Development of All-Atom Empirical Potentials for Phloroglucinol (Phg) and Hexachlorocyclotriphosphazene (HCCP) Based on their Crystal Phase Structures.

Two existing generic force fields have been augmented with partial charges and tuned in order to give intercompatible all-atom empirical potentials that can satisfactorily represent the known crystal phase structures of the organic phloroglucinol (Phg) (C6H6O3) and inorganic hexachlorocyclotriphosphazene (HCCP) (N3P3Cl6) molecules at several temperatures. It has been proposed that HCCP-Phg network polymers could act as efficient H2 barrier layers in hydrogen storage tanks for cars. However, essential requirements for modeling such networks are adequate representations of both monomers in their pure dense phases using a common form of force field. Tests of their ability to maintain stable crystal structures have been made using classical molecular dynamics (MD) simulations on large 800-molecule supercells. The force fields have been optimized to match the densities calculated from the experimental unit cell dimensions at ambient conditions as well as the intermolecular potential energy, as estimated from experimental enthalpies of sublimation. For Phg, the crystal structure is stabilized by a network of hydrogen bonds and the Coulombic interactions contribute to over 55% of the total intermolecular potential energy. In contrast, the crystal structure of HCCP is intrinsically stabilized by the van der Waals terms. Both optimized force fields reproduce very well the orthorhombic symmetry of their respective crystals under constant-pressure NPT conditions. The model parameters tuned at ambient temperature also give reasonable agreement with crystallographic data at lower temperatures.

Comparison of Eight Classical Lennard-Jones-Based H2 Molecular Models in the Gas Phase at Temperatures and Pressures Relevant to Hydrogen On-Board Storage Tanks.

This work compares eight classical H2 molecular models in the gas phase taken from the existing literature. All models are based on Lennard-Jones (LJ) 12-6 terms for the van der Waals interactions and hence easier to transfer to multiphase molecular simulations than more sophisticated potentials. The H2 potentials tested include one-site, two-site, three-site, and five-site models, with the sites being either the H atoms, the center-of-mass of the H2 molecule, or massless sites. For the multisite models, high-frequency H-H stretching modes can lead to poor equipartition of the kinetic energy, and the timestep for molecular dynamics (MD) simulations should be reduced to maintain a stable numerical integration of the equations of motion. As such, only those models with rigid bonds are considered. In the present case, 600 MD simulations of H2 gas were carried out over a large range of temperatures (-50 to +90 °C) and at densities corresponding to a pressure range of 50 to 2000 bar, which include the operating conditions of on-board storage tanks in hydrogen-fueled vehicles. Most of the models under study were found to reproduce reasonably well the experimental pVT phase diagram as well as the solubility. Discrepancies only became significant at the highest densities tested, and these could be used to rank the different models. All model diffusion coefficients were essentially indistinguishable from experimental results, and as such, kinetically dominated dynamic properties could not be used as a criterion for the choice of model. Among the eight models tested, two of them, i.e., the two-site model of Yang and Zhong and the one-site model derived from Buch performed very well over the range of conditions tested. They represent a good compromise between realism, simplicity, and computational efficiency.

Single-gas and mixed-gas permeation of N2/CH4 in thermally-rearranged TR-PBO membranes and their 6FDA-bisAPAF polyimide precursor studied by molecular dynamics simulations.

High-performance polymers with polybenzoxazole (PBO) structures, formed via thermal rearrangement (TR) of aromatic polyimide precursors, have been developed for gas separation applications. The present work compares the transport of N2 and CH4 in a 6FDA-bisAPAF polyimide precursor and in its TR-PBO derivative using molecular dynamics (MD) simulations. The modelling closely mimicked the experimental approach by transforming a 6FDA-bisAPAF atomistic model into its corresponding TR-PBO structure via a specific algorithm. The densities and void spaces of both precursor and TR polymers were found to compare well to experimental data. An iterative technique was used to obtain the single-gas sorption isotherms of N2 and CH4 at 338.5 K in both polymers over a range of feed pressures up to and exceeding 65 bar. CH4 was systematically found to be more soluble than N2. Solubilities in both matrices were quite similar with those in TR-PBO being slightly higher due to its larger fraction of significant volume. Volume dilation analyses confirmed a higher resistance to plasticization for TR-PBO. Extended single-gas N2 and CH4 simulations and 2 : 1 binary CH4/N2 mixed-gas simulations were then conducted in both matrices at 338.5 K and at a pressure of ∼65 bar corresponding to natural gas processing conditions. Mixed-gas sorption was modelled using a modification of the aforementioned iterative method, which fixed the pressure and iterated to convergence the number of molecules of each type of penetrant. The gas diffusion coefficients were estimated using the Trajectory-Extending Kinetic Monte Carlo (TEKMC) procedure. As found experimentally, significantly higher diffusivities and permeabilities were observed in the TR polymer, which led to a slightly lower ideal N2/CH4 permselectivity for TR-PBO (∼2.6) when compared to its 6FDA-bisAPAF precursor (∼3.8). However, both models showed a reduced N2/CH4 separation efficiency under 2 : 1 binary CH4/N2 mixed-gas conditions bordering on the loss of selectivity. For 6FDA-bisAPAF, both permeabilities decreased in the mixed-gas case, but more for N2 than for CH4. For TR-PBO, the permeability of the faster N2 decreased while the permeability of the slower CH4 increased under mixed-gas conditions. This confirms that single-gas simulations are not sufficient for the prediction of the actual mixed-gas permselectivity behaviour in such polymers.

