A molecular simulation study of cavity size distributions and diffusion in para and meta isomers

Comparison of Homo-Polyimide Films Derived from Two Isomeric Bis-Benzimidazole Diamines

Heteroaromatic polyimides (PIs) containing benzimidazole have attracted tremendous attention due to their positive impact on the properties of PIs. Some research on PIs containing 4,4'-[5,5'-bi-1H-benzimidazole]-2,2'-diylbis-benzenamine (4-AB) has been reported. However, reports are lacking on homo-polyimides (homo-PIs) containing 3,3'-[5,5'-bi-1H-benzimidazole]-2,2'-diylbis-benzenamine (3-AB), which is one of the isomers of 4-AB. In this paper, the influence of amino groups' positions on the performance of homo-PIs was investigated. It was found that the net charge of the amine N group in 4-AB was lower than that of 3-AB, resulting in higher reactivity of 4-AB. Consequently, PIs containing 4-AB displayed better mechanical performance. Molecular simulation confirmed that 3-AB and its corresponding PI chain exhibited distorted conformation, leading to the PI films containing 3-AB having a lighter color. In addition, the 3-AB structure was calculated to have higher rotational energy compared to 4-AB, resulting in a higher glass transition temperature (Tg) in PIs prepared from 3-AB. On the other hand, PIs containing 4-AB exhibited a higher level of molecular linearity, leading to a lower coefficient of thermal expansion (CTE) compared to PIs prepared from 3-AB. Furthermore, all PIs showed higher thermal stability with a 5% weight loss temperature above 530 °C and Tg higher than 400 °C.

A theoretical probe into the separation of CO2/CH4/N2 mixtures with polysulfone/polydimethylsiloxane-nano zinc oxide MMM

In the current investigation, molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) simulation as remarkable and competent approaches have been employed for understanding structural and transport properties of MMMs in the realm of gas separation. The two commonly used polymers i.e. polysulfone (Psf) and polydimethylsiloxane (PDMS) as well as zinc oxide (ZnO) nanoparticle (NP) were used to carefully examine the transport properties of three light gasses (CO₂, N₂ and CH₄) through simple Psf, Psf/PDMS composite loaded by different amounts of ZnO NP. Also, the fractional free volume (FFV), X-ray diffraction (XRD), glass transition temperature (T_g), and Equilibrium density were calculated to scrutinize the structural characterizations of the membranes. Moreover, the effect of feed pressure (4-16 bar) on gas separation performance of simulated MMMs was investigated. Results obtained in different experiments showed a clear improvement in the performance of simulated membranes by adding PDMS to PSf matrix. The selectivity of studied MMMs was in the range from 50.91 to 63.05 at pressures varying from 4 to 16 bar for the CO₂/N₂ gas pair, whereas the corresponding value for CO₂/CH₄ system was found to be in the range 27.27-46.24. For 6 wt% ZnO in 80%PSf + 20%PDMS membrane, high permeabilities of 78.02, 2.86 and 1.33 barrers were observed for CO₂, CH₄ and N₂ gases, respectively. The 90%PSf + 10%PDMS membrane with 2% ZnO had a highest CO₂/N₂ selectivity value of 63.05 and its CO₂ permeability at 8 bar was 57 barrer.

A molecular simulation study on amine-functionalized silica/polysulfone mixed matrix membrane for mixed gas separation

Polysulfone (PSF) based mixed matrix membranes (MMMs) are one of the most broadly studied polymeric materials used for CO₂/CH₄ separation. The performance of existing PSF membranes encounters a bottleneck for widespread expansion in industrial applications due to the trade-off amongst permeability and selectivity. Membrane performance has been postulated to be enhanced via functionalization of filler at different weight percentages. Nonetheless, the preparation of functionalized MMMs without defects and its empirical study that exhibits improved CO₂/CH₄ separation performance is challenging at an experimental scale that needs prior knowledge of the compatibility between the filler and polymer. Molecular simulation approaches can be used to explore the effect of functionalization on MMM's gas transport properties at an atomic level without the challenges in the experimental study, however, they have received less scrutiny to date. In addition, most of the research has focused on pure gas studies while mixed gas transport properties that reflect real separation in functionalized silica/PSF MMMs are scarcely available. In this work, a molecular simulation computational framework has been developed to investigate the structural, physical properties and gas transport behavior of amine-functionalized silica/PSF-based MMMs. The effect of varying weight percentages (i.e., 15-30 wt.%) of amine-functionalized silica and gas concentrations (i.e., 30% CH₄/CO₂, 50% CH₄/CO₂, and 70% CH₄/CO₂) on physical and gas transport characteristics in amine-functionalized silica/PSF MMMs at 308.15 K and 1 atm has been investigated. Functionalization of silica nanoparticles was found to increase the diffusion and solubility coefficients, leading to an increase in the percentage enhancement of permeability and selectivity for amine-functionalized silica/PSF MMM by 566% and 56%, respectively, compared to silica/PSF-based MMMs at optimal weight percentage of 20 wt.%. The model's permeability differed by 7.1% under mixed gas conditions. The findings of this study could help to improve real CO₂/CH₄ separation in the future design and concept of functionalized MMMs using molecular simulation and empirical modeling strategies.

