Doping and Diffusion in an Extruded Saloplastic Polyelectrolyte Complex

Doping constants and diffusion coefficients for an extruded, stoichiometric, dense polyelectrolyte complex, PEC, were determined for a Hofmeister series of anions. These thermodynamic and kinetic parameters describe the extent and speed to which a complex of poly­(styrenesulfonate) and poly­(diallyldimethylammonium) may be doped. Both parameters followed a Hofmeister ordering and covered a wide range of response. Differences between doping and undoping kinetics were observed, with the latter adhering well to classical diffusion from the cylindrical geometry employed. Tracer diffusion of radiolabeled Na⁺, compared with coupled diffusion of NaCl, revealed slightly faster diffusion of Na⁺ compared to Cl^– ions within the PEC

Asymmetric Growth in Polyelectrolyte Multilayers

Radioactive counterions were used to track the ratio of positive to negative polymer repeat units within a polyelectrolyte multilayer made from poly­(diallyldimethylammonium chloride), PDADMAC, and poly­(styrene sulfonate), PSS. For this widely employed pair of “linearly” assembled polyelectrolytes it was found that the accepted model of charge overcompensation for each layer is incorrect. In fact, overcompensation at the surface occurs only on the addition of the polycation, whereas PSS merely compensates the PDADMAC. After the assembly of about a dozen layers, excess positive sites begin to accrue in the multilayer. Treating the surface as a reaction–diffusion region for pairing of polymer charges, a model profile was constructed. It is shown that different reaction–diffusion ranges of positive and negative polyelectrolyte charge lead to a blanket of glassy, stoichiometric complex growing on top of a layer of rubbery, PDADMAC-rich complex. Though overcompensation and growth was highly asymmetric with respect to the layer number, entirely conventional “linear” assembly of the multilayer was observed. The impact of asymmetric growth on various properties of multilayers is discussed

Roughness and Salt Annealing in a Polyelectrolyte Multilayer

The surface roughness of polyelectrolyte multilayers made from poly­(diallyldimethylammonium chloride), PDADMAC, and poly­(styrene sulfonate), PSS, was measured as a function of film deposition conditions. For dry multilayers, the significant roughness which builds up for thicker films is much more apparent for multilayers terminated with PSS. Corresponding roughness for PDADMA-capped multilayers may be seen by imaging in situ under electrolyte. Roughness may be substantially reduced, but not eliminated, by annealing in salt. Annealing does not lead to loss of polyelectrolyte from the film, even under conditions where the salt concentration is high enough to place the film properties beyond the glass transition. Roughness does not correlate with the molecular weight of the polyelectrolyte and is thus not caused by solution or film polymer chain conformations. The wavelength of the roughness features is approximately proportional to film thickness, which supports a mechanism whereby roughness is generated by anisotropic swelling due to water and polyelectrolyte addition in a manner similar to water uptake in hydrogels. Roughness is preserved by the glassy PSS layer and probably incorporated within the film as it grows

Thermal Transformations in Extruded Saloplastic Polyelectrolyte Complexes

Extruded, salt-plasticized complexes of hydrated poly­(styrenesulfonate), PSS, and poly­(diallyldimethylammonium), PDADMA, were analyzed by differential scanning calorimetry and dynamic mechanical thermal analysis. Whereas the enthalpic signatures were weak, the latter technique revealed a strong transition in modulus, identified as a glass transition. The temperature of this transition, <i>T</i>_g, varied with deformation rate as expected from time/temperature superposition. <i>T</i>_g also decreased with increasing salt doping, which breaks ion pairing in the complexes, confirming the plasticizing effect of doping. Time, temperature, and salt concentration data were superposed to demonstrate the trends/equivalence of these three variables, and an empirical equation was used to connect them. Measurement time regimes were discussed with reference to the average lifetime of an ion pair