New self-assembling biocompatible-biodegradable amphiphilic block copolymers

New biodegradable-biocompatible amphiphilic block copolymers were prepared in good yields by SnOct₂ catalyzed ring opening polymerization of ε-caprolactone initiated by monomethoxy-terminated poly(ethylene glycol) (MPEG). Coupling of the AB copolymers with hexamethylene diisocyanate afforded ABA (formally ABBA) block copolymers. Both AB and ABA copolymers were thoroughly characterized by IR and NMR spectroscopy, size exclusion chromatography, TGA and DSC thermal analysis. In particular, DSC measurements evidenced that the copolymer hydrophilic-lipophilic balance appreciably affected the state of adsorbed water. Polarized optical microscopy of bulk materials and pyrene fluorescence emission of polymer water solutions highlighted the copolymer tendency to phase separation and self-organization, respectively. Most of the prepared materials formed micelles in water and the copolymer structure appreciably affected their critical micellar concentration. In vitro cytocompatibility tests confirmed the low toxicity of the prepared polymeric materials which enhances their potential for biomedical applications.     

Ceramic membranes from reaction-sintered silicon nitride on nitride and oxide substrates

The gas permeability and pore sizes in membranes and porous materials from reaction-sintered silicon nitride (RSSN) are studied. The substrates for the RSSN membranes are highly porous (80% pores) silicon nitride with pores 50–80 μm in size and alumina-based ceramics with pores about 10 μm in diameter. Silicon powder with a specific surface of 1.4 g/cm² is deposited onto the substrates in the form of an aqueous dispersion by various methods including filtering and sintered in nitrogen at 1300–1400°C. The main difficulty of the process is in preventing the formation of defects at the places where the largest channel pores reach the surface of the substrate. Membranes with 0.5 -1 μm pores can be obtained on both types of substrate after a single deposition.     

Ultra-small angle X-ray scattering from bulk nanoporous carbon produced from silicon carbide

The technique of ultra-small angle X-ray scattering has been applied in parallel with the X-ray diffraction method for structural studies of bulk nanoporous carbon materials chemically produced from polycrystalline SiC powders with certain variations in the technology. The experiments were performed with MoKα and CuKα radiation. The integral absorption and scattering coefficients have been estimated for different types of samples. The experimental data reveal a strong correlation between the diffraction patterns and angular distributions of the scattering intensity. The correlation is associated with graphite-like nanoclusters that can be formed on carbonization of the SiC precursor under certain technological conditions. By analyzing the combined scattering and diffraction data, it is shown that the nanoclusters are quasi-two-dimensional. Their size parameters, concentration, volume fraction, and contribution to the scattering coefficient have been found.     

Molecular transport of aromatic hydrocarbons through crosslinked styrene-butadiene rubber membranes

Molecular transport of aromatic hydrocarbons through crosslinked SBR has been carried out in the temperature range (25–65°C). SBR has been vulcanized by four different vulcanizing techniques viz., conventional, efficient, dicumyl peroxide and a mixture consisting of sulfur and peroxide. SBR vulcanized with EV system showed highest solvent uptake tendency and that vulcanized with peroxide showed the lowest. The influence of penetrant size on sorption behaviour of SBR has been examined. The thermodynamic constants such as standard entropy, standard enthalpy and first-order kinetic rate constant have been evaluated. A correlation between theoretical and experimental sorption results was evaluated. The polymer—solvent interaction parameter (χ) was also computed from the diffusion data.     

多孔石墨烯气体分离膜分子渗透机理

通过分子动力学方法模拟了4种不同气体分子（He,H₂,N₂和CH₄）在多孔石墨烯气体分离膜中的穿透过程,揭示了气体分子穿透石墨烯纳米孔的渗透机理,指出分子的渗透不仅与其动力学参数有关,如分子直径和质量,还与分子在石墨烯表面的吸附有关。石墨烯表面的吸附层给气体分子的渗透提供了一个额外的路径,因此分子在石墨烯表面的吸附越强,分子的渗透通量越大。同时,不同大小的纳米孔下H₂分子的渗透通量都随着压力的增加而线性增加。     

Electrical resistivity of silicon nitride produced by various methods

It has been shown that the grain growth and amount of the glass phase influence the electrical resistivity of pressureless sintered and spark plasma sintered silicon nitride. Sintering additives strongly affect the impurity conductivity of pressureless sintered silicon nitride and slightly influence the intrinsic conductivity due to the longer sintering process as compared with the spark plasma sintering. It was demonstrated that Al₂O₃-Y₂O₃ lead to decrease in the electrical resistivity of SPSed silicon nitride due to increase in the band gap width as opposed to Al₂O₃-MgO. Effect of the sintering additive on the impurity conductivity is practically absent but there is a strong dependence of the sintering temperature for reported spark plasma sintered silicon nitride. However, intrinsic conductivity of SPSed silicon nitride is affected by both sintering temperature and sintering additive. It was also shown that electrical resistivity of produced ceramics is linearly depends on the content of β-Si₃N₄ and microhardness. Electrical resistivity of manufactured silicon nitride varied from 3.16·10⁹ to 1.73·10¹¹ Ω?m. It has been observed strong influence of the sintering additive and sintering temperature on the electrical properties of SPSed and pressureless sintered silicon nitride.     

