Ultrahigh‐molecular‐weight polyethylene: Raman spectroscopic study of melt anisotropy

Abstract

AbstractThe Raman spectrum of ultrahigh‐molecular‐weight polyethylene (UHMWPE) has been obtained in the temperature interval 135–208°C, a region where optical anisotropy was observed to exist. On the basis of our spectroscopic evidence, we believe that ordered regions persist in the melt above the calorimetrically determined melting point, and that part of the polyethylene chain is in an enviroment which is similar to that of the orthorhombic crystal. These ordered domains disappear with increasing temperature, but no calorimetric phase transition is associated with this change. We postulate that the very long relaxation times associated with the highly viscous melt keep the polyethylene chains in ordered environments which persist until decreased viscosity at increased temperature allows long‐range segmental motion. Our evidence supports the view that the melt anisotropy of UHMWPE arises from oriented slowly melting superheated crystals and not from a smectic liquid‐crystalline phase.