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Chemical Engineering 378

Science of Materials Engineering

Lecture 17Polymers and Applications I

Spiritual Thought

Isaiah 1:1818. Come now, and let us reason together, saith the LORD: though your sins be as scarlet, they shall be as white as snow; though they be red like crimson, they shall be as wool.

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Materials Roadmap3

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What is a Polymer?

Poly mermany repeat unit

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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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

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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

Organic Chemists8

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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 CHH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

HH H3C CH2 CH2 CH2 CH2 CH2 CH2 CH3=

H3C CHCH3

CH2 CHCH2

CH3

CH3

H3C CH2 CH3( )6

⇓

10Polymerization and Polymer Chemistry

• Free radical polymerization

• Initiator: example - benzoyl peroxide

CH

HO O C

H

H

CH

HO2

C CH H

HHmonomer(ethylene)

R +

free radical

R C CH

H

H

H

initiation

R C CH

H

H

HC CH H

HH

+ R C CH

H

H

HC CH H

H H

propagation

dimer

R= 2

propagation

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Chemistry and Structure of Polyethylene

Adapted from Fig. 14.1, Callister & Rethwisch 10e.

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

Polymers – Molecular Shape12

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Bulk or Commodity Polymers

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Bulk or Commodity Polymers (cont)

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Bulk or Commodity Polymers (cont)

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To very complicated

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And Super Complicated!

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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

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Chain End-to-End Distance, r

Fig. 14.6, Callister & Rethwisch 10e.

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xi = number fraction of chains in size range i

MOLECULAR WEIGHT DISTRIBUTIONFig. 14.4, Callister & Rethwisch 10e.

wi = weight fraction of chains in size range i

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

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Molecular Weight Calculation

Example: average mass of a classStudent Weight

mass (lb)1 1042 1163 1404 1435 1806 1827 1918 2209 22510 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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Molecular Weight Calculation (cont.)

Solution: The first step is to sort the students into weight ranges. Using 40 lb ranges gives the following table:

total number

total weight

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

For example: for the 81-120 lb range

Sheet1

Weight

mass (lb)

104

116

140

143

180

182

191

220

225

380

Sheet2

weightnumber ofmeannumberweight

rangestudentsweightfractionfraction

NiWixiwiStudentWeight

mass (lb)mass (lb)mass (lb)

81-12021100.20.1171104

121-16021420.20.1502116

161-20031840.30.2943140

201-24022230.20.2374143

241-2800-00.0005180

281-3200-00.0006182

321-3600-00.0007191

361-40013800.10.2028220

9225

SNiSNiWiMnMw10380

101881188 lb218 lb

188.1217.7529682793

Sheet2

1

Sheet3

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Molecular Weight Calculation (cont.)

Sheet1

Weight

mass (lb)

104

116

140

143

180

182

191

220

225

380

Sheet2

weightmeannumberweight

rangeweightfractionfraction

WixiwiStudentWeight

mass (lb)mass (lb)mass (lb)

81-1201100.20.1171104

121-1601420.20.1502116

161-2001840.30.2943140

201-2402230.20.2374143

241-280-00.0005180

281-320-00.0006182

321-360-00.0007191

361-4003800.10.2028220

9225

10380

188.1217.7529682793

MnMw

188 lb218 lb

Sheet3

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

C C C C C C C CHH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

HC C C CH

H

H

H

H

H

H

HH( ) DP = 6

mol. wt of repeat unit iChain fraction

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Adapted from Fig. 14.7, Callister & Rethwisch 10e.

Molecular Structures for Polymers

Branched Cross-Linked NetworkLinear

secondarybonding

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Molecular Configurations for Polymers

Configurations – to change must break bonds• Stereoisomerism

EB

A

D

C C

D

A

BE

mirror plane

C CR

HH

HC CH

H

H

R

or C CH

H

H

R

Stereoisomers are mirrorimages – can’t superimposewithout breaking a bond

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Tacticity

Tacticity – stereoregularity or spatial arrangement of R units along chain

C CH

H

H

R R

H

H

HCC

R

H

H

HCC

R

H

H

HCC

isotactic – all R groups on same side of chain

C CH

H

H

RC CH

H

H

RC CH

H

H

R R

H

H

HCC

syndiotactic – R groups alternate sides

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Tacticity (cont.)

atactic – R groups randomlypositioned

C CH

H

H

R R

H

H

HCC

R

H

H

HCC

R

H

H

HCC

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cis/trans Isomerism

C CHCH3

CH2 CH2C C

CH3

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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Copolymers

two 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

Fig. 14.9, Callister & Rethwisch 10e.

alternating

Thermoplastic or Thermosetting32

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Crystallinity in Polymers

• Ordered atomic arrangements involving molecular chains

• Crystal structures in terms of unit cells

• Example shown– polyethylene unit cell

Fig. 14.10, Callister& Rethwisch 10e.

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Polymer Crystallinity

• Crystalline regions – thin platelets with chain folds at faces– Chain folded structure

Fig. 14.12, Callister& Rethwisch10e.

≈ 10 nm

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Polymer Crystallinity (cont.)

Polymers rarely 100% crystalline• Difficult for all regions of all chains to

become aligned

• Degree of crystallinityexpressed as % crystallinity.-- Some physical properties

depend on % crystallinity.-- Heat treating causes

crystalline regions to grow and % crystallinity to increase.

Fig. 14.11, Callister 6e. (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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Polymer Single Crystals

• Electron micrograph – multilayered single crystals (chain-folded layers) of polyethylene

• Single crystals – only for slow and carefully controlled growth rates

Fig. 14.11, Callister & Rethwisch 10e. [From A. Keller, R. H. Doremus, B. W. Roberts, and D. Turnbull (Eds.), Growth and Perfection of Crystals. General Electric Company and John Wiley & Sons, Inc., 1958, p. 498. Reprinted with permission of John Wiley & Sons, Inc.]

1 μm

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Semicrystalline Polymers

Spherulitesurface

Fig. 14.13, Callister & Rethwisch 10e.

• Some semicrystallinepolymers form spherulite structures

• Alternating chain-folded crystallites and amorphous regions

• Spherulite structure for relatively rapid growth rates

38Photomicrograph – Spherulites in Polyethylene

Fig. 14.14, Callister & Rethwisch 10e.

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

Cou

rtesy

F. P

. Pric

e, G

ener

al E

lect

ric C

ompa

ny

100 μm

Chemical Engineering 378��Science of Materials Engineering��Spiritual ThoughtMaterials RoadmapWhat is a Polymer?Ancient PolymersSlide Number 6Unsaturated HydrocarbonsOrganic ChemistsIsomerismPolymerization and �Polymer ChemistryChemistry and Structure of PolyethylenePolymers – Molecular ShapeBulk or Commodity PolymersBulk or Commodity Polymers (cont)Slide Number 15Slide Number 16Slide Number 17Slide Number 18MOLECULAR WEIGHTChain End-to-End Distance, rMOLECULAR WEIGHT DISTRIBUTIONMolecular Weight CalculationMolecular Weight Calculation (cont.)Molecular Weight Calculation (cont.)Degree of Polymerization, DPMolecular Structures for PolymersMolecular Configurations for PolymersTacticityTacticity (cont.)cis/trans Isomerism CopolymersThermoplastic or ThermosettingCrystallinity in PolymersPolymer CrystallinityPolymer Crystallinity (cont.)Polymer Single CrystalsSemicrystalline PolymersPhotomicrograph – Spherulites in Polyethylene