Vapor Deposition of Silicon-Containing Microstructured Polymer Films onto Silicone Oil Substrates.

In this study, a silicon-containing cross-linked polymer, poly(1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane-co-ethylene glycol diacrylate) (p(V4D4-co-EGDA)), was deposited onto high-viscosity silicone oil using initiated chemical vapor deposition (iCVD). The ratio of the feed flow rate of V4D4 to EGDA was systematically studied, and the chemical composition and morphology of the top and bottom surfaces of the films were analyzed. The films were microstructured, and the porosity and thickness of the films increased with increasing V4D4 content. The top of the film was composed of densely packed and loosely packed microstructured regions. X-ray photoelectron spectroscopy on the top and bottom surfaces of the films showed a heterogeneous chemical composition along the thickness of the film, with higher silicon content on the top surface compared to that on the bottom surface. To the best of our knowledge, this is the first study of iCVD deposition of a silicon-containing polymer film onto silicone oil. The results of this study can be used for the synthesis of polymer precursor films for the fabrication, via pyrolysis, of silicon-based inorganic membranes for use in hydrogen production using silicone oil to prevent infiltration of monomer into the underneath membrane support structure during vapor deposition.

First principles-based multiparadigm, multiscale strategy for simulating complex materials processes with applications to amorphous SiC films.

Progress has recently been made in developing reactive force fields to describe chemical reactions in systems too large for quantum mechanical (QM) methods. In particular, ReaxFF, a force field with parameters that are obtained solely from fitting QM reaction data, has been used to predict structures and properties of many materials. Important applications require, however, determination of the final structures produced by such complex processes as chemical vapor deposition, atomic layer deposition, and formation of ceramic films by pyrolysis of polymers. This requires the force field to properly describe the formation of other products of the process, in addition to yielding the final structure of the material. We describe a strategy for accomplishing this and present an example of its use for forming amorphous SiC films that have a wide variety of applications. Extensive reactive molecular dynamics (MD) simulations have been carried out to simulate the pyrolysis of hydridopolycarbosilane. The reaction products all agree with the experimental data. After removing the reaction products, the system is cooled down to room temperature at which it produces amorphous SiC film, for which the computed radial distribution function, x-ray diffraction pattern, and the equation of state describing the three main SiC polytypes agree with the data and with the QM calculations. Extensive MD simulations have also been carried out to compute other structural properties, as well the effective diffusivities of light gases in the amorphous SiC film.

Field-Scale Testing of a High-Efficiency Membrane Reactor (MR)-Adsorptive Reactor (AR) Process for H2 Generation and Pre-Combustion CO2 Capture.

The study objective was to field-validate the technical feasibility of a membrane- and adsorption-enhanced water gas shift reaction process employing a carbon molecular sieve membrane (CMSM)-based membrane reactor (MR) followed by an adsorptive reactor (AR) for pre-combustion CO2 capture. The project was carried out in two different phases. In Phase I, the field-scale experimental MR-AR system was designed and constructed, the membranes, and adsorbents were prepared, and the unit was tested with simulated syngas to validate functionality. In Phase II, the unit was installed at the test site, field-tested using real syngas, and a technoeconomic analysis (TEA) of the technology was completed. All project milestones were met. Specifically, (i) high-performance CMSMs were prepared meeting the target H2 permeance (>1 m3/(m2.hbar) and H2/CO selectivity of >80 at temperatures of up to 300 °C and pressures of up to 25 bar with a <10% performance decline over the testing period; (ii) pelletized adsorbents were prepared for use in relevant conditions (250 °C < T < 450 °C, pressures up to 25 bar) with a working capacity of >2.5 wt.% and an attrition rate of <0.2; (iii) TEA showed that the MR-AR technology met the CO2 capture goals of 95% CO2 purity at a cost of electricity (COE) 30% less than baseline approaches.

Water harvesting using a conducting polymer: a study by molecular dynamics simulation.

