chapter 4: polymer structures - cauisdl.cau.ac.kr/education.data/mat.sci/ch04.pdf · 2012-03-22 1...

2012-03-22

1

Chapter 4 -

Chapter 4: Polymer Structures

School of Mechanical EngineeringChoi, Hae-Jin

Materials Science - Prof. Choi, Hae-Jin 1

Chapter 4 - 2

CHAPTER 4: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

2012-03-22

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Chapter 4 -

What is a Polymer?

Poly mermany repeat unit

3

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

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


Chapter 4 -

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

4Materials Science - Prof. Choi, Hae-Jin

2012-03-22

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Chapter 4 -

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

5

C C

H

H H H

HH


Chapter 4 - 6Materials Science - Prof. Choi, Hae-Jin

2012-03-22

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Chapter 4 -

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 4 -

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

ß


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Chapter 4 -

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 4 -

Chemistry and Structure of Polyethylene

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


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Chapter 4 -

Bulk or Commodity Polymers


Chapter 4 -

Bulk or Commodity Polymers (cont)


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Chapter 4 - 13

Bulk or Commodity Polymers (cont)


Chapter 4 -

MOLECULAR WEIGHT

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• 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


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Chapter 4 -

MOLECULAR WEIGHT DISTRIBUTION

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xi = number fraction of chains in size range i

molecules of #totalpolymer of wttotal

=nM

iiw

iin

MwM

MxM

S=

S=


wi = weight fraction of chains in size range i

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


Chapter 4 -

Molecular Weight CalculationExample: average mass of a class

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Student Weightmass (lb)

1 1042 1163 1404 1435 1806 1827 1918 2209 225

10 380

What is the averageweight of the students inthis 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?


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Chapter 4 -

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

Using 40 lb ranges gives the following table:

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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

SNi SNiWi

10 1881total

numbertotal

weight

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

�

xi =Ni

Niå

�

wi =NiWi

NiWiå

For example: for the 81-120 lb range

�

x81-120 =2

10= 0.2

117.01881

011 x 212081 ==-w


Chapter 4 -

Molecular Weight Calculation (cont.)

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�

Mn = xiMiå = (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 = wiMiå = (0.117 x 110 + 0.150 x 142 + 0.294 x 184

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

�

Mw = wiMiå = 218 lb


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Chapter 4 -

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

mMDP n=

19

iimfm

m

S=

=

: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 fractionMaterials Science - Prof. Choi, Hae-Jin

Chapter 4 -

Molecular Structures for Polymers

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Branched Cross-Linked NetworkLinear

secondarybonding


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Chapter 4 -

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

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Chapter 4 -

Chain End-to-End Distance, r

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Chapter 4 -

Molecular Configurations for Polymers

Configurations – to change must break bonds• Stereoisomerism

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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

Stereoisomers are mirrorimages – can’t superimposewithout breaking a bond


Chapter 4 -

TacticityTacticity – stereoregularity or spatial arrangement of

R units along chain

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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


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Chapter 4 -

Tacticity (cont.)

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atactic – R groups randomlypositioned

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC


Chapter 4 -

cis/trans Isomerism

26

C CHCH3

CH2 CH2

C CCH3

CH2

CH2

H

ciscis-isoprene

(natural rubber)

H atom and CH3 group on same side of chain

transtrans-isoprene (gutta percha)

H atom and CH3 group on opposite sides of chain


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Chapter 4 -

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 –

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random

block

graft


alternating


Chapter 4 -

Crystallinity in Polymers• Ordered atomic

arrangements involving molecular chains

• Crystal structures in terms of unit cells

• Example shown– polyethylene unit cell

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Chapter 4 -

Polymer Crystallinity• Crystalline regions

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

29

10 nm



Chapter 4 -

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

amorphousregion


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Chapter 4 -

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 4 -

Semicrystalline Polymers• Some semicrystalline

polymers form spherulite structures

• Alternating chain-folder crystallites and amorphous regions

• Spherulite structure for relatively rapid growth rates

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Spherulite surface



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Chapter 4 -

Photomicrograph – Spherulites in Polyethylene

33


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


chapter 4: polymer structures - cauisdl.cau.ac.kr/education.data/mat.sci/ch04.pdf · 2012-03-22 1...

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