Ultraselective Pebax Membranes Enabled by Templated Microphase Separation

Thin Film Nanocomposite Membranes Based on Zeolitic Imidazolate Framework-8/Halloysite Nanotube Composites

Thin film nanocomposite (TFN) membranes have proven their unrivaled value, as they can combine the advantages of different materials and furnish membranes with improved selectivity and permeability. The development of TFN membranes has been severely limited by the poor dispersion of the nanoparticles and the weak adhesion between the nanoparticles and the polymer matrix. In this study, to address the poor dispersion of nanoparticles in TFN membranes, we proposed a new combination of m-ZIF-8 and m-HNTs, wherein the ZIF-8 and HNTs were modified with poly (sodium p-styrenesulfonate) to enhance their dispersion in water. Furthermore, the hydropathic properties of the membranes can be well controlled by adjusting the content of m-ZIF-8 and m-HNTs. A series of modified m-ZIF-8/m-HNT/PAN membranes were prepared to modulate the dye/salt separation performance of TFN membranes. The experimental results showed that our m-ZIF-8/m-HNT/PAN membranes can elevate the water flux significantly up to 42.6 L m-2 h-1 MPa-1, together with a high rejection of Reactive Red 49 (more than 80%). In particular, the optimized NFM-7.5 membrane that contained 7.5 mg of HNTs and 2.5 mg of ZIF-8 showed a 97.1% rejection of Reactive Red 49 and 21.3% retention of NaCl.

Nanocarbon-Based Mixed Matrix Pebax-1657 Flat Sheet Membranes for CO2/CH4 Separation

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.

Hydroxypropyl-β-cyclodextrin-incorporated Pebax composite membrane for improved permselectivity in organic solvent nanofiltration

Thanks to the characteristic hollow cavity structure and sustainable and nontoxic macrocycle molecule feature, cyclodextrins have been used as building block to fabricate organic solvent nanofiltration (OSN) membranes with enhanced permeability and selectivity. Herein, hydroxypropyl-β-cyclodextrin (HP-β-CD) was incorporated into a poly(ether-block-amide) (Pebax) layer on a polysulfone support, followed by crosslinking with toluene 2,4-diisocyanate to prepare a crosslinked HP-β-CD/Pebax (CHP) membrane. By adjusting the initial HP-β-CD concentration (x) and crosslinking reaction time (y), the microporous structure and surface morphology of CHP x-y (x = 0, 0.25, 0.5, 0.75; y = 5, 10, 15) membranes could be manipulated. The OSN performances of the CHP x-y membranes were evaluated by the removal of dyes in methanol solution. The results revealed that the optimal CHP0.5-10 membrane exhibited a high methanol permeance of 8.7 L m-2 h-1 bar-1, high dye rejection (>96%), and high running stability (at least 336 h), due to the intrinsically microporous structure and surface morphology. This work would inspire the further development of cyclodextrins and other macrocyclic molecules in the preparation of OSN membranes and provide a promising strategy to fabricate state-of-the-art membranes for the efficient separation of organic solvent reclamation and removal of organic pollutants.

The State-of-the-Art Functionalized Nanomaterials for Carbon Dioxide Separation Membrane

Nanocomposite membrane (NCM) is deemed as a practical and green separation solution which has found application in various fields, due to its potential to delivery excellent separation performance economically. NCM is enabled by nanofiller, which comes in a wide range of geometries and chemical features. Despite numerous advantages offered by nanofiller incorporation, fabrication of NCM often met processing issues arising from incompatibility between inorganic nanofiller and polymeric membrane. Contemporary, functionalization of nanofiller which modify the surface properties of inorganic material using chemical agents is a viable approach and vigorously pursued to refine NCM processing and improve the odds of obtaining a defect-free high-performance membrane. This review highlights the recent progress on nanofiller functionalization employed in the fabrication of gas-separative NCMs. Apart from the different approaches used to obtain functionalized nanofiller (FN) with good dispersion in solvent and polymer matrix, this review discusses the implication of functionalization in altering the structure and chemical properties of nanofiller which favor interaction with specific gas species. These changes eventually led to the enhancement in the gas separation efficiency of NCMs. The most frequently used chemical agents are identified for each type of gas. Finally, the future perspective of gas-separative NCMs are highlighted.

