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POLYMERSTRUCTURES

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Polymers

Naturally occurring Wood, rubber, cotton, wool, leather

Proteins, enzymes, starches, cellulose

Synthetic (from organic molecules) Plastics, synthetic rubbers, Fibers

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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 ?

Polymer Structures

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Hydrocarbon molecules

Most polymers are organicin origin

Many organic materials are

hydrocarbons Covalent bonds

Each C has 4 electrons thatmay participate in covalent

bonding

Single electron pair sharing single bond

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Polymer Composition

Saturated hydrocarbons Each carbon singly bonded to four other atoms

Example:

Ethane, C2H6

C C

H

H HH

HH

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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 C

H

H

H

H

C C HH

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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 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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Polymer Molecules

Much larger in comparison to the hydrocarbonmolecules Macromolecules

A chain of carbon atoms is the backbone

Mer

The repeating unit in a polymer chain Monomer a stable unit from which the

polymer is synthesized

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Polymerization andPolymer Chemistry

Free radical polymerization

Initiator: example - benzoyl peroxide

C

H

H

O O C

H

H

C

H

H

O2 R= 2

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

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Chemistry of Polymer molecules

C C

H H

HH

Monomer

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Types of Polymers

HomopolymerAll repeating units along a chain are of the

same type; polymer formed from a singlemonomer

Eg. polyethylene, formed by polymerization ofethylene

Copolymer

2 or more different mer units involved

PMMA

http://en.wikipedia.org/wiki/Image:Ethylene-2D.png

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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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VMSE: Polymer Repeat Unit Structures

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Manipulate and rotate polymer structures in 3-dimensions

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MOLECULAR WEIGHT

Low M

high M

Not all chains in a polymer are of the same length

i.e., there is a distribution of molecular weights

Large molecular weights for polymers with very longchains

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

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Molecular weights

Number average molecular weight Mn M

n= x

iM

i

Mi represents mean (middle) molecular weight of size

range i

xi is the fraction of the total number of chains within

the corresponding size range

Weight average molecular weight Mw

Mw = wi Mi

wi is the weight fraction of molecules within the same

size interval

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Example

Find the molecular weight

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Mol. Wt. range

(g/mol)

Mean Mi

(g/mol) wi wiMi xi xiMi5000 10,000 7500 0.12 900 0.2 1500

10,000

15,000 12500 0.18 2250 0.1 1250

15,000

20,000 17500 0.26 4550 0.3 525020,000

25,000 22500 0.21 4725 0.1 2250

25,000

30,000 27500 0.14 3850 0.1 2750

30,000

35,000 32500 0.09 2925 0.2 6500

= 1.0

Mw

=wiMi = 1.0

Mw

=xiMi

=

19,200 19500

Example solution

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Degree of Polymerization, DP

DP= average number of repeat units per chain

iimfm

m

S=

=

:followsascalculatedisthiscopolymersfor

unitrepeatofweightmolecularaveragewhere

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 fraction

m

MDP

n

=