Time- and Feedback-Dependent DLO Phenomena in Oxidative Polymer Aging

Abstract

Polymer degradation under oxidative conditions, particularly under accelerated stresses such as increased temperatures and irradiation, often exhibits spatially heterogeneous oxidation profiles. This well-known behavior is the result of diffusion-limited oxidation (DLO), which occurs when the oxidation rate is faster than the resupply of oxygen through diffusion into the material. So far most theoretical model descriptions have focused on DLO in equilibrium situations in which the underlying material properties do not change with increasing degradation levels or are constant (i.e. time-independent) variables. An extension of a previously developed finite element model is now presented, which accommodates time-dependent variables that are either explicitly time-dependent (i.e. changes homogeneously throughout the material), or through a feedback mechanism driven by the localized degree of oxidation, which results in spatial variations in the material properties responsible for specific DLO behavior. This model is realized in COMSOL Multiphysics and is capable of geometries in 1D up to 3D. Additionally, specific theoretical cases in 1D are shown which relate to known non-stationary phenomena in polymer degradation. They illustrate the effect on the resulting oxidation profile, when the oxygen diffusivity, solubility or oxidation rate properties change over time and in space. With COMSOL based FEM, it is now possible to model DLO for whatever material behavior may exist or could be envisaged.