Effects of temperature,strain and strain rate on shape memory thermoplastic polyurethane processed by injection molding

Abstract

Shape memory polymers (SMPs) represent a materials class capable of recovering their initial shape, when submitted to deformations, through an external stimulus. This paper focuses on a Shape Memory Thermoplastic Polyurethane (SMPU), characterized by a one-way shape memory effect (SME) activated by temperature. The material was characterized by Differential Scanning Calorimetry (DSC) to find transition temperature; then, it was manufactured by an injection molding machine to perform a campaign of characterization tests. Thermomechanical tests were conducted varying the prescribed strain (up to 25%, 50%, and 80%), imposed at both different strain rates ( ${10}^{-3}$ , ${10}^{-2}$ , and ${10}^{-1}$ s⁻¹), and temperatures (50°C, 60°C, and 70°C); the results were investigated in terms of shape fixity and shape recovery ratios. An excellent memory capability was observed for all the tested conditions; otherwise, the shape recoverability is strongly influenced by all investigated parameters, especially by the strain rate, resulted to be the most effective one. In fact, the shape recovery ratio can increase as much as 20%, when the strain rate is increased from ${10}^{-3}$ to ${10}^{-1}$ s⁻¹. An opposite effect was observed with the variation of prescribed strain and temperature, the increase of which reduces the ability of the material to return to its original shape.