Deformation of epoxy shape memory polymer foam. Part I: Experiments and macroscale constitutive modeling

Shape memory polymer foams have both shape memory properties and attributes of low density and compressibility of foams. Potential applications for these materials span from embolic sponges for biomedical uses to morphing wings on advanced airframes. This study focuses on how the relative density of the foam affects macroscale response to deformation. Epoxy shape memory foams with relative densities of 20, 30%, and 40% and a glass transition temperature (Tg) near 85 °C as measured by dynamic mechanical analysis were tested in compression. Micro-CT scans were used to characterize the micro-architecture for each relative density. Tensile test data showed a temperature dependence on the effect of relative density on tensile strain-to-failure. Compression tests demonstrated similar effects of relative density at different temperatures. Unconstrained shape recovery tests showed no effect of relative density on free strain recovery, while constrained stress recovery showed a strong effect of relative density. Relative density did have a slight effect on constrained cooling, which was demonstrated to be controlled by viscous relaxation followed by thermal stress relaxation. A model from the Gibson and Ashby on cellular solids was used to simulate the effect of relative density on the macroscale stress-strain properties. The prediction of the effect of relative density on modulus correlated well with the DMA data, and the compressive response was fit to each compression curve. However, it was necessary to modify the prediction for the densification strain and allow it to depend on temperature for the fits to correlate with experimental data at higher strains. The fitting parameters were fairly temperature independent once the material was above Tg.