Gas Transport Properties Research Articles

Blended crosslinked polybenzoxazole (PBO) membranes derived from phenolphthalein-based polyamide and polyimide for CO2/CH4 gas separation

Designing membrane materials with excellent gas separation performance is always a goal of membrane researchers. However, to be able to deliver stable performance and maintain membrane integrity under actual working conditions, a membrane material must be mechanically strong. In the last two decades, thermally rearranged polybenzoxazole (TR-PBO) polymers exhibit out-standing gas transport properties and great anti-plasticization behavior. But the TR reaction often lead to catastrophic deterioration in roughness of the polymer. To overcome this problem, we developed a polymer blending system consisting of a phenolphthalein-based polyamide (PHA, 6FC-DAP) and polyimide (API, OH-6FDA-DAP). Because of their similar chemical structures and the formation of inter-molecular hydrogen bonds, homogenous polymer blended membranes were obtained at molar ratios from 1:3 to 3:1 of PHA and API. Importantly, the B11 (PHA to API molar ratio of 1:1) derived PBO-based membranes exhibited superior mechanical and gas separation performance compared to the B31 (PHA: API=3:1) and B13 (PHA: API=3:1) derived PBO-based membranes. Note that, when the blended membrane B11 was fully converted into PBO, its tensile strength and elongation at break are 5.89 times and 3.27 times higher than those of the API derived TR-PBO. In addition, the B11–350 and B11–400 (thermal treatment at 350 °C and 400 °C in air atmosphere for 2 h, respectively) were not plasticized at a CO2 pressure of 30 atm. The excellent mechanical properties with appealing gas separation properties of these TR-PBO polymers have potential for CO2 relative gas separation applications.

Fabrication of MgO nanorods blended cellulose acetate-based mixed matrix membranes for selective gas separation of H2/CH4, CO2/CH4 and H2/CO2: Effect of loading and pressure

The growing interest in polymer-based mixed matrix membranes (MMMs) in the field of gas separation encouraged the authors to fabricate MgO nanorods and cellulose acetate (MgO-X/CA) MMMs for the separation of H2/CH4, CO2/CH4, and H2/CO2 gases. The facile solution-casting, followed by solvent evaporation technique, was applied to fabricate MMMs. FE-SEM, EDS, FT-IR, XRD, DSC, and TGA characterization techniques were used to investigate the physico-chemical properties of fabricated MMMs, whereas the gas transport properties of MMMs were investigated using a gas permeation setup. Upon uniform MgO nanorods incorporation in CA polymer, the gas permeation through MMMs is improved due to disruption of the polymer chain. The influence of feed pressure and various loadings of nanorods (5, 10, and 15 wt%) on gas permeability and selectivity of CA based MMMs were investigated at 25 °C and 1–4 bar gas pressure. Based on the results, the MgO-15/CA MMMs achieved the highest H2 permeability of 77.80 Barrer which is higher than pure CA by 45%, with a two-fold increase in H2/CO2 selectivity and unchanged H2/CH4 selectivity. Meanwhile, MgO-5/CA MMMs revealed the highest CO2 permeability of 62.90 Barrer with an increased in CO2/CH4 selectivity from 22.30 to 24.30.

Transport cross sections and collision integrals for C+(2P)–H(2S), C(3P)–H+(1S), C(1D)–H+(1S), and C+(4P)–H(2S) interactions

Transport collision integrals of interacting atoms or ions play a crucial role in modeling transport properties of high-temperature gases and plasmas. Here, we obtained the potential energy curves (PECs) of CH+ using the internally contracted multireference configuration interaction method with the Davidson correction (icMRCI+Q) method. The PECs were then used to investigate the transport cross sections and transport collision integrals for the C+(2P)–H(2S), C(3P)–H+(1S), C(1D)–H+(1S), and C+(4P)–H(2S) interactions using the classical mechanical approach and a quantum mechanical treatment of the scattering with Wentzel–Kramers–Brillouin approximations of the scattering phase shifts. The transport cross sections were obtained in the collision energy of 10−6–1 hartree, which were used to compute transport collision integrals for C–H+ and C+–H systems over the temperature range of 500–40 000 K. The C(1D)–H+(1S) and C+(4P)–H(2S) interactions are considered for the first time. Our transport collision integrals can provide data references for computing transport properties of high-temperature plasmas involving C and H atoms/ions.

