Constitutive modeling of diffusion-limited oxidation coupled with a large deformation theory for polymer degradation

Abstract

The influence of oxidation on the degradation of polymers is one of the most critical aging processes. The oxidation is usually limited to the sample’s surface, commonly called diffusion-limited oxidation (DLO). DLO occurs through the competition of simultaneous oxygen absorption, diffusion transport, and the reaction of oxygen in the elastomers. A new kinetics-based oxygen absorption model is developed and validated against multiple experimental data. In addition, the diffusion-reaction equation is extended in 3 dimensions and solved by the Alternating Direction Implicit (ADI) method. Various reaction rate functions are considered for chain scission and network reformation reactions to describe non-uniform degradation. An enhanced model is utilized to simulate the heterogeneous oxidation and to demonstrate the influence of contributing factors on the oxidation behavior of nitrite rubber. Our proposed model’s results agree with the empirical data on polymers’ oxidation degree. In addition, a constitutive model is developed that incorporates the coupling between diffusion, chemical reaction, and large deformation of polymers. The finite element implementation of the coupled multi-physics is explained in detail. The proposed constitutive model illustrates the effect of diffusion-limited oxidation (DLO) on the mechanical properties of cross-linked polymers. It numerically analyzes the coupled diffusion–reaction and mechanical behavior of polymers undergoing DLO.