CHAPTER 7CHAPTER 7

POLYMERIC MATERIALSPOLYMERIC MATERIALS

2

Chapter 14 – PolymersWhat is a polymer?

Polymers are organic materials made of very large molecules contPolymers are organic materials made of very large molecules containing aining

hundreds of thousands of unit molecules called hundreds of thousands of unit molecules called ““mersmers”” linked in a linked in a

chainchain--like structure (repeated pattern)like structure (repeated pattern)

Poly mer

many repeat unit

Adapted from Fig. 14.2, Callister 7e.

C C C C C C

HHHHHH

HHHHHH

Polyethylene (PE)

ClCl Cl

C C C C C C

HHH

HHHHHH

Polyvinyl chloride (PVC)

HH

HHH H

Polypropylene (PP)

C C C C C C

CH3

HH

CH3CH3H

repeat

unit

repeat

unit

repeat

unit

3

• Originally natural polymers were used

� Wood – Rubber

� Cotton – Wool

� Leather – Silk

• Oldest known uses

� Rubber balls used by Incas

� Noah used pitch (a natural polymer) for the ark

Ancient Polymer History

�� Polymers are characterized by:Polymers are characterized by:

�� Low density materials (replace metals such as Low density materials (replace metals such as

steel, steel, aluminiumaluminium etc)etc)

�� Versatility in synthesis Versatility in synthesis –– processing processing –– properties properties

relationshiprelationship

�� Raw materials and processing are costRaw materials and processing are cost--effectiveeffective

�� Recycling is possible and practicalRecycling is possible and practical

Applications of PolymersApplications of Polymers

PPPP

PplycabonatePplycabonate roofroof

Bottles extrusion blow Bottles extrusion blow

moldingmolding

Classification of PolymersClassification of Polymers

POLYMERSPOLYMERS

ELASTOMER ELASTOMER

(RUBBER)(RUBBER)

NATURALNATURAL SYNTHETICSYNTHETIC

THERMOPLASTICSTHERMOPLASTICS

PE, PVC, PP, PSPE, PVC, PP, PS

THERMOSETSTHERMOSETS

Epoxy, Epoxy, phenolicphenolic

resinsresins

e.ge.g; wood, cotton, leather, ; wood, cotton, leather,

skin, hairskin, hair

Characteristics of Polymers when compared to metals and ceramicsCharacteristics of Polymers when compared to metals and ceramics

Characteristic Characteristic AdvantageAdvantage / disadvantage/ disadvantage

Low melting pointLow melting point

High elongationHigh elongation

Low densityLow density

Low thermal conductivityLow thermal conductivity

Electrical resistanceElectrical resistance

Easily coloredEasily colored

Flammable Flammable

Ease of processing/ Ease of processing/ lower useful temperature rangelower useful temperature range

Low brittleness/ Low brittleness/ high creep and lower strengthhigh creep and lower strength

Lightweight products/ Lightweight products/ low structural strengthlow structural strength

Good thermal insulation/ Good thermal insulation/ dissipates heat poorlydissipates heat poorly

Good electrical insulation/ Good electrical insulation/ do not conduct electricitydo not conduct electricity

Use without painting/ Use without painting/ difficult to match colorsdifficult to match colors

Waste can be burned/ Waste can be burned/ may cause fume or fire hazardmay cause fume or fire hazard

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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 HH

HH

Polymer

9 10

• Double & 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

C C

H

H

H

H

C C HH

Unsaturated Hydrocarbons

11

• Isomerism

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

Ex: C8H18

n-octane

2-methyl-4-ethyl pentane (isooctane)

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

⇓⇓⇓⇓

Isomerism

12

Bulk or Commodity Polymers

13 14

Chemical Composition of PolymersChemical Composition of Polymers

�� Polymers are classified into:Polymers are classified into:

1.1. HomopolymersHomopolymers

�� Only 1 type of repeat unitOnly 1 type of repeat unit

Chemical Composition of PolymersChemical Composition of Polymers

2.2. Copolymers Copolymers

�� At least 2 types of repeat unitAt least 2 types of repeat unit

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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

block

graft

Adapted from Fig.

14.9, Callister 7e.

alternating

Copolymers

Polymer ArchitecturePolymer Architecture

ThermostettingThermostetting

polymerspolymersRubbers Rubbers

Thermoplastics: PVC, Thermoplastics: PVC,

acrylic, polyethyleneacrylic, polyethylene

polyethylenepolyethylene

Linear polymersLinear polymers

PVCPVC

PolypropylenePolypropylene

Polycarbonate (PC)Polycarbonate (PC)

Nylon Nylon

Complex polymersComplex polymers

POLYMER FORMATIONPOLYMER FORMATION

�� The properties and processing of polymers depend on their structThe properties and processing of polymers depend on their structure and ure and

chemical compositionchemical composition

�� Polymers are formed by causing small units (monomers) to chemicaPolymers are formed by causing small units (monomers) to chemically bond lly bond

together and form very long molecules (polymers)together and form very long molecules (polymers)

