Latest development on the membrane formation for gas ...rdo.psu.ac.th/sjst/journal/24-Suppl-1/28gas-separation.pdfMembrane gas separation by polymer membrane is a proven technology

REVIEW ARTICLE

Latest development on the membrane formation forgas separation

Ahmad Fausi Ismail1, Norida Ridzuan

2, and Sunarti Abdul Rahman

3

AbstractIsmail, A.F., Norida, R., and Sunarti, A.RLatest development on the membrane formation for gas separationSongklanakarin J. Sci. Technol., 2002, 24(Suppl.) : 1025-1043

The first scientific observation related to gas separation was encountered by J.K Mitchell in 1831.However, the most remarkable and influential contribution to membrane gas separation technology wasthe systematic study by Thomas Graham in 1860. However only in 1979, membrane based gas separationtechnology was available and recognized as one of the most recent and advanced unit operations for gasseparation processes. Membrane is fabricated by various methods and the parameters involved to a certainextent are very complicated. The phase inversion technique that is normally employed to produce mem-branes are dry/wet, wet, dry and thermal induced phase separation. Other techniques used to producemembrane are also reviewed. This paper reports the latest development in membrane formation for gasseparation. The route to produce defect-free and ultrathin-skinned asymmetric membrane is also presentedthat represents the cutting edge technology in membrane gas separation process.

Key words : membrane formation, gas separation, phase inversion, asymmetric membrane,defect-free

1Ph.D. (Chemical Engineering), Prof.,

2M.Sc. (Gas Engineering), Post Graduate Student,

3M.Sc. (Gas Engi-

neering), Lecturer, Membrane Research Unit, Faculty of Chemical and Natural Resources Engineering,Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia.Corresponding e-mail : [email protected], 10 January 2003 Accepted, 23 April 2003

Latest development on the membrane formationIsmail, A.F. et al.

Songklanakarin J. Sci. Technol.Vol. 24 (Suppl.) 2002 : Membrane Sci. & Tech. 1026

Membrane separation processes has be-come one of the emerging technologies, whichhave undergone a rapid growth during the pastfew decades. A membrane is defined as a selec-tive barrier between two fluid phases (Sirkar andHo, 1992, Mark and Menges, 1985 and Pandey andChauhan, 2001). Gas separation became a majorindustrial application of membrane technologyonly during the past 15 years, but the study of gasseparation actually began long before that period(Baker, 2000). Gas separation processes require amembrane with high permeability and selectivitythat is the asymmetric membrane (Lin et al.,1996). Asymmetric membranes that are suitablefor gas separation should have thin and denseskin layers supported by thick porous sublayers(Wang et al., 1996, Pinnau et al., 1990 and Peine-mann and Lauenburg, 1988). Membrane struc-

tures can be classified according to the cross sec-tion microstructure as shown in Figure 1.

There are six major membrane processes,which are widely used, in industrial application.They are microfiltration, ultrafiltration, reverseosmosis, electrodialysis, gas separation and per-vaporation. Gas separation is known as a devel-oping process and most gas separation mem-branes are of the solution-diffusion mechanismtype. The key membrane performance variablesare selectivity, permeability and durability. Forsolution-diffusion membranes, permeability isdefined as the product of the solubility and dif-fusivity. Traditionally, there has been a trade-off between selectivity and permeability; highselectivity membranes tend to exhibits less per-meability and vice versa.

Gas membranes are now widely used invariety of application, as shown in Table 1. They

Figure 1. Membrane classifications

Table 1. Gas separation membrane application

Common gas separation Application

O2/N

2Oxygen enrichment, inert gas generation

H2/Hydrocarbons Refinery hydrogen recovery

H2/N

2Ammonia Purge gas

H2/CO Syngas ratio adjustment

CO2/ Hydrocarbons Acid gas treatment, landfill gas upgrading

H2O/ Hydrocarbons Natural gas dehydration

H2S/ Hydrocarbons Sour gas treating

He/ Hydrocarbons Helium separationHe/N

2Helium recovery

Hydrocarbons/Air Hydrocarbons recovery, pollution controlH

2O/Air Air dehumidification



offer low capital cost, low energy con-sumption,ease of operation, cost effectiveness even at lowgas volumes and good weight and space effi-ciency. Membrane processes for gas separationapplications were commercially introduced in theearly 1970’s (Lokhandwala et al., 1995).

1. The mechanism of membrane formation byphase inversion separation

Membrane gas separation by polymermembrane is a proven technology and has beenfound in a wide range of industrial applications.Membranes can be prepared by phase inversiontechniques and can be categorized into fourdifferent techniques as shown in Figure 2.

In the air casting technique process, thepolymer is dissolved in a mixture of a volatilesolvent and less volatile nonsolvent. During theevaporation of the solvent, the solubility of thepolymer decreases and then phase separationtake place (Van, 1996).

In the precipitation from the vapor phaseprocess, phase separation of the polymer solutionis induced by penetration of nonsolvent vapor inthe solution. Thermal induced phase separation(TIPS) technique is based on the phenomenonthat the solvent quality usually decreases whenthe temperature is decreased. After demixing isinduced, the solvent is removed by extraction,evaporation or freeze drying. In the immersion

precipitation case, a polymer is cast as a thin filmon a support or extruded through a die, and issubsequently immersed in a nonsolvent bath. Pre-cipitation can occur because of the good solventin the polymer solution is exchanged with non-solvent in the coagulation bath. The differencesbetween the four techniques originate from differ-ences in the desolvation mechanisms (Van et al.,1996).

Among these techniques, immersion preci-pitation is widely used to produce commercial gasseparation membranes and other membranebased separation (Van et al., 1996). Immersion pre-cipitation technique can be further divided intothree categories namely wet, dry and dry/wetphase inversion technique as shown in Figure 3.

2. Improving the separation characteristicsOne of the major problems confronting the

use of membrane-based gas separation processesin a wide range of applications is the lack ofmembranes with high flux and high selectivity.During fabrication, membrane formation processplays an important role and certain factors needproper attention in order to produce a good gasseparation membrane. Currently, gas separationmembranes technologies are challenged to main-tain their favorable economics while improvingtheir gas selectivity, flux and durability. The im-proved membranes would be attractive in large

Figure 2. Phase inversion techniques



potential markets such as CO2/CH

4 , hydrocar-

bon/H2 and olefin/paraffin separations (Koenhen

et al., 1997 and Kawakami et al., 1996). One po-tential route is through the use of inorganic mem-branes, which have demonstrated improvementin permeation property, and durability. However,the formation of inorganic materials is presentlyestimated at 1-3 orders of magnitude greater thanpolymeric materials (Kawakami et al., 1996).

As a result, polymeric based materials suchas mixed-matrix materials, and cross-linkablepolymers might address the needs for improve-ment in selectivity and durability in next genera-tion membranes (Kawakami et al., 1996). All theattractive alternatives for advanced membranesrequire the ability to form essentially defect-freeselective skins in order to achieve the full benefitfrom the intrinsically improved membrane mat-erials are discussed in this paper.

3. Development of new polymer- zeolite-mixedmatrix membranes

Membranes used for gas separation at themoment are of solution-diffusion type polymericmembranes. One way of improving the separationcharacteristics of solution-diffusion type polyme-

ric membranes is to incorporate specific ad-sorbent such as zeolites into the polymericmatrix. A mixed-matrix material with inorganiczeolites or carbon molecular sieves with causeexcellent gas separation properties embeddedinto the matrix of a prospective polymer (Car-ruthers, 2001).

