4. Aging by migration of additives

4.1 Mechanisms

A polymer sample containing an additive is, in principle, out of equilibrium, since the vapour pressure of the additive in the environment (except in the case of confinement) is zero. The additive therefore tends to migrate in the environment and within the material, i.e. to tend towards negligible concentration in both media.

The process is symmetrical with that of solvent absorption. The latter can be described as the sequence of two elementary acts: dissolution at the medium-material interface and diffusion. In the case of admixture migration, molecules close to the interface first leave (passing into the environment), then diffuse from the core to the surface (in the direction of the concentration gradient).

As before,...