High-frequency transport properties of a 2D electron gas with spin-orbit interaction under magnetic field driven topological transition

We study the transport properties of a two-dimensional electron system with an essential spin-orbit interaction under topological phase transition due to changing in-plane magnetic field. We have analyzed the contribution of diagonal and off-diagonal components of density matrix with respect to number of spin zone in the high-frequency conductivity. Analytical formulas for the conductivity tensor have been obtained. A numerical analysis giving full enough representation about the dependence of the conductivity on frequency and magnetic field is presented. We have shown that the contribution of the off-diagonal elements of density matrix is the greater than the higher the frequency of the alternating electromagnetic field. It has been established that for magnetic field value at which the minimum electron energy of upper spin zone exceeds the Fermi energy, the conductivity as a function of magnetic field change in discrete step.

Fabrication of high-performance poly(benzoxazole-co-imide) membrane by utilizing synergistic effect of thermal rearrangement and Zn2+ modification

Developing membranes that combine excellent gas separation performance with robust resistance to plasticization is highly demanded for membrane separation but remains a challenge. Herein, a series of Zn2+ functionalized poly(benzoxazole-co-imide) membrane was constructed via co-polymerizing (4,4′-hexafluoroisopropylidene) diphthalic anhydride (6FDA) with o-hydroxyl diamine 2,2′-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (BisAPAF) and 1,3,5-tris(4-aminophenyl)benzene (TAPB) followed by thermal rearrangement (TR) and Zn2+ modification processes. The TR conversion opened the polymer chains with larger d-spacing and higher BET surface area, thus, greatly increased the gas permeability. Meanwhile, the post Zn2+ modification resulted in a huge improved selectivity because of the π-complexation reactions between Zn2+ and CO2 and the formation of a Zn2+ coordination crosslinking network. As a result, due to the synergistic effect of TR and Zn2+ modification, the overall gas transport properties of these newly developed membranes were greatly improved. Furthermore, the dual network structures that arise from the remaining network PI and the Zn2+ coordination crosslinking contribute to the resultant membrane with excellent anti-plasticization capability. A represent membrane termed as PBOI-Zn2+-14h membrane exhibits great CO2/CH4 separation performance, delivering a CO2 permeability of 1048.1 Barrer and a CO2/CH4 selectivity of 46.1 that is exceeded the 2008 Roberson limitation. Furthermore, this membrane possesses remarkable resistance to CO2-induced plasticization, withstanding pressures above 35 Bar. Overall, this study presents an effective way for constructing polymer membranes with both high gas separation performance and strong resistance to plasticization issue.

Copolyimide Brushes as a Component of a Hybrid Poly(phenylene Oxide) Membrane for Controlling Gas Separation: Effect of Water, Methanol, and Hexane Vapors.

The effect of water, methanol, and hexane vapors on gas permeability was studied in a hybrid membrane containing 5 wt% copolyimide brushes with poly(methyl methacrylate) side chains (PI-g-PMMA) in a poly(phenylene oxide) (PPO) matrix, and in a pristine PPO membrane. These membranes in the form of dense nonporous films were further examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM), as well as by measuring their mechanical and gas transport properties. A gas separation study of the membranes in a dry state and the membranes saturated with water, methanol, and hexane vapors was performed to estimate the effect of each vapor on the H2, CO2, N2 permeability and selectivity in the separation of H2/N2 and CO2/N2 pairs. In general, saturation with water, methanol, and hexane vapors caused a decrease in the gas permeability of both membranes. The hybrid membrane containing copolyimide brushes demonstrated enhanced selectivity in the separation of H2/N2 and CO2/N2 pairs. It was found that a special effect of the vapors used for membrane saturation is associated with their molar volume. The solubility and diffusion coefficients of N2 and CO2 were obtained by Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations.

