Phase behaviour of n-alkanes and polyethylene. A thermodynamic study

Abstract

Free energies of fusion, and derived thermodynamic properties, are reported for a series of pure long chain n-alkanes, containing from 19 to 42 carbon atoms, in the temperature range –35°C → melting point. Orthorhombic, monoclinic (for which the sub-cell is orthorhombic), triclinic, and hexagonal phases are investigated and the properties of the corresponding polyethylene structures extrapolated using the Flory-Vrij equation. The orthorhombic form (extrapolated m.p., 141.6°C) is always stable under normal conditions but the triclinic modification is only 3-4 J g^–1 less stable and, once formed, will remain indefinitely at all temperatures until these are such that there is considerable lattice mobility.