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Chain Mobility in Polymer Systems; on the Border Line between Solid and Melt

Polymorphism is a well-established phenomenon in crystalline materials and is important for pharmaceutical and polymeric materials. In our study concerning the processability of polymers, we came across an unusual observation related to polymorphism induced by pressure. The experimental observation is that polyethylene crystals transform from the stable orthorhombic crystal into a transient hexagonal phase. The occurrence of a transient hexagonal phase is shown to be dependent on the polymer crystal size; smaller crystals transform into the transient hexagonal phase at temperatures and pressures much below the thermodynamic critical point Qow hich is located at P = 3.6 kbar and T = 230 oC. The crystal size dependence in the phase transition was investigated by in-situ X-ray studies. Since the chain mobility is rather high in the hexagonal phase, sintering has been attempted via this transient phase using ultra-high molecular weight polyethylene (UHMW-PE) as a model system. UHMW-PE is an intractable polymer due to its high molar mass but possesses excellent abrasion resistance properties. For this reason it is used as an inlay in demanding applications such as artificial hip- and knee-joints. The service life of UHMW-PE in these artificial joints, however, is limited due to the poor processing characteristics notably during sintering and, often a second operation is needed to replace the UHMW-PE interface. Sintering via the transient hexagonal phase could provide a solution for this important problem which concerns an increasing number of people.

UHMW-PE; artificial joints; nascent structure; processing

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