Free Volume Cavities Research Articles

Hierarchical microstructure changes of oriented poly(vinylidene fluoride) foam via supercritical carbon dioxide with increased piezoelectric performance

Light weight poly (vinylidene fluoride) (PVDF) has attracted wide attention for energy supply. In this work, oriented PVDF foams were prepared using supercritical carbon dioxide (CO2) physical foaming associated with a constraint mold. The corresponding micro-nanostructures and electrical performances were researched comprehensively. It reveals that the cell structure and molecular chains of PVDF foams are oriented due to the limitation of gas diffusion paths. Because of the realization of uniaxial stretching during cell growth, the β phase content is increased. Besides, after foaming, the regularity of the chain segments in amorphous region is improved, which is caused by the decrease in the quantity of free-volume cavities. The 1D-149 °C oriented PVDF foam exhibits a high voltage output (8 V), due to the ultralow dielectric constant (1.8) and high compressive strength. This study provides a theoretical foundation for the design of oriented PVDF foams, showing the potential of energy collector.

Bifunctional side-chains decorating a distorted poly(aryl ether sulfone) backbone to endow an anion exchange membrane with high perm-selectivity for chloride ions

Mono-valent selective ion-exchange membrane, as the core component of selective electrodialysis (SED), has received extensive attention. In this work, we report a mono-valent selective anion-exchange membrane (AEM), which was fabricated with distorted poly(aryl ether sulfone) and decorated with bifunctional side-chains providing anion exchange groups and hydrophobic segments. By tuning the length of the hydrophobic side-chains, the nanophase separation within AEMs was finely tuned, evidenced by an ionic cluster size of about 0.47 nm from SAXS analysis. The optimized AEM (PAES-TA-7 AEM) shows a very low water swelling ratio of 6.4 %, signaling good dimensional stability. The perm-selectivity (PSO42–Cl–) of PAES-TA-7 AEM during ED reached 107 at 2.5 mA∙cm−2 (ion flux: 2.69 × 10−8 mol cm−2 s−1 at 90 min) significantly exceeding that of the commercial Neosepta ACS (11; 2.08 × 10−8 mol cm−2 s−1). It is inferred that the ion channels, resulting from nanophase separation between the side-chains and the backbone, as well as the free volume cavity created by the distorted backbone, significantly contribute to the smooth transport of the Cl− ions through the AEM with less resistance. As a result, it is believed that this work offers a useful strategy to advance the monovalent anion-selective membranes.

Mechanism Insights in Anticorrosion Performance of Waterborne Epoxy Coatings Reinforced by PEI-Functionalized Boron Nitride Nanosheets.

Functionalized hexagonal boron nitride nanosheets (BNNSs) have arisen as compelling anticorrosive additives, yet the precise mechanism of their corrosion resistance enhancement in coatings remains unclear. Here, polyethylenimine functionalized BNNSs (PEI-BNNSs) with approximately 6-11 layers were prepared through a "one-step" method. Then, the PEI-BNNSs/Waterborne epoxy (WEP) composite coatings were incorporated via the waterborne latex blending method for the anticorrosion of the Q235 substrate. The impedance modulus (|Z|f = 0.01 Hz) of 0.5 wt % PEI-BNNSs/WEP composite coating soaked in 3.5 wt % NaCl solution for 35 days increased by 4 orders of magnitude compared to pure WEP coating, exhibiting exceptional long-term resistance against corrosion. The positron annihilation lifetime spectroscopy and corrosion product analysis demonstrated that the reinforced anticorrosion capabilities are not solely ascribed to the "tortuous path effect" arising from BNNSs impermeability. These mechanisms also encompass the reduction in free volume fraction and radius of the free volume cavities within the composite coating brought about by the PEI molecules. Additionally, the increase in coating adhesion, promoted by PEI, plays an important role in augmenting the barrier properties against corrosive agents. This study provided a full comprehension of the role played by functionalized BNNSs in fortifying the anticorrosion attributes of WEP coatings.

