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Separation Science and Technology

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Effect of Stabilization Condition on PEI/PVP-BasedCarbon Hollow Fiber Membranes Properties

W. N. W. Salleh & A. F. Ismail

To cite this article: W. N. W. Salleh & A. F. Ismail (2013) Effect of Stabilization Condition on PEI/PVP-Based Carbon Hollow Fiber Membranes Properties, Separation Science and Technology,48:7, 1030-1039, DOI: 10.1080/01496395.2012.727938

To link to this article: http://dx.doi.org/10.1080/01496395.2012.727938

Accepted author version posted online: 15Jan 2013.Published online: 15 Jan 2013.

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Effect of Stabilization Condition on PEI/PVP-Based CarbonHollow Fiber Membranes Properties

W. N. W. Salleh1,2 and A. F. Ismail1,21Advanced Membrane Technology Research Centre, Universiti Teknologi Malaysia, Skudai, JohorBahru, Malaysia2Faculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia, Skudai,Johor Bahru, Malaysia

The concept of carbon membrane fabrication has attracted agreat deal of attention among membrane researchers around theworld in recent years. Stabilization is one of the important stepsinvolved during the fabrication of the carbon membranes. The stabi-lization of PEI/PVP membrane was carried out in a tubular furnaceunder two types of environments (air and N2). The resulting set ofexperiments from thermogravimetry analysis (TGA), Fourier trans-form infrared spectroscopy (FTIR), scanning electron microscopy(SEM), and X-ray diffraction (XRD) was used to investigate theeffect of stabilization environments on the properties of the preparedcarbon hollow fiber membranes (CHFM)s in terms of their morpho-logical structure, thermal property, chemical structure, and micro-structure. The detailed studies with regard to the chemical reactionmechanism occurring during the heat treatment process of the PEI/PVP-based CHFMs were explored. During the heat treatment pro-cess, PEI/PVP membranes underwent various physical and chemicalchanges, such as coloration, shrinkage, crosslinking reaction, andrandom scission. Based on overall properties, the stabilization stepunder air environment prior to carbonization step showed the bestcondition for the preparation of CHFMs derived from PEI/PVP.

Keywords carbon membrane; environments; hollow fiber; stabi-lization; thermal properties

INTRODUCTION

Generally, a majority of carbon membrane research hasdealt with synthesizing new carbon membrane materialsusing new polymeric precursor materials, new heat treat-ment techniques or modifying existing precursors, andcarbon membranes. During the conversion of polymericmembranes into carbon membranes, an intermediate steptermed stabilization is crucial. The quality of the resultingcarbon membranes depends strongly upon the degree ofstabilization (1,2). Surprisingly, there are very few studies

devoted to the real understanding of the effect of stabiliza-tion conditions on the properties of the resultant carbonmembranes. The chemical reaction mechanism involvedduring the heat treatment process is also not well character-ized, although such research has clearly indicated that thestabilization history in the carbon membrane preparationhas a profound impact on the qualities of the resultantcarbon membranes. So far, the studies on the stabilizedPEI=PVP-based carbon membrane are generally confinedto surface morphology structure using techniques likeSEM (3,4). The present study has, therefore, been underta-ken not only to establish the structure of the stabilizedPEI=PVP-based CHFMs but also to follow the gradualtransformation of the structure as the stabilization pro-gresses under different gas environments (air and N2) usingthe TGA, FTIR, and XRD.

In order to achieve an optimum stabilization conditionfor polymeric precursor membrane, it is necessary to under-stand the mechanism and reaction taking place during theheat treatment process. Basically, the stabilization step thatis performed under air atmosphere would proceed viadiffusion of oxygen, which results in a variety of oxygencontaining functional groups on the aromatic rings, dehy-dration, and cross linking of the oligomers. During thisprocess, polymeric precursor membranes were slowly con-verted to carbon material. Oxygen in the air would readilychemisorb into the carbon surface and carbon-oxygencomplexes were formed (5,6). Besides that, during thermaltreatment, a degradation process takes place where theproperties of the polymer deteriorate. The heat treatmentof polymer material consists of a series of degradationreactions along the polymer chain. As the temperatureincreases, linkages along the polymer chain are broken,starting from the weakest bonds and progressing to thestrongest. At the same time, H2, H2O, CO, and CO2 evolveas the remaining carbon becomes more aromatic (7,8).

