From plastic-crystal paraffins to liquid-crystal polyethylene: Continuity of the mesophase in hydrocarbons

The relationship is explored between the plastic "rotator" phase (RP), which occurs in n-alkanes up to C₄₀, and the "liquid-crystalline" high-pressure hexagonal phase (HPHP) in polyethylene. The radiation-induced hexagonal phase (RIHP) of polyethylene, which is intermediate between the two, has been shown in a previous study to behave with increasing pressure essentially like HPHP. In this work a link is demonstrated between RP and RIHP through a study of paraffin C₁₁, binary paraffins C₂₃-C₂₅ and C₃₆-C₄₀, as well as irradiated polyethylene by Fourier-transform infrared spectroscopy. Absence of GG defects and presence of GTG* kinks was confirmed in both RP and RIHP. A smooth continuity was established between the two phases by the defect concentration criterion. More evidence for such continuity is provided by a systematic comparison of published pressure-temperature diagrams for paraffins and polyethylenes. Moreover, the available data for high-temperature forms in related systems (high-modulus polyethylene fibers, cycloalkanes, and substituted n-alkanes) all suggest that a single master relation between the degree of order and variables such as temperature and chain stem length can, to a good approximation, describe the qualitative behavior of the wide range of hexagonal or pseudohexagonal phases in hydrocarbon chain systems.