Influence of poly(methyl methacrylate) on the structure and phase transition behavior of poly(vinylidene fluoride)

The structure and organization of poly(vinylidene fluoride) (PVDF) spherulites in blends with poly(methyl methacrylate) of high (PMMA-H) and low (PMMA-L) molecular weights, respectively, have been studied in details. It is confirmed that the addition of PMMA-H and PMMA-L have different influence on the morphologies of PVDF spherulites in several aspects, such as the spacing of ring-band α-PVDF spherulites, the kinetics of both α-PVDF crystallization and α−γ′ solid phase transition, as well as the formation of Wagon-Wheel spherulites. First, incorporation of PMMA-L enlarges the band spacing, which is associated to the increased chain mobility of crystallizable PVDF that helps to release the surface stresses of lamellae. In contrast, the low mobility of PMMA-H chains enhances the congestion at fold surfaces, which results in a vigorous twisting and thus the formation of tightened ring-band α-PVDF structure. Second, the PMMA-H reduces the crystallization rate of PVDF more severely than the PMMA-L, resulting in an extremely slow growth rate of α-PVDF spherulites in PVDF/PMMA-H sample at 160 °C, only about one-twenty-third compared to the neat PVDF. Third, the remarkably reduced α-PVDF spherulite growth rate and enhanced α−γ′ transition rate at 160 °C lead to the formation of Wagon-Wheel PVDF spherulites with inner γ′-PVDF crystals and outer γ crystals. It originates from the synchronization of slow α-PVDF growth and accelerated α−γ′ transition rate through homoepitaxy. That is, when the α−γ′ transition with exceeded rate reaches the growth front of the original α-PVDF spherulite, the transformed γ′-PVDF crystals trigger the growth of γ-PVDF crystals directly from melt with non-band feature. These results help to unravel the formation mechanism of Wagon-Wheel PVDF spherulites to be the competition of α-γ′ crystal transformation and α-spherulite radial growth rates.