Sorption and Diffusion of Methane and Carbon Dioxide in Amorphous Poly(alkyl acrylates): A Molecular Simulation Study

Copper-Free Synthesis of Cationic Glycidyl Triazolyl Polymers

Copper-free synthesis of cationic glycidyl triazolyl polymers (GTPs) is achieved through a thermal azide-alkyne cycloaddition reaction between glycidyl azide polymer and propiolic acid, followed by decarboxylation and quaternization of the triazole unit. For synthesizing nonfunctionalized GTP (GTP-H), a microwave-assisted method enhances the decarboxylation reaction of carboxy-functionalized GTP (GTP-COOH). Three variants of cationic GTPs with different N-substituents [N-ethyl, N-butyl, and N-tri(ethylene glycol) monomethyl ether (EG3)] are synthesized. The molecular weight of GTP-H is determined via size exclusion chromatography. Thermal properties of all GTPs are characterized using differential scanning calorimetry and thermogravimetric analysis. The ionic conductivities of these cationic GTPs are assessed by impedance measurements. The conducting ion concentration and mobility are calculated based on the electrode polarization model. Among three cationic GTPs, the GTP with the N-EG3 substituent exhibits the highest ionic conductivity, reaching 6.8 × 10-6 S cm-1 at 25 °C under dry conditions. When compared to previously reported reference polymers, the reduction of steric crowding around the triazolium unit is considered to be a key factor in enhancing ionic conductivity.

Adsorption-Induced Deformation of Zeolites 4A and 13X: Experimental and Molecular Simulation Study

Gas adsorption in zeolites leads to adsorption-induced deformation, which can significantly affect the adsorption and diffusive properties of the system. In this study, we conducted both experimental investigations and molecular simulations to understand the deformation of zeolites 13X and 4A during carbon dioxide adsorption at 273 K. To measure the sample's adsorption isotherm and strain simultaneously, we used a commercial sorption instrument with a custom-made sample holder equipped with a dilatometer. Our experimental data showed that while the zeolites 13X and 4A exhibited similar adsorption isotherms, their strain isotherms differed significantly. To gain more insight into the adsorption process and adsorption-induced deformation of these zeolites, we employed coupled Monte Carlo and molecular dynamics simulations with atomistically detailed models of the frameworks. Our modeling results were consistent with the experimental data and helped us identify the reasons behind the different deformation behaviors of the considered structures. Our study also revealed the sensitivity of the strain isotherm of zeolites to pore size and other structural and energetic features, suggesting that measuring adsorption-induced deformation could serve as a complementary method for material characterization and provide guidelines for related technical applications.

Modelling across Multiple Scales to Design Biopolymer Membranes for Sustainable Gas Separations: 1—Atomistic Approach

In this work, we assessed the CO2 and CH4 sorption and transport in copolymers of 3-hydroxybutyrate and 3-hydroxyvalerate (PHBV), which showed good CO2 capture potential in our previous papers, thanks to their good solubility–selectivity, and are potential biodegradable alternatives to standard membrane-separation materials. Experimental tests were carried out on a commercial material containing 8% of 3-hydroxyvalerate (HV), while molecular modelling was used to screen the performance of the copolymers across the entire composition range by simulating structures with 0%, 8%, 60%, and 100% HV, with the aim to provide a guide for the selection of the membrane material. The polymers were simulated using molecular dynamics (MD) models and validated against experimental density, solubility parameters, and X-ray diffraction. The CO2/CH4 solubility–selectivity predicted by the Widom insertion method is in good agreement with experimental data, while the diffusivity–selectivity obtained via mean square displacement is somewhat overestimated. Overall, simulations indicate promising behaviour for the homopolymer containing 100% of HV. In part 2 of this series of papers, we will investigate the same biomaterials using a macroscopic model for polymers and compare the accuracy and performance of the two approaches.

Two Facile Aniline-Based Hypercrosslinked Polymer Adsorbents for Highly Efficient Iodine Capture and Removal

Effective capture and safe disposal of radioactive iodine (¹²⁹I or ¹³¹I) during nuclear power generation processes have always been a worldwide environmental concern. Low-cost and high-efficiency iodine removal materials are urgently needed. In this study, we synthesized two aniline-based hypercrosslinked polymers (AHCPs), AHCP-1 and AHCP-2, for iodine capture in both aqueous and gaseous phases. They are obtained by aniline polymerization through Friedel-Crafts alkylation and Scholl coupling reaction, respectively, with high chemical and thermal stability. Notably, AHCP-1 exhibits record-high static iodine adsorption (250 wt%) in aqueous solution. In the iodine vapor adsorption, AHCP-2 presents an excellent total iodine capture (596 wt%), surpassing the most reported amorphous polymer adsorbents. The rich primary amine groups of AHCPs promote the rapid physical capture of iodine from iodine water and iodine vapor. Intrinsic features such as low-cost preparation, good recyclability, as well as excellent performance in iodine capture indicate that the AHCPs can be used as potential candidates for the removal of iodine from radioactive wastewater and gas mixtures.

