Structure-Piezoelectric Property Relationships of Thin Films Composed of Electrospun Aligned Poly(vinylidene fluoride) Nanofibers.

In the past two decades, many studies on piezoelectric nanofibers (NFs) prepared from poly(vinylidene fluoride) (PVDF) and its copolymers, including single NFs, randomly oriented nonwoven mats, and aligned NFs, have been reported. However, studies on the relationships between the PVDF NF diameter, the orientation of the ß-phase crystals inside NFs, and the piezoelectric properties of the NFs are still limited. In this study, the effect of the fiber diameter on the internal molecular packing/orientation and piezoelectric properties of aligned PVDF NF thin films was investigated. Herein, piezoelectric thin films composed of densely packed, uniaxially aligned, PVDF NFs with diameters ranging from 228 to 1315 nm were prepared by means of electrospinning with a rotating collector and successive hot-pressing and poling. The effect of the diameters of PVDF NFs on their internal structures, as well as the piezoelectric properties of the thin films, was investigated. All prepared NFs mainly contained ß-phase crystals with a similar total crystallinity. The orientation of the ß-phase crystals inside the NFs increased with an increase in the fiber diameter, resulting in an improved transverse piezoelectric coefficient (d31) for the thin films. The output voltage of the prepared thin films reached a maximum of 2.7 V at 104 Hz.

Current-Voltage Characteristics and Solvent Dissociation of Bipolar Membranes in Organic Solvents.

In this work, the chronopotentiometric responses, pH changes, and current-voltage (I-V) characteristics of bipolar membrane (BPM)/LiCl-organic solvent systems were measured and compared with those of the BPM/LiCl-water system. Monohydric alcohols, polyhydric alcohols, and amides were used as organic solvents. The chronopotentiograms and pH changes supported that the organic solvents can dissociate into cations and anions at the BPM interface. It is found that amides cannot dissociate easily at the BPM compared with alcohols. The I-V characteristics showed that both the viscosity and acid-base property of organic solvents substantially influences the dissociation behaviors in addition to the autoprotolysis constant and relative permittivity of the solvents.

Enhancing the effect of the nanofiber network structure on thermoresponsive wettability switching.

This letter reports the enhancing effects of a nanofiber network structure on stimuli-responsive wettability switching. Thermoresponsive coatings composed of nanofibers were prepared by electrospinning from thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAAm). The nanofiber coatings showed a large amplitude of thermoresponsive change in the wettability from hydrophilic to hydrophobic states compared to a smooth cast film. In particular, the combination of the surface chemistry and unique topology of the electrospun nanofiber coatings enables a transition from the Wenzel state to the metastable Cassie-Baxter state with an increase in temperature and consequently an enhanced amplitude of change in the water contact angles: the apparent contact angle differences between 25 and 50 °C are Δθ*(25-50 °C )= 108 and 10° for the nanofiber coatings with a diameter of 830 nm and a smooth cast film, respectively. The fabrication of the 3D nanofiber network structure by electrospinning from stimuli-responsive materials is a promising option for highly responsive surfaces in wettability.

Stabilization of organic field-effect transistors by tert-butyl groups in dibenzotetrathiafulvalene derivatives.

For a material for organic thin-film transistors, not only high mobility but also low threshold voltage and long-term stability are important requirements. In order to realize these properties, materials with relatively large oxidation potentials, namely weak donors, have been designed as p-channel organic semiconductors. Here we propose a different strategy; transistor properties of dibenzotetrathiafulvalene (DBTTF) are significantly improved by the introduction of tert-butyl groups. Although this chemical modification does not much change the ionization potential, small threshold voltage and stability over several months are attained together with the improved mobility, probably due to some kind of passivation effect of the bulky tert-butyl groups. In contrast, the systematic fluorine substitution rapidly diminishes the transistor performance. There are two kinds of herringbone structures with much different dihedral angles of about 50° and 130°, and the tert-butyl compound falls into the former category.

De Novo Ion-Exchange Membranes Based on Nanofibers.

The unique functions of nanofibers (NFs) are based on their nanoscale cross-section, high specific surface area, and high molecular orientation, and/or their confined polymer chains inside the fibers. The introduction of ion-exchange (IEX) groups on the surface and/or inside the NFs provides de novo ion-exchangers. In particular, the combination of large surface areas and ionizable groups in the IEX-NFs improves their performance through indices such as extremely rapid ion-exchange kinetics and high ion-exchange capacities. In reality, the membranes based on ion-exchange NFs exhibit superior properties such as high catalytic efficiency, high ion-exchange and adsorption capacities, and high ionic conductivities. The present review highlights the fundamental aspects of IEX-NFs (i.e., their unique size-dependent properties), scalable production methods, and the recent advancements in their applications in catalysis, separation/adsorption processes, and fuel cells, as well as the future perspectives and endeavors of NF-based IEMs.

