Introductory lecture: levels of order in amorphous polymers

Abstract

Early theoretical predictions that amorphous and liquid polymers should be devoid of significant order down to dimensions commensurate with the diameter of the chain are supported by an extensive body of evidence from X-ray scattering at small and at wide angles, neutron scattering at small and at intermediate angles, recent electron microscopic investigations, analysis of rubber elasticity including, especially, its dependence on temperature and dilution, and cyclization equilibrium constants. If the nodular domains previously observed in electron micrographs are dismissed as artifacts, as studies documented in this Discussion indicate, then depolarized Rayleigh scattering (DRS) offers the only substantial evidence for a detectable degree of order in liquids consisting of long-chain molecules. This evidence is examined in detail. The analysis presented leads to the conclusion that the enhancement of the DRS of higher n-alkanes by factors of 2–2.5 over the values that would be observed for uncorrelated molecules is attributable to: (i) steric constraints on relative orientations of neighbouring chains which restrict them to orientations generated by rotation about one axis only, this axis being normal to their plane of contact, and (ii) a small preference in the alignment of neighbouring chains. These correlations do not imply order in the sense in which this term customarily is used. Rather, they complement the distance correlations manifested in the radial distribution functions deduced from wide-angle X-ray scattering; like them, they are of very short range.