High-pressure X-ray diffraction and vibrational spectroscopy of polyethylene: Evidence for a structural phase transition

High-pressure synchrotron x-ray diffraction and vibrational spectroscopy (infrared absorption and Raman) in a diamond-anvil cell have been performed for polyethylene samples at pressures up to -10 GPa. These two techniques yield complementary information on the interand intra-molecular bonds and how they change as a function of volume. Three different samples of polyethylene were examined, including a high-density polyethylene, an ultra-high molecular weight polyethylene, and a crosslinked polyethylene. The diffraction peaks for the common orthorhombic Pnam phase continuously decreases in intensity as the pressure is increased and become unobservable at high pressure, while those for a monoclinic A2/m impurity phase initially present in all three samples remain relatively constant in intensity. The monoclinic A2/m phase has two chains per unit cell, as does the ambient pressure orthorhombic Pnam phase, which is consistent with the lack of a change in the vibrational mode splitting patterns as pressure is increased and the monoclinic phase becomes dominant. In addition, the pressure-induced correlation splitting of the infrared active C-H stretching vibrational modes has been observed for the first time.