Chapter 14/15-

ISSUES TO ADDRESS...• What are the basic microstructural features?

1

• How do these features dictate room T tensile response?

• What dictates polymer mechanical properties?

• How does elevated temperature mechanicalresponse compare to ceramics and metals?

CHAPTERS 14/15:Polymer structures, applications, &

processing

Chapter 14/15- 2

• Polymer = many mers

C C C C C CHHHHHH

HHHHHH

Polyethylene (PE)

mer

ClCl ClC C C C C C

HHH

HHHHHH

Polyvinyl chloride (PVC)

mer

Polypropylene (PP)CH3

C C C C C CHHH

HHHHHH

CH3 CH3

mer

Adapted from Fig. 14.2, Callister 6e.

Adapted from Fig. 14.7, Callister 6e.

Polymer microstructure

The C-C bond has rotational freedom; provided there are no sterichindrance from bulky side groups (polystyrene)

Chain rotation

C C

C

Weakness of polymers is due to weak van der Waals bond betweenchains

Chapter 14/15-

Polymerization of ethylene

C CHH

HHC C

HH

HH=C C

HH

HH= X+ C C

HH

HHX * C C

HH

HHX * + C C

HH

HHX *

C CHH

HHC C C C

HHHH

HHHH

CCH

H HH

CC

CCH

HH

H

HH

HH C C

HH

HHC C C C

HHHH

HHHHCH

C CHH

HHC C C C

HHHH

HHHHC C

HH

HHC C C C

HHHH

HHHHC C

HH

HHC C C C

HHHH

HHHHC C

HH

HHC C C C

HHHH

HHHHX X

However, mistakes do happen during polymerization, leading to branching

Polymerization and termination

Addition polymerization

Chapter 14/15-

Condensation polymerization

C OH

H

H

C

O

CO H

H

H

C

OH

C CH

HH

HO HH O

C OH

H

H

C

O

C

O

C CH

HH

HO HO CO H

H

H

H

Methyl alcohol by-productRepeating unit for polyethylene terephtalate(PET)

+

Dimethyl terephtalate Ethylene glycol

Chapter 14/15-

Polymer propertiesMolecular characteristics

BranchedLinear crosslinked network

Configuration

Stereoisomers Geometrical isomers

cis transSyndiotacticIsotactic Atactic

Chemistry(Mer composition)

Size(Molecular weight)

Shape(Chain twisting, entanglement)

Structure

Chapter 14/15-

Polymer chemistry

Chapter 14/15-

Degree of polymerization

Degree of polymerization =Average molecular weight of polymer

Molecular weight of repeat unit

Mn =ΣxiMi=ΣwiMiMw

Weight-average molecular weight

Number-average molecular weight

5 10 15 20 25 30 40 (103 g/mol)35 5 10 15 20 25 30 40 (103 g/mol)35

Wei

ght f

ract

ion

Num

ber f

ract

ion

Chapter 14/15-

Chapter 14/15-

Chain stiffeness

C CHH

HC C C C

HH

HHHH

Polystyrene (PP)

CH3C C C C C C

HHH

HHHCH

C CH3

OCH3

OCH3

OCH3

O

Bulky side groups offer steric hindrance to polymer chains and kinking

Polymethyl Methacrilate (PMMA)

Chapter 14/15-

Chain crystallization

Chain-folded structure

Chain crystallinity influenced by:• Chemistry (simple mers)• Regularity in

• Structure (linear) • stereoisomerism (isotactic,

syndiotactic)• Co-polymerization (alternating and block)

Chapter 14/15-

Polyethylenes

HDPE LDPEDensity g/cm3 0.96 0.92Tensile strength (psi) 5,500 3,000Young’s modulus (psi) 180,000 40,000Applications

Propane gas Baby oil Parafin wax PolyethylenesChain length (Molecular Weight)

Plastic wrap Plastic bagsShort length branched

• low density• amorphous• weaker

Garbage cansdish pans, pipes

Some crystallinity

UHMWPE has very long molecular chains (70,000 C atoms), crystalline, inert lines sockets of artificial joints

Chapter 14/15-

Structure

• Structure – 4 Covalent chain configurations and strength:

Direction of increasing strength

Branched Cross-Linked NetworkLinear

secondarybonding

Chapter 14/15-

Chain configuration

Stereoisomers Geometrical isomers

cis transSyndiotacticIsotactic Atactic

• isotactic (side group is ordered on one side)

• syndiotactic (side group is ordered on opposite sides)

• atactic (side group is random along the chain)

CH

H

CH3C C

H

H

HC=

cis

CH

H

CH3C C

H H

HC=

trans

Natural Rubber

Gutta-percha

Chapter 14/15-

Other polymer characteristicsCo-polymers

Block

Graft

Alternating

Random

HIP – High Impact PolystyreneStyrene mer (PS)Butadiene mer (PB)

ABS – High Impact PolystyreneStyrene + Acrylonitrile (PS + PA)Butadiene + Acrylonitrile (PB + PA)

C CHH

HC C C C

HH

HHHH

C C

C

N N

N

Polyacrylonitrile

C CHH

HC C C CH

HHHH H

H

Polybutadiene

Chapter 14/15-

• Molecular weight, Mw: Mass of a mole of chains.

3

smaller M w larger M w

• Tensile strength (TS):--often increases with Mw.--Why? Longer chains are entangled (anchored) better.

