Polyelectrolytes (PE), chain molecules bearing chargeable units, possess a variety of fascinating properties that are both theoretically intriguing and have found numerous applications ranging from drug delivery to surface modification. In favorable conditions, oppositely charged PEs have long since been demonstrated to undergo aggregation into soluble supramolecular complexes ranging from 5 to 15 nm in diameter. Above the dilute regime, the mesoscale complexation turns into a macroscopic phase separation characterized by the coexistence of a dilute liquid-like phase in thermodynamic equilibrium with a dense gel (solid)-like phase, called the coacervate (precipitate), predominantly driven by electrostatic interactions. Strength of electrostatic interactions, pH, counter ion valence and salt concentration, among other parameters, control the phase behavior of oppositely charged PEs. Electrostatic attraction of oppositely charged PEs can also be used to prepare stratified coatings comprising alternating layers of two PEs in Layer-By-Layer (LBL) deposition where a film, 10 to 1000 nm thick, is grown upon sequential dipping of a suitable substrate into pure PEs solutions with rinsing steps in between. The growth of PEs in LBL process is believed to be the result of surface charge overcompensation after each dipping step. Not surprisingly, the key parameters controlling the phase behavior of PE, affect the growth rate of PE multilayer films during LBL process in the similar fashion.
The ultimate focus of this project is to establish a connection between the bulk behavior of oppositely charged PEs and their growth regime during LBL process. To that end, we investigate the phase behavior of model PEs in order to construct a phase map delineating the effects of different parameters on the extent and type of PEs phase separation. Concurrent LBL experiments are conducted in order to map the phase behavior onto the corresponding LBL results. Kinetic and thermodynamic modeling is coupled with experimental data in order to obtain predictive tools and a deeper understanding of the underlying physical processes involved.
Salehi, A.; Desai, P.S.; Li, J.; Steele, C.A.; Larson, R.G. "Relationship between Polyelectrolyte Bulk Complexation and Kinetics of Their Layer-by-Layer Assembly" Macromolecules 2015 48(2) pp. 400-409