X-RAY STRUCTURAL STUDY OF ORIENTED VINYLIDENE FLUORIDE TRIFLUOROETHYLENE COPOLYMERS

The orientation of the crystalline regions in the vinylidene fluoride/trifluoroethylene (VDF/TrFE)-copolymer with molecular ratio 70:30 has been studied by wide-angle X-ray diffraction. This copolymer crystallizes in the beta-phase or form I upon compression moulding. Dumb-bells 90 mm wide by 50 mm long were drawn at 0.25 mm min-1 and 80-85-degrees-C to the natural draw ratio of 7; the oriented sheets were void-free, and the crystal phase remained form I. Some samples were then poled at 200 MV m-1. Strips 0.5 mm wide of both unpoled and poled samples were examined in a two-circle X-ray diffractometer, and all equatorial omega-scans corrected for absorption of the X-ray beam. Both (200/110) and (310/020) composite reflections then showed six distinct intensity maxima round the draw direction, separated by 60-degrees from each other, indicating a high degree of uniplanar-axial orientation within the drawn sheets. Calculation of the structure factors from two crystal models and comparison with the measured intensity ratios round the draw direction by curve-fitting methods showed that most of the polar b axes of the unpoled material are in the sheet plane (bc sheets), and that poling increases the number of these axes taking on positions 60-degrees from this plane, in line with similar results in the poly(vinyldene fluoride) homopolymer.