�� The process used to cause this bonding is called The process used to cause this bonding is called ““polymerisationpolymerisation””

�� Polymerisation reactions can be:Polymerisation reactions can be:

1.1. ChainChain-- Growth Polymerisation (Growth Polymerisation (addition polymerisationaddition polymerisation))

�� No byNo by--product formationproduct formation

2.2. StepStep--Growth Polymerisation (Growth Polymerisation (condensation polymerisationcondensation polymerisation))

�� Formation of a byFormation of a by--productproduct

1.1. Chain Chain -- Growth PolymerisationGrowth Polymerisation

�� Applies to monomers that have double Applies to monomers that have double

bondsbonds

�� Proceeds by several sequential stepsProceeds by several sequential steps

1.1. Initiation stepInitiation step: active initiator (peroxide) : active initiator (peroxide)

interact with monomer double bondinteract with monomer double bond

2.2. Propagation stepPropagation step: linear growth of : linear growth of

molecule as monomer units become molecule as monomer units become

attached to one another producing chain attached to one another producing chain

molecule molecule

3.3. Termination stepTermination step: chain will eventually : chain will eventually

stop when the active end of two stop when the active end of two

propagating chains react or link together propagating chains react or link together

to form a nonto form a non--reactive moleculereactive molecule

Chain polymerisation of polyethyleneChain polymerisation of polyethylene

2. Step 2. Step –– Growth Polymerisation of 6,6 nylonGrowth Polymerisation of 6,6 nylon

Basic PropertiesBasic Properties1.1. Molecular Weight:Molecular Weight:

�� In all real polymer systems the nature of the polymerisation proIn all real polymer systems the nature of the polymerisation process results in cess results in

chains with many different lengths. chains with many different lengths.

�� That is, the polymer molecules (chains) are usually different moThat is, the polymer molecules (chains) are usually different molecular lecular

weights. weights.

�� Molecular weight is defined as the average of the weight of eachMolecular weight is defined as the average of the weight of each species or species or

size of the molecule present.size of the molecule present.

�� Molecular weight is most frequently characterised by:Molecular weight is most frequently characterised by:

�� Weight average, MWeight average, Mww

�� Based on the weight fraction of each specie or size present in tBased on the weight fraction of each specie or size present in the he

moleculemolecule

Basic PropertiesBasic Properties

�� Number average, Number average, MMnn

�� Based on the sum of the number of fractions of the weight of eacBased on the sum of the number of fractions of the weight of each h

specie or size of the molecule presentspecie or size of the molecule present

2.2. Degree of Polymerisation (DP)Degree of Polymerisation (DP)

�� Polymers with high molecular Polymers with high molecular

weight are tougher and weight are tougher and

chemically resistant: this is chemically resistant: this is

because long chains are easily because long chains are easily

entangled (anchored).entangled (anchored).

�� Polymers with low molecular Polymers with low molecular

weight are weak and more weight are weak and more

brittlebrittle

�� The melting point (plastics) also The melting point (plastics) also

increases with increasing Mincreases with increasing Mww

Effect of molecular weight and degree Effect of molecular weight and degree of polymerization on the strength and of polymerization on the strength and

viscosity of polymers.viscosity of polymers.

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Ex: polyethylene unit cell

• Crystals must contain the polymer chains in some way

� Chain folded structure

10 nm

Adapted from Fig. 14.10, Callister 7e.

Adapted from Fig.

14.12, Callister 7e.

Polymer Crystallinity

27

Polymers rarely 100% crystalline

• Too difficult to get all those chains

aligned

• % Crystallinity: % of material

that is crystalline.-- TS and E often increase

with % crystallinity.-- Annealing causes

crystalline regionsto grow. % crystallinity

increases.

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 Wiley and Sons, Inc., 1965.)

crystalline region

amorphousregion

Polymer Crystallinity

28

• Single crystals – only if slow careful growth

Adapted from Fig. 14.11, Callister 7e.

Polymer Crystal Forms

29

Spherulite

surface

Adapted from Fig. 14.13, Callister 7e.

• Spherulites – fast growth – forms lamellar (layered) structures

Polymer Crystal Forms

Nucleation site

30Adapted from Fig. 14.14, Callister 7e.

Maltese cross

Spherulites – crossed polarizersA transmission photomicrograph showing the spherulite structure of ployethylene.

Linear boundaries form between adjacent spherulites, and within each spheruliteappears a Maltese cross (Courtesy, FP Price, General Electric company).

