The Crystalline Structure of Nascent Ultra High Molecular Weight Single Particles and Its Change on Heating, as Revealed by in-situ Synchrotron Studies

A solvent-free route to high performance ultra-high molecular weight polyethylene (UHMWPE) film threads is currently under intensive development. It involves compaction/sintering of UHMWPE reactor powder at T < T-m followed by orientation hardening of the sintered film cut into narrow strips. However, not any kind of reactor powder can be transformed into the desired high-performance material. The presence of a monoclinic crystalline phase (MP) in the powder is considered as one of the key parameters indicating its applicability for solvent-free processing. Since the MP is stable only under stress, the assumption has been made that the observed MP is generated during tableting for X-ray analyses rather than during synthesis of the nascent powder. We show that comparative X-ray analysis of a tablet and a single "virgin" particle using synchrotron radiation indicates that the MP content in the virgin particle was far less than that in the compressed tablet. Only the (001) MP peak was resolved while the others were severely overlapped with the normal, orthorhombic reflections. Thus, it supports our idea that the widely observed MP phase is, for the most part, generated during the sample preparation for the X-ray analyses.