CHAPTER 14:
POLYMER STRUCTURES
ISSUES TO ADDRESS...
• What are the basic microstructural features?
• How are polymer properties effected by
molecular weight?
• How do polymeric crystals accommodate the
polymer chain?
Chapter 14 - 1
Chapter 14 – Polymers
What is a polymer?
Poly
many
mer
repeat unit
repeat
unit
repeat
unit
repeat
unit
H H H H H H
C C C C C C
H H H H H H
H H H H H H
C C C C C C
H Cl H Cl H Cl
Polyethylene (PE)
Polyvinyl chloride (PVC)
H
C
H
H H
C C
CH3 H
H H
C C
CH3 H
H
C
CH3
Polypropylene (PP)
Adapted from Fig. 14.2, Callister 7e.
Chapter 14 - 2
Ancient Polymer History
• 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
Chapter 14 - 3
Polymer Composition
Most polymers are hydrocarbons
– i.e. made up of H and C
• Saturated hydrocarbons
– Each carbon bonded to four other atoms
H
H
C
H
H
C
H
H
CnH2n+2
Chapter 14 - 4
Chapter 14 - 5
Unsaturated Hydrocarbons
• Double & triple bonds relatively reactive – can
form new bonds
– Double bond – ethylene or ethene - CnH2n
H
H
C C
H
H
• 4-bonds, but only 3 atoms bound to C’s
– Triple bond – acetylene or ethyne - CnH2n-2
H C C H
Chapter 14 - 6
Isomerism
• Isomerism
– two compounds with same chemical formula can
have quite different structures
Ex: C8H18
• n-octane
H H H H H H H H
H C C C C C C C C H
= H3C CH2 CH2 CH2 CH2 CH2 CH2 CH3
H H H H H H H H

H3C ( CH2 ) CH3
• 2-methyl-4-ethyl pentane (isooctane)
6
CH3
H3C CH CH2 CH CH3
CH2
CH3
Chapter 14 - 7
Chemistry of Polymers
• Free radical polymerization
R
+
H H
H H
H H
C C
R C C
H H
monomer
(ethylene)
free radical
R C C
H H
+
initiation
H H
H H
H H H H
C C
R C C C C
H H
H H H H
propagation
dimer
• Initiator: example - benzoyl peroxide
H
C O O C
H
H
H
H
2
C O
=2R
H
Chapter 14 - 8
Chemistry of Polymers
Adapted from Fig.
14.1, Callister 7e.
Note: polyethylene is just a long HC
- paraffin is short polyethylene
Chapter 14 - 9
Bulk or Commodity Polymers
Chapter 14 - 10
Chapter 14 - 11
Chapter 14 - 12
MOLECULAR WEIGHT
• Molecular weight, Mi: Mass of a mole of chains.
Lower M
higher M
total wt of polymer
Mn 
total # of molecules
M n  x i M i
M w  w i M i
Mw is more sensitive to
higher molecular
weights
Adapted from Fig. 14.4, Callister 7e.
Chapter 14 - 13
Molecular Weight Calculation
Example: average mass of a class
Ni
Mi
# of students
mass (lb)
1
1
2
3
2
1
100
120
140
180
220
380
xi
wi
0.1
0.1
0.2
0.3
0.2
0.1
0.054
0.065
0.151
0.290
0.237
0.204
Mn
186 lb
Mw
216 lb
M n   xi Mi
M w   w i Mi
Chapter 14 - 14
Degree of Polymerization, n
n = number of repeat units per chain
H H H H H H H H H H H H
H C C (C C ) C C C C C C C C H
ni = 6
H H H H H H H H H H H H
Mn
nn   x i ni 
m
Mw
nw   w i ni 
m
where m  average molecular weight of repeat unit
m  fi mi
Chain fraction
mol. wt of repeat unit i
Chapter 14 - 15
End to End Distance, r
Adapted from Fig.
14.6, Callister 7e.
Chapter 14 - 16
Molecular Structures
• Covalent chain configurations and strength:
secondary
bonding
Linear
Branched
Cross-Linked
Network
Direction of increasing strength
Adapted from Fig. 14.7, Callister 7e.
Chapter 14 - 17
Polymers – Molecular Shape
Conformation – Molecular orientation can be
changed by rotation around the bonds
– note: no bond breaking needed
Adapted from Fig.
14.5, Callister 7e.
Chapter 14 - 18
Polymers – Molecular Shape
Configurations – to change must break bonds
• Stereoisomerism
H
H
C C
H
H H
H R
or
C C
R
C C
H R
H H
A
A
C
B
E
C
E
D
D
B
mirror
plane
Chapter 14 - 19
Tacticity
Tacticity – stereoregularity of chain
isotactic – all R groups on
same side of chain
H H H H H H H H
C C C C C C C C
H R H R H R H R
syndiotactic – R groups
alternate sides
H H H R H H H R
C C C C C C C C
H R H H H R H H
H H H H H R H H
atactic – R groups random
C C C C C C C C
H R H R H H H R
Chapter 14 - 20
cis/trans Isomerism
CH3
H
CH3
C C
CH2
CH2
C C
CH2
CH2
H
cis
trans
cis-isoprene
(natural rubber)
trans-isoprene
(gutta percha)
bulky groups on same
side of chain
bulky groups on opposite
sides of chain
Chapter 14 - 21
Copolymers
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–
Adapted from Fig.
14.9, Callister 7e.
random
alternating
block
graft
Chapter 14 - 22
Polymer Crystallinity
Adapted from Fig.
14.10, Callister 7e.
Ex: polyethylene unit cell
• Crystals must contain the
polymer chains in some
way
– Chain folded structure
Adapted from Fig.
14.12, Callister 7e.
10 nm
Chapter 14 - 23
Polymer Crystallinity
Polymers rarely 100% crystalline
• Too difficult to get all those chains
aligned
crystalline
region
• % Crystallinity: % of material
that is crystalline.
-- TS and E often increase
with % crystallinity.
-- Annealing causes
crystalline regions
to grow. % crystallinity
increases.
amorphous
region
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.)
Chapter 14 - 24
Polymer Crystal Forms
• Single crystals – only if slow careful growth
Adapted from Fig. 14.11, Callister 7e.
Chapter 14 - 25
Polymer Crystal Forms
• Spherulites – fast
growth – forms lamellar
(layered) structures
Spherulite
surface
Nucleation site
Adapted from Fig. 14.13, Callister 7e.
Chapter 14 - 26
Spherulites – crossed polarizers
Maltese cross
Adapted from Fig. 14.14, Callister 7e.
Chapter 14 - 27
ANNOUNCEMENTS
Reading:
Core Problems:
Self-help Problems:
Chapter 14 - 28