CHAPTER 14/15: POLYMER STUDIES
Issues:
• What are the basic microstructural features of a polymer?
• How are polymer properties affected by molecular weight?
• How do polymeric materials accommodate the polymer chain?
• What are the tensile properties of polymers and how are they affected by
basic microstructural features?
• Changing Polymer Properties: Hardening, anisotropy,
and annealing in polymers.
• How does the elevated temperature mechanical response of
polymers compare to ceramics and metals?
• What are the primary polymer processing methods?
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
Polypropylene (PP)
Adapted from Fig. 14.2, Callister 7e.
H
C
CH3
Ancient Polymer History
• Originally many natural polymers were used
– Wood
– Rubber
– Cotton
– Wool
– Leather
– Silk
• Oldest known uses of “Modern Polymers”
– Rubber balls used by Incas
– Noah used pitch (a natural polymer)
for the ark – as had all ancient mariners!
Polymer Composition
Most polymers are hydrocarbons
– i.e. made up of H and C
(we also recognize Si-H ‘silicones’)
• Saturated hydrocarbons
– Each carbon bonded to four other atoms
H
H
C
H
H
C
H
CnH2n+2
H
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
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
6
• 2-methyl-4-ethyl pentane (isooctane)
CH3
H3C CH CH2 CH CH3
CH2
CH3
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
H
=2R
Chemistry of Polymers
• Free radical polymerization (addition 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
H
=2R
Condensation Polymerization
Water is “Condensed out”
during polymerization of Nylon
• Some of the original monomer’s materials are shed
(condensed out) during polymerization process
• Process is conducted in the presence of a catalyst
• Water, CO2 are commonly condensed out but other
compounds can be emitted including HCN or other acids
Bulk or Commodity Polymers
NOTE: See Table 15.3 for commercially important
polymers – including trade names
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.
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
M n   xi M i
M w   wi M i
xi 
Ni
N
i
all i
Mn
186 lb
Mw
216 lb
wi 
Ni  M i
N M
i
all i
i
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
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.
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.
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
mirror
plane
B
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
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
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–
Adapted from Fig.
14.9, Callister 7e.
random
alternating
block
B–
graft
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.
Polymer Crystallinity
Polymers rarely exhibit 100% crystalline
• Too difficult to get all those chains aligned
crystalline
region
• % Crystallinity: how much
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.)
Mechanical Properties
• i.e. stress-strain behavior of polymers
brittle polymer
FS of polymer ca. 10% that of metals
plastic
elastomer
elastic modulus
– less than metal
Adapted from Fig. 15.1,
Callister 7e.
Strains – deformations > 1000% possible
(for metals, maximum strain ca. 100% or less)
Tensile Response: Brittle & Plastic
Near Failure
(MPa)
fibrillar
structure
x brittle failure
onset of
necking
near
failure
plastic failure
x
Initial
unload/reload
e
aligned, networked
crosscase
linked
case
crystalline
regions
slide
semicrystalline
case
amorphous
regions
elongate
crystalline
regions align
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, PrenticeHall, Inc., 1974, pp. 500-501.)
Predeformation by Drawing
• Drawing…(ex: monofilament fishline)
-- stretches the polymer prior to use
-- aligns chains in the stretching direction
• Results of drawing:
-- increases the elastic modulus (E) in the
stretching direction
-- increases the tensile strength (TS) in the
stretching direction
-- decreases ductility (%EL)
• Annealing after drawing...
-- decreases alignment
-- reverses effects of drawing.
• Comparable to cold working in metals!
Adapted from Fig. 15.13, Callister
7e. (Fig. 15.13 is from J.M.
Schultz, Polymer Materials
Science, Prentice-Hall, Inc.,
1974, pp. 500-501.)
Tensile Response: Elastomer Case
(MPa)
x brittle failure
x
plastic failure
x
elastomer
e
initial: amorphous chains are
kinked, cross-linked.
final: chains
are straight,
still
cross-linked
Stress-strain curves
adapted from Fig. 15.1,
Callister 7e. Inset
figures along elastomer
curve (green) adapted
from Fig. 15.15, Callister
7e. (Fig. 15.15 is from
Z.D. Jastrzebski, The
Nature and Properties of
Engineering Materials,
3rd ed., John Wiley and
Sons, 1987.)
