Modeling the relaxation mechanisms of amorphous shape memory polymers. 
In this progress report, we review two common approaches to constitutive modeling of thermally activated shape memory polymers, then focus on a recent thermoviscoelastic model that incorporates the time-dependent effects of structural and stress relaxation mechanisms of amorphous networks.  An extension of the model is presented that incorporates the effects of multiple discrete structural and stress relaxation processes to more accurately describe the time-dependent behavior.  In addition, a procedure is developed to determine the model parameters from standard thermomechanical experiments.  The thermoviscoelastic model was applied to simulate the unconstrained recovery response of a family of (meth)acrylate-based networks with different weight fractions of the crosslinking agent.  Results showed significant improvement in predicting the temperature-dependent strain recovery response.