Dielectric properties of silicon nitride ceramics produced by free sintering

The silicon nitride ceramics with a beneficial combination of low dielectric losses and improved physical properties was fabricated by cold isostatic pressing and pressureless sintering. The fine grain microstructure, three-phase composition based on the β-SiAlON, the small amount of the glass phase and relatively small porosity promote a unique combination of a low thermal conductivity 14.51 W m⁻¹ K⁻¹ and low dielectric loss 1.4·10⁻³. A novel method is proposed to overcome the main drawbacks of the commercial and high-cost technologies.     

Silica transport through charged membranes

The possibility of using the electrodialysis process for the desalination of brackish waters having a high silica content, was investigated. The apparent transport numbers of silica through an anion membrane, under controlled hydrodynamic and chemical experimental conditions, were measured.Results indicate a direct dependence of the silica transport number on the SiO (OH^-₃^- ion concentration in the solution. It was found that the silica transport number decreases by increasing current density if chloride ions are present in the solution and that it is negligible with current density higher than 4 mA/cm² if bicarbonate ions are present.Some desalination tests were carried out by electrodialyzing brackish water with silica concentration of about 150 mg/l as SiO₂.Results showed that both transport and scaling effects of silica in the membrane were negligible with feed solutions containing HCO^-₃ ions and with brine acidified at pH ? 4, in accordance with transport number determinations.     

Novel nanoporous membranes from regenerated bacterial cellulose

Bacterial cellulose (BC) in an NaOH/urea aqueous solution was used as a substrate material for thefabrication of a novel regenerated cellulose membrane. The dissolution of BC involved swelling BC in a 4 wt % NaOH/3 wt % urea solution followed by a freeze–thaw process. The BC solution was cast onto a Teflon plate, coagulated in a 5 wt % CaCl₂ aqueous solution, and then treated with a 1 wt % HCl solution. Supercritical carbon dioxide drying was then applied to the formation of a nanoporous structure. The physical properties and morphology of the regenerated bacterial cellulose (RBC) films were characterized. The tensile strength, elongation at break, and water absorption of the RBC membranes were 4.32 MPa, 35.20%, and 49.67%, respectively. The average pore size of the RBC membrane was 1.26 nm with a 17.57 m²/g surface area. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Light-controlled ion transport through spiropyran-modified nanoporous silica colloidal films

The transport of positively charged redox active species through spiropyran-modified nanopores in silica colloidal films was controlled using light. The silica colloidal films were comprised of 18 layers of face centered cubic (fcc)-packed 170 nm silica spheres. The surface of the films was first modified with amines, which were then used to attach the spiropyran moiety to the surface. The limiting current of a positively charged redox active species through the spiropyran-modified nanopore decreased after irradiation with UV light at pH 6.6. When the silica colloidal film was subsequently irradiated with visible light the initial limiting current was restored.     

聚偏氟乙烯纳米多孔膜结构调控及离子传递特性

在纳微米尺度调控膜孔结构对发展高性能膜分离材料具有重要意义。使用半结晶性高分子材料聚偏氟乙烯（PVDF）,利用非溶剂诱导相分离（NIPS）、蒸汽诱导相分离（VIPS）、溶剂蒸发诱导相分离（EIPS）方法,成功制备了不同形貌的多孔膜。提出了根据聚合物的结晶生长机制调控膜孔结构概念,根据溶剂蒸发时间调控结晶生长。利用SEM和BET对膜孔形貌进行表征,XRD和DSC对结晶进行检测,氢离子（H⁺）和四价钒离子（VO²⁺）以及其他常用离子的扩散系数表征传质特性。在溶剂蒸发诱导结晶的过程中,随着溶剂蒸发时间的延长,膜断面的球晶比例逐渐增加,最后至球晶完全融合,膜孔结构发生了显著变化,且膜的结晶度和结晶形态随之发生变化,离子选择性能随膜孔尺寸减小而逐渐增大。     

