Miscible blends of two crystalline polymers .2. Crystallization kinetics and morphology in blends of poly(vinylidene fluoride) and poly(1,4-butylene adipate)

The crystallization kinetics and morphology in miscible blends of poly(vinylidene fluoride) (PVF2) and poly(1,4-butylene adipate) (PBA) have been investigated using optical microscopy and differential scanning calorimetry. Blends of PVF2 and PBA are unique in the sense that both components are capable of crystallization over a wide range of compositions. The kinetics of crystallization of both PVF2 and PBA in these blends have been investigated. During the crystallization of the high-T-m component (PVF2, T-m approximate to 175 degrees C), the low-T-m component (PBA, T-m approximate to 60 degrees C) acts as a noncrystalline diluent. Addition of PBA depresses the radial spherulitic growth rate and overall crystallization rate of PVF2 and affects the texture of PVF2 spherulites but does not significantly alter the nature of the nucleation and growth processes of this polymer. During the crystallization of PBA, the PVF2 phase is always partially solidified and the presence of the spherulitic microstructure of PVF2 profoundly influences the crystallization behavior and morphology of PBA. The overall crystallization rate of PBA goes through a maximum as a function of PVF2 content, which is explained in terms of the combined effects of enhanced heterogeneous nucleation and reduced linear growth rate. The mechanism of crystal growth is modified in the presence of PVF2, as indicated by a change in the Avrami exponent. In the blends, spherulitic crystallization of PBA is not observed, in contrast to the bulk crystallization behavior of this polymer. Crystallization of PBA from the blends initially takes place at the boundaries of PVF2 spherulites, but at longer crystallization times, crystallization of PBA within PCF2 spherulites is also observed. The observed phenomena are unique to blends of two crystalline polymers.