Crystal orientation behavior and shape-memory performance of poly(vinylidene fluoride)/acrylic copolymer blends

The crystal orientation behavior and shape-memory performance of miscible poly(vinylidene fluoride) (PVDF)/acrylic copolymer blends in various ratios have been investigated. With the incorporation of amorphous acrylic copolymer into the gallery of PVDF lamellae, the molecular connection (tie molecule concentration) between the neighboring PVDF crystals decreases gradually. For the blends with more than 80 wt % PVDF, most of the PVDF α-crystals are transformed into β-crystals upon deformation, and the molecular chains of the β-crystals are aligned along the stretching direction because tie molecules transfer the loading effectively. For the blends with less than 30 wt % PVDF, almost all of the PVDF crystals are isolated from each other with very few tie molecules. Mechanical deformation induces the perpendicular crystal molecular chain arrangement with no crystal form transitions. For the blends with 40 wt % to 70 wt % PVDF, the c-axis-oriented β-phase and the tilt-oriented α-phase coexist in the uniaxially stretched blends. The shape-memory properties of the blends were also evaluated over the same blend composition range. The maximum shape-memory recovery properties were observed for the blend sample containing 50 wt % PVDF, in which a small amount of tie molecules connect the PVDF crystallites to form a deformable elastic network. This network contributes to the good shape-memory properties of the blend. For the blends with very few tie molecules or high tie molecule concentrations, the deformation induces the slipping of the amorphous molecules or the large fibrillar crystal structure, respectively; thus, these samples exhibit relatively low shape-memory performance.