Publications

Controlled co-solvent vapor annealing and the importance of quenching conditions in thin-film block copolymer self-assembly

by Stahl, BC; Kramer, EJ; Hawker, CJ; Lynd, NA

Journal of Polymer Science Part B – Polymer Physics; Aug 1 2017; Volume: 55; Issue: 15; Pages: 1125-1130; DOI: 10.1002/polb.24366

A controlled co-solvent vapor annealing system was designed and constructed to investigate the effects of solvent vapor activity during the rapid ambient quenching process on the morphology of a cylinder-forming poly(styrene)-b-poly (ethylene oxide) (PS-PEO) annealed in toluene and water vapor. A phase transformation from cylinders in the bulk to close-packed spheres in swollen thin films occurred, which was reversed upon quenching with dry nitrogen. Quenching with humidified nitrogen preserved the spherical morphology but could significantly alter domain spacing and reduce long-range order in the dried films under some circumstances. Specifically, long-range order in the quenched films was found to decrease as the quenching humidity decreased from the humidity used during annealing, and the best long-range order was obtained when the humidity remained consistent throughout both annealing and quenching.

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Dopant Mediated Assembly of Cu2ZnSnS4 Nanorods into Atomically Coupled 2D Sheets in Solution

by Singh, A; Singh, A; Ong, GK; Jones, MR; Nordlund, D; Bustillo, K; Ciston, J; Alivisatos, AP; Milliron, DJ

Journal of Nano Letters; Jun 2017; Volume: 17; Issue: 6; Pages: 3421-3428; DOI: 10.1021/acs.nanolett.7b00232

Assembly of anisotropic nanocrystals into ordered superstructures is an area of intense research interest due to its relevance to bring nanocrystal properties to macroscopic length scales and to impart additional collective properties owing to the superstructure. Numerous routes have been explored to assemble such nanocrystal superstructures ranging from self-directed to external field-directed methods. Most of the approaches require sensitive control of experimental parameters that are largely environmental and require extra processing steps, increasing complexity and limiting reproducibility. Here, we demonstrate a simple approach to assemble colloidal nanorods in situ, wherein dopant incorporation during the particle synthesis results in the formation of preassembled 2D sheets of close-packed ordered arrays of vertically oriented nanorods in solution.

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Charge carrier concentration dependence of ultrafast plasmonic relaxation in conducting metal oxide nanocrystals

by Johns, RW; Blemker, MA; Azzaro, MS; Heo, S; Runnerstrom, EL; Milliron, DJ; Roberts, ST

Journal of Materials Chemistry C; Jun 21 2017; Volume: 5; Issue: 23; Pages: 5757-5763; DOI: 10.1039/c7tc00600d

Electronically doped metal oxide nanocrystals exhibit tunable infrared localized surface plasmon resonances (LSPRs). Despite the many benefits of IR resonant LSPRs in solution processable nanocrystals, the ways in which the electronic structure of the host semiconductor material impact metal oxide LSPRs are still being investigated. Semiconductors provide an alternative dielectric environment than metallically bonded solids, such as noble metals, which can impact how these materials undergo electronic relaxation following photoexcitation. Understanding these differences is key to developing applications that take advantage of the unique optical and electronic properties offered by plasmonic metal oxide NCs. Here, we use the two-temperature model in conjunction with femtosecond transient absorption experiments to describe how the internal temperature of two representative metal oxide nanocrystal systems, cubic WO3-x and bixbyite Sn-doped In2O3, change following LSPR excitation. We find that the low free carrier concentrations of metal oxide NCs lead to less efficient heat generation as compared to metallic nanocrystals such as Ag. This suggests that metal oxide NCs may be ideal for applications wherein untoward heat generation may disrupt the application’s overall performance, such as solar energy conversion and photonic gating.