Adjustable free volume of polyamide membrane medicated by confined organic alkane molecules for efficient hydrogen separation

Rational design of polyamide cross-linked network is of vital significance for fabricating advanced separation membranes. Herein, polyamide membranes with tunable free volume cavity are constructed utilizing an in-situ confinement strategy during interfacial polymerization process, where organic alkane molecules with different alkyl chain such as n-octane, n-dodecane, and n-hexadecane are confined in stiff polyamide skeleton and entangled with nearby polyamide chains. The efficient confinement of organic alkane molecules is confirmed by thermogravimetric coupled with mass spectrometry analysis, and corresponding variations in chain architecture of polyamide network are verified by X-ray diffraction and positron annihilation lifetime spectroscopy techniques. Upon thermal stimulation, the inherent flexibility of confined organic alkane molecules along with stiffness of polyamide skeleton, allow enlargement of chain d-spacing and dynamic regulation of membrane microstructure for fascinating H2 permeation separation. The optimized modified membrane exhibits a H2 permeance of 116.1 GPU and a H2/CH4 selectivity of 70.8, which can be stably operated at 100 °C for over 400 h. This study provides a deep understanding of polyamide membranes with confinement of organic molecules and thermally induced separation performances, advancing the design of novel membranes with promising H2 separation performances at elevated temperatures.

Molecular-Level Control over Oxygen Transport and Catalyst–Ionomer Interaction by Designing Cis–Trans Isomeric Ionomers

We here demonstrate a monographic study to investigate the effects of cis–trans conformation on the properties of anion exchange ionomers for the first time. Twisted cis-conformation possessed of abundant subnanoscale free volume cavities facilitates oxygen permeability and water adsorption. Moreover, its nonplanar conformation can alleviate the undesirable absorption of ionomer on Pt and Pt–Ru surfaces, thus improving the electrochemical performance of the catalyst layers. Theoretical and electrochemical investigations are mutual corroborations, demonstrating our proof-of-concept. The catalyst layer equips cis-ionomer showing 51% and 54% increases in limiting current density and electrochemically active surface area. The H2/O2 single-cell peak power density of the cis-membrane electrode assembly (c-MEA) is increased by 56% (1.0 vs 0.64 W cm–2) compared with that of the t-MEA at 100% relative humidity (RH). Moreover, the performance of c-MEA is particularly prominent at low humidity (increased by 114% and 113% at 80% and 60% RH, respectively).

Toughening mechanism of phthalonitrile polymer: MD simulation and experiment

Based on the demand for materials in the aerospace field, we are researching the brittleness problem of phthalonitrile resin (PN75), a high-temperature resistant resin material. Combining molecular dynamics simulations with experiments in time-consuming and costly studies has become a meaningful way to reduce research and development cycle time and cost. In a world where high-performance computer simulations are capable of enormous computational scales with guaranteed reliability, experiments and characterization at the atomic-molecular scale are still challenging tasks. Revealing microscale mechanisms through atomic simulations and providing guidance for laborious, time-consuming, and expensive experiments is one of the current scientific hotspots. The free volume pore distribution of PN75 was analyzed using molecular dynamics (MD) simulations to reveal the primary mechanism of PN75 during toughening. Improving the scalability of the free volume cavity of PN75 creates an appreciable energy dissipation mechanism. The combination of experimental and MD simulation results verifies the usefulness of simulation as a guide for experiments. MD simulations can reduce complex experimental and characterization efforts, reveal experimental mechanisms at the nanoscale, and guide the design of polymers.

