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The gas separation performance of biopolymers is still scarcely characterized, mostly because of their poor thermomechanical properties and high crystallinity which is associated to low permeability. In this work we characterize the gas transport in a poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) random copolymer, that has a relatively low crystallinity and good mechanical properties, combined with a renewable origin, biodegradability and biocompatibility. In the study we also compared several solvents for membrane casting, with different toxicity levels. We found that dimethyl carbonate allows the production of polymer films with transport properties similar to those obtained with the more toxic CHCl3, and it leads also to stable crystallinity of the samples over time. PHBV films show a size-sieving gas separation behaviour, as the permeability decreases significantly with the gas kinetic diameter. However, the strong energetic interactions of CO2 with the polymer matrix, confirmed by the Flory-Huggins model, induce a marked solubility-driven CO2/N2 and CO2/CH4 selectivity, which could make the material potentially interesting for CO2 removal processes.

This work aims at obtaining a systematic description and prediction of the multicomponent gas separation behavior of membranes, using the Non-Equilibrium Lattice Fluid (NELF) model for gas sorption and the Standard Transport (ST) model for gas permeation. The scheme is applied to a comprehensive analysis of CO2/CH4 separation with cellulose acetate membranes. A dedicated experimental campaign (pressure-volume-temperature (pVT), differential scanning calorimetry (DSC) and pure gas sorption tests) was performed to obtain reliable model parameters, accounting also for crystallinity. The approach was validated against a complete set of literature data, including mixed gas sorption and permeation.The parameters obtained were used to perform predictive simulations of mixed CO2/CH4 sorption and permeation in a wide mixture composition range. The NELF model accurately predicts the effect of temperature on sorption, as well as the strong competitive exclusion of CH4 when CO2 is present, that enhances the solubility-selectivity. The ST model correctly estimates the experimentally observed lower-than-ideal CO2/CH4 perm-selectivity. The model shows that low perm-selectivity is due to mixed gas diffusivity-selectivity (which is not available experimentally): even though solubility-selectivity increases in the mixed gas case, diffusivity-selectivity suffers a larger departure from pure gas conditions, ultimately leading to a lower perm-selectivity under multicomponent conditions.

The transport of H2, He, CO2, O2, CH4, and N2 at three temperatures up to 65 °C was measured in dense, thick composite films formed by amorphous Polysulfone (PSf) and particles of the size-selective zeolitic imidazolate framework 8 (ZIF-8) at loadings up to 16 wt%. The morphological and structural properties of the membranes were analyzed via SEM and density measurement. The addition of ZIF-8 to PSf enhances the H2 and He permeabilities up to 480% with respect to the pure polymer, while the ideal H2/CO2 and He/CO2 selectivities of MMMs reach values up to 30–40% higher than those of pure PSf. The relative permeability and diffusivity enhancements are higher than those obtained in other polymers, such as PPO, with the same amount of filler. The Maxwell–Wagner–Sillars model is able to represent the MMM H2/CO2 separation performance for filler volume fractions below 10%.

We used catalytic arene−norbornene annulation (CANAL) polymerization to synthesize high-molecular weight (degree of polymerization 500−800 based on Mn) rigid ladder polymers with methyl, ethyl, and isopropyl substituents that can form self-standing films. The short alkyl substitution on CANAL ladder polymers significantly impacted gas transport properties and their chain packing as revealed by variable-temperature pure-gas permeation and high-pressure sorption experiments as well as wide-angle X-ray scattering. Interestingly, a combination of methyl and isopropyl substituents enhanced both the sorption capacity and permeation of all gases tested without compromising permselectivity. Our findings suggest that varying short alkyl substitutions on ladder polymers with high fractional free volume represents an effective strategy to tune their chain packing and fractional free volume, which can enhance permeability without compromising permselectivity.