Commercial Pressure Retarded Osmosis Systems for Seawater Desalination Plants.

The development of renewable energy technologies is of global importance. To realize a sustainable society, fossil-resource-independent technologies, such as solar- and wind-power generation, should be widely adopted. Pressure retarded osmosis (PRO) is one such potential renewable energy technology. PRO requires salt water and fresh water, both of which can be found at seawater desalination plants. The total power generation capacity of PRO, using concentrated seawater and fresh water, is 3 GW. A large amount of energy is required for seawater desalination; therefore, the introduction of renewable energy should be prioritized. Kyowakiden Industry Co., Ltd., has been working on introducing PRO to seawater desalination plants since 2001 and is attracting attention for its ongoing PRO pilot plant with a scale of 460 m3/d, using concentrated seawater and treated sewage water. In this study, we evaluated the feasibility of introducing PRO in existing desalination plants. The feasibility was examined based on technology, operation, and economy. Based on the number of seawater desalination plants in each country and the electricity charges, it was determined whether the introduction of PRO would be viable.

Control over Internal Structure of Liquid Crystal Polymer Nanofibers by Electrospinning.

Liquid crystal polymer nanofibers with a diameter ranging from 0.13 to 4.71 µm were prepared by electrospinning from a main-chain liquid crystalline polyester, BB-5(3-Me). WAXD measurements showed that the formation and orientation of the ordered structure in the electrospun fibers were controlled by the fiber diameter formed during electrospinning. For BB-5(3-Me), the SmA structure with two layer spacings was formed in the fiber during the electrospinning. Under optimal spinning conditions, the SmA structure is highly oriented in the fiber. In addition, annealing transformed the metastable SmA structure in the BB-5(3-Me) fiber into stable SmCA one.

Characterization of chitosan nanofiber fabric by electrospray deposition: electrokinetic and adsorption behavior.

Cationic biopolymer nanofiber fabrics were prepared from a chitosan/poly(ethylene oxide) blend solution by electrospray deposition. Their electrokinetic properties and DNA adsorption behavior were analyzed as a function of pH. The zeta potential was determined from streaming potential/streaming current measurements. The adsorption of DNA onto the fabrics was investigated by spectrophotoscopy. The adsorption behavior of DNA correlated well with the electrokinetic properties of the fabrics. This revealed that the electrokinetic approach was a useful option for characterization of novel nanofiber assemblies made by the electrostatic spray process. In addition, these results provided fundamental information about chitosan nanofiber fabrics for both biomedical and analytical applications.

Membrane potential across reverse osmosis membranes under pressure gradient.

Membrane potential measurement has been widely used for the characterization of ionic membranes such as ion-exchange membranes without solvent permeability. However, there have been few studies on membrane potentials across pressure-driven processes such as reverse osmosis (RO) membranes with solvent permeability. In the present study, the membrane potential across RO membranes in NaCl and MgCl2 under the pressure gradient, DeltaP=0-0.3 MPa, was measured. The experimental results were analyzed by the theoretical model based on the Donnan equilibrium and the extended Nernst-Planck flux equation considering the pressure effect. The theoretical values agreed well with the experimental ones. This indicates that membrane potential is useful for characterizing the effective charge density of the active layer of RO membranes under pressure gradient.

Preparation of ion-exchange fiber fabrics by electrospray deposition.

Ion-exchange fiber (IEF) fabrics were prepared by electrospray deposition (ESD) and post-deposition chemical modification of their surfaces. Nonwoven fibrous fabrics were obtained from the solutions of synthetic polymers-polystyrene (PS) and poly(4-vinylpyridine) (P4VP)-of various concentrations. The diameter of the fiber in the fabrics ranged from 600 nm to 1.70 microm. Cation- and anion-exchange fiber (CEF and AEF) fabrics were obtained from the sulfonation of PS fabrics and the quaternization of P4VP fabrics, respectively. These fabrics were thoroughly characterized by a series of techniques, such as scanning electron microscopy (SEM), permporometry, nitrogen adsorption measurements, and potentiometric titrations. The SEM images showed that the fabrics had a porous structure after their chemical modification. The mean pore size, porosity, and specific surface area of the flow-through pores were 1.67-3.53 microm, about 80%, and 13 m(2)/g, respectively. The ion-exchange capacity was in the range from 0.78 to 1.34 mmol/g. The AEF fabric, on the other hand, showed a high specific surface area, i.e., the Brunauer-Emmett-Teller (BET) surface area of 600 m(2)/g, due to the formation of much smaller pores on the surface of the fiber structure in the fabric. The secondary chemical modification of the nano-microfiber fabrics by ESD provides novel functional materials with a large adsorption capacity and a high catalytic activity.

