Constitutive modelling of hydrolytic degradation and viscoplasticity in glassy polymers: An effective temperature approach.

Hydrolysis is the primary degradation mechanism in biodegradable polymers in aqueous environments, involving water diffusion and polymer chain scission. These two processes dynamically alter the composition of the polymer, significantly influencing its thermomechanical properties and deformation behaviour. In this work, we develop a constitutive modelling approach that couples water diffusion, hydrolytic chain scission and viscoplastic deformation in glassy polymers. The effect of water concentration and hydrolytic degradation on the mechanical properties is captured through an effective temperature, reflecting the reduction in glass transition temperature brought about by water uptake and the reduction in average molecular weight. The model is calibrated using experimental data for polylactic acid (PLA), including thermo-mechanical characterisation in the wet degraded state and dry undegraded state at different temperatures. Our model accurately captures the evolution of molecular weight and water concentration distributions measured experimentally, and successfully predicts the deformation behaviour at different degradation stages. The potential of the model for weakly coupled simulations is also illustrated in representative case studies. Overall, this study supports the use of the effective temperature as a practical yet physically-motivated method for capturing the effect of degradation on mechanical properties, while providing a robust tool for the design and analysis of degradable polymer devices.