Shingjiang Jessie Lue

Effect of UV-Ozone Treatment on Poly(dimethylsiloxane) Membranes: Surface Characterization and Gas Separation Performance

A thin SiO(x) selective surface layer was formed on a series of cross-linked poly(dimethylsiloxane) (PDMS) membranes by exposure to ultraviolet light at room temperature in the presence of ozone. The conversion of the cross-linked polysiloxane to SiO(x) was monitored by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX) microanalysis, contact angle analysis, and atomic force microscopy (AFM). The conversion of the cross-linked polysiloxane to SiO(x) increased with UV-ozone exposure time and cross-linking agent content, and the surface possesses highest conversion. The formation of a SiO(x) layer increased surface roughness, but it decreased water contact angle. Gas permeation measurements on the UV-ozone exposure PDMS membranes documented interesting gas separation properties: the O(2) permeability of the cross-linked PDMS membrane before UV-ozone exposure was 777 barrer, and the O(2)/N(2) selectivity was 1.9; after UV-ozone exposure, the permeability decreased to 127 barrer while the selectivity increased to 5.4. The free volume depth profile of the SiO(x) layer was investigated by novel slow positron beam. The results show that free volume size increased with the depth, yet the degree of siloxane conversion to SiO(x) does not affect the amount of free volume.

Crystallinity and Stability of Poly(vinyl alcohol)‐Fumed Silica Mixed Matrix Membranes

Mixed matrix membranes (MMMs) containing poly(vinyl alcohol) (PVA) and 10–30% fumed silica (FS) were prepared using a solvent casting method. The FS particles were distributed evenly in the MMMs as revealed in the scanning electron microscopy/energy dispersive X‐ray spectroscopy (SEM/EDX) micrographs. The surface roughness increased with higher FS content. The thermogravimetric analysis (TGA) results showed that the MMM decomposition temperature was slightly improved with increasing FS content. The MMM crystallinity was determined using Fourier‐transform infrared spectroscopy (FT‐IR), X‐ray diffraction (XRD), and differential scanning calorimetry (DSC). All results indicated that the crystallinity was decreased with FS content. This was attributed to the fact that the FS particles restricted the polymer chain mobility during membrane formation. The confined chain mobility also slowed or inhibited crystal unfolding and, therefore, suppressed the membrane dissolution in water. The lower crystallinity drop of the swollen membranes with FS addition confirmed this finding. The incorporation of FS improved the thermal stability and resistance to water dissolution. These MMMs exhibit potential for use in alkaline direct methanol fuel cell applications.

Sorption, diffusion, and pervaporation of benzene/cyclohexane mixtures on silver-Nafion membranes☆

Abstract Pervaporation (PV) of benzene (Bz) and cyclohexane (Cx) was performed using Nafion-117 membranes containing silver and sodium ions. The effects of membrane conditioning methods and operating conditions on the permeability and selectivity of PV processing were studied. Membranes treated with NaOH, ion-exchanged with AgNO3, and swollen in glycerol yielded higher Bz fluxes and better separations. Increasing operating temperatures enhanced the Bz and Cx fluxes, and the activation energy was 50.1 kJ/mol for Bz and 55.1 kJ/mol for Cx. The solution-diffusion model was used to elucidate the mass-transfer mechanism. The sorption isotherms of the liquid mixtures in the membranes were determined. The diffusivity coefficients were calculated and compared using PV results and sorption kinetics data. Results indicate that PV selectivity is governed by the sorption and/or the diffusion phenomena depending diffusivity on the membrane—permeant characteristics. Although the pure Bz uptake by the Ag+ membranes was only 1.5 times that of the pure Cx uptake, the sorption selectivity in the binary solutions was increased to 15–19. This indicates that the extent of solvent sorption is greatly influenced by competition between Bz and Cx.

