Formation of poly(vinylidene fluoride) crystalline phases from tetrahydrofuran/N, N-dimethylformamide mixed solvent

Abstract

Poly(vinylidene fluoride) (PVDF) is one of the semicrystalline polymers with at least four crystalline forms referred to as a, b, c, and d phase, among which the b phase has gained substantial importance due to its advantageous pyroand piezo-electric properties [1, 2]. It requires a better method to induce the formation of b phase crystal in order to improve the electrical properties of PVDF. b phase is not usually obtained by crystallization from the melt, but it is normally obtained by various methods including tensile deformation and uniaxial compressional deformation of a phase [3, 4], blending with small contents of poly(methyl methacrylate) (PMMA) [5, 6] or poly(o-methoxyaniline) (POMA) [7], quenching and then annealing process [8], applying a strong electric field [9] and crystallizing from solution at appropriate conditions [10–12]. In all these methods which used to prepare b phase of PVDF, the one grown from a single solvent is widely used because of its facility and controllability. However, few literatures are concentrated on the solvent-induced b phase formation in the mixed solvent. In this letter a novel method to achieve the b phase of PVDF is carried out by crystallizing from swelling agent/solvent mixture. Dissolution of a semicrystalline polymer, such as PVDF, requires disruption of strong interchain forces in order regions. So the rate of polymer dissolution is dependent on the type of liquid [13]. It has been reported that tetrahydrofuran (THF) is a good swelling agent at 60 C for PVDF; N, N-dimethylformamide (DMF) is a solvent for it at 20 C [13]. The aim of this study is to include the effects of the THF/ DMF mixture with various mass ratios of each component on the crystalline phase formation and crystallization behavior in the PVDF film by solution casting. It was characterized by Fourier transform infrared (FTIR), wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and scanning electron microscope (SEM) techniques. The PVDF (Kynar K-761, Mw = 441000) utilized in this work was supplied by Elf Atochem of North America Inc. (USA). PVDF resin was dissolved in the THF/ DMF mixture with different mass ratio (m/m composition of THF/DMF = 9:1, 8:2, and 5:5) at 50 C. The solubility of the PVDF was improved by an increase of DMF content in the mixture of solvents because of the reinforced disruption of the intermolecular polymer bonds (via interaction of the C = O dipole with the CH2CF2 dipole or by limited hydrogen bonding) [14, 15]. Films (thickness: *50 lm) of PVDF were quiescently cast from each solution with 10 wt% PVDF (initial polymer concentration) at 50 C, and the glass substrate was used for solution casting. After the solvents evaporated the films were peeled off from the glass substrate at room temperature. The residue of DMF and THF was allowed to evaporate in an air oven for about 1 week at room temperature. FTIR spectra of films were obtained on a FTIR spectrometer (Bruker Vector-22). WAXD was done in a Shimadzu XRD-6000 diffractometer (Cu Ka radiation, 40 kV and 30 mA) at a scanning velocity of 4 /min. The melting behavior of the films was measured with a Perkin– Elmer DSC-7C differential scanning calorimeter using a heating rate of 10 C/min. SEM micrographs were taken on a Jeol JSM-5900 microscope. W. Ma J. Zhang (&) College of Materials Science and Engineering, Nanjing University of Technology, No.5, Xin Mo Fan Road, Nanjing 210009, P.R. China e-mail: zhangjun@njut.edu.cn