Structure and Dynamics of Polyelectrolyte Multilayers and Complexes

Polyelectrolyte complexes (PECs) are made by mixing oppositely charged polymers. In this dissertation, a pair of pH-independent polyelectrolytes was used: a polycation, poly(diallyldimethylammonium), PDADMA, and a polyanion, poly(styrenesulfonate), PSS. Different morphologies of complexes can be obtained depending on the preparation technique. Polyelectrolyte multilayers, PEMUs, are formed by alternately depositing “layers” of oppositely charged polymers on different surfaces, while traditional complexes are produced by solution precipitation of the dissolved polyelectrolytes. A plethora of applications have been proposed for these polymeric materials, from antibacterial coatings, to electronic devices, and drug reservoirs. Doping the PEC refers to adding a certain concentration of salt to the material, causing many polymer/polymer interactions, known as intrinsic sites, to turn into polymer/counterion interactions, or extrinsic sites. The doping level is usually determined by the salt type and concentration, as well as the polymer pair. This work can be divided into two large sections. The dilemma at the center of the first one was an excess of one polyelectrolyte in as-made PEMUs – PDADMA in this case. This created an imbalance and brought extra ions in the film rendering it unpredictable for applications. The solution was to cycle the film between 2 M NaCl and PSS, the missing polyanion, in 1 M NaCl to compensate for the excess polycation. The technique turned PEMUs into stoichiometric, ion-free, and exceedingly smooth films. With this homogeneous PEC film at hand, the next steps aimed at understanding the reasons that lead to the properties of the original multilayer. The two main targets were to elucidate the overcompensation or overcharging behavior of polyelectrolytes in these complexes, and their kinetics. Long known as the driving force behind the growth of these layered systems, overcompensation had not been previously extensively explored. Both polyelectrolytes were separately added to the stoichiometric films, and the amount of excess was monitored using radiolabeling and Fourier transform infrared spectroscopy, FTIR. Independently from the polymer nature, concentration, molecular weight (to a certain extent), and the film thickness, the overcompensation limit was around 40%. Calculations showed that this maximum is related to the breakdown of a Donnan equilibrium that forms during the addition of the polyelectrolytes to the PEC. A slight dependence of this value on the salt concentration hinted at the second target of this study: the kinetics of polyelectrolytes. Through the same techniques, and using deuterated polymers in FTIR, the transport of species within the PEC was monitored over time. Site-diffusion, or the movement of counterions from one excess site to another, revealed by radioactive labeling, was around three orders of magnitude faster than polymer diffusion, shown by the deuterated polymer moving throughout a protiated film, or vice versa (protiated polymer in deuterated film). Moreover, site-dominated diffusion was closer to the timescale of the multilayering procedure. The information obtained from reaching these two targets allowed us to tailor PEMU growth to obtain different thicknesses and buildup regimes. In the second main section, the internal structure of the polyelectrolyte complex/coacervate continuum, accessed by the doping of the solid complex with increasing concentrations of KBr, was probed using small-angle neutron scattering, SANS. Deuterated PSS inserted at a proper concentration within the protiated PEC exhibits a detectable contrast in this technique. The chain maintained its radius of gyration in the solid samples, but decreased slightly in size in the phase-separated coacervate and liquid samples. In KBr solutions, the decrease in the macromolecule’s size was sharper, as revealed by light scattering, suggesting that the chain behavior in the PEC is under opposing forces. On one hand, the network volume increase promotes polymer extension. On the other hand, higher salt concentrations induce what is known as the polyelectrolyte effect, shrinking the polymer. The result is a coiled chain throughout the continuum. Pores in the solid regime were also observed and the reason behind their presence was explored. These pores were more thoroughly investigated in a second study that focused on the long-term swelling behavior in extruded PEC fibers. The material exhibited significant swelling, and porosity, in solutions with an NaCl concentration below 0.01 M. Osmotic pressure was quantified inside and outside the polymer network, and exposed as the culprit behind this comportment. PECs were also shown to swell more in a range of polar solvents that have a small molecule size. These investigations of the shape and mobility of charged polymers within their complexes provide a better understanding of their often-puzzling behavior. They also serve as a starting point for studies that can benefit from this knowledge to build better materials.