On the morphology of poly(vinylidene fluoride) crystals in blends

The crystallization of poly(vinylidene fluoride) (PVF 2) in homogeneous blends with poly(methyl methacrylate) (PMMA), a copolymer of styrene and MMA (PSMMA) or poly(methyl acrylate) (PMA) was investigated to establish the conditions leading to dendritic crystals. These unusual crystals feature branches, from which grow twigs, so that a tree-like, i.e. dendritic, structure results. In blends with PMMA or PSMMA, but not in blends with PMA, the α modification of PVF 2 forms such dendrites, while the γ modification forms more or less perfect spherulites. The dendritic crystals appear at moderate undercoolings, up to surprisingly high PVF 2 concentrations. The various spherulitic and dendritic structures observed in the blends can be explained in terms of a competition of crystallization and diffusion that produces concentration gradients in the melt.