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Chapter 14:

ISSUES TO ADDRESS

Polymer StructuresISSUES TO ADDRESS... What are the general structural and chemical

characteristics of polymer molecules?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 - 1

What is a Polymer?

Poly mermany repeat unit

C C C C C CHHHHHH

C C C C C CHHHHHH HHH H

C C C C C CHH

repeatunit

repeatunit

repeatunit

C C C C C CHHHHHH

Polyethylene (PE)ClCl Cl

C C C C C CHHH

Poly(vinyl chloride) (PVC)HH

Polypropylene (PP)

C C C C C CCH3CH3CH3H

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

Chapter 14 - 2

Ancient Polymers Originally natural polymers were used

Wood Rubber Wood Rubber Cotton Wool

L th Silk Leather Silk

Old t k Oldest known uses Rubber balls used by Incas Noah used pitch (a natural polymer)

for the ark

Chapter 14 - 3

Polymer CompositionMost polymers are hydrocarbons

i.e., made up of H and Cp Saturated hydrocarbons

Each carbon singly bonded to four other atoms Example:

Ethane, C2H6

C C

HHH

C C

H H H

Chapter 14 - 4

Chapter 14 - 5

Unsaturated Hydrocarbons Double & triple bonds somewhat unstable

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

H HC C

H

H

H

H

Triple bond found in acetylene or ethyne - C2H22 2

C C HH

Chapter 14 - 6

Isomerism Isomerism

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

for example: C8H18 normal-octane

C C C C C C C CH

H H H H H H H H

H H3C CH2 CH2 CH2 CH2 CH2 CH2 CH3=

2 4 dimethylhexane

H H H H H H H H

H3C CH2 CH3( )6

2,4-dimethylhexane

H3C CH

CH3CH2 CH

CH

CH3

Chapter 14 - 7

CH2CH3

Polymerization and Polymer ChemistryPolymer Chemistry

Free radical polymerizationH H H H

C C

HHmonomer

R +

free radical

R C C

H H

initiation

(ethylene)

R C C

H H

C C

H H+ R C C

H H

C C

H Hpropagation

Initiator: example benzoyl peroxide

R C C

H H

C C

HH

+ R C C

H H

C C

H H

propagation

dimer Initiator: example - benzoyl peroxide

H H H

Chapter 14 - 8

C

H

O O C

H

C

H

O2 R= 2

Chemistry and Structure of PolyethylenePolyethylene

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

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

Chapter 14 - 9

- paraffin wax for candles is short polyethylene

Bulk or Commodity Polymers

Chapter 14 -10

Bulk or Commodity Polymers (cont)

Chapter 14 -11

Bulk or Commodity Polymers (cont)

Chapter 14 -12

VMSE: Polymer Repeat Unit Structures

Chapter 14 -13

Manipulate and rotate polymer structures in 3-dimensions

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

Low M

high Mhigh M

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

Chapter 14 -14

MOLECULAR WEIGHT DISTRIBUTION

moleculesof#totalpolymer of wttotalnM


molecules of #total

M n x MM n xi MiM w wi Mi

xi = number fraction of chains in size range iMi = mean (middle) molecular weight of size range i

Chapter 14 -15

wi = weight fraction of chains in size range i

Molecular Weight Calculation

Example: average mass of a classStudent Weight

mass (lb)1 104

What is the averageweight of the students in1 104

2 1163 140

this class:a) Based on the number

fraction of students in4 1435 1806 182

fraction of students in each mass range?

b) Based on the weight fraction of students in6 182

7 1918 220

fraction of students in each mass range?

Chapter 14 -16

9 22510 380

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

Using 40 lb ranges gives the following table:

weight number of mean number weightrange students weight fraction fraction

Ni Wi xi wi

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

mass (lb) mass (lb)81-120 2 110 0.2 0.117121-160 2 142 0.2 0.150161 200 3 184 0 3 0 294

xi NiNi wi

NiWiNiWi

161-200 3 184 0.3 0.294201-240 2 223 0.2 0.237241-280 0 - 0 0.000281 320 0 0 0 000

For example: for the 81-120 lb range

x81120 210 0.2281-320 0 - 0 0.000321-360 0 - 0 0.000361-400 1 380 0.1 0.202

10

117.01881

011 x 212081 w

Chapter 14 -17

Ni NiWi10 1881

total number

total weight

Molecular Weight Calculation (cont.)weight mean number weightrange weight fraction fraction