According to Bos et al. (9) and Drbohlay and Stevenson(10), the stabilization step prior to carbonization is recom-mended to prevent thermoplastic polymers from the

Received 20 February 2012; accepted 3 September 2012.Address correspondence to A. F. Ismail, Advanced Membrane

Technology Research Centre, Universiti Teknologi Malaysia,81310 Skudai, Johor Bahru, Malaysia. Tel.: þ60 7 5535592;Fax: þ60 7 5581463. E-mail: afauzi@utm.my; fauzi.ismail@gmail.com

Separation Science and Technology, 48: 1030–1039, 2013

Copyright # Taylor & Francis Group, LLC

ISSN: 0149-6395 print=1520-5754 online

DOI: 10.1080/01496395.2012.727938

1030

melting stage at high temperatures and helps to maintainthe morphology and structure of the precursor in the result-ant carbon membranes. This is because the thermoplasticpolymers favor to go through a melting or a softening stageduring the carbonization step. In the carbon membranefabrication, the melting stage is unfavorable. It is becausethe microporous structure evolved would be destroyeddue to the removal of functional groups in the form ofvolatiles or gases during carbonization (11–14).

EXPERIMENTAL

Membrane Fabrication

In this study, PEI (Ultem 1000) as the basic polymer andPVP (Fluka, K90) as the second polymer were used formaking the blend membrane. Both polymers were driedovernight at 120�C prior use. The chemical N-methyl-2-pyrrolidone (NMP) with analytical grade was used assolvent without any purification. PEI=PVP hollow fibermembranes with 17wt% PEI=6wt% PVP were used as aprecursor membrane (15). Precursor membranes were sub-jected to stabilization step under two different environ-ments before undergoing the carbonization step at hightemperature. The heat treatment process was carried outby placing the precursor membrane in the center of Carbo-lite (Model CTF 12=65=550) wire wound tube furnace withEurotherm 2416CC temperature control systems. In thestabilization step, the precursor membranes were treatedunder compressed air or N2 atmosphere (200mL=min) forup to 300�C with a heating rate of 3�C=min. At this stage,the membranes were held for 30min. Subsequently, thetemperature was increased to a final carbonization tempera-ture of 650�C with the heating rate of 3�C=min and holdingit constant for 30min under N2 flow (200mL=min). At last,the membranes were cooled down naturally to room tem-perature. The detailed heat treatment profile is illustratedin Fig. 1.

Membrane Characterization

The physical and chemical changes during the stabiliza-tion of PEI.PVP membranes were examined by means ofTGA, FTIR, SEM, and XRD. The thermal property ofthe membrane during the heat treatment process was char-acterized by using Mettler TGA TSO800GC1 in flowingN2 at a ramp of 10�C=min from 50 to 900�C. FTIR analysiswas taken via a Nicolet (Magna-IR 560) spectrometer to dis-play the change of the functional groups and elemental inthe membranes when they were heated from room tempera-ture to carbonization temperature. The FTIR spectra wererecorded at a resolution between 600 and 2500 cm�1 at roomtemperature with a spectral resolution of 8 cm�1 and aver-aged over 20 scans. The morphological structure micropho-tographs were obtained using a JEOL JSM-5610LV SEM.The microstructure properties of resulting CHFMs were

examined using X’Pert PRO XRD from PANalytical withthe diffraction angle 2h from 10 to 50�. Ni-filtered CuKaradiation with a wavelength of k¼ 1.54 A was applied inthe experiments. The interplanar distance (d-spacing) ofthe CHFMs was calculated using the well-known Braggequation and can be determined from the maximumintensity.

nk ¼ 2d sin h ð1Þ

where d is the dimension spacing (A), h is the diffractionangle (�), k is the X-ray wavelength (A), and n is an integralnumber (1, 2, 3, . . .).

