A method of solid state deformation processing of crosslinked polymers includes deforming a polymer bulk material by compressing it in a direction orthogonal to a main axis of the bulk material and optionally cooling the bulk material while maintaining the deformation force. When the polymeric material is made of UHMWPE and the crosslinking is by irradiation such as γ-irradiation, products of the method are particularly suitable for use in bearing components and implants for total hip replacement and the like.
In a further aspect, a method for solid state deformation processing of γ-irradiated crosslinked UHMWPE comprises deforming the UHMWPE by extruding it at a temperature below its melt transition or crystalline melting point. In subsequent steps, the extruded UHMWPE is cooled to a temperature below its solidification temperature, optionally while maintaining the extruded rod in a deformed state.
In various embodiments, the solid state deformation process provides polymers that are characterized by a crystal and molecular orientation. By molecular orientation is meant that polymer chains are oriented perpendicular to the direction of compression. By crystalline orientation it is meant that crystal planes in polyethylene, such as the 200 plane and the 110 plane are oriented to the direction parallel to the compression plane. In this way the crystal planes are oriented. The presence of the orientations can be shown by means of birefringent measurements, infrared spectra, and x-ray diffraction.