Chung Tai-Shung,Neal
Email Address
chencts@nus.edu.sg
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Publication Carbon molecular sieve membranes derived from pseudo-interpenetrating polymer networks for gas separation and carbon capture(2011-05) Low, B.T.; Chung, T.S.; NUS NANOSCIENCE & NANOTECH INITIATIVE; CHEMICAL & BIOMOLECULAR ENGINEERINGThe design of polyimide-based pseudo-interpenetrating polymer networks (IPNs) is proposed to tailor the molecular structure of polymeric precursors for fabricating carbon molecular sieve membranes (CMSMs). To demonstrate the feasibility of this concept, pseudo-IPNs comprising of poly(2,3,5,6-phenylene-2, 2′-bis(3,4-carboxylphenyl)hexafluoropropane) diimide (6FDA-TMPDA) and 2,6-bis(4-azidobenzylidene)-4-methylcyclohexanone (azide) are used to fabricate CMSMs. The gas transport properties of CMSMs are dependent on the azide loading and heat treatment temperature. During the pyrolysis, two competing processes of pore evolution from the released gases and molecular transformation are occurring simultaneously. The creation of pores determines the structural morphology of the CMSM at a low pyrolysis temperature of 550 °C while the molecular rearrangement is the governing factor for carbonization at an elevated temperature of 800 °C. The CMSMs prepared at 550 °C display good CO2/N2 separation performance. The 6FDA-TMPDA/azide (90-10) CMSM pyrolyzed at 550 °C shows a CO2 permeability of 9290 ± 170 Barrer and an ideal CO2/N2 selectivity of 26.0 ± 0.8. CMSMs with high CO2/CH4 selectivity can be fabricated by carbonization at 800 °C. The 6FDA-TMPDA/azide (70-30) CMSM prepared at 800 °C has a CO2 permeability of 280 ± 7.0 Barrer and CO2/CH4 selectivity of 164 ± 6.0. The CMSMs derived from polyimide/azide pseudo-IPNs exhibit potential use in pre- and post-combustion CO2 capture. © 2011 Elsevier Ltd. All rights reserved.Publication Molecular interactions between polybenzimidazole and [EMIM]OAc, and derived ultrafiltration membranes for protein separation(2012-05) Xing, D.Y.; Chan, S.Y.; Chung, T.-S.; PHARMACY; CHEMICAL & BIOMOLECULAR ENGINEERINGThis study has investigated the molecular interactions between ionic liquids and PBI and discovered a potentially green solvent to fabricate polybenzimidazole (PBI) membranes for water reuse and protein separation. Ionic liquid, 1-ethyl-3-methylimidazolium acetate ([EMIM]OAc), exhibits superior efficiency in dissolving PBI under much lower temperatures and pressures compared to the traditional toxic N,N-dimethylacetamide (DMAc). this is because the acetate anions of [EMIM]OAc could form hydrogen bonding with PBI chains and effectively break up the interchain hydrogen bonding in PBI molecules. Molecular simulations have verified this mechanism by showing the largest amount of hydrogen bonding and the lowest interaction energy in the PBI-[EMIM]OAc system among the studied PBI-ionic liquid systems. The PBI-[EMIM]OAc solution also displays unique rheological properties significantly deviated from the traditional Cox-merz rule, and the shear thinning rheology at low shear rates implies a strong charge-ordered structure resulting from the intense hydrogen bonding. In addition, PBI ultrafiltration membranes have been prepared from PBI-[EMIM]OAc solutions by the non-solvent induced phase separation method and displayed a relatively thick sponge-like structure with a few macrovoids. After thermal treatment in ethylene glycol at 140 °C and chemical cross-linking by dichloro p-xylene, the PBI membranes achieve a high separation factor of 94.55 for a binary protein mixture containing bovine serum albumin and hemoglobin with the aid of combined effects from size exclusion and charge repulsion. This study provides a viable alternative to the current fabrication technology of PBI membranes in a "green" process with the aid of ionic liquids. © 2012 The Royal Society of Chemistry.Publication Enhanced double-skinned FO membranes with inner dense layer for wastewater treatment and macromolecule recycle using Sucrose as draw solute(2012-04-01) Su, J.; Chung, T.