Influence of structural changes on gas transport properties of a cycled CaO/Ca(OH)2 powder bulk for thermochemical energy storage

Thermochemical energy storage using reversible gas-solid reactions has a great potential to evolve into a vital part of a future energy system. A major obstacle for technical application, however, is caused by insufficient gas transport within storage reactors leading to ineffective charging and discharging as well as lower storage densities. Although significant changes in the solid bulk structure are reported throughout the literature, the relevant material property – the bulk permeability – is rarely experimentally studied. In the presented study, the evolution of the gas permeability of a technical grade CaO/Ca(OH)2 powder bulk is studied experimentally. To this end, the pressure drop at ambient conditions is recorded before thermochemical cycling as well as after each dehydration respectively rehydration reaction. The pressure gradient and thereby the permeability is determined based on Darcy's law. The permeability of the fresh Ca(OH)2 bulk after compaction due to the gas flow is 1·10−13 m2. Due to restructuring during thermochemical cycling the permeability increases to 1.6…3.3·10−12 m2 for the cycled Ca(OH)2 bulk and 9.2·10−12 m2 to 2.4·10−11 m2 for the cycled CaO bulk. Further analysis of fresh and cycled samples shows a general increase in particle size and decrease in specific surface area as well as a reduction in reactivity due to agglomeration of the powder. Noticeable is the inhomogeneity of the cycled bulk with powder mixed with larger agglomerates at the top of the bulk and highly porous but seemingly homogeneous solid material at the bottom. Based on the experimental results, gas transport within the agglomerates is identified as a major limiting factor within thermochemical reactors. It is therefore proposed to consider cycled CaO/Ca(OH)2 bulks on a macro- and mesoscale (respectively bulk and agglomerate) simultaneously if transport properties – and their impact on effective reaction dynamics – are relevant.

Understanding hydrogen solubility and free volume characteristics in charge-transfer phthalonitrile prepolymer and polyimide blend membranes

The increasing demand for hydrogen production has necessitated the development of H2-selective membranes. Polyimides are excellent membrane materials for gas separation; however, commercial polyimides generally lack sufficient H2 selectivity due to their low H2 affinity. Understanding the relationship between gas transport properties and free volume microstructure is critical to advancing H2-selective membrane design. Herein, we report a facile material strategy to adjust the free volume characteristics and H2 separation performance via blending Matrimid (PI) and crosslinkable resorcinol-based phthalonitrile prepolymer (RPN) with electron donor/acceptor properties. The novel RPN30/PI70 membrane exhibits H2/N2 and H2/CO2 permselectivity of 1637 and 66.4, respectively, with H2 permeability of 2.7 Barrer in pure gas test, surpassing Robeson upper bounds (2008). The increased H2 permselectivity of RPN/PI membranes was attributed to the narrowed free volume size and distribution, giving rise to the considerably improved H2 solubility and selectivity of the blends. Moreover, the H2 permeability of crosslinked RPN30/PI70 membranes can be further improved via thermal treatment. The H2/CO2 mixed-gas test reveals that the H2 gas separation performance of the RPN30/PI70 membrane is influenced by plasticization effect and competitive sorption. This study demonstrates a new versatile strategy for designing high-performance hydrogen-selective polymeric membranes.

Gas Permeation through Mechanically Resistant Self-Standing Membranes of a Neat Amorphous Organic Cage.

The synthesis and characterization of a novel film-forming organic cage and of its smaller analogue are here described. While the small cage produced single crystals suitable for X-ray diffraction studies, the large one was isolated as a dense film. Due to its remarkable film-forming properties, this latter cage could be solution processed into transparent thin-layer films and mechanically stable dense self-standing membranes of controllable thickness. Thanks to these peculiar features, the membranes were also successfully tested for gas permeation, reporting a behavior similar to that found with stiff glassy polymers such as polymers of intrinsic microporosity or polyimides. Given the growing interest in the development of molecular-based membranes, for example for separation technologies and functional coatings, the properties of this organic cage were investigated by thorough analysis of their structural, thermal, mechanical and gas transport properties, and by detailed atomistic simulations.