Interfacial properties of the hexane + carbon dioxide + water system in the presence of hydrophilic silica

Molecular dynamics simulations were conducted to study the interfacial behavior of the CO₂ + H₂O and hexane + CO₂ + H₂O systems in the presence of hydrophilic silica at geological conditions. Simulation results for the CO₂ + H₂O and hexane + CO₂ + H₂O systems are in reasonable agreement with the theoretical predictions based on the density functional theory. In general, the interfacial tension (IFT) of the CO₂ + H₂O system exponentially (linearly) decreased with increasing pressure (temperature). The IFTs of the hexane + CO₂ + H₂O (two-phase) system decreased with the increasing mole fraction of CO₂ in the hexane/CO₂-rich phase x_CO2 . Here, the negative surface excesses of hexane lead to a general increase in the IFTs with increasing pressure. The effect of pressure on these IFTs decreased with increasing x_CO2 due to the positive surface excesses of carbon dioxide. The simulated water contact angles of the CO₂ + H₂O + silica system fall in the range from 43.8° to 76.0°, which is in reasonable agreement with the experimental results. These contact angles increased with pressure and decreased with temperature. Here, the adhesion tensions are influenced by the variations in fluid-fluid IFT and contact angle. The simulated water contact angles of the hexane + H₂O + silica system fall in the range from 58.0° to 77.0° and are not much affected by the addition of CO₂. These contact angles increased with pressure, and the pressure effect was less pronounced at lower temperatures. Here, the adhesion tensions are mostly influenced by variations in the fluid-fluid IFTs. In all studied cases, CO₂ molecules could penetrate into the interfacial region between the water droplet and the silica surface.

Temperature Effects in Flexible Adsorption Processes for Amorphous Microporous Polymers

A collection of atomistic molecular simulations is reported that illustrate the impact of adsorption temperature on species uptake and adsorbate-induced structural rearrangement for amorphous polymers of intrinsic microporosity. Temperature-sensitive structural rearrangement is evaluated by contrasting two methods: standard grand canonical Monte Carlo simulations using a rigid framework approximation and a combined Monte Carlo/molecular dynamics approach that fully incorporates framework flexibility. We report single-component gas phase adsorption isotherms for CH₄, C₂H₄, C₂H₆, C₃H₆, C₃H₈, and CO₂ across a temperature range of 250-400 K for models of an archetypal polymer of intrinsic microporosity, PIM-1. A quadratic model is presented that captures two main mechanisms of temperature-dependent adsorption-induced deformation of PIM-1 up to a relative swelling of 1.15: thermal expansion and an increased propensity to swell as a function of species uptake. Two case studies are reported that highlight the critical role of operating temperature in industrial storage and separation applications. The first study focuses on methane storage and delivery applications using a pressure-temperature swing adsorption application (PTSA). We demonstrate that larger working capacities are accompanied by increased volumetric strain between adsorption-desorption steps. The second case study considers PIM-1 as an adsorbent to separate an exemplar ternary syngas mixture at operating temperatures ranging 300-550 K. A temperature threshold of ∼400 K is identified, beyond which adsorption-induced PIM-1 swelling is negligible and the solubility selectivity-loading curve transitions to exhibiting a nearly linear relationship.

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 effect of external salts on the aggregation of the multiheaded surfactants: All-atom molecular dynamics studies

Tailoring the molecular design of the surfactants leads to changes in the aggregation properties. The role of external salts on the aggregation properties of the multiheaded surfactants is investigated using molecular dynamics simulations. The multiheaded surfactants show differential aggregation properties on addition of external salts, as reported earlier from experimental studies. We have modelled the multiheaded surfactants to study the effect of external salts (potassium bromide and sodium salicylate) at three different concentrations using the all-atom modelling and explicit solvation. The influence of external salts on the hydration and aggregation propensity, hydrogen bonding, and the structural characteristics of the surfactant aggregates are probed using various analyses across the four groups of multiheaded surfactants. The larger salicylate ion masks the repulsion between the cationic head groups and acts as an effective promoter of aggregation.