-S.; Helmer, B.J.; de Wit, J.S.; CHEMICAL & BIOMOLECULAR ENGINEERINGInternal concentration polarization (ICP) that occurs in the membrane sublayer is considered a serious problem restricting the performance of forward osmosis (FO) processes. Aiming at reducing ICP and fouling propensity, novel nanofiltration (NF) hollow fiber membranes with two apparently dense skins have been designed from cellulose acetate (CA) for FO applications by manipulating different phase inversion rates and degrees of annealing at the inner and outer layers. For the CA hollow fibers precipitated rapidly and then annealed at the lumen side, the surface pores within the inner skin layer show a very narrow size distribution with a mean radius of 0.34nm. Being also relatively dense, the outer skin layer may keep the feed solutes from entering the sublayer and avoid their accumulation within the sublayer if the feed solutes are macromolecules or multi-valence ions with relatively larger sizes. Thus, the double-skinned FO membrane would have improved performance by suppressing ICP at a cost of additional external concentration polarization (ECP) at the outer surface. In the FO process, the CA hollow fiber membrane with inner selective layer generates a water flux of 17.1LMH (Lm -2h -1) with 2.0M MgCl 2 draw solution running at the lumen side of the fibers and DI water feed at the shell side. When using 1.0M Sucrose (26.7bar osmotic pressure) as the draw solution at the shell side and DI water feed at the lumen side, a water flux of 12.9LMH is obtained with a negligible reverse Sucrose flux. This FO performance is comparable to that created by 1.0M MgCl 2 draw solution although 1.0M MgCl 2 has a much higher osmotic pressure of 93.7bar. With wastewater feed containing 200-2000mgL -1 mixed metal ions at the lumen side and 0.5M Sucrose draw solution at the shell side, water fluxes in the range of 9.9-6.5LMH with minimal reverse Sucrose fluxes are observed. These results have revealed great potential of the newly developed double-skinned CA hollow fiber membranes as well as using Sucrose as the draw solute for wastewater reclamation and macromolecule recycle. © 2012 Elsevier B.V.Publication Effect of polyvinylpyrrolidone molecular weights on morphology, oil/water separation, mechanical and thermal properties of polyetherimide/polyvinylpyrrolidone hollow fiber membranes(1999-11) Xu, Z.-L.; Chung, T.-S.; Huang, Y.; CHEMICAL & ENVIRONMENTAL ENGINEERINGWe prepared polyetherimide (PEI) hollow fiber membranes using polyvinylpyrrolidones (PVP) with different molecular weights (PVP 10,000, PVP 40,000, and PVP 1,300,000) as additives for oil/water separation. Asymmetric hollow fiber membranes were fabricated by wet phase inversion technique from 25 wt % or 30 wt % solids of 20:5:75 or 20:10:70 (weight ratio) PEI/PVP/N-methyl-2-pyrrolidone (NMP) solutions and a 95:5 NMP/water solution was used as bore fluid to eliminate resistance on the internal surface. Effects of PVP molecular weights on morphology, oil-surfactant-water separation characteristics, mechanical, and thermal properties of PEI/PVP hollow fiber membranes were investigated. It was found that an increase in PVP molecular weight and percentage in PEI/PVP dope solution resulted in the membrane morphology change from the finger-like structure to the spongy structure. Without sodium hypochlorite posttreatment, hollow fiber membranes with higher PVP molecular weights had a higher rejection but with a lower water flux. For oil-surfactant-water emulsion systems (1600 ppm surfactant of sodium dodecylbenzenesulfonate and 2500 ppm oil of n-decane), experimental results illustrated that the rejection rates for surfactant, total organic carbon, and oil were 76.1 ≈ 79.8%, 91.0 ≈ 93.0%, and more than 99%, respectively. Based on the glass transition temperature values, PVP existed in hollow fiber membranes and resulted in the hydrophilicity of membranes. In addition, using NaOCl as a posttreatment agent for membranes showed a significant improvement in membrane permeability for PVP with a molecular weight of 1300 K, whereas the elongation at break of the treated hollow fiber membranes decreased significantly.Publication Enhanced Matrimid membranes for pervaporation by homogenous blends with polybenzimidazole (PBI)(2006-03-01) Chung, T.-S.; Guo, W.F.; Liu, Y.; CHEMICAL & BIOMOLECULAR ENGINEERINGWe have demonstrated that the incorporation of a small amount of polybenzimidazole (PBI) into Matrimid polyimide can significantly stabilize Matrimid's polymeric chains for high-temperature pervaporation and remarkably enhance the separation factor/selectivity and flux/permeance for the dehydration of tert-butanol/water mixtures. The strong interactions between the carbonyl group of Matrimid and the NH group of PBI are the cause for the enhanced chain stability, while the hydrophilic nature of PBI and the close chain packing may be the reasons for the high separation performance. Data from DSC, TMA, DMA and FTIR confirm that the PBI/Matrimid blends studied are miscible in molecular level, but the degree of miscibility apparently decreases with an increase in PBI. It is found that annealing plays an important role on membrane structure and separation performance. Not only can it induce chains rearrangement towards a dense packing and the formation of charge transfer complexes (CTC), but also significantly improve separation factor and selectivity. Pervaporation experiments conducted at room and elevated temperatures all indicate that the blend membrane with 3.55 wt.