Effect of ion-exchange nanofiber fabrics on water splitting in bipolar membrane.

In the present study, the effect of ion-exchange fiber fabric made by electrospray deposition (ESD) on water splitting in a composite bipolar membrane (CBM) was investigated. Cation- and anion-exchange fiber (CEF and AEF) fabrics, which were composed of very thin fibers, were prepared by ESD and postdeposition chemical modification and then used as the intermediate layer of a CBM. The current-voltage characteristics under reverse bias conditions showed that the AEF fabrics enhanced water splitting. The water dissociation is accelerated by the AEF fabric, which contains both tertiary pyridyl groups and quaternary pyridinium groups and has a high specific surface area. On the other hand, the CEF fabric, which contains sulfonic acid groups and has an insufficient specific surface area, reduced water splitting. These results indicate that fiber fabric with catalytic activity and a high surface area obtained by ESD can improve the performance of a CBM.

Membrane potential across low-water-content charged membranes: Effect of ion pairing.

In the present paper, we systematically examined the ion-pairing effect in low-water-content charged membranes. Cation- and anion-exchange membranes with various water contents and homogeneous fixed-charge distribution were prepared by radical copolymerization and then characterized by membrane potential measurements. The experimental results were analyzed by our recently developed theoretical model (Yamamoto, R.; Matsumoto, H.; Tanioka, A. J. Phys. Chem. B 2003, 107, 10615), which is based on the Donnan equilibrium, the Nernst-Planck equation for ion flux, and the Fuoss formalism for ion-pair formation between the fixed-charge group and the counterion in the membrane. The theoretical predictions agreed well with the experimental results for both cation- and anion-exchange membranes. This supported the belief that the ion-pairing effect was substantial in a low-water-content membrane system. Our theoretical analysis also showed the following results: (i) the dielectric constant in the membrane, epsilon(r), was smaller than the value in bulk water, (ii) the center-to-center distance of the ion pair, a, was independent of the water content of the membranes, and (iii) the charge effectiveness of all membranes, Q, was small (<0.35).

Membrane potential across anion-exchange membranes in acidic solution system.

The membrane potential across anion-exchange membranes in H2SO4 and Na2SO4 solutions was measured, and the experimental results were fitted to the theory in the 2-1 electrolyte system based on the Donnan equilibrium and the Nernst-Planck flux equations. For the Na2SO4 solution, the Donnan potential makes a significant contribution to the membrane potential, but for the H2SO4 solution, the diffusion potential significantly contributes to the membrane potential. The diffusion potential has a greater contribution to the membrane potential across AEM-2 with a high water content than that across AEM-1. These results suggest that a proton with a high mobility can move without substantial influence of electrostatic interaction in a positively charged membrane.

Organic/inorganic hybrid nano-microstructured coatings on insulated substrates by electrospray deposition.

Organic/inorganic hybrid nano-microstructured coatings on insulated polymer films were prepared by electrospray deposition (ESD) from an acrylic resin/silica sol blend solution. The surface morphologies of the coated films were observed using scanning electron microscopy (SEM). The SEM images showed that a nano-microscaled fibrous structure was formed on the film. The fiber diameter decreased from 4.4 microm to 600 nm with the increase in the silica sol content. Energy-dispersive X-ray analysis also revealed that silica atoms were homogeneously distributed in the fibrous structure on the polymer film. These results indicated that the ESD method is potentially a useful option for producing nano-microstructured coatings on not only conductive, but also insulating surfaces.

Molecular Dynamics Study of Carbon Nanotubes/Polyamide Reverse Osmosis Membranes: Polymerization, Structure, and Hydration.

Carbon nanotubes/polyamide (PA) nanocomposite thin films have become very attractive as reverse osmosis (RO) membranes. In this work, we used molecular dynamics to simulate the influence of single walled carbon nanotubes (SWCNTs) in the polyamide molecular structure as a model case of a carbon nanotubes/polyamide nanocomposite RO membrane. It was found that the addition of SWCNTs decreases the pore size of the composite membrane and increases the Na and Cl ion rejection. Analysis of the radial distribution function of water confined in the pores of the membranes shows that SWCNT+PA nanocomposite membranes also exhibit smaller clusters of water molecules within the membrane, thus suggesting a dense membrane structure (SWCNT+PA composite membranes were 3.9% denser than bare PA). The results provide new insights into the fabrication of novel membranes reinforced with tubular structures for enhanced desalination performance.

