Cross-linking of Polymer of Intrinsic Microporosity (PIM-1) via nitrene reaction and its effect on gas transport property

Polymer of Intrinsic Microporosity (i.e. PIM-1) has been crosslinked thermally via nitrene reaction using polyethylene glycol biazide (PEG-biazide) as a crosslinker. The crosslinking temperature was optimized using TGA coupled with FT-IR spectroscopy. The dense membranes containing different ratios of PIM-1 to PEG-biazide were cast from chloroform solution. Crosslinking of PIM-1 renders it insoluble even in excellent solvents for the uncrosslinked polymer. The resulting crosslinked membranes were characterized by FT-IR spectroscopy, TGA and gel content analysis. The influence of crosslinker content on the gas transport properties of PIM-1, its density and fractional free volume (FFV) were investigated. Compared to the pure PIM-1 membrane, the crosslinked PIM-1 membranes showed better gas separation performance especially for CO₂/N₂, CO₂/CH₄ and propylene/propane (C₃H₆/C₃H₈) gas pairs and as well as suppressed penetrant-induced plasticization under high CO₂ pressure.     

Sorption of CO2, C2H4, N2O and their binary mixtures in poly(methyl methacrylate)

Sorption of carbon dioxide, ethylene, and nitrous oxide in poly(methyl methacrylate) (PMMA) at 35°C has been characterized for each gas as a pure component and for mixtures of carbon dioxide/ethylene and carbon dioxide/nitrous oxide. Pressures up to 20 atm were examined. Pure-component sorption isotherms are concave to the pressure axis for each of the gases. This behavior is accurately described by the dual-mode sorption model. Using only the purecomponent dual-mode parameters and the generalization of the model for gas mixtures, one can predict the total concentration of gas sorbed in the polymer to within an average deviation of ±2.01% for the CO₂/C₂H₄/PMMA system and ±0.98% for the CO₂/N₂O/PMMA system. In both systems, for each component of the mixture, sorption levels were lower than corresponding pure-component sorption levels at pressures equal to the partial pressure of the respective components in the mixture. Depression of the sorbed concentration in mixture situations appears to be a general feature of the above systems and can be substantial in some situations. For the CO₂/C₂H₄/PMMA system, use of pure-component sorption data to estimate the total sorbed concentration in the mixture would be in error by as much as 40% if one failed to account for competition phenomena responsible for depression in mixed-gas situations. Mixture pressures as high as 20 atm were studied for both systems and in the CO₂/N₂O/PMMA system sorbed concentrations reach 33.90 [cm³(STP)/cm³ polymer] without any significant deviation from model predictions.     

Sorption equilibria of CO2/CH4 mixture on activated carbon in presence of water

The sorption isotherms of CO2 + CH4 mixtures on an activated carbon were collected in the presence of water at a temperature suitable for hydrate formation. The equilibrium composition of both phases was determined. The initial concentration of CO2 in mixtures was set at 33, 38 and 42%, and the total pressure was up to 10 MPa. CO2 hydrates were firstly formed following the increase of total pressure, and CO2 dominates the sorbed phase composition. CO2 concentration in the sorbed phase begins to decrease when the partial pressure of methane allows for the formation of methane hydrates. Competition for hydrate cavities was observed between CO2 and CH4 as reflected in the isotherm shape and phase composition at equilibrium. The formation pressure of hydrates is lower for mixtures than for pure gases, and the highest sorption capacity of each gas decreased in the mixture sorption either.     

Evaluation of gas sorption parameters and prediction of sorption isotherms in glassy polymers

A model of continuous‐site distribution for gas sorption in glassy polymers is examined with sorption data of CO₂ and Ar in polycarbonate. A procedure is presented for determining from a measured isotherm the number of sorption sites in a polymer, an important parameter that previously had to be assumed. With this parameter value and solubility data obtained at zero pressure, the model can reasonably predict sorption isotherms of CO₂ in glassy polycarbonate for a wide temperature range. The number of sorption sites and the average site volume evaluated from CO₂ sorption isotherms are employed for the prediction of Ar sorption isotherms with zero‐pressure solubility data and the independently measured partial molar volume of Ar. A reasonable fit to the measured isotherms of Ar is achieved. With the proposed procedure, the continuous‐site model shows several advantages over the conventional dual‐mode sorption model. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 883–888, 2000     

