Boron nitride colloidal solutions, ultralight aerogels and freestanding membranes through one-step exfoliation and functionalization

Manufacturing of aerogels and membranes from hexagonal boron nitride (h-BN) is much more difficult than from graphene or graphene oxides because of the poor dispersibility of h-BN in water, which limits its exfoliation and preparation of colloidal solutions. Here, a simple, one-step mechano-chemical process to exfoliate and functionalize h-BN into highly water-dispersible, few-layer h-BN containing amino groups is presented. The colloidal solutions of few-layer h-BN can have unprecedentedly high concentrations, up to 30 mg ml(-1), and are stable for up to several months. They can be used to produce ultralight aerogels with a density of 1.4 mg cm(-3), which is ∼1,500 times less than bulk h-BN, and freestanding membranes simply by cryodrying and filtration, respectively. The material shows strong blue light emission under ultraviolet excitation, in both dispersed and dry state.

Next generation membranes for membrane distillation and future prospects

This chapter presents the most recent advances in membrane materials technology and module configurations for membrane distillation (MD), as well as their novel applications. The impact of the material morphology, surface energy, and pore structure on MD performance is discussed. The differences between hollow fibres and flat sheet membrane materials are also examined and promising novel structures or module configurations reviewed. In addition, novel materials and module configuration strategies are also identified that may lead to superior MD systems. Also, this chapter identifies current gaps where further work may lead to ongoing improvements to MD.

Morphology-properties relationship of gas plasma treated hydrophobic meso-porous membranes and their improved performance for desalination by membrane distillation

The impact on performance of the surface energy and roughness of membrane materials used for direct contact membrane distillation are critical but yet poorly investigated parameters. The capacity to alter the wettability of highly hydrophobic materials such as poly(tetra-fluoro-ethylene) (PTFE) by gas plasma treatments is reported in this paper. An equally important contribution from this investigation arises from illustrating how vaporized material from the treated sample participates after a short while in the composition of the plasma and fundamentally changes the result of surface chemistry processes. The water contact angle across the hydrophobic membranes is generally controlled by varying the plasma gas conditions, such as the plasma power, chamber pressure and irradiation duration. Changes to surface porosity and roughness of the bulk material as well as the surface chemistry, through specific and partial de-fluorination of the surface were detected and systematically studied by Fourier transform infra-red analysis and scanning electron microscopy. It was found that the rupture of fibrils, formed during membrane processing by thermal-stretching, led to the formation of a denser surface composed of nodules similar to these naturally acting as bridging points across the membrane material between fibrils. This structural change has a profound and impart a permanent effect on the permeation across the modified membranes, which was found to be enhanced by up to 10% for long plasma exposures while the selectivity of the membranes was found to remain unaffected by the treatment at a level higher than 99.99%. This is the first time that an investigation demonstrates how the permeation characteristics of these membranes is directly related to data from spectral, morphological and surface charge analyses, which provide new insights on the impact of plasma treatments on both, the surface charge and roughness, of PTFE porous materials.

Towards enhanced performance thin-film composite membranes via surface plasma modification

Advancing the design of thin-film composite membrane surfaces is one of the most promising pathways to deal with treating varying water qualities and increase their long-term stability and permeability. Although plasma technologies have been explored for surface modification of bulk micro and ultrafiltration membrane materials, the modification of thin film composite membranes is yet to be systematically investigated. Here, the performance of commercial thin-film composite desalination membranes has been significantly enhanced by rapid and facile, low pressure, argon plasma activation. Pressure driven water desalination tests showed that at low power density, flux was improved by 22% without compromising salt rejection. Various plasma durations and excitation powers have been systematically evaluated to assess the impact of plasma glow reactions on the physico-chemical properties of these materials associated with permeability. With increasing power density, plasma treatment enhanced the hydrophilicity of the surfaces, where water contact angles decreasing by 70% were strongly correlated with increased negative charge and smooth uniform surface morphology. These results highlight a versatile chemical modification technique for post-treatment of commercial membrane products that provides uniform morphology and chemically altered surface properties.

