The path to “single-component composites”: An in situ X-ray study of melting and crystallization of extended-chain polyethylene fibers under pressure

“Single component” polymer composites, in which both the reinforcement and the matrix are made of the same polymer, are currently fabricated by subjecting oriented fibers to moderate pressure and temperature in a processing scheme termed “reversed pressure compaction”. In order to elucidate the nature of the transformations that lead from fibers to a monolithic composite, an X-ray diffraction study with a pressure cell was performed using synchrotron X-ray radiation (Beamline ID-11, ESRF). We observed a transition from the orthorhombic crystal to the meso-morphic hexagonal phase upon heating even at a low pressure (100 bar). The hexagonal phase also reappears even upon cooling from the melt. These indicate that the path to high-performance composites from the oriented PE fibers occurs via sintering of the hexagonal phase. This phase allows enough chain mobility for the compaction process yet maintains the chain orientation to a large extent.