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PolymersMade up of chain molecules

long-range connectionsthroughout material

In practice, mostly carbon atoms along length of the chain +various side-groups (e.g. -H, -Cl,-OH, -CH3) attached to it atregular intervals

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Polymer materials

High Density Polyethylene forstorm water drainage system.

Replaces fired clay ceramics

(brittle, difficult to make longsections, difficult to join) and castiron (heavy, low corrosion

resistance)Also used in domestic plumbing,displacing copper (expensive)

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Polymer materials

Kevlar for body armour.

Replaces iron (heavy, stiff)

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Polymer materials

Polyvinyl chloride for cable insulation:flexible, easily formed, thermally andelectrically insulating.

Nylon for fabrics, replacingsilk (expensive)

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Natural polymer materials

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Basic unit = mer

e.g. polyethylene polypropylene

polyvinyl chloride

polystyrene

Mixtures

copolymers

Describing polymers

C C( )nC C( )

n

CH3

C C( )n

Cl

C C( )n

C6H5

C C( )n

=

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Describing polymers (cont.)

Chain length (typically 103 - 105 mers)

Extent of branching

Extent of cross-linking thermoplastic

thermosetting

Tacticity: spatial arrangement of side-groupsisotactic, syndiotactic, atactic

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Describing polymers (cont.)

amorphous

(random arrangement)Arrangement of chains

crystalline

(chains aligned)

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Polymer crystallinity

Callister (6th ed.) p. 470

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In practice, polymers are semi-crystalline

Callister (6th ed.) p. 471

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Factors influencing crystallinity

Cooling rate

Chain complexity and regularity Side group size

Tacticity

Cross-linking Branching

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HDPE and LDPE

High density polyethylene: linear chains, branching

minimised

efficient alignment of chains, 90%crystallinity.

E = 1 GPa

Low density polyethylene: branched chainsinefficient alignment of chains, 50-60% crystallinity.

E = 0.2 GPa

Uses e.g. cling film

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Polymer categories

Fibres (e.g. Kevlar, silk): chains aligned parallel to

fibre axis

Elastomers (e.g. rubber): loosely cross-linkednetwork

Plastics

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Mechanical properties

4 possible responses to loading:1. stretching C-C bonds

2. stretching secondary bonds (weak forces between differentchains)

3. chains unravel, straighten out4. chains slide past each other }

only happen above glasstransition temperature (Tg)

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temperature

specificvolume

TmTg

liquid

supercooled

liquid

glassy solid

crystallisation

crystalline solid

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temperature

density

TmTg

liquid

supercooledliquid

glassy solidcrystallisation

crystalline solid

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Factors influencing Tg

Cooling rate

Chain flexibility Side group size

Cross-linking

Branching

Chain length

Typical glass transition temperatures:

Polyethylene: -90CPolypropylene: -18C

Polystyrene: +100C

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Polymer behaviour above and below Tg

Below Tg:

Polymer chains frozen into place, only bond stretchingpossible.

Polymer is glassy: stiff, strong and brittle

Above Tg:Polymer chains free to move around and change shape

Elastomers: chain motion restricted by cross-linking

Semi-crystalline polymers: amorphous sections pinned by

crystalline regions

Amorphous polymers: slight restriction on chain motionprovided by entanglements, but resistance to deformationdrops rapidly with temperature, until material flows

Amorphous polymers generally used below Tg

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Callister (6th ed.) p. 488

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Deformation of semi-crystalline polymers

Above Tg, behaviour is viscoelastic (leathery):

Elastic component from bond stretching (mainly incrystalline region) Hookean behaviour

Viscous component from chain segments sliding pasteach other in amorphous region

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Viscous component is time-dependent!

Load

stress

time time

strain

Two effects: (i) Creep

Response

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Viscous component is time-dependent!

Load

strain

time time

stress

Two effects: (ii) Stress relaxation

Response

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Viscous component is time-dependent!

Compliance / Elastic modulus are time-

dependent

= D(t)

Under creep conditions ( = constant):

D(t) = (t)/ 0

definition of creep compliance

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How do viscoelastic materials deform?

Viscoelastic materials can be considered as compositematerials:

Viscous component (amorphous areas) + elastic component(crystalline areas)

model using a Maxwell element:

spring dashpot

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Deformation of elastomers

Typical modulus: 10 MPa

Extent of reversible deformation: ~400%stress

strain

50 MPa

5

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r

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Callister (6th ed.) p. 497