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Chapter 15 - 1
Chapter 15:
Characteristics, Applications &
Processing of Polymers (1)
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?
Chapter 15 - 2 2
Mechanical Properties of Polymers –
Stress-Strain Behaviors
• Strength of polymers ~ 10% of those for metals
• Deformation strains for polymers > 10 times of those for metals
– for most metals, deformation strains < ~0.3
brittle polymer
Plastic polymer
Elastomer/rubber
elastic moduli
– less than for metals Adapted from Fig. 15.1,
Callister & Rethwisch 8e.
Chapter 15 - fig_15_02
Tensile Strength & Yield Strength for
Polymers
Chapter 15 - 4 4
• Decreasing T: -- increases E
-- increases TS
-- decreases %EL
• Increasing strain rate: -- same effects
as decreasing T.
Adapted from Fig. 15.3, Callister & Rethwisch 8e. (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.)
Influence of T and Strain Rate on Thermoplastics
Mechanical Property
20
4 0
6 0
8 0
0 0 0.1 0.2 0.3
4ºC
20ºC
40ºC
60ºC to 1.3
s (MPa)
e
Plots for
semicrystalline
PMMA (Plexiglas)
Chapter 15 - fig_15_04
Change in Shape for Polymers during
Tensile Test
Different from typical plastic deformation for
metals where necking concentrates locally,
polymer necking section elongate significantly
Chapter 15 - 6 6
Mechanisms of Deformation for Brittle,
Crosslinked or Network Polymers
brittle failure
plastic failure
s (MPa)
e
x
x
aligned, crosslinked
polymer Stress-strain curves adapted from Fig. 15.1,
Callister & Rethwisch 8e.
Initial
Near
Failure Initial
network polymer
Near
Failure
Chapter 15 - 7 7
Mechanisms of Deformation for
Semicrystalline Polymers
brittle failure
plastic failure
s (MPa)
x
x
crystalline
block segments
separate
fibrillar
structure
near
failure
crystalline
regions align
onset of
necking
undeformed
structure amorphous
regions
elongate
unload/reload
Stress-strain curves adapted
from Fig. 15.1, Callister &
Rethwisch 8e. Inset figures
along plastic response curve
adapted from Figs. 15.12 &
15.13, Callister & Rethwisch
8e. (15.12 & 15.13 are from
J.M. Schultz, Polymer
Materials Science, Prentice-
Hall, Inc., 1974, pp. 500-501.)
e
Chapter 15 -
fig_15_12
Elastic Deformation in
Semicrystalline Polymers
Chapter 15 -
fig_15_13
Plastic Deformation in
Semicrystalline Polymers
Chapter 15 - 10 10
Predeformation by Drawing
• Drawing…(e.g.: monofilament fishline) -- stretches the polymer prior to use
-- aligns chains in the stretching direction
• Results of (pre-) drawing:
-- increases the elastic modulus (E) in the
stretching direction
-- increases the tensile strength (TS) in the
stretching direction
-- decreases ductility (%EL) Pre-deformation for polymer has similar strengthening
effects as cold working for metals
• Annealing after drawing...
-- decreases chain alignment
-- reverses effects of drawing (reduces E and
TS, enhances %EL) Annealing for polymers has similar effect of annealing
for metals
Adapted from Fig. 15.13, Callister
& Rethwisch 8e. (Fig. 15.13 is
from J.M. Schultz, Polymer
Materials Science, Prentice-Hall,
Inc., 1974, pp. 500-501.)
Chapter 15 - 11 11
Stress-strain curves
adapted from Fig. 15.1,
Callister & Rethwisch 8e.
Inset figures along
elastomer curve (green)
adapted from Fig. 15.15,
Callister & Rethwisch 8e.
(Fig. 15.15 is from Z.D.
Jastrzebski, The Nature
and Properties of
Engineering Materials,
3rd ed., John Wiley and
Sons, 1987.)