Wi xi wii i imass (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 0 000

281-320 - 0 0.000321-360 - 0 0.000361-400 380 0.1 0.202

Mn xiMi (0.2 x 110 0.2 x 142+ 0.3 x 184+ 0.2 x 223+ 0.1 x 380) =188 lbMw wiMi (0.117 x 110 0.150 x 142+ 0.294 x 184

Chapter 14 -18

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

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

C 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 = 6H H H H H H H H H H H H

MDP n

m unitrepeatofweightmolecularaveragewherem

DP

mfm

m

:follows as calculated is this copolymers for

unitrepeatofweightmolecularaverage where

Chapter 14 -19

iimfm mol. wt of repeat unit iChain fraction

Molecular Structures for Polymers

secondarybonding


Branched Cross-Linked NetworkLinear

Chapter 14 -20

Polymers Molecular ShapeMolecular Shape (or Conformation) chain

bending and twisting are possible by rotationbending and twisting are possible by rotation of carbon atoms around their chain bonds note: not necessary to break chain bondsnote: not necessary to break chain bonds

to alter molecular shape


Chapter 14 -21

Chain End-to-End Distance, r


Chapter 14 -22

Molecular Configurations for Polymers

Configurations to change must break bonds StereoisomerismStereoisomerism

C CHH

C C

H H

C C

H RC C

RHC C

H R

or C C

H H

E

A

C C

A

E

Stereoisomers are mirrorimages cant superimpose

ith t b ki b d EB D D B

E

mirror

without breaking a bond

Chapter 14 -23

plane

TacticityTacticity stereoregularity or spatial arrangement of R

units along chain

isotactic all R groups on same side of chain

syndiotactic R groups alternate sides

C C

H H HH

CC

HH

CC

HH

CC C C

H H

C C

H H

C C

H R RH

CCH R RH RH RH H R H RH H HH

Chapter 14 -24

Tacticity (cont.)

atactic R groups randomlypositionedpositioned

H H HH RH HH

C C

H R RH

CC

HH

CC

RH

CC

Chapter 14 -25

cis/trans Isomerism

C CHCH3

CH2 CH2C C

CH3

CH2

CH2

HCH2 CH2 CH2 H

cis transcis-isoprene

(natural rubber)

H t d CH

trans-isoprene (gutta percha)

H t d CH H atom and CH3 group on same side of chain

H atom and CH3 group on opposite sides of chain

Chapter 14 -26

VMSE: Stereo and Geometrical Isomers

Chapter 14 -27Chapter 7 - 19

Manipulate and rotate polymer structures in 3-dimensions

Copolymers Adapted from Fig. 14.9, Callister & R th i h 8

two or more monomers polymerized together

random

Rethwisch 8e.

random A and B randomly positioned along chain

alternating A and Balternating A and B alternate in polymer chain

block large blocks of A units alternate with large

alternating

units alternate with large blocks of B units

graft chains of B units f d A b kb

block

grafted onto A backbone

A B

Chapter 14 -28

A B graft

Crystallinity in Polymers Adapted from Fig. 14 10 C lli t &

Ordered atomic arrangements involving

14.10, Callister & Rethwisch 8e.

g gmolecular chains

Crystal structures in terms of nit cellsof unit cells

Example shownpolyethylene unit cell polyethylene unit cell

Chapter 14 -29

Polymer Crystallinity Crystalline regions

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

10 nm


Chapter 14 -30

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

become aligned

Degree of crystallinity

crystalline region

Degree of crystallinity expressed as % crystallinity.-- Some physical properties

depend on % crystallinity.depend on % crystallinity.-- Heat treating causes

crystalline regions to grow and % crystallinity to increase.

Adapted from Fig. 14.11, Callister 6e.

amorphousregion

Chapter 14 -31

p g ,(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 Crystals Electron micrograph multilayered single crystals

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

growth rates

Chapter 14 -32Adapted from Fig. 14.11, Callister & Rethwisch 8e.

Semicrystalline Polymers Some semicrystalline

polymers form yspherulite structures

Alternating chain-folded crystallites andcrystallites and amorphous regions

Spherulite structure for

Spherulite surface

prelatively rapid growth rates

surface


Chapter 14 -33

Photomicrograph Spherulites in PolyethylenePolyethylene

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

Chapter 14 -34Adapted from Fig. 14.14, Callister & Rethwisch 8e.

ANNOUNCEMENTSReading:

Core Problems:

Self help Problems:Self-help Problems:

Chapter 14 -35

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