• % Crystallinity: % of material that is crystalline.--TS and E often increase

with % crystallinity.--Annealing causes

crystalline regionsto grow. % crystallinityincreases.

crystalline region

amorphous region

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.)

Molecular weight & crystallinity

Chapter 14/15- 4

0

unload/reload

0

brittle failure

plastic failure

20

40

60

2 4 6

σ(MPa)

ε

x

x

semi- crystalline

caseamorphous

regions elongate

crystalline regions align

crystalline regions

slide

8

onset of necking

aligned, cross- linked case

networked case

Initial

Near Failure

near failure

Stress-strain curves adapted from Fig. 15.1, Callister 6e. Inset figures along plastic response curve (purple) 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.)

Tensile response: brittle & plastic

Chapter 14/15- 5

• Drawing...--stretches the polymer prior to use--aligns chains to the stretching direction

• Results of drawing:--increases the elastic modulus (E) in the

stretching dir.--increases the tensile strength (TS) in the

stretching dir.--decreases ductility (%EL)

• Annealing after drawing...--decreases alignment--reverses effects of drawing.

• Compare to cold working in metals!

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.)

Predeformation by drawing

Chapter 14/15-

Rubber

Cis-isoprene

CCH3

=H

HC C

H H

HH

HC C

Trans-isoprene

CCH3

=H

HC C

H

HH

HC C

H

Crowding causes kinking and chain rotation

Linear chain is stiffer CH

H

CH3C C

H H

HC=

CH

H

CH3C C

H

H

HC=

Chapter 14/15- 6

• Compare to responses of other polymers:--brittle response (aligned, cross linked & networked case)--plastic response (semi-crystalline case)

Stress-strain curves adapted from Fig. 15.1, Callister 6e. Inset figures along elastomer curve (green) adapted from Fig. 15.14, Callister 6e. (Fig. 15.14 is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.)

Tensile response: elastomer case

initial: amorphous chains are kinked, heavily cross-linked.

final : chains are straight,

still cross-linked

0

20

40

60

0 2 4 6

σ(MPa)

ε 8

x

x

x

elastomer

plastic failure

brittle failure

Deformation is reversible!

Chapter 14/15- 7

• Thermoplastics:--little cross linking--ductile--soften w/heating--polyethylene (#2)

polypropylene (#5)polycarbonatepolystyrene (#6)

• Thermosets:--large cross linking

(10 to 50% of mers)--hard and brittle--do NOT soften w/heating--vulcanized rubber, epoxies,

polyester resin, phenolic resin

Callister, Fig. 16.9

T

Molecular weight

Tg

Tmmobile liquid

viscous liquid

rubber

tough plastic

partially crystalline solidcrystalline

solid

Adapted from Fig. 15.18, Callister 6e. (Fig. 15.18 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.)

Thermoplastics vs thermosets

Chapter 14/15- 8

• Decreasing T...--increases E--increases TS--decreases %EL

• Increasingstrain rate...

--same effectsas decreasing T.

Adapted from Fig. 15.3, Callister 6e. (Fig. 15.3 is from T.S. Carswell and J.K. Nason, 'Effect of Environmental Conditions on the Mechanical Properties of Organic Plastics", Symposium on Plastics, American Society for Testing and Materials, Philadelphia, PA, 1944.)

20

40

60

80

00 0.1 0.2 0.3

4°C

20°C

40°C

60°C to 1.3

σ(MPa)

ε

Data for the semicrystalline polymer: PMMA (Plexiglas)

T and strain rate: thermoplastics

Chapter 14/15-

Melting vs Glass transition temperature

Tg Tm

Den

sity

or s

peci

fic v

olum

e

Temperature

Temperature at which upon cooling a noncrystallinepolymer transforms into from a supercooled liquid to a rigid glass

Supercooledliquid

Liquid

Glass

CrystallineSemi-crystalline

Chapter 14/15- 9

• Stress relaxation test:

Er (t ) =

σ(t )εo

--strain to εο and hold.--observe decrease in

stress with time.

• Relaxation modulus:

• Data: Large drop in Erfor T > Tg. (amorphous

polystyrene)

• Sample Tg(C) (Glass transition) values:PE (low Mw)PE (high Mw)PVCPSPC

-110- 90+ 87+100+150

103

101

10-1

10-3

105

60 100 140 180

rigid solid (small relax)

viscous liquid (large relax)

transition region

T(°C)Tg

Er(10s) in MPa

Adapted from Fig. 15.7, Callister 6e. (Fig. 15.7 is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.)

Selected values from Table 15.2, Callister 6e.

time dependent deformation

time

straintensile test

εo

tσ( )

Chapter 14/15- 10

• General drawbacks to polymers:-- E, σy, Kc, Tapplication are generally small.-- Deformation is often T and time dependent.-- Result: polymers benefit from composite reinforcement.

• Thermoplastics (PE, PS, PP, PC):-- Smaller E, σy, Tapplication-- Larger Kc-- Easier to form and recycle

• Elastomers (rubber):-- Large reversible strains!

• Thermosets (epoxies, polyesters):-- Larger E, σy, Tapplication

-- Smaller Kc

Table 15.3 Callister 6e:

Good overviewof applicationsand trade namesof polymers.

Summary

Chapter 14/15-

Reading:

Core Problems:

Self-help Problems:

0

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