Water vapor/propylene sorption and diffusion behavior in PVA-P(AA-AMPS) blend membranes by GCMC and MD simulation

Detailed atomistic structures of blend membranes (poly vinyl alcohol (PVA)/(acrylic acid-co-2-acrylamido-2-methylpropylsulfonic acid) (P(AA-AMPS)) were constructed to investigate the sorption and diffusion behavior of gas molecules (water and propylene) in the membranes. Interaction and miscibility between PVA and P(AA-AMPS) were calculated, and it was found that strong intermolecular interaction resulted in good miscibility of PVA and P(AA-AMPS) in the blend. The polymer chains mobility and free volume properties of the blend membranes were characterized. The sorption quantities and sorption sites of water and propylene in the blend membranes were calculated using Grand Canonical Monte Carlo (GCMC) method. The diffusion coefficients of water in the blend membranes were calculated by molecular dynamics (MD) simulation. The simulated results of the membrane structure (chain mobility, free volume properties), the sorption quantities and diffusion coefficients of water/propylene in the blend membranes showed the identical changing trends as the experimental results. Hopefully, this study could offer qualitative insight into the mass transport phenomena within the blend membranes.     

Application of the Monte Carlo simulation method to the investigation of peculiar free‐radical copolymerization reactions: Systems with both reactivity ratios greater than unity (rA > 1 and rB > 1)

With a Monte Carlo simulationmethod, copolymer properties have been thoroughly studied, and the influence of the reactivity ratios and feed composition has been taken into consideration. Instantaneous alterations of the copolymer composition and copolymer heterogeneity, which is also called a randomness parameter, have been examined with data obtained from the simulation at each stage of the copolymerization reaction. The results prove the azeotropic behavior of copolymerization reactions in which both reactivity ratios are greater than unity, although some special reactivity ratio combinations ignore the azeotropic behavior. The copolymer composition reaches an azeotrope point at the end of the copolymerization reaction when the copolymerization is an azeotropic reaction. In addition, the randomness parameter takes its maximum value at the azeotrope point when reactivity ratio r_A is equal to reactivity ratio r_B. Finally, increasing the reactivity ratios causes no change in the trend of copolymer composition/feed composition curves when r_A is equal to r_B. However, the curves produced with larger r_A and r_B values show more fluctuations. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Multi-scale modeling of polyimides

A coarse-grained model for a set of three polyimide isomers is developed. Each polyimide is comprised of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and one of three APB isomers: 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene or 1,3-bis(3-aminophenoxy)benzene. The coarse-grained model is constructed as a series of linked vectors following the contour of the polymer backbone. Beads located at the midpoint of each vector define centers for long range interaction energies between monomer subunits. A bulk simulation of each coarse-grained polyimide model is performed with a dynamic Monte Carlo procedure. These coarse-grained models are then reverse-mapped to fully atomistic models. The coarse-grained models show the expected trends in decreasing chain dimensions with increasing meta linkage in the APB section of the repeat unit, although these differences were minor due to the relatively short chains simulated here. Considerable differences are seen among the dynamic Monte Carlo properties of the three polyimide isomers. Decreasing relaxation times are seen with increasing meta linkage in the APB section of the repeat unit. The packing behavior of the three isomers is compared with the atomistic and coarse-grained models.     

On Bayesian analysis of nonlinear continuous‐time autoregression models

Abstract. This article introduces a method for performing fully Bayesian inference for nonlinear conditional autoregressive continuous‐time models, based on a finite skeleton of observations. Our approach uses Markov chain Monte Carlo and involves imputing data from times at which observations are not made. It uses a reparameterization technique for the missing data, and because of the non‐Markovian nature of the models, it is necessary to adopt an overlapping blocks scheme for sequentially updating segments of missing data. We illustrate the methodology using both simulated data and a data set from the S & P 500 index.     

