The influence of the dianhydride precursor in hyper-cross-linked hybrid polyPOSS-imide networks

Covalent Organic Framework Membranes with Regulated Orientation for Monovalent Cation Sieving

Continuous covalent organic framework (COF) thin membranes have garnered broad concern over the past few years due to their merits of low energy requirements, operational simplicity, ecofriendliness, and high separation efficiency in the application process. This study marks the first instance of fabricating two distinct, self-supporting COF membranes from identical building blocks through solvent modulation. Notably, the precision of the COF membrane's separation capabilities is substantially enhanced by altering the pore alignment from a random to a vertical orientation. Within these confined channels, the membrane with vertically aligned pores and micron-scale stacking thickness demonstrates rapid and selective transportation of Li+ ions over Na+ and K+ ions, achieving Li+/K+ and Li+/Na+ selectivity ratios of 38.7 and 7.2, respectively. This research not only reveals regulated orientation and layer stacking in COF membranes via strategic solvent selection but also offers a potent approach for developing membranes specialized in Li+ ion separation.

Molecular Characterization of Membrane Gas Separation under Very High Temperatures and Pressure: Single- and Mixed-Gas CO2/CH4 and CO2/N2 Permselectivities in Hybrid Networks

This work illustrates the potential of using atomistic molecular dynamics (MD) and grand-canonical Monte Carlo (GCMC) simulations prior to experiments in order to pre-screen candidate membrane structures for gas separation, under harsh conditions of temperature and pressure. It compares at 300 °C and 400 °C the CO₂/CH₄ and CO₂/N₂ sieving properties of a series of hybrid networks based on inorganic silsesquioxanes hyper-cross-linked with small organic PMDA or 6FDA imides. The inorganic precursors are the octa(aminopropyl)silsesquioxane (POSS), which degrades above 300 °C, and the octa(aminophenyl)silsesquioxane (OAPS), which has three possible meta, para or ortho isomers and is expected to resist well above 400 °C. As such, the polyPOSS-imide networks were tested at 300 °C only, while the polyOAPS-imide networks were tested at both 300 °C and 400 °C. The feed gas pressure was set to 60 bar in all the simulations. The morphologies and densities of the pure model networks at 300 °C and 400 °C are strongly dependent on their precursors, with the amount of significant free volume ranging from ~2% to ~20%. Since measurements at high temperatures and pressures are difficult to carry out in a laboratory, six isomer-specific polyOAPS-imides and two polyPOSS-imides were simulated in order to assess their N₂, CH₄ and CO₂ permselectivities under such harsh conditions. The models were first analyzed under single-gas conditions, but to be closer to the real processes, the networks that maintained CO₂/CH₄ and CO₂/N₂ ideal permselectivities above 2 were also tested with binary-gas 90%/10% CH₄/CO₂ and N₂/CO₂ feeds. At very high temperatures, the single-gas solubility coefficients vary in the same order as their critical temperatures, but the differences between the penetrants are attenuated and the plasticizing effect of CO₂ is strongly reduced. The single-gas diffusion coefficients correlate well with the amount of available free volume in the matrices. Some OAPS-based networks exhibit a nanoporous behavior, while the others are less permeable and show higher ideal permselectivities. Four of the networks were further tested under mixed-gas conditions. The solubility coefficient improved for CO₂, while the diffusion selectivity remained similar for the CO₂/CH₄ pair and disappeared for the CO₂/N₂ pair. The real separation factor is, thus, mostly governed by the solubility. Two polyOAPS-imide networks, i.e., the polyorthoOAPS-PMDA and the polymetaOAPS-6FDA, seem to be able to maintain their CO₂/CH₄ and CO₂/N₂ sieving abilities above 2 at 400 °C. These are outstanding performances for polymer-based membranes, and consequently, it is important to be able to produce isomer-specific polyOAPS-imides for use as gas separation membranes under harsh conditions.

High-temperature molecular screening of hybrid polyOAPS-imide networks based on octa(aminophenyl)silsesquioxane for increased thermomechanical resistance

A new family of hybrid hyper-cross-linked thin films based on inorganic polyhedral oligomeric silsesquioxane (POSS) cages covalently bound with short organic imides has recently been developed using interfacial polycondensation followed by high-temperature imidization. These polyPOSS-imide networks were aimed at gas separations under harsh conditions, but the aliphatic arms of the initial POSS precursor, octa(aminopropyl)silsesquioxane, were found to be a weak link. This work investigates the replacement of the aliphatic arm by a phenyl derivative, octa(aminophenyl)silsesquioxane (OAPS). Although this new precursor is expected to be more thermoresistant, it introduces extra degrees of complexity since the functional -NH2 group on the phenyl ring can either be attached at a meta, a para or an ortho position. In order to avoid a costly programme of synthesis and testing, molecular dynamics (MD) simulations have been used to efficiently screen a large number of candidate structures based on mixtures of the three OAPS isomers, the initial POSS and three organic precursors, the PMDA, 6FDA and ODPA dianhydrides. Following cross-linking at room temperature, twenty-two model networks were further relaxed at the imidization temperature and directly tested under harsh conditions at 300 °C. The screening stage included the characterization of their intercage single-links and double-links, which reinforce the structures, and intracage links, which have the opposite effect. Carrying out the cross-linking reactions to completion significantly improved the resistance to isotropic dilation. The initial POSS as well as the flexible 6FDA and ODPA linkers were found to be prone to large deformations, whereas the orthoOAPS, metaOAPS, paraOAPS and the PMDA linker prevented volume dilations. Upon uniaxial tension, the Young's moduli varied in the order paraOAPS < POSS ≈ metaOAPS < orthoOAPS for the inorganic precursors and in the order 6FDA < ODPA < PMDA for the organic precursors. In all cases, the networks based on either orthoOAPS and/or PMDA displayed superior resistance. Nine polyOAPS-imides were further heated up to 400 °C, i.e. closer to the expected degradation, and re-submitted to isotropic dilations and uniaxial tensions. They confirmed the trends found at 300 °C with no signs of structural collapse. Using OAPS as the inorganic precursor thus significantly reinforces the thermoresistance of these hybrid hyper-cross-linked networks.

Dynamic and programmable morphology and size evolution via a living hierarchical self-assembly strategy

Recent advances in the preparation of shape-shifting and size-growing nanostructures are hot topics in development of nanoscience, because many intelligent functions are always relied on their shape and dimension. Here we report a tunable manipulation of sequential self-assembled transformation in situ via a hierarchical assembly strategy based on a living thiol-disulfide exchange reaction. By tailoring the external stimuli, the reactive points can be generated at the ends of initially unimolecular micelles, which subsequently drive the pre-assemblies to periodically proceed into the hierarchically micellar connection, axial growth, bending, and cyclization processes from nanoscopic assemblies to macroscopic particles. Of particular interest would be systems that acquired the shape control and size adjustment of self-assemblies after termination or reactivation of disulfide reshuffling reaction by regulating external stimuli whenever needed. Such a hierarchical strategy for self-assembled evolution is universally applicable not only for other disulfide-linked dendritic polymers but also for exploitation of biological applications.

Rh-catalyzed direct arylation of a polyhedral oligomeric silsesquioxane

Conjugation of a polyhedral oligomeric silsesquioxane (POSS) with the π-electrons of functional organic units has recently attracted much attention. This is the first example of catalytic direct arylation of a POSS.