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 compacted nascent ultra high molecular weight polyethylene (UHMWPE) precursors were investigated. High-temperature solid-state nuclear magnetic resonance studies show that an optimum compaction pressure exists for the compacted precursors to retain maximum chain mobility in the amorphous phase. The optimum compaction condition also produces the maximum mechanical toughness, as determined by universal testing machine and maximum crystal sizes as implied by differential scanning calorimetry. An attempt was also made to elucidate the mechanisms for particle interfacial diffusion between the compacted UHMWPE powders. A sufficiently high compaction pressure is needed to produce a large contact surface area for intimate molecular contact at particle boundaries enabling interfacial diffusion. The diffused chains can either crystallize to give larger crystal sizes or form entanglements with their new neighbouring chains. These recrystallization and re-entanglement processes which occur at the particle boundaries will give enhanced particle interfacial cohesive strength, resulting in high mechanical toughness for the compacted material. However, when the compaction pressure reaches a critical value at which the product of the contact surface area and the free volume available for chain movement reaches maximum, the interfacial diffusion will start to decrease. Further increase in compaction pressures might result in a decrease in particle cohesive strength. Cohesive precursors with high chain mobility are ideal for further tensile drawing into high stiffness films or fibres. Copyright (C) 1996 Elsevier Science Ltd.