In this study, carbon membrane performance wasmeasured using the single gas permeation system. Thedetailed experimental procedures can be found elsewhere(15). Additionally, the length and outer diameter of eachmembrane were measured before and after the heat treat-ment process and the percent of the shrinkage was calcu-lated by using:

Percent Shrinkage ¼ ðLinitial � LfinalÞ=Linitial � 100 ð2Þ

Percent Shrinkage ¼ ðDinitial=DfinalÞ=Dinitial � 100 ð3Þ

RESULTS AND DISCUSSION

Thermal Properties

In order to obtain information on the stabilization con-ditions, the thermal degradation behaviors of membranesunder both air and N2 environments were monitored viaTGA. Figure 2 illustrates the TGA profiles of the membrane

FIG. 1. Heat treatment profile.

CARBON MEMBRANE FABRICATION 1031

recorded during the heat treatment process under air and N2

environments. The main purpose of this measurement is todetermine the suitable environment for stabilization stepduring the heat treatment process in the preparation ofthe PEI=PVP-based CHFMs. This is because the PEI is aplastic polymeric material and the stabilization step is a veryuseful approach in order to stabilize the structure of theprecursor membrane to withstand the rigors of high tem-perature process in carbonization step (11,16).

The TGA profiles of the membrane treated under air andN2 environments showed two stages of weight loss. The firstand second main stage of weight loss taking place in bothenvironments could be observed at the temperature intervalof 350–530� 1�C and 540–700� 1�C, respectively. How-ever, for membrane treated under the N2 environment,there were two steps of weight loss that could be observedat the first main stage. These stages of weight loss werecaused by the removal of moisture and residual solvent,and the scission and decomposition of the polymer mainchains, respectively. It was obvious that the first main stageof weight loss occurred in both membranes treated under airand N2 environment (17,18). The weight loss of the mem-brane treated under air atmosphere shifting towards highertemperature was also traced. It was attributed by the forma-tion of a high performance carbonaceous char on the sur-face of the samples that insulated the underlying materialand minimized the escape of volatile products generatedduring decomposition (19).

In the case of using N2 atmosphere, the degradation pro-cess of the membrane was enhanced, presumably due to theincrease in gas phase heat and mass transfer during the heat

treatment process (7,19). Moreover, the stabilizationenvironment was shown to affect the final carbon mem-brane properties. Under N2 atmosphere, weight loss startedfrom 350� 1�C, and nearly constant above 700� 1�C at50%, representing a higher carbon yield than those mem-branes treated under air atmosphere of 5% in the samecondition. This is because the membranes treated at hightemperature process under air atmosphere would cause asignificant weight loss due to the presence of oxygen inair, whereas a thermostable membrane was formed for themembranes treated under N2.

Based on the results, the stabilization step under airatmosphere was preferred and more suitable for the fabri-cation of the PEI=PVP-based CHFMs, since these mem-branes were more stable in air atmosphere at low processtemperature due to the oxygen containing functionalgroups incorporated into the polymer chains by the crosslinking reaction. On the other hand, the N2 environmentwas more preferred for the carbonization step since thethermostability membrane was better in N2 at higher tem-peratures, indicating by the least weight loss obtained.Carbonization under air atmosphere could also partiallydestroy the original backbone of the polymer (20, 21). Thus,the second step (carbonization) of the heat treatmentprocess in the preparation of PEI=PVP-based CHFMswas performed under N2 atmosphere.

Chemical Structure Properties

FTIR was conducted in order to further elucidate thedecomposition reaction occurring during the heat treatmentprocess. This analysis can also be used to characterize thetransformation of the polymeric precursor membrane intocarbon membranes and examine whether any identifiablefunctional groups remain in the membrane after the heattreatment process. Figure 3 depicts the FTIR spectra ofthe precursor membrane treated at 100 to 300�C under airatmosphere together with their FTIR spectrum of theresulting CHFMs treated up to and 650�C. As clearlyobserved, there was a significant change in the spectra ofthe membrane as the process temperature increased. Whenthe membrane was treated from 100 to 300�C, there were

FIG. 2. TGA profile. (Color figure available online)

FIG. 3. FTIR spectra of membranes treated at different temperatures.