Interfacial behavior of the decane + brine + surfactant system in the presence of carbon dioxide, methane, and their mixture

Molecular dynamics simulations are carried out to get insights into the interfacial behavior of the decane + brine + surfactant + CH₄ + CO₂ system at reservoir conditions. Our results show that the addition of CH₄, CO₂, and sodium dodecyl sulfate (SDS) surfactant at the interface reduces the IFTs of the decane + water and decane + brine (NaCl) systems. Here the influence of methane was found to be less pronounced than that of carbon dioxide. As expected, the addition of salt increases the IFTs of the decane + water + surfactant and decane + water + surfactant + CH₄/CO₂ systems. The IFTs of these surfactant-containing systems decrease with temperature and the influence of pressure is found to be less pronounced. The atomic density profiles show that the sulfate head groups of the SDS molecules penetrate the water-rich phase and their alkyl tails are stretched into the decane-rich phase. The sodium counterions of the surfactant molecules are located very close to their head groups. Furthermore, the density profiles of water and salt ions are hardly affected by the presence of the SDS molecules. However, the interfacial thickness between water and decane/CH₄/CO₂ molecules increases with increasing surfactant concentration. An important result is that the enrichment of CH₄ and/or CO₂ in the interfacial region decreases with increasing surfactant concentration. These results may be useful in the context of the water-alternating-gas approach that has been utilized during CO₂-enhanced oil recovery operations.

Acrylic Paints: An Atomistic View of Polymer Structure and Effects of Environmental Pollutants

Most of the artwork and cultural heritage objects are stored in museums under conditions that are difficult to monitor. While advanced technologies aim to control and prevent the degradation of cultural heritage objects in line with preventive conservation measures, there is much to be learned in terms of the physical processes that lead to the degradation of the synthetic polymers that form the basis of acrylic paints largely used in contemporary art. In museums, stored objects are often exposed to temperature and relative humidity fluctuations as well as airborne pollutants such as volatile organic compounds (VOCs). The glass transition of acrylic paints is below room temperature; while low temperatures may cause cracking, at high temperatures the sticky surface of the paint becomes vulnerable to pollutants. Here we develop fully atomistic models to understand the structure of two types of acrylic copolymers and their interactions with VOCs and water. The structure and properties of acrylic copolymers are slighlty modified by incorporation of a monomer with a longer side chain. With favorable solvation free energies, once absorbed, VOCs and water interact with the polymer side chains to form hydrogen bonds. The cagelike structure of the polymers prevents the VOCs and water to diffuse freely below the glass transition temperature. In addition, our model forms the foundation for developing mesoscopic and continuum models that will allow us to access longer time and length scales to further our understanding of the degradation of artwork.

High-Barrier Polyimide Containing Carbazole Moiety: Synthesis, Gas Barrier Properties, and Molecular Simulations

A high-barrier polyimide (2,7-CPI) was synthesized through the polymerization of pyromellitic dianhydride (PMDA) and a novel diamine (2,7-CDA) containing carbazole moiety. The synthesized diamine and polyimide were fully characterized by elemental analyses, FTIR and NMR. The 2,7-CPI displays very attractive barrier performances, with oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) low to 0.14 cm³·m⁻²·day⁻¹ and 0.05 g·m⁻²·day⁻¹, respectively. Meanwhile, 2,7-CPI also exhibits exceptional thermal stability with a glass transition temperature (T_g) of 467 °C, 5% weight-loss temperature (T_d5%) of 550 °C under N₂ and coefficient of thermal expansion (CTE) of 3.4 ppm/K. The barrier performances of 2,7-CPI are compared with those of a structural analogue (2,7-CPPI) and a typical polyimide (Kapton). Their barrier performances with respect to microstructure were investigated by molecular simulations, wide angle X-ray diffraction (WAXD), and positron annihilation lifetime spectroscopy (PALS). The results show that 2,7-CPI possesses better coplanar structure and more number of intermolecular hydrogen bonds among the three PIs, which result in tight chain packing and thereby high crystallinity, low free volume, and decreased chains mobility. That is, the high crystallinity and low free volume of 2,7-CPI reduce the diffusion and solubility of gases. Meanwhile, the poor chains mobility further decreases the gases diffusion. The reduced diffusion and solubility of gases consequently promote the improvement of barrier properties for 2,7-CPI. The polyimide has a wide application prospect in the field of flexible electronic packaging industries.