Growth Of Macrovoids Research Articles

Strategy to prepare skin-free and macrovoid-free polysulfone membranes via the NIPS process

The non-solvent induced phase separation (NIPS) process applied to polysulfone (PSf) casting solutions traditionally leads to the formation of membranes decorated with large macrovoids in their cross-section on top of which lies a dense skin offering a significant resistance to flow. Macrovoids, are considered weak points. They are prone to compaction leading to poor filtration performance over time. Here, we used 2-pyrrolidone (2P) to prepare skin-free PSf membranes by NIPS, and studied the effect of water as an additive to help suppress the formation of macrovoids. The surface images showed the presence of multiple pores while the cross-section images exhibited the successful suppression of macrovoids. The latter was attributed to the enhancement of the solution viscosity causing the mass transfer kinetics to be slowed down, consequently affecting the demixing rates. Moreover, the rheological behavior of the casting solutions revealed that high degree of chain entanglement helped mitigate macrovoid formation. Using 2P as the solvent for PSf permitted the retention of interconnected pores from the top surface, due to the high viscosity of the solution which prevented coarsening. The high porosity (>75%) and large pores (>0.3 μm) of the PSf membranes were still maintained. The permeability of membranes was maintained after suppressing the macrovoids but their mechanical attributes were improved. The feasibility of utilizing water to eliminate macrovoid growth was further validated when applied to other PSf/solvent systems which showed similar trends in the SEM images. These results imply that water as a casting solution additive can provide a highly cost-effective and simple strategy to obtain macrovoid-free p olysulfone membranes.

A Biofouling Resistant Zwitterionic Polysulfone Membrane Prepared by a Dual-Bath Procedure.

This study introduces a zwitterionic material to modify polysulfone (PSf) membranes formed by a dual bath procedure, in view of reducing their fouling propensity. The zwitterionic copolymer, derived from a random polymer of styrene and 4-vinylpyrridine and referred to as zP(S-r-4VP), was incorporated to the PSf solution without any supplementary pore-forming additive to study the effect of the sole copolymer on membrane-structuring, chemical, and arising properties. XPS and mapping FT-IR provided evidence of the modification. Macrovoids appeared and then disappeared as the copolymer content increased in the range 1–4 wt%. The copolymer has hydrophilic units and its addition increases the casting solution viscosity. Both effects play an opposite role on transfers, and so on the growth of macrovoids. Biofouling tests demonstrated the efficiency of the copolymer to mitigate biofouling with a reduction in bacterial and blood cell attachment by more than 85%. Filtration tests revealed that the permeability increased by a twofold factor, the flux recovery ratio was augmented from 40% to 63% after water/BSA cycles, and irreversible fouling was reduced by 1/3. Although improvements are needed, these zwitterionic PSf membranes could be used in biomedical applications where resistance to biofouling by cells is a requirement.

ZrO2-TiO2 Incorporated PVDF Dual-Layer Hollow Fiber Membrane for Oily Wastewater Treatment: Effect of Air Gap.

Dual-layer hollow fiber (DLHF) nanocomposite membrane prepared by co-extrusion technique allows a uniform distribution of nanoparticles within the membrane outer layer to enhance the membrane performance. The effects of spinning parameters especially the air gap on the physico-chemical properties of ZrO2-TiO2 nanoparticles incorporated PVDF DLHF membranes for oily wastewater treatment have been investigated in this study. The zeta potential of the nanoparticles was measured to be around –16.5 mV. FESEM–EDX verified the uniform distribution of Ti, Zr, and O elements throughout the nanoparticle sample and the TEM images showed an average nanoparticles grain size of ~12 nm. Meanwhile, the size distribution intensity was around 716 nm. A lower air gap was found to suppress the macrovoid growth which resulted in the formation of thin outer layer incorporated with nanoparticles. The improvement in the separation performance of PVDF DLHF membranes embedded with ZrO2-TiO2 nanoparticles by about 5.7% in comparison to the neat membrane disclosed that the incorporation of ZrO2-TiO2 nanoparticles make them potentially useful for oily wastewater treatment.

