Miscible Blends of Two Crystalline Polymers. 1. Phase Behavior and Miscibility in Blends of Poly(vinylidene fluoride) and Poly(1,4-butylene adipate)

Abstract

The phase behavior and miscibility of blends of poly(vinylidene fluoride) (PVF 2 ) 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 °C. A depression of the equilibrium melting point of both PVF 2 and PBA is observed. From the melting point data of the high- T m component, PVF 2, a value for the polymer−polymer interaction parameter of χ 12 = −0.19 was derived using the Flory−Huggins equation. This implies that PVF 2 /PBA blends are thermodynamically miscible in the melt. The extent of the melting point depression for the low- T m component, PBA, is much smaller than it is for PVF 2, which is attributed to the fact that PVF 2 is semicrystalline at temperatures of PBA melting. Infrared spectroscopy measurements focusing on the carbonyl absorption band of PBA reveal a slight shift in peak position toward lower frequencies due to blending, indicating that the thermodynamic miscibility of the PVF 2 /PBA pair arises from weak specific interactions involving the polyester carbonyl group. In spite of their miscibility, blends of PVF 2 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 (PBA and PVF 2 ) coexist with an intimately mixed amorphous phase. This morphology is observed at room temperature over a very broad composition range.