Transport collision integrals for C(5S)-H(2S), C(1S)-H(2S), C(1D)-H(2S), and C(3P)-H(2S) interactions

Transport collision integrals of interacting atoms or ions are essential in modeling transport properties of high-temperature gases and plasmas. Here, we obtained the potential energy curves (PECs) of CH using the state-of-the-art ab initio methods. The PECs were also extrapolated to investigate the transport collision integrals for C(3P)-H(2S), C(5S)-H(2S), C(1S)-H(2S), and C(1D)-H(2S) interactions, in which the interactions between the excited C(5S), C(1S), and C(1D) atoms and the ground H(2S) atoms were calculated for the first time. The resulting transport collision integrals were fitted to simple functional forms for ease of use in plasma modeling. Our transport collision integrals can provide data references for computing transport properties of high-temperature plasmas involving C and H atoms.

Interfacially polymerized nanofilms with triptycene moieties and enhanced micropore interconnectivity for highly permselective gas separations

Highly permeable and selective synthetic polymer membranes are attractive for energy-efficient gas separations, while fabricating such membranes with well-defined micropore architecture and interconnectivity remains a significant challenge. We report the design and fabrication of highly crosslinked and microporous polymer nanofilms via engineering the bridged-bicyclic triptycene triamine moieties into the interfacially polymerized networks. The integrated polyamide nanofilms exhibited hierarchical pore structures with finely tuned microporosity (pore size 0.7–1.0 nm), ultramicroporosity (pore size 0.4–0.7 nm), and submicroporosity (pore size < 0.4 nm) and enhanced micropore interconnectivity due to triptycene-induced non-coplanar orientation and configurational free volume in the networks. Consequently, the composite membranes comprising polyamide nanofilms display exceptional gas separation performance for He and H2 recovery with enhanced permeances and selectivities as well as notable plasticization resistance compared to current state-of-the-art highly crosslinked TFC membranes. It is also proved that the microporosity and gas transport properties of the composite nanofilms are highly tailorable by regulating the reaction conditions. Incorporating hierarchical triptycene units with well-defined and interconnected microvoids provides the potential to fabricate highly permselective thin-film composite membranes applicable for various important gas separations.

Selective gas permeation through polymer-hybridized graphene oxide nanoribbon nanochannels: Towards enhanced H2/CO2 selectivity

Two-dimensional materials have garnered significant attention as membrane building blocks for their tunability, which enables simultaneous enhancement of permeance and selectivity. This study investigates the impact of reduced nanosheet width on H2/CO2 separation by examining the gas permeation behavior of graphene oxide nanoribbon (GONR)/polymer hybrid membranes. Polymers are incorporated into the macroporous GONR scaffold to accelerate H2 transport due to the expansion of GONR nanochannels. The GONR/polymer scaffolds can be coated on porous support by using the scalable shear coating method, resulting in micrometer-scale selective layers. The carbon layers remain intact after polymer hybridization and inhibit polymer crystallization, allowing strong polymer-CO2 interactions to hinder CO2 permeation. However, the excessive polymer composition can compromise GONR-polymer hybridization and crystallization suppression. We also explore the effect of polymer charge on gas separation performance. A relatively small amount of cationic polymer enhances H2/CO2 selectivity, but the enhancement is limited due to strong electrostatic interactions. Anionic polymer-GONR hybridization also improves gas separation performance, but is less effective compared to neutral polymer, as repulsive interactions result in a looser structure. The gas separation performance of the optimized membrane (GONR/PEO 25 wt%) shows H2 permeability of 7,108 Barrer and H2/CO2 selectivity of 10.8, surpassing the Robeson upper bound for polymeric membranes and comparable to that of previous 2D-materials-based membranes. Our finding demonstrates that GONR nanochannels and their gas transport properties can be tuned by simply adding polymers depending on the quantity, molecular weight, and charge of the polymers.

