On the Phase Transitions in Polystyrene-Polyvinylidene Fluoride Blends

Blends of polymeric materials may exhibit complex temperature patterns, resulting in new or modified features. This study focuses on blends between an essentially amorphous polymer (polystyrene, PS) and a semicrystalline polymer (polyvinylidene fluoride, PVDF). Blends of PS and PVDF (PS-PVDF) were obtained by melt mixing and investigated by differential scanning calorimetry (DSC) in the range -75 ^oC to 250 ^oC, at different heating rates ranging from 5 to 50 ^oC/minute. The glass transition of PS was determined by locating the extremum of the derivative of the heat rate with respect to the temperature. The research focuses on the glass transition temperature of the PS component, for both the heating and the cooling branches, as a function of composition and heating rate. The width of the glass transition was also determined. The dependence of the melting/crystallization transition of PVDF on the composition and heating/cooling rates is reported. The enthalpy of melting and crystallization for the PVDF component was estimated. Finally, the glass transition of PVDF on the heating branch was briefly analyzed. It is concluded that the PS-PVDF blend is not perfectly compatible, showing weak interactions. DSC data revealed weak deviations from theoretical predictions, consistent with the formation of an interphase at low concentrations (below 10 % wt.) of the minority phase. The observed behavior was connected to the partial diffusion of PS chains into PVDF domains, with a preference towards amorphous PVDF regions.