
Crystallinity
-- quasi-statically; Thermodynamic Factor;Hm > Tm Sm ; G Minimum
-- Real Practice; Say Quenching; Kinetic Factor; and Nucleation rate.
-- Increase of Tensile Modulus and TS with enhanced % crystallinity
-- Stronger Secondary Bonding; Ordered and Parallel arrangements
PHYSICAL CHARATERISTICS OF POLYETHYLENE
(% Crystallinity and Molecular Weight)
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Modulus Structure: Increase in Modulus and with crystallinity-- Increase in Brittleness
Youngs Modulus, E, Vs % Crystallization
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Rubber Polyethelene
PREDEFORMATION BY DRAWING
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Drawing... ( strain hardening in metals)-- deform the polymer
-- aligns chains to the stretching direction
-- Anisotropic Characteristics Results of drawing:
--increases the elastic modulus (E) in the
stretchin dir.
--increases the tensile strength (TS) in the
stretching dir.--decreases ductility (%EL)
Annealing after drawing...--decreases alignment and strain-induced crystallinity
--reverses effects of drawing.
Adapted from Fig. 15.12,Callister 6e. (Fig. 15.12 is from
J.M. Schultz, Polymer
Materials Science, Prentice-
Hall, Inc., 1974, pp. 500-501.)
DEFORMATION OF ELASTOMERS
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final: chainsare straight,
still-
0
20
40
60
0 2 4 6
(MPa)
8
x
x
x
elastomer
plastic failure
brittle failure
Driving Force-- Increase in entropy, S, when the
elastomer comes from ordered to
kinked and coiled counters
-- Increase in T and E
< Tg , Brittle
initial: amorphous chains arekinked, heavily cross-linked.
-
Deformationis reversible!
Smaller E, Vary with Strain
VULCANIZATION
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Vulcanization-- Cross Linking Process in
Elastomers
-- Non-reversible chemicalReaction
Results of Vulcanization
-- Enhanced; E, TS andResistance to Degradation
Modulus of Elasticity
-- Density of the Crosslinks
STRESS-STRAIN CURVES FOR UNVULKANIZED AND VULKANIZED RUBBER
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600 % elongation
SUMMARY
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General drawbacks to polymers:-- E, y, T application are generally small.
-- Deformation is often T and time dependent.
-- Result: polymers benefit from composite reinforcement.
Thermoplastics (PE, PS, PP, PC):-- Smaller E, y, Tapplication--
Elastomers (rubber):-- Large reversible strains!
Thermosets (epoxies, polyesters):
-- Larger E, y, Tapplication