Isothermal Immersion Precipitation Research Articles

Nano-titania/polyethersulfone composite ultrafiltration membranes with optimized antifouling capacity

TiO2 nanoparticles synthesized via the sol–gel technique were used as an antifouling additive to produce polyethersulfone membranes by the isothermal immersion–precipitation method. In order to finely disperse the TiO2 in the dope, the sol–gel process for their formation involved DMAc as the solvent, same as that used for preparing the dopes. The pore size, water flux, tensile strength, and molecular weight cut-off of the membranes, as affected by the added amounts of TiO2 sol in the dope, exhibited an increase-then-decrease trend. All membranes demonstrated high rejections (≥ 92.3%) on filtration against BSA aqueous solutions. In the optimal case, nearly perfect rejection at the permeation flux of 91 LMH/bar was achieved for the membrane containing 1.68 wt% TiO2. This membrane also possessed the highest water flux (181 LMH/bar) and antifouling capability among all prepared membranes; specifically, the flux recovery ratio value remained as high as 94% even after three cycles of filtration-cleaning tests.

Effect of solvent on the dipole rotation of poly(vinylidene fluoride) during porous membrane formation by precipitation in alcohol baths

Microporous PVDF membranes were prepared by isothermal immersion precipitation of PVDF/DMSO and PVDF/TEP casting dopes in different alcohol baths. The formed membranes exhibited the so-called particulate morphology, being packed by interconnected crystalline particles of a similar size. For the alcohol/TEP/PVDF system, the particles appeared as sheaf-like spherulites consisting of curled lamellae with thickness of ∼15 nm. In contrast, for the alcohol/DMSO/PVDF system, the formed membranes comprise relatively compact globules whose surface was covered with granular objects of ca. 30 nm dia. Wide angle X-ray diffraction patterns of the membranes indicate that the sheaf-like crystals were of the α-type structure, while the compact globules the γ-type structure, as were confirmed by FTIR-ATR analyses. 19F NMR spectroscopy indicated that PVDF chains exhibited TG+TG− conformation in TEP and T3G+T3G− conformation in DMSO. These conformations remained unchanged during immersion-precipitation to yield the α- and γ-type crystal structures. The crystallinity of the membranes, as determined by X-ray diffraction and differential scanning calorimetry, was found to be ∼68%. Furthermore, desalination by membrane-distillation of 3.5% NaCl(aq) feed at 50 °C were carried to see the desalination capability of the membranes. High fluxes (10–13 LMH) and rejections (∼99.8%) were obtained for both types of membranes.

The effect of Tween-20 additive on the morphology and performance of PVDF membranes

The effects of additive, Tween-20, on the morphology and ultra-filtration performance of the PVDF membranes formed by isothermal immersion precipitation from the Tween-20/water/TEP/PVDF system were investigated. High resolution FESEM imaging indicates that the membranes׳ bulk constituted of interlinked crystalline particles, whose shapes change from more sheaf-like to more stick-like as larger amounts of Tween-20 were initially added into the dope. Introduction of Tween-20 caused formation of nano-pores on the top surfaces of the membranes and micron-sized columnar voids underneath the top skin layer. Some small amounts of Tween-20 may reside in the formed membrane if the nascent membrane has not been carefully washed. Presence of Tween-20 in the membrane was analyzed by water contact angle measurements, Fourier Transform Infrared-Attenuated total reflection spectroscopy (FTIR-ATR), Nuclear Magnetic Resonance (NMR), and X-ray Photoelectron Spectroscopy (XPS). Furthermore, water flux and blue dextran filtration experiments were carried out to illustrate the potential applications of the membranes in fine separation processes.

Preparation of the antifouling microfiltration membranes from poly(N,N-dimethylacrylamide) grafted poly(vinylidene fluoride) (PVDF) powder

Poly(vinylidene fluoride) (PVDF) powder is graft polymerized with N,N-dimethylacrylamide (DMAA) by a pre-irradiation induced graft polymerization technique. The existence of the graft chains in grafted PVDF (PVDF-g-PDMAA) powder has been proven by FT-IR spectroscopy and X-ray Photoelectron Spectroscopy (XPS) analysis. Then, the microfiltration (MF) membranes are prepared by isothermal immersion precipitation from PVDF-g-PDMAA powder in 1-methyl-2-pyrrolidone (NMP) solution from a water bath. The hydrophilicity of the MF membranes is determined by measuring the contact angles. The asymmetric morphology of the grafted membranes is studied by scanning electron microscopy (SEM), and water filtration properties are tested. The interaction between the membranes and proteins is studied by comparing the fluorescence microscopy images of these MF membranes cast from pristine PVDF and PVDF-g-PDMAA with a degree of grafting (DG) of 17.1% after surface fouling by Fluorescein Isothiocyanate (FITC)-conjugated Human Albumin solution. The antifouling property is determined by measuring the recovery percentage of pure water flux after the MF membranes have been fouled by bovine serum albumin (BSA) and lysozyme aqueous solution, separately. The results confirm that the existence of PDMAA graft chains improves the hydrophilicity and reduces protein adsorption of these MF membranes cast from PVDF-g-PDMAA powder.

