CHAIN MOBILITY IN POLYMER SYSTEMS - ON THE BORDERLINE BETWEEN SOLID AND MELT - 2 - CRYSTAL SIZE INFLUENCE IN PHASE-TRANSITION AND SINTERINGOF ULTRAHIGH MOLECULAR-WEIGHT POLYETHYLENE VIA THE MOBILE HEXAGONAL PHASE

Polymorphism is a well-established phenomenon in crystalline materials and is important for pharmaceutical and polymeric materials. In our s tudy concerning the processability of polymers, we came across an unus ual observation related to polymorphism induced by pressure. The exper imental observation is that polyethylene crystals transform from the s table orthorhombic crystal into a transient hexagonal phase. The occur rence of a transient hexagonal phase is shown to be dependent on the p olymer crystal size; smaller crystals transform into the transient hex agonal phase at temperatures and pressures much below the thermodynami c critical point Q(o), which is located at P = 3.6 kbar and T = 230 de grees C. The crystal size dependence in the phase transition was inves tigated by in situ X-ray studies in the unirradiated and irradiated so lution-crystallized films. Since the chain mobility is rather high in the hexagonal phase, sintering has been attempted via this transient p hase using ultrahigh molecular weight polyethylene (UHMW-PE) as a mode l system. UHMW-PE is an intractable polymer due to its high molar mass but possesses excellent abrasion resistance properties. For this reas on 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 r eplace the UHMW-PE interface. Sintering via the transient hexagonal ph ase could provide a solution for this important problem which concerns an increasing number of people.