Relative Strength of Polycation Adsorption on Oxide Surfaces.

Polyelectrolyte adsorption onto surfaces is widely employed in water treatment and mining. However, little is known of the relative interaction strengths between surfaces and polymer. This fundamental property is assumed to be dominated by electrostatics, i.e., attractive interactions between opposite charges, which are set by the overall ionic strength ("salt concentration") of the solution, and charge densities of the surface and the polymer. A common, counterintuitive finding is a range of salt concentrations over which the amount of adsorbed polyelectrolyte increases as electrostatic interactions are tempered by the addition of salt. After an adsorption maximum, higher salt concentrations then produce the expected gradual desorption of polyelectrolyte. In this work, the salt response of the adsorption of the same narrow molecular weight distribution polycation, poly(N-methyl-4-vinylpyridinium), PM4VP, to a variety of surfaces was explored. Oxide powders for adsorption included Al2O3, SiO2, Fe2O3, Fe3O4, TiO2, ZnO, and CuO. Planar surfaces included silicon wafers, mica, calcium carbonate, and CaF2 single crystals. The PM4VP was radiolabeled with 14C so that sensitive, submonolayer amounts could be detected. The position of the peak maximum, or the lack of a peak, in response to added salt was used to rank the electrostatic component of the interaction. The importance of charge regulation, a shift in the surface pKa in response to solution species, was highlighted as a mechanism for adsorption on the "wrong" side of the isoelectric point and also as a factor contributing to the difficulty of reaching the totally desorbed state even at the highest salt concentrations.

Gummy Nanoparticles with Glassy Shells in Electrostatic Nanocomposites.

Nanocomposites with unusual and superior properties often contain well-dispersed nanoparticles. Polydimethylsiloxane, PDMS, offers a fluidlike or rubbery (when cross-linked) response, which complements the high-modulus nature of inorganic nanofillers. Systems using PDMS as the nanoparticulate, rather than the continuous, phase are rare because it is difficult to make PDMS nanoparticles. Aqueous dispersions of hydrophobic polymer nanoparticles must survive the considerable contrast in hydrophobicity between water and the polymer component. This challenge is often met with a shell of hydrophilic polymer or by adding surfactant. In the present work, two critical advances for making and using aqueous colloidal dispersions of PDMS are reported. First, PDMS nanoparticles with charged amino end groups were prepared by flash nanoprecipitation in aqueous solutions. Adding a negative polyelectrolyte, poly(styrene sulfonate), PSS, endowed the nanoparticles with a glassy shell, stabilizing them against aggregation. Second, when compressed into a nanocomposite, the small amount of PSS leads to a large increase in bulk modulus. X-ray scattering studies revealed the hierarchical nanostructuring within the composite, with a 4 nm PDMS micelle as the smallest unit. This class of nanoparticle and nanocomposite presents a new paradigm for stabilizing liquidlike building blocks for nanomaterials.

The Thiouronium Group for Ultrastrong Pairing Interactions between Polyelectrolytes.

Various charged groups may be used as a repeat unit in polyelectrolytes to provide physical interactions between oppositely charged polymers leading to phase separation. The materials formed thus are termed polyelectrolyte complexes or coacervates (PECs). The strength of pairing between positive, Pol+, and negative, Pol-, repeat units depends on the specific identity of the monomer repeat unit. In this work, the pairing strength of the thiouronium group, a cation closely related to guanidinium, is evaluated using a polythiouronium polyelectrolyte. Polymers containing guanidinium, notably polyarginine, a peptide, are known for their unusual behavior, such as the formation of like-charge ion pairs and hydrogen bonding. It is shown here that some of this behavior is carried over to polythiouroniums, which results in exceptionally strong interactions with polyanions such as polysulfonates and polycarboxylates. The resilience of the polythiouronium/Pol- interaction was evaluated using the buildup of polyelectrolyte multilayers at various salt concentrations and by breaking up preformed PECs with high concentrations of added salt. The thiouronium group even interacts strongly enough with polymeric zwitterions to enable complexation with this nominally weakly interacting, net-neutral polymer.

Engineering Thiolated Surfaces with Polyelectrolyte Multilayers.

Interfaces bearing firmly attached thiol groups are useful for many applications requiring the versatile and facile chemistry of the -SH functionality. In this work, rugged ultrathin films were prepared on substrates using layer-by-layer assembly. The surface of these smooth films was capped with a co-polymer containing benzyl mercaptan units. The utility of this coating was illustrated by three applications. First, thiol-ene "click" chemistry was used to introduce the Arg-Gly-Asp (RGD) adhesive peptide sequence on a surface that otherwise resisted good adhesion of fibroblasts. This treatment promoted cell adhesion and spreading. Similar Michael addition chemistry was employed to attach poly(ethylene glycol) to the surface, which reduced fouling by (adhesion of) serum albumin. Finally, the affinity of gold for -SH was exploited by depositing a layer of gold nanoparticles on the thiolated surface or by evaporating a tenacious film of gold without using the classical chromium "primer" layer.

Intrinsic Properties of Polyelectrolyte Multilayer Membranes: Erasing the Memory of the Interface.

Polyelectrolyte multilayers (PEMUs) are ultrathin membranes made by alternating adsorption of oppositely charged polyelectrolytes on substrates. Although PEMUs have shown exceptional selectivity for certain ion-filtering applications, they usually contain an excess of one of the polyelectrolytes due to the history- and condition-dependent mode of PEMU assembly. This excess charge provides fixed sites for ion exchange, enhancing the concentration of oppositely charged ions. Thus, the ion-permselective properties of PEMUs cannot be compared unless they are assembled under identical conditions. This work demonstrates the enhanced permeability of PEMUs as-made from poly(diallyldimethylammonium) (PDADMA), and poly(styrene sulfonate) (PSS) to ferricyanide as an example of an anion. Annealing by NaCl followed by pairing of excess PDADMA with additional PSS produces an almost stoichiometric film that better reflects the intrinsic transport properties of PEMUs. This pairing, observed in real time using electrochemical methods, occurs at the PEMU/solution interface under countercurrent transport of PSS from solution and excess PDADMA paired with a counterion, termed PDADMA*, from the PEMU bulk. A quantitative comparison of PSS and PDADMA* diffusion reveals the conditions under which PEMU assembly depends on PSS molecular weight and concentration.

Turning the heat on conjugated polyelectrolytes: an off-on ratiometric nanothermometer.

We report a self-referenced ratiometric nanothermometer based on short conjugated polyelectrolytes. An amphiphilic macromolecule destabilizes the polymer π-π stacking and makes it possible to shift the equilibrium between the less emissive aggregated state (520 nm) and the brighter individual chain (450 nm) within 20.0 °C and 70.0 °C.