Investigations on the peculiar permeation properties of volatile organic compounds and permanent gases through PTMSP

In contrast to common glassy polymers, poly(1-trimethylsilyl-1-propyne) (PTMSP), a high free volume glassy polymer, shows a preferable permeation of large condensable organic vapors in comparison to permanent gases. In order to investigate this phenomenon, a systematic permeability study over a large activity range has been performed on PTMSP with three types of volatile organic compounds (VOCs) as diffusing probes: toluene, dimethylketone and dichloromethane. PTMSP was synthesized with different catalytic systems (Nb or Ta based) able to induce controlled sub-molecular cis–trans structures. Whereas dimethylketone and dichloromethane permeability can be correctly described by a classical dual-mode equation, a peculiar bell shaped pattern was obtained for toluene, with a minimum permeability located at an activity value around a=0.3–0.4. In that case, only a dual-mode expression taking into account a concentration dependent diffusion coefficient can account for the results.

On the other hand some apparent conflicting data recorded from PTMSP brand new films were related to the microstructure of the polymer main chain thanks to ¹³C NMR spectroscopy analysis showing importance of cis- and trans-forms of the main chain of PTMSP. cis-Structure is more flexible and can be responsible for the creation of a higher density physical network (HDN) in polymeric matrix; conversely, trans-structure is more rigid and can provide lower density physical network (LDN). The higher permeability recorded for several probes through PTMSP synthesized with TaCl₅/Al(i-Bu)₃ catalytic system compared to those of NbCl₅ based polymer can be explained by the geometric difference of the macromolecule networks.     

The permeation of organophosphorus compounds in silicone rubber membranes

The permeabilities, solubilities, and diffusivities of eight organophosphorus chemicals in silicone rubber were measured at saturation concentration using two different experimental methods: permeation experiments and absorption experiments. All tests were carried out at 25°C (±3°C). The eight organophosphorus chemicals investigated are dimethyl methylphosphonate, diethyl methylphosphonate, dimethyl hydrogenphosphonate, diethyl hydrogenphosphonate, trimethylphosphate, triethylphosphate, trimethylphosphite, and triethylphosphite. These eight chemicals were selected based on their similarities to organophosphorus chemicals used as pesticides and chemical warfare agents. The experimental data were analyzed using solutions of Fick's second law of diffusion and boundary conditions representative of the experimental settings. An unsteady-state diffusion model using boundary conditions that represent uniform initial concentration in the polymer and constant but different surface concentrations was used to interpret the permeation experimental data. In this model, the effective diffusivity calculated from the steady-state permeability and equilibrium solubility of each chemical was used and was assumed to be constant.     

Analytical theory for permeation of condensable gases in a nanopore

The permeation of a condensable gas mixture in a pressure gradient is examined within a dynamic density functional theory (DDFT). The non-equilibrium density and flux profiles of gas molecules trapped within a nanopore are calculated for each species as a function of feed gas density. Because of important fluid–fluid interaction close to condensation the flux and density gradients are not related by constant transport diffusivities with the thermodynamic correction of uniform density. For long narrow pores the relation of the selectivity to the equilibrium adsorption isotherms is validated. Improved separation is achieved by combining preferential wall interaction and advantageous attraction between gas molecules of different species and examples are discussed. Results from experiments and simulations of permeation in binary mixtures near condensation are still rare and the theory provides a simple basis to study qualitative trends using known parameters.     

Experimental studies in relation to a new theoretical description of the permeation of dilute adsorbable gases through porous membranes

Precise data on the permeability of porous silica and alumina membranes to dilute gases are reported as a function of the nature of the gas and of temperature. It is shown that the unusual permeability behaviour previously observed only in “Vycor” porous glass at high temperatures [8-10] is a more general phenomenon. These results cannot be accounted for by conventional “surface diffusion” theory [1, 2] even qualitatively, but can be understood on the basis of recent, more advanced, theoretical treatments [3, 4, 7]. The present data provide an experimental test (not possible on the basis of previous data) of the general correlation between permeability and extent of sorption (including both the nature of the gas and temperature) predicted by the new theoretical approach, which is shown to be remarkably successful. Differences in the detailed permeability behaviour noted here, and in the previous porous glass study [8-10], are also satisfactorily accounted for in terms of differences in the mean effective pore size of the respective membranes.     

A model for permeation of mixed gases and vapors in glassy polymers

A model is discussed which explains reported complex effects of feed composition and pressure on component permeabilities in high-pressure gas separators based on glassy polymer membranes. A special form of Fick's law which accounts for the fact that penetrants in glassy polymers sorb into and diffuse through two different molecular environments provides the basis for the analysis of gas mixture permeation. Potential deviations from the theory are discussed in terms of separable solubility-and mobility-related effects.     

