A constitutive model and its numerical implementation for reversible behavior of shape memory hydrogels

Shape memory hydrogels (SMHs) are kinds of smart materials with great importance in many fields, such as drug release and soft robotics. In order to design the structures base on SMHs, it is necessary to reveal the mechanism of the shape memory effect and establish the constitutive model of SMHs. However, the existing constitutive models can not describe some of important mechanical behavior of SMHs, such as reversible shape memory effects. In this paper, a three-dimensional finite deformation constitutive model is developed for SMHs with reversible shape memory effects caused by the conformation transition of N-isopropylacrylamide (PNIPAM). In order to well capture its reversible shape memory effect, the polymer network for PNIPAM is decomposed into two parts, coil PNIPAM and globule PNIPAM, with different reference configurations. Then this model is implemented into a user material subroutine (UMAT) and is used for simulating the equilibrium swelling, isothermal uniaxial tension, reversible shape memory behavior and multiple shape memory cycles of SMHs. Our model is validated by comparing the simulation results with experiments. The deformation of a more complex 3D structure and a bilayer structure containing SMHs are also numerically studied which demonstrates great potential of our model in exploring the application of SMHs.