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

Abstract

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(N,N-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 (C S c ) 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 C S c . 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.

Influence of the degree of ionization on the growth mechanism of poly(diallyldimethylammonium)/poly(acrylic acid) multilayers

“…We show that exponential growth appears when PAA is not fully charged and continues down to very low PAA charge densities where the thickest films are observed. In agreement with earlier work we prove that other than electrostatic interactions are important for the film build‐up, at least for low pH values.…”

“…In general, the occurrence and varying degree of exponential growth in our systems can be explained along the lines of Salehi et al For exponential growth to occur, conversely, a strong driving force for chain incorporation into the multilayer is necessary, such as a strong gradient in the chemical potentials of the chains in the film and the solution, respectively. In charge‐dominated systems like the present one, electrostatic attractive forces can act as the responsible driving force.…”

“…Alonso et al concluded that electrostatic interactions between the polyions are not sufficient to describe their results, but that hydrogen bonding has to be considered as well. In addition, Salehi et al recently investigated the correlation between polyelectrolyte bulk complexation and LbL assembly for PAA in combination with three different polycations. Concerning pH‐dependent layer growth of PDADMA/PAA multilayers, similar to Alonso et al, they claim a supralinear or even exponential behavior at pH values < 7 .…”

“…In addition, Salehi et al recently investigated the correlation between polyelectrolyte bulk complexation and LbL assembly for PAA in combination with three different polycations. Concerning pH‐dependent layer growth of PDADMA/PAA multilayers, similar to Alonso et al, they claim a supralinear or even exponential behavior at pH values < 7 . Finally, they also concluded on the relevance of secondary interactions, for example, hydrogen bonding, ion–dipole interactions and hydration of chains.…”

The influence of ionic strength and mixing ratio on the colloidal stability of PDAC/PSS polyelectrolyte complexes

“…Intermediate salt concentrations yield a coacervate, although the boundary between complex solid and complex coacervate is not definitive. 30 Many of these findings and others [32][33][34][35] have attempted to equate the properties of PECs with that of layer-by-layer assemblies. More recently, Schlenoff and coworkers conducted studies of PECs made from PDAC and PSS.…”