Molecular Insights on the CH₄/CO₂ Separation in Nanoporous Graphene and Graphene Oxide Separation Platforms: Adsorbents versus Membranes

Molecular dynamics simulations were performed to gain fundamental molecular insights on the concentration-dependent adsorption and gas transport properties of the components in a CH₄/CO₂ gaseous mixture in single- and double-layered nanoporous graphene (NPG) and graphene oxide (NPGO) separation platforms. While these platforms are promising for a variety of separation applications, much about the relevant gas separation mechanisms in these systems is still unexplored. Based on the gas adsorption results in this work, at least two layers of CO₂ are formed on the gas side of both NPG and NPGO, while no adsorption is observed for pure CH₄ on the single-layered NPG. In contrast, increasing the CH₄ concentration in the CH₄/CO₂ mixture leads to an enhancement of the CH₄ adsorption on both separation platforms. The through-the-pore diffusion coefficients of both CO₂ and CH₄ increase with an increase in the CH₄ concentration for all NPG and NPGO systems. The permeance of CO₂ is smaller than that of CH₄, suggesting the NPG and NPGO platforms are more suitable as CO₂ adsorbents or membranes for the CH₄/CO₂ (rather than the CO₂/CH₄) separation. The highest observed selectivities for the CH₄/CO₂ separation in the NPG and NPGO platforms are about 5 and 6, respectively

Reactive Molecular Simulation of the Damage Mitigation Efficacy of POSS‑, Graphene‑, and Carbon Nanotube-Loaded Polyimide Coatings Exposed to Atomic Oxygen Bombardment

Reactive molecular dynamics simulation was employed to compare the damage mitigation efficacy of pristine and polyimide (PI)-grafted polyoctahedral silsesquioxane (POSS), graphene (Gr), and carbon nanotubes (CNTs) in a PI matrix exposed to atomic oxygen (AO) bombardment. The concentration of POSS and the orientation of Gr and CNT nanoparticles were further investigated. Overall, the mass loss, erosion yield, surface damage, AO penetration depth, and temperature evolution are lower for the PI systems with randomly oriented CNTs and Gr or PI-grafted POSS compared to those of the pristine POSS or aligned CNT and Gr systems at the same nanoparticle concentration. On the basis of experimental early degradation data (before the onset of nanoparticle damage), the amount of exposed PI, which has the highest erosion yield of all material components, on the material surface is the most important parameter affecting the erosion yield of the hybrid material. Our data indicate that the PI systems with randomly oriented Gr and CNT nanoparticles have the lowest amount of exposed PI on the material surface; therefore, a lower erosion yield is obtained for these systems compared to that of the PI systems with aligned Gr and CNT nanoparticles. However, the PI/grafted-POSS system has a significantly lower erosion yield than that of the PI systems with aligned Gr and CNT nanoparticles, again due to a lower amount of exposed PI on the surface. When comparing the PI systems loaded with PI-grafted POSS versus pristine POSS at low and high nanoparticle concentrations, our data indicate that grafting the POSS and increasing the POSS concentration lower the erosion yield by a factor of about 4 and 1.5, respectively. The former is attributed to a better dispersion of PI-grafted POSS versus that of the pristine POSS in the PI matrix, as determined by the radial distribution function

Self-Assembled Morphologies of Polystyrene-<i>block</i>-poly(ethylene oxide)/1-Ethyl-3-methylimidazolium Thiocyanate Membranes by Mesoscopic Dynamics Simulation

The stability of gas separation membranes cast from diblock copolymer/ionic liquid (IL) blends depends on the resulting self-assembled morphologies of the cast films. Therefore, controlling the copolymer/IL morphology by tuning parameters such as IL loading and copolymer block size ratio is essential to prevent the membrane from leaching out the IL at high transmembrane pressures. In the present study, we used the dynamic mean-field density functional method to investigate the self-assembly of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) copolymers in 1-ethyl-3-methylimidazolium thiocyanate ([EMIM][SCN]) at different PS:PEO block size ratios and IL loadings (10–90 vol %) at room temperature (298 K). The IL was observed to be either confined by the hydrophilic PEO phase (designated as IL-PEO) due to strong IL-PEO interactions or yield a separate partially or fully encapsulated microphase (IL-Micro). The copolymer morphologies observed herein were lamellar (L), cylindrical (C), body-centered cubic (BCC), and spherical micelle (S). The dominant copolymer/IL morphology on the ternary phase diagram was L/IL-PEO, which formed at medium loadings of the three components (40 vol % < PS < 80 vol %, PEO < 90 vol %, and IL < 50 vol %). The IL-Micro morphologies appeared as transition phases to the IL-PEO phases, typically at low IL loadings and PS:PEO block size ratios. We also investigated the morphology evolution of select copolymer/IL compositions. Overall, the G, L, and C copolymer morphologies were observed at low to medium IL loadings, while the BCC and S copolymer morphologies appeared at higher IL loadings. The potential applications of these self-assembled morphologies could be further explored by investigating the role of electrostatic interactions and varying the types and loadings of ILs, as well as the type of the diblock copolymers, to discover new membrane systems with unique properties for gas separations