Cocrystallization of poly(vinylidene fluoride) and vinylidene fluoride-tetrafluoroethylene copolymers: 2. Thermodynamic study

Abstract

The phase diagrams of poly(vinylidene fluoride) (PVF₂; KF and KY) and vinylidene fluoride-tetrafluoroethylene (VF₂-VF₄) copolymers (Cop-1, Cop-2) have been reported. They are the KF/KY, Cop-1/Cop-2, KF/Cop-1 and KY/Cop-1 systems. The head to head (H-H) defect structures of KF, KY, Cop-1 and Cop-2 are 3.5, 5.3, 15.8 and 21.3 mol%, respectively. The KF/KY and Cop-1/Cop-2 blends exhibit linear variation of apparent melting points with composition but the KF/Cop-1 and KY/Cop-1 blends show a bifurcation of melting point-composition diagram at the Cop-1 rich regions. The crystallization temperature (T_c)-composition plots are linear except for the KF/Cop-1 system. The anomaly of the KY/Cop-1 system in this regard has been explained. The equilibrium melting points (T_m⁰) of the blends were determined using the Hoffman-Weeks procedure. The KF/KY blends show a slightly concave upward T_m⁰ vs. composition curve indicating weak interaction of the components. The Cop-1/Cop-2 blends exhibit a linear plot over the same composition due to ideal mixing. Both the KF/Cop-1 and KY/Cop-1 blends exhibit a concave upward curve with some concave downward portion for the T_m⁰ vs. composition plots at the Cop-1 rich regions. Further, at the Cop-1 rich regions (W_Cop-1 = 0.75), three melting peaks of isothermally crystallized KF/Cop-1 and KY/Cop-1 blends were observed when melted from the T_cs. Liquid-liquid phase separation in the melt for the KF/Cop-1 and KY/Cop-1 systems at Cop-1 rich compositions has been attributed to the different behaviour rather than observed from the other pairs. The cocrystallization of this system is complicated by both the crystalline phase segregation and the liquid-liquid phase segregation.