Phase transitions, structure evolution, and mechanical properties of blends of two crystalline polymers: poly(vinylidene fluoride) and poly(butylene adipate)

The emergence of crystalline structures during phase transitions, involving crystallization and phase separation in blends of poly(vinylidene fluoride) (PVF(2)) and poly(butylene adipate) (PBA) has been examined by means of time-resolved depolarized light scattering, polarizing optical microscopy, and differential scanning calorimetry. These PVF(2)/PBA blends are known to exhibit various phase transitions including liquid-liquid phase separation, melting of the PVF(2), melting of the PBA, and a single glass transition, in the order of descending temperature. Several thermal quench experiments were undertaken, from the isotropic melt (200 degrees C) to various crystallization temperatures at several blend compositions, to elucidate the emerging morphology and the crystallization kinetics in competition with phase separation. The development of the crystalline morphology is shown to be strongly dependent on the blend composition and crystallization temperature. Of particular interest is the observation that at PBA-rich compositions, nematic mesophase structures are formed even though the constituents do not contain mesogenic groups. Samples quenched from the isotropic melt (200 degrees C) to low temperatures (e.g., 25 degrees C) develop interconnected spinodal decomposition textures, and this observation is discussed in relation to the possible existence of an upper critical solution temperature (UCST). Tensile properties of high temperature crystallized blends with a spherulitic morphology have been compared with those of the thermal quenched blends having a periodic modulated crystalline structure. It is concluded that the mechanical properties are strongly dependent on the complex interplay of crystalline structure and phase-separated blend morphology. (C) 1998 Elsevier Science Ltd. All rights reserved.