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 alpha-PVDF spherulites, the kinetics of both alpha-PVDF crystallization and alpha- gamma ' 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 alpha-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 alpha-PVDF spherulites in PVDF/PMMA-H sample at 160 degrees C, only about one-twenty-third compared to the neat PVDF. Third, the remarkably reduced alpha-PVDF spherulite growth rate and enhanced alpha- gamma ' transition rate at 160 degrees C lead to the formation of Wagon-Wheel PVDF spherulites with inner gamma '-PVDF crystals and outer gamma crystals. It originates from the synchronization of slow alpha-PVDF growth and accelerated alpha- gamma ' transition rate through homoepitaxy. That is, when the alpha- gamma ' transition with exceeded rate reaches the growth front of the original alpha-PVDF spherulite, the transformed gamma '-PVDF crystals trigger the growth of gamma-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 alpha-gamma ' crystal transformation and alpha-spherulite radial growth rates.