Structure of nanoporous carbon produced from boron carbide

The structure of nanoporous carbon produced by chlorination of powdered boron carbide at 600, 800, 1000, 1300, 1500, and 1800 °C has been studied by scanning electron microscopy, X-ray diffraction analysis, helium pycnometry, and low-temperature adsorption of nitrogen. On the basis of the results obtained, suggestions are made concerning the type of organization of the nanoporous structure of these materials. The evolution of the structure of nanoporous carbon is analyzed in relation to the synthesis temperature of nanoporous carbon. It is shown that, as the chlorination temperature increases, the structure of nanoporous carbon becomes more perfect: it changes from paracrystalline to turbostratic. The specific surface area decreases from 2200 to 36 m²/g, the volume of micropores decreases from 0.93 to 0.01 cm³/g, and that of mesopores first increases from 0.15 cm³/g (600 °C) to 0.57 cm³/g (1000 °C) and then decreases to 0.19 cm³/g (1800 °C). The total pore volume decreases from 1.08 to 0.20 cm³/g.     

Diffusive transport through mesoporous silica membranes

The results of a detailed study of the permeation of several light gases through unmodified and modified mesoporous silica membranes are reported. The base membranes which were synthesized by dip-coating multiple layers of a templated silica onto a macroporous alumina support showed relatively high permeances with evidence of both support resistance and a contribution from viscous flow, in addition to Knudsen diffusion, through the active layer. The behavior of the modified membranes, which were prepared by silanation of the original membranes with octadecyldimethylchlorosilane, was more interesting. Permeances were lower and there was no evidence of support resistance or viscous flow. Permeation through the active layer appeared to occur by a “Knudsen-like” process. Permeance and permeance ratios measured in both single component and binary systems showed the characteristic inverse dependence on the square root of the molecular weight (or the molecular weight ratio) but the temperature dependence was much stronger than expected for classical Knudsen flow. The behavior of CO₂ was somewhat anomalous yielding permeances that were about 25% larger than those for propane which has the same molecular weight.     

Water transport mechanisms through asymmetric membranes

Permeability coefficients and activation energy values for the transport of water through asymmetric cellulose acetate membranes were determined in order to establish the mechanism of the process when different driving forces are applied. A stirred Lucite cell with controlled temperature was used to measure the membrane transport properties under hydraulic and osmotic pressure differences and also in the presence of a tracer concentration gradient across the membrane. The experimental results based on the temperature dependence of water flow show that the controlling step for water transport is diffusion with net flux in the dense zone of the membrane under hydraulic or osmotic pressure gradients. When a tracer concentration gradient is used, equimolar diffusion of water in the thicker, porous zone of the membrane is the controlling mechanism. A mass transport model based on the composed structure of the membrane is presented to provide a general framework for treating the particular cases. Finally, the difference in the controlling barriers, in agreement with a previous work by Hays,¹⁸ is shown to account for the much higher absolute values of osmotic than tracer water permeabilities determined here and frequently reported in the literature.     

Correlation of penetrant transport with polymer free volume: Additional evidence from block copolymers

The transport rates of methanol and water through a series of PEBAX® block copolymers were measured and correlated with the fractional free volume. Excellent agreement between the logarithm of the diffusion coefficient and the inverse of the fractional free volume of the polymer was observed. This provides new evidence of the utility of the free volume theory to describe the transport of highly condensable vapours. Correlation was also quite good when the logarithmic additivity relationship was employed. This relationship has been previously shown to correlate the properties of homogenous blends with copolymer composition. The successful use of this theory for a series of blends that exhibit two glass transition temperature is discussed.     

Ceramic made from SHS silicon nitride powder

A ceramic made from a powder of silicon nitride which is obtained by self-propagating high-temperature synthesis is investigated. Yttrium-aluminum garnet and mullite are used as sintering aids. The strength of the material obtained reaches 540 MPa. __________ Translated from Steklo i Keramika, No. 3, pp. 17–19, March, 2007.     

Making reusable reaction-bonded silicon nitride crucibles for silicon casting from kerf-loss silicon waste

Silicon kerf loss during wafer slicing and the broken quartz crucibles after silicon casting are two major solid wastes from photovoltaic (PV) industry. Especially, the recycle of kerf-loss silicon has become an urgent issue because near 100 000 t of solid wastes are generated every year. One of the most meaningful recycle routes of the kerf-loss silicon is to make silicon nitride crucibles to replace the quartz crucibles. In this study, we demonstrated how this is feasible through acid leaching refining, slip casting, and nitridation. The reaction-bonded silicon nitride (RBSN) crucibles after oxidation were found pure enough for silicon ingot growth. More importantly, they could be reused after ingot growth. With the present examples, the potential of using the kerf-loss silicon for fine ceramics is prominent.