CHAPTER 14: POLYMER STRUCTURES 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 - 1

Chapter 14 – Polymers

What is a polymer? Poly many mer repeat unit H H repeat unit H H H H C C C C C C H H H H H H Polyethylene (PE) H H repeat unit H H H H C C C C C C H Cl H Cl H Cl Polyvinyl chloride (PVC) H C H H C H C repeat unit H C H C H C CH 3 H CH 3 H Polypropylene (PP) CH 3 Adapted from Fig. 14.2,

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Chapter 14 - 2

Ancient Polymer History

• Originally natural polymers were used – Wood – Cotton – Leather – Rubber – Wool – Silk • Oldest known uses – Rubber balls used by Incas – Noah used pitch (a natural polymer) for the ark Chapter 14 - 3

Polymer Composition

Most polymers are hydrocarbons – i.e. made up of H and C • Saturated hydrocarbons – Each carbon bonded to four other atoms H H C H H C H H C n H 2n+2 Chapter 14 - 4

Chapter 14 - 5

Unsaturated Hydrocarbons

• Double & triple bonds relatively reactive – can form new bonds – Double bond – ethylene or ethene - C n H 2n H C H C H H • 4-bonds, but only 3 atoms bound to C’s – Triple bond – acetylene or ethyne - C n H 2n-2 H C C H Chapter 14 - 6

Isomerism

• Isomerism – two compounds with same chemical formula can have quite different structures Ex: C 8 H 18 • n-octane H H C H H C H H C H H C H H C H H C H H C H H C H H = H 3 C CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3  H 3 C ( • 2-methyl-4-ethyl pentane (isooctane) CH 2 ) 6 CH 3 H 3 C CH 3 CH CH 2 CH CH 2 CH 3 CH 3 Chapter 14 - 7

Chemistry of Polymers

• Free radical polymerization R + free radical H C H C H H monomer (ethylene) H R C H H C H H R C H H C H + H C H H C H H R C H H C H H C H dimer • Initiator : example - benzoyl peroxide H C H initiation propagation H C H H O O C H 2 H C H O = 2 R Chapter 14 - 8

Chemistry of Polymers

Adapted from Fig. 14.1,

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Note: polyethylene is just a long HC - paraffin is short polyethylene Chapter 14 - 9

Bulk or Commodity Polymers

Chapter 14 - 10

Chapter 14 - 11

Chapter 14 - 12

MOLECULAR WEIGHT

• Molecular weight ,

M i

: Mass of a mole of chains.

Lower

M M n

 total wt of polymer total # of molecules higher

M M n

 

x i M i M w

 

w i M i M w

is more sensitive to higher molecular weights Adapted from Fig. 14.4,

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Chapter 14 - 13

Molecular Weight Calculation

Example: average mass of a class

N i

# of students 1 1 2 3 2 1

M i

mass (lb) 100 120 140 180 220 380

x i

0.1

0.1

0.2

0.3

0.2

0.1

w i

0.054

0.065

0.151

0.290

0.237

0.204

M n M w

 

x i M i

 

w i M i M n

186 lb

M w

216 lb Chapter 14 - 14

Degree of Polymerization, n

n

= number of repeat units per chain H H C H H H C ( C H H H H C ) C H H H C H H C H H C H H C H H C H H C H H C H H

n i

= 6

n n

 

x i n i



M n m n w

 

w i n i



M w m

where

m

 average molecular weight of repeat unit

m

 

f i m i

Chain fraction mol. wt of repeat unit i Chapter 14 - 15

End to End Distance, r

Adapted from Fig. 14.6,

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Chapter 14 - 16

Molecular Structures

• Covalent chain configurations and strength: secondary bonding Linear Branched Cross-Linked Network Direction of increasing strength Adapted from Fig. 14.7,

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Chapter 14 - 17

Polymers – Molecular Shape

Conformation – Molecular orientation can be changed by rotation around the bonds – note: no bond breaking needed Adapted from Fig. 14.5,

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Chapter 14 - 18

Polymers – Molecular Shape

Configurations – to change must break bonds • Stereoisomerism H C H H C R H C H H C R or H C H R C H A A B C D E E mirror plane D C B Chapter 14 - 19

Tacticity

Tacticity – stereoregularity of chain isotactic – all R groups on same side of chain H C H H C R H C H H C R H C H H C R H C H H C R syndiotactic – R groups alternate sides atactic – R groups random H C H H C R H C H R C H H C H H C R H C H R C H H C H H C R H C H H C R H C H R C H H C H H C R Chapter 14 - 20

cis/trans Isomerism

CH 3 C CH 2 H C CH 2 cis cis-isoprene (natural rubber) bulky groups on same side of chain CH 3 C CH 2 C CH 2 H trans trans-isoprene (gutta percha) bulky groups on opposite sides of chain Chapter 14 - 21

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 Adapted from Fig. 14.9,

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alternating block graft Chapter 14 - 22

Polymer Crystallinity

Ex: polyethylene unit cell Adapted from Fig. 14.10,

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• Crystals must contain the polymer chains in some way – Chain folded structure Adapted from Fig. 14.12,

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10 nm Chapter 14 - 23

Polymer Crystallinity

Polymers rarely 100% crystalline • Too difficult to get all those chains aligned crystalline region • % Crystallinity : % of material that is crystalline.

--

TS

and

E

often increase with % crystallinity.

-- Annealing causes crystalline regions to grow. % crystallinity increases.

amorphous region Adapted from Fig. 14.11,

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(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.) Chapter 14 - 24

Polymer Crystal Forms

• Single crystals – only if slow careful growth Adapted from Fig. 14.11,

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Chapter 14 - 25

Polymer Crystal Forms

• Spherulites – fast growth – forms lamellar (layered) structures Spherulite surface Nucleation site Adapted from Fig. 14.13,

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Chapter 14 - 26

Spherulites – crossed polarizers

Maltese cross Adapted from Fig. 14.14,

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Chapter 14 - 27

Reading:

ANNOUNCEMENTS

Core Problems: Self-help Problems: Chapter 14 - 28