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Generating Large Thermally Stable Marangoni-Driven Topography in Polymer Films by Stabilizing the Surface Energy Gradient

by Jones, AR; Kim, CB; Zhou, SX; Ha, H; Katsumata, R; Blachut, G; Bonnecaze, RT; Ellison, CJ

Journal of Macromolecules; Jun 13 2017; Volume: 50; Issue: 11; Pages: 4588-4596; DOI: 10.1021/acs.macromol.7b00055

Marangoni forces drive a fluid to flow in response to positional differences in surface energy. In thin polymer films, a difference in surface energy between two coincident liquid polymers could offer a useful route to manufacture topographically patterned surfaces via the Marangoni effect. Previously, we have demonstrated a photochemical method using the Marangoni effect for patterning thin polystyrene films. To generalize the approach, a theoretical model that gives the underlying physics of this process was also developed, which further revealed that low viscosities, low diffusivities, and large surface energy gradients favor rapid evolution of large film thickness variations. However, as described by the Stokes Einstein equation or the Rouse model, low viscosity is generally correlated with high diffusivity in a single-component system. Herein, we report a strategy to decouple film viscosity and diffusivity by co-casting a high molecular weight surface energy gradient creating copolymer (low diffusivity) with a low molecular weight majority homopolymer (high diffusivity and low viscosity), which are miscible with each other. Patterned light exposure through a photomask imposes a patterned surface energy gradient between light-exposed and unexposed regions due to photochemical reactions involving only the low diffusivity component. Upon heating the film to the liquid state, the film materials (primarily the low viscosity homopolymer component) flow from the low to high surface energy regions. This strategy either eliminates or greatly slows dissipation of the prepatterned surface energy gradient while maintaining rapid feature formation, resulting in formation of ca. 500 nm high features within only 30 min of thermal annealing. Furthermore, the formed features are stable upon extended thermal annealing for up to one month. It is found that a ratio of Marangoni forces to capillary forces can provide a predictive metric that distinguishes which scenarios produce features that dissipate or persist.

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Supported Cobalt Nanorod Catalysts for Carbon Dioxide Hydrogenation

by Jimenez, J; Bird, A; Santiago, MS; Wen, C; Lauterbach, J

Journal of Energy Technology; Jun 2017; Volume: 5; Issue: 6; Pages: 884-891; Special Issue; DOI: 10.1002/ente.201600575 

To create new CO2 hydrogenation catalysts, we developed a methodology in which the selective surface faceting of nanorods is combined with the practicality of a supported catalyst. The basic Co catalyst with preferential {110} surface faceting was supported with either SiO2, Al-2 O-3, or TiO2. We grafted charged ligands onto the Co nanorods to attract the support precursors to the surface of the Co to allow a large contact area between the Co and the support. The large contact area promoted the chemical interaction between the Co and the support, which was verified by using Raman spectroscopy, XRD, and temperature- programmed reduction. TiO2- and Al2O3- supported Co nanorods showed an increase in activity at low and high reaction temperatures, respectively, whereas the relatively inert SiO2- supported nanorods possessed the same catalytic performance as unsupported rods.

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A geometric framework for monitoring and fault detection for periodic processes

by Wang, R; Edgar, TF; Baldea, M

AICHE Journal; Jul 2017; Volume: 63; Issue: 7; Pages: 2719-2730; DOI: 10.1002/aic.15638

Although cyclical operation systems are relatively widespread in practice (notably in the realm of physical separations, for example, pressure-swing adsorption and chromatography), the development of specific fault detection mechanisms has received little attention compared to the extensive efforts dedicated to continuous or batch processes. Here, a novel geometric approach for process fault detection is proposed. Specifically, a time-explicit multivariable representation of data collected from the process, which provides a natural framework for defining normal operation and the corresponding confidence regions is developed. On this basis, a two-step fault detection approach is proposed, based on detecting intercycle variations to locate a faulty cycle, and intracycle changes to determine the exact timing of a fault. The theoretical developments are illustrated with two simulation case studies.

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Central metabolic nodes for diverse biochemical production.

by Cordova, LT; Alper, HS

Journal of Current Opinion in Chemical Biology; Dec 2016; doi: 10.1016/j.cbpa.2016.08.025

Central carbon metabolism is conserved among all organisms for cellular function and energy generation. The connectivity of this metabolic map gives rises to key metabolite nodes. Five of these nodes in particular, pyruvate, citric acid, tyrosine and aspartate, acetyl-CoA, serve as critical starting points for the generation of a broad class of relevant chemical molecules with ranging applications from fuels, pharmaceuticals and polymer precursors. This review highlights recent progress in converting these metabolite nodes into valuable products. In particular, acetyl-CoA, the most well-connected node, serves as the building block for several classes of molecules including fatty acids and terpenes. Systematic metabolic engineering efforts focused on these metabolic building blocks has enabled the production of industrially-relevant, biobased compounds.

