H. B. Hopfenberg

Diffusion of organic vapors at low concentrations in glassy PVC, polystyrene, and PMMA

Abstract Diffusion coefficients of various C 1 to C 6 organic vapors, at concentrations 0.5 wt. percent, have been determined by gravimetric sorption rate measurements on emulsion and suspension-polymerized powder samples of PVC, polystyrene, and PMMA. Fickian diffusion kinetics were observed at the lowest concentrations, with a second-stage, relaxation-controlled sorption appearing at higher concentrations. In conjunction with published data for diffusivities of fixed gases in these polymers, the results indicate that diffusivity decreases exponentially, and that diffusion activation energy ( E D ) increases linearly, with increasing diameter of “spherical” penetrant molecules (e.g., the noble gases, CH 4 , SF 6 , CCl 4 , and neopentane). Much of the observed scatter in these correlations is attributable to uncertainty in the molecular diameters. For C 4 and larger n-alkanes and other elongated or flattened molecules, diffusivities are higher, and E D lower, than for spherical molecules of similar molar volume. This finding suggests that anisometric molecules are oriented and move along their long axes during diffusion through the glassy polymer matrix. Correlations of diffusivities with molecular dimensions suggests that transport of anisometric molecules is governed by a diameter smaller than the mean (equivalent sphere) diameter but larger than the minimum dimension of their extended-chain conformation. Among the three polymers studied, diffusivity of each penetrant, at a given temperature, decreases in the order polystyrene > PVC ≥ PMMA.

Transport of gases in unmodified and arylbrominated 2,6-dimethyl- 1,4-poly (phenylene oxide)

Abstract Independent solubility and permeability data, measured at 35°C at up to 26 atm, are reported to show the influence of aryl-bromination on the transport of CO2, CH4, and N2 in 2,6-dimethyl-1,4-poly(phenylene oxide) (PPO). The permeability of PPO was found to vary with the extent of bromination, and the magnitude of change depends on the nature of the gas. The apparent solubility coefficients of all three gases at 20 atm in the polymer increased with the extent of bromination, and the percentage of increase was higher for the gas with lower condensability. The concentration-averaged diffusivities of CO2 and CH4 also showed some variation with the extent of bromination. In particular, there was a notable increase in the diffusivity of CO2 but a slight decrease in that of CH4 when the extent of bromination was increased to 91%. The gas-transport data were also analyzed according to the dual-mode model. The dual-mode parameters exhibit similar dependence on the extent of bromination as the apparent solubility coefficient and concentration-averaged diffusivity do. These observations are interpreted in terms of changes in the average packing, torsional mobility of the chain segments, and cohesive energy density of the polymer.

The barrier properties of polyacrylonitrile

Abstract The permeabilities and time lags characterizing transport in machine-cast polyacrylonitrile (PAN) films have been determined for helium, neon, argon, krypton, oxygen, nitrogen, and carbon dioxide. The measured permeabilities were decreased slightly and the observed time lags were increased consequent to scrupulous removal of residual solvent. The activation energies for permeation were not changed consequent to film purification, however. In all cases, the measured permeabilities were remarkably low. The permeability of PAN to helium was inordinately higher than the observed permeability of PAN to the other gases studied. The relatively high permeability to helium and the low activation energies for permeation suggest that the excellent barrier properties of PAN are related to the very small size of the fluctuating free volume elements involved in the rate-determining transport step for fixed gases in and through this particular polymer.

Factors affecting water sorption in and solute release from glassy ethylene—vinyl alcohol copolymers

Abstract Liquid water sorption kinetics and equilibria in ethylene—vinyl alcohol copolymers of 62, 70 and 72 mole percent vinyl alcohol are characterized at 4° and 25°C. The effect of vinyl alcohol content on both sorption kinetics and equilibria is discussed. Particular attention has been devoted also to the description of the desorption kinetics of ionic solutes which have been previously sorbed and trapped in these glassy polymers. p]Solute diffusion coefficients in the swollen polymer and the relaxation parameters associated with the largely relaxation-controlled swelling of the glass have been measured. Sodium chloride release essentially follows water uptake during the swelling of the glass. Conversely, Fickian desorption patterns have been observed for the release of a highmolecular-weight dye, since complete swelling was achieved rapidly and diffusion-controlled solute release lagged behind the rapid swelling process. Comparison between solute diffusive conductance and the rate of penetration of the advancing swelling fronts leads to predictions of the relative contributions of diffusion and relaxation to the observed release patterns.

Pure and mixed gas sorption of carbon dioxide and ethylene in poly(methyl methacrylate)

Abstract The sorption of CO2, C2 H4 and mixtures of these gases in poly(methyl methacrylate) at 35°C has been characterized at pressures up to 6.5 atm. The pure gas sorption isotherms exhibit significant concave curvature to the pressure axis and are described well by the dual mode sorption model. The mixed gas sorption data indicate that significant decrements in the solubility of each component in the mixture occur relative to the pure component case at equivalent partial pressures. This phenomenon is predicted accurately by the dual mode sorption model, which accounts for competition between mixture components for sorption in the polymer. The model requires only pure component sorption parameters to describe the mixture behaviour over the full range of composition. The mixture data reveal that the model should be formulated in terms of the gas phase fugacity rather than the partial pressures of the gases at high pressures where non-ideal gas mixture behavior may occur. Deviations between mixed gas sorption data and model predictions based on pure component dual mode parameters are small (± 2.1%) and probably negligible for most purposes for the pressure range studied.