6FDA-DABA polyimide, polyphenylene, and polyimide-polyphenylene multiblock synthesis, characterization, and membrane gas separation performance and modeled transport

A 6FDA-DABA (6FD) polyimide, polyphenylene (PP), and multiblock copolymer (6FD-PP) materials were synthesized using sequential polycondensation and Diels-Alder reactions. PP and 6FD solubility parameters were 19.4 (MPa)1/2 and 22.4 (MPa)1/2, as predicted using Group Contribution Theory. Due to their solubility parameter differences, PP and 6FD solutions in tetrahydrofuran became immiscible when mixed at all concentrations. However, 6FD-PP multiblock copolymer solutions were clear, and their solution-cast films were robust and creasable. Homopolymer and block copolymers estimated fractional free volume (FFV) ranged from 0.174 (6FD) to 0.196 (PP). Their densities and FFV largely obeyed the rule of mixtures, suggesting a "blend-like" morphology. The glass transition temperature (Tg) of 6FD and PP were 285 °C and 400 °C, which became two distinct Tgs at 340 °C and 420 °C for 6FD-PP-[14k:13k]. Larger PP block lengths appeared to suppress the first-order polyimide transition entirely. SAXS and AFM morphological analysis revealed two distinct domains with an average separation distance ranging from 9.0 to 47.7 nm, dependent on PP block length. Membrane gas separation performance was compared to Robeson's upper bound. 6FD-PP multiblock copolymer gas transport properties were found to be highly dependent upon copolymer composition and FFV. Increasing PP content within a copolymer significantly improved gas permeability at the expense of He/CH4 and CO2/CH4 selectivity. Membrane CO2 permeability ranged from 4.36 Barrers using 6FD to 155 Barrers with PP. Their CO2/CH4 selectivity varied between 80.6 (6FD) to 12.7 (PP). 6FD-PP-[14k:13k] membrane's O2 permeability was between these polymers. The membrane had a 30% improvement in its ideal O2/N2 selectivity. The N2/CH4 selectivity ranged from 3 to 1.70, suggesting that 6FD-PP multiblock systems could be efficiently tailored from N2 to CH4 selectivity. Five gas permeability models (rule-of-mixture, Maxwell, EBM, laminate, and blend) were used to study 6FD-PP block copolymer transport behavior. Among them, a blend model had the most reasonable gas permeability prediction. 6FD-PP multiblock membranes improved several ideal gas pairs leading to more permeable and selective materials. The synthetic approach using block copolymerization creates an opportunity for gas separation improvement and tuneability of specific gas pairs.

Enhanced gas separation performance of PIM-1 blend membranes incorporating ionic liquid (3-(trimethoxysilyl) propan-1-aminium acetate ([APTMS][Ac])) as filler: Investigation of morphology, compatibility and transport properties

Blend membranes were fabricated by incorporating an ionic liquid, 3-(trimethoxysilyl) propan-1-aminium acetate ([APTMS][Ac]), as a filler into the polymer of intrinsic microporosity PIM-1. The analyses using FTIR spectroscopy, AFM, and SEM confirmed a uniform dispersion and excellent polymer-filler compatibility, even at higher loadings. Gas transport properties were assessed with filler loadings up to 10 wt% of IL, revealing a slight reduction in permeability but an enhancement in selectivity. CO2 permeability of the blend membrane decreased by 37% at 10% filler loading, while the ideal selectivity increased from 19.6 to 29.0 for CO2/CH4 and from 20.9 to 31.7 for CO2/N2 upon increasing [APTMS][Ac] content (0 wt% − 10 wt%). The CO2/CH4 gas mixture selectivity rose by 58.62% and the CO2 permeability dropped by 40.44% compared to the pristine PIM-1 membrane. For the CO2/N2 gas mixture, the selectivity and CO2 permeability were enhanced by 40.11% and reduced by 39.07%, respectively. The permeability reduction can be ascribed to the filling of free volume voids in the PIM-1 polymer matrix by the IL [APTMS][Ac], while the selectivity improvement can be attributed to the strong interaction and high solubility of CO2 in the ionic liquid. The CO2 solubility coefficient increased with the rising IL concentration, indicating the high solubility of the IL in the polymer matrix.

