Orthorhombic-hexagonal Phase Transition in High Density Polyethylene Crystals

Hydrogenation of High-Density Polyethylene during Decompression of Pressurized Hydrogen at 90 MPa: A Molecular Perspective

To investigate changes in the physical and chemical properties of high-density polyethylene (HDPE) upon the rapid release of hydrogen gas at a pressure of 90 MPa, several characterization techniques have been employed, including optical microscopy, scanning electron microscopy, X-ray diffraction, differential scanning thermal analysis, and attenuated total reflectance Fourier-transform infrared spectroscopy. The results showed that both physical and chemical changes occurred in HDPE upon a rapid release of hydrogen gas. Physically, a partial hexagonal phase was formed within the amorphous region, and the overall crystallinity of HDPE decreased. Chemically, hydrogenation occurred, leading to the addition of hydrogen atoms to the polymer chains. Oxidation also occurred, for example, the formation of ester -C=O groups. Crosslinking and an increase in -CH₃ end termination were also observed. These changes suggest that structural transformation and chemical modification of HDPE occurred upon the rapid release of hydrogen gas.