•• Mechanical behavior of polymers Mechanical behavior of polymers

(amorphous or semi(amorphous or semi--crystalline) is crystalline) is

strongly dependent on the strongly dependent on the glass glass

transition temperaturetransition temperature ,,TTgg (state where, (state where,

liquid transform to glass)liquid transform to glass)

•• Polymers with high Polymers with high TTgg (above service (above service

temperature) are strong, stiff and brittletemperature) are strong, stiff and brittle

•• Polymers with low Polymers with low TTgg (below service (below service

temperature) are weak, less rigid and temperature) are weak, less rigid and

ductileductile

•• TTgg is due to a reduction in motion of is due to a reduction in motion of

large segments of molecular chains large segments of molecular chains

with decreasing temperaturewith decreasing temperature

•• At At TTgg the polymer changes from the polymer changes from

rubbery to rigid staterubbery to rigid state

(Amorphous)

semi-crystalline

Factors Affecting Factors Affecting TTgg

1.1. Melting temperature of polymer Melting temperature of polymer

(T(Tmm): as T): as Tmm increasesincreases TTgg

increasesincreases

2.2. Chain stiffness or flexibility: as Chain stiffness or flexibility: as

stiffness stiffness increasesincreases (or flexibility (or flexibility

decreases), decreases), TTgg increasesincreases

3.3. Molecular weight: Molecular weight: TTgg increasesincreases

with increasing molecular weightwith increasing molecular weight

4.4. Degree of branching or crossDegree of branching or cross--

linking (restrict chain movement) linking (restrict chain movement)

increasesincreases TTgg

POLYMER PROPERTIESPOLYMER PROPERTIES

�� Both Both time time and and temperaturetemperature affect the affect the

mechanical properties of polymers: they mechanical properties of polymers: they

are are viscovisco--elasticelastic materials (have both materials (have both

viscous and elastic behaviour)viscous and elastic behaviour)

�� Polymer properties also depend Polymer properties also depend

whether the material is: amorphous, whether the material is: amorphous,

semisemi--crystalline or rubbercrystalline or rubber

�� 3 different types of stress 3 different types of stress –– strain strain

behaviour:behaviour:

1.1. Curve A: brittle polymer; fracture while Curve A: brittle polymer; fracture while

deforming elasticallydeforming elastically

2.2. Curve B: plastic polymer: similar to Curve B: plastic polymer: similar to

metallic materials behaviour (metallic materials behaviour (e.ge.g; nylon; nylon))

3.3. Curve C: totally elastic; typical for Curve C: totally elastic; typical for

elastomerelastomer (rubber) ((rubber) (e.ge.g; polyethylene; polyethylene))

35

• i.e. stress-strain behavior of polymers

brittle polymer

plastic

elastomer

σFS of polymer ca. 10% that of metals

Strains – deformations > 1000% possible

(for metals, maximum strain ca. 10% or less)

elastic modulus

– less than metal

Adapted from Fig. 15.1, Callister 7e.

Mechanical Properties

36

brittle failure

plastic failure

σ (MPa)

ε

x

x

crystallineregions

slide

fibrillar

structure

near failure

crystallineregions align

onset of necking

Initial

Near Failure

semi-crystalline

case

aligned,cross-linkedcase

networkedcase

amorphousregions

elongate

unload/reload

Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from Figs. 15.12 & 15.13, Callister 7e. (Figs. 15.12 & 15.13 are from J.M. Schultz, Polymer Materials Science, Prentice-

Hall, Inc., 1974, pp. 500-501.)

Tensile Response: Brittle & Plastic

37

• Compare to responses of other polymers:

-- brittle response (aligned, crosslinked & networked polymer)

-- plastic response (semi-crystalline polymers)

Stress-strain curves

adapted from Fig. 15.1, Callister 7e. Inset

figures along elastomercurve (green) adapted

from Fig. 15.15, Callister7e. (Fig. 15.15 is from Z.D. Jastrzebski, The

Nature and Properties of Engineering Materials, 3rd ed., John Wiley and

Sons, 1987.)

σ(MPa)

ε

initial: amorphous chains are kinked, cross-linked.

x

final: chainsare straight,

stillcross-linked

elastomer

Deformation is reversible!

brittle failure

plastic failurex

x

Tensile Response: Elastomer CaseEffect of Temperature on StressEffect of Temperature on Stress--Strain BehaviourStrain Behaviour

ViscoelasticViscoelastic Deformation of PolymersDeformation of Polymers

�� An amorphous polymer may behave like a:An amorphous polymer may behave like a:

1.1. Glass at low temperaturesGlass at low temperatures

�� Elastic deformation (conform to Elastic deformation (conform to HookeHooke’’ss law)law)

2.2. Rubbery solid at intermediate temperatures (above Rubbery solid at intermediate temperatures (above TTgg))

�� Exhibit the combined characteristics of the low and high Exhibit the combined characteristics of the low and high

temperaturestemperatures

�� This condition is calledThis condition is called ““ViscoelasticityViscoelasticity””

3.3. Viscous liquid at higher temperaturesViscous liquid at higher temperatures

�� Viscous or liquidViscous or liquid--like behaviourlike behaviour

Viscoelastic – stress relaxation