Molecular structure and orientation of gel-spun polyethylene fibers

Characterization of molecular structure and orientation of six commercially available gel-spun polyethylene fibers have been carried out using infra-red and Raman spectroscopy, thermal and X-ray diffraction analysis, together with optical microscopy techniques. Thermal and X-ray diffraction analysis revealed the existence of highly oriented orthorhombic and monoclinic crystallites together with a highly oriented intermediate phase known as pseudohexagonal mesophase structure. The results suggest the existence of a three-phase structure consisting, at room temperature, of orthorhombic and monoclinic polymorphic crystallites, oriented noncrystalline and un-oriented noncrystalline (amorphous) phases, respectively. The crystallinity measurements have been carried out using density, thermal and X-ray diffraction analysis together with infra-red and Raman spectroscopy techniques whereas the molecular orientation measurements have been carried out using birefringence and polarized IR spectroscopy, respectively. The results obtained from density, thermal analysis, and Raman spectroscopy based on a simple two-phase modeling approach lead to the overestimated amorphous fractions and appears to ignore the presence of an intermediate phase known as oriented noncrystalline structure. X-ray analysis has also been used for the measurement of the apparent crystallite sizes. The birefringence values have been used to determine the overall orientation parameters whereas the dichroic measurements of IR bands have been used to determine the crystalline and oriented noncrystalline orientation parameters. The results show that the orthorhombic and monoclinic phases are more highly oriented than the oriented pseudohexagonally packed noncrystalline chains. (c) 2006 Wiley Periodicals, Inc.