Impact of humidity on gas transport in polybenzimidazole membranes

Polybenzimidazoles (PBIs) are promising materials for high temperature H2/CO2 separation in applications such as steam reforming and pre-combustion carbon capture where significant amounts of water are often present. However, PBIs are hydrophilic, and the impact of humidity on PBI gas separation properties is relatively unexplored. Furthermore, opportunity exists to elucidate the interplay between plasticization, free volume, and gas transport in glassy polymer membranes such as PBIs. This study investigates the effect of humidity on H2, O2, and CO2 permeabilities at 35 °C in a commercial PBI and two sulfone-containing PBIs. Water uptake significantly reduces PBI gas permeabilities at low humidities due to competitive sorption and antiplasticization. At high humidities, plasticization increases the permeabilities of larger gases in more hydrophilic PBIs. Effective fractional free volumes evaluated from gas permeation data and previously reported water sorption and dilation data suggest water plasticizes PBIs by increasing accessible free volume via enhanced molecular dynamics rather than by creating new free volume cavities.

Studies on ion dynamics of polymerized ionic liquids through the free volume theory

Over the last decade, the volume has been envisioned as a critical variable controlling the dynamics of glass-forming systems in general, and charge transport of ionic conductors in particular. Herein, we use theoretical and experimental approaches to investigate the volume effects in a specific 1,2,3-triazolium-based poly(ionic liquid) (PIL) at both the macro and nano scales. The size of the local free volume cavity in the studied system was determined by positron annihilation lifetime spectroscopy (PALS). On the other hand, the theoretical approach enabled us to estimate the hard-core volume (Vhc) and thus free volume (Vf) in the studied PIL. Since the experimental value determined for Vf is relatively large it can be considered as the main factor controlling the superionic properties of this material. Furthermore, the analysis of specific volume and relaxation dynamics in T-P plane in terms of the locally correlated lattice (LCL) and cooperative free volume (CFV) models shows that none of them works for the studied system.

Optimizing microstructure of polymer composite membranes by tailoring different ionic liquids to accelerate CO2 transport

This study researched a strategy to optimize the microstructure of polymer composite membranes by designing and tailoring different anions and cations for ionic liquids (ILs). A series of ILs containing the 1-alkyl-3-methylimidazolium cation ([CnMIM]+) and anion of tetrafluotoborate ([BF4]−), acetate ([Ac]−) or glycine ([Gly]−) were incorporated into a Pebax® MH 1657 (Pebax) matrix to fabricate polymer composite membranes to accelerate CO2 transport. The [Gly]− anion and [C6MIM]+ cation gave the membranes the optimal microstructure, which had free volume cavities with a favorable radius (0.338 nm) only for CO2 molecule transport, rendering a precise molecular sieving ability. Additionally, the free amino acid anions of IL in the polymer composite membranes contained abundant amino and carboxyl groups, which served as mobile CO2 carriers, facilitating and accelerating highly efficient CO2 transport. The resultant membranes containing 20 wt% [C6MIM][Gly] showed CO2 permeability of 1490 ± 30 Barrer. This study may stimulate research on optimizing polymer microstructures by tailoring the types of ILs.

Insight into the spontaneous breakdown of ‘Toughened glass’: From nano- to macroscale

Instances of sudden breakdown of ‘toughened glass’ (TG) without apparent causes have grave consequences. Here we present results of investigations on two types of samples – pieces of a TG door that broke down spontaneously (STG) and pieces of a TG sheet broken down intentionally (ITG). In both cases, the broken pieces have the same morphology and were studied through X-ray Diffraction (XRD), X-ray Fluorescence Spectroscopy (XRF), X-ray Absorption Near-edge Spectroscopy (XANES) at Na K-edge, Fourier Transform Infrared Spectroscopy (FTIR), Twin Mercury Porosimetry (TMP), Atomic Pair Distribution Function (PDF) and Positron Annihilation Spectroscopy (PAS). XRD and PAS showed the same structure and high defect concentration for both the samples indicating no major difference in microstructure due to aging in STG. No trace of Ni or S could be observed from XRF, casting doubt on the presence of NiS inclusions. PAS studies indicated very large concentration of free volume cavities in the STG sample, implying that mechanical instability leading to the erosion of the structure and loss of rigidity is a result of atomic motions within the glass and the associated weakening of the molecular bonds. From XRF, a small depletion in Na content was seen in STG, which became much more apparent from low angle XANES, pointing to a surface depletion. From FTIR, a reduction in the Si–O–Si bonds and an enhancement in the O–H bonds could be identified indicating a conversion of the former to Si(OH)4. The bond distances of different pair of atoms were calculated from atomic Pair Distribution Function experiments. An increase in total porosity especially in larger pores (diameter ≥100–200 µm, from TMP) were observed in STG over ITG. Based on these results, we propose a simple model in which the Si–O network is visualized as suffering from destabilization due to the leaching out of the Na-ions from the STG over time, leading to the accumulation of large pores, increase of overall porosity and the final collapse of the structure.

