Abstract:The influence of the temperature on the mechanical properties of gel-spun hotdrawn ultra-high molecular weight polyethylene fibers has been investigated.From these experiments two different fracture mechanisms could be distinguished.The results indicate that above 20 ~ a stress-induced orthorhombic-hexagonal phase transition is responsible for fiber failure. In the hexagonal or rotator phase the chains can easily slip past one another and fiber fracture is initiated by creep. Below 20 ~ the phase transition cannot be introduced because the stress needed for the phase transition would exceed the covalent-bond strength in the polyethylene chain. The strength temperature data of the low temperature region was treated with Zhurkov's kinetic concept, leading to a bond-fracture activation energy of 160 kJ/mol and an activation volume of 0.01 nm 3. These values, together with the data from irradiation and shrinkage experiments, indicated that in the low temperature region fiber failure might be initiated by the fracture of trapped entanglements instead of that by overstressed, taut tie molecules.

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SynopsisA study has been made of the mechanism of the fibrous crystallization process of high-molecular-weight polyethylene from dilute solution subjected to Couette flow. Fiber growth has been observed to take place in a gel layer adhering to the surface of the rotating inner cylinder. The fiber is both taper and ribbonlike shaped, which suggests that growth occurs primarily on the lateral surfaces. The crystal growth rate, as determined by measurements of the fiber cross section, is found to depend on the roughness and the chemical nature of the rotor surface, and is independent of the take-up speed. These observations have led to the following conclusions concerning the mechanism. Long-chain molecules adsorb onto the rotor surface. By reptation an entanglement network is formed. It has long relaxation times due to the tendency of the supercooled chains to form embryonic crystallites upon slight orientation. This network is stretched by the entanglement formation with the cilia of a seed crystal, followed by shear stresses. Stretching will lead to further crystallization. The elastically active chain parts between entanglements will lead to the extended-chain backbone crystals. Inelastic chain parts will crystallize as folded platelets of the shish kebab.