Ca2+ and CU2+ induced conformational changes of sodium polymethacrylate in dilute aqueous solution

The coil contraction of sodium polymethacrylate in dilute aqueous sodium chloride solution (0.1 N, pH = 9) was monitored by static and dynamic light scattering when the Na+ ions were partly replaced by Ca2+ or Cu2+ ions. The effect was quantified as a function of polyion and bivalent metal ion concentration. The impact of Cu2+ ions on the chain conformation and the solubility of sodium polymethacrylate was observed to be larger than the corresponding effect of Ca2+ ions. This difference was ascribed to complexation of the carboxylate group with bivalent metal cations which is larger for the Cu2+ than for the Ca2+ ion. Even at the largest extent of contraction observed close to the precipitation threshold, the ratio of the radius of gyration and the hydrodynamic radius remained unchanged in contrast to the coil-globule transition observed for neutral polymer systems below the Theta-temperature.