| Ultrafiltration has been widely applied not only in the biological products, pharmaceutical products and food industry; but also used for blood, wastewater treatment and ultra pure water preparation. Because of its competitive mechanical properties, thermal and chemical stability, radiation resistance, etc. As one of the most popular materials for membranes preparation, polyvinylidene fluoride (PVDF) has earned lots of attention. However, PVDF membranes possess high hydrophilicity, which leads to poor wetting ability and prone to fouling, especially in the oil/water separation system, including protein or organism. This results in decreasing of performance properties, reducing the membrane life and increasing the operation cost. Thus, the fine configurations modulation of PVDF membranes with excellent hydrophilicity and high performance are always highly sought after. The self-assembled of surfactants with various molecular weights (Tween-80and amphiphilic copolymer) in organic solvents was utilized to tune the hydrophilicity, morphology and performances of PVDF membranes. Then, after optimization of preparation parameters of flat sheet PVDF membrane preparation, PVDF hollow fiber membranes possessed good permeability, hydrophilicity and excellent mechanical properties were successfuly fabricated. The results were as follows.Firstly, PVDF membranes were fabricated by non-solvent induced phase separation (NIPS) process using Tween-80and H2O as mixture additive from both60℃and room-temperature (RT) casting solution. PVDF membranes revealed improved pure water flux (PWF), enlarged mechanical properties and well Bovine serum albumin (BSA) and Dextran rejection as a result of addition of H2O into the PVDF-DMAc-Tween-80system. The PWF of resultant PVDF was192L·m-2·h-1with0.1MPa operation pressure, and its rejection of BSA (MW67K) and Dextran (MW70K) were88.3%and83.8%, respectively. Its break strength, elongation at break and Young’s modules were1.3MPa,42.2%and32.2MPa, respectively. The PWF increased to1.7times (prepared with60℃casting solution) and2.3times (prepared with ambient temperature casting solution) after addition of H2O into the casting solution. Besides, PVDF membranes’mechanical properties and dynamic contact angle increased as H2O concentration increasing. However, the rejection of BSA and Dextran of resultant PVDF membranes changed little as H2O concentration increasing. The improved performance was attributed to the existence of non-solvent, which was solubilized by the polar head groups of Tween-80reverse micelle to form the water pool. Furthermore, the interaction between polar head of surfactant and water provided a balance resistance to the interconnection between PVDF and hydrophobic chains of the surfactant, which enhanced the thermodynamics stability of casting solution.Secondly, to obtain the target modulation path of Tween-80and H2O dopants, various compositions of coagulants and temperatures of casting solutions were utilized. PVDF membranes possessed interconnected bi-continuous structures with superior mechanical properties and fine hydrophilicity improvement were obtained from PVDF-DMAc-Tween-80-H2O system with60℃and ambient temperature casting solution, respectively. Tween-80-H2O mixtures were adopted as dope additive; water-ethanol (50:50, mass ratio) and ethanol were chosen as coagulants. The effects of process parameters in terms of variation contents of dope additives, casting solution temperatures and coagulant compositions on phase inversion process and performances of resultant PVDF membranes were investigated. Results showed that the compositions of coagulants modulated the sequence and the extent of liquid-liquid and liquid-solid demixing (crystallization), while the contents of dope additives and casting solution temperatures changed the precipitation rate of the casting solutions. During demixing process, water diffused from interior of Tween-80reverse micelles, resulting in the accelerated precipitation rate and surface segregation process of polar head groups of Tween-80. And the high temperature of casting solution contributed to the enhancing diffusion rate of liquid-liquid demixing on crystallization. The coagulant compositions changed the extent of liquid-liquid and solid-liquid demixing dynamic of casting solutions. Ethanol coagulant contributed to the crystallization of PVDF-DMAc-Tween-80-water system occurred prior to liquid-liquid demixing. This longer time-delay demixing process favored the formation of porous foliage type top-structures with fibrils or laths bi-continuous fine structure of membrane bulk, contributing to the increased flux and significant hydrophilicity improvement. While casting solutions in water-ethanol coagulant exhibited the shorter time-delay demixing process with both liquid-liquid demixing and crystallization, resulting in the formation of the fine structure in the form of the strings or stripes and limited the hydrophilic improvement. The predominant typical α and β type crystalline with crystallinity degree of59.1%in PVDF was attributed to the existence of dope additives, high temperature of casting solution and water-ethanol coagulant. This was consistent with the superior mechanical properties of corresponding PVDF membrane.In order to avoid the high activity, soluble problem of Tween-80during PVDF membrane preparation and operating process, as