Conformational changes and phase transformation mechanisms in PVDF solution-cast films

The supramolecular crystal structure in poly(vinylidene fluoride) (PVDF) solution-cast films is studied through changing crystallization conditions in two solvents of different structures and polarities. The crystalline-state chain conformations of isothermally solution-crystallized PVDF in N, N-dimethylacetamide (DMAc), and cyclohexanone are studied through the specific FTIR absorption bands of alpha, beta, and gamma phase crystals. There are no changes in the FTIR spectra of cyclohexanone solution-crystallized films in the temperature range of 50-120 degreesC. In the case of DMAc solution-crystallized films, low temperature crystallization mainly results in formation of trans states (beta and gamma phases), whereas at higher temperatures gauche states become more populated (alpha phase). This is due to the variations in solvent polarity and ability to induce a specific conformation in PVDF chains, through the changes in chain coil dimensions. This indicates that in spite of cyclohexanone solutions, the intermolecular interactions between PVDF and DMAc are temperature-sensitive and more important in stabilizing conformations of PVDF in crystalline phase than temperature dependence of PVDF chain end-to-end distance . The high-resolution F-19 NMR spectroscopy also showed little displacement in PVDF characteristic chemical shifts probably due to changes in PVDF chain conformation resulting from temperature variations. Upon uniaxial stretching of the prepared films under certain conditions, contribution of trans state becomes more prominent, especially for the originally higher alpha phase-containing films. Due to formation of some kink bands during film stretching and phase transformation, alpha phase absorption bands are still present in infrared spectra. Besides, uniaxial stretching greatly enhances piezoelectric properties of the films, maybe due to formation of oriented beta phase crystals, which are of more uniform distribution of dipole moments. (C) 2004 Wiley Periodicals, Inc.