The effect of photodegradation on effective properties of polymeric thin films: A micromechanical homogenization approach

An analytical model is developed to study the impact of photodegradation on the elastic properties of polymeric thin films. The multi-phase heterogeneous aged polymer material is considered as a two-phase functionally graded material with varying volume ratios as functions of both time and depth. The concentration gradations are obtained using a three-species chemical kinetic model with the kinetics being driven by light that is absorbed as it passes through the film. Concentration gradations are connected to the elastic stiffness through volume averaging and a micromechanics approach is employed to find effective properties of functionally graded composites. Concise matrix expressions of the effective properties are presented. The derived formulas are applied to study the effective responses of an aged simply-supported polymer film under surface loads. The obtained results are then compared with and validated by those from a multi-layer analytical model. The present formulas are also applied to predict the evolution of effective properties of a polymer thin film under photodegradation, and the variation of effective responses under surface loads. The present solution could be useful in studying the relation between polymer molecular structure and mechanical properties, and in the evaluation of long-term mechanical responses of polymeric structures.