Formation of Macrovoid-Free PMDA-MDA Polyimide Membranes Using a Gelation/Non-Solvent-Induced Phase Separation Method for Organic Solvent Nanofiltration

For the first time, we demonstrated that the gelation/nonsolvent induced phase separation (Gel-NIPs) method could be applied to fabricate macrovoid-free flat-sheet nanofiltration membranes based on organo-insoluble polyimide (pyromellitic dianhydride–4,4′-diaminodiphenylmethane; PMDA-MDA). Specifically, the PMDA-MDA polyamic acid (PAA)/N-methyl pyrrolidone (NMP) dope was cast on a polyester-based nonwoven, gelatinized in a fridge, and immersed in water. Gelation greatly inhibited the growth of macrovoids and resulted in an asymmetric membrane with high permeability but low rejection. After adding 4 wt % TiO2 sol into the dope, the gelation process was accelerated, and the PMDA-MDA/TiO2 membrane had a dense skin layer with a macrovoid-free cross-sectional structure. Its permeability and rejection to Rose Bengal (RB, Mw = 1017 Da) were 8.3–9.2 L·m–2·h–1·bar–1 and 89.5–95.7%, respectively, in solvents including DMF, THF, and DMSO. The separation performance was stable at a feed pressure of 25 bar, indicating...

Insight into the role of amphiphilic pluronic block copolymer as pore-forming additive in PVDF membrane formation

The use of additives in the polymer dope solutions is one of the most powerful strategies to control the pore structure of poly(vinylidene fluoride) (PVDF) membranes for various applications. In present work, the effects of amphiphilic pluronic block copolymer as an additive on PVDF membrane formation has been studied and a membrane formation mechanism involving pluronic has been proposed. The surface tension–concentration curve reveals that no micelles are formed in pluronic/N-methyl-2-pyrrolidone (NMP) solutions while the thermodynamics and kinetics are found to be similar for the systems containing pluronic and polyethylene glycol (PEG), respectively. The membranes prepared with pluronic as the additive exhibit larger surface pore, porosity, and macrovoids than that with PEG, indicating that pluronic has a stronger pore-forming ability. Instead of the commonly used concept of instantaneous or delayed demixing, it is believed that the stronger pore-forming ability of pluronic is caused by the hydrophobic–hydrophobic interaction between pluronic and PVDF that delays the solidification process. Since pluronic is not completely stable in PVDF membrane matrix, the membranes are soaked in 2-propanol (IPA) to purposely wash out pluronic. The loss of pluronic after IPA-treatment causes an increase in water permeation and a decrease in rejection, indicating that the presence of pluronic on the membrane pore surface can narrow down the pore size. By adding lithium chloride as a second additive, the growth of macrovoids is suppressed. Moreover, the resultant membranes exhibit both higher permeability and selectivity.

Macrovoids in solution-cast membranes: Direct probing of systems exhibiting horizontal macrovoid growth

Abstract Macrovoids are defects that are frequently encountered in phase inversion as well as in other solution-cast membranes. Macrovoid formation continues to hold many surprises despite several decades of pertinent studies and frequent claims for definitive answers. We have discovered conditions leading to macrovoid growth parallel to a transparent membrane skin (horizontal macrovoids, HMVs); the benefits from the macrovoid-study point of view are slow growth, easy observation and possibility of designing modes of interference that facilitate the understanding of the parameters that affect the initiation and growth (including orientation) of macrovoids. Our study emphasizes the case of dopes consisting of atactic PMMA dissolved in acetone and solidified in a hexane non-solvent bath but dopes of other polymers also exhibit the same features. While a host of parameters affect the occurrence of HMVs we find that they are ultimately a possible response to the mechanical force field that evolves during progressing solidification of the concentrated polymeric solution; as a result HMVs can occur, under appropriate conditions, regardless if there is phase separation (which leads ultimately to a macroporous bulk membrane microstructure) or not.

Correlation between macrovoid formation and the ternary phase diagram for polyethersulfone membranes prepared from two nearly similar solvents

The morphological structure of membranes prepared from two nearly similar systems consisting of water/ N, N-dimethylacetamide (DMAc)/polyethersulfone (PES) and water/ N-methyl-2-pyrrolidone (NMP)/polyethersulfone (PES) has been studied. The morphology of the prepared membranes showed that both systems exhibit an instantaneous liquid–liquid demixing that leads to the formation of macrovoids in the resulting structures. Nevertheless, the resulting macrovoid structures were contrary to the generally accepted concepts concerning macrovoid formation. The membranes morphologies showed that in spite of better miscibility between water and DMAc, which must promote the formation of channel- and finger-like structures, more sponge-like structures were observed in membranes prepared from the water/DMAc/PES system compared to those prepared from the water/NMP/PES system. To find the source of this unexpected phenomenon, the complete ternary phase diagrams consisting of theoretical binodal curves, vitrification boundaries, and gelation boundaries were constructed for both systems and it was shown that gelation process occurs earlier in the water/DMAc/PES system compared to the other system, which inhibits the growth of macrovoids in this system.