In addition, those mixed matrix materialshave the potential to provide membranes withhigher permselectivity and equivalent productivitycompared to existing membrane materials. It hadbeen proven by Mahajan et al., that successfulmixed matrix materials can be formed using rela-tively low glass transition (T

g) polymers that have

a favorable interaction with the sieves. One of theimmediate challenges facing membrane materialdesign is achieving higher permselectivity withequal or greater productivity compared to existingmaterials. Molecular sieving membrane materials,such as zeolites, are capable of overcoming thischallenge, but not in an economical way (Mahajanet al, 2002).

Synthetic zeolite incorporation is reportedto enhance the separation characteristics of rub-bery polymers. The majority of the work on zeo-lite filled polymeric membranes utilises synthetic

Figure 3. Schematic representations of immersion precipitation phaseinversion processes: (A) dry, (B) wet, (C) dry/wet



zeolites, such as zeolite A, X, Y and silicalite.However, there have not been many studies re-porting the use of natural zeolites.

The effect of incorporating filler into thepolymeric matrix of silicone rubber membraneson their gas separation properties was investi-gated in case of a natural zeolite, clinoptilolite. Itis also expected that this study will help to deter-mine the role of zeolite during gas transportthrough the membrane and for better understand-ing of the transport mechanism.

Recent studies have shown that zeolitesmay be useful in enhancing the permeability andselectivity of polymeric materials when the cor-rect zeolite–polymer pair is selected. Tantekin-Ersolmaz et al. reported that the permeabilitiesof the silicalite–PDMS mixed-matrix membranesincrease with the increasing of silicate crystallitesparticle size. This behavior may originate fromdifficulties in permeation due to the enhancedarea and number of the zeolite–polymer interfacesin the cases of employing relatively smaller par-ticle sizes (Tantekin-Ersolmaz et al., 2000).

According to Paul and Kemp, the incorpo-ration of zeolite 5A into silicone rubber did notimprove the separation properties of the polymer(Paul and Kemp, 1973). This observation, though,cannot be generalized and quite a few contradic-tory results have been reported, indicating the ex-istence of favorable effects of employing zeolitesas fillers in the mixed matrix membranes. Somestudies revealed that silicalite has significant po-tential for increasing the permeability and selec-tivity of various polymers for the separation ofO

2 from N

2, CO

2 from H

2 and CO

2 from CH

4

(Kulprathipanja et al., 1988 and Jia et al., 1991and 1992).

The zeolites 13X and 4A have also beenshown to be useful for enhancing the perform-ances of zeolite filled polymeric membranes forthe separation of H

2 from N

2, CO

2 from N

2 and

CO2 from O

2 ( S˙̇uer et al., 1994 and G˙̇urkan et al.,

1989). It is known that the permeability of gasthrough a zeolite filled polymeric membrane de-pends on the intrinsic properties of the zeoliteand the polymer, the interaction between the two

and the percentage of zeolite loading in themixed-matrix membrane (Zimmerman et al., 1997).However, the possible effect of the zeolite parti-cle size on the permeation characteristics of thezeolite filled polymeric membranes has not beeninvestigated.

The incorporation of zeolites into rubberypolymers has been shown experimentally to en-hance both the permeability and selectivity inpervaporative separation of organic compoundsout of water (Dotremont et al., 1989, Boom et al.,1998, Chandak et al., 1997 and Netke et al., 1995).Since then, there is a great interest in zeolite-filledpolymeric membranes for gas separation (Yonget al., 2001). The mixed-matrix membrane is acombination between polymer and zeolite. Previ-ous studies indicated that the mixed matrix mem-brane give a good advantage in gas separation.

The advantages using polymer membranesare good processability, inexpensive productionand low operating cost and modular design. Zeo-lite membrane advantages are in their separationperformance and their chemical and thermal sta-bility as shown in Figure 4 (A) and (B). The ad-

Figure 4. Schematic representations of (A) poly-mer membrane, (B) zeolite membrane(C) mixed-matrix membrane



Figure 5. Fabrication of mixed matrix membrane

Figure 6. Homogeneous blend of polymer and zeolite membrane

Figure 7. Incomplete blend of polymer and zeolite

vantages of mixed-matrix membrane are in gasseparation performance in the processability andlow cost operation as shown in Figure 4 (C).

Zeolite-filled rubbery polymer membraneswere first investigated by Jia et al. (Jia et al.,1991). They studied the permeation properties ofvarious gases through poly- dimethylsiloxane(PDMS) membranes filled with silicalite-1, fromwhich the permeabilities of He, H

2, O

2 and CO

2

were observed to increase, while those of N2, CH

4

and C4H

10 were observed to decrease. They con-

cluded that silicalite plays the role of a molecularsieving, although its pore size was larger than thekinetic diameters of gases. This means that theshape-selective effect is not only inherent in theequilibrium adsorption of gas molecules intozeolites but also in the kinetic adsorption and dif-fusion. Duval et al. studied the effect of zeolites



incorporated into PDMS, ethylene-propylene rub-ber (EPDM), polychloroprene (PCP) and nitrilebutadiene rubber (NBR) (Duval et al., 1993). Theresults showed that silicalite-1, zeolites 13X andKY improved the gas separation properties ofmembranes, which was attributed to both CO

2

sorption enhancement and molecular sieving ef-fects embodied by zeolites. However, zeolites3A, 4A and 5A were totally ineffective in im-proving the permselectivity of the rubbery poly-mers. This behavior was attributed to the slowdiffusion of the absorbed molecules from zeoliteto polymer phase.

Subsequently, zeolite-filled glassy poly-mer membranes were investigated. S˙̇uer et al.studied the permeation rates of N

2, O

2, Ar, CO

2

and H2 of polyethersulfone (PES) membranes

filled with zeolites 13X and 4A ( S˙̇uer et al., 1994).They concluded that both permeabilities andselectivities increased at high zeolite loading(42-50 wt.%). Permeabilities decreased first andthen increased with increasing zeolite loading.For PES-13X membranes, permeability showed arecovery above 8 wt.% zeolite loading. However,the permeability recovery of PES-4A membraneswere observed to be above 25 wt.% zeolite load-ing. This result was interpreted by the fact thatzeolite 13X crystals seemed to be more discrete,whereas zeolite 4A crystals were partly aggre-gated forming wider cavities. Pechar et al., hadsuccessfully fabricated mixed matrix membranesof 6FDA-6FpDA-DABA, a glassy polyimide, andmodified zeolites (ZSM-2) and used He, N

2, O

2

and CO2 as a test gases. The ZSM-2 zeolites were

fictionalized with amine groups by reacting themwith aminopropyltri methoxysilane in toluene(Pechar et al., 2002).

