David M. Ford

Engineering nanospaces: ordered mesoporous silicas as model substrates for building complex hybrid materials

This contribution summarizes investigations of organic-inorganic hybrid materials wherein the inorganic phase is ordered mesoporous silica such as MCM-41 and SBA-15. The review, which covers work performed in the last three years, emphasizes studies of: (1) covalently attached functional groups, (2) new approaches to functionalization, (3) approaches for achieving high densities of uniform functional groups, (4) periodic mesoporous organosilicas (PMOs) with hierarchical ordering, (5) new functional chemistries, and (6) the application of new materials to enantioselective catalysis and emerging areas. The review concludes with the authors outlining some outstanding problems in the field.

Thermodynamic and mechanical properties of epoxy resin DGEBF crosslinked with DETDA by molecular dynamics.

Fully atomistic molecular dynamics (MD) simulations were used to predict the properties of diglycidyl ether of bisphenol F (DGEBF) crosslinked with curing agent diethyltoluenediamine (DETDA). This polymer is a commercially important epoxy resin and a candidate for applications in nanocomposites. The calculated properties were density and bulk modulus (at near-ambient pressure and temperature) and glass transition temperature (at near-ambient pressure). The molecular topology, degree of curing, and MD force-field were investigated as variables. The models were created by densely packing pre-constructed oligomers of different composition and connectivity into a periodic simulation box. For high degrees of curing (greater than 90%), the density was found to be insensitive to the molecular topology and precise value of degree of curing. Of the two force-fields that were investigated, cff91 and COMPASS, the latter clearly gave more accurate values for the density as compared to experiment. In fact, the density predicted by COMPASS was within 6% of reported experimental values for the highly crosslinked polymer. The predictions of both force-fields for glass transition temperature were within the range of reported experimental values, with the predictions of cff91 being more consistent with a highly cured resin.

Direct molecular simulation of gradient-driven diffusion

Recent work in the active area of grand canonical molecular dynamics methods is first briefly reviewed followed by an overview of the dual control volume grand canonical molecular dynamics (DCV-GCMD) method, designed to enable the dynamic simulation of a system with a steady-state chemical potential gradient. A short review of the methods and systems used to prototype the DCV-GCMD method and its parallel implementation follows. Finally a new, novel implementation of the DCV-GCMD method that enables the establishment of a steady-state chemical potential gradient in a multicomponent system without having to insert or delete one of the components is presented and discussed.

A transition-state theory approach to adsorbate dynamics at arbitrary loadings

There has been much recent interest in using transition-state theory (TST) to extend the time and length scales accessible to molecular-level simulations of adsorbate transport in microsporous materials. However, the vast majority of this work has been performed on systems at infinite dilution. The objective of this paper is to obtain fundamental rate constants for adsorbate motion at nonzero loadings using multidimensional TST. More specifically, we focus on systems where the adsorption of a molecule is not highly localized in a single site, but rather distributed throughout an uncorrugated cage. We develop a theory in which high-dimensional TST integrals are approximated using exact lower-dimensional information. The evaluation of the resulting integrals is performed with an importance sampling method involving the insertion of a single molecule, thus improving the statistical quality of the results. The theory is applied to the motion of methane and xenon in the zeolite ZK4, where hopping between α cag...

Massively parallel dual control volume grand canonical molecular dynamics with LADERA II. Gradient driven diffusion through polymers

This paper, the second part of a series, extends the capabilities of the LADERA FORTRAN code for massively parallel dual control volume grand canonical molecular dynamics (DCVGCMD). DCV-GCMD is a hybrid of two more common molecular simulation techniques (grand canonical Monte Carlo and molecular dynamics) which allows the direct molecularlevel modelling of diffusion under a chemical potential gradient. The present version of the code, LADERA-B has the capability of modelling systems with explicit intramolecular interactions such as bonds, angles, and dihedral rotations. The utility of the new code for studying gradient-driven diffusion of small molecules through polymers is demonstrated by applying it to two model systems. LADERA-B includes another new feature, which is the use of neighbour lists in force calculations. This feature increases the speed of the code but presents several challenges in the parallel hybrid algorithm. There is discussion on how these problems were addressed and how our implement...

Molecular modelling of polymers 17. Simulation and QSPR analyses of transport behavior in amorphous polymeric materials

Quantitative structure-property relationships (QSPRs) were constructed to predict O 2 , N 2 and CO 2 diffusion in amorphous polymers. The major polymer property governing diffusion in these QSPR models is bulk modulus. Oxygen permeation and solubility QSPR models were also constructed. Cohesive energy of the polymer matrix is the polymer property most highly correlated to permeation. Oxygen solubility appears to be about equally dependent on polymer density, bulk modulus or cohesive energy. A limited QSPR exploration of aqueous diffusion in polymer matrices indicates that cohesive energy of the polymeric material governs aqueous diffusion. The QSPR models can be used to efficiently predict transport properties from molecular dynamics simulations since only bulk modulus and/or cohesive energy need to be determined from a simulation in order to estimate a transport property using the appropriate QSPR model. An original computational technique to generate close-to-equilibrium dense polymeric structures is proposed. Diffusion of small gases are studied on the equilibrated structures using massively parallel molecular dynamics simulations running on the Intel Teraflops (9200 Pentium Pro processors) and Intel Paragon (1840 processors). Compared to the current state-of-the-art equilibration methods, the new technique appears to be faster by some orders of magnitude. The main advantage of the technique is that one can circumvent the bottlenecks in configuration space that inhibit relaxation in molecular dynamics simulations. The technique is based on the fact that tetravalent atoms (such as carbon and silicon) fit in the centre of a regular tetrahedron and that regular tetrahedrons can be used to mesh three-dimensional space. Thus, the problem of polymer equilibration described by continuous equations in molecular dynamics is reduced to a discrete representation where solutions are approximated by simple algorithms.

