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Cylindrical Pores (cylindrical + pore)

Distribution by Scientific Domains

Polymers and Materials Science	36%
Physics and Astronomy	27%
Chemistry	18%

Selected Abstracts

Temperature dependence of positronium lifetime in cylindrical pores

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007
R. Zaleski
Abstract Positron Annihilation Lifetime Spectroscopy (PALS) results for ordered mesoporous silica sieves (MCM-41 and its fibre form MSF) are used to verify predictions of the Extended Tao-Eldrup (ETE) model. Cylindrical pores (R , 1,2 nm) in these materials have well defined geometry and size, thus the structure of MCM-41 and MSF reproduces the model assumptions. Positron lifetimes were measured in the range 100-500K. In MCM-41, due to low intensity of pore related ortho-positronium component, it is difficult to determine the component's lifetime precisely. Accuracy of the lifetime fitting is better for MSF, where temperature dependence is in a good agreement with the ETE model results. For comparison, the ETE model predictions fit the lifetime of the pore related component in the Vycor sample only for T > 200 K. Below this value the lifetime exceeds the model predictions by over 30%. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Structure of Polymer Brushes in Cylindrical Tubes: A Molecular Dynamics Simulation

MACROMOLECULAR THEORY AND SIMULATIONS, Issue 7 2006
Dimitar I. Dimitrov
Abstract Summary: Molecular dynamics simulations of a coarse-grained bead-spring model of flexible macromolecules tethered with one end to the surface of a cylindrical pore are presented. Chain length N and grafting density , are varied over a wide range and the crossover from "mushroom" to "brush" behavior is studied for three pore diameters. The monomer density profile and the distribution of the free chain ends are computed and compared to the corresponding model of polymer brushes at flat substrates. It is found that there exists a regime of N and , for large enough pore diameter where the brush height in the pore exceeds the brush height on the flat substrate, while for large enough N and , (and small enough pore diameters) the opposite behavior occurs, i.e. the brush is compressed by confinement. These findings are used to discuss the corresponding theories on polymer brushes at concave substrates. Snapshot picture of a brush grafted inside of a cylinder, for , , displaying different chains in distinct colors in order to be able to distinguish them. Top shows a side view of the cylinder, and the lower part a view of the cross-section. Note that the particles forming the cylindrical wall are not displayed. [source]

Computer simulations of protein translocation

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2006
Serdal Kirmizialtin
Many biological processes involve translocation of protein molecules across pores. Shown in the cover picture of this issue is a snapshot from a computer simulation of the protein ubiquitin that is forced mechanically to enter a narrow cylindrical pore. As the force f applied at one end of the molecule pulls it along the pore, the confinement within the pore causes the protein to unfold. The ensuing changes in the protein's entropy and energy lead to a free energy barrier, which has to be surmounted for the translocation to occur. The magnitude of this barrier is estimated by Kirmizialtin et al. in the article [1]. The plot was generated by using the PyMol software. This paper is a presentation from the 1st Institute for the Theory of Advanced Materials in Information Technology Workshop on Computational Materials and Electronics held 20,22 October 2005 in Austin, USA. The present issue has been guest-edited by James Chelikowsky and Alex Demkov. The background of the cover shows a photograph of the tower of the University of Texas at Austin. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Photoswitchable Gas Permeation Membranes Based on Liquid Crystals

ADVANCED FUNCTIONAL MATERIALS, Issue 17 2010
Eric G, owacki
Abstract We have fabricated switchable gas permeation membranes in which a photoswitchable low-molecular-weight liquid crystalline (LC) material acts as the active element. Liquid crystal mixtures are doped with mesogenic azo dyes and infused into commercially available track-etched membranes with regular cylindrical pores (0.40 to 10.0 ,m). Tunability of mass transfer can be achieved through a combination of (1) LC/mesogenic dye composition, (2) surface-induced alignment, and (3) reversible photoinduced LC-isotropic transitions. Photo-induced isothermal phase changes in the imbibed material afford large and fully reversible changes in the permeability of the membrane to nitrogen. Both the LC and photogenerated isotropic states demonstrate a linear permeability/pressure relationship, but they show significant differences in their permeability coefficients. Liquid crystal compositions can be chosen such that the LC phase is more permeable than the isotropic,or vice versa , and can be further tuned by surface alignment. Permeability switching response times are 5 s, with alternating UV and >420-nm radiation at an intensity of 2 mW/cm2 being sufficient for complete and reversible switching. Thermal and kinetic properties of the confined LC materials are evaluated and correlated with the observed permeation properties. We demonstrate for the first time reversible permeation control of a membrane with light irradiation. [source]

Bifurcated Mechanical Behavior of Deformed Periodic Porous Solids

ADVANCED FUNCTIONAL MATERIALS, Issue 9 2009
Srikanth Singamaneni
Abstract The transformation of periodic microporous structures fabricated by interference lithography followed by their freezing below glass transition is described. Periodic porous microstructures subjected to internal compressive stresses can undergo sudden structural transformation at a critical strain. The pattern transformation of collapsed pores is caused by the stresses originated during the polymerization of acrylic acid (rubbery component) inside of cylindrical pores and the subsequent solvent evaporation in the organized microporous structure. By confining the polymerization of acrylic acid to localized porous areas complex microscopic periodic structures can be obtained. The control over the mechanical instabilities in periodic porous solids at a sub-micron scale demonstrated here suggests the potential mechanical tunability of photonic, transport, adhesive, and phononic properties of such periodic porous solids. [source]

Self-Assembled PEO-Peptide Nanotapes as Ink for Plotting Nonwoven Silica Nanocomposites and Mesoporous Silica Fiber Networks

