Relationship between Polyelectrolyte Bulk Complexation and Kinetics of Their Layer-by-Layer Assembly

The effects of pH and salinity on both the bulk phase behavior and the layer-by-layer (LbL) growth kinetics are investigated for polyanion poly­(acrylic acid) or PAA with two polycations, namely poly­(<i>N</i>,<i>N</i>-dimethylaminoethyl methacrylate) or PDMAEMA and poly­(diallyldimethylammonium chloride) or PDADMAC, with the goal of relating the phase behavior to the LbL growth kinetics. Depending on salinity, pH, and mixing ratio, the complex formed in the bulk is either a powdery precipitate or a gel-like coacervate, and the multilayers grow either linearly or exponentially with deposition time. In addition to primary Coulombic interactions, we observe that polymer-specific interactions have a profound effect on both bulk complexation and LbL growth of the three PE pairs studied here. The overall strength of interaction between polyelectrolytes, as indicated by their phase behavior, has a nonmonotonic effect on LbL growth rate, apparently because stronger interactions not only increase the driving force for diffusion but also reduce the effective diffusion coefficient of a polyelectrolyte molecule through the LbL multilayer. As a result, there is little correspondence between coacervation and exponential growth on one hand and precipitation and linear growth on the other. Salt concentration has a nonmonotonic effect on LbL growth kinetics at pH 7, with exponential growth found over the range 15–60% of the critical salt concentration (<i>C</i><sub>S</sub><sup>c</sup>) needed to transition from coacervation to a clear solution in the bulk, regardless of the physical chemistry of polyelectrolytes employed, whereas salt concentrations both below and above this range result in linearly growing films. Finally, for our polyelectrolyte pairs at pH 7, we report a “universal curve” for the dependence of LbL growth rate, normalized by its maximum value, against the salt concentration, normalized by <i>C</i><sub>S</sub><sup>c</sup>. If it proves to be robust, this correlation could be used to estimate optimal salinity for LbL growth from bulk measurements of the critical salt concentration needed to suppress complexation.