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Membrane separation has been an efficient and energy saving technique in dealing with greenhouse gases, but the traditional membrane designs might not be able to handle the concomitant environmental pollution due to their fixed properties. Correspondingly, the use of an active-responsive smart membrane appears to be a new trend for membrane development in the coming future, which shows great potential to deal with the obstacles. In this research, we demonstrate a smart graphene-organic framework membrane to enable reversed electrical-switchable permselectivity in CO2 separation. The addition of polydopamine (PDA) to the polyvinylidene fluoride/Graphene (PVDF/G) membranes was done to (i) induce the β-phase of PVDF, since –NH2-functionalized graphene has specific interactions (dipole induced dipole interaction) between graphene-PDA and PVDF; (ii) modify the organic PVDF-inorganic graphene interface; and (iii) facilitate CO2 selective separation. Permeability and permselectivity was increased after applying voltage that resulted in the increase of gas permselectivity in response to the lowest applied voltage range (0–3 V) to the membrane. Digital image correlation method depicted the response of the membrane to voltage and proved that the membrane has high piezoelectric properties that is switchable. Furthermore, PALS studies confirmed the free volume and interlayers in the membrane. This membrane has unique properties because the pore changes from bimodal to single pore distribution.

Graphene quantum dots(GQDs) were used to possibly cover the structural defects on the graphitic regions of reduced graphene oxide(rGO). The resulting homostructure nanocomposite fillers(rGO + GQD) were incorporated in an alginate solution creating a dense composite membrane for pervaporation. The membranes were applied to separate alcohol/water mixtures in which the successful covering of structural defects was observed with good separation performance of lower molecular weight alcohols. The Raman spectra and elemental analyses confirmed the successful changes on the physical and chemical structures of the homostructured fillers. The X-ray diffraction pattern showed that the crystallinity of the membrane was greatly affected when incorporated with rGO + GQD. The alginate incorporated with 3 wt% of rGO + GQD(Alg-rGO + GQD) has the best pervaporation performance in separating methanol/water mixture with a permeation flux of 2323 g m−2 h−1 and water concentration in permeate of 92.7% at 70 °C. While the positron annihilation lifetime spectroscopy results supported that the minimization of the structural defects enhanced the water selectivity of the membrane by easy transport of water molecules through the free volume spaces and blocks methanol molecules. The results in this study may provide the possible sealing or curing the defects on rGO with GQDs and improve the interfacial interaction with the polymer matrix.

Carbon molecular sieve membranes (CMSMs) are regarded as promising membranes for gas separation due to its high gas separation performance. Polyimide (ODPA-FDA) precursor films were prepared and characterized for the formation of carbon molecular sieve membranes. This was compared to a commercial polyimide Matrimid. The precursor membranes were characterized with dynamic mechanical analysis, thermogravimetric analysis, X-ray diffraction, FTIR spectroscopy, positron annihilation lifetime spectroscopy in order to determine the effect of the polyimide structure on its permeation properties. The carbonized membranes were characterized by determining its pore size distribution and sorption isotherms. Results showed that ODPA-FDA polyimide has a higher Tg and d-spacing than that of Matrimid. The permeability and selectivity of ODPA-FDA precursor and carbonized films were higher or similar than that of Matrimid precursor and carbonized films due to the addition of a bulky aromatic group in the ODPA-FDA amine moiety which causes narrow free volume distribution. The unique structure of ODPA-FDA polyimide makes it an attractive candidate for the precursor of CMSM.

