Porous Substructure Research Articles

Preparation, characterization, and fouling analysis of PVC/ND-PEG ultrafiltration membranes for whey separation

In this work, polyvinyl chloride (PVC) membranes were embedded with nanodiamond-Polyethylene glycol (ND-PEG) nanoparticles. Fabricated membranes were characterized by FTIR, FESEM, AFM, pore size distribution, contact angle, porosity, pure water flux (PWF), and tensile strength. Successful functionalization of ND nanoparticles by PEG was proved by the FTIR test. According to the FESEM and pore size distribution results, there was an increase in the quantity of surface pores, a shift toward smaller pores, and larger macro-voids in the porous substructure of the nanocomposite membranes as the ND-PEG content in the dope solution increased. The decrease water contact angle from 79.8° for neat PVC membrane to 59.5° for 2.00 wt% ND-PEG containing membrane along with the increase of porosity from 53.89 % to 80.80 % respectively showed that the hydrophilicity of the nanocomposite membranes was improved by increasing nanoparticles content, and this phenome along with surface roughness increment caused PWF improvement from 206.8 LMH to 266.4 LMH. In addition, due to the uniform dispersion of ND-PEG nanoparticles and the high mechanical strength of NDs, the tensile strength of nanocomposite membranes reached a maximum value of 2.91 Mpa for 1.5 wt% ND-PEG membrane. The performance results showed that PVC/ND-PEG nanocomposite membranes had acceptable whey rejection along with a significant increment in flux recovery ratio. Moreover, PVC/ND-PEG membranes with 2.00 wt% ND-PEG had lower flux reduction in long-term filtration, improved antifouling properties and more turbidity removal efficiency in comparison with neat PVC membranes. The analysis of the fouling mechanism based on Hermia's model revealed that the cake formation is the prevailing mechanism for all fabricated membranes.

Ethanol-regulated gelation of chitosan dissolved in an alkali/urea aqueous solution: Morphological characteristics for capturing particulate species

Gelation of chitosan dissolved in an alkali/urea aqueous solution promotes the mechanistic diversity of forming a membrane via aqueous phase separation (APS), which is conceptualized to highlight the advantages of using environmentally friendly solvents. When aimed at deepening the knowledge of regulating the structural formation, the alkaline dope of chitosan was employed in this study to investigate how the presence of ethanol in the coagulant could regulate the gelation behavior in APS and thereby change the particle-trapping mechanism. Both the gelling films and the particle-filtering membranes were in-situ characterized by optical coherence tomography (OCT) to compare the film-formation kinetics and the morphological variations in the resulting porous substructures, respectively. It was revealed that the ethanol-containing coagulant could induce the shrinkage of the gelling film and thereby favor the transverse coalescence of polymer-rich phases; that is, increasing the content of ethanol in the coagulant (over a range less than a critical value to avoid film deformation) would give rise to more tortuous substructures (for effectively capturing particles) primarily owing to the lower connectivity in the transverse direction. The theoretical analysis was supported by numerically evaluating the topological analog. This study not only confirms effectiveness of the ethanol-based regulation, but also establishes a paradigm for fabricating membranes with tailored topological characteristics.

Evaluating 3D-printed bioseparation structures using multi-length scale tomography

X-ray computed tomography was applied in imaging 3D-printed gyroids used for bioseparation in order to visualize and characterize structures from the entire geometry down to individual nanopores. Methacrylate prints were fabricated with feature sizes of 500 µm, 300 µm, and 200 µm, with the material phase exhibiting a porous substructure in all cases. Two X-ray scanners achieved pixel sizes from 5 µm to 16 nm to produce digital representations of samples across multiple length scales as the basis for geometric analysis and flow simulation. At the gyroid scale, imaged samples were visually compared to the original computed-aided designs to analyze printing fidelity across all feature sizes. An individual 500 µm feature, part of the overall gyroid structure, was compared and overlaid between design and imaged volumes, identifying individual printed layers. Internal subvolumes of all feature sizes were segmented into material and void phases for permeable flow analysis. Small pieces of 3D-printed material were optimized for nanotomographic imaging at a pixel size of 63 nm, with all three gyroid samples exhibiting similar geometric characteristics when measured. An average porosity of 45% was obtained that was within the expected design range, and a tortuosity factor of 2.52 was measured. Applying a voidage network map enabled the size, location, and connectivity of pores to be identified, obtaining an average pore size of 793 nm. Using Avizo XLAB at a bulk diffusivity of 7.00 × 10−11 m2s−1 resulted in a simulated material diffusivity of 2.17 × 10−11 m2s−1 ± 0.16 × 10−11 m2s−1.Graphical abstract