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.

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

Hybrid organic/inorganic hyper-cross-linked membranes based on imides covalently bonded with polyhedral oligomeric silsesquioxanes (POSS) have recently been developed for gas-separation applications under high pressure and/or temperature conditions. Their molecular sieving capabilities have been shown to depend on the nature of the organic dianhydride precursor. In the present work, realistic molecular models of such polyPOSS-imide films based on the flexible 6FDA dianhydride are compared to those based on the shorter and more rigid PMDA dianhydride. The models creation procedure closely mimicks the mixing, polycondensation and imidization steps of the experimental scheme. The resulting networks are found to be highly heterogeneous in terms of both the number of links (from zero to the maximum possible of eight per POSS cage with an average of four) and their structure (interPOSS, intraPOSS, single-links, double-links) because of the eight-equivalent-arms nature of the POSS precursor. For both dianhydride precursors, crosslinking with POSS and the subsequent imidization step decrease the density, create additional void-space and increase the solubility of the resulting membranes. However, when compared to PMDA, the added flexibility of the central 6FDA bridge leads to a larger thermally-induced dilation of the networks and a larger volume loss per H2O over the imidization step. With their better ability to redensify and to adapt to the added constraints, the cagecage distances and cage(organic bridge)cage angles in the 6FDA polyPOSS-imides span a larger range than in their PMDA counterparts. In addition, the stiffness of the PMDA moiety results in more unrelaxed free volume remaining trapped in the PMDA polyPOSS-imides upon imidization, and as such, to significantly more open structures with less favourable interactions. As expected from their enhanced flexibility, the thermomechanical properties of the 6FDA networks are slightly lower than those based on PMDA. However, the better mechanical resistance of PMDA over 6FDA does not really become significant before very large volume dilations.

A Molecular Dynamics Study of Single-Gas and Mixed-Gas N2 and CH4 Transport in Triptycene-Based Polyimide Membranes.

Fluorinated polyimides incorporated with triptycene units have gained growing attention over the last decade since they present potentially interesting selectivities and a higher free volume with respect to their triptycene-free counterparts. This work examines the transport of single-gas and mixed-gas N2 and CH4 in the triptycene-based 6FDA-BAPT homopolyimide and in a block 15,000 g mol-1/15,000 g mol-1 6FDA-mPDA/BAPT copolyimide by using molecular dynamics (MD) simulations. The void-space analyses reveal that, while the free volume consists of small-to-medium holes in the 6FDA-BAPT homopolyimide, there are more medium-to-large holes in the 6FDA-mPDA/BAPT copolyimide. The single-gas sorption isotherms for N2 and CH4 over the 0-70 bar range at 338.5 K show that both gases are more soluble in the block copolyimide, with a higher affinity for methane. CH4 favours sites with the most favourable energetic interactions, while N2 probes more sites in the matrices. The volume swellings remain limited since neither N2 nor CH4 plasticise penetrants. The transport of a binary-gas 2:1 CH4/N2 mixture is also examined in both polyimides under operating conditions similar to those used in current natural gas processing, i.e., at 65.5 bar and 338.5 K. In the mixed-gas simulations, the solubility selectivities in favour of CH4 are enhanced similarly in both matrices. Although diffusion is higher in 6FDA-BAPT/6FDA-mPDA, the diffusion selectivities are also close. Both triptycene-based polyimides under study favour, to a similar extent, the transport of methane over that of nitrogen under the conditions studied.

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.

The stability of amino-functionalized polyhedral oligomeric silsesquioxanes in water.

Octa(aminopropylsilsesquioxane) Si8O12[(CH2)3NH2]8 is a very important precursor for many other hybrid organic/inorganic polyhedral oligomeric silsesquioxanes (POSS) because of the reactivity of its primary amine groups. Unfortunately, it is unstable in water, which can lead to the cleavage of its siloxane cage. In the present work, such a degradation was confirmed using solid-state (29)Si NMR spectroscopy, and the molecular features at the basis of this instability were studied using molecular dynamics simulations (MD). It was also investigated whether replacing the primary amine end groups by secondary amines or by amides with long aliphatic chains could lead to an improvement in the water stability of the Si/O framework. In the pure bulk models, all POSS interdigitate with their pendant organic arms intertwined. Upon insertion of isolated molecules into water, the dimensions of the primary amine POSS remain close to those of the bulk, while the secondary amine and the amide POSS favor conformations that optimize the intramolecular chain-chain interactions. When there are several POSS molecules in water, they cluster with each other through both intra- and intermolecular chain-chain interactions. This tendency for the organic chains to intertwine whenever possible provides some protection to the siloxane cages from water, but also leaves some of the siloxane O exposed. As such, the latter are accessible to form transient hydrogen bonds with the water molecules, which could be a precursor step to hydrolysis and thus cage breakage. In the molecular models, a better protection was obtained in the amide POSS for two reasons: its chains tended to wrap efficiently around its cage, and its ketone O kept water from getting close to the siloxanes. The molecular modeling characterizations were found to agree very well with experimental evidence.