Duval et al. focused on the formation ofinterfacial voids due to the poor adhesion of theglassy polymer and the zeolite surface (Duval etal., 1994). When silicalite-1 was added into glassypolymers such as cellulose acetate (CA), poly-sulfone (PSF), polyetherimide (PEI) and polyi-mide (PI), permeabilities increased but selecti-vities decreased or have maintained. This resultis attributed to void formation. In order to make

zeolites more compatible with polymers, theymodified the external surface of zeolites withsilane, or evaporated the solvent used to preparethe polymer solution above the T

g of the poly-

mers during membrane fabrication. However,permeation properties of membranes were hardlyimproved. G˙̇ur approaches this problem with an-other method ( G˙̇ur , 1994). A melt extrusion proc-ess fabricated zeolite 13X filled PSF membranes,and then CO

2, O

2, N

2 and CH

4 permeabilities of

the membranes were measured. However, a pro-nounced effect of the filler was not observed. Heconcluded that the pore size of zeolite 13X waslarger than the kinetic diameters of any gas mol-ecules tested. Hence, separation due to size ex-clusion did not take place. The results mentionedabove could not show any clear explanation onwhy zeolite-filled glassy polymer membranescould not improve the permselectivity. It was as-sumed that voids still remained after silylationand extrusion or that the large pore size of the zeo-lite precluded a molecular sieving effect. 3.1 Fabrication of mixed-matrix membranes

Figure 5 below shows the complete fabric-ation of the mixed-matrix membrane while thehomogeneous blend of polymer and zeolite mem-brane is illustrated in Figure 6.

It is difficult to obtain a homogeneousblend of polymer and zeolite membrane since theprevious attempts had failed due to the presenceof voids between the polymer and zeolite. In thiscase, adhesion between polymer and zeolite isessential. Incomplete blend of polymer and zeo-lite can be illustrated in Figure 7 where the exist-ence of voids are shown clearly.

So in this case, amine–carboxylic acid inter-action will promote adhesion via acid-base inter-action between –OH and-NH

2. A polyimide-zeo-

lite mixed-matrix membrane was successfullyfabricated without defects as shown in Figure 8.Attractive molecular forces exist between thepolymer and modified zeolite.

Yong et al., indicated that interfacial void-free Matrimid polyimide (PI) membranes filledwith zeolites were prepared by introducing 2, 4,6-triaminopyrimidine (TAP). TAP enhanced the



contact of zeolite particles with polyimide chainspresumably by forming hydrogen bonding bet-ween them. 2, 4, 6-triaminopyrimidine (TAP) isintroduced as a kind of compatibilizer to elimi-nate the interfacial voids. This chain essentiallyenhanced the contact between the two compo-nents. The resulting membranes displayed anincrease in permeability without much change inthe selectivity. In fact, filling the space betweenzeolite particles and polymer chains would bemore convenient and effective than surface treat-ment of zeolites (Duval et al., 1993).

4. Development of high performance asymmet-ric membrane for gas separation throughrheological approach

The phase inversion process determinedthe general morphology of a membrane such asskin layer thickness and surface porosity. Hence,it determined the basic capability of the mem-brane for gas separation. Besides the inversionprocess, another aspect that is equally importantis the rheological conditions during manufactur-ing. These parameters involved the shear andelongation experienced during casting or hollowfiber spinning. The degree of shear can be studiedby altering casting speed or the dope extrusionrate and the effect of elongation can be studied byaltering the jet stretch ratio in spinning that is the

ratio wind-up speed to extrusion speed.The physical properties of films and fibers,

such as tensile strength and Young’s modulus,were related to molecular orientation inducedduring the various steps in fabrication. The sepa-ration performance of membranes is also signi-ficantly affected by molecular orientation. There-fore, it is important to closely relating the productperformance to the fabrication process and tofully understand the mechanism behind the de-velopment of molecular orientation. In amorphouspolymeric materials, molecular orientation wasdefined as a preferential alignment of randomlycoiled chain molecules (Cleeremen, 1974). Vari-ous mechanisms have been postulated in the lit-erature. There are three main mechanisms estab-lished named, phase inversion, shear flow andelongational or stretching flow.

By phase inversion, Serkov and Khanchih(Serkov and Kanchih, 1977) examined the devel-opment of molecular orientation during the pre-cipitation process of polymer solution. They foundthat preorientation was developed during preci-pitation, which significantly determined the sub-sequent stretchability, structure and physicalproperties of the spun fibers. Molecular orienta-tion by ionotropic coagulation is only applied topoly electrolyte precipitation. Due to this limita-tion, the mechanism cannot be adopted to explain

Figure 8. Modified zeolite in mixed-matrix membrane



the structural alignment in polymeric solutionsgenerally. Serkov and Khanchih suggested thatas the front moves through the solidifying me-dium, orientation is developed. They also postu-lated that as the polymer rich phase solidifies,spherulites could form if the energy of interactionwith the solvent is high. This process can lead tohighly ordered structure at the molecular level.Referring to shear flow, Takuechi et al. (Takuechiet al., 1988) studied the morphological develop-ment in a copolyester fiber with increasing shearrate. They observed a skin-core structure in theresultant extrudate and characterized this in termsof band pattern, which were monitored througha polarizing microscope. Bousmina and Muller(Bousmina and Muller, 1996) also proposed amechanism responsible for the development ofordered structure in an extruded filament accord-ing the influence of shear. They described a proc-ess which particles become aligned in the sheardirection. Serkov and Khanchih (Serkov andKanchih, 1977) also postulated that shear-in-duced orientation was frozen into the skin of afiber during formation, while the molecular orien-tation was induced by elongation flow. Perepelkin(Perepelkin, 1977) outlined the principle aspectsin the structural reorganization of polymer solu-tions that are; 1) a change in the orientational andthree dimensional order of the structure, 2) anincrease in the degree of orientation of the crystal-line and amorphous sections of the structure, 3)a change in the conformational arrangement ofthe molecule chain and 4) a change in the mole-cular structure.

In gas separation membrane, it has beenacknowledged that molecular orientation in theactive layer will affect the selectivity. In recentyears, besides the phase inversion process beingthe primary factor to determine the separationperformance of gas separation membrane, moreresearches have been focusing on the rheologicalaspect. For that reason, rheological conditionsduring manufacture will affect membrane per-formance by altering molecular orientation. Theshear during casting is one of the important rhe-ological factors. It was shown that molecular ori-

entation was intensified with the shear rate incre-ment during casting and spinning and that thereis a favorable effect on selectivity. The shear ratefor flat sheet membrane varies by drag time. Onthe other hand, the hollow fiber membrane variesby dope extrusion rate.

Shilton et al. found that permeability andselectivity rose with increasing dope extrusionrate for coated fibers due to the combined effectof enhanced polymer selectivity and increased insurface porosity (Shilton et al., 1994). However,elongation experienced during spinning wasfound to be detrimental to membrane perform-ance. Permeability increased but selectivity de-creased with increasing jet stretch ratio or elonga-tion. This was due to increase in porosity in theskin layer and unfavorable polymer structure. Toexplain their findings, Shilton et al. suggestedthat shear and elongation affected the selectivityof the solid polymer in the membrane by alteringmolecular orientation (Shilton et al., 1994). Theactual rise in selectivity of the coated membraneswith increasing dope extrusion rate could be ex-plained by observing the increase in the selecti-vity of the polymer itself. This was related to therheological behavior of the spinning dope undershear.

Aptel et al. (Aptel et al., 1985) spun poly-sulfone ultrafiltration hollow fibers and foundthat the performance of the membrane dependedon the extrusion rates of the polymer solution andon the bore liquid. According to them, shear forcesin the spinneret caused orientation of polymermolecules which in turn affecting pore shape. Ifgelation of the solution due to contact with thecoagulant liquid is faster than the relaxation timeof the polymer solution, the polymer chain align-ment is frozen into the membrane wall.

Ismail et al indicated that increasing mole-cular orientation occurred in the high-shearmembranes. Furthermore, the selectivity of thesemembranes was heightened and even surpassedthe recognized intrinsic selectivity of the mem-brane polymer. The results suggested that increasedshear during spinning would increase molecularorientation and, in turn, enhance selectivity. The



selectivities were much greater for high shearmembrane (Ismail et al., 1997 and 1998).

