EFFECT OF AGING ON THE MECHANICAL-PROPERTIES OF POLYMERIC MATERIALS

Aging can modify polymer structure at the molecular, macromolecular, a nd/or the morphological level and thus induce changes in the mechanica l properties. Stiffness is generally not modified for nonrubbery mater ials, except for mass transfer (solvent plasticization or plasticizer loss) in amorphous polymers or phase transfer (crystallization or crys tal destruction) in semicrystalline polymers. The most significant mod ulus changes occur in the radiochemical aging of semicrystalline polym ers whose amorphous phase is in the rubbery state. Yield properties ge nerally vary in the same way as stiffness. Physical aging at T < T-g c an lead to a significant increase in the yield stress. Very general fe atures can be observed for rupture properties, for instance: 1) Only u ltimate elongation E is a pertinent variable in kinetic studies of agi ng involving tensile testing and related methods, 2) the amplitude of E variation for a given degradation conversion is considerably higher for initially ductile materials than for brittle ones, and 3) the rupt ure envelope sigma = f(epsilon), i.e., the ultimate stress, is often v ery close to the initial tensile curve except for rubbery materials un dergoing predominant crosslinking. The mechanisms of ultimate property changes are reviewed. A kinetic approach is proposed for the very imp ortant case of heterogeneous, diffusion-controlled aging.