Detection of hexagonal fluid crystalline phase in polymer suspensions as suggested by de Gennes via anomalous small-angle x-ray scattering II. Continuation-melting of the liquid crystalline phase as observed in semi-dilute solutions of polyacrylate macroions with concentrations of 55 and 110 mM surrounded by thallium counterions

Abstract

Element specific scattering techniques known as anomalous small-angle x-ray scattering have been employed in the analysis of the thallium counterion distribution in aqueous polyacrylate solutions of concentrations 25, 55, and 110 mM. The three basic scattering functions—pure-resonant, mixed-resonant, and non-resonant—–have been deduced explicitly, thereby revealing decisive structural and quantitative information of the involved chemical components. In continuation of a previous paper, at a molar concentration of 55 mM, a hexagonal lattice structure of cylinders with an axis of h = 5.3 nm and a lattice parameter α0 = 8.4 nm was identified, with about 87% of the Tl+-ions located in the cylinders. From the Lindemann criterion, the stability of the hexagonal phase is deduced. At a concentration of 110 mM, a strong disordering of the hexagonal phase is observed, and only residuals of the Bragg reflections are detected with h = 7.6 nm and α0 = 14.5 nm, which can be interpreted as melting of the hexagonal phase, the latter being confirmed by the Lindemann criterion.