Chapter 14,15: Polymersisdl.cau.ac.kr/education.data/mat.sci/ch14.15.polymers.… · ·...

Chapter 14

Chapter 14,15: Polymers

School of Mechanical Engineering Choi, Hae-Jin

Materials Science - Prof. Choi, Hae-Jin 1

http://www.cau.ac.kr/

Chapter 14

2

CHAPTER 14,15: POLYMER STRUCTURES

ISSUES TO ADDRESS... • What are the general structural and chemical characteristics of polymer molecules? • What are some of the common polymeric materials, and how do they differ chemically? • How is the crystalline state in polymers different from that in metals and ceramics ?

Materials Science - Prof. Choi, Hae-Jin

• What are the various types/classifications of polymers? • What are the polymerization and basic fabrication and processing techniques for polymers?


Chapter 14

What is a Polymer?

Poly mer many repeat unit

3

Adapted from Fig. 4.2, Callister & Rethwisch 3e.

C C C C C C H H H H H H

H H H H H H

Polyethylene (PE) Cl Cl Cl

C C C C C C H H H

H H H H H H

Poly(vinyl chloride) (PVC) H H

H H H H

Polypropylene (PP)

C C C C C C CH3

H H

CH3 CH3 H

repeat unit

repeat unit

repeat unit



Chapter 14

Ancient Polymers

• Originally natural polymers were used – Wood – Rubber – Cotton – Wool – Leather – Silk

• Oldest known uses

– Rubber balls used by Incas – Noah used pitch (a natural polymer)

for the ark

4 Materials Science - Prof. Choi, Hae-Jin


Chapter 14

Polymer Composition Most polymers are hydrocarbons – i.e., made up of H and C • Saturated hydrocarbons

– Each carbon singly bonded to four other atoms – Example:

• Ethane, C2H6

5

C C

H

H H H

HH



Chapter 14



Chapter 14

Unsaturated Hydrocarbons • Double & triple bonds somewhat unstable

– can form new bonds – Double bond found in ethylene or ethene - C2H4

– Triple bond found in acetylene or ethyne - C2H2

7

C CH

H

H

H

C C HH



Chapter 14

Isomerism • Isomerism

– two compounds with same chemical formula can have quite different structures

for example: C8H18 • normal-octane

• 2,4-dimethylhexane

8

C C C C C C C CH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H H3C CH2 CH2 CH2 CH2 CH2 CH2 CH3=

H3C CH

CH3

CH2 CH

CH2

CH3

CH3

H3C CH2 CH3( )6

⇓



Chapter 14

Polymerization and Polymer Chemistry

• Free radical polymerization

• Initiator: example - benzoyl peroxide

9

C

H

H

O O C

H

H

C

H

H

O2

C C

H H

HHmonomer(ethylene)

R +

free radical

R C C

H

H

H

H

initiation

R C C

H

H

H

H

C C

H H

HH

+ R C C

H

H

H

H

C C

H H

H H

propagation

dimer

R= 2



Chapter 14

Chemistry and Structure of Polyethylene

10


Note: polyethylene is a long-chain hydrocarbon - paraffin wax for candles is short polyethylene



Chapter 14

Bulk or Commodity Polymers



Chapter 14

Bulk or Commodity Polymers (cont)



Chapter 14

13

Bulk or Commodity Polymers (cont)



Chapter 14

MOLECULAR WEIGHT

14

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

Low M

high M

Not all chains in a polymer are of the same length — i.e., there is a distribution of molecular weights



Chapter 14

MOLECULAR WEIGHT DISTRIBUTION

15

xi = number fraction of chains in size range i

molecules of #totalpolymer of wttotal

=nM

iiw

iin

MwM

MxM

Σ=

Σ=


wi = weight fraction of chains in size range i

Mi = mean (middle) molecular weight of size range i



Chapter 14

Molecular Weight Calculation Example: average mass of a class

16

Student Weight mass (lb)

1 104 2 116 3 140 4 143 5 180 6 182 7 191 8 220 9 225 10 380

What is the average weight of the students in this class: a) Based on the number

fraction of students in each mass range?

b) Based on the weight fraction of students in each mass range?