MACROMOLECULAR RAPID COMMUNICATIONS, Issue 4 2008
Stefanie Kessel
Abstract Macroscopic networks of oriented polymer-silica composite fibers can be accessed via a convenient 2D-plotting process. By using self-assembled PEO-peptide nanotapes as an ink to draw the composite fibers, the macroscopic form of the fiber networks, the line width, and both network orientation as well as network anisotropy can be defined. The plotting process relies on a biomimetic silicification route, which combines self-assembly and peptide-directed silicification in a cooperative manner. The local injection of PEO-peptide nanotapes into a thin layer of a dilute solution of pre-hydrolyzed TMOS leads to the rapid formation of the composite fibers, which exhibit several levels of hierarchical order. It was shown, that the rate of plotting is a parameter, enabling one to control the line width and the orientation of the nano- and sub-micrometer structure elements in the network. Moreover, the plotted composite fibers can be used as precursors for networks of oriented, mesoporous silica-fibers. After calcination procedures, nonwoven silica fabrics can be obtained with high surface areas and cylindrical pores aligned in plot direction. [source]

Simple models for evaluating effects of small leaks on the gas barrier properties of food packages

PACKAGING TECHNOLOGY AND SCIENCE, Issue 2 2003
Donghwan Chung
Abstract A detailed theoretical analysis and calculations were made for providing a simple and explicit means to evaluate the effects of small leaks on the barrier properties of food packages. Small leaks, such as pinholes and channel leaks, were approximated as cylindrical pores with diameters of 50,300,,m. The first part of the current study proposes a simple mathematical model based on Fick's law of diffusion, which accounts for both the gas leakage across small leaks and the gas permeation across package walls. The model uses an effective permeability that depends on leak dimensions, type of diffusing gas, type of packaging material and gas conditions around the leak ends. In the second part of the study, three practical cases are presented to illustrate the application of the proposed model in examining the significance of leaks. These demonstrate in a simple and explicit manner that for LDPE packages: (a) leaks affect the oxygen transfer more than the water vapour transfer; (b) leak effects are more significant at lower storage temperatures; and (c) that for high gas barrier packages, the effect of leaks is very important and should not be neglected. The model can be also used to arrive at conclusions about the significance of leaks in other packaging situations (e.g. other than LDPE packaging materials) and to correct the shelf-life estimation of gas- and water vapour-sensitive foods for errors from package leaks. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Temperature dependence of positronium lifetime in cylindrical pores

Enhanced Chondrogenic Responses of Human Articular Chondrocytes Onto Silk Fibroin/Wool Keratose Scaffolds Treated With Microwave-Induced Argon Plasma

ARTIFICIAL ORGANS, Issue 5 2010
Young Woo Cheon
Abstract Silk fibroin (SF) is a natural, degradable, fibrous protein that is biocompatible, is easily processed, and possesses unique mechanical properties. Another natural material, wool keratose (WK), is a soluble derivative of wool keratin, containing amino acid sequences that induce cell adhesion. Here, we blended SF and WK to improve the poor electrospinability of WK and increase the adhesiveness of SF. We hypothesized that microwave-induced argon plasma treatment would improve chondrogenic cell growth and cartilage-specific extracellular matrix formation on a three-dimensional SF/WK scaffold. After argon plasma treatment, static water contact angle measurement revealed increased hydrophilicity of the SF/WK scaffold, and scanning electron microscopy showed that treated SF/WK scaffolds had deeper and more cylindrical pores than nontreated scaffolds. Attachment and proliferation of neonatal human knee articular chondrocytes on treated SF/WK scaffolds increased significantly, followed by increased glycosaminoglycan synthesis. Our results suggest that microwave-induced, plasma-treated SF/WK scaffolds have potential in cartilage tissue engineering. [source]

Consideration of the Effect of Irregular Catalytic Active Component Distributions in Mesopores , Extension of a Model for Wall Catalyzed Reactions in Microchannel Reactors

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 7 2003
B. Platzer
Abstract Data available from the literature and experimental results have shown that the distribution of the catalytic active components can be irregular already for fresh catalysts. The determination of the local concentrations of the catalytic active components using wavelength dispersive X-ray spectroscopy confirms this for microstructured wafers used in microchannel reactors. Considering this nonuniform distribution, the used model gives the relation between the local concentration profiles of the reactants inside the pores and the product yield in the entire pore. These results were used in an equation for the diffusion flux at the pore mouth, which is useful for a microchannel model developed in a recent paper [1]. The theoretical considerations deal with cylindrical pores with known reactant concentrations at the pore mouth and known distribution of the catalytic active component within the pore. Beside numerical results, some analytical solutions with low mathematical expense, applicable to special cases, are discussed. The nonconsideration of the irregular distribution of the catalytic active component can be the reason for difficulties during the extrapolation of experimental results to slightly different conditions and can have a great influence on the reaction results. The regarded examples are typical of wall-catalyzed reactions in microchannel reactors with mesopores. [source]

Theory of Ion Transport in Electrochemically Switchable Nanoporous Metallized Membranes

CHEMPHYSCHEM, Issue 1 2009
Christian Amatore Dr.
Abstract A physicomathematical model of ion transport through a synthetic electrochemically switchable membrane with nanometric metal-plated pores is presented. Due to the extremely small size of the cylindrical pores, electrical double layers formed inside overlap, and thus, strong electrostatic fields whose intensities vary across the cross-sections of the nanopores are created. Based on the proposed model a relationship between the relative electrostatic energies experienced by ions in the nanopores and the potential applied to the membrane is established. This allows the prediction of transference numbers and explains quantitatively the ion-transport switching capability of such synthetic membranes. The predictions of this model agree satisfactorily with previous experimental data obtained for this type of devices by Martin and co-workers. [source]