Rong-Yao Wang

Characterization of hollow fiber membranes in a permeator using binary gas mixtures

We have studied the CO2=CH4 mixed gas permeation through hollow ber membranes in a permeator. An approach to characterize the true separation performance of hollow ber membranes for binary gas mixtures was provided based on experiments and simulations. Experiments were carried out to measure the retentate and permeate 5ow rates and compositions at each outlet. The in5uences of pressure drop within the hollow bers, non-ideal gas behavior in the mixture and concentration polarization were taken into consideration in the mathematics model. The calculation results indicate that the net in5uence of the non-ideal gas behavior, competitive sorption and plasticization yields the calculated CO2 permeance in a mixed gas permeator close to that obtained in pure gas tests. Whereas the CH4 permeance is higher in the mixed gas tests than that in the pure gas tests, as the plasticization caused by CO2 dominates the permeation process. As a result, the CO2=CH4 mixed gas selectivity is smaller than those obtained in pure gas tests at equivalent pressures. The calculated membrane performance shows little changes with stage cut if the e9ect of concentration polarization is accounted for in the calculation. The integration method developed in this study could provide more accurate characterizations of mixed gas permeance of hollow membranes than other estimation methods, as our model considers the roles of non-ideal gas behavior and concentration polarization properly. ? 2002 Elsevier Science Ltd. All rights reserved.

Chiral assembly of gold nanorods with collective plasmonic circular dichroism response

In this work, a chiral hybrid superstructure has been achieved through a hierarchical, cooperative self-assembly/self-organization process in a multiple-component system comprising of gold nanorods, surfactants, and phospholipids film. This chiral hybrid superstructure presented a signate signal in plasmonic circular dichroism with negative collective plasmonic modes on one side of the plasmon resonance and positive modes on the other side. The origin of such an unusual optical activity is fundamentally different from that of chiral metal nanoparticles/nanoclusters in previous studies. We highlight here a possibility that helical alignment of gold nanorods in the hybrid superstructure would resemble mesogenic molecules in a cholesteric liquid crystalline mesophase, and the Coulomb dipole–dipole interactions between gold nanorods in such a chiral liquid crystalline-like mesophase would give rise to the plasmonic circular dichroism effect. This study also demonstrated a facile approach for fabricating chiral superstructures from preformed metal nanoparticles with defined shapes, sizes, and compositions, which may have values in exploiting novel plasmonic nanostructures with tunable optical activity responses in the visible/NIR region for a variety of bioscience and biomedicine applications.

Nanoengineering of a biocompatible organogel by thermal processing

The formation of most organogels requires the compatibility of both the gelator and solvent. It is very desirable if the rheological properties of a gel can be manipulated to achieve the desired performance. In this paper, a novel organogel was developed and its rheological properties and fiber network were engineered by controlling the thermal processing conditions. The gel was formed by the gelation of 12-hydroxystearic acid as a gelator in benzyl benzoate. It was observed that the degree of supercooling for gel formation has a significant effect on the rheological properties and fiber network structure. By increasing supercooling, the elasticity of the gel was enhanced, and the correlation length of the fibers was shortened, leading to the formation of denser fiber networks. The good biocompatibility of both the gelator and solvent makes this gel a promising vehicle for a variety of bioapplications such as controlled transdermal drug release and in vivo tissue repair.

Effect of diamine composition on the gas transport properties in 6FDA-durene/3,3′-diaminodiphenyl sulfone copolyimides

Abstract The gas permeation behavior of hexafluoro-dianhydride (6FDA)-durene/3,3′-diaminodiphenyl sulfone (DDS) polyimides was investigated by systematically varying the diamine ratios. Generally, the gas permeability, diffusion and solubility coefficients were found to decrease with increasing 3,3′-DDS diamine content. The permeability coefficients of H 2 , O 2 , N 2 and CO 2 decreased with the increasing order of kinetic diameters of the penetrant gases. However, the decrease in diffusion coefficients of O 2 , N 2 and CO 2 was in accordance with the effective diameters of the gas molecules. The permselectivity of gas pairs such as CO 2 /N 2 , O 2 /N 2 and H 2 /N 2 was enhanced with the incorporation of 3,3′-DDS moiety. The gas transport coefficients of the copolyimides predicted from the addition rule were compared with the experimental results. It was seen that gas permeation coefficients for the copolyimides slightly deviated from the predicted values. In contrast, the logarithm of gas permeation coefficients and the reciprocal of fractional free volume (1/FFV) exhibited a better correlation.

The physical and gas permeation properties of 6FDA-durene/ 2,6-diaminotoluene copolyimides

Abstract The physical and gas transport properties of homo-polyimides, 6FDA-durene and 6FDA-2, 6-diaminotoluene (2,6-DAT), and their copolyimides, 6FDA-durene/2,6-DAT with different diamine ratios, were characterized. The glass transition temperatures of the copolyimides obtained from DSC experiments were about 6–10°C lower than that calculated from the Fox equation. The experimental results for the gas permeability, diffusivity and solubility of this series of polyimides fitted well with that predicted from the logarithm of property versus volume fraction. The gas permeability of 6FDA-durene/2,6-DAT decreased with increasing 6FDA-2,6-DAT content. However, the selectivity of gases pairs, such as CO2/N2, O2/N2 and H2/N2, increased with the addition of 6FDA-2,6-DAT. The permeability coefficients of H2, He, O2, N2 and CO2 decreased with the kinetic molecular diameters of the gas molecules, with an exception of helium. The diffusion coefficients for the gases of O2, N2 and CO2 were found to decrease with the effective diameters of the penetrant molecules. The solubility coefficients of the gases increased with the condensability of the gas molecules. By decoupling the permselectivity of gas pairs of CO2/N2, O2/N2, it was found that the permselectivity of CO2/N2 was mainly controlled by the solubility selectivity, while in the gas pair of O2/N2, the permselectivity was dominated by the diffusivity selectivity.