In this study, we prepared and characterized composite films formed by amorphous poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and particles of the size-selective Zeolitic Imidazolate Framework 8 (ZIF-8). The aim was to increase the permselectivity properties of pure PPO using readily available materials to enable the possibility to scale-up the technology developed in this work. The preparation protocol established allowed robust membranes with filler loadings as high as 45 wt% to be obtained. The thermal, morphological, and structural properties of the membranes were analyzed via DSC, SEM, TGA, and densitometry. The gas permeability and diffusivity of He, CO2, CH4, and N2 were measured at 35, 50, and 65 &deg, C. The inclusion of ZIF-8 led to a remarkable increase of the gas permeability for all gases, and to a significant decrease of the activation energy of diffusion and permeation. The permeability increased up to +800% at 45 wt% of filler, reaching values of 621 Barrer for He and 449 for CO2 at 35 &deg, C. The ideal size selectivity of the PPO membrane also increased, albeit to a lower extent, and the maximum was reached at a filler loading of 35 wt% (1.5 for He/CO2, 18 for CO2/N2, 17 for CO2/CH4, 27 for He/N2, and 24 for He/CH4). The density of the composite materials followed an additive behavior based on the pure values of PPO and ZIF-8, which indicates good adhesion between the two phases. The permeability and He/CO2 selectivity increased with temperature, which indicates that applications at higher temperatures than those inspected should be encouraged.

In this work, a new experimental protocol was designed to measure sorption of ternary gas mixtures in polymer membranes for the first time. We measured the sorption isotherms of binary and ternary mixtures of C2H6, CO2, and CH4 in PIM-1, using a pressure decay apparatus, working at constant composition and variable pressure of the gas phase. The results show competitive sorption effects in all gas mixtures analyzed, affecting each species in proportion to the total sorbed concentration of the other components. It was found that the presence of C2H6 in the mixture reduces the sorption of both CH4 and CO2 to a similar extent, so that the CO2/CH4 solubility-selectivity is not markedly affected. The experimental data were used to validate predictions of the Non-Equilibrium Lattice Fluid (NELF) model of binary and ternary sorption isotherms. The NELF model is generally in close agreement, both in the prediction of pressure and mixture composition effects, proving to be a reliable tool to assess mixed-gas sorption, even in complex scenarios.

The gas solubility in polymeric membranes affects the separation performance, particularly in the case of CO2 capture processes. Solubility and solubility-selectivity in membranes of multicomponent mixtures can deviate rather markedly from the corresponding pure gas values, due to swelling and competition phenomena, and require dedicated time-consuming measurements. Many experiments can be avoided by using a suitable thermodynamic tool, such as an Equation of State (EoS) model, to represent the gases sorption in the membrane. Such models require, for parameterization, knowledge of the polymer behavior above the glass transition Tg, which is a limit for membrane modeling, because the most attractive polymers for gas separation are rigid matrices characterized by very high Tg values, difficult to reach experimentally. In this work, we study the sorption of CO2/CH4 mixtures in a high-Tg polyimide membrane (Matrimid®) using a bottom-up approach. Pressure-volume-temperature data for Matrimid® above Tg are generated using NPT Molecular Dynamics simulations: the results are regressed to find Matrimid® parameters for the PC-SAFT Equation of State. Finally, the Non Equilibrium PC-SAFT macroscopic model (NE-PC-SAFT) is used to calculate CO2 and CH4 solubility and solubility-selectivity as a function of gas mixture pressure, composition and temperature. The approach is tested successfully over many experimental pure gas and vapor sorption data in Matrimid®. Mixed gas calculations predict a marked competition, which affects more methane than CO2 sorption, and results in a higher-than-ideal value of solubility-selectivity. Combined with the fact that experimental mixed gas permeability-selectivity is lower than the ideal value, such results indicate that the diffusivity of CH4 in Matrimid is significantly enhanced in presence of CO2, causing a decrease of diffusivity-selectivity. The gas solubility in polymeric membranes affects the separation performance, particularly in the case of CO2 capture processes. Solubility and solubility-selectivity in membranes of multicomponent mixtures can deviate rather markedly from the corresponding pure gas values, due to swelling and competition phenomena, and require dedicated time-consuming measurements. Many experiments can be avoided by using a suitable thermodynamic tool, such as an Equation of State (EoS) model, to represent the gases sorption in the membrane. Such models require, for parameterization, knowledge of the polymer behavior above the glass transition Tg which is a limit for membrane modeling, because the most attractive polymers for gas separation are rigid matrices characterized by very high Tg values, difficult to reach experimentally. In this work, we study the sorption of CO2/CH4 mixtures in a high-Tg polyimide membrane (Matrimid®) using a bottom-up approach. Pressure-volume-temperature data for Matrimid® above Tg are generated using NPT Molecular Dynamics simulations: the results are regressed to find Matrimid® parameters for the PC-SAFT Equation of State. Finally, the Non Equilibrium PC-SAFT macroscopic model (NE-PC-SAFT) is used to calculate CO2 and CH4 solubility and solubility-selectivity as a function of gas mixture pressure, composition and temperature. The approach is tested successfully over many experimental pure gas and vapor sorption data in Matrimid®. Mixed gas calculations predict a marked competition, which affects more methane than CO2 sorption, and results in a higher-than-ideal value of solubility-selectivity. Combined with the fact that experimental mixed gas permeability-selectivity is lower than the ideal value, such results indicate that the diffusivity of CH4 in Matrimid® is significantly enhanced in presence of CO2, causing a decrease of diffusivity-selectivity.