The shear-induced oriented structures phe-nomenon had been treated well by Takuechi etal. (Takuechi et al., 1988) and more recently byBousmina and Muller (Bousmina and Muller,1996). Takuechi et al. studied the morphologicaldevelopment in a copolyester fiber with increas-ing shear rate. They observed a skin-core structurein the resultant extrudate and characterized thisin terms of band pattern. Takuechi et al. also stud-ied the texture of the extrudate by birefringenceand showed that with shear rate induced, molecu-lar orientation was concentrated only in the skinlayer. Both Takuechi et al. and Bousmina andMuller suggested that understanding and quanti-fying, whenever possible, the relationship betweenextrudate morphology and rheology was of theoutmost importance in optimizing processingconditions.

Spectroscopy has become an importanttool to investigate the morphological characteris-tics of a membrane at the molecular level. Polar-ized transmission infrared spectroscopy has beenused previously to study molecular orientation inthin polymer films, but transmission spectroscopyis applicable only if the polymer films are suffi-ciently transparent in the wavelength regions ofinterest. Fourier transform infrared spectroscopytechnique has also been employed to study the de-position of polymide outer layer in composite gasseparation membranes (Ancelin et al., 1992). Theauthors used non-polarized infrared spectroscopyto analyze the polyimide polymerization mecha-nism using various polyamic acid precursors.Then, they tried to relate chemical structure tothe selectivity of the prepared membranes. Theirstudy did not involve direct measurement of mo-lecular orientation in the membrane. By usingFTIR, plane polarized infrared absorbed morestrongly when the plane polarization was parallel.There was a difference in absorption betweenparallel polarized and perpendicularly polarizedradiation and this phenomenon is known as li-near dichroism (Shilton et al., 1996).

Molecular orientation in membranes can

now be directly measured by spectroscopictechniques (Khulbe et al., 1995). Khulbe et al.employed electron spin resonance and Ramanspectroscopy techniques to study the differentmorphologies in integral-asymmetric and sym-metric membranes of poly(phenylene oxide) (PPO)and polyamide (PA). They found that molecularorientation existed in the skin layer of the asym-metric membranes and this was responsible forgreater selectivities. This orientation was subse-quently related to molecular vibration in specificbonds in the membranes. Pronounced infrared di-chroism (the difference in absorption betweenparallel and perpendicularly polarized light) in-dicates that the alignment of molecules, with theabsence of dichroism showed that a sample hadbeen randomly orienting the molecules. PolarizedIR spectra of hollow fiber membranes had beenrecorded by IR reflection from samples of thefibers wound several time round a KBr plate ofrectangular cross section. In this study the mem-branes were cast at low and high shear rate. Bothmembranes exhibit dichroism in the infrared, butthe effect was more pronounced in the high shearmembranes suggesting greater molecular orienta-tion (Ismail et al., 1997).

Molecular orientation in the active layerof these membranes was directly and quantita-tively measured using plane-polarized reflectanceinfrared spectroscopy. This technique could revealanisotropy on the molecular level within a sampleby pronounced infrared (Ismail et al., 1997 and1998 and Lai, 2002). Shear and elongation duringspinning have been shown to affect the permeat-ion performance of polysulfone hollow fibermembranes (Shilton et al., 1996 and Aptel et al.,1985) and this was attributed to molecular ori-entation in the active layer. If special skin forma-tion conditions prevail, increased shear can createan oriented and highly ordered membrane activelayer which can exhibit selectivities significantlygreater than the recognized intrinsic value forthe isotropic polymer. In addition, the membraneshave been tested recently using positron annihi-lation lifetime spectroscopy (PALS) to furtherinvestigate the conformation of the active layer at



the molecular level (Ismail et al., 1997).The calculated results suggest that hollow

fiber membranes spun with high shear rates ap-parently have a thicker dense skin layer, indicatingincreasing gas transport resistance because ofshear-induced chain orientation and packing. Thisresult might also imply that a high shear rate mayyield a hollow fiber with a “denser” selective layerwith a lower permeability (Wang and Chung, 2001).

Diffuse reflectance infrared Fourier trans-form spectroscopy (DRIFTS) has been used tocharacterize the surface structure, in particularthe molecular orientation, in wide variety of poly-mer films, fibers and glass fibers. In polymerfilms, DRIFTS was capable of providing goodquality spectra and structural information of poly-mer surfaces up to nanolevel thickness (Jansenand Haas, 1988). Reflection infrared spectroscopyhad the advantage of being able to analyze thesurface layers of opaque films or fibers. Recently,the technique has been developed to determineorientation of polymer molecules in asymmetrichollow fiber and flat sheet membranes for gasseparation.

5. Chemical cross-linking modification of poly-mer membranes for gas separation

From previous researchers, experimentalresults suggested that cross-linking modificationimparted membranes with anti-plasticization,good chemical resistance and even with goodlong-term performance. Besides, the gas permea-bilities or permselectivity relationships of somecross-linked polymers such as polyimides werehigher than the normal trade-off line (Liu et al.,2001). Cross-linking modification can be inducedby two methods. The first method is by using UVlight induced photochemically cross-linking re-actions and the second method is using thermaltreatment by elevated temperatures. Cross-linkingof the polymer matrix is a convenient way to im-prove resistance to chemical dissolution whilesimultaneously enhancing the high temperaturemodulus of the material. However, cross-linkinghas been shown to result in a significant decreasein the gas permeability.

UV light or other energy sources inducedphotochemically cross-linking reactions is amethod of cross-linking modification but the dif-ficulties of implementing it uniformly for hollowfiber membranes limit the application of thismethod. The radiative processes can activate re-active groups primarily restricted to the incidentsurface layer, depending on dosage rate and ex-posure time (Dudley et al., 2001). This is espe-cially true for typical UV irradiation.

Ouyang et al. reported that a thin SiOx sur-

face layer was formed on porous Nylon mem-branes coated with a cross-linked polysiloxane byexposure to UV light at room temperature in thepresence of atmospheric oxygen (Ouyang et al.,2000). The transformations manifest by changesin both the chemical and physical properties ofthe films. The majority of carbonaceous speciesare removed from the film surface, indicative ofthe conversion of an organic siloxanepolymerinto an inorganic SiO

x surface film. The gas per-

meation result showed that before irradiation,CO

2/N

2 selectivity was 22 and after UV-ozone

induced formation of the SiOx surface layer, the

selectivity for CO2/N

2 increased to 48.

Liu et al. prepared a series of copolyimidesand modificated copolyimides by UV irradia-tion to photochemically cross-linked copolyi-mides (Liu et al., 1999). Photochemically cross-linking modification will result in decrease of gaspermeability and increase in both, energy (E

p)

and H2/N

2 selectivity of all copolyimides. UV ir-

radiation of the copolyimides can determine acti-vation energies for gas permeation. The cross-linked copolyimides obtained exhibited highergas permeabilitites and selectivities than normalpolymers for H

2/N

2 separation.

