A simulation of gas transport in polymeric membranes. Application to heterogeneous aging

A simplified mechanism of polymer aging has been proposed to simulate the effect of diffusion on degradation processes. Simple observations have been made on different events simulated using a modelization of the diffusion of low mass species: (1) the bulk oxidation rate, in the case of heterogeneous oxidation, is proportional to the square root of the initiation rate (formation of radicals). (2) The relative oxidation profile (relative to the surface oxidation) as a function of aging time is not constant. (3) The initial concentration of oxygen has, in some cases, a large influence on the apparent bulk oxidation rate during all the degradation processes. (4) The same observation is made for model samples stored in air and aged in inert atmosphere. (5) When heterogeneous oxidation occurs in samples containing antioxidant additives, the concentration of antioxidant has an effect on the induction period and on the bulk oxidation rate (this is not the case for homogeneous oxidation). (6) When heterogeneous oxidation occurs, the effect of antioxidant mobility can be practically neglected when D-oxygen/D-antioxidant (D = diffusion coefficient) varies from to 100 to 1. (7)The time required to reach a pseudostationary state (corresponding to the egality of oxidation rate and oxygen diffusion rate) can be interpreted by a simple induction period (antioxidant consumption and/or hydroperoxide accumulation). (C) 1997 John Wiley & Sons, Inc.