Temperature Dependence of Gas Permeation and Diffusion in Triptycene-Based Ultrapermeable Polymers of Intrinsic Microporosity

Diels-Alder Reactivity of Triisopropylsilyl Ethynyl Substituted Acenes

We investigated the Diels-Alder reaction of 6,13-bis(triisopropylsilylethynyl)pentacene (1) with small dienophiles such as (bridged) dihydronaphthalenes/cyclohexenes that yielded adducts at the central ring, the other dienophiles predominantly or exclusively attacked the unsubstituted off-center ring. The difference in regioselectivity was investigated by DFT calculations. Apart from dispersion interactions, it is due to the steric demand of the dienophiles, which need to fit in between the silylethynyl substituents to react at the central ring. Epoxynaphthalene adducts of 1 as well as its anthracene and tetracene congeners were deoxygenated, easily furnishing triarenobarrelenes with TIPS-ethynyl substituents at the bridgeheads, attractive building blocks for porous solids and higher acene-based trimers.

High-Performance Carbon Molecular Sieve Membranes Derived from a PPA-Cross-linked Polyimide Precursor for Gas Separation

Carbon molecular sieve (CMS) membranes have emerged as attractive gas membranes due to their tunable pore structure and consequently high gas separation performances. In particular, polyimides (PIs) have been considered as promising CMS precursors because of their tunable structure, superior gas separation performance, and excellent thermal and mechanical strength. In the present work, polyphosphoric acid (PPA) was employed as both cross-linker and porogen, it created pores within the PI polymeric matrix, while it also effectively acting as a cross-linker to regulate the ultramicropores of the CMS membranes, thus simultaneously improving both permeability and selectivity of the CMS membranes. By employing PI/PPA hybrid with PPA content of 5 wt % as a precursor, the obtained CMS membrane exhibited a CO2 and He permeability of 1378.3 Barrer and 1431.4 Barrer, respectively, which was an approximately 10-fold increase compared to the precursor membrane. Under optimized conditions, the CO2/CH4 and He/CH4 selectivity of the obtained CMS membrane reached 81.5 and 89.9, respectively, which was 278% and 307% higher than that of the pristine PI membrane. In addition, the membrane exhibited good long-term stability during a one-week continuous test. This study clearly denoted PPA can be used for precisely tailoring the ultramicroporosity of CMS membranes.

Triptycene-like naphthopleiadene as a readily accessible scaffold for supramolecular and materials chemistry

Triptycene derivatives are used extensively in supramolecular and materials chemistry, however, most are prepared using a multi-step synthesis involving the generation of a benzyne intermediate, which hinders production on a large scale. Inspired by the ease of the synthesis of resorcinarenes, we report the rapid and efficient preparation of triptycene-like 1,6,2',7'-tetrahydroxynaphthopleiadene directly from 2,7-dihydroxynaphthalene and phthalaldehyde. Structural characterisation confirms the novel bridged bicyclic framework, within which the planes of the single benzene ring and two naphthalene units are fixed at an angle of ∼120° relative to each other. Other combinations of aromatic 1,2-dialdehydes and 2,7-disubstituted naphthalenes also provided similar triptycene-like products. The low cost of the precursors and undemanding reaction conditions allow for rapid multigram synthesis of 1,6,2',7'-tetrahydroxynaphthopleiadene, which is shown to be a useful precursor for making the parent naphthopleiadene hydrocarbon. The great potential for the use of the naphthopleiadene scaffold in supramolecular and polymer chemistry is demonstrated by the preparation of a rigid novel cavitand, a microporous network polymer, and a solution-processable polymer of intrinsic microporosity.

Ultra-thin polymer foil cryogenic window for antiproton deceleration and storage

We present the design and characterization of a cryogenic window based on an ultra-thin aluminized biaxially oriented polyethylene terephthalate foil at T < 10 K, which can withstand a pressure difference larger than 1 bar at a leak rate <1×10-9 mbar l/s. Its thickness of ∼1.7 μm makes it transparent to various types of particles over a broad energy range. To optimize the transfer of 100 keV antiprotons through the window, we tested the degrading properties of different aluminum coated polymer foils of thicknesses between 900 and 2160 nm, concluding that 1760 nm foil decelerates antiprotons to an average energy of 5 keV. We have also explicitly studied the permeation as a function of coating thickness and temperature and have performed extensive thermal and mechanical endurance and stress tests. Our final design integrated into the experiment has an effective open surface consisting of seven holes with a diameter of 1 mm and will transmit up to 2.5% of the injected 100 keV antiproton beam delivered by the Antiproton Decelerator and Extra Low ENergy Antiproton ring facility of CERN.

