Aspects of the morphology and melting behavior of constrained polyethylene fibers

The internal microstructure of a high performance polyethylene fiber, both as received and after heating, has been investigated by electron microscopy following permanganic etching. There is longitudinal fibrosity extending over many mu m but, in common with other high performance fibers, this is accompanied by internal lateral cratering on the scale of similar to 1 mu m. The craters, which are a response to preferential penetration by the etchant into regions of reduced density and high free volume related to processing conditions, are centered on the first regions to melt. Melting of constrained fibers is from the hexagonal phase to an ordered melt which relaxes making the hexagonal phase unstable. The internal fiber/melt interfaces then move outwards from these centers towards the higher melting regions. Those which melt last are the intersections of the etched surface with surviving crater walls but appear as spines many mu m long. They are almost certainly the remnants of the initial entangled molecular network which sustained the stress of processing in accordance with the new model of fiber structure.