David J. Moll

Dynamics of light gases in rigid matrices of dense polymers

Transition‐state theory was employed to study the dynamics of light gases dissolved in rigid microstructures of glassy polycarbonate and rubbery polyisobutylene modeled in atomistic detail. The gaseous molecules migrated through the polymer structures in a sequence of ‘‘hops’’ between local minima of the potential energy. The solute dynamics was characterized by three time domains: at short times, <10−12 s, the mobility was very high and the molecules traveled on a scale of 5 A; this was followed by a domain of anomalous diffusion; at long times the tiniest molecules (He and H2) followed the Einstein diffusion law. Larger molecules (Ar, O2, and N2) did not reach the diffusive regime at the time scale of simulation (up to ca. 10−3 s) but were trapped instead in the vicinity of their initial sites without any progress in translational motion. It was concluded that the rigid‐matrix approach is inadequate for studying the dynamics of light gases in dense polymers, except for He.

Characterization and effect of biofouling on polyamide reverse osmosis and nanofiltration membrane surfaces

Biofouling is a major reason for flux decline in the performance of membrane-based water and wastewater treatment plants. Initial biochemical characterization of biofilm formation potential and biofouling on two commercially available membrane surfaces from FilmTec Corporation were investigated without filtration in laboratory rotating disc reactor systems. These surfaces were polyamide aromatic thin-film reverse osmosis (RO) (BW30) and semi-aromatic nanofiltration (NF270) membranes. Membrane swatches were fixed on removable coupons and exposed to water with indigenous microorganisms supplemented with 1.5 mg l−1 organic carbon under continuous flow. After biofilms formed, the membrane swatches were removed for analyses. Staining and epifluorescence microscopy revealed more cells on the RO than on the NF surface. Based on image analyses of 5-μm thick cryo-sections, the accumulation of hydrated biofoulants on the RO and NF surfaces exceeded 0.74 and 0.64 μm day−1, respectively. As determined by contact angle the biofoulants increased the hydrophobicity up to 30° for RO and 4° for NF surfaces. The initial difference between virgin RO and NO hydrophobicities was ∼5°, which increased up to 25° after biofoulant formation. The initial roughness of RO and NF virgin surfaces (75.3 nm and 8.2 nm, respectively) increased to 48 nm and 39 nm after fouling. A wide range of changes of the chemical element mass percentages on membrane surfaces was observed with X-ray photoelectron spectroscopy. The initial chemical signature on the NF surface was better restored after cleaning than the RO membrane. All the data suggest that the semi-aromatic NF surface was more biofilm resistant than the aromatic RO surface. The morphology of the biofilm and the location of active and dead cell zones could be related to the membrane surface properties and general biofouling accumulation was associated with changes in the surface chemistry of the membranes, suggesting the validity of the combination of these novel approaches for initial assessment of membrane performance.

Water Transport in Ionic Polymers

Hydrophilic polymers such as perfluorinated ionomers with sulfonic acid functional groups have high water permeabilities and good selectivities for water over most gases and organic liquids. Understanding the water transport mechanism in these water plasticized membranes is necessary in order to effectively utilize them for dehydration applications. Water diffusion in these polymers is non-Fickian. Water permeabilities and solubility coefficients are a function of water vapor pressure. A careful analysis is required to arrive at the concentration dependent diffusion coefficients. An approach for calculating thermodynamic diffusion coefficients based on steady state permeabilities and equilibrium sorption measurements is presented in this paper. At very low water vapor pressures, strong ion-water interactions give rise to high solubility coefficients and low diffusion coefficients. At water activities of 0.1 to 0.6, the solubility coefficient decreases and the diffusion coefficient increases with water activity. At very high water activities (≥0.6), the diffusion coefficient appears to attain a limiting value while the solubility coefficient sharply increases. An anomalous thickness effect was also observed. Thickness normalized permeabilities increased with increasing membrane thickness. Several mechanisms were considered.