Comparative investigation of short‐range order in unbranched alkanes and polyethylene

This paper presents basic results obtained in the X‐ray investigation of liquid unbranched alkanes of type C n H 2n+2 and polyethylen (PE) melts of molecular weights from 86 (hexane) to 2.10 6 (PE) in the temperature range from 0to 350°C. The position of the maximum of the X‐ray scattering curve 2θ max , and the half‐width Δ 1/2 of the amorphous halo, with average errors of ±1,5′ and ±6′, respectively were measured. The experimental results were used for a comparative analysis of short‐range ordering in polyethylene melts and their low‐molecular analogues. The thermal expansion coefficients of the liquids and their “X‐ray densities” were calculated (assuming that the packing is hexagonal). The analytical dependences on the molecular weight for the melting and boiling point 2θ = f( M ) and 2θ = f( M ) were derived, and a “critical molecular weight” was calculated for the transition from oligomer to polymer. The data revealed jump‐wise changes of the short‐range order parameters in unbranched alkanes with lower number of carbon atoms in passing from an odd to the next even homologue, which indicates a high degree of ordering in the liquid.