A hierarchical approach to the molecular modeling of diffusion and adsorption at nonzero loading in microporous materials

Abstract A new hierarchical approach is presented for the modeling of small molecules at nonzero concentrations in microporous materials. This approach is complementary to other methods recently appearing in the literature; it is targeted for systems with pores that are well defined, large enough to host multiple molecules, and energetically uncorrugated in the interior. Statistical mechanical partition functions are calculated on molecular-level models and used as input to coarse-grained models, to predict both adsorption isotherms and self-diffusion coefficients. Certain physically reasonable simplifying approximations are employed to make the partition functions tractable. The approach is demonstrated on the model system of methane in siliceous zeolite ZK4 at 300 K , and the results are judged in comparison to those from traditional grand canonical Monte Carlo and molecular dynamics simulations. The adsorption isotherm is predicted to a high degree of accuracy across a large pressure range. The predicted trends in the self-diffusion coefficient are in qualitative agreement with the molecular dynamics results, but there is some quantitative disagreement at the lowest and highest adsorbate loadings.

Permeation of small molecules through polymers confined in mesoporous media

Abstract The use of hybrid organic–inorganic materials in gas separation is a topic of great interest. One recent example is the observed increase in separation performance of polymers upon confinement in a mesoporous medium [M. Moaddeb, W.J. Koros, Journal of Membrane Science 125 (1997) 143–163]. In this paper, we use computer simulation to probe molecular level phenomena and mechanisms in such systems. Molecular dynamics simulations were employed to study the permeation of small penetrants (helium and neon) in model polymers confined between solid surfaces. A planar graphite mesopore model was used for the solid surfaces, and the polymer was modeled as polymethylene chains. The diffusion coefficients and Henry’s Law solubilities of the penetrants were calculated in a variety of membrane models with varying polymer loading between the solid surfaces. The state of the polymer was varied between rubbery and glassy by changing the temperature. Changes in the microstructure and dynamics of the polymer were observed upon confinement, similar to those seen in previous literature studies. Dramatic changes in permeability and selectivity of the polymer material were also observed. The selectivity for helium over neon changed by as much as a factor of two, while the penetrant permeability changed by as much as two orders of magnitude. The polymer loading in the pore was found to be a key variable; very small changes in loading could produce order of magnitude changes in permeability. The implications of our results on the rational design of hybrid inorganic–organic separation media are discussed.

Solubility-based gas separation with oligomer-modified inorganic membranes: Part II. Mixed gas permeation of 5 nm alumina membranes modified with octadecyltrichlorosilane

Following the ideas presented in a previous publication [J. Membr. Sci. 187 (2001) 141], hybrid organic/inorganic membranes were prepared by the surface-derivatization of commercially available mesoporous alumina membranes with octadecyltrichlorosilane (OTS). Ideal and mixed gas selectivities for propane/nitrogen and butane/methane systems were obtained at different inlet feed pressures and compositions. The membranes were always selective for the heavier species over the lighter, which is indicative of a solubility-based separation. The mixed gas selectivities were generally greater than or equal to the corresponding ideal values. Interestingly, the dependence of mixed gas selectivity on feed pressure was qualitatively different for the two different gas pairs. Apparent competitive and cooperative effects between the permeating gas species were observed under different conditions. Overall, the permeation characteristics of the hybrids were more consistent with a dense polymeric material rather than a porous one.

A Smoluchowski model of crystallization dynamics of small colloidal clusters

We investigate the dynamics of colloidal crystallization in a 32-particle system at a fixed value of interparticle depletion attraction that produces coexisting fluid and solid phases. Free energy landscapes (FELs) and diffusivity landscapes (DLs) are obtained as coefficients of 1D Smoluchowski equations using as order parameters either the radius of gyration or the average crystallinity. FELs and DLs are estimated by fitting the Smoluchowski equations to Brownian dynamics (BD) simulations using either linear fits to locally initiated trajectories or global fits to unbiased trajectories using Bayesian inference. The resulting FELs are compared to Monte Carlo Umbrella Sampling results. The accuracy of the FELs and DLs for modeling colloidal crystallization dynamics is evaluated by comparing mean first-passage times from BD simulations with analytical predictions using the FEL and DL models. While the 1D models accurately capture dynamics near the free energy minimum fluid and crystal configurations, predictions near the transition region are not quantitatively accurate. A preliminary investigation of ensemble averaged 2D order parameter trajectories suggests that 2D models are required to capture crystallization dynamics in the transition region.