Sorption and diffusion of organic vapors in two fluoroelastomers

Immersion experiments with Aflas (I), poly(tetrafluoroethylene‐co‐propylene), and Fluorel (II) [poly(vinylidene fluoride‐co‐perfluoropropylene)], showed greater swelling of I in nonpolar liquids and much higher swelling of II in polar liquids: over 100% (wt/wt) in two ketones and a phosphate ester. Sorption isotherms determined for toluene and acetone at 25 and 35°C were fitted with the Flory–Rehner relation, employing a concentration‐dependent solvent–polymer interaction parameter. The fitted K parameters indicated that the degree of crosslinking in II was lower than in I. However, the high swelling of II by polar solvents is attributed primarily to the polar nature of II resulting from the asymmetric CF(CF₃) moiety. Diffusion coefficients determined from sorption kinetics, corrected for nonisothermal effects, and converted to solvent self‐diffusion coefficients were fitted with the Fujita free‐volume relation. The values were much higher for I than II with acetone and also slightly higher for I with toluene. The estimated zero‐concentration values were 1.5 E‐09 cm²/s for Aflas–acetone, 0.3 E‐09 cm²/s for Fluorel–acetone, and even lower for toluene. The low diffusion coefficients, which contribute to the superior barrier performance of these elastomers, is due, in part, to the high glass transition temperatures of I and II, −7 and −21°C, respectively. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1525–1535, 1999     

Anomalous diffusion of water in glassy polymers

Anomalous diffusion of water has been observed in polystyrene and in epoxy resin. Sudden changes in the boundary conditions can cause the absorbed water to agglomerate and form a second phase within these polymers. The agglomerated water can force the polymer segments apart and create permanent crazes. The diffusion of water in crazed polymers is non-Fickian. The solubility becomes a function of the total pressure and the amount of water taken up from liquid water is much more than that absorbed in saturated water vapour.     

Sorption and diffusion of alcohols in amorphous polymers

The effects of polymer composition and penetrant molecular size on the solubility and diffusivity of alcohol vapors in a series of well characterized isoprene-methyl methacrylate copolymers and their corresponding homopolymers has been investigated at room temperature. The rate of sorption behavior changes progressively from Fickian to non-Fickian, to Case II to “Super Case II” transport with increasing methyl methacrylate (MMA) content in the polymers. The equilibrium solubility of the alcohols increases linearly with increasing penetrant molecular size for polymers which are above their glass transition temperature and decreases for polymers which are below their T_g. The solubility also initially increases as an approximately linear function of MMA content in the copolymers. At about 55 mole percent MMA, the sorbed concentration either levels off or passes through a maximum depending on the size of the penetrant. The apparent “diffusion coefficients” (D) decrease with increasing molecular volume of the penetrants. An exponential dependence was found between these two variables for PMMA. These “diffusion coefficients” also decrease exponentially with increasing MMA content in these polymers. However, at 55 mole percent MMA the copolymer undergoes a rubber to glass transition at the temperature of the experiments. On this basis, it is suggested that the hindered chain segmental motion contributes to the sorption process in addition to strictly thermodynamic considerations. Free volume theory can be used to explain the mechanism of diffusion through the rubbery polymers while the “hole” theory can be applied to explain the transport of the penetrants through the glassy polymers.     

Sorption of vapors in cellulose film

The rates of sorption of water vapor and of methanol vapor in cellulose film were examined by stepwise increments of vapor pressure. In addition, the rates of sorption of the series of vapors water, methanol, ethanol, n-propanol, and n-butanol were studied at pressures just below the respective saturated vapor pressures. The results obtained from both parts of the study are explained in terms of a bimodal diffusion mechanism. This consists of the simultaneous diffusion of vapor into the microporous structure of the cellulose film and diffusion from the micropores into the polymer matrix.     

Estimation of diffusion coefficients for trace amounts of solvents in glassy and molten polymers

Two methods based on the free-volume theory of transport are developed for the estimation of diffusion coefficients for trace amounts of solvents in amorphous polymers. The first method uses diffusivity data for a polymer–solvent system above the glass transition temperature to estimate the temperature dependence of the mutual diffusion coefficient below this temperature. In the second method, mutual diffusion coefficients are estimated for a particular polymer–solvent system both above and below the glass transition temperature using no diffusivity data for the system. The predictions of the proposed theory are compared with diffusivity data for the n-pentane–polystyrene and ethylbenzene–polystyrene systems.     