An Insight into the Spontaneous Breakdown of 'Toughened Glass' from Nano- to Macroscale

Sudden breakdown of 'toughened glass' (TG) without apparent causes have grave consequences. We present results of investigations on two samples - pieces of a TG door that broke down spontaneously (STG) and pieces of a TG sheet broken down intentionally (ITG). Both samples have same morphology and X-ray Diffraction showed the same structure for both samples indicating no major difference in microstructure. No trace of Ni or S could be observed from X-ray Fluorescence Spectroscopy whereas a small depletion in Na content was seen in STG, which became much more apparent from low angle X-ray Absorption Near-edge Spectroscopy at Na K-edge, pointing to a surface depletion. Positron Annihilation Spectroscopy studies indicated very large concentration of free volume cavities in STG, implying that the erosion of the structure and loss of rigidity is a result of atomic motions within the glass and the associated weakening of the molecular bonds. From Fourier Transform Infrared Spectroscopy, a reduction in the Si-O-Si bonds and an enhancement in the O-H bonds could be identified indicating a conversion of the former to Si(OH)_4. The bond distances of different pair of atoms were calculated from Atomic Pair Distribution Function experiments. An increase in total porosity especially in larger pores were observed in STG over ITG, from Twin Mercury Porosimetry. We propose a simple model in which the Si-O network is destabilized due to leaching out of Na-ions from STG over time, leading to accumulation of large pores, increase of overall porosity and final collapse of the structure.

Investigation of polysulfone film on high-performance anode with stabilized electrolyte/electrode interface for lithium batteries

Abstract Lithium metal has been considered to be the most promising anode material for the new generation of energy-storage system. However, challenges still stand in protecting lithium metal from spontaneous reactions with electrolytes and preventing the dendritic propagation, both of which would lead to undesirable decrease in Coulombic efficiency. Polysulfone (PSf) membrane with high rigidity and free-volume cavities of approximately 0.3 nm was employed to provide a stable interface on the surface of anodic electrode. The isotropic channels were constructed by the interconnected and uniformly distributed free volumes in the polymer matrix, and were expected to be swelled by solvent molecules and anions of lithium salt and to allow Li+ ions to pass through onto the electrode surface. As a result, dendrite-free morphology of deposited lithium was observed. The stabilized interface arose from the PSf film was verified by the promoted performances of Cu|Li cells and steady voltage polarization of Li|Li cells. The full cell with PSf coated anode exhibited excellent cyclability (85% capacity retention rate over 400 cycles at 1 C) and an outstanding rate capability (117 mAh g−1 at 5 C). The beneficial performances were further verified by the EIS results. This work provides a new strategic idea to settle the dendritic problems of Li metal anodes.

Thermal crosslinking of a novel membrane derived from phenolphthalein-based cardo poly(arylene ether ketone) to enhance CO2/CH4 separation performance and plasticization resistance