Modeling Microstructure Formation in Block Copolymer Membranes Using Dynamical Self-Consistent Field Theory.

Block copolymers have attracted recent interest as candidate materials for ultrafiltration membranes, due to their ability to form isoporous integral-asymmetric membranes by the combined processes of self-assembly and nonsolvent-induced phase separation (SNIPS). However, the dependence of surface layer and substructure morphologies on the processing variables associated with SNIPS is not well understood nor is the interplay between microphase and macrophase separation in block copolymers undergoing such coagulation. Here, we use dynamical self-consistent field theory to simulate the microstructure evolution of block copolymer films during SNIPS and find that such films form the desired sponge-like asymmetric porous substructure only if the solvent and nonsolvent have opposite block selectivities and that otherwise they form a dense nonporous microphase-separated film. Our results could have important implications for the choices of solvent and nonsolvent in the processing of block copolymer membranes.

Design, fabrication, and physicochemical characterization of polyamideimide membranes containing ZIF-8 and CMS particles for potential gas separation applications

The development of hybrid membrane materials with excellent permeance and selectivity applied for gas separation has been the focus of world attention. However, the development of membranes with efficient separation performance remains a great challenge.The ZIF-8 nanocrystals were prepared by a precipitation reaction with Zinc metal cluster and 2-methylimidazole, whereas the carbon molecular sieves (CMS) were synthesized by pyrolysis of polyamideimide (PAI) polymer. The successful formation of ZIF-8 nanocrystals was evident from the rhombic octahedron shape, and EDX confirms the presence of Zn metal cluster and methylimidazole linker. The ZIF-8 and CMS nanoparticles incorporated PAI membranes were prepared using the phase inversion technique with varying loading wt.% of 1, 2, and 3%. PAI/ZIF-8 and PAI/CMS membranes cross-sectional morphology confirmed that synthesized nanoparticles were well embedded through the PAI membrane. The PAI/ZIF-8 (3 %) membrane, thin dense skin top layer, and well-defined honeycomb porous substructure were observed. It was noticed that the PAI/ZIF-8 (1 wt%) membrane had shown endurance for ultimate strain up to 8.25 % which is comparatively higher than the PAI/CMS membranes. However, both the ZIF-8 and CMS membranes had shown a decrease in the mechanical stability at the higher filler loading of 3 wt% which could be due to the increased brittleness due to the aggregation of filler particles at the higher loading. Thus, the inclusion of ZIF-8 and CMS particles in the PAI matrix positively altered the physicochemical properties of the resulting hybrid membranes, which could help them achieve remarkable gas permeance and selectivity.

One-step preparation of bilayered films from kraft lignin and cellulose acetate to mimic tree bark

This contribution presents the development of a dry-cast method for the one-step preparation of bio-based films from wood polymers that mimic the bilayered structure of tree bark, the natural protective layer of the tree. In a simplified view, natural bark can be considered as the superposition of an external homogeneous and non-porous layer (outer bark) and a porous substructure layer (inner bark). This work is a first step for the future development of bio-based biomimetic wood coatings. The film had a bark-like appearance and its total density, bulk density and porosity were similar to values measured in natural bark. Furthermore, the structural characteristics of the studied film, namely specific surface area (BET) and pore size distribution, as well as the performance of the water adsorption ability were investigated and discussed.