Diffusion in ethylene–propylene rubber

A study of sorption in a copolymer of ethylene and propylene is presented. Long-time sorption and desorption measurements provided the actual diffusion coefficient in the limit of zero concentration gradient. An analysis of the diffusion–sorption data reinforced the Frisch hypothesis about diffusion in a polymer matrix. The better solvent deforms the microstructure, allowing a more marked dependence of the diffusivity upon concentration.     

Liquid permeation through nonporous barrier materials

Permeation of chemical warfare agents (CWAs) and relatively nontoxic simulants in several elastomer compounds is examined using a new design flooded surface, liquid permeation cell. Diffusivity (D) and permeability are determined for sulfur mustard and sarin CWA, and for the simulants: dichlorohexane, chloroethyl phenyl sulfide, diethyl methylphosphonate and diisopropyl methylphosphonate. Values of D calculated by several kinetic methods at different stages of the permeation process and from steady-state permeability are generally in agreement but lower than those obtained from immersion kinetics. Barrier effectiveness of the elastomer compounds towards the CWA and simulant liquids decreases in the order: butyl > EPDM > nitrile ≫ silicone. Simulant and CWA permeation are correlated in terms of relative permeability and reduced breakthrough time.     

Inert gas permeation through homopolymer membranes

New data are reported for the permeation of inert gases through polyethylene, polytetrafluoroethylene, and silicone and natural rubbers. Additional data are compiled from the literature. The relative solubilities of these gases are practically insensitive to chemical variations in the homopolymer. Hence variations in structure at the glass transition (T_g) and melting (T_m) temperatures that affect diffusion also unambiguously affect permcation. Consequently an equivalence results between permeation at a given temperature for different polymers and permeation at different temperatures for a given polymer. Although the diffusion coefficient changes continuously with temperature, the Arrhenius parameters D_o and E_d apparently change discontinuously at T_g and T_m. Their magnitudes and variations with atomic weight reach maxima at about T_g. These data indicate a dependence of the classical correlation between D_o and E_d on polymer properties. A perturbed diameter for the permeant, specific for each polymer, is proposed for correlating the D_o and E_d data. This correlation makes the changes observed at T_g and T_m more perceptible.     

Gas permeation through water-swollen hydrogel membranes

This work deals with water-swollen hydrogel membranes for potential CO₂ separation applications, with an emphasis on elucidating the role of water in the membrane for gas permeation. A series of hydrogel membranes with a wide range of water contents (0.9–10 g water/g polymer) were prepared from poly(vinyl alcohol), chitosan, carboxyl methyl cellulose, alginic acid and poly(vinylamine), and the permeation of CO₂, H₂, He and N₂ through the membranes at different pressures (200–800 kPa) was studied. The gas permeabilities through the dry dense membranes were measured as well to evaluate the resistance of the polymer matrix in the hydrogel membranes. It was shown that the gas permeability in water-swollen membrane is lower than the gas permeability in water, and the selectivity of the water-swollen membranes to a pair of gases is close to the ratios of their permeabilities in water. The permeability of the water-swollen membranes increases with an increase in the swelling degree of the membrane, and the membrane permeability tends to level off when the water content is sufficiently high. A resistance model was proposed to describe gas permeation through the hydrogel membranes, where the immobilized water retained in the polymer matrix was considered to form transport passageways for gas permeation through the membrane. It was shown that the permeability of hydrogel membranes was primarily determined by the water content in the membrane. The model predictions were consistent with the experimental data for various hydrogel membranes with a wide range of water contents (0.4–10 g water/g polymer).     

“Second component” effects in sorption and permeation of gases in glassy polymers

Data for CO² permeability through Kapton polyimide at 60°C are reported for upstream pressures up to 240 psia (16.33 atm) in the presence and absence of water vapor in the feed. The carbon dioxide flux was depressed by the presence of the water vapor. This phenomenon is analyzed in terms of the dual mode sorption and transport models. Together with other recent sorption and permeation data, this study suggests that competition of mixed penetrants for sorption sites and transport pathways associated with unrelaxed volume in glassy polymers is a general feature of gas/glassy polymer systems. The permselectivity of a membrane to a mixture of penetrants is strongly related to its ability to maintain a size and shape differentiating matrix, that is, to remain essentially unplasticized under operating conditions. Under such conditions, competition among penetrants for excess volume will be a generally important consideration for modeling gas permeation in permselective membranes.     