A sacrificial-layer approach to prepare microfiltration membranes

The preparation of hydrophilic microfiltration membranes by a sacrificial layer via co-casting is reported in this paper. The membranes were fabricated using two polymer solutions. Selection of the sacrificial coating layer was based on solution blending between coating solution/PSf solution and co-casting of two solutions. Based on delamination guideline, PEI was selected as the sacrificial layer for the investigation on PSf membrane structure. PSf membranes prepared as such showed improved permeability and surface porosity. Concave structure was observed in PSf membrane surface, hypothetically caused by the growth of macrovoids in PEI layer during phase separation.

Surface effect on nanosized void growth in a rigid-perfectly plastic material

The influence of the surface effect on the nanosized spherical void growth in a rigid-perfectly plastic material is analyzed and the mechanism of the nanosized void growth with high triaxiality is given. Based on the Rice and Tracey model for a macro void growth, the present model is proposed to account for the nanosized void growth under a uniform remote strain rate field with consideration on the surface effect. It is concluded that the surface effect yields an evident resistant influence on the nanosized void growth. That is, this influence decays as the void radius increases. With high triaxiality, the nanosized void growth is divided into two stages: the initial stage and the mature stage. At the first stage, the void grows slowly and the influence of surface effect is relatively weak, whereas at the second stage, the influence is significant and the void grows drastically.

Theoretical phase diagram calculation and membrane morphology evaluation for water/solvent/polyethersulfone systems

Theoretical ternary phase diagrams with very good agreement with experimental cloud point data were constructed for water/ N, N-dimethylacetamide (DMAc)/polyethersulfone (PES) and water/ N-methyl-2-pyrrolidone (NMP)/polyethersulfone systems. Theoretical phase diagrams were determined based on the extended Flory–Huggins theory of polymer solutions. To construct the theoretical phase diagrams, all binary interaction parameters were determined accurately and thoroughly revisited. Also, the structures of membranes prepared of these systems by phase separation process were investigated. The morphological studies showed that in spite of better miscibility between water and DMAc compared to water and NMP, channel-like structures were observed in membranes prepared of water/NMP/PES systems but tear-like structures with more spongy areas were observed in membranes prepared of water/DMAc/PES system. According to the constructed theoretical ternary phase diagrams of these systems, these unexpected observations were attributed to the higher concentration of polymer in the polymer-rich phase of water/DMAc/PES system, which causes an early vitrification in this system which suppresses the growth of macrovoids.

Macrovoid growth during polymer membrane casting

The Solutocapillary Convection (SC) hypothesis contends that macrovoid (MV) growth in dry-cast membranes is governed by a solutal-Marangoni convection-induced force caused by the rapid evaporation of volatile solvent from the liquid/gas interface, a viscous drag force, and a gravity-induced buoyancy force. Two different sets of experiments using the cellulose acetate-acetone-water system were conducted to test the SC hypothesis. Membranes were cast aboard a KC-135 aircraft that enabled short periods of microgravity (∼0-g) as well as 2-g conditions. The studied process variables included the solvent/non-solvent (S:NS) ratio, surface tension, and the magnitude of the body force (buoyancy). SEM analysis of the resulting membrane morphologies indicated that the MV morphology was strongly influenced by the S:NS ratio. However, dependence of MV size and number density on the buoyancy force could not be established. In the second set of experiments, videomicroscopy flow-visualization (VMFV) was utilized to measure fluid velocities at the MV/casting-solution interface and in the bulk solution. The magnitude of the solutocapillary convection was controlled via surfactant additions. A comparison of the ratio of the edge to the bulk velocity for MVs made from surfactant-free and surfactant-containing casting solutions did not provide evidence of a statistically significant surfactant effect. However, the presence of the surfactant did affect the MV number density. In addition, the presence of tracer particles inside the MVs indicated that a convective flow enables their transfer from the bulk to the interior of the MV.