well better modulation configuration of PVDF membranes, P(PEGMA-r-MMA) amphiphilic copolymer with Mn of66500g/mol and Mw of34200g/mol was synthesized via free radical polymerization choosing polyethylene glycol monomethyl ether methyl methacrylate (PEGMA) and methyl methacrylate (MMA) as reaction monomers, and its chemical structure and composition were characterized. And polyvinylidene fluoride (PVDF)-P(PEGMA-r-MMA) blend membranes were fabricated from water and ethanol coagulants via simplified blend method by directly blending PVDF and P(PEGMA-r-MMA) amphiphilic copolymer solution (including the reaction mixture) to form casting solution. The formation of the supramolecular aggregates of PVDF-P(PEGMA-r-MMA) in PVDF solution containing the copolymer were confirmed by dynamic light scattering (DLS) and scanning electron microscopy (SEM). This contributed to the micro-structure adjustment of PVDF solution and resulted in its decreasing surface tension, accelerating precipitation rate and increasing viscosity with trivial strain thinning behavior. Furthermore, the effects of the variations in dopant contents and coagulant compositions on the performances of those blend membranes were investigated. All PVDF-P(PEGMA-r-MMA) blend membranes possessed narrow distribution mean effective pore size (p.), molecular weight cut off (MWCO), improved recovery water flux after filtration experiments of BSA and tuned configurations. Compared with the instantaneous demixing process in water coagulant, the delayed demixing process in ethanol favored the pore-forming and surface segregated of the polar head group of the copolymer, which induced the increasing μ, MWCO, tunable morphologies and hydrophilicity improvement.The results of simpilified blend method showed that the formation of the large-sized nanoparticulate aggregates (size:100～200nm) of PVDF-P(PEGMA-r-MMA) in the casting solution limited the function of the P(PEGMA-r-PEGMA) amphiphilic copolymer with respect to pore-forming and morphology modulation of the resultant membranes. Then the in situ free radical polymerization was creatively designed to obtain narrow distribution PVDF-P(PEGMA-r-MMA) supramolecular aggregates with small size and enhance better modulation configration of P(PEGMA-r-MMA); PVDF membranes with superior mechanical behaviors and enhanced antifouling properties was proven to be successfully fabricated. The DLS and SEM results verified the formation of supramolecular polymer-copolymer aggregates with small size (size:0-10nm). The narrow distribution of the supramolecular aggregates explained the decreasing surface tension, increasing viscosity with trivial strain thinning behavior and accelerating precipitation rate of the PVDF casting solution. The resultant PVDF membranes possessed narrowly distributed pore size and MWCO of the final filtration properties, which were attributed to the aggregates; additionally, their corresponding μ and MWCO increased, including the crystallinity of the a and β phase (largely in the β phase form) varied from56.1%to84.1%with increasing concentration of the monomers via in situ polymerization. Furthermore, in situ polymerization not only enlarged the recovery water flux after filtration experiments with BSA but also improved the hydrophilicity of both the top-surface and the bottom-surface of the PVDF membranes. Additionally, in situ polymerization modulated the configurations of PVDF membrane, varying from stripe-shaped grains to agglomerates of globule (with one of the globules bearing on the surface of another). The tunable morphologies, combined with the progressive enhanced crystallinity of the a and P phase were used to interpret the superior mechanical properties (Its break strength, elongation at break and Young’s modules were8.8MPa,343%and229MPa, respectively) of the resulting PVDF membranes.Finally, basic previous study, and choosing polytetrahydrofuran dimethacrylate ester (PTMGDA) that synthesied by our own lab with high reaction activity as reaction monomers instead of MMA, and reacted with PEGMA to synthesis P(PTMGDA-r-PEGMA) amphiphilic copolymer. PVDF hollow fiber UF membranes were prepared from the PVDF-TEP-DMAc-PTMGDA-PEGMA-PVP casting solution system after in situ polymerization. Combined with basic physical-chemical properties of the casting solution caused by the formation of PVDF-P(PTMGDA-r-PEGMA)-PVP supramoleculat aggregates, the configuration modulation, permeability, mechanical properties and antifouling BSA of resulant PVDF hollow fiber membranes were investigated. Results indicated that the round role appreared in the sponge-like cross-section of PVDF hollow fiber membranes, and the nanograins that constrcuted membrane bulk became widen and loose-packing as PVP concentration increasing. Besides, the membranes’pure water flux increased from15.7L.m-1.h-1to60.2L.m-1.h-1, and their mechanical properties and the rejection of Dextran (40K and70K) changed little. This showed that the synergy effects of PVP and the P(PTMGDA-r-PEGMA) amphiphilic copolymer formed via in situ polymerization on configration modulation and tuned performances of PVDF hollow fiber membranes. And the newly developed hydrophilicity PVDF hollow fiber membranes with superior mechanical properties and low-fouling of BSA are anticipated to be suitable not only for wastewater treatment, but also for bio-separation. |
PDF Full Text Request