Dar-Jong Lin

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 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.

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.

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 .

Observation of Nano-Particles in Silica/poly(HEMA) Hybrid by Electron Microscopy

The sol–gel method was employed to prepare an inorganic–organic hybrid, silica/poly(HEMA). The chemical structure of this material was analyzed by means of FTIR and solid 29Si-NMR. Using TEM and low voltage FESEM imaging at high magnifications (e.g., 300 KX), the hybrid was found to comprise nano-particles whose sizes fall largely over the range 20–30 nm.

Immobilization of L-Lysine on Microporous PVDF Membranes for Neuron Culture

Microporous poly(vinylidene fluoride) (PVDF) membranes with dense or porous surface were prepared by immersion precipitation of PVDF/TEP solutions in coagulation baths containing different amounts of water. Onto the membrane surface, poly(glycidyl methacrylate) (PGMA) was grafted by plasma-induced free radical polymerization. Then, L-lysine was covalently bonded to the as-grafted PGMA through ring-opening reactions between epoxide and amine to form amino alcohol. The highest attainable graft density of PGMA on a PVDF membrane was 0.293 mg/cm2. This was obtained when the reaction was carried out on a porous surface under an optimized reaction condition. For immobilization of L-lysine, the yield was found to depend on the reaction temperature and L-lysine concentration. The maximal yield was 0.226 mg/cm2, a value considerably higher than reported in the literature using other immobilization methods. Furthermore, neurons were cultured on L-lysine-immobilized PVDF membranes. The results indicated that these membrane surfaces were suited to the growth of neurons, with a MTT value higher than that of the standard culture dish.

Effect of compatible nucleation seeds on the morphology of porous Nylon 6 membrane

Recent study has revealed that when the nucleation takes place by adding compatible particles in Nylon 6 medium, it is possible to change greatly the nucleation intensity and the growing speed of crystallites. The present work demonstrates the employment of compatible PPO which was prepared by reactive blending of PPO and Nylon 6. A compatible phase of PPO was dispersed in meta-solution containing Nylon, formic acid and water. Dopes in meta-solution condition were prepared by varying the quantity and compatibility of dispersed PPO phase. It was observed that the morphology of membranes was dependent on the number of dispersed PPO. Firstly, the quantity of growing crystallite numbers initiated in such meta-solution dopes was much more than that when there is no compatible dispersed PPO in dopes. The resultant structure of membranes is due to kinetic mechanism of nucleation in addition to the thermodynamic phase behavior, both of which are prevalent for membrane formation process. Secondly, different compatibility between PPO and Nylon 6 results in different size of dispersed PPO phase in blends. Therefore even with the same weight fraction of PPO, different compatibility gives different quantity in number of dispersed PPO in meta-solution dopes and thus produces different effects of nucleation induction. The better is the interface compatibility of original blend, the greater will be the initial nucleation and thus, the porosity of the structure of membranes will increase.

Immobilization of DNA on Microporous PVDF Membranes by Plasma Polymerization

Microporous poly(vinylidene fluoride) (PVDF) membranes with different porous surface morphologies were prepared by immersion-precipitation from coagulation baths of different strengths. On these membranes single-strand deoxyribonucleic acid (ss-DNA) was covalently immobilized by a dual-step procedure. First, poly(glycidyl methacrylate) (PGMA) was grafted on PVDF membranes by plasma-induced free radical polymerization. Then, ss-DNA was bonded to PGMA through ring-opening reactions of epoxies with amine to form amino alcohols. The highest attainable graft yield of PGMA on PVDF membrane was 0.3 mg/cm2, which was the case when a highly porous PVDF membrane was employed as the substrate. For immobilization of ss-DNA, the yield was found to depend significantly on the reaction temperature and pH. The maximal value was 48.5 μg/cm2. Furthermore, adsorption tests of anti-DNA antibody were carried out on membranes with and without immobilized ss-DNA using serum obtained from systemic lupus erythematosus patients. The results indicated that the immobilized DNA could effectively adsorb the antibody in the serum.