From kinetic–structure analysis to engineering crystalline fiber networks in soft materials

Understanding the role of kinetics in fiber network microstructure formation is of considerable importance in engineering gel materials to achieve their optimized performances/functionalities. In this work, we present a new approach for kinetic-structure analysis for fibrous gel materials. In this method, kinetic data is acquired using a rheology technique and is analyzed in terms of an extended Dickinson model in which the scaling behaviors of dynamic rheological properties in the gelation process are taken into account. It enables us to extract the structural parameter, i.e. the fractal dimension, of a fibrous gel from the dynamic rheological measurement of the gelation process, and to establish the kinetic-structure relationship suitable for both dilute and concentrated gelling systems. In comparison to the fractal analysis method reported in a previous study, our method is advantageous due to its general validity for a wide range of fractal structures of fibrous gels, from a highly compact network of the spherulitic domains to an open fibrous network structure. With such a kinetic-structure analysis, we can gain a quantitative understanding of the role of kinetic control in engineering the microstructure of the fiber network in gel materials.

Fabrication of chiral plasmonic oligomers using cysteine-modified gold nanorods as monomers

Generation of circular dichroism (CD) beyond the UV region is of great interest in developing chiral sensors and chiroptical devices. Herein, we demonstrate a simple and versatile method for fabrication of plasmonic oligomers with strong CD response in the visible and near IR spectral range. The oligomers were fabricated by triggering the side-by-side assembly of cysteine-modified gold nanorods. The modified nanorods themselves did not exhibit obvious plasmonic CD signals; however, the oligomers show strong CD bands around the plasmon resonance wavelength. The sign of the CD band was dictated by the chirality of the absorbed cysteine molecules. By adjusting the size of the oligomers, the concentration of chiral molecules, and/or the aspect ratio of the nanorods, the CD intensity and spectral range were readily tunable. Theoretical calculations suggested that CD of the oligomers originated from a slight twist of adjacent nanorods within the oligomer. Therefore, we propose that the adsorbed chiral molecules are able to manipulate the twist angles between the nanorods and thus modulate the CD response of the oligomers.

Control of crystallization in supramolecular soft materials engineering

As one class of the most important supramolecular functional materials, gels formed by low molecular weight gelators (LMWGs) have many important applications. The key important parameters affecting the in-use performance of a gel are determined by the hierarchical fiber network structures. Fiber networks consisting of weakly interacting multiple domains are commonly observed in gels formed by LMWGs. The rheological properties, particularly the elasticity, of a gel with such a fiber network are weak due to the weak interactions between the individual domains. As achieving desirable rheological properties of such a gel is practically relevant, in this work, we demonstrate the engineering of gels with such a type of fiber network by controlling crystallization of the gelator. Two example gels formed by a glutamic acid derivative in a non-ionic surfactant Tween 80 and in propylene glycol were engineered by controlling the thermodynamic driving force for crystallization. For a fixed gelator concentration, the thermodynamic driving force was manipulated by controlling the temperature for fiber crystallization. It was observed that there exists an optimal temperature at which a gel with maximal elasticity can be fabricated. This will hopefully provide guidelines for producing high performance soft materials by engineering their fiber network structures.

Plasmon-induced strong interaction between chiral molecules and orbital angular momentum of light

Whether or not chiral interaction exists between the optical orbital angular momentum (OAM) and a chiral molecule remains unanswered. So far, such an interaction has not been observed experimentally. Here we present a T-matrix method to study the interaction between optical OAM and the chiral molecule in a cluster of nanoparticles. We find that strong interaction between the chiral molecule and OAM can be induced by the excitation of plasmon resonances. An experimental scheme to observe such an interaction has been proposed. Furthermore, we have found that the signal of the OAM dichroism can be either positive or negative, depending on the spatial positions of nanocomposites in the cross-sections of OAM beams. The cancellation between positive and negative signals in the spatial average can explain why the interaction has not been observed in former experiments.

Strong superchiral field in hot spots and its interaction with chiral molecules

We have found that strong superchiral fields created by surface plasmon resonance exist in hot spots of nonchiral plasmonic structure, which showed a chiral density greater than that of circularly polarized light by hundreds of times. We have demonstrated a direct correlation between the chirality of the local field and the circular dichroism (CD) response at the plasmon resonance bands induced by chiral molecules in the hot spots. Our results reveal that the wavelength-dependent superchiral fields in the hot spots can play a crucial role in the determination of the plasmonic CD effect. This finding is in contrast to the currently accepted physical model in which the electromagnetic field intensity in hot spots is a key factor to determine the peak intensity of the plasmonic CD spectrum. Some related experimental phenomena have been explained by using our theoretical analysis.