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Spherulitic morphologies and overall crystallization kinetics of miscible poly(vinylidene fluoride) (PVDF)/poly(butylene succinate-co-butylene adipate) (PBSA) blends were studied by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD). PVDF and PBSA crystallize separately in the blends. For the high-T m component PVDF, the crystallization mechanism does not change, while both the spherulitic growth rate and the overall crystallization rate of PVDF decrease with increasing crystallization temperature and the PBSA content in the blends. Much more attention has been directed to the spherulitic morphologies and overall crystallization kinetics of the low-T m component PBSA, which are affected not only by blend composition and crystallization temperature but also strongly by the preexisting crystals of the high-T m component PVDF in the blends. PBSA spherulites nucleate in the matrix of the PVDF spherulites and continue to grow until impinging on other PBSA spherulites. The overall crystallization rate of PBSA is accelerated in the blends compared with that of neat PBSA, which reduces with increasing crystallization temperature despite blend composition for the PBSA-rich blends. The presence of the preexisting PVDF crystals shows two opposite effects on the overall crystallization of PBSA, i.e., enhanced nucleation ability and slower crystal growth rates.

In a previous short communication (Macromolecules 2002, 35, 8251-8254), we reported a unique morphology in binary miscible crystalline poly(ethylene succinate) (PES)/poly(ethylene oxide) (PEO) polymer blends, in which two kinds of spherulites, corresponding to each component, respectively, grew simultaneously and PEO spherulites continued to crystallize inside PES spherulites on contact, forming interpenetrated spherulites. In the present work the effects of blend composition and crystallization temperature on the crystalline morphologies of PES/PEO blends were studied by optical microscopy in detail. The unique morphology of PES spherulites penetrated by PEO spherulites was observed for the blends with PEO content ranging from 50 to 90% in a wide crystallization temperature range. Furthermore, three novel and interesting crystalline morphologies were found depending on blend composition and crystallization temperature. First, new boundaries were formed by the growth of a single PEO spherulite itself after penetrating into PES spherulites because of the change in the growth direction. Second, both components penetrated each other and formed interpenetrated spherulites. Third, PES and PEO formed a special kind of blended spherulite that nucleated from the same site and showed two growth fronts with PEO growing faster than PES. The growth rates of both components were studied; furthermore, much more attention has been directed to the study of the observed growth rates of both types of spherulites when they developed freely in the undercooled melt and when they were forced to penetrate each other. The blend composition and crystallization temperature effects on the various crystalline morphologies were summarized in PES/PEO polymer blends, which should be of great interest and importance for a better understanding of the crystallization and morphology in miscible crystalline/crystalline polymer blends.

Poly(vinylidene fluoride) (PVDF) and poly(butylene succinate-co-butylene adipate) (PBSA) are crystalline/crystalline polymer blends with PVDF being the high-T(m) component and PBSA being the low-T(m) component, respectively. PVDF/PBSA blends are miscible as shown by the decrease of crystallization peak temperature and melting point temperature of each component with increasing the other component content and the homogeneous melt. The low-T(m) component PBSA presents various confined crystalline morphologies due to the presence of the high-T(m) component PVDF crystals by changing blend composition and crystallization conditions in the blends. There are mainly three different types of crystalline morphologies for PBSA in its miscible blends with PVDF. First, crystallization of PBSA commenced in the interspherulitic regions of the PVDF spherulites and continued to develop inside them in the case of PVDF-rich blends under two-step crystallization conditions. Second, PBSA spherulites appeared first in the left space after the complete crystallization of PVDF, contacted and penetrated the PVDF spherulites by forming interpenetrated spherulites in the case of PVDF-poor blends under two-step crystallization condition. Third, PBSA spherulites nucleated and continued to grow inside the PVDF spherulites that had already filled the whole space during the quenching process in the case of PBSA-rich blends under one-step crystallization condition. The conditions of forming the various crystalline morphologies were discussed.