Diffusion through colloidosome shells

Colloidosomes are aqueous cores surrounded by a shell composed of packed colloidal particles. Recent studies suggest that these colloidal shells reduce, or even inhibit, the transport of molecular species (diffusants). However, the effect of the colloidal shell on transport is unclear: In some cases, the reduction in transport of diffusants through the shell was found to be independent of the size of the colloidal particles composing the shell. Other studies find, however, that shells composed of small colloidal particles of order 100nm or less hindered transport of diffusants more than those composed of micro-scale colloidal particles. In this paper we present a simple diffusion model that accounts for three processes that reduce diffusant transport through the shell: (i) a reduction in the penetrable volume available for transport, which also increases the tortuousity of the diffusional path, (ii) narrow pore size which may hinder transport for larger diffusants through size exclusion, and (iii) a reduction in interfacial area due to 'blocking' of the surface by the adsorbed particles. We find that the colloidal particle size does not affect the reduction in transport through the colloidal shell when the shell is a monolayer. However, in closely packed, thick layers where the thickness of the multi-layer shell is fixed, the rate of transport decreases significantly with colloidal particle dimensions. These results are in excellent agreement with previously published experimental results.     

New differential permeation rate method for determination of membrane transport parameters of gases

A new method for determining the membrane transport parameters (diffusivity, permeability and solubility) of gases through nonporous polymeric membranes is described. The method employs a continuous permeation chamber containing a flat membrane. The most important feature of this method is that, instead of a step concentration change, a rectangular pulse or impulse is sent to the upstream side of the membrane. Consequently, no steady state is approached but a signal peak of typical form can be recorded. The permeability and the diffusivity can be estimated from the height and half-width of the peak, respectively. The method was applied to measure the permeability of hydrocarbons through a polyethylene membrane, the permeation rate being measured by a flame ionization detector. The method and the derived relations are valid for other detectors and gas—membrane combinations as well. The advantages of this novel method are that all the membrane transport parameters can be directly evaluated from data of the response peak, whilst approaching the steady state is not necessary and thus the measuring time can be shortened. Finally, the known and new differential permeation rate methods are compared by generalization of the relationship between the input and output (response) functions.     

Gas permeation through PSF-PI miscible blend membranes

The permeation rates of He, H₂, CO₂, N₂ and O₂, are reported for a series of miscible polysulfone-polyimide (PSF-PI) blend membranes synthesized in our laboratory. For gases which do not interact with the polymer matrix (such as He, H₂, N₂ and O₂), gas permeabilities in the miscible blends vary monotonically between those of the pure polymers and can be described by simple mixture equations. In the case of CO₂, which interacts with PI, blend permeabilities decrease somewhat, compared to pure PSF and PI. This, however, is accompanied by a two-fold improvement in the critical pressures of plasticization vs. polyimide. Permselectivities of CO₂/N₂ and H₂/CO₂ in the blends deviate from mixing theory predictions, in contrast to selectivities of gas pairs which do not interact with PI. Differential scanning calorimetry measurements of pure and PSF/PI blend membranes show one unique glass transition temperature, supporting the miscible character of the PSF/PI mixture. Optical micrographs of the blend membranes clearly indicate perfect homogenization and no phase separation. Frequency shifts and absorption intensity changes in the FTIR spectra of the blends, as compared with those of the pure polymers, indicate mixing at the molecular level. This compatibility in mixing PSF and PI, results essentially in a new blend polymer material, suitable for the preparation of gas separation membranes. Such membranes combine satisfactory gas permeation properties, reduced cost, advanced resistance to harsh chemical and temperature environments, and improved tolerance to plasticizing gases.     

Permeability of pure and mixed gases in silicone rubber at elevated pressures

The transport properties of silicone rubber are reported at 35°C for a series of pure gases (He, N₂, CH₄, CO₂, and C₂H₄) and gas mixtures (CO₂/CH₄ and N₂/CO₂) for pressures up to 60 atm. The effects of pressure and concentration on the permeability of various gases have been analyzed to consider plasticization and hydrostatic compression effects. Over an extended pressure and concentration range, both compression of free volume and eventual plasticization phenomena were observed for the various penetrants. In pure component studies, plasticization effects tended to dominate hydrostatic compression effects for the more condensible penetrants (C₂H₄ and CO₂) while the reverse was true for the low sorbing N₂ and He. These issues are discussed in terms of penetrant diffusion coefficients versus pressure to clarify the interplay between the opposing effects for the penetrants of markedly different solubilities. Additional insight into the somewhat complex interplay of the plasticization and hydrostatic compression effects are given by mixed gas permeation results. It was found that the permeability of nitrogen in a 10/90 CO₂/N₂ and a 50/50 CO₂/N₂ mixture was increased by the presence of CO₂ because the plasticizing nature of CO₂ is able to overcome nitrogen's compression effect.     