Mechanisms of Deformation of
Elastomers/Rubbers s (MPa)
e
initial: amorphous chains are kinked, cross-linked.
x
final: chains are straighter,
still cross-linked
elastomer
deformation is reversible (elastic)!
brittle failure
plastic failure x
x
Chapter 15 - 12 12
• Relaxation:
-- strain in tension to eo
and hold.
-- observe decrease in
stress with time.
o
r
ttE
e
s
)()(
• Relaxation modulus: modulus change (decreases) with time
or
Relaxation & Creep for
Semicrystalline Polymers
time
strain
tensile test
eo
σ(t)
• Creep:
-- stress in tension to σo
and hold.
-- observe increase in
strain with time.
time
stress
tensile test
σo e (t)
)()( 0
ttEr
e
s
Chapter 15 - fig_15_05
Strain vs. Time Behaviors
Pure Elastic (Metal/Ceramics)
Viscous (honey, pitch)
Viscoelastic (typical plastics above glass transition Tg)
Chapter 15 -
fig_
15
_0
7
(Relaxation) Modulus vs. Temperature
Rigid and brittle
(as glass)
Viscoelastic
(as typical
plastics)
Elastomer
(as rubber)
Viscous liquid
(polymer melts)
Chapter 15 -
fig_15_11
Fatigue in Polymer
Chapter 15 - 16 16
Fracture of Polymers (1)
fibrillar bridges microvoids crack
aligned chains
Adapted from Fig. 15.9,
Callister & Rethwisch 8e.
Craze formation prior to cracking
– during crazing, plastic deformation of spherulites
– and formation of microvoids and fibrillar bridges
Chapter 15 -
fig_
15
_1
0
Fracture of Polymers (2)
Chapter 15 - 18 18
Melting & Glass Transition Temps. Tm – Melting Temperature:
For crystalline material the temperature at
which phase transition between (free
flowing) liquid and crystalline solid happen
Tg – Glass transition temperature
For glassy material, the temperature
at which the highly viscous liquid
transforms to rigid amorphous solid,
accompanied by sudden change in other
physical/chemical properties
Tm > Tg
Adapted from Fig. 15.18,
Callister & Rethwisch 8e.
What factors affect Tm and Tg?
• Both Tm and Tg increase with increasing
chain stiffness
• Chain stiffness increased by presence of
1. Bulky side-groups
2. Polar groups or side-groups
3. Chain double bonds and aromatic
chain groups
Chapter 15 -
Examples for Polymers with
Different Tg and Tm • Rubbers for auto tires:
– Tg of about -50 oC https://www.perkinelmer.com/CMSResources/Images/44-
141192APP_007771B_15_Characterization_of_Car_Tire_Rubber.pdf
• High performance nylon for under-the-
hood applications
– Tm about 250 oC
– Tg about 50 oC http://fiberpolymer.invista.com/en/advantages.html
19
Chapter 15 - 20 20
• Thermoplastics: -- little crosslinking
-- ductile
-- soften with heating easy reshaping and recycle
-- polyethylene
polypropylene
polycarbonate
polystyrene
• Thermosets: -- significant crosslinking
(10 to 50% of repeat units)
-- hard
-- do NOT soften with heating usually not recycled
-- vulcanized rubber, epoxies, polyester resin, phenolic resin
Thermoplastics vs. Thermosets
Chapter 15 - 21
• Polymers mechanical properties: -- E, sy, Kc, Tapplication are generally small.
-- Deformation is often time and temperature dependent.
• Thermoplastics (PE, PS, PP, PC):
-- Smaller E, sy, Tapplication -- Larger Kc
-- Easier to form and recycle
• Elastomers (rubber):
-- Large reversible strains!
• Thermosets (epoxies, polyesters):
-- Larger E, sy, Tapplication
-- Smaller Kc
Table 15.3 Callister &
Rethwisch 8e:
Good overview
of applications
and trade names
of polymers.
Summary