Sorption and transport of organic vapors in poly[1-(trimethylsilyl)-1-propyne]

Abstract Sorption kinetics and apparent equilibrium isotherms were determined gravimetrically at 35°C for organic vapors in glassy poly[1-(trimethylsilyl)-1-propyne] (PMSP). The sorption kinetics are characterized by diffusion coefficients on the order of 10 −8 cm 2 /sec for the normal hydrocarbons (C 3 -C 9 ). These diffusivities are more than one million times larger than corresponding diffusivities characterizing the transport of corresponding penetrants in conventional glassy polymers such as polystyrene, poly(vinyl chloride), and poly(methyl methacrylate). The sorption capacity of this polymer is also quite high compared with values observed for more conventional polymeric glasses. The sorption isotherms of the organic vapors in PMSP follow a generalized dual mode behavior which is often observed for vapors in polymeric glasses. Specifically, at low activities the isotherms are concave toward the activity axis, then become convex to the activity axis at higher activities. The transition separating these two portions of the isotherms does not become evident until the sorption level in PMSP reached more than 30 weight percent, compared with less than 10 weight percent for other glassy polymers. These unusual properties can be related to the low packing density of PMSP. However, the results also reveal that although the quantitative values of diffusivities and solubilities are extraordinary, the qualitative behavior regarding sorption and diffusion is consistent with comparable observations on conventional glassy polymers.

Super Case II Transport of Organic Vapors in Glassy Polymers

Alfrey, Turner, and Lloyd have defined Fickian transport and Case II transport as the two limiting cases for the transort of an organic penetrant through an amorphous glassy polymer.1 Fickian transport refers to the interdiffusion of penetrant and polymer described by Fick’s equations. In general, Fickian diffusion is only observed in glassy systems when the penetrants are simple gases, solvents with small molecular diameters, or partial solvents at very low temperatures and penetrant activities. At temperatures and penetrant activities where partial solvents swell the polymer, transport is often controlled by a combination of polymer relaxation and Fickian diffusion mechanisms. Case II transport occurs when the sorption is entirely controlled by stress-induced relaxations taking place at a sharp boundary separating an outer swollen shell, essentially at equilibrium penetrant concentration, from an unpenetrated glassy core. Ideally, this sharp boundary moves through the polymer at a constant velocity during Case II transport.

Mixed gas sorption in glassy polymers: Equipment design considerations and preliminary results

Abstract A novel apparatus to measure precisely both pure and mixed gas sorption in glassy polymers is described. A method for accurate calibration and operation of the equipment is discussed, based upon complete closure of the material balance of all gaseous components in all phases. The description is accompanied by pure and mixed gas sorption equilibria measured with this equipment for the system CO 2 /C 2 H 4 /Poly(methyl methacrylate) (PMMA) at 35°C. The observed reduction in sorption of individual components is explained by competition between the penetrants for the unrelaxed volume fraction in a glassy polymer. This effect is a natural consequence of the dual mode sorption model applied to mixed penetrants.

The effect of film thickness and sample history on the parameters describing transport in glassy polymers

Abstract The effect of a characteristic dimension of a glassy polymeric specimen on the kinetic mechanism controlling unidirectional absorption of organic vapors and liquids in glassy polymers is demonstrated by comparing n-alkane absorption experiments in microspheres, spheres, films, and sheets of polystyrene. Absorption in submicron microspheres is controlled by Fickian diffusion whereas, under otherwise identical boundary conditions, films (ca. 75μm thick) and spheres (ca. 184 μm in diameter) sorb according to Case II absorption kinetics. Thinner films (35 μm thick) sorb by Super Case II kinetics and relatively thick sheets (2000 μm thick) sorb initially by Case II kinetics but, at long times, diffusion through the outer swollen region contributes significantly to the overall resistance to mass transfer and the rate of absorption decreases progressively with time. p]The rather short experimental times, afforded by the exceedingly small mean diameter of the narrowly distributed microsphere powder sample, permitted convenient characterization of the effects of preswelling, sorption-desorption cycling, and annealing on the kinetics and apparent equilibria of sorption. History effects were quite dramatic and were related to glassy state relaxations initiated by the various thermal and swelling histories imposed upon the glassy microspheres. p]Prediction of sorption and permeation behavior in membranes, from kinetic and equilibrium parameters determined experimentally on film and powder samples, requires explicit recognition of these dimensional and history effects. These effects do not appear to be related to any special properties of this polymer-penetrant system.

A note on the effects of mono-and di-bromination on the transport properties of poly(2,6-dimethylphenylene oxide)

Abstract The permeability of poly (2,6-dimethylphenylene oxide) (PPO) to carbon dioxide and methane at 35°C can be increased more than 100% by aryl-bromination, depending on the extent of bromination, whereas, the carbon dioxide/methane permselectivity of the polymer, which ranges from 17–20, is changed only slightly by bromination. The increase in permeability results primarily from a marked increase in the penetrant diffusivity, although the penetrant solubility is also moderately increased by bromination. These results are interpreted in terms of changes in packing density and chain torsional mobility brought about by aryl-bromination of the polymer.