Surface modification of microporous PVDF membranes for neuron culture

Abstract Microporous poly(vinylidene fluoride) (PVDF) membranes with either dense or porous surface were prepared by isothermal immersion–precipitation of a casting solution in coagulation baths of different strengths. Onto the membrane surface, amino acid ( l -lysine) or 1,6-hexanediamine (HMDA) was chemically immobilized. The membrane was first grafted with poly(glycidyl methacrylate) (PGMA) by means of plasma-induced free radical polymerization. Then, l -lysine or 1,6-hexanediamine was reacted with epoxy groups in the grafted PGMA to create an ECM environment suited to cell culture. Neuronal cells were cultivated on the formed pristine and surface-modified PVDF membranes. It is found that neurons tend to aggregate into large clusters with neuritic branches on pristine PVDF membranes, whereas for those cultured on the PGMA/PVDF membranes, serious aggregation takes place as well, but without neurites being formed. In contrast, neurons did not aggregate on the l -lysine or HMDA-immobilized membrane. It is also interesting to find that a neuritic network inter-connecting cells is constructed for the former, yet no neurite is observed for the latter membrane. Such phenomenon is thought to be associated with the extra –COOH group in l -lysine with respect to HMDA.

Membranes of Polyvinylidene Fluoride and PVDF Nanocomposites with Carbon Nanotubes via Immersion Precipitation

Microporous polyvinylidene fluoride (PVDF) and PVDF nanocomposite membranes were prepared via an isothermal immersion precipitation method using two different antisolvents (ethanol and water). The structure and morphology of the resulting membranes were investigated by wide angle X‐ray diffraction (WAXD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). The effects of the type of the antisolvent and the presence of multiwalled carbon nanotubes (MWNTs) on membrane morphology and the crystal structure developed within the membranes were studied. The crystallization of the PVDF upon immersion precipitation occurred predominantly in the α‐phase when water is used as the antisolvent or in the absence of the carbon nanotubes. On the other hand, β‐phase crystallization of the PVDF was promoted upon the use of ethanol as the antisolvent in conjunction with the incorporation of the MWNTs. The morphology and the total crystallinity of the PVDF membranes were also affected by the incorporation of the MWNTs and the antisolvent used, suggesting that the microstructure and the ultimate properties of the PVDF membranes can be engineered upon the judicious selection of crystallization conditions and the use of carbon nanotubes.

Preparation and characterization of microporous PVDF/PMMA composite membranes by phase inversion in water/DMSO solutions

PMMA/PVDF composite membranes were prepared by isothermal immersion–precipitation of dope solutions consisting of PMMA, PVDF, and DMSO into both harsh and soft nonsolvent baths. The effects of PMMA and DMSO contents on the membrane morphology, crystal structure, thermal behavior and tensile strength of the formed membrane were investigated. For a PMMA-free casting dope immersed in a harsh bath, such as pure water, the formed membrane exhibited a typical asymmetric morphology characterized by skin, finger-like macrovoids, and cellular pores. In contrast, when a soft 70% DMSO bath was adopted, PVDF crystallized to form a membrane packed by spherulitic globules. Incorporation of PMMA gave rise to interesting morphological features; e.g., PVDF globules were observed to adhere to the interlocked polymer branches coexisting with the continuous porous channels, as revealed by high resolution FESEM imaging. XPS analysis of the surfaces of the composite membranes suggested the occurrence of a surface segregation phenomenon, wherein PVDF preferentially migrated to the top surface region of the membrane such as to minimize the interfacial energy. XRD analyses indicated that PVDF crystallized into ‘α’ structure in both PVDF and PMMA/PVDF composite membranes. The crystallinity of the membranes was found to decrease with increasing PMMA content, which was confirmed by DSC thermal analyses. The latter results also indicated a significant decrease in membrane’s melting temperature as the PMMA content was increased. Tensile strengths of the membranes were improved by inclusion of PMMA in either harsh or soft baths. However, elongation at break showed a reversed trend.

