The spontaneous association of oppositely charged polyelectrolytes is an example of liquid−liquid phase separation. The resulting hydrated polyelectrolyte complexes or coacervates, both termed "PECs", display a wide range of viscosities. In addition to the usual dependence of viscosity on molecular weight and volume fraction expected for condensed neutral polymers, PECs also contain dense charge pairing between positive, Pol + , and negative, Pol − , repeat units. These "stickers" slow polymer chain dynamics on multiple length scales. Pol + Pol − charge pairs may be broken by the addition of salt to solutions contacting PECs, reducing viscosity ("saloplasticity"). Here, the dynamics of matched pairs of a polycation, poly(methacryloylaminopropyltrimethylammonium chloride), and polyanion, sodium poly(methacrylate), with molecular weights considerably above the entanglement concentration, were measured as a function of temperature and salt concentration. The dynamics of NaCl ions in PECs were also determined and correlated to the segmental relaxation times, which control viscosity. A suite of relaxation times corresponding to ion, monomer, Pol + Pol − pair exchange, entanglement, and reptation was determined or estimated. The zero-shear viscosity, η 0 , was found to be an unusually strong function of molecular weight, with the scaling η 0 ∼ M 5 . A polymer coil size, measured by small-angle neutron scattering, was used in concert with new quantitative expressions to provide a good fit of theory to experiment for this unusual scaling.

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.

Though the strong transformation in mechanical properties of glass-forming materials such as amorphous polymers near the glass transition, T g, has long been recognized and exploited, efforts to understand and predict this phenomenon at a molecular level continue to this day. Close to T g, where relaxation is considerably slower than predicted by the well-known Arrhenius equation, one of the most versatile and widely used expressions to describe the dynamics or relaxation of glass formers is that of Vogel, Fulcher, and Tammann (VFT). The VFT equation, introduced nearly 100 years ago, contains three adjustable fit parameters. In the present work, polyelectrolyte complexes, hydrated amorphous blends of charged polymers, are used to investigate ion transport phenomena reporting the dynamics of individual polymer repeat units, which, in turn, control macroscopic dynamics. A simple analytical expression, containing no freely adjustable fit parameters, is derived to quantitatively model relaxation from T g to temperatures well into the Arrhenius region. The new expression, which also fits a selection of three common neutral polymers, will advance the understanding and use of the glass-forming phenomenon.

Salt bridges, pairings between oppositely charged amino acids, are dispersed throughout proteins to assist folding and interactions. Biopolyelectrolyte complexes (BioPECs) were made between the homopolypeptides poly-l-arginine (PLR) and poly-l-lysine (PLK) with sodium triphosphate (STPP), as well as from polypeptide-only combinations. Viscoelastic measurements on these high salt bridge density materials showed many were solid, even glassy, in nature. Although the polypeptide–phosphate complexes had similar moduli at room temperature, the PLR–STPP complex displayed an unusual melting event above 70 °C not seen in PLK–STPP. This event was supported with differential scanning calorimetry. Infrared spectroscopy showed the PLK–STPP system contained β-sheets, while PLR–STPP did not. Stoichiometric, macroscopic BioPECs of PLR and PLK with poly-l-aspartic acid (PLD) and poly-l-glutamic acid (PLE) were made. PLR–PLD was found to undergo a melting event similar to that in PLR–STPP. ATR-FTIR studies showed that BioPECs made with PLD do not contain β-sheets, while those composed of PLE do. This work illustrates an expanded palette of unique properties from these biomaterials, such as strong viscoelastic differences between PECs containing PLE and PLD, even though they differ by only one carbon on the side chain.

Pinhole-free ultrathin films of polyelectrolyte complex assembled using layerby-layer deposition were used to evaluate electron transfer from a redox species in solution to an electrode over the distance range of 1−9 nm. Over this thickness, the polyelectrolytes employed wet the surface and the polymer molecules flattened to less than their equilibrium size in three dimensions. A decay constant β for current as a function of distance of about 0.3 nm −1 placed this system in the regime expected for multistep hopping versus a one-step tunneling event. Discreet hopping sites within the films were identified as ferrocyanide ions with an equilibrium concentration of 0.032 M and an average separation of 3.7 nm. The Butler−Volmer (BV) expression for electron transfer as a function of overpotential was modified by distributing the applied voltage evenly among the hopping sites. This modified BV expression fits both the distance dependence and the applied potential dependence well, wherein the only freely adjustable parameter was the electron transfer coefficient. The finding that β is simply the inverse of the hopping range is consistent with previous conclusions that electrons within conjugated molecule sites are delocalized, or, for nonconjugated systems, spread over more than one repeat unit by lattice distortions.