Plasma modification on a Nafion membrane for direct methanol fuel cell applications

This research focuses on Nafion modification using plasma techniques for direct methanol fuel cell applications. The results indicated the both argon (Ar) and carbon tetrafluoride (CF4) plasma treatments modified the Nafion surface substantially without altering the bulk properties. The Nafion surface exposed to CF4 plasma resulted in a more hydrophobic layer and an even lower MeOH permeability than the Ar-treated membrane. The plasma operating conditions using CF4 were optimized by utilizing an experimental design. The minimum MeOH permeability was reduced by 74%. The conductivity was 1–2×10-3 S/cm throughout the entire experimental range. Suppressed MeOH permeability can be achieved while maintaining the proton conductivity at a satisfactory level by adjusting the plasma operating conditions.

Tuning of Lower Critical Solution Temperature (LCST) of Poly(N-Isopropylacrylamide-co-Acrylic acid) Hydrogels

Temperature-responsive hydrogel with a lower critical solution temperature (LCST) close to human body temperature was prepared. Crosslinked N-isopropylacrylamide (NIPAAm) and acrylic acid (AAc) copolymer networks were synthesized at various monomer ratios in the presence of ammonium persulfate (APS), N,N′-methylenebisacrylamide (NMBA) and N,N,N′,N′-tetramethylethylenediamine (TEMED) via a redox polymerization method. The resulting hydrogels possessed thermo- and pH-responsive characteristics. They were characterized in terms of swelling ratio, volume change, water uptake and diffusivity, water vapor uptake and diffusivity, and phase transition temperature. The water liquid and vapor diffusion coefficients for all the synthesized hydrogels were higher than the literature data, implying higher rates for drug release. The LCST of the hydrogel increased with higher AAc content in the copolymer. The gel containing 1.8% AAc exhibited an LCST similar to human body temperature, demonstrating a potential use in drug controlled release and biomedical applications.

Permeation of xylene isomers through supported liquid membranes containing cyclodextrins

The permeation of p- and m-xylenes through supported liquid membranes (SLM) containing cyclodextrins (CDs) was investigated in this study. Three CDs were used in the experiments, α-CD, β-CD, and hydroxypropyl-β-CD (HP-β-CD). The mass transfer mechanisms with and without facilitating carriers (CDs) were studied individually by applying the solution–diffusion model. The facilitated mass transfer mechanism was elucidated as partitioning of xylenes from the organic phase into an aqueous phase, formation of an inclusion complex with CDs, diffusion of the complex, and extraction into the receiving phase. The formation constants in aqueous solutions for CDs and p-xylene were 1.6–2.4 times higher than that of CDs and m-xylene. The diffusivity coefficient of the xylene–CD complex (D m) and the equilibrium extraction constant (K ex) associated with each CD were determined. Meanwhile, K ex, the parameter used to describe the ability of CD to extract xylene and to form a complex, of p-xylene and CDs was almost twice as much as that for m-xylene and CDs. Thus, the addition of CD into the membrane phase for xylene separation yields a two-fold benefit in favor of p-xylene: an increase in selectivity and an enhancement of the mass transfer flux.

In vitro biocompatibility of magnetic thermo-responsive nanohydrogel particles of poly(N-isopropylacrylamide-co-acrylic acid) with Fe3O4 cores: effect of particle size and chemical composition.

Biocompatibility is a critical factor in the design and development of candidate materials for biomedical use. This paper reports on the in vitro biocompatibility of magnetic stimuli-sensitive nanohydrogel particles composed of magnetite cores in poly(N-isopropylacrylamide-co-acrylic acid) shells referred to Fe(3)O(4)/P(NIPAAm-co-AAc). The AAc concentration and polymerization time were varied to fabricate magnetic nanoparticles with various AAc levels (1.80-2.37%) and particle sizes (74-213 nm). The P(NIPAAm-co-AAc) shell exhibited thermo-sensitive properties and the Fe(3)O(4) core constituted 2.25-4.10% of the particles by weight. After a 2-day incubation of L929 cells with extract media that had been conditioned with various test samples, the cellular responses were monitored in terms of cell viability and growth. The Live/Dead assays showed that high levels of cellular viability (97.3-98.1%) were observed in all groups, indicating that none of the nanoparticles were cytotoxic. However, the 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymetho-xyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assays demonstrated that the activity of mitochondrial dehydrogenase varied significantly in cultures exposed to different magnetic nanohydrogel particles. The murine fibroblasts exposed to the NIP-(AAc5.1-Fe)-2 sample, which contained the highest AAc content and largest particle sizes, were the least metabolically active. In contrast, the activity levels in the cultures treated with the low AAc content and small size particles (NIP-(AAc2.6-Fe)-1) were not significantly different from those in the control group. Our findings suggest that smaller magnetic stimuli-sensitive nanohydrogel particles with a lower AAc content may have little inhibitory impact on cell proliferation. Overall, the in vitro biocompatibilities of the nanoparticles depend on the chemical composition and size of the Fe(3)O(4)/P(NIPAAm-co-AAc) particles.

