Miscible blends of two crystalline polymers .1. Phase behavior and miscibility in blends of poly(vinylidene fluoride) and poly(1,4-butylene adipate)

The phase behavior and miscibility of blends of poly(vinylidene fluoride) (PVF2) and poly(1,4-butylene adipate) (PBA), both semicrystalline polymers, have been investigated using differential scanning calorimetry and light-scattering techniques. The phase diagram of this blend system exhibits a single glass transition temperature over the entire composition range, two distinct melting transitions, and a cloud-point curve above a lower critical solution temperature of 235 degrees C. A depression of the equilibrium melting point of both PVF2 and PEA is observed. From the melting point data of the high-T-m, component, PVF2, a value for the polymer-polymer interaction parameter of (chi 12) = -0.19 was derived using the Flory-Huggins equation. This implies that PVF2/PBA blends are thermodynamically miscible in the melt. The extent of the melting point depression for the low-(Tm) component, PBA, is much smaller than it is for PVF2, which is attributed to the fact that PVF2 is semicrystalline at temperatures of PEA melting. Infrared spectroscopy measurements focusing on the carbonyl absorption band of PEA reveal. a slight shift in peak position toward lower frequencies due to blending, indicating that the thermodynamic miscibility of the PVF2/PBA pair arises from weak. specific interactions involving the polyester carbonyl group. In spite of their miscibility, blends of PVF2 and PBA exhibit a complex phase behavior and may form multiphase systems. Of particular interest is the three-phase morphology in which two distinct crystalline phases (PEA and PVF2) coexist with an intimately mixed amorphous phase. This morphology is observed at room temperature over a very broad composition range.