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 0 is imposed in the creep test.

0

ttER

Modulus is a function of temperature.

As we expect, the moduli are higher for higher temperatures.

REGIMES OF BEHAVIOR – DEPEND ON TEMPERATURE We have 1. Glassy, E nearly

const.2. Leathery Big

change in E3. Rubbery, E

nearly constant4. Rubber Flow, E

falling5. 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 starts at yield

Neck propagates

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

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brittle failure

plastic failure

(MPa)

x

x

crystalline regions

slide

fibrillar structure

near failure

crystalline regions align

onset of necking

Initial

Near Failure

semi-crystalline

case

aligned,cross-linkedcase

networkedcase

amorphousregions

elongate

unload/reload

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.)

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TENSILE RESPONSE: ELASTOMER CASE

• Compare to responses of other polymers: -- brittle response (aligned, crosslinked & networked polymer) -- plastic response (semi-crystalline polymers)

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.)

(MPa)

initial: amorphous chains are kinked, cross-linked.

x

final: chainsare straight,

stillcross-linked

elastomer

Deformation is reversible!

brittle failure

plastic failurex

x