Gas separation properties of PIM-1 films treated by elemental fluorine in liquid perfluorodecalin

Gas transport properties of PIM-1 films treated by elemental fluorine in a liquid perfluorodecalin were investigated. The investigation included an estimation of fluorine content in a fluorinated layer, pure and mixed gas permeability, diffusion and solubility coefficients measurement, ideal selectivity assessment and analysis of fluorination time effect on gas transport properties. An increase of treatment duration resulted in an increase of concentration of fluorine in the fluorinated layer as well as a depth of fluorination. The depths of fluorination linearly increased with square of fluorination time indicating a diffusion of fluorine in the fluorinated layer to be a restrictive stage of kinetics of fluorination. The effect of the drop in gas permeability was demonstrated to be dependent on the kinetic size of the penetrant. The comparison of liquid-phase and gas-phase regimes was performed, and it was shown that sharp decrease in gas permeability is observed for gas-phase regime at short times of fluorination already, whereas this decrease is more gradual for liquid-phase fluorination. According to a Barrie method, the gas transport parameters of the fluorinated layer of the modified PIM-1 films were calculated, which turned out to be much lower than those for the virgin PIM-1 polymer and slightly lower than the gas transport parameters for dry Nafion. The reduction of the gas permeability coefficients is mostly induced by a sharp decrease in the gas diffusion coefficients. According to combination of permeability and selectivity, the fluorinated PIM-1 samples show excellent results exceeding the 2008 Robeson upper bounds for H2/CH4, H2/CO2, H2/N2, He/CH4 and others gas pairs. Mixed gas experiments were also conducted for hydrogen-containing mixtures and fluorinated PIM-1 samples have demonstrated good separation characteristics for the model binary mixtures attaining separation factors as high as 115–143 (H2/CH4), 4.4–6.7 (H2/CO2) and 49–146 (He/CH4), which makes these materials promising for helium recovery from natural gas and hydrogen recovery from various catalytic and biohydrogen production mixtures. Ten-months aging of the PIM-1 sample fluorinated during 30 min showed a decline of permeability by 25 and 40%, respectively and a slight decrease of ideal selectivity.

Effect of monomer moieties on the aggregate structure and gas transport properties of polyimides: Insights from experiments and simulations

AbstractTFDB‐based PIs (BTDA‐TFDB, PMDA‐TFDB and 6FDA‐TFDB) were prepared by the polymerization of 2,2′‐bis(trifluoromethyl) benzidine (TFDB) with three different dianhydrides, which contained flexible carbonyl bridged unit, rigid‐rod moiety without bridged unit, and bulky CF3 bridged unit in dianhydride moieties, respectively. The influence of dianhydride moieties on the aggregate structure and gas permeation performances of polyimide was systematically investigated in relation to their chain packing and free volumes, using the methods of molecular simulation, positron annihilation measurement and X‐ray diffractogram. Changing the dianhydride from benzophenonetetracarboxylic dianhydride (BTDA) to pyromellitic dianhydride (PMDA) and 4,4′‐(hexafluoroisopropylidene) diphthalic anhydride (6FDA), the gas permeation coefficients of polyimide increased, and the gas permeation selectivity (O2/N2, CO2/N2, H2/N2 and CO2/CH4) decreased. With the introduction of flexible carbonyl bridged unit into dianhydride moiety, the high rotational mobility favored tight chains packing and decreased free volumes, which in turn led to the lowest permeation coefficients of BTDA‐TFDB. The bulky CF3 bridged unit in dianhydride moiety greatly restricted chain packing and increased free volumes, thus resulting in the highest permeation coefficients of 6FDA‐TFDB. From BTDA‐TFDB to PMDA‐TFDB and 6FDA‐TFDB, the permeation selectivity decreased. This was mostly ascribed to the reduction of diffusive selectivity, which was resulted from the increase in free volumes.

The Effect of Complex Modifier Consisting of Star Macromolecules and Ionic Liquid on Structure and Gas Separation of Polyamide Membrane.

A novel hybrid membrane was developed on the basis of poly(m-phenylene isophthalamide) (PA) by introducing an original complex modifier into the polymer; this modifier consisted of equal amounts of heteroarm star macromolecules with a fullerene C60 core (HSM) and the ionic liquid [BMIM][Tf2N] (IL). The effect of the (HSM:IL) complex modifier on characteristics of the PA membrane was evaluated using physical, mechanical, thermal, and gas separation techniques. The structure of the PA/(HSM:IL) membrane was studied by scanning electron microscopy (SEM). Gas transport properties were determined by measuring He, O2, N2, and CO2 permeation through the membranes based on PA and its composites containing a 5 wt% modifier. The permeability coefficients of all gases through the hybrid membranes were lower than the corresponding parameters for the unmodified membrane, whereas the ideal selectivity in the separation of He/N2, CO2/N2, and O2/N2 gas pairs was higher for the hybrid membrane. The position of the PA/(HSM:IL) membrane on the Robeson's diagram for the O2/N2 gas pair is discussed.