Raman spectroscopic studies on methane + tetrafluoromethane mixed-gas hydrate system

Raman spectra of intramolecular vibration mode for each guest species in the methane + tetrafluoromethane (CF₄) mixed-gas hydrate crystal have been measured at 291.1 K. Both of pure guest species generate the structure-I hydrate in the present pressure ranges. Isothermal phase-equilibrium curve exhibits two discontinuous points around the equilibrium methane compositions (water-free) in the gas phase of 0.3 and 0.8. At the above points, the Raman spectra of both guest molecules have been drastically changed. One of the most important findings is that the crystal of methane + tetrafluoromethane mixed-gas hydrate shows the structural phase-transition (from the structure-I to the structure-II and back to the structure-I) caused by composition changes.     

Swelling and sorption in polymer–CO2 mixtures at elevated pressures

Experimental data on gas sorption and polymer swelling in glassy polymer—gas systems at elevated pressures are presented for CO₂ with polycarbonate, poly(methyl methacrylate), and polystyrene over a range of temperatures from 33 to 65°C and pressures up to 100 atm. The swelling and sorption behavior were found to depend on the occurrence of a glass transition for the polymer induced by the sorption of CO₂. Two distinct types of swelling and sorption isotherms were measured. One isotherm is characterized by swelling and sorption that reach limiting values at elevated pressures. The other isotherm is characterized by swelling and sorption that continue to increase with pressure and a pressure effect on swelling that is somewhat greater than the effect of pressure on sorption. Glass transition pressures estimated from the experimental results for polystyrene with CO₂ are used to obtain the relationship between CO₂ solubility and the glass transition temperature for the polymer. This relationship is in very good agreement with a theoretical corresponding-states correlation for glass transition temperatures of polystyrene-liquid diluent mixtures.     

14CO2 and CO2 Transport in polycarbonate: Measurement of the time lag and permeability

Transport of CO₂ across polycarbonate films has been studied using a diffusion cell technique employing a radioactively labelled tracer (¹⁴CO₂). Because the ¹⁴CO₂ driving force could be established independently of the unlabelled CO₂ driving force, several classes of experiments not possible with conventional techniques were performed. These different classes of experiments showed measurably different time lags. Formally, these experiments all are limiting cases of the more general mixed-gas permeation problem; however, simplifying assumptions in the dual sorption theory are possible because the tracer concentration approaches zero and because the two species in this special mixed-gas problem exhibit the same dual sorption parameters. These simplifications allow derivation of analytical expressions for the time lag for both the unlabelled and labelled gas species. The experimental measurements are in good agreement with the dual sorption model formulated for the mixed-gas tracer diffusion problem.     

Interaction of CO2 gas with silicone elastomer at high ambient pressures

Interaction of high-pressure CO₂ gas with a silicone elastomer, and to a lesser extent, with a nitrile rubber and a PTFE have been investigated. Sorptive dilations of the polymers were measured with the help of custom-made piezoelectric ultrasonic transducers under gas pressures of up to ca. 22 MPa at 42°C. The gas mass sorption was determined by a vibrating reed probe. For the silicone elastomer system the dilation isotherm mimics the sorption isotherm. The partial molar volume (PMV) of the absorbed CO₂ gas in the silicone elastomer has been computed. A significant drop in the PMV value is observed when the CO₂ gas becomes supercritical. In the transition region, the transmission of ultrasonic signals through the specimen indicated the formation of discrete small (estimated as about 60 μm in diameter) high density zones of CO₂ in the rubber matrix. The plasticization effects of the absorbed high pressure CO₂ gas have been identified from the interpretation of the changes in the acoustic longitudinal modulus obtained from ultrasonic transmission measurements. The effects of rapid gas decompression on the structural integrity of the various polymers have also been determined. Significant inflation of certain specimens occur toward the latter stages of the decompression cycle. The initiation and development of internal cracks or bubbles was followed by monitoring the ultrasonic signal attenuation.     