Transient permeation of organic vapors through elastomeric membranes

The permeation of benzene and acetone vapors through sulfur-cured natural rubber was studied by the time-lag method. The experimental results were analyzed by a method suggested by Meares. The zero concentration diffusion coefficient D₀ was obtained by the early-time method. The Frisch time-lag equation was utilized to estimate both the solubility coefficient s and the additional parameter b required to define the concentration dependence of the diffusion coefficient: D(c) = D₀ exp {bc}. This form of concentration dependence was manifested by the corresponding permeability coefficient values. At low entering penetrant pressure, where the transport coefficients are constant, indirect evidence was obtained that D₀ is the mechanistically correct diffusion coefficient. The solubility coefficient values calculated for benzene vapor in natural rubber are in reasonable agreement with published equilibrium sorption data for a similar rubber compound. At higher entering penetrant pressures, average diffusion coefficients obtained at steady state tended to be larger than the corresponding average diffusion coefficients derived from the time lags. This same effect has been detected by other experimental approaches. Permeation experiments designed for this rapid method of analysis appear capable of yielding information consistent with that obtained by more time-consuming traditional methods.     

Enantioselective permeation of racemic alcohol through polymeric membrane

Copolymer of 1,2-bis(2-methyl-1-triethylsiloxy-1-propenyloxy)ethane and dialdehyde have been synthesized by Mukaiyama Aldol polymerization using lipase as the catalyst. The chirality of the polymer was tested by optical rotation and circular dichroism study. The membrane forming ability of this chiral polymer was examined by casting the membrane in three different solvents viz., N-methyl-2-pyrilidone (NMP), dimethyl formamide (DMF) and dimethyl acetamide (DMAc) using the phase inversion method and it was found that chiral polymer–NMP membranes formed more uniform and regular surface morphology as was evident from SEM analysis. The enantioselective membranes prepared in the solvents was tested for resolution of racemic alcohol and it was found that NMP is the best solvent for obtaining highest enantioselectivity value. It was also found that the enantioselectivity for adsorption favoured the (S)-isomer whereas permeation favoured the (R)-isomer which is confirmed from interpretation of the adsorption isotherm by Langmuir model. Accordingly, the enantioselective permeation was caused by suppression of the (S)-isomer permeation. Optical resolution of (±)trans-sobrerol was achieved by pressure driven permeation through the membrane. The highest enantioselectivity, enantiomeric excess and permeation co-efficient was obtained as 98.59%, 20.42 and 13.627 m² h⁻¹, respectively. With an increase in polymer content in the membrane, the permeation rate increases.     

Diffusion and permeability of aldehydes into blends of natural rubber and chemically modified low molecular weight natural rubber

Low molecular weight natural rubber (LMWNR) obtained from natural rubber (NR) by depolymerization of natural rubber latex (NRL) was modified by epoxidation to 35% epoxide level to yield epoxidized low molecular weight natural rubber (ELMWNR). The ELMWNR was in turn modified in a solution of thioglycollic acid (TGA) (0.5 mol phr) to yield a product of 20% conversion of its initial LMWNR epoxide. Blends of NR with LMWNR, ELMWNR and epoxidized low molecular weight natural rubber thioglycollic acid (ELWMNR‐TGA) adduct were made. The mixes were vulcanized at 150°C for 20 min before determining the system parameters (n and k), the sorption (S), diffusion (D) and permeability (P) of the vulcanizates in acetaldehyde and formaldehyde solutions at three different temperatures (25, 40 and 60°C) for a period of 90 days. The sorption, diffusion and permeability of the vulcanizates were found to be temperature dependent. The vulcanizates containing ELMWNR were found not to be easily penetrated by both acetaldehyde and formaldehyde when compared with base mix A that is vulcanizates with only NR. The reaction system was found not to be spontaneous but dependent on the activation energies. Copyright © 2005 John Wiley & Sons, Ltd.     

Transient and steady-state water vapor permeation through polymer films

Water vapor transport properties for the polymers Kapton H

1 Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U.S. Atomic Energy Commission to the exclusion of others that may be suitable.

and Parylene C were determined over a temperature range of 20 to 55°C. Activation energies and entropies for permeation as well as partial molar free energies, heats, and entropies of dilution were calculated for water vapor concentrations ranging from 3 × 10^?6 to 1 × 10^?3 mole H₂O per cm³ of polymer. Mylar A was tested to extend the available data for partial molar heats and entropies of dilution and to compare permeation and diffusion results with the corresponding values in the literature. Diffusion coefficients were measured using the time-lag technique of Barrer but employing a modified test apparatus. Equilibrium sorption isotherms at 30°C were obtained for Mylar A and Kapton H with a Cahn microbalance. The ratios of the permeability to diffusion coefficients as measured from time-lag experiments agreed with solubility coefficients within 3% for Mylar A and within 12% for Kapton H. Both polymers obeyed Henry's law. The results were interpreted in light of polymer polarity and morphology.