Molecular simulation of enhanced separation of humid air components using GO-PVA nanocomposite membranes under differential pressures

Hydrophilic nanocomposite membranes have significant advantages in the separation of water vapor which is the core process in air dehumidification. This paper focuses on exploring the micro-mechanism of enhanced separation using graphene oxide-polyvinyl alcohol (GO-PVA) nanocomposite membranes. The sorption and diffusion behaviors of water vapor and nitrogen in GO-PVA membranes were investigated using molecular dynamics (MD) and Monte Carlo (MC) methods. The study showed that embedding GO into a PVA matrix results in a higher glass transition temperature and fractional free volume. The latter is believed to enhance the diffusivity of gas molecules in polymeric membranes. The interaction between the polymer chains and GO nanoparticles notably promotes the adsorption capacity of water vapor and inhibits nitrogen adsorption in the membrane. A water vapor permeance of 8844.07 Barrer and a separation factor of 3.53 could be achieved with the GO-PVA-0.5 membrane. The analysis confirmed that GO has the same effect on single gas and binary gas mixtures, i.e., increasing the water vapor permeability and selectivity. The calculated water vapor permeance of binary gas is 83% lower than that of single gas permeation. It is expected that this research could provide fundamentals for the optimization and synthesis of gas separation membranes.

Dielectric Properties of Fluorinated Aromatic Polyimide Films with Rigid Polymer Backbones

Fluorinated aromatic polyimide (FAPI) films with rigid polymer backbones have been prepared by chemical imidization approach. The polyimide films exhibited excellent mechanical properties including elastic modulus of up to 8.4 GPa and tensile strength of up to 326.7 MPa, and outstanding thermal stability including glass transition temperature (T_g) of 346.3-351.6 °C and thermal decomposition temperature in air (T_d5) of 544.1-612.3 °C, as well as high colorless transmittance of >81.2% at 500 nm. Moreover, the polyimide films showed stable dielectric constant and low dielectric loss at 10-60 GHz, attributed to the close packing of rigid polymer backbones that limited the deflection of the dipole in the electric field. Molecular dynamics simulation was also established to describe the relationship of molecular structure and dielectric loss.

A Molecular Simulation Study of Silica/Polysulfone Mixed Matrix Membrane for Mixed Gas Separation

Polysulfone-based mixed matrix membranes (MMMs) incorporated with silica nanoparticles are a new generation material under ongoing research and development for gas separation. However, the attributes of a better-performing MMM cannot be precisely studied under experimental conditions. Thus, it requires an atomistic scale study to elucidate the separation performance of silica/polysulfone MMMs. As most of the research work and empirical models for gas transport properties have been limited to pure gas, a computational framework for molecular simulation is required to study the mixed gas transport properties in silica/polysulfone MMMs to reflect real membrane separation. In this work, Monte Carlo (MC) and molecular dynamics (MD) simulations were employed to study the solubility and diffusivity of CO₂/CH₄ with varying gas concentrations (i.e., 30% CO₂/CH₄, 50% CO₂/CH₄, and 70% CO₂/CH₄) and silica content (i.e., 15–30 wt.%). The accuracy of the simulated structures was validated with published literature, followed by the study of the gas transport properties at 308.15 K and 1 atm. Simulation results concluded an increase in the free volume with an increasing weight percentage of silica. It was also found that pure gas consistently exhibited higher gas transport properties when compared to mixed gas conditions. The results also showed a competitive gas transport performance for mixed gases, which is more apparent when CO₂ increases. In this context, an increment in the permeation was observed for mixed gas with increasing gas concentrations (i.e., 70% CO₂/CH₄ > 50% CO₂/CH₄ > 30% CO₂/CH₄). The diffusivity, solubility, and permeability of the mixed gases were consistently increasing until 25 wt.%, followed by a decrease for 30 wt.% of silica. An empirical model based on a parallel resistance approach was developed by incorporating mathematical formulations for solubility and permeability. The model results were compared with simulation results to quantify the effect of mixed gas transport, which showed an 18% and 15% percentage error for the permeability and solubility, respectively, in comparison to the simulation data. This study provides a basis for future understanding of MMMs using molecular simulations and modeling techniques for mixed gas conditions that demonstrate real membrane separation.