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Modeling gas permeability and diffusivity in HAB-6FDA polyimide and its thermally rearranged analogs

by Galizia, M; Stevens, KA; Paul, DR; Freeman, BD

Jounral of Membrane Science; Sep 1 2017; Volume: 537; Pages: 83-92; DOI: 10.1016/j.memsci.2017.05.015

Gas permeability in HAB-6FDA polyimide and its thermally rearranged analogs was described using a thermodynamic model based on the non-equilibrium lattice fluid (NELF) theory. This study is part of an ongoing effort to describe gas sorption and transport behavior of TR polymers theoretically. Hydrogen, nitrogen and methane permeability over a broad range of pressures (up to 32 atm) and temperatures (-10 to 50 degrees C) was calculated with one adjustable parameter at each temperature, i.e., the infinite dilution mobility coefficient. For highly soluble, swelling gases, such as CO2, matrix plasticization was accounted for by a second adjustable parameter, the plasticization factor, which describes the dependence of penetrant mobility on concentration. Model parameters correlate with membrane structure and gas properties. At fixed temperature, the infinite dilution mobility correlates with penetrant critical volume and polymer fractional free volume. For each penetrant, the temperature dependence of infinite dilution mobility is described by the Arrhenius law. Based on the modeling results, unique separation performance of TR polymers is a manifestation of their strong size-sieving ability. Finally, diffusion coefficients and ideal selectivities were predicted with no adjustable parameters.

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Accounting for frame of reference and thermodynamic non-idealities when calculating salt diffusion coefficients in ion exchange membranes

by Kamcev, J; Paul, DR; Manning, GS; Freeman, BD

Journal of Membrane Science; Sep 1 2017; Volume: 537; Pages: 396-406; DOI: 10.1016/j.memsci.2017.05.034

Accurate evaluation of salt diffusion coefficients from transport rate data in ion exchange membranes requires accounting for frame of reference and non-ideal thermodynamic effects. Due to a lack of models and experimental data quantifying membrane ion activity coefficients, it has been impossible to evaluate the impact of non-ideal thermodynamic effects on observed salt diffusion coefficients. Here, a framework is presented that includes both frame of reference (i.e., convection) and non-ideal thermodynamic effects in calculating salt diffusion coefficients in ion exchange membranes. Effective concentration averaged NaCl diffusion coefficients were determined as a function of upstream NaCl concentration in commercial ion exchange membranes from NaCl permeability and sorption measurements via the solution-diffusion model. Frame of reference effects were evaluated using a version of Fick’s law that accounts for convection. The factors necessary to account for non-ideal thermodynamic effects were developed using Manning’s counter-ion condensation theory. At low upstream NaCl concentrations, frame of reference and non-ideal thermodynamic effects on diffusion coefficients were negligible. However, at higher upstream NaCl concentrations (e.g., > 0.1 M), both effects contribute measurably to NaCl diffusion coefficients. Correcting for frame of reference effects increased apparent NaCl diffusion coefficients. However, correcting for thermodynamic non-idealities of the ions sorbed into the membranes reduced apparent NaCl diffusion coefficients. Fortuitously, for the materials considered in this study, frame of reference and non-ideal thermodynamic effects nearly cancel each other.

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Autocovariance-based plant-model mismatch estimation for linear model predictive control

by Wang, S; Simkoff, JM; Baldea, M; Chiang, LH; Castillo, I; Bindlish, R; Stanley, DB

Journal of Systems & Control Letters; Jun 2017; Volume 104; Pages: 5-14;  DOI: 10.1016/j.sysconle.2017.03.002

In this paper, we present autocovariance-based estimation as a novel methodology for determining plant-model mismatch for multiple-input, multiple-output systems operating under model predictive control. Considering discrete-time, linear time invariant systems under reasonable assumptions, we derive explicit expressions of the autocovariances of the system inputs and outputs as functions of the plant-model mismatch. We then formulate the mismatch estimation problem as a global optimization aimed at minimizing the discrepancy between the theoretical autocovariance estimates and the corresponding values computed from historical closed-loop operating data. Practical considerations related to implementing these ideas are discussed, and the results are illustrated with a chemical process case study.

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