Structures and properties of newly synthesized semi-aromatic polyamide thermoplastic elastomers

Novel semi-aromatic polyamide based thermoplastic elastomers containing both strong and weak H-bond units were fabricated via a facile “two-step” melt polycondensation method. The structures and properties of a series of TPAEs are discussed.

A Novel Composite Material UiO-66@HNT/Pebax Mixed Matrix Membranes for Enhanced CO2/N2 Separation

Mixing a polymer matrix and nanofiller to prepare a mixed matrix membrane (MMM) is an effective method for enhancing gas separation performance. In this work, a unique UiO-66-decorated halloysite nanotubes composite material (UiO-66@HNT) was successfully synthesized via a solvothermal method and dispersed into the Pebax-1657 matrix to prepare MMMs for CO2/N2 separation. A remarkable characteristic of this MMM was that the HNT lumen provided the highway for CO2 diffusion due to the unique affinity of UiO-66 for CO2. Simultaneously, the close connection of the UiO-66 layer on the external surface of HNTs created relatively continuous pathways for gas permeation. A suite of microscopy, diffraction, and thermal techniques was used to characterize the morphology and structure of UiO-66@HNT and the membranes. As expected, the embedding UiO-66@HNT composite materials significantly improved the separation performances of the membranes. Impressively, the as-obtained membrane acquired a high CO2 permeability of 119.08 Barrer and CO2/N2 selectivity of 76.26. Additionally, the presence of UiO-66@HNT conferred good long-term stability and excellent interfacial compatibility on the MMMs. The results demonstrated that the composite filler with fast transport pathways designed in this study was an effective strategy to enhance gas separation performance of MMMs, verifying its application potential in the gas purification industry.

Rational Design of Halloysite Surface Chemistry for High Performance Nanotube-Thin Film Nanocomposite Gas Separation Membranes

The interfacial region has a critical role in determining the gas separation properties of nanofiller-containing membranes. However, the effects of surface chemistry of nanofillers on gas separation performance of thin film nanocomposite (TFN) membranes, prepared by the interfacial polymerization method, have been rarely studied in depth. In this work, pristine and three differently surface-modified halloysite nanotubes (HNTs), by non- (SHNT), moderately (ASHNT), or highly CO₂-philic (SFHNT) agents, are embedded in the polyamide top layer of thin film nanocomposite (TFN) membranes for CO₂/N₂ and CO₂/CH₄ separations. Trimethoxyoctyl silane, 3-(2-aminoethylaminopropyl)trimethoxysilane, and poly(styrenesulfonic acid) are used as modifying agents to quantitatively investigate the effects of interfacial interactions between the polyamide and HNTs on the gas permeation of TFNs. This allows us to provide an interfacial design strategy to fabricate high-performance gas separation membranes. Pure gas permeations conducted on the TFNs at the feed gas pressure of 10 bar showed that CO₂ permeance and CO₂/N₂ and CO₂/CH₄ selectivities were increased by 145%, 130%, and 108%, respectively, after addition of 0.05 w/v% of sulfonated HNTs. The experimental gas permeations through all TFNs/HNTs, except TFNs/SFHNTs, agree well with predictions of a recently developed model, which suggests the importance of considering the neglected role of CO₂ interactions with the HNT/polyamide interface in the model. These results unambiguously proved that designing the interfacial layer thickness in the nanotube-containing membranes is an effective approach to tuning the gas separation properties. The results show that the dispersion of HNTs in the polyamide top layer and the experimental CO₂/gas selectivity was increased with increasing interfacial thickness, a_int, upon surface modification. Moreover, it is quantitatively demonstrated that the thickness of the interfacial layer between the filler and polymer matrix is a function of gas pressure applied on the membrane.