Dudley et al. (Dudley et al., 2001) studiedthe cross-linking of polymer blend membranescontaining a reactive additive using a variety ofenergy sources (UV, γ and electron bean irradia-tion) into integrally skinned asymmetric mem-branes. They reported that polyetherimide as theprimary membrane component and the polymerwas cross-linked by blending with an ethynyl-ter-minated monomer (ETM). The thermosetting



ETM can be cross-linked to provide materialswith improved solvent resistance without evolu-tion of volatile products and in the absence ofcatalysts. Results fall into two categories, depend-ing upon the activation procedure employed: sur-face treatments and thermal treatments. Surfacetreatment resulted in improvements in separationselectivity with only modest reduction in fast gasflux. Thermal treatments were conducted at atemperature well below the maximum reactiontemperature of the ethynyl unit (180oC) and at atemperature near the peak reaction temperature(279oC). In general, increasing the time or temp-erature of the annealing process resulted in anincrease in the glass transition temperature andan increase in the extent of reaction. Completeconversion of the ethynyl units resulted in markedimprovements in the resistance to chemicals,thermal stability and gas selectivity.

Thermal treatment at elevated tempera-tures is another method of cross-linking modi-fication. The thermal process activates reactivegroups uniformly throughout the material. Themajor drawback of thermal treatment at an ele-vated temperature is it decreases the fine struc-tures of asymmetric membranes and their gaspermeation properties. Therefore, cross-linkingmethods performed at a low temperature are nec-

essary for the successful modification of asym-metric membranes (Liu et al., 2001). Figure 9describes the cross-linking modification processstep-by-step by using immersing polyimide filmin p-xlenediamine methanol solution (Liu et al.,2001).

Most of previous researchers used cross-linking method at high temperature. Bos et al.used homogenous matrimide films as the modelmaterial and successfully stabilized the mem-brane by forming a semi-interpenetrating net-work with oligo-polyimide containing acetyleneend groups at 265oC and by cross-linking with aheat treatment at 300oC without curing agents(Liu et al., 2001). Staudt-Bickel and Koros foundthat cross-linking reaction between ethylene gly-col and the carboxylic acid groups in the diami-no benzoic acid by thermal treatment is muchbetter than antiplasticization at 150oC. Liu et al.studied the fundamentals of the chemical cross-linking modification of a polyimide with p-xy-lenediamine at ambient temperature (Liu et al.,2001). The advantages of this method are theroom temperature curing process and the easyusage. The cross-linking modification result insignificant decrease in gas permeabilities of cross-linked polyimides to He, O

2, N

2 and CO

2. The de-

crease in diffusion coefficients plays a dominant

Figure 9. Schematic representation of chemical cross-linking process by thermal treatment.A) swelling B) cross-link C) removing from the solution D) drying (Liu et al., 2001).



role to the decrease in gas permeability. With in-crease in the degree of cross-linking, the perm-selectivies of He/N

2 and O

2/N

2 increase too but

CO2/N

2 selectivity decreases (Dudley et al., 2001).

Rezac et al. analyzed the influence ofsolid-state cross-linking polyimides to achievethis unique properties combination (Dudley et al.,2001). They did comparison to other cross-linkedpolymer system. As a result, they found out thatcross-linking p polyimide blend resulted in amaterial which simultaneously had outstandingpermselectivity properties and chemical resist-ance, as well as no measurable decrease in perme-ability observed upon cross-linking by annealingat 240oC.

One specific technique is to synthesize po-lymers with cross-linkable groups on the polymerbackbone, enabling the polymer to be cross-linkedduring a reactive post-treatment step after mem-brane formation. To avoid magnifying existingdefects during the cross-linking step, it is highlydesirable to start with a defect-free selective layer.Various other attractive membrane materials arealso being developed though successful commer-cialization of any potential membrane materialrelying on retaining its intrinsic dense film prop-erties in an integrally skinned asymmetric hollowfiber structure. Therefore, the issue of being ableto produce an essentially defect-free asymmetricmembrane structure represents one of the majorproblems in the gas separation membrane indus-try (Carruthers, 2001).

In order to improve gas separation per-formance, thus, to obtain a high performance mem-brane, the combination of the above mentionedthree techniques should be adopted. Various anddifferent modifications from many inventors willdirect this technology toward the production ofgood performance membrane with high selecti-vity and high permeability or high permeabilitywith constant selectivity.

6. Development of defect-free and ultrathin-skinned asymmetric membranes for gasseparation

The fabrication of polymer membranes

having both high gas permeability and selectivityhas become a subject of strong research interestbecause of its importance for the development ofthe membrane gas separation process. One way ofthe achieving the gas transport performance is tofabricate asymmetric polymer membrane consist-ing of a defect-free skin layer, which has high gaspermeability without a significant decrease in gasselectivity (Niwa et al., 2000). Asymmetric flatsheet and hollow fiber membranes were definedto be “defect free” if the ideal A/B for example O

2/

N2 selectivity was found to be greater than 80% of

intrinsic O2/N

2 selectivity of dense polysulfone

membrane film (Pesek and Koros, 1993). Inter-estingly, the asymmetric membrane exhibitedgreater gas selectivity than the dense membrane.The following are researchers that have donestudies on the development of defect-free andultrathin-skinned asymmetric membranes for gasseparation.

Lai (Lai, 2002) did the study by using acasting solution consist of 22.0 wt.% polysulfone(polymer), 31.8 wt.% N,N-dimethylacetamide(less volatile solvent), 31.8 wt.% tetrahydrofuran(more volatile solvent), and 14.4 wt.% ethanol(nonsolvent). Manipulation of rheological factorsand evaporation time in membrane formationprocess is a novel approach in membrane tech-nology, which provides a potential platform todevelop defect-free and ultrathin-skinned asym-metric membranes for gas separation. Rheologi-cal factors would affect morphology, physicalproperties and separation performance of asym-metric membrane. Therefore, this study investi-gated the effects of rheological factors with re-gard to membrane properties and structures (Lai,2002). Molecular orientation is enhanced by shearduring casting and this has a favorable effect onmembrane selectivity. Molecular orientation ofopaque polymeric films was directly measuredusing diffuse reflectance infrared Fourier trans-form spectroscopy (DRIFTS), which was firstreported by Fuller and Griffiths (Ismail, 1997).By manipulation the evaporation time, typicalrange time that lay within 9s to 21s were chosenin this study to investigate their effects on mem-



brane properties and structures. The result eluci-dated that by increasing the evaporation time dur-ing casting cause a decreased in pore size andsurface porosity with an increased in skin thick-ness. Decreasing the evaporation time duringcasting will cause an increase in both pore sizeand surface porosity accompanied by a decreasein skin thickness. A combination on both of theeffects of phase inversion and rheological factorshas successfully developed defect-free and ultra-thin-skinned asymmetric membranes for gas se-paration. Overall, the optimum conditions forfabricating an approximately high performance,ultrathin polysulfone flat sheet asymmetric mem-brane from the results obtained in this study wasthe membrane cast at highest shear (367s-1) andat 20s of forced convective evaporation in dryphase inversion steps.

