High-pressure crystalline polyethylene studied by x-ray diffraction and ab initio simulations

Crystalline polyethylene was investigated under pressure between 0 and 40 GPa, up to 280 °C, by means of synchrotron x-ray powder diffraction and ab initio calculations. A rich polymorphism was unveiled, consisting of two new high-pressure monoclinic phases, in addition to the well-known orthorhombic one, which appear reversibly, although with strong hysteresis, upon increasing pressure above 6 GPa (P2₁∕m, Z_chain=1) and 14–16 GPa (A2∕m, Z_chain=2), respectively. The equation of state was determined for the three solid phases. We find that polyethylene is characterized by a sharp separation between strong covalent intrachain and weaker interchain interactions up to the maximum investigated pressure, which, in turn, places the ultimate chemical stability limit of polyethylene far beyond these thermodynamic conditions.