(Color figure available online)

1032 W. N. W. SALLEH AND A. F. ISMAIL

two intense peaks appearing at around 1677 to 1748 cm�1

and 1025 cm�1. These peaks were associated to C=Ostretching and Ar-O-Ar (aryl ether bonds) stretching,respectively. However, when the carbonization temperatureincreased up to 650�C, the peak assigned for C=O stretch-ing completely disappeared. The intensity in the FTIR spec-trum of the resulting CHFMs also reduced and becamebroad as process temperature increased. It was plausiblethat oxygen-containing functional groups such as carboxylgroups were introduced mainly on the surface of the mem-brane by the oxidation (stabilization under air atmosphere)at 100 to 300�C and those oxides were continuously decom-posed during the high temperature process.

Figure 4 illustrates the FTIR spectra of the stabilizedmembrane prepared under different stabilization environ-ments. Similar absorption band characteristics with quite dif-ferent intensity were observed for stabilized membranesperformed under air and N2 atmosphere. The region from1900 to 2500 cm�1 and 1600 to 600 cm�1 of the stabilizedmembrane prepared under air atmosphere showed a closeresemblance to that of stabilized membrane prepared underN2 atmosphere. Important changes observed on stabilizedmembranes could be indicated by the presence of a broadpeak at 1677 cm�1 as shown for stabilized membrane treatedunder air environment. This might be attributed to carbonylformed by oxidation during the stabilization. Meanwhile forthe stabilized membrane prepared under N2 environment, apeak associated to C=O stretching of the aromatic rings wereobserved at 1722 cm�1. Besides that, the absorption bandassigned for Ar-O-Ar (aryl ether bonds) stretching wasobserved at 1025 cm�1, whereas the deformation vibrationsmodes of the aromatic ring and phthalimide ring bendingvibrations could be seen at 796 cm�1 for both membranes.

Furthermore, FTIR spectra of the resultant CHFMsprepared under different stabilization environments areshown in Fig. 5. It was clearly revealed that, both CHFMsshowed similar absorption band characteristics and therewere no significant differences that could be observed.Two intense absorption bands appeared in both of theresultant CHFMs. The bands at 1552 and 1558 cm�1

observed for CHFMs prepared under N2 and air stabiliza-tion, respectively, were referred to the C=C or C-N stretch-ing (22,23). Meanwhile, the absorption bands assigned forAr-O-Ar (aryl ether bonds) stretching were observed at1066 and 1047 cm�1.

Overall, the changes of the functional groups in themembrane structure during the heat treatment processcould be identified and observed by means of FTIR. TheFTIR spectra of the resultant CHFMs changed as the pro-cess temperature increased. This was because the membranehad decomposed and almost all the chemical structures inthe polymeric precursor membranes had been convertedinto carbon structure. The change in the chemical structureof the polymeric precursor membrane during the heat treat-ment process was also irreversible. It can be seen from theFTIR spectra of the resulting membrane shown above.The result was in agreement with the Musto et al. (22)and Zhou et al. (23).

According to FTIR measurements, polymer blends ofPEI=PVP underwent thermal degradation without interact-ing chemically with each other as found in the case of PEIblend with polybenzimidazole (PBI) (22). The chemicalstructure of PEI is shown in Fig. 6. During the heat treat-ment process under oxygen-containing atmosphere, the ther-mal degradation of the PEI polymer occurred in two ways:

i. by cross linking andii. by random scission (24).

FIG. 4. FTIR spectra of stabilized membrane prepared under different

environments. (Color figure available online)

FIG. 5. FTIR spectra of CHFMs prepared under different stabilization

environments. (Color figure available online)

FIG. 6. Chemical structure of PEI polymer.

CARBON MEMBRANE FABRICATION 1033

Cross linking can be formed by chemical reactions that areinitiated by heat. Cross linking generally occurs aftersome stripping of substituent and involves the creationof bonds between two adjacent polymer chains. Theseprocesses are very important in the formation of chars,since they generate a structure with a higher molecularweight that is less easily volatilized (25). Figure 7demonstrates the cross linking reaction that occurred bycombination of two radicals as suggested by Filho andcoworkers (24).

