Structure-property relationships in high-strength high-modulus polyethyelene fibres

Six commercially available high-strength, high-modulus polyethylene fibres have been studied with the aim of developing a deeper understanding of their structure-property relationships. Structural studies have been carried out on the measurement of crystallite size and orientation parameters using a wide range of techniques including optical microscopy, thermal analysis and wide-angle X-ray diffraction techniques, together with polarised IR spectroscopy. Birefringence, melting temperatures, melting enthalpies and crystallinity values are found to increase with increasing tensile strength and modulus as a direct result of the increased chain alignment induced during the fibre-drawing stages. Careful examination of the results show that improving the chain alignment encourages the alignment and lateral perfection of unoriented non-crystalline chains. Analysis of wide-angle X-ray diffraction, polarised IR spectroscopy and thermal analysis confirms the presence of a three-phase structure consisting of polymorphic crystalline (i.e. orthorhombic and monoclinic), highly oriented non-crystalline (i.e. paracrystalline) and unoriented non-crystalline (i.e. amorphous) structures, as opposed to the classical two-phase (crystalline and amorphous) structures. Analysis of polarised IR data shows the existence of a polymorphic structure consisting of highly oriented orthorhombic and monoclinic crystallites, together with highly oriented non-crystalline structure containing pseudo-hexagonally packed chains. Analysis of second order orientation parameter <P-200>, shows that the monoclinic crystallites are more oriented than the orthorhombic crystallites. The analysis of the X-ray diffraction data suggests that increasing crystallinity and lateral perfection leads to a consistent reduction in the proportion of the oriented non-crystalline structure. A scanning electron microscopy (SEM) examination of the longitudinal views showed fibrillar striations along the fibre axis direction, together with relatively smooth surfaces with increasing tensile moduli. The samples showed excellent bending behaviour advantageous for the formation of fabrics. The cross-sectional images appeared to vary from deformed triangular to deformed circular shapes.