Thermo-aqueous coupling behaviors for shape memory hydrogels: A statistical–mechanical model and simulations faced with experiments

The shape memory behavior of hydrogels endows themselves with the capacities of shape programming and shape recovering, which have great potential in many fields, such as drug release and soft robotics. In thermally responsive hydrogel, particularly, there are two critical factors, temperature and water, that can induce the shape memory effect(SME). Temperature-induced SME has been extensively studied, while water-induced SME has been rarely investigated. In our work, the combined effect of water and temperature on shape memory behavior of hydrogels is experimentally studied and deeply analyzed from a physical perspective. We found that the shape fixed ratio of shape memory hydrogels (SMHs) increases monotonically with temperature, but first increases and then decreases with water content. Moreover, it exits a nonlinear coupling between the water and temperature effects on the SME. On the basis of these findings, a constitutive model is developed to capture this thermo-aqueous coupling using the statistical theory of transient networks, from which we obtain an explicit evolution equation of deformation gradient. In the proposed theory, the temperature-induced SME is investigated through the temperature-water responsive association and dissociation of dynamic bonds while the water-induced SME is explained by the variation of the glass transition temperature according to the Fox–Flory model. Finally, and to provide a final test of the theoretical work, it is implemented into a user element (UEL) subroutine which provide simulations in agreement with our experiments. Our work is expected to provide guidance for future application of SMHs. •The coupling effect of water and temperature on shape memory behavior of hydrogels was experimentally studied.•The mechanics of thermo-aqueous shape memory effect was investigated.•A constitutive model was developed to capture this thermo-aqueous coupling using the statistical theory of transient networks, and was implemented into a user element (UEL) subroutine which provide simulations in agreement with our experiments.•We proposed a new solution for the fabrication of micro-devices with sophisticated configurations of actuators and functional components based on the thermo-aqueous coupling effect of SMHs.