Dibenzomethanopentacene-Based Polymers of Intrinsic Microporosity for Use in Gas-Separation Membranes

Dibenzomethanopentacene (DBMP) is shown to be a useful structural component for making Polymers of Intrinsic Microporosity (PIMs) with promise for making efficient membranes for gas separations. DBMP-based monomers for PIMs are readily prepared using a Diels-Alder reaction between 2,3-dimethoxyanthracene and norbornadiene as the key synthetic step. Compared to date for the archetypal PIM-1, the incorporation of DBMP simultaneously enhances both gas permeability and the ideal selectivity for one gas over another. Hence, both ideal and mixed gas permeability data for DBMP-rich co-polymers and an amidoxime modified PIM are close to the current Robeson upper bounds, which define the state-of-the-art for the trade-off between permeability and selectivity, for several important gas pairs. Furthermore, long-term studies (over ≈3 years) reveal that the reduction in gas permeabilities on ageing is less for DBMP-containing PIMs relative to that for other high performing PIMs, which is an attractive property for the fabrication of membranes for efficient gas separations.

Side-Chain Length and Dispersity in ROMP Polymers with Pore-Generating Side Chains for Gas Separations

Bottlebrush polymers with flexible backbones and rigid side chains have shown ultrahigh CO₂ permeability and plasticization resistance for membrane-based gas separations. To date, this class of polymers has only been studied with polydisperse side chains. Herein, we report gas transport properties of a methoxy (OMe) functionalized polymer synthesized via ring-opening metathesis polymerization (ROMP) with uniform side-chain lengths ranging from n = 2 to 5 repeat units to elucidate the role of both side-chain length and dispersity on gas transport properties and plasticization resistance. As side-chain length increased, both Brunauer-Emmett-Teller (BET) surface area and gas permeability increased with minimal losses in gas selectivity. Increased plasticization resistance was also observed with increasing side-chain length, which can be attributed to increased interchain rigidity from longer side chains. Controlling the side-chain length provides an effective strategy to rationally control and optimize the performance of ROMP polymers for CO₂-based gas separations.

Hydrocarbon ladder polymers with ultrahigh permselectivity for membrane gas separations

Membranes have the potential to substantially reduce energy consumption of industrial chemical separations, but their implementation has been limited owing to a performance upper bound-the trade-off between permeability and selectivity. Although recent developments of highly permeable polymer membranes have advanced the upper bounds for various gas pairs, these polymers typically exhibit limited selectivity. We report a class of hydrocarbon ladder polymers that can achieve both high selectivity and high permeability in membrane separations for many industrially relevant gas mixtures. Additionally, their corresponding films exhibit desirable mechanical and thermal properties. Tuning of the ladder polymer backbone configuration was found to have a profound effect on separation performance and aging behavior.

Features of the Gas-Permeable Crystalline Phase of Poly-2,6-dimethylphenylene Oxide

Poly-2,6-dimethylphenylene oxide (PPO) film samples with varying degrees of crystallinity (from 0 to 69%) were obtained by means of different techniques. The films were studied by various physicochemical methods (Fourier-transform infrared spectroscopy, positron annihilation lifetime spectroscopy, X-ray diffraction, and ¹H nuclear magnetic resonance relaxation). Solubility coefficients of gases in the PPO samples were measured via sorption isotherms of gases by volumetric technique with chromatographic detection. The apparent activation energy of permeation and the activation energy of diffusion of all gases were estimated based on temperature dependences of gas permeability and diffusivity for amorphous and semi-crystalline PPO in the range of 20–50 °C. The peculiarities of free volume, density, and thermal properties of gas transport confirm the nanoporosity of the gas-permeable crystalline phase of PPO. So, the PPO can be included in the group of organic molecular sieves.

Membrane-Assisted Removal of Hydrogen and Nitrogen from Synthetic Natural Gas for Energy-Efficient Liquefaction

Synthetic natural gas (SNG) production from coal is one of the well-matured options to make clean utilization of coal a reality. For the ease of transportation and supply, liquefaction of SNG is highly desirable. In the liquefaction of SNG, efficient removal of low boiling point impurities such as hydrogen (H2) and nitrogen (N2) is highly desirable to lower the power of the liquefaction process. Among several separation processes, membrane-based separation exhibits the potential for the separation of low boiling point impurities at low power consumption as compared to the existing separation processes. In this study, the membrane unit was used to simulate the membrane module by using Aspen HYSYS V10 (Version 10, AspenTech, Bedford, MA, United States). The two-stage and two-step system designs of the N2-selective membrane are utilized for SNG separation. The two-stage membrane process feasibly recovers methane (CH4) at more than 95% (by mol) recovery with a H2 composition of ≤0.05% by mol, but requires a larger membrane area than a two-stage system. While maintaining the minimum internal temperature approach value of 3 °C inside a cryogenic heat exchanger, the optimization of the SNG liquefaction process shows a large reduction in power consumption. Membrane-assisted removal of H2 and N2 for the liquefaction process exhibits the beneficial removal of H2 before liquefaction by achieving low net specific power at 0.4010 kW·h/kg·CH4.

