Effects of stretching on crystalline phase structure and morphology of hard elastic PVDF fibers

Abstract

ABSTRACT:The phase characteristics and morphology of stretched hard elastic poly(vinylidene fluoride) (PVDF) fibers were investigated by X-ray diffraction (XRD) and wide-angle and small-angel X-ray scattering (WAXS and SAXS). It was indicated that a and b phases coexisted in stretched PVDF fibers, stretching assisted in a to b phase transformation. The b/a ratios of stretched PVDF fibers were affected by stretching temperature, rate, and ratio. The b phase content of stretched PVDF fibers had an abrupt increase when stretched near 708C, and then it decreased with increasing stretching temperature. Besides, the b/a ratio of PVDF fibers increased with stretching rate and ratio. The total crystallinity of PVDF fibers did not change much even on different stretching conditions. WAXS results indicated that the unstretched and stretched PVDF fibers all exhibited three strong equatorial streaks, with d-spacing (0.964, 0.488, and 0.439 nm) and (0.946, 0.494, and 0.480 nm), which suggested that PVDF fibers still remained the crystalline reflections of c-axis orientation even after being stretched. The long periods of stretched PVDF fibers, calculated from SAXS curves, increased from 19.04 to 39.75 nm. On the basis of these results, the b transformation mechanism of stretched PVDF fibers was also discussed.

Thermal Analysis of Phase Transitions and Crystallization in Polymeric Fibers

“…Possible applications include direction sensitive and spatially resolved strain measurement, which is useful for health monitoring in medical / smart textiles or structural health monitoring in fiber reinforced composites. However, suitable process conditions for fiber spinning, drawing and further processing steps have to be found, whereas phase crystallites have to be formed and not be destroyed along the process chain [39][40][41][42]. Therefore, methods of thermal analysis were developed to identify the presence of the phase, which are validated by additional X-ray diffraction measurements (WAXD) and dynamic mechanical analysis (DMA) [40,42].…”

The improvement of water flux and mechanical strength of PVDF hollow fiber membranes by stretching and annealing conditions

“…3 However, the above two methods both waste a large amount of solvent and the mechanical properties of the membranes made by these methods are relatively poor. 4 Compared with the TIPS and phase inversion processes, the stretching process does not use a large amount of organic solvent; therefore, it may be useful to develop this method for the preparation of PVDF membranes. One of the features that distinguishes PVDF from other polymers is its polymorphism; it can occur in at least four crystalline phases: , , , and .…”

Effect of annealing conditions on crystallization behavior and mechanical properties of NIPS poly(vinylidene fluoride) hollow fiber membranes

“…Generally, strength is one of the key factors that influence the application of PVDF hollow fiber membranes. In addition, the crystalline structure and total crystallinity of PVDF are very important in determining the mechanical properties of membranes as reported by Liu et al17 Therefore, in recent years lots of investigations have been conducted regarding the factors that affect the crystalline phase structure of PVDF hollow fiber membranes,18–28 and a plenty of valuable results about the relationship between fabrication conditions and PVDF polymorph have been obtained. However, the influences of annealing conditions on crystallization behavior and mechanical properties of PVDF hollow fiber membranes have seldom been reported and little is known on how to control the crystalline phase structure of PVDF hollow fiber membranes through convenient choice of annealing conditions.…”