Chapter 14

Molecular Weight Calculation (cont.) Solution: The first step is to sort the students into weight ranges.

Using 40 lb ranges gives the following table:

17

weight number of mean number weightrange students weight fraction fraction

Ni Wi xi wi

mass (lb) mass (lb)81-120 2 110 0.2 0.117

121-160 2 142 0.2 0.150161-200 3 184 0.3 0.294201-240 2 223 0.2 0.237241-280 0 - 0 0.000281-320 0 - 0 0.000321-360 0 - 0 0.000361-400 1 380 0.1 0.202

ΣNi ΣNiWi

10 1881total

number total

weight

Calculate the number and weight fraction of students in each weight range as follows:

xi =Ni

Ni∑

wi =NiWi

NiWi∑

For example: for the 81-120 lb range

x81−120 =2

10= 0.2

117.01881

011 x 212081 ==−w



Chapter 14

Molecular Weight Calculation (cont.)

18

Mn = xiMi∑ = (0.2 x 110 + 0.2 x 142 + 0.3 x 184 + 0.2 x 223 + 0.1 x 380) =188 lb

weight mean number weightrange weight fraction fraction

Wi xi wi

mass (lb) mass (lb)81-120 110 0.2 0.117

121-160 142 0.2 0.150161-200 184 0.3 0.294201-240 223 0.2 0.237241-280 - 0 0.000281-320 - 0 0.000321-360 - 0 0.000361-400 380 0.1 0.202

Mw = wiMi∑ = (0.117 x 110 + 0.150 x 142 + 0.294 x 184

+ 0.237 x 223 + 0.202 x 380) = 218 lb

Mw = wiMi∑ = 218 lb



Chapter 14

Degree of Polymerization, DP DP = average number of repeat units per chain

mMDP n=

19

iimfm

m

Σ=

=

:follows as calculated is this copolymers forunit repeat of weightmolecular average where

C C C C C C C CH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

C C C C

H

H

H

H

H

H

H

H

H( ) DP = 6

mol. wt of repeat unit i Chain fraction Materials Science - Prof. Choi, Hae-Jin


Chapter 14

Molecular Structures for Polymers

20


B ranched Cross-Linked Network Linear

secondary bonding



Chapter 14

Polymers – Molecular Shape

Molecular Shape (or Conformation) – chain bending and twisting are possible by rotation of carbon atoms around their chain bonds – note: not necessary to break chain bonds to

alter molecular shape

21




Chapter 14

Chain End-to-End Distance, r

22




Chapter 14

Molecular Configurations for Polymers

Configurations – to change must break bonds • Stereoisomerism

23

E B

A

D

C C

D

A

B E

mirror plane

C CR

HH

HC C

H

H

H

R

or C C

H

H

H

R

Stereoisomers are mirror images – can’t superimpose without breaking a bond



Chapter 14

Tacticity Tacticity – stereoregularity or spatial arrangement of

R units along chain

24

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC

isotactic – all R groups on same side of chain

C C

H

H

H

R

C C

H

H

H

R

C C

H

H

H

R R

H

H

H

CC

syndiotactic – R groups alternate sides



Chapter 14

Tacticity (cont.)

25

atactic – R groups randomly positioned

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC



Chapter 14

cis/trans Isomerism

26

C CHCH3

CH2 CH2

C CCH3

CH2

CH2

H

cis cis-isoprene

(natural rubber)

H atom and CH3 group on same side of chain

trans trans-isoprene (gutta percha)

H atom and CH3 group on opposite sides of chain



Chapter 14

Copolymers two or more monomers

polymerized together • random – A and B randomly

positioned along chain • alternating – A and B

alternate in polymer chain • block – large blocks of A

units alternate with large blocks of B units

• graft – chains of B units grafted onto A backbone A – B –

27

random

block

graft


alternating



Chapter 14

Crystallinity in Polymers • Ordered atomic

arrangements involving molecular chains

• Crystal structures in terms of unit cells

• Example shown – polyethylene unit cell

28




Chapter 14

Polymer Crystallinity • Crystalline regions

– thin platelets with chain folds at faces – Chain folded structure

29

10 nm




Chapter 14

Polymer Crystallinity (cont.) Polymers rarely 100% crystalline • Difficult for all regions of all chains to

become aligned

30

• Degree of crystallinity expressed as % crystallinity. -- Some physical properties depend on % crystallinity. -- Heat treating causes crystalline regions to grow and % crystallinity to increase.

