EFFECTS OF COMPACTION PRESSURE ON COHESIVE STRENGTH AND CHAIN MOBILITY OF LOW-TEMPERATURE COMPACTED NASCENT UNMWPE

The effects of compaction pressure on the deformability of the compact ed nascent ultra high molecular weight polyethylene (UHMWPE) precursor s were investigated. High-temperature solid-state nuclear magnetic res onance studies show that an optimum compaction pressure exists for the compacted precursors to retain maximum chain mobility in the amorphou s phase. The optimum compaction condition also produces the maximum me chanical toughness, as determined by universal testing machine and max imum crystal sizes as implied by differential scanning calorimetry. An attempt was also made to elucidate the mechanisms for particle interf acial diffusion between the compacted UHMWPE powders. A sufficiently h igh compaction pressure is needed to produce a large contact surface a rea for intimate molecular contact at particle boundaries enabling int erfacial diffusion. The diffused chains can either crystallize to give larger crystal sizes or form entanglements with their new neighbourin g chains. These recrystallization and re-entanglement processes which occur at the particle boundaries will give enhanced particle interfaci al cohesive strength, resulting in high mechanical toughness for the c ompacted material. However, when the compaction pressure reaches a cri tical value at which the product of the contact surface area and the f ree volume available for chain movement reaches maximum, the interfaci al diffusion will start to decrease. Further increase in compaction pr essures might result in a decrease in particle cohesive strength. Cohe sive precursors with high chain mobility are ideal for further tensile drawing into high stiffness films or fibres. Copyright (C) 1996 Elsev ier Science Ltd.