Comparative acoustic performance and mechanical properties of silk membranes for the repair of chronic tympanic membrane perforations

The acoustic and mechanical properties of silk membranes of different thicknesses were tested to determine their suitability as a repair material for tympanic membrane perforations. Membranes of different thickness (10-100μm) were tested to determine their frequency response and their resistance to pressure loads in a simulated ear canal model. Their mechanical rigidity to pressure loads was confirmed by tensile testing. These membranes were tested alongside animal cartilage, currently the strongest available myringoplasty graft as well as paper, which is commonly used for simpler procedures. Silk membranes showed resonant frequencies within the human hearing range and a higher vibrational amplitude than cartilage, suggesting that silk may offer good acoustic energy transfer characteristics. Silk membranes were also highly resistant to simulated pressure changes in the middle ear, suggesting they can resist retraction, a common cause of graft failure resulting from chronic negative pressures in the middle ear. Part of this strength can be explained by the substantially higher modulus of silk films compared with cartilage. This allows for the production of films that are much thinner than cartilage, with superior acoustic properties, but that still provide the same level of mechanical support as thicker cartilage. Together, these in vitro results suggest that silk membranes may provide good hearing outcomes while offering similar levels of mechanical support to the reconstructed middle ear.

Amino-functionalized mesoporous silica based polyethersulfone-polyvinylpyrrolidone composite membranes for elevated temperature proton exchange membrane fuel cells

It is important to find alternative membranes to the state-of-the-art polybenzimidazole based high temperature proton exchange membranes with high proton conductivity at elevated temperature but with simple synthesis procedures. In this work, inorganic-organic nanostructured hybrid membranes are developed based on a polyethersulfone-polyvinylpyrrolidone (PES-PVP) polymeric matrix with hollow mesoporous silica (HMS), amino-functionalized hollow mesoporous silica (NH2-HMS) and amino-functionalized mesoporous silica (NH2-meso-silica). The composite membranes show a significant increase in proton conductivity and a decrease in the activation energy for proton diffusion in comparison with the phosphoric acid (H3PO4, PA) doped PES-PVP membrane. And the composite membrane with NH2-HMS shows the best performance under the conditions in this study, achieving the highest proton conductivity of 1.52 × 10-1 S cm-1 and highest peak power density of 480 mW cm-2 at 180 °C under anhydrous conditions, which is 92.7% higher than that of the PA doped PES-PVP membrane at identical conditions. Such enhancement results from the facilitated proton transportation in the ordered mesoporous channels via the hydrogen bond between the -NH2 groups and H3PO4. The high water retention capability of silica materials with a hollow structure also contributes to the decrease of the activation of proton diffusion. Consequently, the results show promising potential of the NH2-HMS based PES-PVP composite membrane for the elevated temperature proton exchange membrane fuel cells.

Layer-by-layer structured membranes of silk fibroin and polyethylenimine on electrospun silk fibroin nanofibers

Self-assembled silk fibroin (SF)-polyethylenimine (PEI) multilayered films were fabricated on ethanol treated electrospun SF nanofibrous substrates via the electrostatic layer-by-layer (LBL) adsorption. The film coated membranes were characterized using scanning electron microscopy (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectrophotometer (XPS). The SEM images showed that the multilayers of SF-PEI were formed on the surface of the ethanol treated SF nanofibres. The characteristics such as the fiber shape and porous structure were well maintained as the number of the coated SF-PEI bilayers was less than five. However, obvious adhesive substances and blocked pores were observed on the surface of the fibers as the number of bilayers of SF-PEI increased to six. Furthermore, the obvious core-shell structures were observed by TEM. The thickness of five SF-PEI bilayers was approximately 80nm. Additionally, the XPS results also revealed that the SF-PEI multilayer composite membranes formed. The adsorption mainly depended on a simple electrostatic interaction between the layers of SF and PEI. These SF-PEI multilayer assembled nanofibrous membranes could be a promising material for use as a sensor, gene delivery agent and scaffolds.