Liao-Ping Cheng

Mechanisms of PVDF membrane formation by immersion-precipitation in soft (1-octanol) and harsh (water) nonsolvents

Abstract Crystalline PVDF was precipitated, respectively, from water/DMF and 1-octanol/DMF solutions to produce membranes with asymmetric and uniform morphologies. The formation mechanisms of these specific structures were described both in the aspects of thermodynamics (equilibrium phase behavior) and the kinetics (diffusion trajectory). The phase diagrams of the investigated systems indicated the possibility of liquid–liquid demixing or crystallization or both during the immersion-precipitation process. The sequence of these phase separation events, which determined the ultimate membrane structure, was attributed to the kinetic factors. Into this context, a quantitative model describing the immersion-precipitation process was considered. The calculated diffusion trajectory and concentration distribution in the nascent membrane were found to be consistent with the experimental membrane morphologies. Moreover, the precipitation rate of the PVDF solution in water and 1-octanol were examined by the light transmission experiments. The latter results further confirmed the validity of the mass transfer calculations.

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

Abstract 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.

Equilibrium phase behavior of the membrane forming water-DMSO-EVAL copolymer system

The equilibrium phase behavior of ethylene vinyl alcohol (EVAL) copolymer in mixtures of DMSO (dimethylsulfoxide, solvent) and water (nonsolvent) was studied for different temperatures. Both crystallization-induced gelation and liquid-liquid demixing were observed. From the determined phase diagram of this system at 25°C, three regions may be identified, i.e., a homogeneous region, a gel region, and binodal region in which both types of phase transition take place. At higher temperatures, crystallization isotherm was found to intersect the binodal phase boundary, which is analogous to the phase behavior reported by Stokes and Berghmans for several binary systems.

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

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

Abstract 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.

Microporous PVDF membrane formation by immersion precipitation from water/TEP/PVDF system

For membranes synthesized from crystalline polymers by phase inversion method, crystallization can sometimes dominate the precipitation process to form a membrane characterized by the so-called particulate structure. Such is the case for PVDF membranes prepared by immersion-precipitation from water/ triethylphosphate solutions. The structure of the membrane formed from this system was studied; in particular, the nano-scale fine structure of the crystallites, which form the matrix of the membrane. The phase diagram of the water/TEP/PVDF was determined using the cloud point method. Membranes were observed using LVSEM at low voltage (e.g., 1 KV) and high magnifications (e.g., 100 KX) to reveal the fine structure of the membranes.

Preparation of EVAL membranes with smooth and particulate morphologies for neuronal culture

In this work, the in vitro interaction of cerebellar granule neurons prepared from 7-day-old Wistar rats and poly ethylene-co-vinyl alcohol (EVAL) membranes was investigated. Cells were cultured in smooth and particulate EVAL membranes for up to 7 days. Particulate membranes were prepared by using 1-octanol to precipitate EVAL solutions in DMSO. Such a membrane was microporous characterized by a packed bed of particles. Voids left between the aggregated particles formed a continuous and interconnected porous network. Crystallization of the EVAL polymer induced by 1-octanol is responsible for the formation of particulate morphology. The membrane structure and its relationship with cells were examined by scanning electron microscopy and the MTT assay. It was observed that the particulate membrane was more favorable for the neuron culture than the smooth membrane. Neurons seeded on the particulate membrane were able to regenerate with formation of an extensive neuritic network. Therefore, the particulate structure may spatially mediate cellular response that can promote neuronal cell attachment, differentiation and neuritic growth, indicating that the particulate structure should be useful as a new polymer scaffold for nerve repair.

Morphology of crystalline Nylon-610 membranes prepared by the immersion-precipitation process : competition between crystallization and liquid-liquid phase separation

Abstract Nylon-610 membranes were prepared at 25°C by direct immersion of various dope solutions into either formic acid/water or 1-octanol bath. Depending on the dope and bath conditions, the precipitated membranes demonstrated characteristics derived from crystallization and/or liquid–liquid phase separation during the precipitation process. As a good dope solution was immersed in a harsh bath, e.g., water, precipitation occurred initiated by liquid–liquid phase separation. The formed membrane exhibited a cellular structure similar to that commonly observed in amorphous membranes. Alternatively, when a metastable dope (with respect to crystallization) was immersed in a soft bath containing a substantial amount of formic acid, crystallization dictated the precipitation process to yield bi-continuous, particulate membranes. Membranes with extensive macrovoids were observed, in the event that the dope contained a large amount of solvent. In the latter case, precipitation took place immediately after immersion, consistent with Strathmann and Smolder’s results for several membrane forming systems. In addition, skinless microporous membranes were prepared by precipitation of the dope solutions in a 1-octanol bath, in which precipitation occurred slowly and the formed membrane was composed of “sheaf-like” crystallites that interlocked into a homogeneous bi-continuous network .

Effects of precipitation conditions on the membrane morphology and permeation characteristics

The permeability and permselectivity of asymmetric and particulate membranes towards glucose and proteins of various molecular sizes were studied. It was found that the skin layer of asymmetric membranes was permeable to glucose and insulin but effectively prevent the permeation of immunoglobulins. This result parallels our interest for the development of artificial pancreas. It was also found that skinless particulate membranes exhibited not only high permeation rates with respect to albumin and immunoglobulins but also good selectivity between these components. Thus, particulate membranes has the potential to be used in separating albumin from immunoglobulins for treating disorders related to immunoglobulin abnormalities.

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.