A poroviscoelasticity model based on effective temperature for water and temperature driven phase transition in hydrogels

Hydrogels exhibit a wide range of responses to various stimuli, which makes them useful in myriad applications including the biomedical industry. To capture certain features of this varied response, several constitutive models have been proposed. However, few attempts have been made at the predictive modelling of temperature and moisture driven transition between fast and slowly relaxing states in physical hydrogels. We in this article focus on the changes that occur in the mechanical response due to such phase transitions. We propose a unified and thermodynamically consistent poro-visco-elastic framework for modelling the transition and the associated mechanical behaviour. Towards that, the effective temperature framework that has been previously exploited for such transitions by the authors and other researchers is used. The constitutive model is numerically solved for the uniaxial tensile response of equilibrated hydrogels and also validated against experimental data from the literature. It is also used for certain transient problems involving solvent diffusion with or without mechanical constrains. The aim of this exercise is to assess the predictive fidelity of the model against some standard poro-elastic problems and examine the influence of inter-molecular interactions on the saturation. The numerical studies are concluded with a set of simulations to showcase certain other capabilities of our approach such as modelling of visco-hyperelastic phase transitions. We also investigate through these simulations a few other coupling effects caused by changes in moisture, temperature, loading rates and prior ageing.