The results of extensive molecular simulations of adsorption and diffusion of water vapor in polyaniline, made conducting by doping it with HCl or HBr over a broad range of temperatures, are reported. The atomistic model of the polymers was generated using energy minimization, equilibrium molecular dynamics simulations, and two different force fields. The computed sorption isotherms are in excellent agreement with the experimental data. The computed activation energies for the diffusion of water molecules in the polymers also compare well with what has been reported in the literature. The results demonstrate the potential of conducting polyaniline for water harvesting from air.

Image-based modeling of gas adsorption and deformation in porous media.

Understanding adsorption of CO2 in porous formations is crucial to its sequestration in geological formations. We describe a model for adsorption of CO2 and the deformation that it induces in a sandstone formation over wide ranges of temperature and pressure. The model couples the thermodynamics of sorption with elastic deformation of the solid. Finite-element computations are then used in order to compute CO2 adsorption isotherms along with the induced strain in the formation. We also compute the Darcy permeability of the porous medium using the lattice-Boltzmann method. All the computations are carried out with a three-dimensional image of a core sample from Mt. Simon sandstone, the target porous formation for a pilot CO2 sequestration project that is currently being carried out by Illinois State Geological Survey. Thus, no assumptions are made regarding the shape and sizes of the pore throats and pore bodies. The computed CO2 sorption isotherm at 195 K is in excellent agreement with our experimental data. The computed permeability is also in good agreement with the measurement. As a further test we also compute the sorption isotherm of N2 in the same formation at 77.3 K, and show that it is also in good agreement with our experimental data. The model is capable of predicting adsorption of CO2 (or any other gas for that matter) in porous formations at high pressures and temperatures. Thus, it is used to study the effect of hydrostatic pressure on adsorption and deformation of the porous formation under various conditions. We find that the effect of the confining pressure is more prominent at higher temperatures. Also computed is the depth-dependence of the capacity of the formation for CO2 adsorption, along with the induced volumetric strain.

Molecular dynamics simulation of diffusion of gases in a carbon-nanotube-polymer composite.

Extensive molecular dynamics (MD) simulations were carried out to compute the solubilities and self-diffusivities of CO2 and CH4 in amorphous polyetherimide (PEI) and mixed-matrix PEI generated by inserting single-walled carbon nanotubes into the polymer. Atomistic models of PEI and its composites were generated using energy minimizations, MD simulations, and the polymer-consistent force field. Two types of polymer composite were generated by inserting (7,0) and (12,0) zigzag carbon nanotubes into the PEI structure. The morphologies of PEI and its composites were characterized by their densities, radial distribution functions, and the accessible free volumes, which were computed with probe molecules of different sizes. The distributions of the cavity volumes were computed using the Voronoi tessellation method. The computed self-diffusivities of the gases in the polymer composites are much larger than those in pure PEI. We find, however, that the increase is not due to diffusion of the gases through the nanotubes which have smooth energy surfaces and, therefore, provide fast transport paths. Instead, the MD simulations indicate a squeezing effect of the nanotubes on the polymer matrix that changes the composite polymers' free-volume distributions and makes them more sharply peaked. The presence of nanotubes also creates several cavities with large volumes that give rise to larger diffusivities in the polymer composites. This effect is due to the repulsive interactions between the polymer and the nanotubes. The solubilities of the gases in the polymer composites are also larger than those in pure PEI, hence indicating larger gas permeabilities for mixed-matrix PEI than PEI itself.

Supercritical fluids in porous composite materials: direction-dependent flow properties.

The results of extensive nonequilibrium molecular dynamics simulations of flow and transport of a pure fluid, as well as a binary fluid mixture, through a porous material composed of a macropore, a mesopore, and a nanopore, in the presence of an external pressure gradient, are reported. We find that under supercritical conditions, unusual phenomena occur that give rise to direction-dependent and pressure-dependent permeabilities for the fluids' components. The results, which are also in agreement with a continuum formulation of the problem, indicate that the composite nature of the material, coupled with condensation, give rise to the direction-dependent permeabilities. Therefore, modeling flow and transport of fluids, in the supercritical regime, in porous materials with the type of morphology considered in this paper (such as supported porous membranes) would require using effective permeabilities that depend on both the external pressure drop and the direction along which it is applied to the materials.

