ABSTRACT
Aligned with the recent trend and imperative to reduce separation layer thickness in gas separation membranes to the nanometer scale in order to raise permeance to levels that can render them competitive with respect to other gas separation technologies, a novel approach and device for fabricating defect-free composite hollow fiber (HF) membranes by dip-coating is described. The presented method avoids the fundamental drawbacks of state-of-the-art techniques for applying a thin gas separation layer onto a porous HF substrate, providing a safe but, at the same time, easily up-scalable way of producing HF membranes at a relatively high production rate. As a basic concept, hanging HF substrates are coated by allowing the coating solution to flow and drip along their external surface. The adaptability of this method, stemming from the array of available coating solutions (a plethora of dispersed nanofillers) and the multitude of substrate options, holds great promise for the fabrication of highly selective and defect-free composite HF membranes.
ABSTRACT
In the present work, Pebax-1657, a commercial multiblock copolymer (poly(ether-block-amide)), consisting of 40% rigid amide (PA6) groups and 60% flexible ether (PEO) linkages, was selected as the base polymer for preparing dense flat sheet mixed matrix membranes (MMMs) using the solution casting method. Carbon nanofillers, specifically, raw and treated (plasma and oxidized) multi-walled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) were incorporated into the polymeric matrix in order to improve the gas-separation performance and polymer's structural properties. The developed membranes were characterized by means of SEM and FTIR, and their mechanical properties were also evaluated. Well-established models were employed in order to compare the experimental data with theoretical calculations concerning the tensile properties of MMMs. Most remarkably, the tensile strength of the mixed matrix membrane with oxidized GNPs was enhanced by 55.3% compared to the pure polymeric membrane, and its tensile modulus increased 3.2 times compared to the neat one. In addition, the effect of nanofiller type, structure and amount to real binary CO2/CH4 (10/90 vol.%) mixture separation performance was evaluated under elevated pressure conditions. A maximum CO2/CH4 separation factor of 21.9 was reached with CO2 permeability of 384 Barrer. Overall, MMMs exhibited enhanced gas permeabilities (up to fivefold values) without sacrificing gas selectivity compared to the corresponding pure polymeric membrane.
ABSTRACT
All-inorganic self-arranged molecular transition metal oxide hyperstructures based on polyoxometalate molecules (POMs) are fabricated and tested as electronically tunable components in emerging electronic devices. POM hyperstructures reveal great potential as charging nodes of tunable charging level for molecular memories and as enhancers of interfacial electron/hole injection for photovoltaic stacks. STM, UPS, UV-vis spectroscopy and AFM measurements show that this functionality stems from the films' ability to structurally tune their HOMO-LUMO levels and electron localization length at room temperature. By adapting POM nanocluster size in solution, self-doping and current modulation of four orders of magnitude is monitored on a single nanocluster on SiO2 at voltages as low as 3 Volt. Structurally driven insulator-to-semi-metal transitions and size-dependent current regulation through single electron tunneling are demonstrated and examined with respect to the stereochemical and electronic structure of the molecular entities. This extends the value of self-assembly as a tool for correlation length and electronic properties tuning and demonstrate POM hyperstructures' plausibility for on-chip molecular electronics operative at room temperature.
