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In a previous work it was shown that, by appropriate melt processing, oriented filament polyethylene plugs could be produced such as possessed ultra-high modulus in spite of containing predominantly lamellar structures, the advantageous properties being due to the parallel and interlocking arrangement of the lamellae. In the present work it is demonstrated how the extreme high end of the molecular weight distribution is instrumental in the attainment of such structures. The longest chains, even if present in very small amounts, produce fibrous crystals during extrusion which are inadequate to influence the properties in themselves but by serving as nuclei for lamellar crystallization determine the detailed arrangements and hence the mechanical effectiveness of the lamellar texture. We show examples of how sensitively these structures and the resulting properties can be influenced by slight variations in the high molecular weight content of the material. In fact, materials which otherwise would not produce the desired effect can be made to do so through prior blending with a few percent of ultra high molecular weight material. By judicious creation of bimodal distributions, however, the previous batch production of high modulus plugs with lamellar structures can be turned into a continuous spinning process, thus for the first time achieving the production of oriented, high modulus filaments in the course of a single-step continuous extrusion.

In a previous work, it was shown that highly oriented fibres with 10GPa modulus could be obtained by continuous single-stage melt extrusion of a medium molecular weight polyethylene to which 3% ultra-high molecular weight (/ldw ~ 3 to 5 x 106 ) material had been added by solution blending. It was demonstrated that a special interlocking shish kebab structure was responsible for the favourable mechanical properties. In the present work, we succeeded in achieving the same effect from an unblended polyethylene by choosing starting materials with an inherently suitable molecular weight distribution. Both the high and low molecular weight tails of the distribution are very influential: the high tail contributes to the formation of extended-chain fibrils (which constitute the backbones of the shish kebabs), while the low tail affects melt extrudability and strength. Melt strength is important because unusually high tensile stresses are required during wind-up. The wind-up stress was measured and found to be an order of magnitude greater than that encountered in conventional melt spinning -where no shish kebabs are formed. The implications of the above findings for polymer processing, crystal morphology and melt rheology are discussed.

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