Crystallization of shape memory polymer blends

Shape Memory Polymers (SMP) blends are of particular interest because they are easily processible, commercially available and recyclable. In this paper we study blends of PVDF (semi-crystalline, T_g=-35 °C, T _m=173 °C) and PVAc (amorphous, T_g=33 °C) which are miscible at all blending ratio (presence of a single T_g). Effects of composition as well as thermal histories on crystallization were investigated via differential scanning calorimetry (DSC), parallel plate rheology and polarizing optical microscopy (POM). For the first time, evidence for two distinct crystalline forms was observed in a blend with PVAc and PVDF crystallized for all blending ratios. Rheological measurements show that the rubber plateau of the blend decreases with lower PVDF content because of lower density of physical crosslinks. Surprisingly, in the case of 30:70 PVDF:PVAc the highest rubber plateau was achieved by isothermal crystallization at temperature close to, but above, the crystallization point measured by DSC Comparing rheological and optical results for crystallization kinetic, it can be inferred that PVDF crystallizes faster near a solid interface than in the bulk.