Sorption thermosiphon apparatus (STA): A novel and accurate system for gas mixtures sorption measurements

One of the main challenges in membrane gas separation is the plasticizing effect that reduces selectivity. For a better understanding of this phenomenon, the knowledge of the sorption behavior of each component of the mixture is necessary. For this purpose, the sorption thermosyphon apparatus (STA), was successfully designed and tested with gas sorption measurements. One of the main advantages of the STA compared to actual other methods is to ensure concentration uniformity at the headspace using a thermosiphon, as pressure decay is recorded. The equilibrium condition is not disturbed during the sampling and allow the obtention of accurate data at the end of the sorption experiment. To validate the novel system, the sorption, diffusion and permeation coefficients of pure CO2 and CH₄, as well as for a CO₂/CH₄ (50/50) mixture, in polydimethylsiloxane (PDMS) were obtained through STA and other experimental techniques showing good agreement with literature data.     

Isothermal phase equilibria for the (xenon + cyclopropane) mixed-gas hydrate system

Isothermal three-phase equilibria of gas, aqueous, and hydrate phases for the {xenon (Xe) + cyclopropane (c-C₃H₆)} mixed-gas hydrate system were measured at two different temperatures (279.15 and 289.15) K. The structural phase transitions from structure-I to structure-II and back to structure-I, depending on the mole fraction of guest mixtures, occur in the (Xe + c-C₃H₆) mixed-gas hydrate system. The isothermal pressure–composition relations have two local pressure minima. The most important characteristic in the (Xe + c-C₃H₆) mixed-gas hydrate system is that the equilibrium pressure–composition relations exhibit the complex phase behavior involving two structural phase transitions and two homogeneous negative azeotropes. One of two structural phase transitions exhibits the heterogeneous azeotropic-like behavior.     

Pervaporation of alcohols through highly permeable PIM-1 polymer films

Polycondensation material PIM-1 has attracted the attention of researchers owing to its high transport parameters in gas separation and a high free volume. The pervaporation characteristics of PIM-1 have been systematically studied. Lower aliphatic alcohols (CH₃OH, C₂H₅OH, and n-C₄H₉OH) and water were selected as objects of research. The rates of mass transfer for individual components at various temperatures have been estimated, and for the ethanol-water binary mixture, the process of separation has been examined. The films based on the polymer under study exhibit the properties of organophilic membranes and are characterized by high permeability with respect to alcohols. The apparent activation energy of permeability is low. This behavior is common for pervaporation membranes based on glassy polymers with a high free volume. The parameters of separation for the water-alcohol mixture surpass corresponding values for typical organophilic membranes based on PDMS. These parameters are commensurable with the values observed for membranes based on poly(trimethylsilylpropyne). At the same time, PIM-1 does not demonstrate a rapid decrease in permeability in the course of time.     

Carbon dioxide sorption and transport in polycarbonate

The solubility, permeability, and diffusion time lag for carbon dioxide in polycarbonate are reported at 35°C for pressures ranging from 1 atm to 23 atm. The solubility data are very well described by the dual sorption mechanism model, Henry's law plus Langmuir adsorption, proposed for glassy polymers. Both the permeability and time lag decrease with increased CO₂ pressure. These observations are not consistent with the proposal that CO₂ sorbed by the Langmuir contribution is totally immobilized; however, all of the results are entirely consistent with an extension of this proposal which considers partial immobilization. The data have been quantitatively analyzed in terms of this partial immobilization model, and the results suggest for polycarbonate at 35°C that the CC₂ sorbed by the Langmuir portion of the isotherm behaves as if it has only about 10% of the mobility of the gas sorbed by the Henry's law part of the isotherm. The results have also been interpreted in terms of a concentration-dependent diffusion coefficient which is shown to be mathematically equivalent to the partial immobilization model. The latter model was also formulated in thermodynamic terms, whereby fugacity was used rather than pressure, and diffusion coefficients were defined in terms of chemical potential gradients rather than concentration, but the consequences of these changes proved to be minor and no better. The significance of these observations and their interpretation is discussed.     