Unexpected permeability enhancement of thermally rearranged (TR) copolybenzoxazole membranes

Gas transport properties of novel thermally rearranged copolybenzoxazole (TR-coPBO) membranes with various copolymerization ratios were investigated. The TR-coPBO membranes were obtained by thermal rearrangement of corresponding copoly(o-hydroxy imide)s with fluorene ring moiety via several rearranged temperatures. The TR-coPBO membranes had larger d-spacing values than corresponding neat TR-PBO membranes without copolymerization and larger specific volumes than those expected from an additive rule, suggesting excessively increased fractional free volumes. Permeability coefficients of the TR-coPBO membranes possessed positive deviations from the values expected by a semilogarithmic additive rule against the copolymerization ratio. The enhanced permeability might be brought by steric repulsion and suppressed molecular chain packing due to the partial introduction of fluorene ring moiety.

The Effect of Introduction of Phenolic Antioxidants on the Thermal-Oxidative Stability and Gas Permeability of Poly(4-methyl-2-pentyne)

Abstract: In this work, we studied the thermal-oxidative stability and gas transport properties of poly(4-methyl-2-pentyne) (PMP) film samples with mixed configurational composition (50% cis-units) in the presence of phenolic antioxidants Irganox 1076, Irganox 1010, Vulkanox BHT and Rianox 1520, which differ in the nature and structure of the substituents on the benzene ring. Annealing of PMP films at 140°C showed that films with the addition of Irganox 1076 and Irganox 1010 retain their strength and stability of gas transport properties for at least 150 h of total heating after a moderate decrease in permeability (∼30%) in the first 48 h of heating. PMP films with Vulkanox BHT and Rianox 1520 after 48 h of heating at 140°C demonstrate an increase in gas permeability coefficients, indicating the onset of polymer degradation, and after 100 h of heating they are destroyed. The wide-angle X-ray diffraction of films with the most effective stabilizers Irganox 1076 and Irganox 1010 revealed a decrease in interchain distances after annealing, which indicates an increase in the packing density of macromolecules as a result of thermally activated relaxation.

A new apparatus for investigating gas transport property in geomaterials with ultralow permeability

The gas flow in geological materials with ultra-low permeability can vary dramatically, i. e., from almost no measurable quantity of gas before breakthrough to a massive gas flux at breakthrough. It is a great challenge to realize the whole process measurement of gas transport in saturated geomaterials with ultra-low permeability. In this work, a newly designed temperature-controlled triaxial apparatus was developed. In which, a water-to-gas-exchanger was designed for precise injection of gas at a constant pressure or constant injection rate, due to that water is less compressible and much more difficult to leak through the seal of a regulator. Meanwhile, eddy current sensors and displacement transducer with high resolutions were utilized to accurately measure the specimen deformation at different locations in a non-contact way to explore the mechanism of gas entry and breakthrough. In addition, a gas flow detection device that contains four gas flowmeters with different measurement ranges and can automatically switch between these flowmeters was developed to monitor the gas outflow of the whole process. To validate the performance of the developed apparatus, systematic gas injection tests on Gaomiaozi (GMZ) bentonite were conducted, during which the helium gas was injected in a constant rate. Real-time data on inlet gas pressure, gas outflow rate, stress state (both isotropic and axial stresses) and volume deformation (both axially and radially) were monitored. Results show that the developed apparatus could successfully capture the gas entry and the subsequent breakthrough points of the compacted bentonite with ultralow permeability. For compacted bentonite with high saturation, capillary flow, dilatant flow and interfacial leakage could co-exist during the gas transport process and the dominated mechanism was closely related to the gas injection pressure.