Effect of Bridgehead Methyl Substituents on the Gas Permeability of Tröger's-Base Derived Polymers of Intrinsic Microporosity

A detailed comparison of the gas permeability of four Polymers of Intrinsic Microporosity containing Tröger's base (TB-PIMs) is reported. In particular, we present the results of a systematic study of the differences between four related polymers, highlighting the importance of the role of methyl groups positioned at the bridgehead of ethanoanthracene (EA) and triptycene (Trip) components. The PIMs show BET surface areas between 845-1028 m2 g-1 and complete solubility in chloroform, which allowed for the casting of robust films that provided excellent permselectivities for O2/N2, CO2/N2, CO2/CH4 and H2/CH4 gas pairs so that some data surpass the 2008 Robeson upper bounds. Their interesting gas transport properties were mostly ascribed to a combination of high permeability and very strong size-selectivity of the polymers. Time lag measurements and determination of the gas diffusion coefficient of all polymers revealed that physical ageing strongly increased the size-selectivity, making them suitable for the preparation of thin film composite membranes.

Thermal Cross Linking of Novel Azide Modified Polymers of Intrinsic Microporosity-Effect of Distribution and the Gas Separation Performance

The synthesis of polymers of intrinsic microporosity (PIM) modified with azide groups, the cross linkage by nitrene reaction and their performance as gas separation membranes are reported. The azide modification of the spirobisindane units in the polymer backbone was done by post functionalization of methylated spirobisindane containing polymers. These polymers differ in distribution and concentration of the azide group containing spirobisindane units by applying perfectly alternating and randomly distributed copolymers along the polymer chains. To investigate the influence of concentration of the azide groups, additionally the homopolymer of methylated spirobisindane was synthesized and subjected to identical treatments and characterizations as both copolymers. Cross linkage by nitrene reaction was examined by different temperature treatments at 150, 200, 250 and 300 °C. Characterization of the new polymers was performed by NMR, SEC and FT-IR. Furthermore, the crosslinking process was investigated by means of solid state NMR, TGA-FTIR, DSC and isoconversional kinetic analysis performed with TGA. Gas permeability of CO2, N2, CH4, H2 and O2 was determined by time lag experiments and ideal selectivities for several gas pairs were calculated. The two azide groups per repeating unit degrade during thermal treatments by release of nitrogen and form mechanically stable PIM networks, leading to an increase in gas permeability while selectivity remained nearly constant. Measured diffusivity and solubility coefficients revealed differences in the formation of free volume elements depending on distribution and concentration of the azide groups. Aging studies over about five months were performed and physical aging rates (βP) were evaluated with regard to the concentration and distribution of curable azide functionalities. Subsequently, the enhanced sieving effect during aging resulted in membrane materials that surpassed the Robeson upper bound in selected gas pairs.

Temperature dependence of the interfacial bonding characteristics of silica/styrene butadiene rubber composites: a molecular dynamics simulation study

Based on our previous studies on the modification of in-chain styrene butadiene rubber (SBR) using 3-mercaptopropionic acid as well as its composites filled with silica, we further constructed two types of models (amorphous and layered) to investigate the temperature dependence of the interfacial bonding characteristics of silica/SBR composites via molecular dynamics (MD) simulation. The competing effects of rubber–rubber interactions and filler–rubber interactions were identified, and the relationship between the competing effects and the temperature was determined. Besides this, the effect of temperature on the mobility and distribution of SBR chains on the surface of silica was investigated. It was found that the stronger the interfacial interactions, the less sensitive the motion of SBR chains to temperature. Finally, the number and length of hydrogen bonds as a function of temperature were analyzed. These simulated results deepened the understanding of interface temperature dependence of the silica/SBR composites and gave a molecular level explanation for the existence of an optimum modifier content (14.2 wt%) that is temperature independent.

Temperature dependence of the interface between silica and styrene butadiene rubber modified by 3-mercaptopropionic acid was investigated by molecular dynamics simulation.

Temperature and Pressure Dependence of Gas Permeation in a Microporous Tröger's Base Polymer

Gas transport properties of PIM-EA(H₂)-TB, a microporous Tröger’s base polymer, were systematically studied over a range of pressure and temperature. Permeability coefficients of pure CO₂, N₂, CH₄ and H₂ were determined for upstream pressures up to 20 bar and temperatures up to 200 °C. PIM-EA(H₂)-TB exhibited high permeability coefficients in absence of plasticization phenomena. The permeability coefficient of N₂, CH₄ and H₂ increased with increasing temperature while CO₂ permeability decreased with increasing temperature as expected for a glassy polymer. The diffusion and solubility coefficients were also analysed individually and compared with other polymers of intrinsic microporosity. From these results, the activation energies of permeation, diffusion and sorption enthalpies were calculated using an Arrhenius equation.