Molecular orientational correlations and local order in n-alkane liquids

Abstract

The molecular orientation correlation, the short-range order and the chain conformation in liquid n-alkanes have been studied by various experimental methods supposing that the nature of “order” in these simple systems will reflect the typical features of the organization of polymer molecules.

The orientational correlation has been investigated by means of the dependence of depolarized light scattering and magnetic birefringence on concentration of diluents, on chain lengths and on temperature. There exists a very weak orientational correlation, which is far from a nematic-like state and which can be characterized by a correlation length ξ < 10å. The orientational correlation shows a temperature dependence as predicted by de Gennes' theory of pretransitional ordering in the isotropic phase of a liquid. Alternatively the temperature dependence of order in the n-alkanes can be described in terms of a heterophase fluctuation (Frenkel), but again the number of CH₂ units in an “ordered phase” is extremely small (≈ 8 to 9 units).

The conformation of the chains has been studied by small-angle neutron scattering and Raman spectroscopy. In spite of the local segmental orientation correlation it was found that the conformation of the single chains behaves as expected from Flory's theory: no conformation changes occur in the melt.