Effect of miscibility state on crystallization behavior and polymorphism in crystalline/crystalline blends of poly(vinylidene fluoride)/poly(ethylene oxide)

Overall crystallization behavior and polymorphism of partially miscible blends of poly(vinylidene fluoride)/ poly(ethylene oxide) (PVDF/PEO) were systematically investigated. The PVDF/PEO blends were prepared from solution in the whole composition range. Isothermal and non-isothermal experiments were carried out using differential scanning calorimetry (DSC). Since the components of the blend crystallize separately due to high difference of the melting points, the changes in melting, crystallization, and equilibrium melting temperatures of PVDF and PEO as well as the degree of crystallization were evaluated. As the binary blends display an asymmetric lower critical solution temperature (LCST)-type phase diagram, the crystallization behavior and kinetics were strongly affected by the miscibility state of the blends in the melt. Particularly, the overall crystallization of PEO was accelerated in the blends with high level of PEO (> 50 wt%) by two different mechanisms. The formation of phase separated domains at this composition range enhanced the nucleation ability due to the "spinodal decomposition/ interface assisted crystallization". Moreover, presence of preexisting crystals of PVDF could act as a substrate for PEO crystallization and increased the crystallization growth rate. Wide angle X-ray diffraction (WAXD) and Fourier transform infrared spectroscopy (FTIR) were also utilized to study the crystalline polymorphism of PVDF and PEO in the blends.