Polymorphic Extended-Chain and Folded-Chain Crystals in Poly(vinylidene fluoride) Achieved by Combination of High Pressure and Ion-Dipole Interaction

Manipulating polymorphism in extended chain-crystals (ECCs), which are commonly achieved by crystallization under high pressures, is important for enriching our understanding of basic polymer crystallization as well as for achieving high performance materials. In this study, the influence of high pressure and ion-dipole interaction on the polymorphism was investigated by comparing neat poly(vinylidene fluoride) (PVDF) and PVDF with 1 wt % cetyltrimethylammonium bromide (CTAB) nonisothermally crystallized from the melt at 210 degrees C. Under low pressures (<= 10 MPa), gamma folded-chain crystals (FCCs), rather than alpha FCCs, were obtained for PVDF/1 wt % CTAB because of the ion dipole interaction. Under a moderate pressure (100 MPa), pure beta FCCs were formed in PVDF/1 wt % CTAB, owing to the synergistic effect of both high pressure and ion dipole interaction. Under high pressures (>= 200 MPa), mixtures of beta/gamma FCCs and ECCs were obtained for PVDF/1 wt % CTAB. This was different from the neat PVDF, where mixtures of alpha FCCs and alpha/gamma/beta ECCs coexisted when the pressure was between 200 and 400 MPa. The formation mechanisms of various crystalline forms and FCCs versus ECCs during the nonisothermal crystallization are discussed using the T-P phase diagram for PVDF.