Polymeric Structures

8/10/2019 Polymeric Structures

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Chapter 14 - 1

ISSUES TO ADDRESS...

What are the basic microstructural features?

How are polymer properties effected bymolecular weight?

How do polymeric crystals accommodate thepolymer chain?

CHAPTER 14:

POLYMER STRUCTURES


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Chapter 14 - 2

Chapter 14 Polymers

What is a polymer?

Poly mermany repeat unit

Adapted from Fig. 14.2, Callister 7e.

C C C C C CHHHHHH

HHHHHH

Polyethylene (PE)ClCl Cl

C C C C C CHHH

HHHHHH

Polyvinyl chloride (PVC)HH

HHH H

Polypropylene (PP)

C C C C C CCH3

HH

CH3CH3H

repeatunit

repeatunit

repeatunit


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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

American Indians used natural rubber on

the soles of their feet


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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 H H

HH


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Chapter 14 - 5


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Chapter 14 - 6

Unsaturated Hydrocarbons

Double & triple bonds relatively reactive canform new bonds Double bond ethylene or ethene - CnH2n

4-bonds, but only 3 atoms bound to Cs

Triple bond acetylene or ethyne - CnH2n-2

C C

H

H

H

H

C C HH


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Chapter 14 - 7

Isomerism

Isomerism two compounds with same chemical formula can

have quite different structures

Ex: C8H18

n-octane

2,4-dimethyl hexane

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


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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

Q: how does termination take place?


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Chapter 14 - 9

Chemistry of Polymers

Adapted from Fig.14.1, Callister 7e.

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


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Chapter 14 -10

Bulk or Commodity Polymers


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Chapter 14 - 11


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Chapter 14 -12


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Chapter 14 -13

MOLECULAR WEIGHT

moleculesof#total

polymerofwttotal=nM

iiw

iin

MwM

MxM

!=

!=

Mwis more sensitive tohigher molecular weights

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

Lower M higher M



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Chapter 14 -14

Molecular Weight Calculation

Example: average mass of a classN

i M

i x

i w

i

# of students mass (lb)

1 100 0.1 0.054

1 120 0.1 0.0652 140 0.2 0.151

3 180 0.3 0.290

2 220 0.2 0.237

1 380 0.1 0.204

Mn

Mw

186 lb 216 lb

!= iiw MwM!

=iin MxM


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Chapter 14 -15

Degree of Polymerization, n

n= number of repeat units per chain

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

Poly dispersity (DP) =Mw/Mn and is typically ~2-3

C C H

H

H

H

H

H

n

n= 6


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Chapter 14 -16

End to End Distance, r

Adapted from Fig.

14.6, Callister 7e.


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Chapter 14 -17

Covalent chainconfigurations and strength:

Direction of increasing strengthAdapted from Fig. 14.7, Callister 7e.

Molecular Structures

Branched Cross-Linked NetworkLinear

secondarybonding


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Chapter 14 -18

Polymers Molecular Shape

Conformation Molecular orientation can bechanged by rotation around the bonds

note: no bond breaking needed



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Chapter 14 -19

Polymers Molecular Shape

Configurations to change must break bonds

Stereoisomerism

EB

A

D

C C

D

A

BE

mirror

plane

C C

R

HH

H

C C

H

H

H

R

or C C

H

H

H

R


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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 Rgroups onsame side of chain

syndiotactic Rgroups

alternate sides

atactic Rgroups random


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Chapter 14 -21

cis/trans Isomerism

C C

HCH3

CH2 CH2

C C

CH3

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


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Chapter 14 -

Polymer Topology

Topology or skeletalstructure of a polymer molecule:

Linear(thermopastics, eg. nylon)

Branched(thermoplastics)

Network(thermosets, e.g. epoxy)

(elastomers, e.g. rubber)

branch-point

main-chain

Soluble, Tm Soluble, Tm insoluble, no Tm

crosslink-point


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Chapter 14 -

Polymer Types and Properties?

Thermoplastics

--

Thermosets

--

Elastomers

--


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Chapter 14 -24

Copolymers

two or more monomerspolymerized together

random A and B randomlyvary in chain

alternating A and B

alternate in polymer chain block large blocks of A

alternate with large blocks ofB

graft chains of B graftedon to A backbone

A B

random

block

graft


alternating


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Chapter 14 -25

Polymer Crystallinity

Ex: polyethylene unit cell

Crystals must contain thepolymer chains in some

way

Chain foldedstructure

10 nm




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Chapter 14 -26

Polymer Crystallinity

Polymers rarely 100% crystalline

Too difficult to get all those chains

aligned

%Crystallinity: % of material

that is crystalline.-- TSand Eoften increase

with % crystallinity.

-- Annealing causes

crystalline regions

to grow. % crystallinityincreases.

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 Wileyand Sons, Inc., 1965.)

crystallineregion

amorphousregion


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Chapter 14 -27

Polymer Crystal Forms

Single crystals only if slow careful growth



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Chapter 14 -28

Polymer Crystal Forms


Spherulites fastgrowth forms lamellar

(layered) structures

Spherulitesurface

Nucleation site


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Chapter 14 -29

Spherulites crossed polarizers


Maltese cross Grain boundary!


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Chapter 14 -30

Summary


Polymers are long entangled chains, typically comprisedof C,H, O and N (covalent bonding!)

linear, branched, network, hyperbranched!.

Isomers are possible: Tacticity! Syndio-, Iso-, A-tactic

Polymers are a mixture of chain lengths: Mn, Mw, PD

Random, homopolymers, block-co-polymers, alternating!

.

Polymers can crystallize but are seldom 100% crystalline

Polymeric Structures

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