Chapter 15: Characteristics, Applications & Processing of Polymers

Chapter 15:
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 - 1
Mechanical Properties of Polymers –
Stress-Strain Behavior
brittle polymer
plastic
elastomer
elastic moduli
– less than for metals
Adapted from Fig. 15.1,
Callister & Rethwisch 8e.
Chapter 15 - 2
Mechanisms of Deformation—Brittle
Crosslinked and Network Polymers
Initial
Near
Failure
(MPa)
Near
Failure
Initial
x brittle failure
x plastic failure
aligned, crosslinked
polymer

network polymer
Stress-strain curves adapted from Fig. 15.1,
Callister & Rethwisch 8e.
Chapter 15 - 3
Mechanisms of Deformation —
Semicrystalline (Plastic) Polymers
(MPa)
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, PrenticeHall, Inc., 1974, pp. 500-501.)
fibrillar
structure
x brittle failure
onset of
necking
plastic failure
near
failure
x
unload/reload

crystalline
block segments
separate
undeformed
structure
amorphous
regions
elongate
crystalline
regions align
Chapter 15 - 4
Mechanisms of Deformation —
Semicrystalline (Plastic) Polymers
Elastic Deformation
two adjacent chain-folded lamemllae
and the interlamellar amorphous
material
Stage 1 : chain molecules in amorphous regions elongating in the direction of the
applied tensile stress
Stage 2 : amorphous chains continue to align and become elongated; bending
and stretching of the strong chain covalent bonds within the lamellar crystallites.
→ slight, reversible increase in the lamellar crystallite thickness (∆t)
Chapter 15 - 5
Mechanisms of Deformation —
Semicrystalline (Plastic) Polymers
Plastic Deformation
Stage 3 : adjacent
chains in the
lamellae slide past
one another
→ tilting of the
lamellae so that the
chain folds become
more aligned
Stage 4 : crystalline block segments separate from the lamella, with the
segments attached to one another by tie chains.
Stage 5 : the blocks and tie chains become oriented in the direction of the tensile
axis → highly oriented structure by drawing.
Chapter 15 - 6
Mechanical Properties of Polymers –
Stress-Strain Behavior
x
Tensile stress-strain curve for
a semicrystalline polymer
Chapter 15 - 7
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
& Rethwisch 8e. (Fig. 15.13 is
-- decreases ductility (%EL)
from J.M. Schultz, Polymer
Materials Science, Prentice-Hall,
• Annealing after drawing...
Inc., 1974, pp. 500-501.)
-- decreases chain alignment
-- reverses effects of drawing (reduces E and
TS, enhances %EL)
Chapter 15 - 8
Degree of Crystallinity
Influence of degree of crystallinity and
molecular weight on the physical
characteristics of polyethylene
Increasing the crystallinity of a polymer enhances its strength;
however, the material tends to become more brittle
Chapter 15 - 9
Heat-treating of semicrystalline polymers
-
Heat-treating(annealing) can lead to an increase in the percent
crystallinity, and crystallite size and perfection, as well as
modifications of the spherulite structure
-
Heat treatment of undrawn materials; increasing temp. leads to
1) an increase in tensile modulus
2) an increase in yield strength
3) a reduction in ductility
-
Heat treatment of drawn polymers (ex. fibers),
Modulus decreases with increased annealing temp. because of
a loss of chain orientation and strain-induced crystallinty
Chapter 15 - 10
Mechanisms of Deformation—
Elastomers
(MPa)
x brittle failure
x
plastic failure
elastomer
x