We have explored the effect of temperature, in the range between 25 and 50 °C, on the mixed gas CO2/CH4 sorption in a polymer of intrinsic microporosity (PIM-1), that has a solubility-selectivity higher than diffusion-selectivity for this mixture. The new data obtained in this work at 25 and 50 °c were combined with previously published data obtained at 35 °C to determine the temperature-dependence of mixed gas solubility and solubility-selectivity, a type of information that has not been obtained before in the literature. The data were collected at different total pressures and gas mixture compositions using a pressure decay − gas chromatographic device. The data collected indicate that the sorption of such mixture in PIM-1 is dominated by competition, whose major effect is to reduce the solubility of gases with respect to the pure gas value at same fugacity. The competitive phenomena follow a generalized trend that is not dependent on total gas pressure, composition and gas type but is only a function of the second gas concentration in the polymer and temperature. In particular the competition, expressed as reduction of gas solubility with respect to pure gas value, decreases with the concentration of the second gas, and increases with increasing temperature. Such effects are however generally favourable to separation, with positive deviations of the CO2/CH4 solubility-selectivity from ideal values calculated from pure gas solubility, by factors as high as 4. As a rule of thumb, observed at all temperatures and also in other glassy polymers, the real solubility-selectivity deviates positively from ideal value calculated from pure gas behaviour if the molar content of CO2 in the membrane is higher than that of methane, which is usually the case, unless low CO2 gas fractions are considered. A new type of generalized plot, reporting departures of multicomponent properties from the corresponding pure gas values, has been traced for this system and indicates that solubility selectivity departure is univocally correlated to CH4 solubility departure, independent of the operative conditions (pressure, composition, temperature) explored during the experiments.

This Special Issue of Membranes provides an updated and comprehensive overview of the state of fundamental knowledge on the fluid sorption and transport in glassy polymers, combining original experimental and modeling works, as well as reviews, prepared by renowned experts [...]

Membranes obtained by adding small amounts (1 wt %) of nanoplatelets of graphene (G) and graphene oxide (GO) to poly(1-trimethylsilyl-1-propyne) (PTMSP) were fabricated with a simple route, and their gas permeability was measured at 30 °C over 9 months. In most cases, variations of PTMSP permeability due to the addition of filler are limited, while the ideal selectivity of CO2/He, CH4/He, and CH4/N2 is slightly enhanced by addition of filler. Specific measurements indicate that the CO2 and CH4 diffusivity are more strongly affected by addition of graphene than their solubility: such behavior indicates that the filler modifies mainly the microstructure of the polymer rather than its interactions with the gas, as it is reasonable. The most significant quantitative effect observed after filler incorporation is the reduction of PTMSP aging, that was monitored by studying gas permeability after 9 months of aging at room temperature and after annealing at 200 °C. The reduction of aging observed after adding graphene is more significant than that obtained with large amounts (up to 20 vol %) of other inorganic fillers, like MgO and TiO2, even though the amount of filler added in this work is small (