Role of Intrinsic Factors of Polyimides in Glass Transition Temperature: An Atomistic Investigation

Polyimides (PIs) are in high demand in the field of active matrix organic light-emitting diode displays because of their excellent heat resistance, chemical stability, and mechanical properties. However, the most critical key to their application is to further enhance their glass transition temperature (T_g), which directly affects the processing temperature of thin-film transistors on the PI films. Therefore, it is of great importance to study the factors that have an influence on the T_g of PIs. To accomplish this goal, PIs derived from pyromellitic acid dianhydride and three sets of isomeric imidazole-based diamines were investigated. The investigation, by computational methods, was to clarify the effect of intrinsic factors associated with the molecular structure of the PIs on their T_g and to construct a structure-T_g relationship for these PIs. For each model system, all-atom molecular dynamics simulations were used to identify and distinguish the effects of chain rigidity, fractional free volume (FFV), cohesive energy density, hydrogen-bonding interactions, and charge-transfer complex interactions on T_g. The results showed that the physical property, chain rigidity, has a direct impact on T_g regardless of the polymer backbone structure. A linear correlation between the increase of FFV and the decrease of T_g was not established due to the existence of hydrogen-bonding interactions, but the tendency was maintained. Furthermore, the formation of hydrogen bonds was found to have an indirect relationship with T_g. That is, the increase of intrachain hydrogen bonds would lead to a decrease in chain rigidity and consequently reduce the T_g value.

Gas Separation Properties of Polyimide Thin Films on Ceramic Supports for High Temperature Applications

Novel selective ceramic-supported thin polyimide films produced in a single dip coating step are proposed for membrane applications at elevated temperatures. Layers of the polyimides P84^®, Matrimid 5218^®, and 6FDA-6FpDA were successfully deposited onto porous alumina supports. In order to tackle the poor compatibility between ceramic support and polymer, and to get defect-free thin films, the effect of the viscosity of the polymer solution was studied, giving the entanglement concentration (C*) for each polymer. The C* values were 3.09 wt. % for the 6FDA-6FpDA, 3.52 wt. % for Matrimid^®, and 4.30 wt. % for P84^®. A minimum polymer solution concentration necessary for defect-free film formation was found for each polymer, with the inverse order to the intrinsic viscosities (P84^® ≥ Matrimid^® >> 6FDA-6FpDA). The effect of the temperature on the permeance of prepared membranes was studied for H₂, CH₄, N₂, O₂, and CO₂. As expected, activation energy of permeance for hydrogen was higher than for CO₂, resulting in H₂/CO₂ selectivity increase with temperature. More densely packed polymers lead to materials that are more selective at elevated temperatures.

Computational insights on the role of film thickness on the physical properties of ultrathin polysulfone membranes

A pioneering work to elucidate physical properties of ultrathin membrane films from atomistic point of view in Materials Studio.

Molecular simulation of the effect of temperature and architecture on polyethylene barrier properties

We present a multiscale approach for calculating the low-concentration solubility, diffusivity, and selectivity of small molecules through polymer matrixes. The proposed modeling scheme consists of two main stages; first, thoroughly equilibrated and representative poly(ethylene) (PE) atomistic melt configurations were obtained through the application of a Monte Carlo (MC) scheme based on advanced chain-connectivity altering moves (linear architectures) or the combination of localized MC moves followed by molecular dynamics. In the second phase, transition-state theory (TST), as proposed by Gusev and Suter [Gusev, A. A.; Suter, U. W. J. Chem. Phys. 1993, 99, 2228], was invoked in a coarser level of description to calculate the barrier properties of the studied macromolecules to small gas molecules at infinite dilution. The multiscale methodology was successfully applied on PE melts characterized by various molecular weights (MW) (from C78 up to C1000) and polydispersity indices at a wide range of temperature conditions. The effect of molecular architecture on the barrier properties was examined through the comparison between linear and short-chain branched structures bearing the same total number of carbon atoms. Simulation results were found to be in very good agreement with available experimental data. Additionally, the new scheme has been further validated by comparing the qualitative behavior of solubility, diffusivity, and selectivity with previously reported trends in the literature based on both experimental and simulation studies. The present study concludes that density plays a dominant role that determines the behavior of the polymer as a barrier material, especially in terms of diffusivity. Additionally, it is evidenced that short-chain branching has a small effect on the barrier properties of PE when the comparison is performed on purely amorphous samples. The hierarchical method presented here not only is faster when compared against conventional molecular dynamics simulations, but in some cases, like the vicinity of the glass transition temperature or for long polymer chain melts, it opens the way to the calculation of the barrier properties at realistic simulation times.