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The melting behaviour of gel-spun, ultra-high molecular weight polyethylene fibres was investigated in an attempt to characterize their morphology after various stages of hot-drawing at 148 ~ In this drawing process a shish-kebab morphology is transformed into a smooth fibrillar structure. It was concluded that this transition initially proceeds by pulling elastically inactive loops, originally present in the folded chain lamellae of the shish-kebabs, taut between entanglements. Thereafter a considerable amount of entanglements is removed by pulling molecular chain ends through them, until ca. 2.5 entangelements per molecule remain in the ultimately drawn fibres. The fibrils in the fully drawn fibres were found to be composed of chain-extended orthorhombic crystallites with an average length of 70 nm, which are interrupted by defect regions (containing trapped entanglements and chain ends) of about 4 nm in length. If free shrinkage of the fibre was allowed, this structure had an equilibrium melting temperature of 140.5 ~ Upon constrained melting of the filaments, a solid-solid phase transition could be observed in the DSC thermograms (at a temperature of ca. 150 ~ associated with a transition of the chain-extended orthorhombic blocks in the fibrils into a hexagonal phase. The heat effect associated with this solid-solid transition in perfectly crystalline polyethylene (AHo_ h) was estimated to be 205 kJ/kg, whereas for the heat of fusion of the hexagonal phase (AHh _ m) a value of 81 kJ/kg was assessed.

SynopsisThe nature of the deformation process involved in hot drawing of porous high-molecular-weight polyethylene was examined by apparent elongational viscosity measurements at drawing temperatures between 100°C and 150°C and deformation rates in the range of 10-6-10-3 m/s. The temperature dependence of the apparent elongational viscosity revealed three distinguishable intervals with different activation energies. In the range of 100-133"C, the activation energy amounted to 50 kJ/mol, indicating that hot-drawing in this region proceeds by a sliding motion of separate fibrillar units. The interval between 133°C and 143°C was characterized by an activation energy of about 150 kJ/mol. Moreover, the porous character of the polyethylene fibers was found to decrease in the drawing process above 133OC. These observations were ascribed to an aggregation of the elementary fibrils upon hot-drawing due to partial melting at the surface of the fibrils. At temperatures above 143°C the activation energy was strongly affected by the initial morphology and the draw ratio of the fibers and amounted to values in the range of 200-600 kJ/mol. Molecular orientation in this region is accomplished by a slippage of individual chains, with entanglements acting as semipermanent crosslinks. Decreasing of the rate of elongation in the drawing process resulted in premature fiber breakage, indicating that the crosslinking action of the entanglements is limited by the time scale of the process.

High molecular weight polyethylene can be processed into fibers with tensile strengths at break of about 5 GPa and Young's moduli upto 200 GPa by either applying the 'surface growth" technique or a combination of gel-spinning and hot-drawing. The mechanism of the piplecular process in both techniques involves the oriented crystallization:-ytretching of an entanglement network. The high strength levels are only achieved either by crystallizing the stretched network or by drawing the semi-crystalline gel at relatively high temperatures in order to transform the elastically ineffective chain ends of the entanglement network into the strong backbone structure. Evidence is presented for the presence of entanglements in the ultra-strong polyethylene fibers and for the necessity of entanglement in achieving high strengths by hot-drawing. High fiber production rates of the order of 270 rn/mm. can be attained by adding 1 wt.% of aluminium stearate to the paraffin oil solution. This additive avoids the adsorption of polyethylene on the wall of the die and suppresses meltfracture during extrusion. 'N '6.Aeq.Aeq.5'y 'ss'1 'L17 'j27

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