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 mer
many repeat unit
repeat repeat repeat
unit unit unit
H H H H H 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 C C C C C C
H H H H H H H Cl H Cl H Cl H CH3 H CH3 H CH3
Polyethylene (PE) Polyvinyl chloride (PVC) 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
H
C C
H 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
Chemistry of Polymers
Adapted from Fig.
14.1, Callister 7e.
Note: polyethylene is just a long HC
- paraffin is short polyethylene
Chapter 14 - 7
Bulk or Commodity Polymers
Chapter 14 - 8
Chapter 14 - 9
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 = Σx i M i
M w = Σw i M i
Mw is more sensitive to
higher molecular
weights Adapted from Fig. 14.4, Callister 7e.
Chapter 14 - 10
Molecular Weight Calculation
Example: average mass of a class
Ni Mi xi wi
# of students mass (lb)
1 100 0.1 0.054
1 120 0.1 0.065 M n = ∑ xi Mi
2 140 0.2 0.151
M w = ∑ w i Mi
3 180 0.3 0.290
2 220 0.2 0.237
1 380 0.1 0.204
Mn Mw
186 lb 216 lb
Chapter 14 - 11
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 Mw
nn = ∑ x i ni = nw = ∑ w i ni =
m m
where m = average molecular weight of repeat unit
m = Σfi mi
Chain fraction mol. wt of repeat unit i
Chapter 14 - 12
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 - 13
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 - 14
Copolymers Adapted from Fig.
14.9, Callister 7e.
two or more monomers random
polymerized together
• random – A and B randomly
vary in chain
• alternating – A and B
alternate in polymer chain alternating
• block – large blocks of A
alternate with large blocks of block
B
• graft – chains of B grafted
on to A backbone
A– B–
graft
Chapter 14 - 15
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 - 16
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 - 17
Polymer Crystal Forms
• Single crystals – only if slow careful growth
Adapted from Fig. 14.11, Callister 7e.
Chapter 14 - 18
Polymer Crystal Forms
• Spherulites – fast
growth – forms lamellar
(layered) structures
Spherulite
surface
Nucleation site Adapted from Fig. 14.13, Callister 7e.
Chapter 14 - 19
Spherulites – crossed polarizers
Maltese cross
Adapted from Fig. 14.14, Callister 7e. Chapter 14 - 20
Characteristics, Applications &
Processing of Polymers
ISSUES TO ADDRESS...
• What are the tensile properties of polymers and how
are they affected by basic microstructural features?
• 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 15 - 21
Mechanical Properties
• i.e. stress-strain behavior of polymers
brittle polymer
σFS of polymer ca. 10% that of metals
plastic
elastomer
elastic modulus
– less than metal
Adapted from Fig. 15.1,
Strains – deformations > 1000% possible Callister 7e.
(for metals, maximum strain ca. 10% or less)
Chapter 15 - 22
Tensile Response: Brittle & Plastic
Near Failure σ(MPa) fibrillar
structure
x brittle failure
near
onset of
failure
necking plastic failure
x
Initial
unload/reload
ε
aligned, networked
cross- case crystalline
linked regions
case slide
semi-
crystalline amorphous
crystalline
case regions
regions align
elongate
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.) Chapter 15 - 23
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 Adapted from Fig. 15.13, Callister
-- decreases ductility (%EL) 7e. (Fig. 15.13 is from J.M.
Schultz, Polymer Materials
• Annealing after drawing... Science, Prentice-Hall, Inc.,
1974, pp. 500-501.)
-- decreases alignment
-- reverses effects of drawing.
• Compare to cold working in metals!
Chapter 15 - 24
Tensile Response: Elastomer Case
σ(MPa)
x brittle failure Stress-strain curves
adapted from Fig. 15.1,
Callister 7e. Inset
figures along elastomer
curve (green) adapted
from Fig. 15.15, Callister
plastic failure
x 7e. (Fig. 15.15 is from
Z.D. Jastrzebski, The
Nature and Properties of
x Engineering Materials,
elastomer 3rd ed., John Wiley and
Sons, 1987.)
final: chains
ε are straight,
still
cross-linked
initial: amorphous chains are Deformation
kinked, cross-linked. is reversible!
• Compare to responses of other polymers:
-- brittle response (aligned, crosslinked & networked polymer)
-- plastic response (semi-crystalline polymers)
Chapter 15 - 25
Thermoplastics vs. Thermosets
T
• Thermoplastics: Callister,
-- little crosslinking viscous rubber
Fig. 16.9
mobile liquid Tm
-- ductile tough
liquid
-- soften w/heating plastic
-- polyethylene Tg
polypropylene
partially
polycarbonate crystalline
polystyrene crystalline
solid
solid
• Thermosets: 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
Chapter 15 - 26
T and Strain Rate: Thermoplastics
• Decreasing T...
σ(MPa)
-- increases E 80 4°C Data for the
-- increases TS semicrystalline
-- decreases %EL 60 polymer: PMMA
20°C (Plexiglas)
• Increasing 40 40°C
strain rate...
-- same effects 20
as decreasing T. to 1.3
60°C
0
0 0.1 0.2 ε 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.)
Chapter 15 - 27
Time Dependent Deformation
• Stress relaxation test: • Data: Large drop in Er
-- strain to εο and hold. for T > Tg. (amorphous
5
-- observe decrease in 10 rigid solid polystyrene)
Er (10s) 3 (small relax) Adapted from Fig.
stress with time. in MPa 10
15.7, Callister 7e.
transition (Fig. 15.7 is from
1
tensile test 10 region A.V. Tobolsky,
Properties and
εo strain 10-1
Structures of
Polymers, John
viscous liquid Wiley and Sons, Inc.,
σ(t) 10-3 (large relax) 1960.)
60 100 140 180 T(°C)
time Tg
• Relaxation modulus: • Sample Tg(°C) values:
σ(t ) PE (low density) - 110
E r (t ) = PE (high density) - 90
εo PVC + 87
Selected values from
Table 15.2, Callister
PS +100 7e.
PC +150
Chapter 15 - 28
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
Chapter 15 - 29
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
Chapter 15 - 30
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
Chapter 15 - 31
Summary
• General drawbacks to polymers:
-- E, σy, 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, σy, Tapplication
-- Larger Kc Table 15.3 Callister 7e:
-- Easier to form and recycle
• Elastomers (rubber): Good overview
-- Large reversible strains! of applications
and trade names
• Thermosets (epoxies, polyesters): of polymers.
-- Larger E, σy, Tapplication
-- Smaller Kc
Chapter 15 - 32
