Thermomechanical Characterization of Shape Memory Polymers

Abstract

Data from comprehensive thermomechanical tests of shape memory polymers are reported, with specimens tested up to 75% strain and between 30—120°C temperatures. The data is analyzed and key observations are drawn. The stress/strain behavior during loading at temperatures above glass transition for the Veriflex TM shape memory polymer tested was linear and did not show much variation with the actual temperature. When the polymer is cooled with end constraints, thermally-induced tensile stresses developed, but only after the temperature reduced below glass transition and the material stiffened. When the constraints were then released, 97—98% of the original strain was locked in. Reheating the shape memory polymer beyond the glass transition temperature resulted in shape recovery (shape memory effect). When the polymer was reheated while constraining the strain, the full recovery stress developed was about the stress that the polymer was initially loaded to during deformation at high temperature. Examining the Young's modulus at elevated and low temperature showed that the loading modulus at high temperatures is around 60 times lower than the low temperature unloading modulus of the stretched sample, and around 3000 times lower than the low temperature loading modulus of an unstretched sample.