D AY 30: M ECHANICAL B EHAVIOR Temperature dependence of Moduli Mechanism of plastic deformation. Cold work and annealing mean different things for polymers.

T EMPERATURE D EPENDENCE OF M ODULUS Here is the definition of relaxation modulus for a polymer. The strain  0 is imposed in the creep test. Modulus is a function of temperature. As we expect, the moduli are higher for higher temperatures.

R EGIMES OF B EHAVIOR – D EPEND ON T EMPERATURE We have 1. Glassy, E nearly const. 2. Leathery Big change in E 3. Rubbery, E nearly constant 4. Rubber Flow, E falling 5. Viscous Flow, E drops greatly, it’s a liquid. Glass temp. middle of leathery

N OTE THE EFFECTS OF CRYSTALLINITY / TACTICITY Three forms of PS behave a lot differently.

D EFORMATION IN S EMI -C RYSTALLINE T HERMOPLASTIC

S TRESS S TRAIN CURVE Neck starts at yield Neck propagates

P REDEFORMATION BY D RAWING 8 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 -- decreases ductility (%EL) Annealing after drawing decreases alignment -- reverses effects of drawing. Compare to cold working in metals! Adapted from Fig , Callister 7e. (Fig is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp )

D RAWING AND A NNEALING 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.

T ENSILE R ESPONSE : B RITTLE & P LASTIC 10 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- linked case networked case amorphous regions elongate unload/reload Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from Figs & 15.13, Callister 7e. (Figs & are from J.M. Schultz, Polymer Materials Science, Prentice- Hall, Inc., 1974, pp )

11 T ENSILE R ESPONSE : E LASTOMER C ASE 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 , Callister 7e. (Fig 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: chains are straight, still cross-linked elastomer Deformation is reversible! brittle failure plastic failure x x