Collapse of a weak polyelectrolyte star in a poor solvent

Abstract

We present a theory of intramolecular collapse transition in a weak (pH sensitive) polyelectrolyte (PE) star induced by a decrease in the solvent strength and/or variation in the ionic strength in the solution. Our system mimics conformational coil-to-globule transitions in individual star-shaped thermo- and pH-sensitive (e.g. poly(dimethylaminoethyl methacrylate)) macromolecules in dilute aqueous solutions. Systematic comparison with the behaviour of non-ionic and strong (quenched) polyelectrolyte stars in poor solvents enables us to unravel specific features of the collapse transition in weak polyelectrolyte stars. We demonstrate that, depending on temperature and the ionic strength of the solution, a vast diversity of different scenarios for the salt- or temperature-induced collapse transitions may take place. Both at high or low ionic strength a collapse transition induced by a decrease in the solvent strength occurs continuously resembling the collapse of a neutral polymer star. On the contrary, at intermediate salt concentrations the collapse of a weak polyelectrolyte star may feature a first order phase transition, which involves the co-existence of a collapsed, weakly ionized state with a swollen, strongly ionized state. At a fixed temperature the collapse transition can be triggered by an increase in salt concentration. In the latter case the transition may occur via a sequence of two co-existence regimes separated by continuous though non-monotonous variation in the star dimensions as a function of salt concentration.