Increasing the knowledge of the influence of water vapor in new mixed matrix membranes (MMMs) could favor the integration of novel membrane materials in the recovery of CO2 from wet industrial streams. In this work, the water vapor effect on the N2, CH4 and CO2 permeability through MMMs comprised of 20 wt% hydrophilic zeolite 4A in hydrophobic PTMSP polymer were investigated in the relative humidity range 0-75%. While in the pure PTMSP membranes, the permeability of all gases decreases with water vapor activity, with almost unchanged CO2/N2 and CO2/CH4 selectivities, in zeolite A/PTMSP MMMs, the CO2 permeability increases with increasing water content in the system up to 50% R.H., resulting in an increase in CO2/N2 and CO2/CH4selectivities with respect to pure PTMSP. Gas sorption was studied so that the effect the residual humidity in the zeolite 4A has on the sorption of the different gases helped explaining the permeability observations. The sorption and humid permeation behavior were evaluated by a simple model equation based on the NELF theory, taking into account the multicomponent gas sorption and diffusion in the presence of humidity, as well as the counteracting effects of the hydrophobic PTMSP and hydrophilic zeolite A in a very accurate way. Financial support is gratefully acknowledged to the Spanish Ministry of Economy and Competitiveness (MINECO) under projects CTQ2016-76231-C2-1-R and CTQ2012-31229 at the University of Cantabria. A. F. B. also thanks the MINECO for the Early Stage Researcher (BES2013-064266) contract and the short stay grant to work at the University of Bologna for 3 months (EEBB-I-17-12097).

It was previously proven that small loadings of few-layer graphene (G) and monolayer graphene oxide (GO) adjust the permselectivity and reduce the ageing of thick poly(trimethyl silyl propyne) (PTMSP) membranes, which currently inhibits their application in real separations. In this work, we extend the analysis to thin film composite membranes with top layers of PTMSP/G and PTMSP/GO between 1 and 7 µm. The good quality of the films, obtained by spin coating the polymer solution on porous flat supports, indicates that graphenic sheets are aligned parallel to the film surface, without defects, and that the large-scale production of such composite films is feasible. The addition of G and GO to PTMSP can improve the gas permeability and selectivity, and even change the behaviour from CO2-selective to He-selective. GO produces more repeatable effects than graphene, and it generally enhances the PTMSP permeability. The ageing was studied by tracking permeability of 4 gases with time: thin films age faster than thick ones, because ageing is due to free volume diffusion across the film. Incorporation of 1 wt% of G and GO visibly reduces the ageing of thin PTMSP films, and the effect is repeated on many different samples. The ageing can be correlated, with a single power law, to the ratio between time and squared film thickness. The exponent of such function, that quantifies the ageing rate, decreases when G and GO are added to PTMSP thin films, by factors as high as 40%. The temperature effect was studied, up to 60 °C, on annealed samples: the membranes, CO2-selective at room temperature, can become easily He-selective by raising the temperature. The He/CO2 selectivity increases with temperature: such effect could be exploited syngas purification and pre-combustion capture processes.

We fabricated a new type of nanostructured materials for CO2 capture processes, based on poly (acrylonitrile), PAN, a polymer almost impermeable to CO2, but easily functionalizable and spinnable. The preparation involved amine functionalization of PAN powder, electrospinning of the powder to form a nanofibrous mat with a large surface area, and compression of the mat to obtain dense membranes for facilitated transport of humid CO2. The functionalization step was carried out with different routes: amination with hexamethylene diamine or ethylene diamine, and basic hydrolysis. The final amine content in the polymer could be tuned varying the reaction type and conditions, although high functionalization degrees led to crosslinking, which made the powder insoluble. The dry CO2 uptake was measured at various stages of the preparation, in order to assess separately the effect of chemical functionalization and surface area enhancement on the material capture ability. Such tests indicated that both chemical and morphological changes of the neat polymer enhanced the dry CO2 sorption capacity, although the increase of surface area yielded the largest improvement. CO2 permeation tests in humid conditions were carried out on the compacted membranes, indicating that the materials functionalized via direct amination exhibit a behavior compatible with the facilitated transport mechanism, with CO2 permeability reaching 83 Barrer, increasing by 17 times with respect to the dry state value, at a relative humidity of 50%. Membranes functionalized via hydrolysis did not show such a behavior, maybe because the amine functionalities were consumed by an unwanted reaction. The procedure can be further improved by optimizing each processing step, or changing the order of the steps. The electrospinning process seemed the key factor of the approach, as the large surface area of electrospun mats allowed to obtain membranes with a higher permeability than the original ones, and a large availability of amine groups useful for humid CO2 capture.