A novel gas separation membrane was fabricated from a commercially available polymer of phenolphthalein-based cardo poly (arylene ether ketone) (PEK-C). Thermal treatment was employed to improve the gas permeability and anti-plasticization property of PEK-C polymeric membrane via inducing interchain crosslinking. The changes of chemical structure, thermal crosslinking reaction, interchain distance, gas separation performance and anti-plasticization properties of PEK-C membranes were investigated by the FT-IR, XPS, TG-MS, XRD and gas permeation tests. Results show that the gas separation performance and anti-plasticization properties of PEK-C membranes are significantly enhanced after the thermal crosslinking, which is induced by the decomposition of the lactone rings in cardo moieties and crosslinking with the formation of biphenyl linkages. The dramatic enhancement in gas permeability for the crosslinked membrane is attributed to the enlargement of the interchain distance and free volume cavity, and the great improvement on the anti-plasticization property is due to the formation of a rigid crosslinked network structure. Compared to PEK-C polymeric membrane, the CO2 permeability of the crosslinked membrane increased by more than 110 times with an adequate CO2/CH4 selectivity, especially for the separation of CO2/CH4 mixed gas (50:50 mol%). The CO2 plasticization pressure substantially increased from 2 atm to the highest tested pressure of 30 atm. The gas separation performance of the crosslinked membrane surpassed the 2008 upper bound for CO2/CH4, exhibiting that the thermal crosslinking membrane derived from PEK-C membrane material is an attractive candidate for the natural gas purification.

Interface engineering of mixed matrix membrane via CO2-philic polymer brush functionalized graphene oxide nanosheets for efficient gas separation

A facile method to improve CO2 separation performance of polymer is to incorporate CO2-philic particles. In this study, sulfonated polymer brush functionalized graphene oxide nanosheets (S-GO) as fillers, with sulfonated polymer brush as CO2-philic chain anchored on GO were introduced into SPEEK membrane at nanoscale level to enhance CO2 selectivity of the membrane. The S-GO enhanced the interface compatibility as shown by SEM, and the “sieve-in-a-cage” morphology in MMMs disappeared by the introduction of CO2-philic polymer brush. Benefiting from the introduction of brush at the interface, on one hand, the CO2-philic brush on S-GO helped to widen CO2 transport pathways verified by the increased radius of free volume cavity (r3) and the fractional free volume (FFV), which resulted in the increase in gas permeability. On the other hand, the addition of the polymer brush on GO increased CO2-philic sites, which imparted enhancement in the CO2 solubility selectivity. Besides, the transport of gases through the GO nanosheets created the increment in CO2 diffusion selectivity. SPEEK/S-GO MMMs showed pronounced enhancements in both CO2 permeability and CO2/CH4(N2) selectivity. Specifically, for the SPEEK/S-GO membrane doped with 8 wt% S-GO nanosheets, the CO2 permeability reached 1327 Barrer, and CO2/CH4 selectivity increased by 179% in comparison with the SPEEK control membrane, surpassing the 2008 Robeson upper bound. The distinctive virtues of polymer brush functionalized filler may be extended to fabricate a series of MMMs for efficient molecule separation.

Enhanced pervaporation performance of PDMS membranes based on nano-sized Octa[(trimethoxysilyl)ethyl]-POSS as macro-crosslinker

A homogeneous distribution of inorganic fillers and their integration with polymer matrix is the primary challenge for advancement of mixed matrix membranes (MMMs) in pervaporation. Due to well-defined structure and multi-functional groups attached to the apex silicon atoms, polyhedral oligomeric silsesquioxane (POSS) provides a high possibility of good compatibility with diverse polymer matrix at the molecular level. In this work, soluble and reactive octa[(trimethoxysilyl)ethyl]-POSS(OPS) was synthesized, which was used as macro-crosslinker and reinforced porous fillers of polydimethylsiloxane (PDMS) synchronously to prepare a series of OPS/PDMS MMMs. The OPS/PDMS MMMs were characterized by FT-IR, XRD, SEM, TGA, DSC, PALS, vapor adsorption measurement and tensile test. It was found that OPS dispersed homogeneously in PDMS matrix and achieved good integration with PDMS based on SEM and DSC characterization. PALS results indicated that compared with pure PDMS membrane, OPS/PDMS MMMs with proper OPS loading showed much larger radii of free volume cavities (r3, r4) and fractional free volumes (fv3, fv4 and fv). The numerous hydrophobic POSS cages and enlarged PDMS’ free volume would effectively improve the diffusion selectivity towards ethanol. Vapor adsorption results showed that the chemical incorporation of moderate amount OPS into PDMS not only improved ethanol vapor adsorption amount, but also depressed water adsorption amount in PDMS, which would obviously improve PDMS’ sorption selectivity to ethanol. As applied to the bio-ethanol recovery from aqueous solutions, the prepared OPS/PDMS MMMs exhibited a simultaneous increase in permeation flux and separation factor, breaking the permeation flux-separation factor trade-off limitation. As OPS loading was 7.5 wt%, the separation factor of OPS/PDMS MMMs reached the maximum value of 16.4 (109% higher than that of pure PDMS membrane), with permeation flux of 253.1 g/m2h (32% higher than that of pure PDMS membrane). Experimental results showed that an increase in operation temperature resulted in an increase in permeation flux, but decrease in permeability, separation factor and selectivity. The pervaporation performance of OPS/PDMS MMMs kept stable during two days operation time. This work may provide useful insights of high-performance MMMs with a POSS embedded selective layer for biofuel separation.

