CHAPTER 4:POLYMER STRUCTURES

ISSUES TO ADDRESS...• What are the general structural and chemical What are the general structural and chemical

characteristics of polymer molecules?• What are some of the common polymeric p y

materials, and how do they differ chemically?• How is the crystalline state in polymers different

from that in metals and ceramics ?

Chapter 4 - 1

What is a Polymer?y

Poly merPoly mermany repeat unit

repeatunit

repeatunit

repeatunit

C C C C C CHHHHHH

HHHHHH

ClCl ClC C C C C C

HHH

HHHHHH

HH

HHH HC C C C C C

CH

HH

CHCH H

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

HHHHHHPolyethylene (PE)

ClCl ClHHHPoly(vinyl chloride) (PVC)

HHPolypropylene (PP)

CH3CH3CH3H

Chapter 4 - 2

Ancient Polymersy• Originally natural polymers were used

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

• Oldest known uses– Rubber balls used by Incas– Noah used pitch (a natural polymer)Noah used pitch (a natural polymer)

for the ark

Chapter 4 - 3

Polymer Compositiony pMost polymers are hydrocarbons

i d f H d C– i.e., made up of H and C• Saturated hydrocarbons

E h b i l b d d f h– Each carbon singly bonded to four other atoms– Example:

• Ethane, C2H6

C C

HHH

H H H

Chapter 4 - 4

Chapter 4 - 5

Unsaturated Hydrocarbonsy• Double & triple bonds somewhat unstable –

f b dcan form new bonds– Double bond found in ethylene or ethene - C2H4

C CH H

H H

– Triple bond found in acetylene or ethyne - C2H2

C C HH

Chapter 4 - 6

Isomerism• Isomerism

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

f l C Hfor example: C8H18• normal-octane

H H H H H H H H

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=

• 2,4-dimethylhexaneH3C CH2 CH3( )

6

H3C CH

CH3

CH2 CH

CH2

CH3

Chapter 4 - 7

2

CH3

Polymerization and Polymer ChemistryPolymer Chemistry

• Free radical polymerization

C C

H H

HH

R + R C C

H

H

H

H

initiation

HHmonomer(ethylene)

free radicalH H

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

• Initiator: example - benzoyl peroxide

H H HH H H H H

dimer

C

H

O O C

HC

H

O2 R= 2

Chapter 4 - 8

C

H

O O C

H

C

H

O2 R= 2

Chemistry and Structure of PolyethylenePolyethylene

Adapted from Fig. 4 1 Callister &4.1, Callister & Rethwisch 3e.

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

Chapter 4 - 9

paraffin wax for candles is short polyethylene

Bulk or Commodity Polymersy y

Chapter 4 - 10

Bulk or Commodity Polymers (cont)y y ( )

Chapter 4 - 11

Bulk or Commodity Polymers (cont)y y ( )

Chapter 4 - 12

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

Low MLow M

high M

Not all chains in a polymer are of the same lengthi th i di t ib ti f l l i ht— i.e., there is a distribution of molecular weights

Chapter 4 - 13

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

H H H H H H H H H H H HDP 6C C C C C C C CH

H H H H H H H H

C C C C

H H H H

H( ) DP = 6

mMDP n

m unit repeat of weightmolecular average where

m

iimfm

:follows as calculated is this copolymers for

Chapter 4 - 14

iimfm mol. wt of repeat unit iChain fraction

Molecular Structures for Polymersy

secondarybonding

Branched Cross-Linked NetworkLinear

Adapted from Fig. 4.7, Callister & Rethwisch 3e.

Chapter 4 - 15

Polymers – Molecular Shapey pMolecular 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 shapep

Adapted from Fig. 4 5 Callister &4.5, Callister & Rethwisch 3e.

Chapter 4 - 16

Chain End-to-End Distance, r,

Adapted from Fig. 4.6, Callister & R th i h 3Rethwisch 3e.

Chapter 4 - 17

Copolymers Adapted from Fig. 4 9 Callister &p y

two or more monomers polymerized together

random

4.9, Callister & Rethwisch 3e.

polymerized together • random – A and B randomly

positioned along chainpositioned along chain• alternating – A and B

alternate in polymer chain alternating

• block – large blocks of A units alternate with large blocks of B units blockblocks of B units

• graft – chains of B units grafted onto A backboneg

A – B –ft

Chapter 4 - 18

graft

Crystallinity in Polymers Adapted from Figy y y• Ordered atomic

t i l i

Adapted from Fig. 4.10, Callister & Rethwisch 3e.

arrangements involving molecular chainsCr stal str ct res in terms• Crystal structures in terms of unit cells

• Example shown• Example shown– polyethylene unit cell

Chapter 4 - 19

Polymer Crystallinityy y y• Crystalline regions

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

Adapted from Fig. 4.12, Callister & Rethwisch 3e.

10 nm

Rethwisch 3e.

Chapter 4 - 20

Polymer Crystallinity (cont.)y y y ( )Polymers rarely 100% crystalline

Diffi lt f ll i f ll h i t• Difficult for all regions of all chains to become aligned crystalline

region

• Degree of crystallinity expressed as % crystallinity.

g

-- Some physical properties depend on % crystallinity.

-- Heat treating causes crystalline regions to grow and % crystallinity to increase. hc ease

Adapted from Fig. 14.11, Callister 6e.(Fig 14 11 is from H W Hayden W G Moffatt

amorphousregion

Chapter 4 - 21

(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.)

Polymer Single Crystalsy g y• Electron micrograph – multilayered single crystals

( h i f ld d l ) f l th l(chain-folded layers) of polyethylene• Single crystals – only for slow and carefully controlled

growth ratesgrowth rates

Chapter 4 - 22Adapted from Fig. 4.11, Callister & Rethwisch 3e.

Semicrystalline Polymersy y• Some semicrystalline

l fpolymers form spherulite structures

• Alternating chain folder• Alternating chain-folder crystallites and amorphous regionsamorphous regions

• Spherulite structure for relatively rapid growth

Spherulite surface

y p grates

Adapted from Fig. 4.13, Callister & Rethwisch 3e.

Chapter 4 - 23

Photomicrograph – Spherulites in PolyethylenePolyethylene

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

Chapter 4 - 24Adapted from Fig. 4.14, Callister & Rethwisch 3e.