In-Situ Simultaneous Synchrotron Small- and Wide-Angle X-ray Scattering Measurement of Poly(vinylidene fluoride) Fibers under Deformation

Abstract

Poly(vinylidene fluoride) (PVDF) fibers spun at different take-up speeds (10.6−61.0 m/min) were subjected to stretch−hold deformation at room temperature and in-situ simultaneous synchrotron small- and wide-angle X-ray scattering measurements. Crystal transformation from α to β form and morphological changes in lamellar and fibrillar structures were analyzed in detail. All fibers were found to yield at an early stage of deformation, resulting in alternating necked and unnecked regions along the fiber. From the two-dimensional (2-D) wide-angle X-ray diffraction (WAXD) patterns, mixed α and β (minor fraction) forms were found to coexist in the undrawn fibers. Deformation assisted in the conversion from the α-form into the β-form. In necked regions, more α to β transformation took place than in unnecked regions. The overall crystallinity index and unit cell parameters of the α form did not change significantly prior to necking. From the 2-D small-angle X-ray scattering (SAXS) patterns, two kinds of equatorial streaks were observed. The first kind originated from fiber of high take-up speed (61.0 m/min) under zero or small strain, and the second kind came from highly deformed fibers (all take-up speeds) in both necked and unnecked regions. These two kinds of equatorial streaks were attributed to the formation of microfibrils and microvoids, respectively. The dimensions of the lengths of microfibrils and microvoids were estimated by Ruland's method. Meridianal scattering maxima from a lamellar morphology were observed in the SAXS patterns in fibers under zero or low strain. The long period of the lamellar structure, estimated using correlation function analysis, increased with strain. Results from SAXS and WAXD analysis suggest that the formation of defects during yielding and plastic flow facilitates the α to β crystal phase transformation, and a phase, similar to conformationally disordered phase, whose density is close to that of crystal, is induced out of the amorphous phase in lamellar structure with application of strain.