% PBI has the best separation performance, which is consistent with the smallest d-space value measured by XRD. In addition, both DMA and pervaporation experiments verify that the blending of a small amount of PBI with Matrimid can efficiently overcome the plasticization of Matrimid at high temperature operation. © 2005 Elsevier B.V. All rights reserved.Publication Molecular design of the morphology and pore size of PVDF hollow fiber membranes for ethanol-water separation employing the modified pore-flow concept(2011-05-15) Sukitpaneenit, P.; Chung, T.-S.; CHEMICAL & BIOMOLECULAR ENGINEERINGIn this study, we have established the fundamental science and engineering of fabricating poly(vinylidene fluoride) (PVDF) asymmetric hollow fiber membranes for ethanol-water separation and elucidated the complicated relationship among membrane morphology, pore size, pore size distribution and separation performance using the concept of the modified pore-flow model proposed in our previous work. The variation of bore-fluid composition, air-gap distance and take-up speed results in membranes with various morphologies ranging from large-finger-like macrovoid to nearly perfect macrovoid-free structures. Interestingly, an increase in air-gap distance or take-up speed not only effectively suppress the formation of macrovoids but also results in the reduction of membrane pore size and the narrowing of pore size distribution, hence leading to the enhancement of membrane performance. The permeation flux is found to be mainly controlled by the overall porosity and the contribution of large pore sizes of the membrane, while the selectivity or separation factor is greatly determined by membrane pore size and pore size distribution, which is consistent with the modified pore-flow model proposed in our previous works. The newly developed PVDF asymmetric hollow fiber membranes demonstrates remarkable high fluxes of 3500-8800gm-2h-1 and reasonable ethanol-water separation factors of 5-8 compared to existing polymeric-based pervaporation membranes. © 2011 Elsevier B.V.Publication Fabrication of mixed matrix hollow fibers with intimate polymer-zeolite interface for gas separation(2006-08) Jiang, L.Y.; Chung, T.S.; Kulprathipanja, S.; NUS NANOSCIENCE & NANOTECH INITIATIVE; CHEMICAL & BIOMOLECULAR ENGINEERINGIt has been demonstrated that a novel p-xylenediamine/methanol soaking method could efficiently remove the polymer-zeolite interface defects of the mixed-matrix structure. In this work, the mixed-matrix structure is in the form of an ultrathin (1.5-3 μm) polysulfone/zeolite beta mixed-matrix layer that is supported by a neat Matrimid® layer in dual-layer composite hollow fibers. The particle's loading in this thin layer has reached 30 wt %. The ideal selectivities of the mixed-matrix hollow fibers (30 wt % of zeolite) for O 2/N 2 and CO 2/CH 4 separation were roughly 30 and 50% superior to that of the neat PSF/ Matrimid® hollow fibers, respectively. Investigation of the morphology of the mixed-matrix selective layer and its relation with gas separation performance indicated that without p-xylenediamine/methanol solution treatment, the outer layer showed various polymer-zeolite interface structures in different fibers with the same heat treatment procedures; this situation might lead to the different selectivities after coating. However, by applying p-xylenediamine/methanol processing on the fibers before thermal treatment, the fibers obtained a more uniform structure and improved attachment between polymer matrix and zeolite surface. Hydrogen bonding was proposed as the possible mechanism for the tighter attachment between the two phases. The improvement of separation efficiency was presumably related to the polymer chain rigidification, partial pore blockage, and/or favorable interaction between the gas penetrants and zeolite framework. © 2006 American Institute of Chemical Engineers.Publication Processing and engineering of pervaporation dehydration of ethylene glycol via dual-layer polybenzimidazole (PBI)/polyetherimide (PEI) membranes(2011-08-15) Wang, Y.; Chung, T.S.; Neo, B.W.; Gruender, M.; CHEMICAL & BIOMOLECULAR ENGINEERINGOperating conditions play a significant role in determining the separation performance of a pervaporation process, because they not only manipulate the driving forces to transport permeants but also affect the physicochemical properties of the pervaporation membrane itself. In this study, fundamental governing equations have been derived to correlate separation performance with system operation conditions and intrinsic separation characteristics of the