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.)

crystalline region

amorphous region



Chapter 14

Polymer Single Crystals • Electron micrograph – multilayered single crystals

(chain-folded layers) of polyethylene • Single crystals – only for slow and carefully controlled

growth rates

31




Chapter 14

Semicrystalline Polymers • Some semicrystalline

polymers form spherulite structures

• Alternating chain-folder crystallites and amorphous regions

• Spherulite structure for relatively rapid growth rates

32

Spherulite surface




Chapter 14

Photomicrograph – Spherulites in Polyethylene

33


Cross-polarized light used -- a maltese cross appears in each spherulite



Chapter 14

Point Defects in Polymers • Defects due in part to chain packing errors and impurities such

as chain ends and side chains

34




Chapter 14

35

Mechanical Properties of Polymers – Stress-Strain Behavior

35

• Fracture strengths of polymers ~ 10% of those for metals • Deformation strains for polymers > 1000% – for most metals, deformation strains < 10%

brittle polymer

plastic elastomer

elastic moduli – less than for metals Adapted from Fig. 7.22,

Callister & Rethwisch 3e.


Chapter 14

36

Mechanisms of Deformation—Brittle Crosslinked and Network Polymers

36

brittle failure

plastic failure

σ (MPa)

ε

x

x

aligned, crosslinked polymer Stress-strain curves adapted from Fig. 7.22,

Callister & Rethwisch 3e.

Initial Near

Failure Initial

network polymer

Near Failure


Chapter 14

37

Mechanisms of Deformation — Semicrystalline (Plastic) Polymers

37

brittle failure

plastic failure

σ (MPa)

x

x

crystalline block segments

separate

fibrillar structure

near failure

crystalline regions align

onset of necking

undeformed structure amorphous

regions elongate

unload/reload

Stress-strain curves adapted from Fig. 7.22, Callister & Rethwisch 3e. Inset figures along plastic response curve adapted from Figs. 8.27 & 8.28, Callister & Rethwisch 3e. (Figs. 8.27 & 8.28 are from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.)

ε


Chapter 14

38

Predeformation by Drawing • Drawing…(ex: monofilament fishline) -- stretches the polymer prior to use -- aligns chains in the stretching direction • Results of drawing: -- increases the elastic modulus (E) in the stretching direction -- increases the tensile strength (TS) in the stretching direction -- decreases ductility (%EL) • Annealing after drawing... -- decreases chain alignment -- reverses effects of drawing (reduces E and TS, enhances %EL) • Contrast to effects of cold working in metals!

Adapted from Fig. 8.28, Callister & Rethwisch 3e. (Fig. 8.28 is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.)


Chapter 14

39

Mechanisms of Deformation—Elastomers

• Compare elastic behavior of elastomers with the: -- brittle behavior (of aligned, crosslinked & network polymers), and -- plastic behavior (of semicrystalline polymers) (as shown on previous slides)

Stress-strain curves adapted from Fig. 7.22, Callister & Rethwisch 3e. Inset figures along elastomer curve (green) adapted from Fig. 8.30, Callister & Rethwisch 3e. (Fig. 8.30 is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.)