The Impact of Plasticizer and Degree of Hydrolysis on Free Volume of Poly(vinyl alcohol) Films

The effect of plasticizer species and the degree of hydrolysis (DH) on the free volume properties of poly(vinyl alcohol) (PVA) were studied using positron annihilation lifetime spectroscopy. Both glycerol and propylene glycol caused an increase in the free volume cavity radius, although exhibited distinct plasticization behavior, with glycerol capable of occupying existing free volume cavities in the PVA to some extent. The influence of water, normally present in PVA film under atmospheric conditions, was also isolated. Water added significantly to the measured free volume cavity radius in both plasticized and pure PVA matrices. Differences in plasticization behavior can be attributed to the functionality of each plasticizing additive and its hydrogen bonding capability. The increase in cavity radii upon plasticizer loading shows a qualitative link between the free volume of voids and the corresponding reduction in Tg and crystallinity. Cavity radius decreases with increasing DH, due to PVA network tightening in the absence of acetate groups. This corresponds well with the higher Tg observed in the resin with the higher DH. DH was also shown to impact the plasticization of PVA with glycerol, indicating that the larger cavities—created by the weaker hydrogen bonding acetate groups—are capable of accommodating glycerol molecules with negligible effect on the cavity dimensions.

Properties of polyamide 612/poly(vinyl alcohol) blends and their impact on free volume and oxygen barrier properties

Oxygen transmission rates and free volume properties (i.e. average volumes of free-volume-cavities (Vf), mean number of the free volume cavities per unit volume (I3) and fractional free volume (Fv)) values of bio-based polyamide 612 (PA612)/poly(vinyl alcohol) (PVA) (i.e. PA612xPVA03y, PA612xPVA05y, PA612xPVA08y and PA612xPVA14y) blend films were reduced to a minimum value, when their PVA content reached corresponding optimal values of 25, 20, 15 and 10 wt%, respectively. The minimum oxygen transmission rate, Vf, I3 and Fv value obtained for the best PA61290PVA1410, PA61285PVA0815, PA61280PVA0520 and PA61275PVA0325 bio-based blown films reduced considerably with decreasing PVA degrees of polymerization. As evidenced by the results of dynamical mechanical analysis, differential scanning calorimetry, wide angle X-ray diffraction and Fourier transform infrared spectroscopic experiments, PA612 and PVA are miscible to some extent at the molecular level when their PVA contents are ≤ the corresponding optimal values. The significantly improved oxygen barrier and free volume properties for the PA612xPVAzy blend films with optimized compositions is at least in part to the enhanced intermolecular interactions between PA612 carbonyl groups and PVA hydroxyl groups.

