chapter 14 – polymers · 2/7/2011 · • originally natural polymers were used wood – rubber...
TRANSCRIPT
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
8
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
17
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.
26
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