σ (MPa)

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

x

final: chains are straighter,

still cross-linked

elastomer

deformation is reversible (elastic)!

brittle failure

plastic failure x

x


Chapter 14

40

Polymer Types – Fibers Fibers - length/diameter >100 • Primary use is in textiles. • Fiber characteristics:

– high tensile strengths – high degrees of crystallinity – structures containing polar groups

40

• Formed by spinning – extrude polymer through a spinneret (a die containing many small orifices) – the spun fibers are drawn under tension – leads to highly aligned chains - fibrillar structure


Chapter 14

41

Polymer Types – Miscellaneous • Coatings – thin polymer films applied to surfaces – i.e.,

paints, varnishes – protects from corrosion/degradation – decorative – improves appearance – can provide electrical insulation

41

• Adhesives – bonds two solid materials (adherands) – bonding types:

1. Secondary – van der Waals forces 2. Mechanical – penetration into pores/crevices

• Films – produced by blown film extrusion • Foams – gas bubbles incorporated into plastic


Chapter 14

42

Advanced Polymers

• Molecular weight ca. 4 x 106 g/mol • Outstanding properties

– high impact strength – resistance to wear/abrasion – low coefficient of friction – self-lubricating surface

• Important applications – bullet-proof vests – golf ball covers – hip implants (acetabular cup)

42

UHMWPE

Adapted from chapter-opening photograph, Chapter 22, Callister 7e.

Ultrahigh Molecular Weight Polyethylene (UHMWPE)


Chapter 14

43

Polymer Formation • There are two types of polymerization

– Addition (or chain) polymerization – Condensation (step) polymerization


Chapter 14

44 44

Addition (Chain) Polymerization

– Initiation

– Propagation

– Termination


Chapter 14

45 45

Condensation (Step) Polymerization


Chapter 14

46 46

Polymer Additives Improve mechanical properties, processability,

durability, etc. • Fillers

– Added to improve tensile strength & abrasion resistance, toughness & decrease cost

– ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc.

• Plasticizers – Added to reduce the glass transition temperature Tg below room temperature – Presence of plasticizer transforms brittle polymer to a ductile one – Commonly added to PVC - otherwise it is brittle


Chapter 14

47 47

Polymer Additives (cont.) • Stabilizers

– Antioxidants – UV protectants

• Lubricants – Added to allow easier processing – polymer “slides” through dies easier – ex: sodium stearate

• Colorants – Dyes and pigments

• Flame Retardants – Substances containing chlorine, fluorine, and boron


Chapter 14

48 48

Processing of Plastics • Thermoplastic

– can be reversibly cooled & reheated, i.e. recycled – heat until soft, shape as desired, then cool – ex: polyethylene, polypropylene, polystyrene.

• Thermoset – when heated forms a molecular network (chemical reaction) – degrades (doesn’t melt) when heated – a prepolymer molded into desired shape, then chemical reaction occurs – ex: urethane, epoxy


Chapter 14

49 49

Processing Plastics – Compression Molding Thermoplastics and thermosets • polymer and additives placed in mold cavity • mold heated and pressure applied • fluid polymer assumes shape of mold

Fig. 14.29, Callister & Rethwisch 3e. (Fig. 14.29 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley & Sons, 1984.)


Chapter 14

50 50

Processing Plastics – Injection Molding

Fig. 14.30, Callister & Rethwisch 3e. (Fig. 14.30 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 2nd edition, John Wiley & Sons, 1971.)

Thermoplastics and some thermosets • when ram retracts, plastic pellets drop from hopper into barrel • ram forces plastic into the heating chamber (around the

spreader) where the plastic melts as it moves forward • molten plastic is forced under pressure (injected) into the mold

cavity where it assumes the shape of the mold

Barrel


Chapter 14

51 51

Processing Plastics – Extrusion

Fig. 14.31, Callister & Rethwisch 3e. (Fig. 14.31 is from Encyclopædia Britannica, 1997.)

thermoplastics • plastic pellets drop from hopper onto the turning screw • plastic pellets melt as the turning screw pushes them

forward by the heaters • molten polymer is forced under pressure through the

shaping die to form the final product (extrudate)


Chapter 14

52 52

Processing Plastics – Blown-Film Extrusion

Fig. 14.32, Callister & Rethwisch 3e. (Fig. 14.32 is from Encyclopædia Britannica, 1997.)


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