Gas Separation Hollow Fiber Membranes: Processing Conditions for Manipulating Morphology and Performance

Currently, membrane unit operations are widely applied at industrial level, replacing conventional separation systems. Membrane gas separation represents a successful case, with an increasing number of installed plants in chemical processes, petrochemical plants and refineries for the production of nitrogen from air, the hydrogen separation and recovery and the carbon dioxide separation from natural gas. Owing to the low space footprint and low energy consumption, membrane separation is an environmental friendly technique that meets the Process Intensification requirements.The present study concentrates on the hollow fiber (HF) configuration that is the most used in applications of industrial interest. HF modules, having a high membrane packing density, are compact devices with thousands of square meters of membrane area per unit of volume.The required membranes have an asymmetric structure, in which a thin dense layer performs the separation and a porous substructure provides the needed mechanical resistance. The manipulation of the HFs morphology, according to a dry-wet spinning process in a pilot plant apparatus, investigating the effect of the operating conditions adopted for the spinning on the membrane performance and microstructure, is experi-mentally examined. Commercially available glassy polymers are used to make a comparison of a conventional double orifice spinneret with a triple orifice spinneret. The prepared HF batches are characterized by means of a morphological characterization (SEM analysis) and gas permeation rate measurements.

Rigid double-stranded siloxane-induced high-flux carbon molecular sieve hollow fiber membranes for CO2/CH4 separation

Carbon molecular sieve (CMS) membranes are a promising candidate for natural gas processing due to their peculiar pore structure-induced excellent separation performance. Formulating ultrathin, defect-free CMS hollow fiber membranes is, however, still challenging due to damage on porous sub-structures induced by thermal relaxation of polymer chains during pyrolysis. Herein, we report a new methodology enabling high separation performance and good plasticization resistance in CMS fiber membranes by uniform integration of double-stranded polysilsesquioxanes into the polyimide matrix. Our polyimide/ladder-structured polysilsesquioxane CMS fibers substantially enhanced CO2 permeance by as much as 546% compared to the precursor fiber analogues due to the thin molecular sieve selective layer. Also, poly(dimethylsiloxane) coating delayed physical aging, still showing a high CO2 permeance of 354 GPU with CO2/CH4 selectivity of 56 after 72 days of aging. Furthermore, they exhibited excellent plasticization resistance up to a CO2 partial pressure of 13.2 bar with CO2/CH4 separation factor of 74 for an equimolar CO2/CH4 feed mixture.

Preparation of High-Flux Nanoporous Solvent Resistant Polyacrylonitrile Membrane with Potential Fractionation of Dyes and Na2SO4

In this study, a high-flux organic solvent nanofiltration (OSN) membrane with the permeance of 66.7 Lm–2 h–1 bar–1 in methanol and 38.0 Lm–2 h–1 bar–1 in ethanol was successfully prepared from nanoporous PAN by phase inversion and hydrolyzation in sodium hydroxide solution. The polymer concentration and hydrolysis time were varied to control the final morphology and performance. The ternary phase diagram and viscosity measurements were used to describe precipitation thermodynamics and kinetics of the phase inversion process. The prepared membranes had a typical asymmetric structure, which could be observed from images of the cross section, comprising a dense skin layer and a porous substructure in the sublayer. It was found that the polymer concentration has an apparent influence on the morphology, selectivity and permeability of the prepared membranes. FTIR analysis, ζ-potential and water contact angle measurements confirm a molecular chain rearrangement as hydrogen bonds were formed between CONH2 and CO...