initial: amorphous chains are
kinked, cross-linked.
final: chains
are straighter,
still
cross-linked
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.)
deformation
is reversible (elastic)!
- Moduli of elasticity are quite small and vary with strain; non-linear S-S
- Elastic deformation, upon application of a tensile load, is the partial
uncoiling, untwisting, and straightening
Chapter 15 - 11
Mechanisms of Deformation—
Elastomers
the crosslinks act as anchor points between the
chains and prevent chain slippage.
“Entropy” is the part of the driving force for elastic
deformation.
Requirement for elastomers
- Must not easily crystallize; amorphous, having molecular chains that are
naturally coiled and kinked in the unstressed state.
- Chain bond rotations must be relatively free for the coiled chains to
readily respond to an applied force
- Restricting the motions of chains past one another by crosslinking
→ delay the onset of plastic deformation
Chapter 15 - 12
Mechanisms of Deformation—
Elastomers
Vulcanization
- Crosslinking process in
elastomers
- Achieved by a nonreversible
chemical reaction at an
elevated temp.
- Sulfur compounds are added to
the heated elastomer
Chapter 15 - 13
Mechanisms of Deformation—
Elastomers
Vulcanization
- Unvulcanized rubber contains
very few crosslinks: soft, tacky
and has poor resistance to
abrasion
- Modulus of elasticity, tensile
strength, and resistance to
degradation by oxidation are all
enhanced by vulcanization
- To produce a rubber that is capable of large extensions without
rupture of the primary chain bonds, there must be relatively few
crosslinks, and these must be widely seprated.
- Increasing the sulfur content further hardens the rubber and reduces
the extensibility.
Chapter 15 - 14
Crystallization, Melting, and Glasstransition in polymers
-
Crystallization : the process by which, upon cooling, an ordered
(i.e. crystalline) solid phase is produced from a liquid melt
having a highly random molecular structure.
-
Melting : the reverse process that occurs when a polymer is
heated.
-
Glass-transition : when cooled from a liquid melt, amororhpous
or noncrystallizable polymer become rigid solids yet retain the
disordered molecular structure; named because glass changes from
a rigid sheet to a flexible plastic-like material at its transition point.
-
For semicrystalline polymers, crystalline regions will experience
melting (and crystallization), whereas noncrystalline area pass
through the glass transition.
Chapter 15 - 15
Melting & Glass Transition Temps.
- Crystalline material(C):
Discontinuous change in
specific volume at Tm.
Specific volume versus
temp., upon cooling
from the liquid melt
- Totally amorphous
material(A): continuous but
experiences a slight
decrease in slope at Tg
- Semicrystalline material(B):
both Tm and Tg are
observed.
Adapted from Fig. 15.18,
Callister & Rethwisch 8e.
Chapter 15 - 16
Melting & Glass Transition Temps.
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
•
Regularity of repeat unit arrangements – affects Tm only
Chapter 15 - 17
Melting & Glass Transition Temps.
Chapter 15 - 18
Melting & Glass Transition Temps.
Dependence of polymer properties as well as melting
and glass transition temp. on molecular weight
T
viscous
liquid
mobile
liquid
Callister,
rubber
Fig. 16.9
tough
plastic
crystalline
solid
Tm
Tg
Tg = 0.5~0.8Tm
partially
crystalline
solid
Molecular weight
Adapted from Fig. 15.19, Callister & Rethwisch 8e. (Fig.
15.19 is from F.W. Billmeyer, Jr., Textbook of Polymer
Science, 3rd ed., John Wiley and Sons, Inc., 1984.)
Chapter 15 - 19
Thermoplastics vs. Thermosets
• Thermoplastics:
-- little crosslinking
-- ductile
-- soften w/ heating
-- polyethylene, polypropylene, polycarbonate, polystyrene
• Thermosets:
-- significant crosslinking (10 to 50% of repeat units)
-- hard and brittle
-- do NOT soften w/ heating
-- vulcanized rubber, epoxies, polyester resin, phenolic resin
Chapter 15 - 20
Influence of T and Strain Rate on Thermoplastics
• Decreasing T...
-- increases E
-- increases TS
-- decreases %EL
• Increasing
strain rate...
-- same effects
as decreasing T.
(MPa)
80 4ºC
60
40
40ºC
20
0
For intermediate temp.,
the polymer is rubbery solid
that exhibits the combined
mechanical characteristics of
elastic and viscous behaviors
→ viscoelasticity
20ºC
Plots for
semicrystalline
PMMA (Plexiglas)
60ºC
0
0.1
0.2

to 1.3
0.3
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.)
Chapter 15 - 21
Viscoelastic Materials
An amorphous polymer may behave like a glass at low temp.(T<Tg), a viscous
liquid at high temp.(T>Tg), and a rubbery solid at intermediate temp.(T≈Tg)
Viscoelasticity
Chapter 15 - 22