Barrier properties and structure of liquid crystalline epoxy and its nanocomposites

There is ever increasing demand for a suitable sealant material to protect electronic devices, in particular, different displays. We present a new approach to generate a liquid processible material that possesses excellent barrier properties and flexibility that can be used for electronic encapsulation. The liquid processable material is based on liquid crystalline (LC) epoxy diglycidyl ether of 4, 4′-dihydroxy-α-methylstilbene (DGE-DHAMS or EDHAMS) and its nanocomposite comprising organo-modified montmorillonite clay. The best-achieved water vapor transmission rate is 4.5 g-mil/m2-day at ambient pressure, which is more than 20 times better than that of a conventional bisphenol A epoxy. The improved barrier performance comes from the reduction in the water diffusivity. The diffusivity in the liquid crystalline epoxy is 1.25 × 10−9 cm2/s, one order of magnitude lower than that of bisphenol A epoxy. The addition of 1 vol% of organoclay further reduces the diffusivity by 2 times. The cavity size of the free volume of the liquid crystalline epoxy is much smaller than that of conventional bisphenol A epoxy as measured by positron annihilation lifetime spectroscopy, suggesting that the improved barrier property originates from this reduced free volume cavity size. The free volume cavity size of the film with an addition of 1 vol% of an organically modified montmorillonite clay is similar to that of the liquid crystalline epoxy. We hypothesize that the additional barrier performance improvement by the nanoclay is due to its effect on the liquid crystalline phase morphology (reduced LC phase domain sizes) or the tortuosity effect of the high aspect ratio particles. The material also has excellent adhesion, optical transparency, and thermal stability.

Water-selective permeation in hybrid membrane incorporating multi-functional hollow ZIF-8 nanospheres

Incorporating metal-organic frameworks (MOFs) into polymer to fabricate hybrid membranes is an effective approach of breaking the trade-off between permeability and selectivity. In this study, hollow zeolitic imidazolate framework-8 (HZIF-8) nanospheres were prepared and blended into sodium alginate (SA) matrix to fabricate water-selective hybrid membrane, which intensify the diffusion process of water molecules dramatically owing to their well-designed hierarchical structures. The micropores on HZIF-8 nanosphere shell have a molecular sieving effect, which enhances diffusion selectivity of water molecules. The hollow structure of HZIF-8 nanospheres ensures the rapid diffusion of water molecules, thus elevating the permeation flux and separation factor simultaneously. Besides, the intervention effects of HZIF-8 nanospheres on the SA chain packing also contribute to the enhancement of permeation flux and separation factor by rendering the membrane more free volume cavities and suitable free volume cavity size for ethanol/water separation. When utilized to separate 90 wt% ethanol aqueous solution at 76 °C, the hybrid membrane containing 6 wt% HZIF-8 displays the highest separation performance with permeation flux of 2485 g/m2 h and separation factor of 1884, much superior to those for pure SA membrane and the hybrid membranes incorporating zeolitic imidazolate framework-8 (ZIF-8) nanocrystals. Desirable long-term stability of the hybrid membranes was also acquired.

Nanocomposite membranes based on alginate matrix and high loading of pegylated POSS for pervaporation dehydration

Fabrication of nanocomposite membranes with high nanoparticle loading is a significant challenge because of the serious agglomeration of nanoparticles at high concentration. In this study, poly(ethylene glycol)-functionalized polyoctahedral oligomeric silsesquioxanes (PEG@POSS) nanoparticles were incorporated into a sodium alginate (SA) matrix to fabricate nanocomposite membranes. A large loading of PEG@POSS up to 50wt% with a well dispersion in the membrane was achieved due to the good compatibility between PEG@POSS and alginate chains. Benefiting from the nanostructured size of PEG@POSS (1–3nm), the obtained nanocomposite membranes possessed very thin active layers of 180–200nm. A large quantity of hydrophilic PEG side chains enhanced the membrane surface hydrophilicity and offered massive water binding sites in the membranes. Homogeneously dispersed high content PEG@POSS nanoparticles increased the free volume of the nanocomposite membranes while maintaining a suitable free volume cavity size for water permeation. The optimum separation performance with a permeation flux of 2500gm−2h−1 and a separation factor of 1077 for dehydration of 90/10wt% ethanol/water feed was achieved. Additionally, the effects of operation temperature and feed concentration on separation performance were investigated systematically.