Henry's law coefficients and partial molar volumes of 34 penetrants (5 inert gases, 6 inorganic gases, 17 hydrocarbon gases, 5 fluorinated gases, and CCl4 vapor) dissolved in poly(dimethylsiloxane) and low-density polyethylene were determined at 25 °C by measuring sorption of the gases and the concomitant dilation of the polymers. From the Henry's law coefficients and the partial molar volumes, Flory−Huggins parameters for polymer/gas interactions were estimated. The partial molar volumes were correlated with critical molar volumes of gases, and the interaction parameters were found to depend on the partial molar volumes. These relationships for the fluorinated gases were clearly different from those of all other gases. For CO2 and CH4 in poly(dimethylsiloxane), partial molar volumes and interaction parameters were obtained as a function of temperature over a range −30 to 95 °C. Thermal expansivities of these dissolved molecules were estimated to be 2 × 10-3 °C-1 from the temperature dependence of partial molar volumes.
Sorption of CO2 in poly(methyl methacrylate) at 35−200 °C and concurrent dilation of the polymer at 35−85 °C over a pressure range up to 50 atm were studied. Dissolution and Flory−Huggins interaction parameters for the gas in the polymer, not only in the rubbery state but also in the glassy state, were estimated by analyzing the sorption data above the glass transition temperature (T g0, 105 °C). Isothermal glass transition of the polymer/gas system was observed on isotherms of sorption and dilation below T g0. Partial molar volumes of sorbed CO2 determined from the sorption and dilation isotherms increased with increasing concentration to the glass transition concentration. These isotherms were also analyzed on the basis of extended dual-mode models of sorption and dilation. From obtained parameters of the dual-mode models, nonequilibrium properties such as mean size and number of microvoids for the pure polymer and the CO2-sorbed polymer in the glassy state were evaluated. The mean size, dependent upon CO2 exposure history of the polymer, was in the range of 20−100 A3, and the number of microvoid ((1−18) × 1020 voids/cm3) was dependent upon both temperature and the exposure history.
Sorption of organic gases (C2H4, C2H6, C3H6, C3H8, n-C4Hi0, iso-C4Hi0, and n-C5Hi2) in two rubbery polymers, 1,2-polybutadiene and poly(ethylene-co-vinyl acetate), and dilation of the polymers due to sorption are measured as a function of gas pressure at 25 °C. Sorption isotherms for all the gases are well described as the Flory-Huggins dissolution. Dilation isotherms are similar in shape to the corresponding sorption isotherms. From sorption and dilation data, partial molar volumes Vr of the dissolved gases are determined. A linear relation is found between partial molar volume and van der Waals volume Vw, i.e., Vr = 1.6 Vw + 18.5 in cm3/mol. This relation is almost the same as that between molar volume and Vw for liquid n-alkanes.
A method is proposed to analyze the effect of pressure on permeation of gases through semicrystalline polymers above the glass transition temperature. The method utilizes similarities in molecular diameters of the gases and differences in their solubilities. Two polymers, polyethylene and polypropylene, and a series of gases are chosen for an application of the method, and the effect of pressure on the permeabilities for 10 gases is measured in the pressure range 1–130 atm at 25°C. For polymers, the logarithm of the permeability coefficient is linear in the pressure for each gas, with negative slope for slightly soluble gases (He, Ne, H2, N2, O2, and Ar) and positive slope for highly soluble gases (CH4, Kr, CO2, and N2O). Analyzing these slopes by the method proposed permits contributions of hydrostatic pressure and concentration to the pressure dependence of permeation to be evaluated. On the basis of the results, the mechanism of gas permeation in rubbery films under high pressures is discussed.
SYNOPSISThe effect of pressure on gas permeability of a rubbery polymer, 1,2-polybutadiene, is investigated for 15 gases with various molecular sizes and solubilities in the ranges of pressure up to 110 atm at 25°C. The permeability for slightly soluble gases (He, Ne, Hz, Nz, 02, and Ar) decreases with increasing pressure, and that for soluble gases (CH,, Kr, COP, N20, C2H4, Xe, C2Hs, C3Hs, and C,H,) increases with increasing pressure. Logarithms of permeability coefficient versus feed-gas pressure for the slightly soluble gases, CH, and Kr, is linear within each pressure range, whereas such plots become convex toward the pressure axis for more soluble gases, such as COz, NzO, C2H4, Xe, C&, C3H6, and C3Hs.By analyzing the pressure dependence of permeability using sorption data of the gases, contributions of concentration and hydrostatic pressure to the gas diffusivity are estimated.
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