T. K. Kwei

The effect of hydrogen bonding on vapor diffusion in water-soluble polymers

Water-vapor transport in poly(acrylic acid) (PAA), poly(N-vinyl-2-pyrrolidone) (PVPo), and their 2 : 1 interpolymer complex was measured between 30 and 50°C. It was found that the formation of interpolymer hydrogen bonding not only caused the Tg of the complex to be higher than those of the individual components, but also affected vapor-sorption behaviors. In the high vapor pressure region (Pr > 0.5), sorption isotherms in pure PAA and PVPo follow the Flory–Huggins solution theory and the sorbed water molecules may form clusters around the COOH groups in PAA and C=O groups in PVPo. However, sorption in the complex is almost linear for the entire range of water activities. The binary polymer—polymer interaction parameter between PAA and PVPo in the complex, ξPAA–PVPo, is less than zero, which is indicative of favorable interaction between PAA and PVPo. At lower water activities, sorption behaviors for water in the three polymers can be approximated by Henrys law and vapor-diffusion coefficients can be ranked as PVPo > complex > PAA. The water vapor diffusion coefficients in each system decrease with increasing Pr, in the region of Pr > 0.5 because the proportion of immobilized water molecules increases with vapor concentration.

Gas transport properties of polyaniline membranes

The transport properties of He, H2, CO2, O2, N2, and CH4 gases in solvent cast, HCl doped, and undoped polyaniline (PANi) membranes were determined. Measurements were carried out at 40 psi pressure from 19°C to 60°C. An excellent correlation was found between the diffusion coefficients and the molecular diameters of gases. The solubility coefficients of gases were found to correlate with their boiling points or critical temperatures. The sepa-ration factors for CO2/N2 and CO2/CH4 are dominated by the high solubility of CO2. These correlations enable us to predict the permeability, diffusion, and solubility coefficients of other gases. After the doping-undoping process, the fluxes of gases with kinetic diameters smaller than 3.5 A increased but those of larger gases decreased. This results in a higher separation factor for a gas pair involving a small gas molecule and a larger one.

Desorption of water vapor in hydrogen‐bonded polymer blends

Successive desorption experiments of water vapor in poly(methyl methacrylate) (PMMA) were performed at temperatures from 31.0 to 45.0°C. The solubility of water in PMMA was found to be independent of temperature in agreement with literature findings. But the results for diffusion showed stronger dependence on water concentration than those in literature. The diffusion coefficients of water in PMMA became almost independent of temperature at high water concentrations. However, at lower water concentrations, the temperature effect on diffusion was more pronounced. The observed weak temperature dependence of diffusivity at high concentrations is likely due to a high degree of clustering of water molecules found in the PMMA we prepared. Two modified polystyrenes containing 5 and 15 mol %, respectively, 4-hydroxystyrene as comonomer units were blended with PMMA to form hydrogen-bonded polymer blends. Successive desorption experiments of water vapor in the hydrogen-bonded polymer blends were carried out at 31.0°C. The solubility of water in both blends was found to increase with increasing composition of PMMA. The diffusion coefficients for PMMA and its blends increased with increasing concentration of water first, reached a maximum, then decreased with water concentration. When the desorption results were plotted with the previous study of absorption, hysteresis phenomenon of sorption existed in all blend compositions for our experimental time span.

Calculation of the Interaction Parameters in Hydrogen-Bonded Polymer Blends

The values of the interaction parameters in the hydrogen-bonded polymer blends were calculated from the sorption isotherms and the glass transition temperatures and found to be mostly negative. The values of the interaction parameters calculated by the sorption isotherms are reasonable in magnitude but those calculated by the glass transition temperatures are much larger in magnitude. However, the values by either method had the same composition order in the studied blends.