STRUCTURAL TRANSFORMATION OF ULTRA-HIGH MODULUS AND MOLECULAR-WEIGHT POLYETHYLENE FIBERS BY HIGH-TEMPERATURE WIDE-ANGLE X-RAY-DIFFRACTION

Wide-angle x-ray diffraction (WAXD) of the ultra-high modulus and mole cular weight polyethylene (UHMWPE) fibers at room temperature shows a predominantly orthorhombic structure with trace amount of nonorthorhom bic crystals and very low amorphous contents. The calculated unit cell dimensions a and b of the orthorhombic crystals are 7.36 (+/-0.04) An gstrom and 4.89 (+/-0.04) Angstrom, respectively. The apparent crystal lite sizes perpendicular to the orthorhombic 110 and 200 reflection pl anes are 169.8 and 143.4 Angstrom, respectively. The crystallite size perpendicular to the nonorthorhombic 010 reflection is 149.4 Angstrom. The crystal density is calculated to be 1.02 g/cc. With increasing te mperature, the thermal expansion coefficient in the a direction is muc h higher than that in the b direction which explains the structural tr ansformation from the orthorhombic crystals to a pseudohexagonal form. Tension along the fiber axis while being heated during the high-tempe rature x-ray diffraction (HTWAXD) scanning has shown enhanced structur al transformation from the orthorhombic form to the monoclinic form. S tructural transformation from the orthorhombic form to the pseudohexag onal phase is not observed on the UHMWPE fibers under axial tension or annealing conditions in HTWAXD. (C) 1997 John Wiley & Sons, Inc.