Effects of Polymer Molecular Weight on Structure and Dynamics of Colloid-Polymer Bridging Systems.

We investigate the effects of polymer molecular weight on the structure and dynamics of a model colloid-polymer bridging system using confocal microscopy. Polymer-induced bridging interactions between trifluoroethyl methacrylate-co-tert-butyl methacrylate (TtMA) copolymer particles and poly(acrylic acid) (PAA) polymers of molecular weight Mw of 130, 450, 3000, or 4000 kDa and normalized concentrations c/c* ranging from 0.05 to 2 are driven by hydrogen bonding of PAA to one of the particle stabilizers. At a constant particle volume fraction ϕ = 0.05, the particles form clusters or networks of maximal size at an intermediate polymer concentration and become more dispersed upon further addition of polymer. Increasing the polymer Mw at a fixed normalized concentration c/c* increases the cluster size: suspensions with 130 kDa polymer contain small clusters that remain diffusive, and those with 4000 kDa polymer form larger, dynamically arrested clusters. Biphasic suspensions with distinct populations of disperse and arrested particles form at low c/c*, where there is insufficient polymer to bridge all particles, or high c/c*, where some particles are sterically stabilized by the added polymer. Thus, the microstructure and dynamics in these mixtures can be tuned through the size and concentration of the bridging polymer.

Fouling Resistance and Release Properties of Poly(sulfobetaine) Brushes with Varying Alkyl Chain Spacer Lengths and Molecular Weights.

We examined the effects of alkyl carbon spacer length (CSL) and molecular weight on fouling resistance and release properties of zwitterionic poly(sulfobetaine methacrylate) brushes. Using surface-initiated atom transfer radical polymerization, we synthesized two series of brushes with CSL = 3 and 4 and molecular weight from 19 to 1500 kg ·mol-1, corresponding to dry brush thickness from around 6 to 180 nm. The brush with CSL = 3 was nearly completely wet with water (independent of molecular weight), whereas the brush with CSL = 4 exhibited a strong increase in water contact angle with molecular weight. Though the two-brush series had distinct wetting properties, both series of brushes exhibited similarly great resistance against fouling by Staphylococcus epidermidis bacteria and Aspergillus niger fungi spores when submerged in water, indicating that neither molecular weight nor CSL strongly affected the antifouling behavior. We also compared the efficacy of brushes against fouling by fungi and silicon oil in air. Brushes grafted to filter paper were strongly fouled by fungi and silicon oil in air. Grafting the polymers to the filter paper, however, greatly enhanced removal of the foulant upon rinsing. The removal of fungi and silicon oil when rinsed with a salt solution was enhanced by 219 and 175%, respectively, as compared to a blank filter paper control. Thus, our results indicate that these zwitterionic brushes can promote foulant removal for dry applications in addition to their well-known fouling resistance in submerged conditions.

Electrostatic Repulsion Slows Relaxations of Polyelectrolytes in Semidilute Solutions.

We investigate the structure and dynamics of unentangled semidilute solutions of sodium polystyrenesulfonate (NaPSS) using small-angle neutron scattering (SANS) and neutron spin-echo (NSE) spectroscopy. The effects of electrostatic interactions and chain structure are examined as a function of ionic strength and polymer concentration, respectively. The SANS profiles exhibit a characteristic structural peak, signature of polyelectrolyte solutions, that can be fit with a combination of a semiflexible chain with excluded volume interactions form factor and a polymer reference interaction site model (PRISM) structure factor. We confirm that electrostatic interactions vary with ionic strength across solutions with similar geometries. The segmental relaxations from NSE deviate from theoretical predictions from Zimm and exhibit two scaling behaviors, with the crossover between the two regimes taking place around the characteristic structural peak. The chain dynamics are suppressed across the length scale of the correlation blob, and inversely related to the structure factor. These observations suggest that the highly correlated nature of polyelectrolytes presents an additional energy barrier that leads to de Gennes narrowing behavior.