Fine characterization of the ethylene and ethane sorption in poly(amide 12-block-tetramethylenoxide) copolymer/AgBF4 membranes

Ethylene/ethane sorption characteristics were determined for dry Pebax™ (poly(amide 12-block-tetramethylenoxide) copolymer)/AgBF₄ membranes by using an electronic microbalance. The membranes containing 0.7 and 22 wt.% AgBF₄ showed a dual-mode sorption isotherm. The ethane isotherms for all the membranes were of the Henry-type, which is the normal sorption for gases in rubbery polymers. The abnormal presence of Langmuir sorption sites only for ethylene in the rubbery copolymer, never reported sofar, is attributed to the silver-based specific complexation sites. The silver salt which dissolved in limited amounts in the rubbery copolymer had a much smaller Langmuir sorption capacity than the salt that crystallized in the copolymer. The sorption kinetics indicate that the crystallized salt did adsorb slowly ethylene according to a zeroth-order kinetics, but not ethane. The gas uptake kinetics resulting from a step of the pressure surrounding the copolymer exhibited one stage for ethane but two stages for ethylene. For the latter, there was first a fast Fickian sorption stage, then a drift of the zeroth-order sorption of ethylene on salt crystals, which contributes for a large part to the total uptake. The zeroth-order sorption suggests that the sorbed ethylene amount in the second-stage is independent of the crystal-surface coverage. The value of the Fickian diffusion coefficient calculated by fitting the kinetics with a solution of the second Fick’s law was 5 × 10⁻¹² m²/s for both ethylene (the first stage) and ethane, and is typical for small organic compounds in a rubbery material.     

Dynamics and mechanism of iodine sorption by polyacetylene

Sorption of iodine by bulk polyacetylene was studied under various I₂ gas pressures at 25°C. The sorption dynamics show that the penetration of iodine into PA is not Fickian and the diffusion coefficient increases with time of sorption. A discontinuous increase in the sorption isotherm is observed at P/P₀ = 0.25 (P is the pressure of the I₂ gas and P₀ is the saturation value at 25°C). It is due to iodine penetration into PA crystals, as evidenced by x-ray analysis. The distribution of iodine within crystals is apparently inhomogeneous: some unit cells are changed into “iodine-PA” cells, while others remain unchanged. The electrical conductivity depends not only on the amount of iodine but also on the I₂ gas pressure under which sorption is carried out. At given iodine content, the conductivity of a sample doped under higher I₂ pressure is greater than that of a sample doped under lower pressure.     

Sorption equilibrium prediction of competitive adsorption of herbicides 2,4-D and MCPA from aqueous solution on activated carbon using ANN

The simultaneous adsorption of two herbicides—2,4-dichlorophenoxyacetic acid (2,4-D) and 4-chloro-2-methylphenoxyacetic acid (MCPA)—from their aqueous binary mixtures onto granular activated carbon was studied. The quantities adsorbed were determined by HPLC with UV detection. The experimental data were analysed using the Freundlich adsorption isotherm. The high correlation coefficients indicated that the adsorption equilibrium fitted the Freundlich isotherm well. A multilayer perceptron (MLP) (an artificial neural network model—ANN) was applied to describe the adsorption equilibrium in multicomponent systems. This enabled sorption isotherms to be predicted for all possible combinations of the two herbicides. The experimental results and the calculated data obtained from MLP for the solutions of the individual components and their mixtures suggest that MCPA is better adsorbed onto activated carbon than 2,4-D.     

Macrochain configuration, stucture of free volume and transport properties of poly(1-trimethylsilyl-1-propyne) and poly(1-trimethylgermyl-1-propyne)

The relationship between poly(1-trimethylsilyl-1-propyne) (PTMSP) and poly(1-trimethylgermyl-1-propyne) (PTMGP) microstructure, gas permeability and structure of free volume is reported. n-Butane/methane mixed-gas permeation properties of PTMSP and PTMGP membranes with different cis-/trans-composition have been investigated. The n-butane/methane selectivities for mixed gas are by an order higher than the selectivities calculated from pure gas measurements (the mixed-gas n-butane/methane selectivities are 20?C40 for PTMSP and 22?C35 for PTMGP). Gas permeability and n-butane/methane selectivity essentially differ in polymers with different cis-/trans-composition. Positron annihilation lifetime spectroscopy investigation of PTMSP and PTMGP with different microstructure has determined distinctions in total amount and structure of free volume, i.e. distribution of free volume elements. The correlation between total amount of free volume and gas transport parameters is established: PTMSP and PTMGP with bigger free volume exhibit higher n-butane permeability and mixed-gas n-butane/methane selectivity. Such behavior is discussed in relation to the submolecular structure of polymers with different microstructure and sorption of n-butane in polymers with different free volume.     

