Prediction of temperature-dependent free recovery behaviors of amorphous shape memory polymers

Shape memory polymers (SMPs) are active materials that can fix a temporary shape and recover the permanent shape in response to environmental stimuli such as temperature, light, moisture or magnetic field. In order to provide insight into the mechanism for shape memory behavior and to predict the behaviors of targeted design, several constitutive models were developed in the past. Most of these models are complicated and require time-consuming experiments to obtain model parameters. However, for many engineers, an estimation of key features of shape memory behaviors, such as time for free recovery, is sufficient. Such estimation should be based on a simplified model involving only a few key parameters that can be quickly identified experimentally. In this paper, a simple theoretical solution was developed to predict the temperature dependent free recovery behaviors of amorphous SMPs. This solution is based on a modified standard linear solid (SLS) model with a Kohlrausch-Williams-Watts (KWW) stretched exponential function and requires only eight parameters that can be determined by stress relaxation tests. The theoretical predictions of free recovery behaviors show a good agreement with experimental results. Parametric studies using this solution reveal that the free recovery time can be reduced by increasing the equilibrium modulus (E-0) or KWW stretching parameter (beta), or by decreasing the nonequilibrium modulus (E-1) or the relaxation time (tau(0)) and is most sensitive to the KWW stretching parameter beta.