Deformation in glassy polymers

On the basis of known nonlinearity of intermolecular force fields, we discuss the interpretation of PVT (pressure, volume, and temperature) behavior, pressure-temperature superposition for polymers, and the relationship between yield stress and tensile modulus. For PVT behavior of polymers, our theoretical results coincide with the experimental data, and their response to pressure is universal. The maximum theoretical yield strain, ϵ_y for glassy polymers is 1.08, and this value is beyond the elastic limit for glassy polymers. The previously established empirical relationship between yield stress, σ_y and tensile modulus, E: σ_y - 0.028 E, which again, is universal for glassy polymers, is predicted also by our phenomenological model. The theoretically predicted values of yield stress for glassy polymers range from 24 MPa to 84 MPa, coinciding with published experimental results. We discuss how the phenomenological model is helpful in the understanding of nonlinear viscoelasticity of glassy polymers © 1996 John Wiley & Sons, Inc.     

Additives in transparent glassy polymers: Concentration profiles obtained by solvent diffusion technique

Concentration profiles of additives, introduced in transparent glassy polymers by solvent diffusion technique, are analyzed by measuring refractive index profiles. Such profiles depend not only on the diffusion conditions (temperature, time, kind of solvent, concentration), but also on the conditions of solvent desorption from the polymer (temperature, pressure, solvent, and additive volatilities). In particular, poly(methyl methacrylate) (PMMA) samples modified in an outer shell by zinc chloride carried by methanol are considered.     

Particulate-filled glassy polymers

Glass beads, NaCl crystals, or other imperfections act as stress concentrators in glassy polymers causing early crazing upon stressing. Stress–strain curves of glassy thermoplastic polymers filled with uncoupled glass beads or NaCl crystals exhibit a knee, or a slope-discontinuity point. The characteristics of the slope change, related to crazing, are studied in the present work. Foams were produced from the polymer/salt samples by soaking them in water resulting in complete leaching of the salt. This preparation procedure gives open-cell foams and there are strong indications that these polymeric foams have a highly cracked structure formed during the extraction step.     

Atomistic molecular dynamics simulations of gas diffusion and solubility in rubbery amorphous hydrocarbon polymers

Diffusion coefficients D of H₂, He, O₂, N₂, and CO₂ in different rubbery amorphous polymeric matrices were estimated by atomistic molecular dynamics simulations at 298 K using the Einstein relationship, and compared with the relevant experimental values, where available. The simulated diffusion coefficients D of all the gases in all polymers considered almost regularly decreased with increasing molecular gas volumes and increasing polymer glass transition temperature. Further, solubility coefficients and heats of solution were obtained for all gases from Grand Canonical Monte Carlo simulations, which were also used to calculate sorption isotherms. In general, there is a good agreement between experimental and simulated values of diffusion and solubility coefficients for all gases considered.     

Sorption kinetics and equilibria in annealed glassy polyblends

The sorption kinetics and equilibria of n-hexane in glassy polyblends of polystyrene and poly(2,6 dimethyl-1,4 phenylene oxide) were studied as a function of annealing conditions. Cast film samples were annealed 20°C above their respective glass transition temperatures for two hours and twenty-four hours. The rate of relaxation-controlled (Case II) sorption of n-hexane in these films was reduced markedly consequent to annealing. The effect of annealing on the sorption kinetics and the independently determined film densities was more pronounced for the poly(phenylene oxide)-rich samples. Although sorption rates were reduced by as much as a factor of 100, the sorption equilibrium was insignificantly affected by annealing. Super Case II transport was observed for the slow absorbing annealed samples whereas the more rapid sorption in the unannealed samples followed ideal Case II kinetics. The more pronounced effects of annealing for the poly(phenylene oxide)-rich samples on sorption rates and film densities were explained by considering the increasing difference between the film T_g and the drying temperature used in the original film preparation for the poly(phenylene oxide)-rich samples. These results suggest that glassy polymers, cast and dried well below their glass transition temperatures, will be subject to large long-term reductions in absorption rates and specific volume. Moreover, residual, excess free volume significantly affects relaxation-controlled absorption of vapors in partially annealed glassy polymers.     