The random scission is another reaction involving themain chain. It may occur at random locations in the chain.The process is a multistep radical chain reaction with all thegeneral features of such reaction mechanism: initiation,propagation, branching, and termination steps. Randomchain scissions generally result in the generation of mono-mers and oligomers (polymer units with 10 or fewer mono-mer units) as well as a variety of other chemical species. Thetype and distribution of volatile products depend on therelative volatility of the resulting molecules (25). The cross-linking in PEI took place at a temperature ranging from 300to 380�C, whereas chain scission occurred above 400�C anda maximum rate of decomposition (rate of weight loss) wasfound to be around 510 to 540�C.

Based on thermal properties, the precursor membranestreated under air atmosphere were more stable comparedto those treated under N2 atmosphere. This was becauseas a small molecule, oxygen (presence in the air) took partin the reaction, where the crosslinking reaction in a ratherlocalized region occurred and it would not affect much ofthe chain mobility. At this stage, the major products wereCO, CO2 and phenol and the C-H transfer proceeded toconvert the precursor membranes into partially carbonizedmembranes. In contrast, for membranes treated under N2

atmosphere, the mobility of chain molecules became restric-ted as the crosslinking reaction proceeded (26–29).

In the case of chain scission, it involved two stages ofreaction, which an early stage occurred at 520�C, wherethe isopropyledene moieties, ether linkages and phenyl–phthalimide rings were broken, and a less pronounced stageoccurred above 600�C, producing CO2 and water by a heatinduced hydrolysis of phthalimide rings. The detailedmechanisms of the random chain scission occurred in thePEI polymer are summarized in References (24,30,31).

As illustrated in Fig. 6, the thermal weak points of PEIwere at ether bonds and isopropylidene bridges, whichbroke at low process temperature. The disproportionationof the isopropylidene bridge followed by C-H transfer reac-tions would form compounds such as intact phthalimiderings containing the aromatic ether moiety and bearinghydrogen, methyl, ethyl, isopropylidene and isopropylend groups (R) (R equals to H, CH3, C2H5, C3H5,C3H7). The scission of the ether bridges would generatecompounds consisting of phthalimide units with the phenylrings substituted with H=OH and=or bisphenol-A, whereasthe scission of phenyl-phthalimide bonds might generatecompounds with N-H and=or N-phenyl as the end groups.The N-H phthalimide end groups which involved the gen-eration of a thermally labile isocyanate intermediate thatdecomposes forming nitriles and CO2 was also believedto occur at a high temperature process. Besides that, stabi-lization in oxygen containing atmosphere (air) wouldalso lead to the formation of compounds such as alcohols,acetones and acids.

Furthermore, the phthalimide unit offers several path-ways that may lead to the loss of CO2 and water duringthe heat treatment process. As the process temperatureincreased (carbonization step), the hydrolysis of the phthali-mide rings to form poly (amic acids) which underwent dec-arboxylation had already contributed to the formation ofchar and CO2. Another possible reaction was water elimin-ation and cross linking that produced Nitrile compoundand char. These kinds of reaction mechanisms were inagreement with the respective thermal degradation of PEIreported by Farong et al. (26), Kuroda and Mita (30),Kuroda et al. (32), Torrecillas et al. (17), Carroccio et al.(31), Perng (27), and Filho et al. (24).

It was pointed out that the cross linking were dominantover the scission reaction in PEI=PVP precursor membranedue to the thermally stable properties attributed from imidelinkage (phthalimide units). This was because, in fact, thegreater the cross linking density, the higher Tg wouldbecome. As the temperature increased above the Tg, thepolymer became denser in chain packing and density(7,29,30,33). Furthermore, it could be concluded that therewere several chemical reactions involved during the stabili-zation in air atmosphere including cross linking, oxidation,and dehydrogenation reaction. In contrast, only the dehy-drogenation reaction occurred in the stabilization undernitrogen atmosphere.FIG. 7. Cross linking reaction in PEI (24).