Molecular dynamics simulation of diffusion and sorption of water in conducting polyaniline.

Energy minimization and molecular dynamics simulations are used to develop, for the first time, atomistic models of HCl- and HBr-doped conducting polyanilines, in order to study diffusion and adsorption of water vapor in the polymers. Various morphological properties of the polymers are computed, including their pair correlation functions that are found to be in good agreement with the experimental data, and their accessible free volumes. Also computed are the sorption isotherms and effective self-diffusivity of water vapor in the polymers. The computed sorption isotherms are in quantitative agreement with the experimental data, while the diffusivities are within an order of magnitude of the data. The reasons for the differences between the computed and measured diffusivities are discussed.

Atomistic simulation of nanoporous layered double hydroxide materials and their properties. II. Adsorption and diffusion.

Nanoporous layered double hydroxide (LDH) materials have wide applications, ranging from being good adsorbents for gases (particularly CO(2)) and liquid ions to membranes and catalysts. They also have applications in medicine, environmental remediation, and electrochemistry. Their general chemical composition is [M(1-x)(II)M(x)(III)(OH(-))(2)](x+)[X(nm)(m-)nH(2)O], where M represents a metallic cation (of valence II or III), and X(nm)(m-) is an m-valence inorganic, or heteropolyacid, or organic anion. We study diffusion and adsorption of CO(2) in a particular LDH with M(II)=Mg, M(III)=Al, and x approximately = 0.71, using an atomistic model developed based on energy minimization and molecular dynamics simulations, together with a modified form of the consistent-valence force field. The adsorption isotherms and self-diffusivity of CO(2) in the material are computed over a range of temperature, using molecular simulations. The computed diffusivities are within one order of magnitude of the measured ones at lower temperatures, while agreeing well with the data at high temperatures. The measured and computed adsorption isotherms agree at low loadings, but differ by about 25% at high loadings. Possible reasons for the differences between the computed properties and the experimental data are discussed, and a model for improving the accuracy of the computed properties is suggested. Also studied are the material's hydration and swelling properties. As water molecules are added to the pore space, the LDH material swells to some extent, with the hydration energy exhibiting interesting variations with the number of the water molecules added. The implications of the results are discussed.

Atomistic simulation of nanoporous layered double hydroxide materials and their properties. I. Structural modeling.

An atomistic model of layered double hydroxides, an important class of nanoporous materials, is presented. These materials have wide applications, ranging from adsorbents for gases and liquid ions to nanoporous membranes and catalysts. They consist of two types of metallic cations that are accommodated by a close-packed configuration of OH- and other anions in a positively charged brucitelike layer. Water and various anions are distributed in the interlayer space for charge compensation. A modified form of the consistent-valence force field, together with energy minimization and molecular dynamics simulations, is utilized for developing an atomistic model of the materials. To test the accuracy of the model, we compare the vibrational frequencies, x-ray diffraction patterns, and the basal spacing of the material, computed using the atomistic model, with our experimental data over a wide range of temperature. Good agreement is found between the computed and measured quantities.

Molecular dynamics simulations of transport and separation of carbon dioxide-alkane mixtures in carbon nanopores.

The configurational-bias Monte Carlo method, which is used for efficient generation of molecular models of n-alkane chains, is combined for the first time with the dual control-volume grand-canonical molecular-dynamics simulation, which has been developed for studying transport of molecules in pores under an external potential gradient, to investigate transport and separation of binary mixtures of n-alkanes, as well as mixtures of CO2 and n-alkanes, in carbon nanopores. The effect of various factors, such as the temperature of the system, the composition of the mixture, and the pore size, on the separation of the mixtures is investigated. We also report the preliminary results of an experimental study of transport and separation of some of the same mixtures in a carbon molecular-sieve membrane with comparable pore sizes. The results indicate that, for the mixtures considered in this paper, even in very small carbon nanopores the energetic effects still play a dominant role in the transport and separation properties of the mixtures, whereas in a real membrane they are dominated by the membrane's morphological characteristics. As a result, for the mixtures considered, a single pore may be a grossly inadequate model of a real membrane, and hence one must resort to three-dimensional molecular pore network models of the membrane.