Polyelectrolyte Gels and Microgels

Basic effects in polyelectrolyte micro- and macrogels, which arise due to the interplay between mobility of counterions, elasticity of the subchains, electrostatic and hydrophobic interactions, will be overviewed. They include superabsorbing properties of polyelectrolyte (macro)gels, peculiarity of their collapse and microphase segregation upon worsening of the solvent quality. Recent results on peculiarities of swelling and collapse of polyelectrolyte nano- and microgels will be reported. 

Internal structure of polyelectrolyte nano- and microgels (mGs) resembles elements of macroscopic polymer network: linear charged chains (subchains) are covalently linked with each other into three-dimensional frame of the size in the range between tens of nanometers and few microns. Similarly to the macroscopic gels, they exhibit different properties at different length scales. Liquid-like behavior is characteristic at the length-scales smaller than the mesh-size, where each subchain does not feel connectivity into the network. Elastic, solid-like response is revealed at the length scales larger than the mesh-size. In contrast to macroscopic polyelectrolyte gels, electric neutrality of the mGs in dilute solutions is violated due to partial escape of monovalent counterions into the outer solvent caused by their thermal motion (entropy). As a result, the mGs are highly swollen in a good solvent due to both long-range repulsion of unscreened charged monomer units and exerting osmotic pressure of mobile counterions which are trapped inside the mG.

In this lecture, we report about new effect comprising equilibrium a center-to-periphery mass redistribution with formation of a quasi-hollow structure of strongly stretched subchains in the center of the mG and dense “skin” at the periphery. This effect is a consequence of a competition between long-range repulsion of unscreened charged monomer units tending to bring them to the periphery of the particle and elasticity of the subchains preventing long-distance migration of the charges.

Acknowledgements