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Metastability in polymers: influence of crystal size

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, which on annealing isothermally and isobarically transforms back into the stable orthorhombic phase. In-situ X-ray, optical microscopy and transmition electron microscopy lead us to conclusion that the occurance of the transient hexagonal phase is dependent on the polymer crystal size; smaller crystals transform into the transient hexagonal phase at temperatures and pressures much below the thermodynamic critical point. Since the chain mobility is rather high in the hexagonal phase, we used these findings in the fusion of UHMW-PE particles, which are nearly impossible to fuse via the conventional methods. The polymer possesses excellent abrasion resistance and for this reason it is used in demanding applications like in artifical hip-joints as the interface between the bone of the hip and the metal bar which is inserted into leg.

polyethylene; phasestability; processing; artifical joints

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