Deformation
is reversible!
• Compare to responses of other polymers:
-- brittle response (aligned, crosslinked & networked polymer)
-- plastic response (semi-crystalline polymers)
Thermoplastics vs. Thermosets
• Thermoplastics:
-- little crosslinking
-- ductile
-- soften w/heating
-- polyethylene
polypropylene
polycarbonate
polystyrene
• Thermosets:
T
viscous
liquid
mobile
liquid
Callister,
rubber
Fig. 16.9
tough
plastic
crystalline
solid
Tm
Tg
partially
crystalline
solid
Molecular weight
Adapted from Fig. 15.19, Callister 7e. (Fig. 15.19 is from F.W. Billmeyer,
-- large crosslinking
Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc.,
1984.)
(10 to 50% of mers)
-- hard and brittle
-- do NOT soften w/heating
-- vulcanized rubber, epoxies,
polyester resin, phenolic resin
T and Strain Rate: Thermoplastics
• Decreasing T...
-- increases E
-- increases TS
-- decreases %EL
• Increasing
strain rate...
-- same effects
as decreasing T.
(MPa)
80 4°C
60
20°C
40
Data for the
semicrystalline
polymer: PMMA
(Plexiglas)
40°C
20
0
60°C
0
0.1
0.2
e
to 1.3
0.3
Adapted from Fig. 15.3, Callister 7e. (Fig. 15.3 is from T.S. Carswell and
J.K. Nason, 'Effect of Environmental Conditions on the Mechanical
Properties of Organic Plastics", Symposium on Plastics, American Society
for Testing and Materials, Philadelphia, PA, 1944.)
Melting vs. Glass Transition Temp.
What factors affect Tm
and Tg?
•
Both Tm and Tg increase with
increasing chain stiffness
•
Chain stiffness increased by
1. Bulky sidegroups
2. Polar groups or
sidegroups
3. Double bonds or aromatic
chain groups
•
Regularity (tacticity) – affects
Tm only
Adapted from Fig. 15.18,
Callister 7e.
Time Dependent Deformation
• Stress relaxation test:
-- strain to eo and hold.
-- observe decrease in
stress with time.
5
10
rigid solid
(small relax)
Er (10s) 3
in MPa 10
transition
region
10
strain
10-1
(t)
10-3 (large relax)
time
• Relaxation modulus:
(t )
E r (t ) 
eo
for T > Tg.
1
tensile test
eo
• Data: Large drop in Er
viscous liquid
(amorphous
polystyrene)
Adapted from Fig.
15.7, Callister 7e.
(Fig. 15.7 is from
A.V. Tobolsky,
Properties and
Structures of
Polymers, John
Wiley and Sons, Inc.,
1960.)
60 100 140 180 T(°C)
Tg
• Sample Tg(C) values:
PE (low density)
PE (high density)
PVC
PS
PC
- 110
- 90
+ 87
+100
+150
Selected values from
Table 15.2, Callister
7e.
Polymer Fracture
Crazing  Griffith cracks in metals
– spherulites plastically deform to fibrillar structure
– microvoids and fibrillar bridges form
alligned chains
fibrillar bridges
microvoids
crack
Adapted from Fig. 15.9,
Callister 7e.
Polymer Additives
Improve mechanical properties, processability,
durability, etc.
• Fillers
– Added to improve tensile strength & abrasion
resistance, toughness & decrease cost
– ex: carbon black, silica gel, wood flour, glass,
limestone, talc, etc.
• Plasticizers
– Added to reduce the glass transition
temperature Tg
– commonly added to PVC - otherwise it is brittle
Polymer Additives
• Stabilizers
– Antioxidants
– UV protectants
• Lubricants
– Added to allow easier processing
– “slides” through dies easier – ex: Na stearate
• Colorants
– Dyes or pigments
• Flame Retardants
– Cl/F & B
Processing of Plastics
• Thermoplastic –
– can be reversibly cooled & reheated, i.e. recycled
– heat till soft, shape as desired, then cool
– ex: polyethylene, polypropylene, polystyrene, etc.