We fabricated novel composite (mixed matrix) membranes based on a permeable glassy polymer, Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), and variable loadings of few-layer graphene, to test their potential in gas separation and CO2 capture applications. The permeability, selectivity and diffusivity of different gases as a function of graphene loading, from 0.3 to 15 wt %, was measured at 35 and 65 degrees C. Samples with small loadings of graphene show a higher permeability and He/CO2 selectivity than pure PPO, due to a favorable effect of the nanofillers on the polymer morphology. Higher amounts of graphene lower the permeability of the polymer, due to the prevailing effect of increased tortuosity of the gas molecules in the membrane. Graphene also allows dramatically reducing the increase of permeability with temperature, acting as a stabilizer for the polymer matrix. Such effect reduces the temperature-induced loss of size-selectivity for He/N-2 and CO2/N-2, and enhances the temperature-induced increase of selectivity for He/CO2. The study confirms that, as observed in the case of other graphene-based mixed matrix glassy membranes, the optimal concentration of graphene in the polymer is below 1 wt %. Below such threshold, the morphology of the nanoscopic filler added in solution affects positively the glassy chains packing, enhancing permeability and selectivity, and improving the selectivity of the membrane at increasing temperatures. These results suggest that small additions of graphene to polymers can enhance their permselectivity and stabilize their properties.

The individual solubility of CH 4 and CO 2 from binary gas mixtures was measured at 35 °C and up to 35 bar in a polymer of intrinsic microporosity (PIM-1), at different compositions of the gas phase (from 0 to 50 mol% of CO 2 ). The experiments were conducted on a pressure-decay apparatus equipped with a gas chromatograph, allowing a highly flexible measuring procedure. The gas solubility was plotted versus gas phase composition, total pressure, gas fugacity and second gas concentration. The mixed gas solubility of both species, CH 4 and CO 2 , is lower than the pure gas value at the same fugacity, but the reduction of methane solubility due to the presence of CO 2 is generally more significant. Such behavior is due to the fact that CO 2 has normally higher solubility than methane: indeed the depression of the solubility coefficient with respect to the pure gas value is similar for both gases, when reported at the same concentration of the second gas. The real, mixed gas solubility selectivity is in general higher than the ideal value calculated from pure gas behavior. The ratio between real and ideal solubility selectivity increases with CO 2 concentration in the membrane, according to a single mastercurve, reaching a maximum value of 4, and it also increases with the ratio between CO 2 and CH 4 concentration in the membrane. In particular, as in the case of other glassy polymers, the real solubility selectivity of CO 2 over CH 4 is higher than the ideal value if c (CO 2 )> c (CH 4 ), and it is lower than the ideal value if the opposite condition holds true. Such behavior occurs because the competition for sorption is normally less effective on the more abundant penetrant in the polymer. A selectivity–solubility performance plot can be drawn for this system.

The effects of adding fumed silica (FS) nanoparticles on the gas permeability, solubility, diffusivity and selectivity of a polymer of intrinsic microporosity (PIM-1) are modeled considering the density of the composite matrix as the key input information. PIM-1 is treated as a homogeneous glassy polymer endowed with a specific free volume that increases with the amount of nanoparticles loaded, as indicated by the experimental values of mixed matrix density. The solubility isotherms of H2, He, O2, N2, CH4 and CO2 in matrices of PIM-1 with different FS loadings are calculated with the Nonequilibrium Lattice Fluid (NELF) model. The gas diffusivity and permeability variation due to FS addition are related to the fractional free volume of the polymer phase according to the semiempirical free volume theory equation. Remarkably, the coefficient amplifying the free volume effect increases with the molecular size of the gas, expressed by the van der Waals volume, thus allowing to estimate the transport properties of gases not investigated experimentally, such as methane. The behavior inspected differs from the one observed in Mixed Matrix Membranes (MMM) formed by PIM-1 and porous selective fillers, that show higher selectivity towards smaller penetrants than the pure polymer, because the effect of silica nanoparticles is only represented by an enhancement of the large free volume domains. The model allows to estimate the ideal selectivity together with its solubility and diffusivity contributions, at various FS contents, for several gas pairs (O2/N2, CO2/N2, CO2/CH4, CO2/H2), which are then compared to the experimental trends available.

