ch-13 14 compatibility mode

7/28/2019 Ch-13 14 Compatibility Mode

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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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PropagationAct 2


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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

ch-13 14 compatibility mode

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