Atomistic molecular dynamics simulations of gas diffusion and solubility in rubbery amorphous hydrocarbon polymers

Diffusion coefficients D of H₂, He, O₂, N₂, and CO₂ in different rubbery amorphous polymeric matrices were estimated by atomistic molecular dynamics simulations at 298 K using the Einstein relationship, and compared with the relevant experimental values, where available. The simulated diffusion coefficients D of all the gases in all polymers considered almost regularly decreased with increasing molecular gas volumes and increasing polymer glass transition temperature. Further, solubility coefficients and heats of solution were obtained for all gases from Grand Canonical Monte Carlo simulations, which were also used to calculate sorption isotherms. In general, there is a good agreement between experimental and simulated values of diffusion and solubility coefficients for all gases considered.     

The thermal characteristics of epoxy resin: Design and predict by using molecular simulation method

Trial-and-error approaches for experimentally designing and optimizing the polymer matrix for advanced composites are time-consuming and expensive. The simulation for curing behavior and structure–property relationships of epoxy resins can provide a guideline for designing resin matrix which will possess desirable properties. So far there are few reports in which the accuracy of the molecular simulation for the different amine-epoxy systems are addressed. In this paper, an atomistic modeling technique was used to theoretically investigate the curing and thermal transition behavior of two epoxy resin matrices containing amine curing agent with different chemical structures i.e. diaminodiphenyl methane (DDM)/diglycidyl-4,5-epoxycyclohexane-1,2-dicarboxylate (TDE85) and diaminodiphenyl sulfone (DDS)/TDE85 to give help for designing high heat-resistant epoxy matrix. The simulated results successfully predicted that the reaction process was catalyzed in the early stage of the curing and the slight modification in the diamine structure resulted in significant change in the curing and glass transition behavior of epoxy resin. As the bridging group of diamine changed from methylene to sulphone, the reactivity of diamine toward epoxy declined and the glass transition temperature increased from about 190 °C to about 230 °C. This simulated method presented a good agreement with experimental data, and can be used to design and predict high performance resin matrix for advanced composites.     

Immobilization of the Laccases from Trametes versicolor and Streptomyces coelicolor on Single‐wall Carbon Nanotube Electrodes: A Molecular Dynamics Study

In this work, we investigate the immobilization of laccases from Trametes versicolor (TvL) and the small laccase (SLAC) from Streptomyces coelicolor on single‐wall carbon nanotube (SWCNT) surfaces. SLAC may potentially offer improved adsorption on the electrode, thus improving bioelectrocatalytic activity via direct electron transfer (DET). Laccase immobilization on SWCNTs is achieved non‐covalently with a molecular tether (1‐pyrene butanoic acid, succinimidyl ester) that forms an amide bond with an amine group on the laccase surface while the pyrene coordinates to the SWCNT by π–π stacking. In our approach, density functional theory calculations were first used to model the interaction energies between SWCNTs and pyrene to validate an empirical force field, thereafter applied in molecular dynamics (MD) simulations. In the simulated models, the SWCNT was placed near the region of the (type 1) Cu(T1) atom in the laccases, and in proximity to other regions where adsorption seems likely. Calculated interaction energies between the SWCNTs and laccases and distances between the SWCNT surface and the Cu(T1) atom have shown that SWCNTs adsorb more strongly to SLAC than to TvL, and that the separation between the SWCNTs and Cu(T1) atoms is smaller for SLAC than for TvL, having implications for improved DET.     

Modeling of the propylene polymerization catalyzed by single‐/multi‐active site catalyst: A Monte Carlo study