聚合物支撑的金属有机骨架膜的制备及其气体分离性能

由于MOF(金属有机骨架)膜与基底之间的作用力较薄弱,所以制备具有高的H_2渗透性和H_2/CO_2选择性的致密连续的大面积金属有机骨架膜仍具有巨大挑战。本文选取多孔Al_2O_3作为基底,在表面涂覆一层PIM-1(一种固有微孔聚合物),并对其进行羧基化处理,使得表面具有大量的羧基基团,随后利用羧基与金属之间的相互作用,原位生长得到了两种致密连续的聚合物支撑的MOF膜(PIM-1-COOH/ZIF-8和PIM-1-COOH/HKUST-1)。通过XRD的表征可以看出MOF膜是纯相的并且具有较高的结晶性;SEM的测试结果表明MOF膜是致密连续的并且MOF膜与基底之间紧密结合。气体分离测试结果表明,这两种MOF膜对H_2具有较高的渗透性以及H_2/CO_2选择性。在常温常压下,对于PIM-1-COOH/ZIF-8和PIM-1-COOH/HKUST-1膜,H_2/CO_2双组分气体的分离系数分别为7.32、9.69,并且它们H_2的渗透通量分别高于3.16×10~(-6)、1.14×10~(-6) mol·m~(-2)·s~(-1)·Pa~(-1)。在单组份测试中,这两种MOF膜的H_2/CO_2的理想分离系数分别为7.70、12.04;H_2的渗透通量分别高达3.73×10~(-6)、3.86×10~(-6) mol·m~(-2)·s~(-1)·Pa~(-1),这就表明这两种MOF膜有望在H_2的纯化和分离方面广泛应用。     

Sorptive dilation of poly(vinyl benzoate) and poly(vinyl butyral) by carbon dioxide

Dilation of poly(vinyl benzoate) and poly(vinyl butyral) accompanying sorption of carbon dioxide is measured with a cathetometer under pressures up to 50 atm at 25°C. Sorption isotherms for carbon dioxide in these polymers were also determined gravimetrically. Each dilation isotherm plotted versus pressure, as well as the sorption isotherm, showed an inflection point corresponding to the glass transition of the polymer-gas system. The dilation isotherms changed their form at that point from concave to convex to the pressure axis or to a straight line. Dilation and sorption isotherms exhibited time-dependent hysteresis below the inflection point but not above the point. Partial molar volumes of carbon dioxide in polymers, which were determined from dilation and sorption data above the point, were found to be independent of concentration and larger than those below the point. The latter volumes depended on concentration. Based upon the extended dual-mode sorption concept, which takes account of plasticization of polymer by sorbed gas, a dilation model was developed. Dilation data were described well by the model.     

Methane adsorption in PIM-1

We report the results of Grand Canonical Monte Carlo (GCMC) simulations of methane adsorption in a prototypical polymer of intrinsic microporosity, PIM-1. Polymer chains were represented with a united-atom model, with Lennard-Jones parameters obtained from the TraPPE potential. Additionally, partial charges were calculated from ab initio methods using Gaussian (HF/6-31G^* basis set). Samples of PIM-1 were built at low density conditions, followed by a Molecular Dynamics compression protocol until densities of 1.2 g?cm^?3 were achieved. This protocol proved to be suitable for the realistic modeling of the amorphous structure of PIM-1. Surface areas and pore size distributions were measured and compared to available experimental data. The simulated pore size distribution present a peak at 4.3 Å, consistent with experimental results. GCMC simulations of methane adsorption were performed, and found to qualitatively reproduce the shape of the available experimental isotherm.