Forward osmosis membranes based on ultrathin graphene oxide (GO) were fabricated. Suitable crosslinking agents were used to tune the interlayer spacing of GO sheets to achieve the desired membrane performance. The physicochemical properties of membranes were evaluated using different techniques. The interlayer spacing of GO-based membranes was controlled the interaction between the surface functionality of GO with the nature of crosslinking agents, such as polyvinyl alcohol, meta-phenylenediamine (MPD) and 1,3,5-benzenetricarbonyl chloride (TMC). The covalent bonds between the layer and crosslinking agents effectively suppressed the d-spacing stretching. Unlike other symmetric structures of membranes, the GO-MPD/TMC behaviour observed in the ultrathin polyamide (36 nm) asymmetric structure for the performance of pressure-retarded osmosis (PRO) mode showed the highest flux of 20.8 LMH and low reverse salt flux of 3.4 gMH. A consistent water flux for a long-term PRO operation was achieved using GO-MPD/TMC membrane (∼98.7%). Therefore, the GO-MPD/TMC membrane can be used to suppress internal concentration polarisation.

Minute amount of poly(vinyl alcohol) (PVA) was used to cross-link graphene oxide (GO) nanosheets to prepare composite graphene oxide-framework (GOF) membranes with a nacre-like structure. The PVA provided additional increase in viscosity of GO suspension which made the solution casting of a large-area composite GOF membrane possible. The membranes were applied to separate an acetic acid-water mixture through pervaporation. The ATR-FTIR and XPS analyses confirmed the covalent bonding between GO and PVA as ether bonds were formed. This influenced the changes in water contact angle where it increased from 40.8° (pristine GO) to 62.7° (GO-PVA 8wt% ). X-ray diffraction showed that d -spacing of GOF was higher than GO due to the intercalation of PVA. The d -spacing of GO layer from dry to wet state has a difference of 2.59 A but addition of PVA lowered the difference to 0.38 A. The newly formed covalent bonds between GO and PVA might suppressed the stretching of d -spacing. It shows that the swelling was prevented and the interlayer spacing can be tuned. The composite GO-PVA 8wt% membrane has a short d -spacing and provided an outstanding pervaporation performance at 80 °C with a permeation flux of 463.9 g m −2 h −1 and water concentration in permeate of 97.7%.

Reduced graphene oxide (rGO) has been prepared through hydrothermal reduction, an environmentally safe reduction of graphene oxide (GO). rGO is used to fabricate a highly compatible and orderly stacked lamellar structure of composite membranes with chitosan (CS). This method also facilitates a continuous large-scale fabrication of membranes. Conventionally, GO and CS are the preferred compounds for this type of fabrication because of their hydrophilic nature. Nevertheless, when they are mixed together, the occurrence of ionic complexation (−COO− H3+ N−R) between the negatively charged carboxylate ions of GO and the positively charged protonated amines of CS create extreme membrane aggregation. Hydrothermal reduction is useful for preventing undesired ionic complexation from mixing rGO and CS because it can remove most carboxylate ions from GO. Consequently, the hydrophilic CS molecular chain is inserted in between the rGO laminates, improving the dispersion and enabling the rGO/CS to stack-up and self-assemble into a lamellar structure. Furthermore, excellent methanol dehydration can be observed through pervaporation, which is attributed to the selective water channels formed in the rGO laminates.

Carbon molecular sieve (CMS) membranes were prepared by controlled pyrolysis of polyetherimide (PEI) blended with polyimide (PI). The microstructure and gas separation properties of the precursor membranes and supported CMS membranes were characterized in terms of the different weight ratios of PI to PEI. The thermogravimetric analysis indicated that the co-pyrolysis in the PEI/PI appeared above 450 °C. Both permeability and selectivity of supported CMS membranes increased with increasing PI content in the precursors. The positron annihilation lifetime (PAL) results showed that the free volume size and the pore size were enhanced as the PI content is increased and that there is a close relationship between the microstructures of polymer blend precursors and CMS membranes. These results confirmed that the microstructure and gas separation performance of CMS membranes can be adjusted by blending two thermally stable polymers. For CMS membrane derived from a precursor containing 75 wt% PI and pyrolyzed at 700 °C, the gas permeance for CO2 was 40 GPU and CO2/N2 selectivity was 39.

