Transcript DAY 30: MECHANICAL BEHAVIOR Temperature dependence of Moduli  Mechanism of plastic deformation.  Cold work and annealing mean different things for polymers. 

DAY 30: MECHANICAL BEHAVIOR
Temperature dependence of Moduli
 Mechanism of plastic deformation.
 Cold work and annealing mean different things
for polymers.

TEMPERATURE DEPENDENCE OF
MODULUS

Here is the definition of relaxation modulus for a
polymer. The strain e0 is imposed in the creep
test.
 t 
ER t  
e0


Modulus is a function of
temperature.
As we expect, the moduli
are higher for higher
temperatures.
REGIMES OF BEHAVIOR – DEPEND ON
TEMPERATURE

1.
2.
3.
4.
5.
We have
Glassy, E nearly
const.
Leathery Big
change in E
Rubbery, E
nearly constant
Rubber Flow, E
falling
Viscous Flow, E
drops greatly,
it’s a liquid.
Glass temp. middle of leathery
NOTE THE EFFECTS OF CRYSTALLINITY /
TACTICITY

Three forms of PS behave a lot differently.
DEFORMATION IN SEMI-CRYSTALLINE
THERMOPLASTIC
STRESS STRAIN CURVE
Neck propagates
Neck starts at yield
DRAWING AND ANNEALING
Drawing, or Cold Work. Take advantage of the
increased strength and stiffness caused by the
orientation of the chains. This can actually be
used as a final step in manufacturing polymers
as it is in metals. Note: drawing just imparts
strength / stiffness in one direction! How is this
different from CW in metals?
 Annealing. (1) If the material is already drawn,
it has much the same effect of softening as in
metals. BUT (2) If the material is not drawn it
can impart strength and stiffness (at least in
some polymers) by enhancing crystallinity.

TENSILE RESPONSE: BRITTLE & PLASTIC
Near Failure
 (MPa)
fibrillar
structure
x brittle failure
onset of
necking
near
failure
plastic failure
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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.)
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)
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