Temperature-Induced Coil-Globule Transition of Alkali Metal Polyacrylates in Aqueous Organic Solvent Mixtures

Abstract

ABSTRACTA counterion-specific coil-globule transition (CGT) has been observed for alkali metal poly(acrylate)s (PAAM, M; Li, Na, K, Cs) in aqueous organic solvent (ethanol, dimethyl sulfoxide, acetone, acetonitrile) mixtures. In the present study we found that the reduced viscosity of PAAM, except for PAALi, in the relevant mixed solvents, except for those containing acetonitrile, significantly increased with increasing temperature. The observed temperature-induced CGT was interpreted as being caused by disintegration of the ion-clusters, i.e., the associated state of contact ion-pairs formed between the polymer charge and the counterion. In fact, the NMR line width of 23Na and 133Cs, which were used as a measure of the counterion mobility, indicated that the counterion binding was significantly weakened with increasing temperature, while the temperature effects for the PAALi and acetonitrile systems were only marginal. All these results suggested that the temperature-induced CGT behavior occurs only for systems where the ion-cluster formation promotes the CGT in the relevant solvent systems, which is also consistent with the known similar behavior of some betaine polymers.