This study fabricates a transparent cyclic olefin copolymer (COC)/thermally reduced graphene oxide (TRGO) composite film with low content of TRGO, which exhibits high water vapor barrier properties. With only 0.06 wt% (0.028 vol%) TRGO, the COC/TRGO composite film reduces the water vapor permeability by ∼21%. The amount of TRGO in this work is 10–100 times lower than that in other polymer/clay or polymer/graphene composites that reduce the water vapor permeability at the same percentage. Low TRGO content has benefits of fabricating a film with low cost, high transparency, and truly excellent flexibility. The water vapor permeabilities of COC/TRGO composites are much lower than those predicted by Cussler or Nielsen model because of the significant rigidification of COC chains near TRGOs. The COC/TRGO composite film was annealed at different temperatures and then surface modified with amphiphilic polymer to further improve its barrier performance. By treating the film with 0.06 wt% TRGO, the water vapor permeability is greatly reduced by ∼65%.

The demand for flexible and transparent barrier films in industries has been increasing. Learning from nature, borate ions were used to cross-link poly(vinyl alcohol) (PVA) and graphene oxide (GO) to produce flexible, transparent high-barrier composite films with a bio-inspired structure. PVA/GO films with only 0.1 wt% GO and 1 wt% cross-linker exhibited an O2 transmission rate 85%; thus, they can be used for flexible electronics. Fourier transform infrared spectrometry and X-ray photoelectron spectroscopy indicated that the outstanding barrier properties are attributed to the formation of chemical cross-linking involving borate ions, GO sheets, and PVA, similar to the borate cross-links in high-order plants. Comparing our experimental data with the Cussler model, we found that the effective aspect ratio was significantly increased after cross-linking, suggesting that cross-linking networks connected GO with each other to form ultra-large impermeable regions. A feasible green technique, with potential for commercial production of barrier films for flexible electronics was presented.

A transparent, gas barrier film comprised of poly(vinyl alcohol) (PVA) and graphene oxide (GO) is synthesized through combined methods of solution blending and isothermal recrystallization. The recrystallized PVA/GO film with only 0.07 vol% GO gives an O2 transmission rate

We prepare highly ordered flexible layers of graphene oxide (GO) on modified polyacrylonitrile substrates by the pressure-assisted self-assembly technique. This composite membrane shows excellent performance during the pervaporation separation of a 70 wt.% isopropyl alcohol (IPA)/water mixture: 99.5 wt% water in permeate and 2047 g m−2 h−1 permeation flux. Despite the specific GO deposition increase from 4.3 to 43.3 × 10−5 g cm−2 (ninefold layer thickness growth), its effect on the permeation flux is not significant, as manifested by only a little decrease in the flux. At 70 °C feed temperature, the permeate water concentration remains 99.5 wt% and the permeation flux reaches 4137 g m−2 h−1. The high selectivity may be due to the dense GO film consisting of highly ordered and packed laminates, allowing water but inhibiting IPA molecules to pass through. GO is demonstrated to be amphiphilic: water molecules adsorb first at the hydrophilic edge (hydroxides) and then rapidly diffuse through the hydrophobic core (mainly carbon), forming a water passage channel that promotes high permeation flux. When water molecules permeate through the GO layers, they accumulate and form a monolayer structure that pushes the successive layers away from each other, leading to widening of the d-spacing.

Pyrolytic carbons were fabricated from a Kapton precursor. The microstructure of the carbon molecular sieve membranes (CMSMs) was examined by means of sorption tests using pure liquid sorbates of different molar volumes and with the application of positron annihilation lifetime spectroscopy (PALS). The pore sizes of the CMSMs were calculated by PALS and our recently proposed equation relating positron lifetime with free volume radius. The mean pore radius in the CMSMs was demonstrated to be smaller than 3 A. Their ability to separate water from bioethanol was determined. Results of aqueous bioethanol sorption showed evident molecular sieving behavior of the CMSMs despite their hydrophobic surface. This behavior became significant at a high pyrolysis end temperature of 700 °C, at which the carbon content was high. The c700 membrane delivered a high permeation flux of 1199 g/m 2 h and a high separation factor of 683. This performance suggests that CMSMs have more potential to separate water from bioethanol through pervaporation or vapor permeation than organic membranes.