Sorption and swelling of alkane and alcohol vapors in addition-type poly(trimethyl silyl norbornene) (PTMSN) obtained in part I of this work were analyzed and compared to the predictions of the Non-Equilibrium Lattice Fluid (NELF) model. The polymer characteristic Lattice Fluid (LF) parameters were determined by best fitting the infinite dilution solubility coefficients, S0, of a wide series of n-alkane penetrants, from ethane to n-dodecane. The solubility of alkanes, which is comparable quantitatively and qualitatively to that measured in PTMSP, is well represented by the model. The solubility isotherms of alcohols (methanol, ethanol and 1-propanol), which exhibit a peculiar sigmoidal behavior that finds no explanation within the dual mode model, are also in line with the NELF model prediction without any ad hoc correction. The model is also used to represent successfully the dependence of the solubility on the alkane size, in PTMSN as well as in PTMSP and Teflon AF2400. The three polymers exhibit different trends: in PTMSP the vapor-pressure normalized solubility coefficient (S0 pvap) increases with the penetrant size, while in Teflon AF2400 the behavior is opposite and in PTMSN the size plays a very little role on the activity-based solubility. Such a behavior, that is useful to determine the selective performance of the polymer, was analyzed and discussed taking advantage of the support offered by the NELF model: it was seen that the three main factors affecting the extent of solubility dependence on the alkane size are the polymer fractional free volume and cohesive energy density, as well as its energetic interaction with the penetrant; an approximate formula is used to estimate a priori the behavior of the solubility based on the knowledge of the polymer properties.

The improvement of the separation performance of polymeric membranes for gas separations has been pursued by the addition of specific fillers, which produced diverse and even unexpected behaviors, hard to rationalize and to predict, both quantitatively and qualitatively. In particular, the addition of fumed silica nanoparticles in glassy polymeric membranes has shown unusual properties that apparently lead to the indication that only a specific experimental analysis could provide the information required for the permeability and separation of the gases of interest. We review transport properties, solubility and permeability in mixed matrix membranes obtained by loading fumed silica nanoparticles in different glassy polymers, interesting for membrane separations, and revise the main modeling procedure suitable to calculate and predict the relevant transport properties in such mixed matrix membranes.

Understanding the dynamics of water in solid-state polymer electrolytes (e.g., Nafion) is important for a variety of applications ranging from membrane-based water purification to hydrogen fuel cells. In this study, the dynamics of water in Nafion was investigated at both low and high humidities with time-resolved Fourier transform infrared−attenuated total reflectance (FTIR−ATR) spectroscopy; a technique that provides a molecular fingerprint of both the diffusant and the polymer simultaneously in real time. At low humidities (0−22% RH), an extended initial time lag resulted in non-Fickian behavior, where dynamic infrared data provided evidence for a reaction between water and sulfonic acid. A diffusion−reaction model was developed and predicted this anomalous behavior, where the time lag was a function of water content. At high humidities (0−100% RH), a slow approach to steady state resulted in non-Fickian behavior, where dynamic infrared data provided evidence of water-induced relaxation in the polymer ...

The gas and vapor transport into films of Hyflon® Ion H, a short-side-chain perfluorosulfonic acid ionomeric (PFSI) membrane, suitable for use in proton exchange membrane fuel cells (PEMFC), has been studied at various temperatures (35°C, 50°C, and 65°C). The permeability and diffusivity values of He, N2, and O2 show an Arrhenius type dependence on temperature in the range inspected. Pure water vapor permeation was studied at 65°C, at low/medium activity values. The determination of water transport parameters has been performed by solving numerically the water mass balance with a variable diffusion coefficient and accounting for a water immobilization reaction onto the hydrophilic sites of the matrix. The boundary conditions vary in time according to the mass balance on the penetrant volume. The solution allows to represent closely the experimental permeation behavior in all its stages.