Composite Membrane Formation by Combination of Reaction‐Induced and Nonsolvent‐Induced Phase Separation

A novel method of preparing skinned asymmetric membranes with two distinctive layers is described: a top layer composed of chemically cross‐linked polymer chains (dense layer) and a bottom layer of non‐cross‐linked polymer chains (porous substructure). The method consists of two simple steps that are compatible with industrial membrane fabrication facilities. Unlike conventional processes to prepare asymmetric membranes, with this approach it is possible to finely control the structure and functionalities of the final membrane. The thickness of the dense layer can be easily controlled over several orders of magnitude and targeted functional groups can be readily incorporated in it. image

Dehydration of industrial isopropanol (IPA) waste by pervaporation and vapor permeation membranes

ABSTRACTWe have investigated the feasibility of using polymeric membranes for the dehydration of industrial isopropanol (IPA) waste from MSD International GmbH by pervaporation and vapor permeation in this study. Four types of membranes were employed. They were (1) thin‐film composite hollow fiber consisting of a polyamide selective layer and a polyetherimide Ultem® substrate, (2) dual‐layer hollow fiber comprising an outer cellulose triacetate (CTA) layer and an inner Ultem® substrate, and (3) two commercially available Sulzer flat‐sheet membranes. The dual‐layer hollow fiber shows the most balanced separation performance in terms of flux and separation factor for the dehydration of industrial IPA waste because it has a porous substructure and a thin dense selective layer. Under vapor permeation tests at 125 °C, it has a flux of 13.41 kg/m2 h and a separation factor of 1332. This study demonstrates the great potential of using polymeric membranes to dehydrate industrial IPA waste for pharmaceutical and electronic industries. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45086.

Influence of Solvent on the Structure of an Amphiphilic Block Copolymer in Solution and in Formation of an Integral Asymmetric Membrane.

Nanoporous membranes with tailored size pores and multifunctionality derived from self-assembled block copolymers attract growing interest in ultrafiltration. The influence of the structure of block copolymer in the membrane casting solution on the formation of integral asymmetric isoporous block copolymer membranes using the nonsolvent induced phase separation process (NIPS) has been one of the long-standing questions in this research area. In this work we studied the principal role of the solvent on the micellization and self-assembly of asymmetric polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymers by using a combination of dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS). Our results indicate a significant effect of the solvent selectivity on the optimal casting concentration and solution structure. In addition, morphological characterization of the resulting membranes demonstrates considerable influence of the solvent system on the ordering and uniformity of the pores and pore characteristics in the separation layer as well as porous substructure of the final membranes.

Electron Transport in Quasi‐Two‐Dimensional Porous Network of Titania Nanoparticles, Incorporating Electrical and Optical Advantages in Dye‐Sensitized Solar Cells

The integration of fast electron transport and large effective surface area is critical to attaining higher gains in the nanostructured photovoltaic devices. Here, we report facilitated electron transport in the quasi-two-dimensional (Q2D) porous TiO2 . Liquid electrolyte dye-sensitized solar cells were prepared by utilizing photoanodes based on the Q2D porous substructures. Due to electron confinement in a microscale porous medium, directional diffusion toward collecting electrode is induced into the electron transport. Our measurements based on the photocurrent and photovoltage time-of-flight transients show that at higher Fermi levels, the electron diffusion coefficient in the Q2D porous TiO2 is about one order of magnitude higher when compared with the conventional layer of porous TiO2 . The results show that microstructuring of the porous TiO2 leads to an approximately threefold improvement in the electron diffusion length. Such a modification may considerably affects the electrical functionality of moderate or low performance dye-sensitized solar cells for which the internal gain or collection efficiency is typically low.

Highly Hydrophobic, Compressible, and Magnetic Polystyrene/Fe3O4/Graphene Aerogel Composite for Oil–Water Separation

A solvothermal technique to process novel graphene aerogel/Fe3O4/polystyrene composites with reticulated graphene structure is described. Porous Fe3O4 nanoparticles as a partial substitute for ethylenediamine assisted cross-linking and interconnections between graphene plates. Fe3O4 and polystyrene incorporation allowed a porous substructure to be formed on the reticulated graphene surfaces, effectively enhancing the hydrophobicity. The compressibility and directional recovery were achieved with the composite having an extremely low density of 0.005 g cm–3, corresponding to a volume porosity of 99.7%. The crude oil intake capacity for the composite was 40 times its own mass after 10 water–oil separation cycles, which is among the highest ever reported for oil absorbents. Furthermore, the incorporation of porous Fe3O4 nanoparticles enabled magnetism which, in combination with compressibility, allowed the oil-soaked graphene composite to be readily collected by a magnet and oil to be squeezed out before the...

