Melting Point Depression and Kinetic Effects of Cooling on Crystallization in Poly ( viny 1 idene 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 experiments conducted in a quasi-equilibrium state show that PVF:! crystallizes in the mixture upon cooling. Furthermore, 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 amorphous polymer. To this end, an analytical expression appropriate to the crystalline-amorphous polymer pair is derived from Scott’s equation for thermodynamic mixing of two polymers. From this expression the interaction parameter 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 cooling 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. In previous studies2 concerned with compatibility of polymer-polymer systems it has been found helpful to interpret the miscibility of the mixtures in terms of their phase diagrams. It has also been shown that the thermal history of the mixture has a pronounced effect on the mixing behavior of the polymer pair when the experimental temperature approaches or falls below the glass transition temperature of the m i x t ~ r e . ~ . ~ These studies have been confined so far to the amorphous polymer pair or, in the case of a crystalline-amorphous polymer pair, the mixing of amorphous fraction of the crystalline polymer and the amorphous polymer. An examination of the compatibility between a crystalline polymer and an amorphous polymer therefore becomes a natural extension of our general study. Recently, it was reported in the literature that poly(vinylidene fluoride) (PVF2) and poly(methy1 methacrylate) (PMMA) form a compatible mixture when blended in the melt.sg It was also reported that PVFz crystallizes in the Macromolecules 910 Nishi, Wang Table I Molecular Weight Distribution Data for Poly(vinylidene fluoride) (PVF,) and Poly(methy1 methacrylate) (PMMA) Sample# ~~ ~ _ _ p.z M w lMn M J M , _____ JTn Z W PVF, 215800 404300 661000 1.87 1.63 PMMA 36600 9150