In-situ simultaneous synchrotron small- and wide-angle X-ray scattering measurement of poly(vinylidene fluoride) fibers under deformation

Poly(vinylidene fluoride) (PVDF) fibers spun at different take-up speeds (1 0.6-61.0 m/min) were subjected to stretch-hold deformation at room temperat ure and in-situ simultaneous synchrotron small- and wide-angle X-ray scatte ring measurements. Crystal transformation from alpha to beta form and morph ological changes in lamellar and fibrillar structures were analyzed in deta il. All fibers were found to yield at an early stage of deformation, result ing in alternating necked and unnecked regions along the fiber. From the tw o-dimensional (2-D) wide-angle X-ray diffraction (WAXD) patterns, mixed alp ha and beta (minor fraction) forms were found to coexist in the undrawn fib ers. Deformation assisted in the conversion from the alpha-form into the be ta-form. In necked regions, more alpha to beta transformation took place th an in unnecked regions. The overall crystallinity index and unit cell param eters of the a 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 fr om highly deformed fibers (all take-up speeds) in both necked and unnecked regions. These two kinds of equatorial streaks were attributed to the forma tion of microfibrils and microvoids, respectively. The dimensions of the le ngths of microfibrils and microvoids were estimated by Ruland's method. Mer idianal scattering maxima from a lamellar morphology were observed in the S AXS patterns in fibers under zero or low strain. The long period of the lam ellar structure, estimated using correlation function analysis, increased w ith strain. Results from SAXS and WAXD analysis suggest that the formation of defects during yielding and plastic flow facilitates the alpha to beta c rystal phase transformation, and a phase, similar to conformationally disor dered phase, whose density is close to that of crystal, is induced out of t he amorphous phase in lamellar structure with application of strain.