Hybrid silica/poly(trimethylsilyl propyne) (PTMSP) membranes, in which the silica particles are generated in the polymer solution with a sol-gel reaction from a tetraethoxysilane (TEOS) precursor, were fabricated with silica loadings up to 30 wt%. The membranes are characterized with microscopic, thermogravimetric and pycnometric analysis; their sorption and diffusion properties with respect to n-C4 and n-C5 vapors are also measured at 25°C. A significant reduction of the polymer specific volume, organic vapour solubility and diffusivity is observed after incorporation of silica, according to a combination of free volume filling and compressive constraints. The behaviour of the hybrid membranes (HM) is opposite to that of mixed matrix membranes (MMM) obtained by physical mixing of preformed silica nanoparticles into PTMSP solutions, which show, on the contrary, free volume, vapour solubility and diffusivity values higher than pure PTMSP. The hybrid membranes are characterized by larger silica domains (around 1 m) than mixed matrix ones. Remarkably, however, the transport behaviour of both type of membranes can be interpreted, by the same theoretical background, on the basis of a single structural parameter, namely the density of the polymer phase. This parameter allows a direct estimation of the variation of free volume and vapour solubility and diffusivity in both type of membranes, through a NELF/free volume approach.

A water vapor soprtion study has been performed on Hyflon Ion membranes obtained by casting and the results have been compared to those obtained in an extruded samples, obtaining lower water uptake.

The sorption of gases (CO2, N2, CH4, O2) and vapors of different nature (n-butane, n-pentane, i-pentane, n-hexane, n-heptane, methanol, ethanol, 1-propanol, 1-butanol, water, chloroform, toluene, 1,4-dioxane, tetrahydrofurane) in films of a polymer of intrinsic microporosity (PIM-1) was studied at room temperature and various pressures. For the first time, the solubility properties of PIM are studied with a rigorous macroscopic model based on the Lattice Fluid (LF) theory; the LF parameters, not available for PIM-1, were obtained from independent gas sorption data at low pressure. The solubility of gases and water vapor follows the typical concave trend of dense glassy polymers and is described accurately by the Non Equilibrium Lattice Fluid (NELF) model, given the dry polymer density value and a slight adjustment of the binary parameter kij. The organic vapor solubility isotherm in PIM-1 can reach very high values and even exhibit a positive concavity at high penetrant contents, that is typical of sorption in rubbery phases. Such portion of the solubility isotherm was thus modeled with the equilibrium LF Equation of State (EoS) and one adjustable parameter kij, while at lower activities the sorption was described with the same value of kij and the NELF model. The value of PIM-1 density in the swollen glassy state required by the NELF model is estimated with an a priori scheme, based on pure penetrant liquid volume. The present work shows that, despite its peculiar microscopic structure, the solubility of PIM-1 can be estimated successfully with the same thermodynamic tools used for dense homogeneous phases and a limited number of parameters.

Fourier transform infrared–attenuated total reflectance (FTIR-ATR) spectroscopy was used to study in detail water vapor sorption in a short-side-chain perfluorosulfonic acid ionomer membrane suitable for use as electrolyte in proton exchange membranes fuel cells. The analysis of the membrane IR spectra, at different values of relative humidity (0.00–0.50) and at 35 °C, allows to identify four types of water molecules, characterized by decreasing strength of interaction with the polymer sulfonate groups. The actual concentration of the different water species inside the membrane was determined by calibrating the IR absorbance data with independent measurements of total water vapor uptake. The sorption of the different populations of water can be represented by Langmuir isotherms: the first population is directly attached to sulfonate sites, while the others form subsequent layers, adsorbed one onto the other in a shell-like structure. To describe the overall sorption behavior of the different populations, four adjustable parameters are required, which are consistent with literature data, thus supporting the validity of the physical model considered.

