Chain mobility in polymer systems: On the borderline between solid and melt. 3. Phase transformations in nascent ultrahigh molecular weight polyethylene reactor powder at elevated pressure as revealed by in situ Raman spectroscopy

The phase transformations under elevated pressure have been followed in sit u by X-ray and Raman spectroscopy for ultrahigh-molecular-weight polyethyle ne (UHMW-PE) reactor powder. Using the in situ X-ray as a reference for the Raman work, it has been shown that Raman spectroscopy is a convenient meth od to follow phase transformations in polyethylene under pressure. The dist inction has been made between different solid phases in polyethylene (monoc linic, orthorhombic, and hexagonal phases) and the melt by studying the con formational changes occurring within the skeletal mode (1000-1200 cm(-1)) u pon the phase transformations. The crystal field splitting (1415/1440 cm(-1 )) in the -CH2-bending mode of the Raman spectrum of polyethylene, related to the orthorhombic unit cell, was confirmed to be sensitive to transformat ions between different crystal structures. By following the changes occurri ng in the methylene bending mode together with the changes within the C-C s keletal mode, it has been shown that the crystalline phases and the melt ca n be mutually distinguished.