Strategy for Fabricating Multiple-Shape Memory Polymeric Materials Based on Solid State Mixing

Traditionally, multiple shape memory polymers (multiple-SMPs) are created by forming either immiscible blends with high phase continuity (cocontinuous or multilayer phase morphology) or miscible blends that exhibit compositional heterogeneity at the nanoscale. Here, a new strategy for the fabrication of multiple-SMPs is proposed. It consists of the possibility of homogeneous mixing of immiscible polymers in the solid state under high pressure and shear deformation conditions. The blends formed in this way exhibit homogeneity of mixing down to the nanoscale, up to 40-95 nm. The transition from immiscible to miscible blends leads to an improvement not only in shape memory but also in the mechanical performance of the blends formed. Polypropylene (PP) and polystyrene (PS) were selected as pairs of immiscible polymers. The method of solid phase mixing is high pressure torsion (HPT). It was shown that the HPT-processed 50% PP/50% PS blend is able to exhibit an excellent triple shape memory effect (shape fixation of ∼94-95%, and recovery of ∼85-95%) with widely tunable (low and high) transition temperatures.