Critical Role of the Molecular Interface in Double-Layered Pebax-1657/PDMS Nanomembranes for Highly Efficient CO2/N2 Gas Separation

In this work, we deposited a CO₂-selective block copolymer, Pebax-1657, as a selective layer with a thickness of 2-20 nm on the oxygen plasma-activated surface of poly(dimethylsiloxane) (PDMS) used as a gutter layer (thickness ∼400 nm). This double-layered structure was subsequently transferred onto the polyacrylonitrile (PAN) microporous support and studied for CO₂/N₂ separation. The effect of interfacial molecular arrangements between the selective and gutter layers on CO₂ permeance and selectivity has been investigated. We have revealed that the gas permeance and selectivity do not follow the conventional theoretical predictions for the multilayer membrane (resistance in series transport model); specifically, more selective CO₂/N₂ separation membranes were achieved with ultrathin selective layers. Detailed characterization of the chemical structure of the outermost membrane surface suggests that nanoscale blending of the ultrathin Pebax-1657 layer with O₂ plasma-activated PDMS chains on the surface takes place. This nanoblending at the interface between the selective and gutter layers played a critical role in enhancing the CO₂/N₂ selectivity. CO₂ permeances in the developed thin-film composite membranes (TFCM) were between 1200 and 3500 gas permeance units (GPU) and the respective CO₂/N₂ selectivities were between 72 and 23, providing the gas separation performance suitable for CO₂ capture in postcombustion processes. This interpenetrating polymer interface enhanced the overall selectivity of the membrane significantly, exceeding the separation ability of the pristine Pebax-1657 polymer.

Exploration of the Synergy Between 2D Nanosheets and a Non-2D Filler in Mixed Matrix Membranes for Gas Separation

Dual-filler MMMs have attracted special interests in recent years because of the possibility of producing synergetic effect. This study is aimed at exploring the underlying synergy between two-dimensional (2D) nanosheets and a non-2D filler in mixed matrix membranes for gas separation. MXene or graphene oxide (GO) as typical nanosheet filler is selected to be in pair with a non-2D filler, SiO₂ or halloysite nanotubes (HNTs), with Pebax as the polymer matrix. In this way, four pairs of binary fillers are designed and the corresponding four groups of MMMs are fabricated. By tuning the mass ratio of binary fillers, synergetic effect is found for each group of MMMs. However, the two 2D fillers found different preferential non-2D partners. GO works better with HNTs than SiO₂, while MXene prefers SiO₂ to HNTs. To be noted, GO/HNTs renders the membranes the maximum enhancement of CO₂ permeability (153%) and CO₂/N₂ selectivity (72%) compared to Pebax control membrane, while each of them as single filler only brought about very limited enhancement of CO₂ separation performance. The possible mechanisms are thoroughly discussed in terms of filler dispersion, nanosheet flexibility, and the tortuosity and connectivity of the surface diffusion pathways along nanosheets.

Ultrathin Membranes with a Polymer/Nanofiber Interpenetrated Structure for High-Efficiency Liquid Separations

Ultrathin-film composite membranes comprising an ultrathin polymeric active layer have been extensively explored in gas separation applications benefiting from their extraordinary permeation flux for high-throughput separation. However, the practical realization of an ultrathin active layer in liquid separations is still impeded by the trade-off effect between the membrane thickness (permeation flux) and structural stability (separation factor). Herein, we report a general multiple and alternate spin-coating strategy, collaborating with the interface-decoration layer of copper hydroxide nanofibers (CHNs), to obtain ultrathin and robust polymer-based membranes for high-performance liquid separations. The structural stability arises from the poly(dimethylsiloxane) (PDMS)/CHN interpenetrated structure, which confers the synergistic effect between PDMS and CHNs to concurrently resist PDMS swelling and avoid CHNs from collapsing, while the ultrathin thickness is enabled by the sub-10 nm pore size of the CHN layer, the rapid cross-linking reaction during spin-coating, and the small thickness of the CHN layer. As a result, the as-prepared membrane possesses an exceptional butanol/water separation performance with a flux of 6.18 kg/(m² h) and a separation factor of 31, far exceeding the state-of-the-art polymer membranes. The strategy delineated in this work provides a straightforward method for the design of ultrathin and structurally stable polymer membranes, holding great potential for the practical application of high-efficiency separations.

A facile one-step grafting of polyphosphonium onto halloysite nanotubes initiated by Ce(iv)

Polyphosphonium was facilely grafted onto HNTs in an aqueous phase by a one-step method initiated by Ce(iv) at a mild temperature.