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Chapter 14 - 1 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 ? Chapter 14: Polymer Structures

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Page 1: ch14mymar19

Chapter 14 - 1

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 ?

Chapter 14:Polymer Structures

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Chapter 14 - 2

Sections 14.1 – 14.4, Equations 14.5a and 14.6,Sections 14.6 – 14.7, Sections 14.9 – 14.13

Reading for Chapter 14

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Chapter 14 - 3

What is a Polymer?

Poly mer many repeat unit

Adapted from Fig. 14.2, Callister & Rethwisch 8e.

C C C C C CHHHHHH

HHHHHH

Polyethylene (PE)ClCl Cl

C C C C C CHHH

HHHHHH

Poly(vinyl chloride) (PVC)HH

HHH H

Polypropylene (PP)

C C C C C CCH3

HH

CH3CH3H

repeatunit

repeatunit

repeatunit

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Chapter 14 - 4

Ancient Polymers• Originally natural polymers were used

– Wood – Rubber– Cotton – Wool– Leather – Silk

• Oldest known uses– Rubber balls used by Incas– Pitch (a natural polymer) used for caulking

on old wooden ships

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Chapter 14 - 5

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

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

• Ethane, C2H6

C C

H

H HH

HH

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Chapter 14 - 6

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Chapter 14 - 7

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

C CH

H

H

H

C C HH

Page 8: ch14mymar19

Chapter 14 - 8

Isomerism• Isomerism

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

for example: C8H18

• normal-octane

• 2,4-dimethylhexane

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

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Chapter 14 - 9

Polymerization and Polymer Chemistry

• Free radical polymerization

• Initiator: example - benzoyl peroxide

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

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Chapter 14 -10

Chemistry and Structure of Polyethylene

Adapted from Fig. 14.1, Callister & Rethwisch 8e.

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

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Chapter 14 -11

Bulk or Commodity Polymers

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Chapter 14 -12

Bulk or Commodity Polymers (cont)

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Chapter 14 -13

Bulk or Commodity Polymers (cont)

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Chapter 14 -14

MOLECULAR WEIGHT• 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

Page 15: ch14mymar19

Chapter 14 -15

xi = number fraction of chains in size range i

molecules of #totalpolymer of wttotal

nM

MOLECULAR WEIGHT DISTRIBUTION

iin MxM

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

Page 16: ch14mymar19

Chapter 14 -16

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

iimfm

m

: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 iChain fraction

mMDP n

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Chapter 14 -17

Adapted from Fig. 14.7, Callister & Rethwisch 8e.

Molecular Structures for Polymers

Branched Cross-Linked NetworkLinear

secondarybonding

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Chapter 14 -18

Polymers – Molecular ShapeMolecular 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

Adapted from Fig. 14.5, Callister & Rethwisch 8e.

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Chapter 14 -19

Chain End-to-End Distance, r

Adapted from Fig. 14.6, Callister & Rethwisch 8e.

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Chapter 14 -20

Copolymerstwo 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 –

random

block

graft

Adapted from Fig. 14.9, Callister & Rethwisch 8e.

alternating

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Chapter 14 -21

Crystallinity in Polymers• Ordered atomic

arrangements involving molecular chains

• Crystal structures in terms of unit cells

• Example shown– polyethylene unit cell

Adapted from Fig. 14.10, Callister & Rethwisch 8e.

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Chapter 14 -22

Polymer Crystallinity• Crystalline regions

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

10 nm

Adapted from Fig. 14.12, Callister & Rethwisch 8e.

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Chapter 14 -23

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

become aligned

• 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

amorphousregion

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Chapter 14 -24

Polymer Single Crystals• Electron micrograph – multilayered single crystals

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

growth rates

Adapted from Fig. 14.11, Callister & Rethwisch 8e.

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Chapter 14 -25

Semicrystalline Polymers

Spherulite surface

Adapted from Fig. 14.13, Callister & Rethwisch 8e.

• Some semicrystalline polymers form spherulite structures

• Alternating chain-folded crystallites and amorphous regions

• Spherulite structure for relatively rapid growth rates

Page 26: ch14mymar19

Chapter 14 -26

Photomicrograph – Spherulites in Polyethylene

Adapted from Fig. 14.14, Callister & Rethwisch 8e.

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

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Chapter 14 -27

Study Problems

14.1, 14.2, 14.3, 14.4, 14.13, 14.14, 14.15, 14,22

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Chapter 14 -28

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Chapter 14 -29

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Chapter 14 -30

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Chapter 14 -31

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Chapter 14 -32

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Chapter 14 -33

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Chapter 14 -34

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Chapter 14 -35