Frequency Distribution, Specific Heat, and Young’s Moduli of Orthorhombic Polyethylene with Skeletal Approximation

In preliminary studies on the frequency distribution, specific heat and Young’s moduli of the orthorhombic polyethylene crystal, methylene groups were treated as single dynamic units. Intermethylene force constants were estimated from the intermolecular potential function of methane and from deBoer’s potential function. Dynamical matrices of acoustic vibrations were constructed. A practical method was worked out for deriving the frequency distribution of acoustic vibrations from the volume enclosed in constant-frequency surfaces in a three-dimensional phase-difference space. With reference to specific heat in the lowest temperature region, inter-methylene force constants were adjusted. An efficient method was used for calculating the frequency distribution of crystal vibrations of chain-polymers. The frequency distribution of orthorhombic polyethylene crystal and specific heats in low temperature region were calculated in agreement with experimental results. A simple approximate method was used for treating Young’s moduli of orthorhombic chain-polymer crystals. Applicability of the skeletal approximation is discussed.