1034 W. N. W. SALLEH AND A. F. ISMAIL

On the other hand, the reaction mechanisms involved inthe thermal degradation of PVP went through the C-Nbonds breaking and C-C main chain random scission asdemonstrated by Peniche et al. (34) and Bastarrachea et al.(35). It was revealed that the main product of the thermaldegradation of the PVP was pyrrolidone. Under nitrogenatmosphere, the thermal degradation of the PVP occurredfrom the release of vinyl pyyrolidone together with oligo-mers via depolymerization reaction. The chemical reactioninvolved during the heat treatment of the polymeric precur-sor membrane at high temperature is also very useful todetermine the thermal resistance of the polymer in orderto successfully produce carbon material. The high tempera-ture process was performed in the second step of heat treat-ment process, which is known as carbonization. The majorreaction in the carbonization stage was the decompositionof the stabilized membranes and remaining imide groupsto form CO and CO2 as the major products along with ben-zene and a small amount of benzonitrile (27).

Morphological Structure Properties

Figures 9 and 10 show the outer surface and cross sec-tion microphotographs of the stabilized membranes treatedunder air and N2 environments, respectively. The SEMmicrophotograph of the precursor membrane (see Fig. 8)and final CHFM (Figs. 11 and 12) also provided as com-parison purposes. It could be observed that there weresome microporous structures and few closed pores on theouter surface of the precursor membrane and stabilizedmembranes treated under air environment. For stabilizedmembranes treated under N2 environment, the size ofmicroporous structure was smaller and had a smooth

surface with almost defect free. Both final CHFM fabri-cated from PEI=PVP and PEI also showed smooth surfacewith almost defect free on the outer surface of the fiber.

As can be seen in Figs. 9, 11, and 12, two dense layers,both outer and inner layers with porous sub-layer inbetween, were obtained for stabilized membranes treatedunder air atmosphere and final CHFM derived fromPEI=PVP and PEI, which were similar to the structure ofthe precursor membrane. In contrast, for stabilized mem-brane treated under N2 environment, a dense structuremembrane was obtained. This was revealed that any

FIG. 9. SEM microphotographs of (a) outer surface and (b) cross

section of the stabilized membrane treated under air environment.

FIG. 8. SEM microphotographs of (a) outer surface and (b) cross

section of the precursor membrane.

FIG. 10. SEM microphotographs of (a) outer surface and (b) cross

section of the stabilized membrane treated under N2 environment.

CARBON MEMBRANE FABRICATION 1035

arrangement of the structure in the precursor membranewould not occur during the stabilization process underair environment. The same structural properties wereobserved in the carbon membrane derived from a polymerblending between PAN and PVP prepared by Linkov et al.(36). In addition, there were no cross section deformationsand irregularities observed for all the membranes.

Membrane Shrinkage

A significant reduction of the diameter of the precursormembrane was observed for the stabilized membrane

treated under N2 environment (26%), compared to thosetreated under air environment (20%). It was indicated thatthe decomposition of the precursor membrane was morepronounced under N2 environment, which resulted in thephysical shrinking of the membrane. The differences inthe structure of the resultant stabilized membranes weresignificantly affected by the stabilization environment usedduring the heat treatment process. The dimensions of theresultant stabilized membranes treated at different stabili-zation environments are summarized in Table 1.

Microstructure Properties

In order to gain further insight into the microstructuralchanges of the membranes, the d-spacing value of the peaksappearing on the XRD patterns were estimated. XRD wasused to investigate the structural change in the amorphousnature during the transformation from PEI=PVP precursorinto carbon materials during heat treatment. The changesin d-spacing upon heat treatment functioned as useful indi-cators of the amount of pores available for the passage ofsmall molecules (14,15). Figure 13 illustrates the XRD pat-terns of the stabilized membranes treated under differentstabilization environments.