Effects of Aluminosilicate Mineral Nano-Clay Fillers on Polysulfone Mixed Matrix Membrane for Carbon Dioxide Removal

Over the last few decades, significant improvements in the performance of polymeric membranes for gas separation have been made. Nevertheless, the existing membrane materials need to be enhanced in order to fully exploit their gas separation application opportunities on industrial scale. Mixed matrix membrane which combines the advantages of the highly selective fillers and desirable properties of the polymer matrix is explored to overcome the trade-off trend between gas permeability and selectivity. This paper describes the effect of aluminosilicate mineral nano-clay fillers on polysulfone (PSF) mixed matrix membrane. In this study, different types of montmorillonite (MMT) nano-clays were used as fillers to fabricate asymmetric flat sheet mixed matrix membrane (MMM) via dry-wet phase inversion method. The interaction between the type of organoclay and its loading is also studied. The synthesis of MMM required N-methyl-2-pyrrolidone (NMP) as solvent and deionized water as coagulant. The fabricated MMMs were characterized by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and field emission scanning electron microsropy (FESEM). Thin asymmetric membrane with dense skin layer and porous sub-structure was successfully fabricated. Results showed enhancement in MMM stability relative to neat PSF membrane. It is also found that higher nano-clay loading resulted in significant agglomeration, nevertheless, there appears to be improved adhesion between the polymer and nano-clay.

Impact of doping level on the metal assisted chemical etching of p-type silicon

The impact of doping level on the metal assisted chemical etching of p-type silicon using silver particles is investigated. 12 boron doped wafers with doping levels ranging from 1012 up to 1019cm−3 were used. Silver particles generated from a sputter-deposited thin film via a dewetting procedure act as catalyst in a HF:H2O2 etch mixture. The characteristics of the etch rate as a function of time was investigated, revealing in addition an enhanced material removal at the beginning (i.e. in the range of several seconds) and an undesired etching of silver free regions at longer etch times (i.e. >15min). Furthermore, a strong decrease in etch rate at doping levels exceeding 1016cm−3 is observed. Only at these highly doped samples, a porous sub-structure is detected located adjacent and in-between the pores which are generated by the Ag particles. It is assumed that the injected holes representing majority charge carriers in p-type silicon diffuse at higher doping levels faster and more efficiently away from the metal-semiconductor interface into the silicon sample resulting in a decreased etch rate.

Electrokinetics in undeveloped flows

For the correct interpretation of results of tangential electrokinetic measurements with porous materials, in particular, composite/asymmetric membranes on porous supports, it is necessary to have the data available for various channel heights. In some kinds of equipment, the variation of channel height is technically possible only for a range of relatively large heights. This communication shows that under these conditions, the fluid flow can become undeveloped and the conventional approaches to the interpretation of electrokinetic measurements should be modified accordingly. In particular, the dependence of streaming-current coefficient on the channel height becomes sub-linear. If the experimental data are available only for larger channel heights, this can be mistakenly taken for the manifestation of contribution of porous sub-structure to the streaming current.In this communication, we investigate electrokinetic phenomena in undeveloped flows both numerically and experimentally. We confirm that the aforementioned sub-linearity occurs for nonporous as well as porous substrates. We also demonstrate that the channel heights estimated from the volume flow rate by using numerical simulations of undeveloped flows are in very good agreement with the reference values obtained from the electrical conductance (in contrast to the values estimated by using the conventional approach of Hagen–Poiseuille equation). The numerical fitting of channel-height dependences of streaming-current coefficient enables us to separate the contributions of external surface and porous sub-structure (in case of porous substrates) and obtain quite reasonable values of (effective) zeta-potentials in both cases. Nonetheless, the accuracy of experimental data deteriorates with increasing channel height, so it is generally advisable to vary the heights within a range below 100–150μm.