Correlation between free-volume properties and pervaporative flux of polyurethane-zeolite composites on organic solvent mixtures.

Micron-sized zeolite particles were incorporated into a polyurethane (PU) matrix to prepare ethylbenzene-selective membranes. The resulting composite membranes were used in the pervaporation (PV) of ethylbenzene/styrene (EB/ST) mixtures. The sorption, diffusion, and PV permeation behaviors as a result of zeolite addition were elucidated. Zeolite is less chemically compatible with organic solvents than PU and the PU-zeolite composites, which exhibited suppressed solvent solubilities compared with pristine PU. However, these membranes favor EB transport by diffusion selectivity. The diffusivity and permeation flux increases in parallel with the enlarged radius of the free-volume hole size (R(4) increasing from 3.46 to 3.64 Å using positron annihilation lifetime spectroscopy analysis) by increasing the zeolite content from 0 to 23%. The enlarged free volume at a zeolite loading of 23% promoted pure solvent diffusivities by 10% higher than that of the unfilled film. During the PV operation on the EB/ST mixture, a significant diffusion-coupling was observed, and the permeant diffusion coefficients from the binary mixture exceeded the pure solvent diffusivity. The permeation flux was greatly improved (up to 0.72 kg/m(2)·h) by zeolite addition without any detrimental effect on the separation efficiency.

Vapor Permeation of Toluene, m-Xylene, and Methanol Vapors on Poly(Dimethylsiloxane) Membranes

Abstract This study examined the simultaneous removal and recovery of volatile organic compounds (VOCs) from nitrogen streams using the vapor permeation (VP) technique. A poly(dimethylsiloxane) (PDMS) membrane was employed to separate toluene, m-xylene, and methanol from nitrogen gas. The effects of operating conditions (including PDMS cross-linker content, membrane thickness, feed flow rate, downstream pressure, and VOC feed concentration) on VOC removal were studied. The sorption isotherms and diffusion coefficients of the vapors in the PDMS were established using the gravimetric method. The Flory-Huggins equation was used to fit the vapor sorption isotherms in PDMS. The diffusivity dependence on vapor concentration were fitted using Longs model. The concentration distribution of the vapor in each layer was determined using a stack of membranes consisting of four layers. The measured permeant concentration distribution agreed excellently with the concentration profile calculation. The solution-diffusion model based on the first Ficks law can describe the mass-transfer mechanism of the vapors in a VP process.

Modeling Sorption Behavior for Ethanol/Water Mixtures in a Cross-linked Polydimethylsiloxane Membrane Using the Flory-Huggins Equation

The sorption behaviors of aqueous ethanol solutions in a polydimethylsiloxane (PDMS) membrane at 25°C were investigated in this study. The sorption isotherms for the ethanol and water binary mixtures were experimentally determined. The water uptake reached a maximum at a concentration of 80 wt% ethanol, and the partial water uptakes were even higher than the pure water solubility for 10–95 wt% ethanol solutions in the PDMS membrane, which implies the presence of a strong synergistic effect due to the ethanol copermeant. The Flory-Huggins equation was utilized to predict the sorption levels at various ethanol/water compositions. The binary Flory-Huggins interaction parameters obtained from pure solvent sorption experiments (χiM ) and the ethanol/water vapor liquid equilibrium data (χ 12) were used in the construction of the model for predicting the partial penetrant solubilities. Using constant χij parameters could not render satisfactory predictions; therefore, concentration-dependent expressions for either χ 12 or χiM were employed to improve the prediction power. We found that constant or concentration dependent χ 12 parameters had little impact on the predicted sorption, whereas the modified concentration-dependent χiM values greatly improved the modeling precision.