As can be seen, a broad weak peak was present for bothmembranes stabilized under air andN2 environments, whichcould be attributed to the turbostratic carbon structure withrandomLy oriented graphitic carbon layers. The reflectionat 2h¼ 22.50� (0.395 nm) and 2h¼ 21.75� (0.409 nm) couldbe observed for stabilized membrane treated under N2 andair environments, respectively. This peak could be indexedto the (100) plane of a hexagonal structure (2). XRD pat-terns revealed that the stabilized membranes were amorph-ous structure. Compared with the stabilized membranestreated under air atmosphere, the d-spacing value of the sta-bilized membrane stabilized under N2 showed lower value.This means that a larger amount of pore structure was cre-ated during the stabilization of precursor membranes from

FIG. 12. SEM microphotographs of (a) outer surface and (b) cross

section of the final PEI-based CHFM.

FIG. 11. SEM microphotographs of (a) outer surface and (b) cross

section of the final PEI=PVP-based CHFM.

TABLE 1Percent shrinkage of the resultant stabilized membrane

Samples

Outerdiameter(mm)

Shrinkage(%)

Length(cm)

Shrinkage(%)

Precursormembranes

0.88� 1 – 43.00� 1 –

Stabilizedmembrane(Air)

0.70� 1 20.45� 1 32.50� 1 24.42� 1

Stabilizedmembrane(N2)

0.65� 1 26.14� 1 31.10� 1 27.67� 1

1036 W. N. W. SALLEH AND A. F. ISMAIL

the air atmosphere. It was in agreement with the resultsfound in Zhang and coworkers’ paper (37).

In addition, the existence of carbon-carbon spacing ongraphitic planes could be observed by the appearance ofsecond weak peak reflection with d-spacing value of0.212 nm and 0.208 nm for membranes stabilized underN2 and air environments, respectively. This indicated thatthe rigid aromatic graphitic planes with more ordered andbetter packed structure were created after the stabilizationprocess of the precursor membranes. In overall, there weretwo d-spacing peaks obtained after stabilization process,indicating the wide pore distribution in the membranestructure (4,38–40).

Gas Permeation Properties

The influence of stabilization environment on the gaspermeation properties of the resultant CHFM is summar-ized in Table 2. The resultant CHFMs treated under airenvironment during the stabilization step demonstrated

higher gas permeance values compared to those treatedunder N2 environment. It was mainly due to the morpho-logical structure properties of the resultant membrane.The pore size of the resultant CHFMs treated under N2

environment during stabilization step probably was toosmall for an effective separation. It was agreed that smallchanges in the pore size of the carbon structure could havea considerable impact on gas permeation properties of theresultant CHFMs since the pore network created in thismembrane was similar in size with the gas penetrant. Basedon the results, it is indicated that the formation of the porestructure in the carbon membrane also depended on thestabilization environment. The selectivity of CO2=CH4

and CO2=N2 as high as 55.33 and 41.50, was achieved forCHFMs prepared under air stabilization. Such selectivityobtained was in the range of the current attractive resultfor CO2=CH4 and CO2=N2 separation. This was probablyowed to the formation of micropores, which connected toeach other, and mesopores, which did not penetratethrough the total thickness of the CHFMs.

On the other hand, the gas permeance of PEI-basedCHFMs prepared at 650�C for N2, CH4, and CO2 was1.34, 1.02, and 13.32 GPU, respectively. This value washigher than those of PEI=PVP-based CHFMs. This isbecause the preparation of PEI-based CHFMs with defectfree surface areas and sufficient mechanical strength forgas separation measurement was more difficult comparedto those PEI=PVP-based CHFMs. This might be due tothe low total polymer concentration used which leads tolow viscosity of the polymer solution. As a result, a more per-meable membrane was obtained for PEI-based CHFMs. Incontrast, the viscosity of the polymer blends were increasedwith the addition of PVP that makes it easier to be fabricatedas CHFMs and fewer cracks was created during heat treat-ment. Based on gas permeation data, the gas permeance ofthe tested gases were in the order CO2>N2>CH4, whichconsistent with the order of the kinetic diameters of the gasmolecules (CO2 (3.3 A)>N2 (3.64 A)>CH4 (3.8 A)). Thisindicates that the gas permeation through these CHFMs