Influence of the blend composition on the properties and separation performance of novel solvent resistant polyphenylsulfone/polyimide nanofiltration membranes

In this study, the preparation of novel solvent resistant nanofiltration membranes made of polyphenylsulfone (Radel-R® – PPSU) and polyimide (Matrimid®(MI)) in blends with three different compositions of PPSU/MI (75/25, 50/50, 25/75wt%) in NMP was investigated. Scanning electron microscopy (SEM) was used to investigate the morphological characteristics and the structure of asymmetric membranes. The prepared membranes had a typical asymmetric structure with a dense skin layer and a porous substructure. Macro-voids were observed in the membrane substructures for all compositions. The permeation of different organic solvents was measured for all prepared membranes in dead-end filtration cells. The flux of organic solvents through these membranes was found to depend directly on the blend composition. The highest flux for alcohol and alkanes was achieved for a 50/50wt% blend while for acetone and methyl ethyl ketone the highest permeability was measured for a 75/25wt% blend of PPSU/MI. A higher permeability of alkyl acetates was observed for the membrane made of 25/75wt% PPSU/MI. Solvent stability of the membranes has been tested by measuring the methanol permeability and Sudan II rejection before and after solvent treatment. Except for pure PPSU, all membranes were stable in the examined solvents. The results confirm that the membranes prepared from these blends are promising to be used for organic solvent nanofiltration. For better insight into the phase behaviour of the PPSU/MI blend, dense reference membranes were prepared by solution casting and solvent evaporation, followed by pure liquid sorption, differential scanning calorimetry and gas permeation measurements.

XPS and contact angle characterization of surface modified cellulose acetate membranes by mixtures of PMHS/PDMS

A mixture of polymethylhydrosiloxane (PMHS) and polydimethylsiloxnae (PDMS) is used for modifying the surface of reverse osmosis and nanofiltration membranes. The porous substructure made of cellulose acetate (CA) was obtained by a phase separation process from acetone/formamide (2/1wt/wt) casting solutions. Two polymer concentrations (22 and 25wt.%) were used with a heat post-treatment temperature (80°C) in order to produce membranes with pore sizes responding to nanofiltration and reverse osmosis needs. Overcoating the CA substructure with a PMHS–PDMS thin layer gave rise to the composite membranes.Surface characterization with X-ray Photoelectron Spectroscopy (XPS) indicates the success of modifying the CA membrane surface with PMHS–PDMS. Contact angle measurements showed an increase in hydrophilicity of the modified membrane surface. Tensile strength measurements indicate that modification by a mixture of PMHS–PDMS has an insignificant decrease in Young's modulus. The pure water flux measurements reveal that layer modification by PMHS–PDMS show an improvement in water permeability which is in agreement with contact angle results.

Advanced electrokinetic characterization of composite porous membranes

The effect of the streaming current flowing through the porous structure of composite membranes during tangential electrokinetic measurements was investigated. It was shown that neglecting this additional path for streaming current may have dramatic implications in the interpretation of the experimental data and on the determination of the membrane zeta potential. Experimental measurements of both streaming current and electrical conductance were performed with two different composite polymer membranes. By following the procedure proposed by Yaroshchuk and Luxbacher, Langmuir 26 (2010) 10882–10889, in the present work it was possible to determine separately the zeta potential of the membrane surfaces and that of their underlying porous structures. This experimental procedure was shown to provide useful information on the functionalization of an ultrafiltration polyethersulfone membrane by positively charged 4-benzyltriphenylphosphonium groups. Notably we found that the chemical modification leads to a charge reversal (from negative to positive) of the porous substructure of the membrane while the overall charge of the external surface remains negative, although with diminished magnitude.