Formation of porous poly(vinylidene fluoride) membranes with symmetric or asymmetric morphology by immersion precipitation in the water/TEP/PVDF system

The phase equilibrium boundaries of the membrane forming system, water/triethyl phosphate (TEP)/PVDF, at 25 °C were determined experimentally using cloud-point and equilibrium absorption methods. Based on the phase diagram, appropriate dope and bath compositions were selected to prepare microporous membranes by means of the isothermal immersion–precipitation technique. As a metastable casting dope with respect to crystallization was adopted, the formed membranes exhibited a uniform cross-section composed of interlocked crystal elements coexisting with the network of continuous pores, as was revealed by high resolution FESEM imaging. Morphologies of the membranes’ top surfaces were found to depend heavily on the bath strength, which was controlled by the TEP content. By changing the bath gradually from pure water to 70% TEP, the top surface evolved from a dense skin (asymmetric membrane) to a totally porous morphology (symmetric membrane). Wide angle X-ray diffraction analysis indicated that PVDF crystallized into α-type structure for all of the synthesized membranes. The crystallinity as determined from diffraction peak deconvolution was ≈65%, which value was confirmed by Differential Scanning Calorimetry (DSC). The obtained thermograms also showed a similar melting peak temperature ( T m ≈ 169 °C) for all membranes. Furthermore, water fluxes and tensile strengths of the membranes were measured. The results were found to correlate with the morphologies of the membranes.

Formation of mica-intercalated-Nylon 6 nanocomposite membranes by phase inversion method

A novel microporous membrane was synthesized from a polymeric nano-composite material, mica-intercalated-Nylon 6, by isothermal immersion-precipitation in a pure water bath. This novel membrane was skinless and was composed of cellular pores and sheaf-like crystallites, which interwove into a bi-continuous structure. By contrast, pure Nylon 6 precipitated under the same condition yielded a skinned asymmetric membrane. Differential scanning calorimetry (DSC) analysis of this membrane revealed a slightly higher degree of crystallinity than the nano-composite pellet. In addition, porosity of the membrane was found to increase with increasing water content in the dope. This was evidenced by the tensile strength and the water permeability measurements.

Formation of particulate microporous poly(vinylidene fluoride) membranes by isothermal immersion precipitation from the 1-octanol/dimethylformamide/poly(vinylidene fluoride) system

Phase diagrams were determined for poly(vinylidene fluoride) (PVDF) and a terpolymer of vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene (VDF/HFP/TFE) in solutions composed of 1-octanol and dimethylformamide at 25°C. From the measured gelation and liquid–liquid phase separation data, binary interaction parameters were computed using a modified Flory–Huggins theory. PVDF microporous membranes were then prepared by isothermal immersion precipitation processes in various doping conditions. The formed membrane exhibited a particulate morphology characterized by a uniform package of spherical particles of identical size. These particles were identified to be full spherulites of PVDF using scanning electron microscopy, differential scanning calorimetry and small angle light scattering techniques.

Formation of crystalline EVAL membranes by controlled mass transfer process in water–DMSO–EVAL copolymer systems

The effect of solvent content in the coagulation bath on the formation of poly(ethylene-co-vinyl alcohol) (EVAL) membranes by isothermal immersion–precipitation in water–dimethylsulfoxide (DMSO)–EVAL system has been investigated. As a homogeneous EVAL dope is immersed in a harsh bath, e.g., water, instant precipitation occurs initiated by the liquid–liquid demixing process. The formed membrane exhibits an asymmetric morphology with extensive finger-like macrovoids, similar to that observed in most amorphous polymeric membranes. On the other hand, if precipitation takes place in a soft bath containing a substantial amount of solvent, crystallization, rather than liquid–liquid demixing, starts to dominate and a skinless, bi-continuous, particulate membrane becomes the precipitated product. The immersion–precipitation process for the present system has been modeled as a ternary mass transfer problem. The calculated diffusion trajectories and concentration profiles illustrate reasonably the membrane morphologies formed in various immersion conditions.

Solute rejection of dextran by EVAL membranes with asymmetric and particulate morphologies

EVAL copolymer was precipitated from water and octanol to form, respectively, asymmetric and particulate membranes in an isothermal immersion precipitation process. Permeability of these membranes was examined with respect to dextran samples of various sizes. The results indicate that asymmetric membranes reject large dextran molecules and let through small molecules for the pressure range of 0.25–0.75 kg/cm 2 . The particulate membranes, however, exhibit an unusual filtration behaviour; i.e. there exists a maximum rejection at intermediate dextran molecular weight. This implies that small molecules tend to be trapped inside the nano-pores within the EVAL particles whereas large molecules travel through the tortuous channels outside the particles during a filtration operation.