Kurdi and Tremblay (Kurdi and Tremblay,1999) have developed defect-free asymmetricpolymeric membranes for separation of oxygenfrom air, by using dry/wet phase inversion pro-cess with polyetherimide (PEI) as polymer, An-hydrus N-methyl-2-pyrrolidinone (NMP) as solventand lithium nitrates (LiNO

3) salt additive. They

discussed the effects of polymer concentration,additive salt concentration and drying process onoxygen permeance and the actual separationfactor of the membrane. The small amount of sol-vent in the coagulation bath improved the leach-ing of the salt additive and produced membraneswith are more open structure. The skin formationis achieved by a diffusion-driven process and thenby using forced convection instead of normalevaporation and produced a defect-free ultrathin-skinned asymmetric membrane suitable for gasseparation. The highest productivity, defined asthe highest simultaneous oxygen permeance andoxygen separation factor was obtained for mem-brane produced from casting solutions containing23% by weight PEI. Increasing LiNO

3 / PEI ratio

to >2% leads to a more open structure, whichcauses a large increase in gas permeance accom-panied by a large decrease in the gas selectivity,due to increase in pore size. The rigidity of themembrane increases on drying at 90oC under

vacuum.Niwa et al. fabricated asymmetric polyi-

mide hollow fiber with a completely defect-freethin skin layer. They focused on effects of thecomposition of spinning solution and the air gapheight on the formation of the skin layer. Thehollow fiber showed a microporous structure onthe inner surface and a defect-free skin layer atthe outer surface. The outer diameter of the fiberwas 500 µm, with a wall thickness of 45 µm andthe calculated apparent skin layer thickness was470 nm. Selectivity of O

2 and N

2 for the hollow

fibers at 35oC and 76 cm Hg was 5.4 and exhi-bited a larger value than that measured in thedense and asymmetric polyimide flat membranes.

Hachisuka et al. (Hachisuka et al., 1996)had developed a new type of asymmetric mem-brane that has a hyper thin skin layer and spongelike porous matrix 6FDA-BAAF polyimide wasused as one of the membrane materials. The skinlayer had hardly any defects and the thickness ofthe membrane was 40-60 nm. The skin layer andsponge like porous matrix, which had finger voidfree morphology, were formed as one body. Thissuggests that the formation of such hyper-thinskin layers depends on the dissolution propertybetween a dope solvent and water as a solidifi-cation solvent. The selectivity and permeationrate of the 6FDA-BAAF polyimide asymmetricmembrane had an α 27 for CO

2/CH

4 and a per-

meation rate of 1.3 [Nm3/m2 h atm] (= 5.5 × 10-4

[cm3(STP)/ cm2 s cmHg ]) for CO2. (The maximum

value had an α of 35 for CO2/CH

4 and a permea-

tion rate of 1.6 [Nm3/m2 h atm] (= 5.8 ×10-4 [cm2 scmHg ]).

Kawakami et al. (Kawakami et al., 2002a)studied the gas permeation stability of an asym-metric polyimide membrane with a thin and de-fect-free skin layer. The polyimide used in thisstudy was synthesized from the fluorinated dian-hydride (6FDA) and the aromatic diamine notcontaining a CF

3 group. The apparent skin layer

thickness of the asymmetric membrane was55 nm. They specifically focused on the CO

2 per-

meation stability of the asymmetric polyimidemembrane with a 55 nm skin layer exposed to a



CO2 pressure of 760 cmHg.

In addition, by Kawakami et al. (Kawakamiet al., 2002b) synthesized the isomeric polyi-mides, 6FDA-m-DDS and investigated the gasselectivity of the asymmetric polyimide mem-branes with an oriented surface skin layer. Inparticular, they focused on the effect of thechemical structure of the polyimide on the mo-lecular orientation. The molecular orientation inthe asymmetric polyimide membranes was meas-ured using polarized ATR–FTIR spectroscopy.The gas selectivity of the asymmetric 6FDA-m-DDS membrane increase with an increased inthe shear stress and were greater than that of thedense membrane. They also clarified that a paral-lel oriented surface formed on the asymmetric6FDA-m-DDS membrane caused an enhance-ment in gas selectivity of the membrane.

Ren et al. (Ren et al., 2002) found thatwhen ethanol (ternary system) as a nonsolventadditive was added to the spinning solution con-tained 6FDA-2,6 DAT /NMP, the resultant fibershad less dense skin thickness compared to fibersspun from the binary dope (6FDA-2,6 DAT/NMP)system at the same shear rate. However interest-ingly, the selectivity in the ternary dope systemincreased slightly with an increase in shear rate.The hollow fiber membranes became more com-pact with an increase in shear rate. From their re-sult, they made a conclusion; the reduction inpermeance is mainly due to elongational inducedmolecular orientation, while the reduction in se-lectivity is probably due to elongational induceddefects.

Chung et al. (Chung et al., 2000) found thatat low shear rates, the permeances of non-polarmolecules such as H

2 ,O

2 ,N

2, and CH

4 decreases,

while their relative selectivities increase with anincrease in shear rates. Once a certain shear rateis reached, all permeances increase, while theirselectivities decrease with an increase in shearrates. In low shear rate regions, the decrease inpermeance or increase in selectivity with increas-ing shear rates arises from the better molecularorientation and chain packing induced by shear.With increasing shear in high shear rate regions,

the increase in permeance or decrease in selectiv-ity is mainly attributed to relatively porous skinstructures induced by the low viscosity nature of apower-law spinning fluid at high shear rates, frac-ture, and modified thermodynamics and kineticsof phase inversion process. This work suggeststhat there may exist an optimum shear rate to yieldoptimal membrane morphology for gas separa-tion. An increase in CO

2 permeance with in-

creasing shear rates are possibly due to enhancedcoupling effect between CO

2 and the highly ori-

ented and closely packed fluoropolyimide mo-lecular chains induced by shear. Chung et al(Chung et al., 2002) also applied the shear rateapplication to the ultrafiltration membrane. Theydemonstrated the effect of shear rate on the outersurface morphology of polyethersulfone (PES)hollow fiber ultrafiltration membrane by anatomic force microscope (AFM). They varyingshear rates from 1305s-1-11,066s-1, and indicatedthat at low shear rates, the outer surface have re-vealed that nodules in the outer skin appeared tobe randomly arranged at low shear rates butformed bands that were aligned in the directionof dope extrusion when the shear rate increased.

The pure water flux of the membranes wasnearly proportional to the mean roughness andresulted in lower separation membranes. AFMdata also imply that there was a certain criticalvalue of shear rate around 3585s-1, the roughnessdecreased significantly with an increase in shearrate below 3582s-1 and almost leveled off or in amuch slower pace above this shear rate.

7. Future directionFormation of defect-free and ultrathin-

skinned layer asymmetric membrane for gasseparation seems to be the most challenging as-pect in the production polymeric asymmetricmembrane for gas separation due to its high per-meability and selectivity. It is foreseen that in thefuture, this aspect will be concentrated in the de-velopment of defect free skin layer from varioustype of polymer especially polyimide, polysul-fone, cellulose acetate, polyethersulfone andpolyaramide.



Since the permeation properties of carbonmembranes have been improving from the past20 years, the future mixed-matrix membraneproduction is preferably to use the polymer. Thecarbonization polymer blend will lead to the for-mation of porous structure due to the thermallyunstable polymer (pyrolyzing polymer) decom-posing to leave pores in the carbon matrix formedfrom the stable polymer (carbonizing polymer)(Ismail and David, 2001). Proper materials selec-tion is the initial step to attaining mixed matrixgas separation membranes with performance ex-ceeding the upper bound trade-off curve. Failureto eliminate defects probably resulted in unfavo-rable performance. The upper bound trade-offcurve still defines the approximate upper boundtoday, despite insensitive theoretical and practicalstructure-permeability studies. However carbonmolecular sieving (CMS) lies well above theupper bound polymeric trade-off curve and in ornear the commercially attractive region. Thesematerials were expensive and difficult to processas membranes (Zimmerman et al., 1997). In ad-dition, mixed-matrix membrane will not only beused in the design and development of mixedmatrix membranes for specific gas separationsapplications, but will also have implications inthe development of various other new and valu-able materials for sensor, optical and electronicdevices. Besides that, current studies clearly dem-onstrate the versatility of chemical cross-linkingmodification of polymer technique for formationof membrane and the potential utility of thistechnique seems limitless.

