Melting Point Depression and Kinetic Effects of Cooling on Crystallization in Poly(vinylidene fluoride)-Poly(methyl methacrylate) Mixtures

Thermal analysis of solution cast mixtures of poly(viny1idene fluoride) (PVF2) and poly(methy1 methacrylate) (PMMA) has been carried out with various programmed temperature profiles. Results from experi- ments conducted in a quasi-equilibrium state show that PVF:! crystallizes in the mixture upon cooling. Further- more, depressions were observed in the melting and the crystallization temperatures. The melting point depression phenomenon is found to be explicable in terms of thermodynamic mixing of a crystalline polymer with an amor- phous polymer. To this end, an analytical expression appropriate to the crystalline-amorphous polymer pair is de- rived from Scott's equation for thermodynamic mixing of two polymers. From this expression the interaction pa- rameter for the polymer pair is found to be -0.295 at 16OoC, indicating that the system is compatible in the molten state. The depression of crystallization temperature which depends strongly on both the composition and the cool- ing rate is attributed to the ability of the PVF2 segments to migrate and to the change of composition in the melt during crystallization. For mixtures with a PVF2 content by weight of less than 0.5, it is possible to suppress the crystal transformation at room temperature with a moderate cooling rate. Finally, it is pointed out that because of the strong kinetic effects of cooling on the thermodynamic state of the mixture, caution should be exercised in the calorimetric study of compatibility of the polymer pair.