The enhancement of gas diffusivity observed after addition of fumed silica nano-particles to glassy polymers can be described and modeled by considering the additional fractional free volume (FFV) created by filler insertion, which can be evaluated from gas solubility data in the composite matrix by means of the NELF model. In this work, a detailed characterization of the solubility of n-C4 and n-C5 vapors, as well as of their diffusivity in matrices of AF 2400 filled with variable amounts of fumed silica has been performed at 25°C. On the bases of n-C4 solubility in the mixed matrix the FFV variation due to filler loading can be estimated and then used to predict the solubility of n-C5 that is found in good agreement with experimental data. Finally, an empirical correlation can be drawn between the FFV value obtained from solubility data and the infinite dilution diffusivity of the penetrants, as well as with the concentration dependence of the penetrant diffusivity.

In this work we evaluated the sorption, diffusion and permeation of a series of volatile organic compounds (VOCs) (acetone, n-butane, n-pentane, n-hexane, ethanol, methanol, chloroform and toluene) into glassy polymers of increasing fractional free volume (FFV): Polycarbonate (PC), Amorphous Teflon AF1600 and AF2400, poly-trimethylsilyl norbornene (PTMSN) and poly[1-(trimethylsilyl)-1-propyne] (PTMSP). Based on some experimental data of sorption and diffusion, and on theoretical and empirical models for the solubility and diffusion coefficients, the permeability for vapor/N2 mixtures was evaluated. These parameters are useful for the membrane separation processes and for other applications such as chemical sensors. The ideal separation factors of glassy polymeric membranes versus mixtures of VOCs and N2 were estimated at various pressures and compositions and at 25°C. The selectivity vs. permeability maps for the mixtures considered were plotted, showing that some of these materials show potentially the same selective ability of rubbery polymeric films. In particular it is shown that, the higher the FFV, the better the vapor/gas selectivity.

The vapor transport properties of nanocomposite materials obtained with different techniques and based on a high free volume glassy polymer suitable for membrane separations, poly[1-(trimethylsilyl)-1-propyne] (PTMSP), have been determined and modeled. The simple mixing in solution of hydrophobic fumed silica nanoparticles with PTMSP leads to mixed matrix membranes, which show higher free volume and higher values of diffusivity and permeability than the pure polymeric material. If a sol-gel route is followed, with PTMSP and Tetraethoxysylane (TEOS) as precursor of the silica phase, one obtains hybrid matrices characterized by lower vapor diffusion and sorption values with respect to the pure polymer. Although the trends observed are very regular functions of the silica content in the composite, none of the behavior observed obeys traditional models for composites permeability, such as the Maxwell’s one. Both types of behaviors were modeled considering the variation of polymer fractional free volume induced by the inorganic phase: in the mixed matrices the poor interactions between silica and polymer chains favor the formation of nanovoids at the interface, increasing the free volume and the vapor diffusivity, while in the more interconnected hybrid matrices the inorganic domains act as constraints, reducing the volume occupied by the polymeric phase, which is naturally endowed with a very high excess free volume.

The addition of fumed silica particles to glassy polymers such as PTMSP and Teflon AF 2400 results in an enhancement of gas diffusivity and permeability that is not captured by traditional models for composites because both volume and gas solubility of the composite material are underestimated by the additive rule based on pure component properties. Such effects have been attributed to the creation of additional free volume due to the insertion of filler in the polymeric matrix. In this work, we use the NELF model to relate gas solubility in the composite material to the variations induced by the addition of filler on polymer density and swellability. From these values, the fractional free volume (FFV) increase produced in the polymeric phase by filler incorporation may be evaluated and, in turn, also the diffusivity in the composite material can be calculated as a function of filler content on the basis of its relation to FFV. The gas diffusivity data for n-C4H10 in AF 2400 and AF 2400-based mixed matrices, as well as those relative to N2, CH4, C2H6, C3H8, and n-C4H10 in PTMSP, follow closely the correlation found, which contains only one adjustable parameter for each penetrant-polymer couple.

Water transport in proton exchange membranes (PEMs) is relevant to several advanced technologies, especially fuel cells. The dynamics of changing water conditions can significantly impact the PEM conductivity and fuel cell performance. Accurate water diffusion coefficients in Nafion, the industry standard PEM, have been determined, non-Fickian regimes identified and modeled, and infrared data used to confirm the validity of these models.