Effects of phase separation on stress development in polymeric coatings

A cantilever deflection technique was used to monitor stress in situ during drying of cellulose acetate coatings. Porosity was introduced in some coatings using dry-cast phase separation. Stress and weight loss profiles for dense coatings, a coating that contained small (∼1μm) pores, and a coating that contained small (∼1μm) pores and macrovoids (∼200μm) are compared. In-plane tensile stress after drying ranged from 30MPa (dense coatings) to 5MPa (macrovoid-containing coating). The stress profiles for dense coatings feature a period of rapidly and then slowly increasing stress due to constrained shrinkage. For a coating that formed small pores, drying and stress development are delayed, stress rises and then drops a small amount due to capillary pressure relief. The stress profiles for the small pore and macrovoid-containing coatings are similar, except for a stress plateau at early stages of drying, which may be caused by macrovoid growth.

Gas separation performance of poly(4-vinylpyridine)/polyetherimide composite hollow fibers

The preparation and gas permeation performance (H 2, CO 2, O 2, N 2, CH 4) of silicone rubber/poly(4-vinylpyridine)/polyetherimide (SR/P4VP/PEI) multi-layer composite hollow fiber membranes are described. Dilute P4VP solutions form a defective coating layer on the top of PEI hollow fiber surface but reduce the surface porosity significantly, which makes the SR plugging feasible to regain the gas permselectivity of P4VP/PEI composite hollow fiber. Polyethylene glycol (PEG) additive in the PEI spinning dopes suppresses the growth of macro-voids and produces a membrane morphology having a more porous skin surface and more compact substructure, which provides a lower skin resistance and a higher substructure resistance for gas permeation. Substantial substructure resistance can deteriorate the membrane performance. Since the unfavorable influence of substructure resistance on the gas permeance is more pronounced for the fast permeating gas and in the order of H 2>CO 2>O 2>N 2>CH 4, the gas selectivities of H 2/N 2, CO 2/CH 4, and O 2/N 2 decrease accordingly. The PEI hollow fibers prepared from a PEI/PEG/NMP (23/0/77) spinning dope, after coated with 0.2 wt.% P4VP and 3 wt.% SR coating solutions, have gas permeances of H 2=41, CO 2=7.4, and O 2=2.0 GPU (1 GPU=1×10 −6 cm 3(STP)/cm 2-s-cmHg) with selectivities of H 2/N 2=117, CO 2/CH 4=62, and O 2/N 2=5.8.

On the formation of macrovoids in PMMA membranes

In this work, the formation of macrovoids in two membrane formation systems was investigated. The polymer and the coagulant used in these two systems were the same: PMMA and water, but the solvents used to dissolve PMMA were different – NMP for one system (system A) and acetone/Tween 80 solution for the other (system B). According to SEM and AFM analyses, it was found that, although the macrostructures of the membranes fabricated by systems A and B are similar, the microstructures are different. On the basis of optical observation of the growth of macrovoids, it is proposed that the difference in the microstructure can be explained by the difference in the growth mechanism of macrovoids: the growth of macrovoids is by convection for system A and by diffusion for system B. The difference in the growth mechanism of macrovoids can also account for some other differences between systems A and B: system B is more sensitive to the casting thickness than system A, and the addition of salt in the coagulation bath can effectively inhibit the growth of macrovoids in system B but not in system A.

Formation mechanism of the macrovoids induced by surfactant additives

In the present work, it is shown that addition of surfactants in the casting solution can induce or suppress macrovoids in polymeric membranes, depending on the miscibility between the added surfactant and the coagulant. Dual-bath experiments, in which casting solution was immersed in one coagulant for a very short time (<2 s) and then removed to a second bath containing different coagulants, were performed to investigate the formation process of macrovoids induced by surfactants. The results suggest that the initiation of macrovoids is completed in the first bath and the growth of macrovoids begins in the first bath but continues in the second bath. It was observed that the size of macrovoids is insensitive to the coagulant used in the second bath, which can be accounted for by proposing that the growth of macrovoids is mainly due to imbibing solvent from the surrounding polymer solution. To verify the above proposal, results from optical study and analysis of the growth speed of macrovoids are also presented.