Conclusion

Increasing amount of research in the for-mation of gas separation membrane indicatesthat membranes technology is growing and be-coming another alternative for industrial gasseparation processes. The fabrication of asym-metric polymer consisting a defect-free and ultra-thin-skinned layer has become the strong subjectof research interest because it stresses on the im-portance of having high permeability and select-

ivity gas separation membranes. High perform-ance zeolite materials as a filler for mixed-matrixmembranes can further take gas separation mem-branes into uncharted areas. Membrane separa-tions employing mixed-matrix materials havethe potential to meet demanding gas separationin a world with shrinking energy resources andincreasing environmental concerns. These zeolitecrystals, which are embedded in a polymermatrix, serve as the supporting material and offerprocessing flexibility unmatched by inorganicmaterials. As for chemical cross-linking modifi-cation by UV light and thermal treatment, thispromising technique will be widely used in thenear future. Covalent cross-linking has been re-ported to be an effective approach for stabilizingmembranes while retaining very attractive trans-port properties. The degree of cross-linking can becontrolled by the amount of carboxylic acid moie-ties incorporated into the polymer backbone andthe thermal treatment given to the polyimidefilms. Though a lot of efforts were being devotedto membrane formation for gas separation in anumber of promising area, more research and de-velopment strategies are still needed in order toexploit the full potential of this technology in or-der to produce high performance membrane forgas separation in the next decade.

ReferencesAncelin, H., Yarwood, J., Willat, A.J and Clint, J.H.

1992. Fourier transform infrared and selecti-vity measurements on polymeric gas separationmembranes, Structure driven design of newmaterial, Vibrational Spectroscopy. J. Membr.Sci., 4: 1-7.

Aptel, P., Abidine, N., Ivaldi, F. and Lafaille, J.P. 1985.Polysulfone hollow fibers-effect of spinningconditions on ultrafiltration properties. J.Membr. Sci., 22: 199-215.

Baker, R.W. 2000. Membrane Technology and Appli-cation. Membrane Technology and Research,Inc.McGrow-Hill, New York.

Boom, J.P., P˙̇unt, I.G.M., Zeijnenberg, H., Boer, R.D.,Bargeman, D., Smolders, C.A. and Strathmann,H. 1998. Transport through zeolite-filled poly-



meric membranes. J. Membr. Sci., 138:237.Bousmina, M. and Muller, R. 1996. Rheology/mor-

phology/flow conditions relationships for poly-methylmethacrylate/rubber blend. Rheol. Acta.,35: 369-381.

Carruthers, S.B. 2001. Integral-skin formation in hol-low fiber membranes for gas separations. Fac-ulty of the Graduate School of The Universityof Texas at Austin. Ph.D. Thesis.

Chandak, M.V., Lin, Y.S., Ji, W. and Higgins, R.J.1997. Sorption and diffusion of VOCs in DAYzeolite and silicalite-filled PDMS membranes.J. Membr. Sci., 133: 231.

Chung, T.S., Lin, W.H and Vora., R.H. 2000. Theeffect of shear rates on gas separation perform-ance of 6FDA durene polyimide hollow fibers.J. Membr. Sci., 167: 55-66.

Chung, T.S., Qin, J.J., Huan, A., and Toh, K.C. 2002.Visualization of the die shear rate on the outersurface morphology o ultrafiltration membraneby AFM. J. Membr. Sci., 196: 251-266.

Cleeremen, K.J. 1974. Influence of molecular orienta-tion on the properties of amorphous polymer.Applied Polymer Symposia., 24: 31-35.

Dotremont, C., Brabants, B., Geeroms, K., Mewis, J.and Vandecasteele, C. 1995. Sorption and dif-fusion of chlorinated hydrocarbons in silical-ite-filled PDMS membranes. J. Membr. Sci.,104: 109.

Dudley, C.N., Schoberl, B., Sturgill, G.K. Beckam,H.W. and Rezac, M.E. 2001. Influence of cross-linking technique on the physical and transportproperties of ethynyl-terminated monomer/poly-etherimide asymmetric membranes. J. Membr.Sci., 191: 1-11.

Duval, J.M., Kemperman, A.J.B., Folkers, B., Mulder,M.H.V., Desgrandchamps, G. and Smolder, C.A.1994. Preparation of zeolite filled glassy poly-mer membranes. J. Membr. Sci., 54: 409.

Duval, J.M., Folkers, B., Mulder, M.H.V., Desgrand-champs, G. and Smolders, C.A. 1993. Adsorb-ent-filled membranes for gas separation. Part 1.Improvement of the gas separation propertiesof polymeric membranes by incorporation ofmicroporous adsorbents. J. Membr. Sci., 80: 189.

Duval, J.M. 1993. Adsorbent-filled polymeric mem-branes. Ph.D. Thesis, University of Twente,

Enschede, The Netherlands.G˙̇ur,T.M. 1994. Permselectivity of zeolite filled poly-

sulfone gas separation membranes. J. Membr.Sci., 93: 283.

G˙̇urkan, T., Baç, N., Kyran,/

G. and G˙̇ur, T. 1989. Anew composite membrane for selective trans-port of gases, in: Proceedings of the 6th Inter-national Symposium on Synthetic Membranesin Science and Industry. T˙̇ubingen, Germany.

Hachisuka, H., Ohara, T. and Ikeda, K. 1996. New typeasymmetric membranes having almost defectfree hyper-thin skin layer and sponge-like po-rous matrix. J. Memb. Sci., 116: 265-272.

Ismail, A.F. 1997. Novel studies of molecular orienta-tion in synthetic polymeric membranes for gasseparation. Department of Chemical and Proc-ess Engineering, University of Strathclyde: Ph.D.Thesis

Ismail, A.F., Shilton, S.J., Dunkin, I.R. and Gallivan,S.L. 1997. Direct measurement of rheological-ly induced molecular orientation in gas sepa-ration hollow fibre membranes and effects onselectivity. J. Membr. Sci., 126: 133-137.

Ismail, A.F. and Shilton, S.J. 1998. Polysulfone gasseparation hollow fiber membranes with en-hanced selectivity. J. Membr . Sci., 139: 285-286.

Ismail, A.F. and David, L.I.B. 2001. A review on thelatest development of carbon membranes forgas separation. J. Memb. Sci., 193:1-18.

Jansen, J.A.J and Haas, W.E. 1988. Applications ofdiffuse reflectance optics for the characteri-zation of polymer surfaces by Fourier transforminfrared spectroscopy. Polymer Commun., 29:77-80.

Jia, M., Peinemann, K.V. and Behling, R.D. 1991.Molecular sieving effect of the zeolite-filledsilicone rubber membranes in gas separation.J. Membr. Sci., 133: 231.

Jia, M.D., Peinemann, K.V. and Behling, R.D. 1992.Preparation and characterization of thin-filmzeolite–PDMS composite membranes. J. Membr.Sci., 73:119.

Kawakami, H., Nakajima, K. and Nagaoka, S. 2002b.Gas separation characteristics of isomeric poly-imide membrane prepared under shear stress. J.Memb. Sci., 211:1-8.

Kawakami, H., Nakajima, K., Shimizu, H. and Na-



gaoka, S. 2002a. Gas permeation stability ofasymmetric polyimide membrane with thin skinlayer: effect of polyimide structure. J. Memb.Sci., 78:1-9.

