Polymer structures

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POLYMER STRUCTURES

Desiree Jane EnriquezSarah Reyes

Bernald CabreraNephtalie CruzArwie Mendoza

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POLYMER

Polymer Polymer HistoryHistoryOriginally natural

polymers were used inWood RubberCotton WoolLeather Silk

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Synthetic PolymersPlastics Fiber materialsRubbers

Polymer CompositionPolymer CompositionMost polymers are hydrocarbons – i.e. made up of H and CSaturated hydrocarbons

◦Each carbon bonded to four other atoms

CnH2n+2

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C C

H

H HH

HH

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Unsaturated Unsaturated HydrocarbonsHydrocarbonsDouble & triple bonds relatively

reactive – can form new bonds Double bond – ethylene or ethene - CnH2n

4-bonds, but only 3 atoms bound to C’s Triple bond – acetylene or ethyne - CnH2n-2

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C C

H

H

H

H

C C HH

IsomerismIsomerism Isomerism

◦ two compounds with same chemical formula can have quite different structures

Ex: C8H18

n-octane

2-methyl-4-ethyl pentane (isooctane)

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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

Chemistry of PolymersChemistry of PolymersFree radical polymerization

Initiator: example - benzoyl peroxide

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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

Chemistry of PolymersChemistry of Polymers

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

Bulk or Commodity Bulk or Commodity PolymersPolymers

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MOLECULAR WEIGHTMOLECULAR WEIGHT

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low M

high M

Not all chains in a polymer are of the same length

◦ The properties of a polymer depend on its length

◦ synthesis yields polymer distribution of lengths

◦ Define “average” molecular weight

◦ Two approaches are typically taken Number average molecular weight (Mn)

Weight-average molecular weight (Mw)

Molecular weightMolecular weight

Molecular mass exampleMolecular mass example

Degree of Polymerization, Degree of Polymerization, nnn = number of repeat units per

chain

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ii

w

iiw

n

iin

mfm

m

m

Mnwn

m

Mnxn

unitrepeat ofweight molecular 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( ) ni = 6

mol. wt of repeat unit iChain fraction

End to End Distance, End to End Distance, rr

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Molecular StructuresMolecular Structures

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• Covalent chain configurations and strength:

Direction of increasing strength

Branched Cross-Linked NetworkLinear

secondarybonding

Polymers – Molecular Polymers – Molecular ShapeShapeConformation – Molecular

orientation can be changed by rotation around the bonds◦note: no bond breaking needed

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Polymers – Molecular Polymers – Molecular ShapeShapeConfigurations – to change must break bondsStereoisomerism

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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

TacticityTacticity

Tacticity – stereoregularity of 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

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 R groups on same side of chain

syndiotactic – R groups alternate sides

atactic – R groups random

cis/trans Isomerismcis/trans Isomerism

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C CHCH3

CH2 CH2

C CCH3

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

CopolymersCopolymerstwo 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 –

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random

block

graft

alternating

Polymer CrystallinityPolymer Crystallinity

Ex: polyethylene unit cell

Crystals must contain the polymer chains in some way ◦ Chain folded structure

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10 nm

Polymer CrystallinityPolymer CrystallinityPolymers rarely 100% crystalline Too difficult to get all those chains

aligned

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• % Crystallinity: % of material that is crystalline. -- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases.

Polymer Crystal FormsPolymer Crystal FormsSingle crystals – only if slow careful growth

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Polymer Crystal FormsPolymer Crystal FormsSpherulites – fast

growth – forms lamellar (layered) structures

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Spherulites – crossed Spherulites – crossed polarizerspolarizers

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Maltese cross