• Thermoset
– when heated forms a network
– degrades (not melts) when heated
– mold the prepolymer then allow further reaction
– ex: urethane, epoxy
Processing Plastics - Molding
• Compression and transfer molding
– thermoplastic or thermoset
Adapted from Fig. 15.23,
Callister 7e. (Fig. 15.23 is from
F.W. Billmeyer, Jr., Textbook of
Polymer Science, 3rd ed.,
John Wiley & Sons, 1984. )
Processing Plastics - Molding
• Injection molding
– thermoplastic & some thermosets
Adapted from Fig. 15.24,
Callister 7e. (Fig. 15.24 is from
F.W. Billmeyer, Jr., Textbook of
Polymer Science, 2nd edition,
John Wiley & Sons, 1971. )
Processing Plastics – Extrusion
Adapted from Fig. 15.25,
Callister 7e. (Fig. 15.25 is from
Encyclopædia Britannica, 1997.)
Polymer Types: Elastomers
Elastomers – rubber
• Crosslinked materials
– Natural rubber
– Synthetic rubber and thermoplastic elastomers
• SBR- styrene-butadiene rubber
butadiene
– Silicone rubber
styrene
Polymer Types: Fibers
Fibers - length/diameter >100
• Textiles are main use
– Must have high tensile strength
– Usually highly crystalline & highly polar
• Formed by spinning
– ex: extrude polymer through a spinnerette
• Pt plate with 1000’s of holes for nylon
• ex: rayon – dissolved in solvent then pumped through
die head to make fibers
– the fibers are drawn
– leads to highly aligned chains- fibrillar structure
Polymer Types
•
Coatings – thin film on surface – i.e. paint, varnish
– To protect item
– Improve appearance
– Electrical insulation
•
Adhesives – produce bond between two adherands
– Usually bonded by:
1. Secondary bonds
2. Mechanical bonding
•
Films – blown film extrusion
•
Foams – gas bubbles in plastic
Blown-Film Extrusion
Adapted from Fig. 15.26, Callister 7e.
(Fig. 15.26 is from Encyclopædia
Britannica, 1997.)
Advanced Polymers
• Ultrahigh molecular weight
polyethylene (UHMWPE)
– Molecular weight
ca. 4 x 106 g/mol
– Excellent properties for
variety of applications
UHMWPE
• bullet-proof vest, golf ball
covers, hip joints, etc.
Adapted from chapteropening photograph,
Chapter 22, Callister 7e.
The Stem, femoral head, and the AC socket are made from Cobalt-chrome metal alloy or ceramic, AC
cup made from polyethylene
ABS – A Polymerized “Alloy”
ABS, Acrylonitrile-Butadiene-Styrene
Made up of the 3 materials: acrylonitrile, butadiene and styrene. The material is located under
the group styrene plastic. Styrene plastics are in volume one of the most used plastics.
Properties
The mechanical properties for ABS are good for impact resistance even in low temperatures.
The material is stiff, and the properties are kept over a wide temperature range. The
hardness and stiffness for ABS is lower than for PS and PVC.
Weather and chemical resistance
The weather resistance for ABS is restricted, but can be drastically improved by additives as
black pigments. The chemical resistance for ABS is relatively good and it is not affected by
water, non organic salts, acids and basic. The material will dissolve in aldehyde, ketone,
ester and some chlorinated hydrocarbons.
Processing
ABS can be processed by standard mechanical tools as used for machining of metals and wood.
The cutting speed need to be high and the cutting tools has to be sharp. Cooling is
recommended to avoid melting of the material. If the surface finish is of importance for the
product, the ABS can be treated with varnish, chromium plated or doubled by a layer of
acrylic or polyester. ABS can be glued to it self by use of a glue containing dissolvent.
Polyurethane based or epoxy based glue can be used for gluing to other materials.
A Processing Movie:
Calloway Golf:
Summary
• General drawbacks to polymers:
-- E, y, Kc, Tapplication are generally small.
-- Deformation is often T and time dependent.
-- Result: polymers benefit from composite reinforcement.
• Thermoplastics (PE, PS, PP, PC):
-- Smaller E, y, Tapplication
-- Larger Kc
-- Easier to form and recycle
• Elastomers (rubber):
-- Large reversible strains!
• Thermosets (epoxies, polyesters):
-- Larger E, y, Tapplication
-- Smaller Kc
And Remember:
Table 15.3 Callister 7e
Is a Good overview of
applications and trade
names of polymers.