Effect of Aging on the Mechanical Properties of Polymeric Materials

Abstract

Aging can modify polymer structure at the molecular, macromolecular, and/or the morphological level and thus induce changes in the mechanical properties. Stiffness is generally not modified for nonrubbery materials, except for mass transfer (solvent plasticization or plasticizer loss) in amorphous polymers or phase transfer (crystallization or crystal destruction) in semicrystalline polymers. The most significant modulus changes occur in the radiochemical aging of semicrystalline polymers whose amorphous phase is in the rubbery state. Yield properties generally vary in the same way as stiffness. Physical aging at T < T _g can lead to a significant increase in the yield stress. Very general features can be observed for rupture properties, for instance: 1) Only ultimate elongation ε is a pertinent variable in kinetic studies of aging involving tensile testing and related methods, 2) the amplitude of ε variation for a given degradation conversion is considerably higher for initially ductile materials than for brittle ones, and 3) the rupture envelope σ = f(ε), i.e., the ultimate stress, is often very close to the initial tensile curve except for rubbery materials undergoing predominant crosslinking. The mechanisms of ultimate property changes are reviewed. A kinetic approach is proposed for the very important case of heterogeneous, diffusion-controlled aging.