Transformations in Polyelectrolyte Complexes and Multilayers

Polyelectrolyte complexes and coacervates (PECs) bear importance in therapeutics, adhesives, and smart coatings, as well as in understanding the origins of life. PECs may be classified as either liquid-like coacervates or solid-like precipitates, in which the solution-coacervate-precipitate boundaries are strongly influenced by composition, salt type, and ionicstrength. Liquid coacervates have been the center of focus for many researchers, but solid-like precipitates are less-explored. This talk will describe the structure and dynamics of solid-like PECs. A special focus will be placed on explaining the methodology developed in our lab that is used to quantify the number of polycation/polyanion intrinsic ion pairs, water content, salt content, polyelectrolyte ratio, glass transition temperature (T_g), and relaxation time (). As a result, it is discovered that the T_g scales with the number of intrinsic ion pairs and water molecules as T_g^-1 ~ ln(n_intrinsic/n_H2O). This result indicates the importance of water’s role in the relaxation of the solid-like PEC. With regard to dynamic mechanical properties of solid-like PECs, time-temperature-water-pH superpositioning is applied and a free-volume relationship is proposed in which the following scalings are identified: ln() ~ ^-1 and ln() ~ n_intrinsic/n_H2O,where is the volume fraction of water in the PEC. These relationships emphasize that the dynamic relaxation of a PEC is mediated by bound water at the intrinsic ion pair, in which water creates free volume in the solid PEC matrix. Last, the solid-liquid-solution phase behavior of PECs will be presented, in which an upper critical solution temperature (UCST) is observed. Theory predicts lower critical solution temperatures, but our contrary observation points to the important role of hydrogen bonding interactions in stabilizing PECs.