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Chapter 14 - 1
ISSUES TO ADDRESS...
What are the basic microstructural features?
How are polymer properties effected bymolecular weight?
How do polymeric crystals accommodate thepolymer chain?
CHAPTER 14:
POLYMER STRUCTURES
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Chapter 14 - 2
Chapter 14 Polymers
What is a polymer?
Poly mermany repeat unit
Adapted from Fig. 14.2, Callister 7e.
C C C C C CHHHHHH
HHHHHH
Polyethylene (PE)ClCl Cl
C C C C C CHHH
HHHHHH
Polyvinyl chloride (PVC)HH
HHH H
Polypropylene (PP)
C C C C C CCH3
HH
CH3CH3H
repeatunit
repeatunit
repeatunit
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Chapter 14 - 3
Ancient Polymer History
Originally natural polymers were used Wood Rubber
Cotton Wool
Leather Silk
Oldest known uses
Rubber balls used by Incas
American Indians used natural rubber on
the soles of their feet
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Chapter 14 - 4
Polymer Composition
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 H H
HH
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Chapter 14 - 5
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Chapter 14 - 6
Unsaturated Hydrocarbons
Double & triple bonds relatively reactive canform new bonds Double bond ethylene or ethene - CnH2n
4-bonds, but only 3 atoms bound to Cs
Triple bond acetylene or ethyne - CnH2n-2
C C
H
H
H
H
C C HH
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Chapter 14 - 7
Isomerism
Isomerism two compounds with same chemical formula can
have quite different structures
Ex: C8H18
n-octane
2,4-dimethyl hexane
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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Chapter 14 - 8
Chemistry of Polymers
Free radical polymerization
Initiator: example - benzoyl peroxide
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
Q: how does termination take place?
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Chapter 14 - 9
Chemistry of Polymers
Adapted from Fig.14.1, Callister 7e.
Note: polyethylene is just a long HC- paraffin is short polyethylene
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Chapter 14 -10
Bulk or Commodity Polymers
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Chapter 14 - 11
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Chapter 14 -12
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Chapter 14 -13
MOLECULAR WEIGHT
moleculesof#total
polymerofwttotal=nM
iiw
iin
MwM
MxM
!=
!=
Mwis more sensitive tohigher molecular weights
Molecular weight, Mi: Mass of a mole of chains.
Lower M higher M
Adapted from Fig. 14.4, Callister 7e.
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Chapter 14 -14
Molecular Weight Calculation
Example: average mass of a classN
i M
i x
i w
i
# of students mass (lb)
1 100 0.1 0.054
1 120 0.1 0.0652 140 0.2 0.151
3 180 0.3 0.290
2 220 0.2 0.237
1 380 0.1 0.204
Mn
Mw
186 lb 216 lb
!= iiw MwM!
=iin MxM
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Chapter 14 -15
Degree of Polymerization, n
n= number of repeat units per chain
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
Poly dispersity (DP) =Mw/Mn and is typically ~2-3
C C H
H
H
H
H
H
n
n= 6
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Chapter 14 -16
End to End Distance, r
Adapted from Fig.
14.6, Callister 7e.
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Chapter 14 -17
Covalent chainconfigurations and strength:
Direction of increasing strengthAdapted from Fig. 14.7, Callister 7e.
Molecular Structures
Branched Cross-Linked NetworkLinear
secondarybonding
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Chapter 14 -18
Polymers Molecular Shape
Conformation Molecular orientation can bechanged by rotation around the bonds
note: no bond breaking needed
Adapted from Fig.14.5, Callister 7e.
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Chapter 14 -19
Polymers Molecular Shape
Configurations to change must break bonds
Stereoisomerism
EB
A
D
C C
D
A
BE
mirror
plane
C C
R
HH
H
C C
H
H
H
R
or C C
H
H
H
R
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Chapter 14 -20
Tacticity
Tacticity stereoregularity of chain
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 Rgroups onsame side of chain
syndiotactic Rgroups
alternate sides
atactic Rgroups random
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Chapter 14 -21
cis/trans Isomerism
C C
HCH3
CH2 CH2
C C
CH3
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
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Chapter 14 -
Polymer Topology
Topology or skeletalstructure of a polymer molecule:
Linear(thermopastics, eg. nylon)
Branched(thermoplastics)
Network(thermosets, e.g. epoxy)
(elastomers, e.g. rubber)
branch-point
main-chain
Soluble, Tm Soluble, Tm insoluble, no Tm
crosslink-point
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Chapter 14 -
Polymer Types and Properties?
Thermoplastics
--
Thermosets
--
Elastomers
--
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Chapter 14 -24
Copolymers
two or more monomerspolymerized together
random A and B randomlyvary in chain
alternating A and B
alternate in polymer chain block large blocks of A
alternate with large blocks ofB
graft chains of B graftedon to A backbone
A B
random
block
graft
Adapted from Fig.14.9, Callister 7e.
alternating
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Chapter 14 -25
Polymer Crystallinity
Ex: polyethylene unit cell
Crystals must contain thepolymer chains in some
way
Chain foldedstructure
10 nm
Adapted from Fig.14.10, Callister 7e.
Adapted from Fig.14.12, Callister 7e.
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Chapter 14 -26
Polymer Crystallinity
Polymers rarely 100% crystalline
Too difficult to get all those chains
aligned
%Crystallinity: % of material
that is crystalline.-- TSand Eoften increase
with % crystallinity.
-- Annealing causes
crystalline regions
to grow. % crystallinityincreases.
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 Wileyand Sons, Inc., 1965.)
crystallineregion
amorphousregion
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Chapter 14 -27
Polymer Crystal Forms
Single crystals only if slow careful growth
Adapted from Fig. 14.11, Callister 7e.
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Chapter 14 -28
Polymer Crystal Forms
Adapted from Fig. 14.13, Callister 7e.
Spherulites fastgrowth forms lamellar
(layered) structures
Spherulitesurface
Nucleation site
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Chapter 14 -29
Spherulites crossed polarizers
Adapted from Fig. 14.14, Callister 7e.
Maltese cross Grain boundary!
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Chapter 14 -30
Summary
Adapted from Fig. 14.14, Callister 7e.
Polymers are long entangled chains, typically comprisedof C,H, O and N (covalent bonding!)
linear, branched, network, hyperbranched!.
Isomers are possible: Tacticity! Syndio-, Iso-, A-tactic
Polymers are a mixture of chain lengths: Mn, Mw, PD
Random, homopolymers, block-co-polymers, alternating!
.
Polymers can crystallize but are seldom 100% crystalline