pervaporation membrane. Polybenzimidazole/polyetherimide (PBI/PEI) dual-layer hollow fiber membranes were chosen to study the pervaporation dehydration of ethylene glycol (EG) under different testing protocols. The effects of operational parameters such as operation temperature, permeate pressure, feed composition and operation duration on performance indicators (flux and separation factor, permeance and selectivity) have been investigated. Experimental results show that an increase in operation temperature results in an increase in flux and selectivity, but a decrease in permeance and separation factor. In addition to other factors, decreasing sorption, less EG-water clusters and lower membrane-EG affinity with increasing temperature, play essential roles for the opposite trends. Both flux and permeance decrease with an increase in permeate pressure, while both separation factor and selectivity have an up-and-down trend. An increase in EG composition in the feed from 50 to 90. wt.% results in a lower water flux and permeance, but EG flux and permeance first increase and then decrease. This is due to the combined effect of water-induced membrane swelling and the formation of an EG boundary layer upon the membrane surface. The long-term test up to 33 days proves the membrane durability for EG dehydration. This work may provide useful insights to pervaporation fundamentals, system design and scale up for the EG dehydration. © 2011 Elsevier B.V.Publication Effects of Si-O-Si agglomerations on CO2 transport and separation properties of sol-derived nanohybrid membranes(2011-08-09) Lau, C.H.; Chung, T.-S.; CHEMICAL & BIOMOLECULAR ENGINEERINGNanohybrid membranes comprising organic and inorganic components with high CO2 affinity are ideal substitutes for traditional high footprint gas separation technologies. The CO2 permeability of these membranes resembles those of polydimethylsiloxane (PDMS), the most permeable rubbery material, while possessing CO2/H2 separation factors that supersede PDMS. Such membranes are synthesized using a simple acid-catalyzed sol-gel process. In this work, we investigate the relationship between the CO2 permeation properties of these nanohybrid membranes and membrane and siloxane network morphology by attuning the reaction kinetics of the sol-gel process. The CO2 permeability of these nanohybrid membranes can reach 1810 barrer, an improvement of 7.5 folds; while H2 permeability increase by 5.7 fold, from 30 to 170 barrer. The mechanism behind gas transport enhancements observed in these nanohybrid membranes is elucidated using positron annihilation lifetime spectroscopy and sorption measurements. Relative fractional free volume (FFV) content and CO2 sorption behaviors in these membranes are augmented as a function of inorganic phase morphology. The CO2 sorption behavior of the inorganic phase is regulated by the organic/inorganic ratio and water/silicon ratio in the sol-gel synthesis process. Harnessing the advantages of a unique combination of organic and inorganic materials, these nanohybrid membranes outperform most other CO 2-philic polymeric membranes. © 2011 American Chemical Society.Publication Effect of air-gap distance on the morphology and thermal properties of polyethersulfone hollow fibers(1997-11-07) Chung, T.-S.; Hu, X.; CHEMICAL ENGINEERINGBy using 30/70 polyethersulfone/NMP (N-methyl-2-pyrrolidone) solutions as an example, we have determined the role of air-gap distance on nascent fiber morphology, performance, and thermal properties. An increase in air-gap distance results in a hollow fiber with a less layer of fingerlike voids and a significant lower permeance. For the first time we have reported that the Tg of a dry-jet wet-spun fiber prepared from one-polymer/one-solvent systems is lower than that of a wet-spun fiber, and Tg decreases with an increase in air-gap distance. These interesting phenomena arise from the fact that different precipitation paths take place during the wet-spinning and dry-jet wet-spinning processes. Wet-spun fibers experience vigorous and almost instantaneous coagulations; it results in hollow fiber skins with a long-range random, unoriented chain entanglement, but loose structure. Dry-jet wet-spun fibers first go through a moisture-induced phase separation process and then a wet-phase inversion process; it results in external fiber skins with a short-range random, compact, and slightly oriented or stretched structure. As a result, the outskin of wet-spun fibers have a greater free volume and a higher first Tg than that of the dry-jet wet-spun ones. Both SEM (scanning electronic microscope) photomicrographs and DSC (differential scanning calorimeter) analyses support our conclusion. © 1997 John Wiley & Sons, Inc.