Kawakami, H., Mikawa, M. and Nagaoka, S. 1996.Gas permeabilitynand selectivity through asym-metric polyimide membranes. J. Applied.Polym. Sci., 62:965-971.

Khulbe, K.C., Gagn ′′e ,S., Tabe Mohammadi, A., Mat-suura T. and Lamarche, A.M. 1995. Investiga-tion of polymer morphology of integral-asym-metric membranes by ESR and raman spec-troscopy and its comparison with homogene-ous films. J. Membr. Sci., 98: 201.

Koenhen, D.M, Mulder, M.H.V. and Smolders, C.A.1977. Phase separation during the formation ofasymmetric membranes. J. Applied. Polym. Sci.,21:199.

Kulprathipanja, S., Neuzil, R.W. and Li, N, N. 1988.Separation of fluids by means of mixed matrixmembranes. U.S. Patent., 4, 740, 219.

Kurdi, J. and Tremblay, A.Y. 1999. Preparation of de-fect-free asymmetric membrane for gas sepa-ration. J. Applied. Polym. Sci., 79:1471-1483.

Lai, P.Y. 2002. Development of defect-free and ultra-thin-skinned asymmetric membranes for gasseparation process. Department of ChemicalEngineering, University Technology of Malay-sia: Msc. Thesis.

Lin, F.C., Wang, D.M. and Lai, J.Y. 1996. AsymmetricTPX membranes with high gas flux. J. Membr.Sci., 110: 25-36.

Liu, Y., Pan, C., Ding, M. and Xu, J. 1999. Gas perme-ability and permselectivity of photochemicallycrosslinked copolyimides. J. Applied. Polym.Sci., 73: 521-526.

Liu, Y., Wang, R. and Chung, T.S. 2001. Chemical cross-linking modification of polyimide membranesfor gas separation. J. Membr. Sci., 189: 231-239.

Lokhandwala, K.A., Schurmans, M. and Jacobs, M.L.1995. Use membranes for improved gas pro-cessing. Membrane Technology & Research,Inc. Menlo Park, CA.U.S.A. 94025.

Mahajan, R., Burns, R., Schaeffer, M., and Koros,W.J. 2002. Challenges in forming successfulmixed-matrix membranes with rigid poly-meric materials. J. Applied. Polym. Sci., 86:

881–890Mark, H.F., Bikare, W.M., Overberger, C.G., and

Menges, G. 1985. Encyclopedia of Science andEngineering (vol. 9, p.50).

Netke, S.A., Sawant, S.B., Joshi, J.B. and Pangarkar,V.G. 1995. Sorption and permeation of aceticacid through zeolite-filled membrane. J. Membr.Sci., 107: 23.

Niwa, M., Kawakami, H., Nagaoka, S. Kanamori, T.and Shinbo, T. 2000. Fabrication of an asym-metric polyimide hollow fiber with a defect-free surface skin layer. J. Membr. Sci., 171:253-261.

Ouyang, M., Muisener, R.J., Boulares, A. and Kober-stein, J.T. 2000. UV-ozone induced growth ofSiO

x surface layer on a cross-linked polysilox-

ane film: characterization and gas separationproperties. J. Membr. Sci., 177: 177-187.

Pandey, P. and Chauhan, R.S. 2000. Membrane for gasseparation. Prog. Polym. Sci., 26: 853-893.

Paul, D.R. and Kemp, D.R. 1973. The diffusion timelag in polymer membranes containing adsorp-tive fillers. Polym. Sym. Ed., 41: 79.

Pechar, T.W., Tsapatsis, M., Marand, E. and Davis, R.2002. Preparation and characterization of aglassy fluorinated polyimide zeolite-mixed ma-trix membrane. Desalination., 146: 3-9.

Peinemann, K.V. and Lauenburg. 1988. Method forproducing an integral asymmetric gas separa-tion membrane and the resultant membrane. USPatent., 4, 746, 333.

Perepelkin, K.E. 1977. The structural factor in the ori-entation process of fibers and films of flexibleand rigid-chain polymers. Khimicheskie vol-okna., 4: 7-12.

Pesek and Koros. 1993. Aqueous Quenched asym-metric polysulfone hollow fibers prepared bydry/wet phase separation. J. Memb. Sci., 81: 71-88.

Pinnau, I., Wind, J. and Peinemann, K.V. 1990. Ultra-thin multicomponent polyethersulpone mem-branes for gas separation made by dry/wetphase inversion. Ind. Eng. Chem. Res., 29: 2028-2032.

Ren, J., Chung, T.S., Li, D., Wang, R. and Liu, Y.2002. Developtment of asymmetric 6FDA-2,6DAT hollow fiber membranes for CO

2/CH

4



separation 1)influence of dope compositionand rheology on membrane morphology andseparation performance. J. Memb. Sci., 207: 227-240.

Serkov, A.T. and Kanchih, O.A. 1977. Formation oforiented structures in the precipitation of poly-mers concentrated solutions. KhimicheskieVolokna., 4: 12-16.

Shilton, S.J., Bell, G. and Ferguson. 1996. The deduc-tion of fine structural details of gas separationhollow fiber membranes using resistance mod-elling of gas permeation. J. Polymer., 37: 485.

Shilton, S.J., Bell, G. and Ferguson. 1994. The rheol-ogy of fiber spinning and the properties ofhollow-fiber membranes for gas separation. J.Polymer., 35:5327.

Sirkar, K.K. and Winston Ho, W.S. 1992. Part I andPart II In Membrane Handbook. New York: VanNostrand Reinhold., 1-25.

S˙̇uer, M.G., Baç, N. and Yylmaz,/

L. 1994. Gas per-meation characteristics of polymer–zeolite mixedmatrix membranes. J. Membr. Sci., 91: 77.

Takuechi, Y., Shuto, Y. and Yamamoto, F. 1988. Bandpattern development in shear-oriented thermo-tropic copolyester extrudates. Polymer., 29:605-612.

Tantekin-Ersolmaz, S.B., Atalay-Oral, C., Tatlyer, /

M.,Erdem-Senatalar, A., Schoeman, B. and Sterte,J. 2000. Effect of zeolite particle size on theperformance of polymer–zeolite mixed matrix.J. Membr. Sci. 175: 285-288

Van de Witte, P., Dijkstra, P. J., Van den Berg, J. W. A.and Feijen, J. 1996. Phase separation processesin polymer solutions in relation to membraneformation. J. Membr. Sci., 117: 1-31.

Wang, D., Li, K. and Teo, W.K. 1996. Polyethersulfonehollow fibre gas separation membranes pre-pared from nmp/alcohol solvent systems. J.Membr. Sci., 115: 85-108.

Wang, R. and Chung, T.S. 2001. Determination of poresizes and surface porosity and the effect ofshear stress within a spinneret on asymmetrichollow fiber membranes. J. Memb. Sci., 188:29-37.

Yong, H.H., Park, H.C., Kang, Y.S., Won, J. and Kim,W.N. 2001. Zeolite-filled polyimide membranecontaining 2,4,6-triaminopyrimidine. J. Memb.Sci., 188: 151-163.

Zimmerman, C.M., Singh, A. and Koros, W.J. 1997.Tailoring mixed matrix composite membranesfor gas separation. J. Memb. Sci., 137:145-154.

Documents

Latest development on the membrane formation for gas ...rdo.psu.ac.th/sjst/journal/24-Suppl-1/28gas-separation.pdfMembrane gas separation by polymer membrane is a proven technology