In the present study, a model is established to describe the propylene polymerization kinetics catalyzed by the typical catalysts with single‐/multi‐active site type in a liquid phase stirred‐tank reactor using the Monte Carlo simulation method, regardless of the mass and heat diffusion effects within the polymer particles. Many kinetic data, including polypropylene yield, concentration transformation of catalyst active sites, number–average molecular weight, etc., are obtained by the model. The simulated kinetic results are found to be in agreement with the reference ones obtained in a population balance model. Furthermore, the comparisons of the kinetic data between the polymerization catalyzed by the catalyst with single‐active site type (typically silica‐supported metallocene) and the catalyst with multi‐active site type (typically MgCl₂‐supported Ziegler‐Natta catalyst) have been studied using the model. Especially, the effects of hydrogen on the polymerization are studied using the model. The studied results show that the theory of catalyst active site can be used to explain the different propylene polymerization kinetics catalyzed by the typical catalyst with single‐/multi‐active site type. In addition, the role of hydrogen in the propylene polymerization needs to be emphasized. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Molecular insights for the optimization of solvent‐based selective extraction of ethanol from fermentation broths

Simulations and experiments were carried out to explore the solvent extraction of ethanol from aqueous solution using a series of seven 10‐carbon alcohols. It is shown that configurational‐bias Monte Carlo simulations in the Gibbs ensemble coupled with the TraPPE‐UA force field can be utilized for predictive screening of the different extraction abilities (in terms of capacity factor and selectivity) of these alcohols. Analysis of the simulation trajectories indicates that extraction capacity is connected to the stabilization of larger ethanol/water cluster in the organic solvent, whereas selectivity is improved when smaller ethanol/water clusters are more prevalent. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3065–3070, 2013     

Porous amorphous carbon models from periodic Gaussian chains of amorphous polymers

An algorithm has been developed to create structural models for amorphous carbons using Monte Carlo simulations in the canonical ensemble. The simulation method used follows the experimental preparation of nanoporous carbons (NPC) by pyrolysis from polyfurfuryl alcohol as a guideline. The resulting structure exhibits properties that compare favorably to those observed experimentally for real NPCs. These atomistic NPC models are approaching a realistic representation of NPCs used for gas separations and as such, are being used to study the diffusion of small gas molecules in these materials. Limitations of the method and possible improvements are discussed.     

Monte Carlo simulation of gas phase polymerization of 1,3‐butadiene. Part II: Kinetics optimization and confirmation

Studies on the deactivations and initiations of gas phase polymerizations of 1,3‐butadiene have been achieved by Monte Carlo simulation. Initiation and deactivation control the reaction before and after the peak of the polymerization rate, respectively. The influence of polymerization temperature has been studied. Monte Carlo modeling of polymerization kinetics and mechanism was confirmed by the agreement of experimental data and simulation results of polymerizations run with a temporary evacuation of monomer. The balance of catalysts and active chains is established by both initiation and chain transfer reactions with cocatalyst, which causes a ‘pseudo‐stability’ stage. © 2003 Society of Chemical Industry     

Enhanced Catalytic Activity with Oxygen for Methyl Acrylate Production via Cross‐Aldol Condensation Reaction

An innovative technology for the production of methyl acrylate via vapor‐phase cross‐aldol condensation is described. Catalytic process intensification was realized through the enhanced catalytic activity with oxygen by catalyst design and adjustment of active sites. Crystal phases with different V valences in vanadium phosphate catalysts were controllably synthesized in the presence of oxygen. Donating electron and withdrawing electron sites arose when oxygen moved from one crystalline phase to another, just for the α‐H removal of methyl acetate and formaldehyde protonation. The reaction pathway and transition states were confirmed via molecular simulation with quantum chemistry theory. The catalytic activity was evaluated in a fixed‐bed reactor by adjusting the percentage of oxygen in the carrier gas. Finally, the mechanism of α‐H removal of methyl acetate through the activated oxygen was clarified.     