FIG. 13. XRD patterns of the stabilized membrane treated under (a) N2

and (b) Air environments (Error analysis is about �0.01�). (Color figureavailable online)

TABLE 2Gas permeation properties of the resultant CHFMs

prepared at different stabilization environments (Erroranalysis is about �10%)

Carbonmembrane

Stabilizationenvironment

Permeance (GPU) Selectivity

N2 CH4 CO2

CO2=CH4

CO2=N2

PEI N2 1.34 1.02 13.32 13.06 9.94PEI=PVP Air 0.04 0.03 1.66 55.33 41.50PEI=PVP N2 0.03 0.02 1.05 52.50 35.00

�Tested at 7 bar, 25�C.

CARBON MEMBRANE FABRICATION 1037

obey molecular sieving mechanism. This result is in agree-ment with the work reported by previous researchers (4,29). Compared to those prepared from PEI-based CHFMs,the selectivity of both gas pairs remarkably increased withthe addition of PVP.

According to the literature review, the study of CHFMsfor gas separation, especially for CO2 separation, was still inthe development stage. Thus, this study is conducted inorder to stimulate the efforts of membranologist towardsa challenging application and convince them that carbonmembrane is one of the materials that need to be consideredfor gas separation applications. One of the challenges inCHFM preparation that hinders their commercializationis their low productivity or permeation value. In this study,the limit of the membrane area that can be used in thehollow fiber test module could be one of the reasons. Themembrane areas that have been used in this study were21 cm2 with an effective length of hollow fiber membranewas 10 cm. Although these membranes have low permeancevalue, it is still capable of being tested in the module for gaspermeation testing in the laboratory scale. To date, anumber of papers regarding systematic studies on the gaspermeation properties of the derived carbon membranesfrom PEI=PVP using hollow fiber geometry are still notsufficient. It is hoped that this finding can serve as a scien-tific platform for researchers and engineers to develop aviable and practical carbon membrane for gas separationprocesses in the future.

CONCLUSIONS

This research is part of an effort to give further clarifi-cation on the effect of stabilization condition on theproperties of the final CHFMs. The stabilization of thePEI=PVP membranes under air and N2 environments wereextensively investigated. Based on the results obtained inthe present study, several conclusions could be derived:

1. The results showed that the stabilization environmentsgave a significant influence on the resultant CHFMsproperties. It was because the mechanism of crosslink-ing and the structure crosslinks created during the stabi-lization step depended on the stabilization environment.

2. The weight loss of the membrane treated under airenvironment shifted towards higher temperature due tothe formation of a high performance carbonaceous charon the surface of the samples that insulated the underly-ing material and minimized the escape of volatileproducts generated during the decomposition.

3. A significant change in the FTIR spectra of themembrane was observed as the process temperatureincreased. This was because the membrane had decom-posed and almost all the chemical structures in the poly-meric precursor membranes had converted into carbonstructure.

4. The morphological structure of the stabilized membranetreated under air environment showed a more openporosity, while dense structures were obtained for thoseprepared under N2 environments.

5. During the heat treatment process, PEI=PVP mem-branes underwent various physical and chemicalchanges, such as coloration, shrinkage, crosslinkingreaction, and random scission.

6. Based on XRD analysis, a broad weak peak wasobtained for both membranes stabilized under air andN2 environments, which could be attributed to theturbostratic carbon structure with randomly orientedgraphitic carbon layers.

7. In overall, the stabilization step under air environmentprior to carbonization step showed the best conditionfor the preparation of CHFMs derived from PEI=PVPsince these membranes were more stable in an airenvironment at low process temperature due to theoxygen containing functional groups incorporated intothe polymer chains by the cross linking reaction.

The findings obtained from this study are worthwhilefor any further research in the area of polymer-basedcarbon membrane.

ACKNOWLEDGEMENTS

One of the authors, W.N.W. Salleh, gratefully acknowl-edges the financial support under the National ScienceFellowship (NSF) from the Ministry of Science, Tech-nology, and Environment of Malaysia.

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CARBON MEMBRANE FABRICATION 1039