Gelation and the production of surface grown polyethylene fibres

Summary

In the course of examining the limiting conditions for growth of polyethylene fibres by the surface growth method (1, 2) an unexpected gelation effect was observed. Accordingly a solution, after having been stirred at such elevated temperatures where no visible crystallization can take place, is found to set as a gel after cooling under quiescent conditions. It is established that the ability to form this gel is a necessary condition for fibre production byPennings andZwijnenburg's surface growth method which by the new findings essentially should consist of the stretching of such gels. We verified that material indeed accumulates at the surface of the rotor used in the surface growth method as was originally envisaged, but we now infer that this material consists of gel particles formed during the preceding treatments. This adhesion of the gel to a rotating surface enables it to be readily stretched by an externally introduced fibre as done in the surface growth method. The gel precursor is of interest in its own right. We find that it is a transient structure but with long, up to several hours, persistence time during which the solution `remembers' that it has been stirred. The structure itself is presumably an incipient network forming crystallization induced by localised chain stretching, where the crystal junctions are likely to provide the nuclei for the more permanent fibrous crystals which arise on cooling causing the gel to set. Electron microscopy indeed identified smooth fibres in the gel which could develop into shish-kebabs on appropriate treatment, while the bulk of the gel consists of large single crystal platelets which arise within the network on final cooling to room temperature. Finally we suggest, that many, if not all, shishkebab structures conventionally observed on stirring arise by the stretching of the gel network formed at a preceding stage of the preparation procedure.