Full‐physics simulations of spray‐particle interaction in a bubbling fluidized bed

Numerical simulations of a gas‐particle‐droplet system were performed using an Euler‐Lagrange approach. Models accounting for (1) the interaction between droplets and particles, (2) evaporation from the droplet spray, as well as (3) evaporation of liquid from the surface of non‐porous particles were considered. The implemented models were verified for a packed bed, as well as other standard flow configurations. The developed models were then applied for the simulation of flow, as well as heat and mass transfer in a fluidized bed with droplet injection. The relative importance of droplet evaporation vs. evaporation from the particle surface was quantified. It was proved that spray evaporation competes with droplet deposition and evaporation from the particle surface. Moreover, we show that adopting a suitable surface coverage model is vital when attempting to make accurate predictions of the particle's liquid content. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2569–2587, 2017     

Atomistic molecular modelling of crosslinked epoxy resin

In the present study, a new method was developed to construct atomistic molecular models of crosslinked polymers based on commercially important epoxy resin. This method employed molecular dynamics/molecular mechanics schemes and assumed close proximity. The generic Dreiding2.21 force-field and advanced compass force-field were used for the construction of models and prediction of properties, respectively. A polymer network with conversion up to 93.7% was successfully generated by this method. Density and elastic constants of the system were calculated from the equilibrated structure for the validation of the generated models. The simulated results compared reasonably with experimental data available. The developed method would hold great promise in further molecular simulations for structure and properties of epoxy resin or other cured systems.     

Application of Monte‐Carlo simulation to estimate the kinetic parameters for pyrolysis—Part I

A mathematical model was developed to represent pyrolysis. The components of primary and secondary pyrolysis reactions were simply lump into different groups and were represented through a set of pseudo‐first‐order reactions. This study presents an algorithm to estimate the kinetic parameters using Monte‐Carlo (MC) simulation. The combination of an analytical reaction model and the MC simulation technique rapidly generates a large number of numerical values. Results show that MC‐simulated data and experimental data are in fair agreement. Though the technique developed in this study proved to have potential, more experimental data are needed to check the robustness of the model. © 2011 Canadian Society for Chemical Engineering     

Molecular Simulations of the Chain Length Dependent Adsorption of C7-C14 n-Alkanes in ZIF-8

Recent experiments show that the diffusivities of C8 and C10 n-alkanes in ZIF-8 are higher than those of C7 and C9, respectively. We investigated this unusual ‘odd-even' effect by simulating the adsorption of C7-C14 n-alkanes in ZIF-8 using hybrid Monte Carlo molecular simulations. The resultant adsorption isotherms, guest-host energies, isosteric heats, and chain length distributions are analyzed for trends among the n-alkanes. ZIF-8 cages filled with n-alkanes are characterized using a combination of image processing and data science to quantify the differences in packing that occur with chain length. Results indicate that packing changes drastically from C7 to C8 and from C9 to C10, which is consistent with the diffusion trends previously reported in experiments.     

Molecular simulation for adsorption of chlorinated hydrocarbon in zeolites

Equilibrium and isosteric heat of adsorption for the system of chloroform and USY-type zeolite were studied. The USY-type zeolite (PQ Co., SiO₂/Al₂O₃=70) was used both as a pure crystalline powder and as granulated particles with binder. Chloroform was reagent grade. The adsorption equilibria were measured using a gravimetric method and were expressed as isotherms. A chromatographic method (i.e. pulse response of chloroform through the USY column with helium carrier) was used to get the initial slope of the isotherms. In the simulation, the GCMC method was used to calculate amounts adsorbed for various conditions. FF parameters were confidently applied. And modified structure model was effective for simulation. This paper was presented at The 5th International Symposium on Separation Technology-Korea and Japan held at Seoul between August 19 and 21, 1999.     

Cavity size, sorption and transport characteristics of thermally rearranged (TR) polymers

Within a polymer thin film, free volume elements have a wide range of size and topology. This broad range of free volume element sizes determines the ability for a polymer to perform molecular separations. Herein, six permeable thermally rearranged (TR) polymers and their precursors were studied. Using atomistic models, cavity size (free volume) distributions determined by a combination of molecular dynamics and Monte Carlo methods were consistent with experimental observation that TR polymers are more permeable than their precursors. The cavity size distributions determined by simulation were also consistent with free volume distributions determined by positron annihilation lifetime spectroscopy. The diffusion, solubility and permeation of gases in TR polymers and their precursors were also simulated at 308 K, with results that agree qualitatively with experimental data.