Structure, Properties, and Phase Transitions of Melt-Spun Poly(vinylidene fluoride) Fibers

Three different experimental techniques were used to study structural phase transitions in melt-spun poly(vinylidene fluoride) fibers, which were produced with different process parameters and processed in the draw-winding process at different temperatures and draw ratios. The fibers are examined with the help of wide-angle X-ray diffraction at elevated temperatures, differential scanning calorimetry with stochastic temperature modulation, and dynamic mechanical analysis. An oriented mesophase and deformed crystal structures can be observed in all fibers and assigned to the mechanical stress occurring in the processes. Furthermore, several phase transitions during melting and two mechanical relaxation processes could be detected. The observed transitions affect the crystal geometry, the orientation distribution, anisotropic thermal expansion, and the mechanic response of the fiber samples. The relaxation processes can be related with an increasing amount of crystalline beta-phase in fibers drawn at different temperatures. The detailed information about phase transitions and the related temperatures are used to produce fibers with an extended amount of b-phase crystallites, which are responsible for piezoelectric properties of the material. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 21-35, 2011