Designing high-modulus polyethylene with lamellar structures

In a preceding work we described a method whereby ultra high modulus filaments of polyethylene of essentially lamellar structure could be produced from the melt by a combination of capillary flow and pressure quenching [1]. Here the lamellae are nucleated by flow induced fibrous crystals formed during the extrusion but present in too small amounts to influence the properties themselves. Yet these microfibrils ensure the particular parallel and mutually interlocking arrangement of lamellae which is the source of the ultra high modulus.In the present work we set out to engineer this interlocking parallel lamellar morphology by utilizing preexisting fibrous crystals, as opposed to relying on their coincidental formation during the extrusion. By a judicious choice of the initial starting material and heat treatment conditions our objective was achieved, illustrating that lamellar self-composites with desirable properties can be achieved by planned design of the micro-morphology.As an additional feature these samples displayed ageing effects which have led to improved properties. Analogous phenomena, termed self stiffening have been observed previously in drawn fibre products [8]. The presently arising example has now allowed its morphological origin to be identified: this is the delayed crystallization by which the interlocking lamellae fill in the residual interstices, the stage at which the corresponding sample acquires its final modulus.