No abstract

Three diamine monomers (ethylenediamine, butylenediamine, and p-phenylenediamine) were selected for cross-linking graphene oxide (GO) to prepare composite graphene oxide-framework (GOF) membranes through filtration using a pressure-assisted self-assembly technique. The membranes were applied to separate an ethanol−water mixture by pervaporation. Unmodified GO comprised only hydrogen bonds and π−π interactions, but after cross-linking it with a diamine, attenuated total reflectance−Fourier transform infrared and X-ray photoelectron spectroscopy demonstrated that the diamine was chemically bonded both to GO and the membrane support. Moreover, GO hydrophilicity was substantially altered; water contact angle increased from 24.4°to 80.6°(from cross-linking with an aliphatic structure of diamine to cross-linking with an aromatic structure). Results of X-ray diffraction showed that d-spacing in GOF layers varied from 10.4 to 8.7 Å. For GOFs presoaked in 90 wt % ethanol−water, covalent bonds between the layer and diamine could effectively suppress stretching of d-spacing. Cross-linking with ethylenediamine produced a composite membrane that exhibited a short interlayer dspacing and delivered an excellent pervaporation performance at 80 °C: permeation flux = 2297 g/(m 2 h); water concentration in permeate = 99.8 wt %. The membrane showed stability during a long-term operation at 30 °C for 120 h.

Separating molecules or ions with sub-Angstrom scale precision is important but technically challenging. Achieving such a precise separation using membranes requires Angstrom scale pores with a high level of pore size uniformity. Herein, we demonstrate that precise solutesolute separation can be achieved using polyamide membranes formed via surfactantassembly regulated interfacial polymerization (SARIP). The dynamic, self-assembled network of surfactants facilitates faster and more homogeneous diffusion of amine monomers across the water/hexane interface during interfacial polymerization, thereby forming a polyamide active layer with more uniform sub-nanometre pores compared to those formed via conventional interfacial polymerization. The polyamide membrane formed by SARIP exhibits highly size-dependent sieving of solutes, yielding a step-wise transition from low rejection to near-perfect rejection over a solute size range smaller than half Angstrom. SARIP represents an approach for the scalable fabrication of ultra-selective membranes with uniform nanopores for precise separation of ions and small solutes.

The free-volume depth profile of asymmetric polymeric membrane systems prepared by interfacial polymerization is studied using positron annihilation spectroscopy coupled with a variable monoenergy slow positron beam. Significant variations of S, W, and R parameters from the Doppler broadened energy spectra vs positron incident energy up to 30 keV and orthopositronium lifetime and intensity are observed at different doping times of triethylenetetraamine (TETA) reacting with trimesoyl chloride (TMC) in an interfacial polymerization on modified porous polyacrylonitrile (PAN) asymmetric membrane. The positron annihilation data are analyzed in terms of free-volume parameters as a function of depth from the surface to nano- and micrometer regions of asymmetric membranes. A multilayer structure is obtained in polymerized polyamide (PA) on modified PAN membranes (m-PAN):  a nanometer scale skin polyamide layer, a nanometer to micrometer scale transition layer from dense to porous m-PAN, and the porous m-PAN support. The results of free-volume parameters and obtained layer thicknesses are compared with the flux (permeability) and water concentration in permeate (selectivity) through the pervaporation separation of 70 wt % 2-propanol aqueous solution. It is found that the water concentration in permeate is mainly controlled by the free-volume properties of skin polyamide and weakly related to the transition layer from the skin to porous m-PAN. The obtained layer structures of asymmetric polymeric membranes are supported by the data obtained by AFM, SEM, and ATR−FTIR.