Sorption of organic liquids and vapors by rigid PVC

The effects of activity and solvent strength of a number of organic penetrants upon their sorption kinetics and equilibria in poly(vinyl chloride) have been studied by gravimetric vapor and liquid sorption experiments at 30°C. For each solvent, the relation of equilibrium sorption to activity is well approximated by the Flory–Huggins equation with a characteristic value of the interaction parameter χ. The glass transition temperature T_g is depressed in direct proportion to the volume fraction of solvent absorbed; the composition corresponding to a T_g of 30°C, C_g (30°C), is in the range of 0.22–0.30 volume fraction for several common solvents. The form of the sorption kinetics varies with the ratio of the equilibrium sorption to C_g, and thus depends on the combined effects of χ, solvent activity, and plasticizing action. When the equilibrium sorption is less than about C_g/2, kinetics are Fickian, with the very low diffusivities typical of the glassy state; for sorption values between C_g/2 and C_g, anomalous kinetics are observed; and when the sorption is greater than C_g, transport in thin PVC films follows Case II kinetics. At high sorption levels, increasing film thickness produces a shift of the kinetics toward Fickian form with apparent diffusivity values typical of rubbery polymers.     

Sorption of vapors and gases by cellulose acetate films

Cellulose diacetate and triacetate films 6–55 µm thick with a total porosity of 0.8–2% were prepared by pouring a solution on an immobile surface and by calendering. Sorption of H₂S and CO₂ gases and acetone vapors on these films was investigated in static and dynamic conditions. It was shown that films 6–10 µm thick prepared by calendering should be used for purification of air from H₂S and CO₂ and films 15–20 µm thick prepared by pouring should be used for removal of acetone vapors.Engels Institute of Technology, Saratov State University. Translated from Khimicheskie Volokna, No. 6, pp. 44–46, November–December, 1995.     

Plastic flow in glassy polymers

Massachusetts Institute of Technology Cambridge, Massachusetts 02.139 The phenomenology of plastic deformation in glassy polymers is described, and a theory based on a molecular model for such deformation is presented and contrasted with other published theories. A phenomenological yield criterion is presented which is consistent with the theory. It is shown that the theory is in very good accord with experimental results on the temperature dependence of the plastic shear resistance of a large group of glassy polymers with widely different molecular structures. Finally, the formation and growth of shear deformation bands is described, based purely on the mechanics of localization processes in inelastically deforming continua.     

Sorption of water vapors by high-strength para-aramid fibres

The water vapor sorption isotherms were measured for different types of high-strength fibres based on carbocyclic and heterocyclic para-aramids: Rusar (copolymers of amidobenzimidazole and p-phenylene terephthalamide), Twaron and Kevlar (poly-p-phenylene terephthalamide), and for comparison, capron (polycaproamide). The parameters of the thermal sorption equation and the thermodynamic characteristics of sorption of water vapor were calculated with previously obtained sorption data with the previously developed theoretical-probability model of sorption. A correlation was found between the integral heat of sorption in amorphous regions of para-aramid fibres and the specific cohesive energy. __________ Translated from Khimicheskie Volokna, No. 5, pp. 50–53, September–October, 2007.     

Adsorption of vapors by amorphous polymers

The equation describing sorption of vapors by solutions of low-molecular-weight substances in the case of formation of solvates with a nonvolatile solvent can coincide in form with the BET equation. In calculating the specific surface area with the sorption value, it is necessary to determine the contributions of absorption (dissolution) processes in bulk and adsorption processes on the phase boundary to the overall sorption. Translated from Khimicheskie Volokna, No. 5, pp. 28–30, September–October, 2008.     

Enthalpy relaxation in glassy polymers

Summary Constitutive equations are derived for enthalpy recovery in amorphous glassy polymers. The model is based on the concept of cooperative relaxation which treats a polymer as an ensemble of flow units rearranging at random times. The rate of rearrangement is determined by the Eyring theory of thermally activated processes. Fair agreement is demonstrated between results of numerical simulation and experimental data for polycarbonate, poly(ether imide), poly(methyl methacrylate), polystyrene and a glycerol glass. Received: 5 December 1999/Revised version: 28 February 2000/Accepted: 2 March 2000