High-performance multi-functional reverse osmosis membranes obtained by carbon nanotube·polyamide nanocomposite.

Clean water obtained by desalinating sea water or by purifying wastewater, constitutes a major technological objective in the so-called water century. In this work, a high-performance reverse osmosis (RO) composite thin membrane using multi-walled carbon nanotubes (MWCNT) and aromatic polyamide (PA), was successfully prepared by interfacial polymerization. The effect of MWCNT on the chlorine resistance, antifouling and desalination performances of the nanocomposite membranes were studied. We found that a suitable amount of MWCNT in PA, 15.5 wt.%, not only improves the membrane performance in terms of flow and antifouling, but also inhibits the chlorine degradation on these membranes. Therefore, the present results clearly establish a solid foundation towards more efficient large-scale water desalination and other water treatment processes.

Dielectric relaxation on the intermediate layer in a bipolar membrane under the water splitting phenomenon. I. Shift of the dielectric properties by means of time-dependent impedance measurements.

In this study, we examined the dielectric properties of an intermediate layer in a bipolar membrane, which is composed of a negatively charged layer and a positively charged layer joined in series. As a result of the time-dependent impedance measurements of charged membranes, the negative increment in electric conductivity and the positive increment in electric capacity were observed only in the case of a bipolar membrane under the application of reverse-biased voltages, which were quite different from the behavior of both monopolar membranes and of a bipolar membrane under forward-biased voltages. Further, the observed shifts showed a nearly constant value against the reverse-biased voltage. It is concluded that these characteristics coincide with the process of ion exclusion in the intermediate layer and are attributed to the water splitting mechanism.

Dielectric relaxation on the intermediate layer in a bipolar membrane under the water splitting phenomenon. II. Double dielectric relaxation and identification of phase parameters.

In this study, we investigated the impedance spectra of bipolar membranes. Under the application of a reverse-biased voltage, the spectra showed a double dielectric relaxation profile due to the heterogeneous structure and it was analyzed in accordance with the three-layered dielectric model. It is defined that one of the compositions of the heterogeneous structure is situated at the membrane interface region between the negatively and the positively charged membrane with a thickness of less than several micrometers, which has an extraordinarily large electric capacity with a magnitude of sub-microfarads. It is concluded that this layer is identified with the intermediate layer in which the water splitting phenomenon occurs on the bipolar membrane.

Effect of proton concentration on membrane potential across a weak amphoteric polymer membrane.

An amphoteric membrane consists of both positively and negatively fixed charge groups chemically bound to the polymer chains. If the external solution is changed from alkali to acid, it is possible to obtain an experimental result in which the membrane potential changes from positive to negative through the isoelectric point. It was characterized by examining the relationship between membrane potential and proton concentration (pH) obtained from both experimental and theoretical considerations. The Nernst-Planck flux equation and the Donnan equilibrium theory were also solved for a four-component system combined with the dissociation constant, in order to discuss the pH dependence of membrane potential in a weak amphoteric membrane by comparing the experimental results with the calculated results. It was proven that the calculated results substantially deviated from the theoretical results despite a similar tendency. Such a deviation was caused by the fact that the original theory disregarded the activity coefficient and the ionic mobility, which were dependent on the fixed charge concentration in a membrane. The original theoretical model was modified by adding the effect of a fixed charge group to the activity coefficient and ionic mobility. The calculated results using the modified model explained well the experimental results if the parameter called charge effectiveness, phi, was introduced into the equations. Introduction of phi into the prediction of membrane potential was already done by Kobatake et al. in a system of a strong polyelectrolyte monopolar membrane/salt aqueous solution. In this study, it was proved that phi can also be introduced into a weak amphoteric polymer membrane/salt aqueous solution system. Finally it was also concluded that the Donnan equilibrium and the Nernst-Planck flux equation were still applicable for examining the transport phenomena for the system of a weak amphoteric charged membrane and electrolyte solutions at various pH.

Ionic rectification properties of a bipolar interface consisting of a cationic surfactant and cation-exchange membrane.

A novel bipolar interface that consists of cationic surfactant and cation-exchange membrane was successfully prepared in an aqueous electrolyte system. This bipolar interface shows a ionic rectification behavior similar to that observed in bipolar membranes. However, different from bipolar membranes, this system has a total rectification behavior, where we cannot observe the occurrence of a water-splitting phenomenon, which always occurs in the bipolar membrane process under reverse bias conditions.