Chapter 14 - 1

ISSUES TO ADDRESS...

• What are the basic microstructural features?

• How are polymer properties effected by molecular weight?

• How do polymeric crystals accommodate the polymer chain?

CHAPTER 14:POLYMER STRUCTURES

Chapter 14 - 2

Chapter 14 – Polymers

What is a polymer?

Poly mer many repeat unit

Adapted from Fig. 14.2, Callister 7e.

C C C C C C

HHHHHH

HHHHHH

Polyethylene (PE)

ClCl Cl

C C C C C C

HHH

HHHHHH

Polyvinyl chloride (PVC)

HH

HHH H

Polypropylene (PP)

C C C C C C

CH3

HH

CH3CH3H

repeatunit

repeatunit

repeatunit

Chapter 14 - 3

Ancient Polymer History

• 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

Chapter 14 - 4

Polymer Composition

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

– Each carbon bonded to four other atoms

CnH2n+2

C C

H

H HH

HH

Chapter 14 - 5

Chapter 14 - 6

Unsaturated Hydrocarbons

• Double & triple bonds relatively reactive – can form new bonds– Double bond – ethylene or ethene - CnH2n

• 4-bonds, but only 3 atoms bound to C’s

– Triple bond – acetylene or ethyne - CnH2n-2

C C

H

H

H

H

C C HH

Chapter 14 - 7

Isomerism

• Isomerism– two compounds with same chemical formula can

have quite different structures

Ex: C8H18

• n-octane

• 2-methyl-4-ethyl pentane (isooctane)

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 - 8

Chemistry of Polymers

• Free radical polymerization

• Initiator: example - benzoyl peroxide

C

H

H

O O C

H

H

C

H

H

O2

C C

H H

HH

monomer(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 - 9

Chemistry of Polymers

Adapted from Fig. 14.1, Callister 7e.

Note: polyethylene is just a long HC - paraffin is short polyethylene

Chapter 14 -10

Bulk or Commodity Polymers

Chapter 14 -11

Chapter 14 -12

Chapter 14 -13

MOLECULAR WEIGHT

molecules of #total

polymer of wttotalnM

iiw

iin

MwM

MxM

Mw is more sensitive to higher molecular weights

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

Lower M higher M

Adapted from Fig. 14.4, Callister 7e.

Chapter 14 -14

Molecular Weight Calculation

Example: average mass of a classN i M i x i wi

# of students mass (lb)

1 100 0.1 0.0541 120 0.1 0.0652 140 0.2 0.1513 180 0.3 0.2902 220 0.2 0.2371 380 0.1 0.204

M n M w

186 lb 216 lb

iiw MwM

iin MxM

Chapter 14 -15

Degree of Polymerization, n

n = number of repeat units per chain

ii

wiiw

niin

mfm

m

m

Mnwn

m

Mnxn

unit 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( ) ni = 6

mol. wt of repeat unit iChain fraction

Chapter 14 -16

End to End Distance, r

Adapted from Fig. 14.6, Callister 7e.

Chapter 14 -17

• Covalent chain configurations and strength:

Direction of increasing strength

Adapted from Fig. 14.7, Callister 7e.

Molecular Structures

Branched Cross-Linked NetworkLinear

secondarybonding

Chapter 14 -18

Polymers – Molecular Shape

Conformation – Molecular orientation can be changed by rotation around the bonds– note: no bond breaking needed

Adapted from Fig. 14.5, Callister 7e.

Chapter 14 -19

Polymers – Molecular Shape

Configurations – to change must break bonds• Stereoisomerism

EB

A

D

C C

D

A

BE

mirror plane

C CR

HH

HC C

H

H

H

R

or C C

H

H

H

R

Chapter 14 -20

Tacticity

Tacticity – stereoregularity of chain

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC

C C

H

H

H

R

C C

H

H

H

R

C C

H

H

H

R R

H

H

H

CC

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

syndiotactic – R groups alternate sides

atactic – R groups random

Chapter 14 -21

cis/trans Isomerism

C CHCH3

CH2 CH2

C CCH3

CH2

CH2

H

cis

cis-isoprene (natural rubber)

bulky groups on same side of chain

trans

trans-isoprene (gutta percha)

bulky groups on opposite sides of chain

Chapter 14 -22

Copolymers

two or more monomers polymerized together

• random – A and B randomly vary in chain

• alternating – A and B alternate in polymer chain

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

• graft – chains of B grafted on to A backbone

A – B –

random

block

graft

Adapted from Fig. 14.9, Callister 7e.

alternating

Chapter 14 -23

Polymer Crystallinity

Ex: polyethylene unit cell

• Crystals must contain the polymer chains in some way – Chain folded structure

10 nm

Adapted from Fig. 14.10, Callister 7e.

Adapted from Fig. 14.12, Callister 7e.

Chapter 14 -24

Polymer Crystallinity

Polymers rarely 100% crystalline• Too difficult to get all those chains

aligned

• % Crystallinity: % of material that is crystalline. -- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases.

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

Chapter 14 -25

Polymer Crystal Forms

• Single crystals – only if slow careful growth

Adapted from Fig. 14.11, Callister 7e.

Chapter 14 -26

Polymer Crystal Forms

Spherulite surface

Nucleation site Adapted from Fig. 14.13, Callister 7e.

• Spherulites – fast growth – forms lamellar (layered) structures

Chapter 14 -27

Spherulites – crossed polarizers

Adapted from Fig. 14.14, Callister 7e.

Maltese cross

Chapter 14 -28

Core Problems:

Self-help Problems:

ANNOUNCEMENTS

Reading: