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Carbon 44 (2006) 2764–2769

Preparation and characterization of carbon membranes madefrom poly(phthalazinone ether sulfone ketone)

Bing Zhang a, Tonghua Wang a,*, Shouhai Zhang b, Jieshan Qiu a,*, Xigao Jian b

a Carbon Research Laboratory, State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology,

158 Zhongshan Road, P.O. Box 49, Dalian 116012, Chinab Department of Polymer Science and Materials, Dalian University of Technology, 158 Zhongshan Road, Dalian 116012, China

Received 11 September 2005; accepted 30 March 2006Available online 30 May 2006

Abstract

Carbon membranes were prepared from a novel polymeric precursor of poly(phthalazinone ether sulfone ketone) (PPESK), of whichthe changes of microstructure and chemical compositions during pyrolysis from 500 �C to 950 �C were monitored by thermal gravimetricanalysis, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. It has been found that theweight loss of the PPESK precursor up to 800 �C is about 43.0 wt%. After the heat treatment, the typical chemical structure of thePPESK precursor disappears, at the same time a graphite-like structure with more aromatic rings is formed. The interlayer spacing(i.e., d value) decreases from 0.471 nm to 0.365 nm as the pyrolysis temperature increases. The gas permeation performance of carbonmembranes has been tested using pure single gases including H2, CO2, O2 and N2. For the carbon membrane obtained by carbonizing thePPESK precursor at 800 �C, the maximum ideal permselectivities for H2/N2, CO2/N2 and O2/N2 gas pairs could reach 278.5, 213.8 and27.5, respectively.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Porous carbon; Pyrolysis; X-ray diffraction; Chemical structure; Microstructure

1. Introduction

Membrane separation as an alternative and competitivetechnology for gas separation has attracted both commer-cial and academic interests since 1980s, of which one of thecrucial materials is the membrane with high performance.Carbon membrane, a new type of membrane materials,has drawn much attention and has been developing rapidlyin the past decade owing to its outstanding gas separationproperties as well as super thermal and chemical stabilityover conventional organic membranes and other inorganicmembranes [1]. Basically, carbon membranes can be pre-pared by carbonizing polymeric materials, of which thegas separation properties are closely related to the precur-

0008-6223/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2006.03.039

* Corresponding authors. Fax: +86 411 8899 3991.E-mail addresses: wangth@chem.dlut.edu.cn (T. Wang), jqiu@dlut.

edu.cn (J. Qiu).

sor’s chemical structure and the microstructure formedduring the pyrolysis or heat treatment step [2,3]. Up tonow, a number of polymer precursors have been testedfor making carbon membranes, which include polyimides[4–6], poly(furfuryl alcohol) (PFA) [7–9], phenolic resin[10], poly(vinylidene chloride–co-vinyl chloride) (PVDC–PVC) [11], polyacrylonitrile (PAN) [12], cellulose derivate[13]. Of these precursors tested, the aromatic polyimidesare regarded as the most promising candidate to preparecarbon membranes for gas separation because the carbonmembranes derived from polyimides show excellent gasseparation performance. However, the high cost of polyi-mides has hampered their wide practical applications.Therefore it is necessary to find more suitable precursorsthat are cheaper and easy to process [14].

Poly(phthalazinone ether sulfone ketone) (PPESK) is anew soluble polymer with high mechanical strength andhigh glass transition temperature [15,16], featuring bulky

B. Zhang et al. / Carbon 44 (2006) 2764–2769 2765

sulfone units and ketone units and a rigid phthalazinonestructure in the main chain, which helps to inhibit the inter-segmental packing and the segmental mobility during theprocessing step [17]. In addition, PPESK is much cheaperthan polyimides and is now available commercially. Inthe present study, the possibility of using PPESK as a pre-cursor to make carbon membranes with high gas perme-ability and permselectivity has been explored, and thepyrolysis behavior of PPESK as well as the changes inmicrostructure of membranes and the permeation proper-ties of the as-prepared carbon membranes have beenaddressed in detail.

2. Experimental

2.1. Materials

The PPESK was supplied by Dalian New Polymer Com-pany of China, of which the detailed synthesis procedurecan be found elsewhere [18]. Fig. 1 shows the typical chem-ical structure of the PPESK material with a sulfone toketone ratio (S/K = m/n) of 1:1, and a glass transition tem-perature of 284 �C.

2.2. Membrane preparation

The starting PPESK material was first dissolved inNMP, in which the concentration of PPESK was con-trolled to be at 15.0 wt%. Then, the PPESK–NMP solutionwas cast into polymeric membranes at room temperatureand a relative humidity of 35–45%. The fresh polymericmembranes were dried under vacuum conditions, first at80 �C for 24 h and then at 100 �C for 24 h. The dried poly-meric membranes were stabilized at 460 �C for 30 min inair, which helps to prevent the melting of the membranesin the subsequent pyrolysis step. The pyrolysis or heattreatment of the polymeric membranes was conducted ina horizontal furnace in flowing argon, in which the furnacewas ramped at a heating rate of 1 �C/min to a temperatureranged from 500 �C to 950 �C and was kept at the finaltemperature for 1 h before cooling back to room tempera-ture. The obtained carbon membranes were designated asCM-X (X stands for the final pyrolysis temperature), ofwhich the thickness was controlled to be in a range of40–80 lm, and it has been found that it is in this thicknessrange that the membrane’s permeability is independent ofthe membrane’s thickness that was measured using an elec-tronic micrometer. The carbon membrane samples were

ONN

OCO

nO

Fig. 1. Chemical structure o

symmetric, and the thickness of every membrane samplevaried within l ± 2 lm, here l stands for the thickness ofmembrane that was an averaged value of the thicknessmeasured at ten different points in the membrane. Beforethe separation performance tests, the carbon membraneswere stored in a desiccator to avoid or minimize the effectof vapor and CO2 species in air.

2.3. Characterization of the precursor and carbon

membranes

The thermal stability of the PPESK sample was studiedusing thermogravimetric analysis (TGA, TGA/SDTA851e

Mettler-Toledo, Switzerland) in flowing nitrogen, whichwas conducted at a heating rate of 15 �C/min and at tem-peratures ranging from 100 �C to 800 �C.

The FT-IR spectra of all samples including the polymerprecursor and the as-prepared carbon membranes wererecorded on a 20DXB FT-IR spectrometer by the KBr pel-let method.

X-ray photoelectron spectroscopy (XPS) was used toanalyze and monitor the elemental compositions of allsamples, which was conducted in a Perkin–Elmer PhI-5300 spectrometer (MgKa radiation at 1253.5 eV, operatedat 12.5 kV and 250 W, the incident angle 45�). For eachrun, the background pressure in the analysis chamberwas kept at 1.3 · 10�8 Pa. All binding energies were cali-brated with the C1s peak at 284.8 eV as reference thatcorresponds to the aromatic hydrocarbon carbons.

XRD patterns of the PPESK precursor and carbonmembranes were recorded on a D/Max-2400 spectrometer(CuKa radiation with a wavelength of 1.54 A, operated at40 kV and 100 mA). The data were collected in a range of5�–60� with a step length of 2h = 0.02�. From the XRDdata, the interlayer spacing (d value) in the samples is cal-culated using Bragg’s equation.

The gas permeation of membranes was measured usingsingle gases with high purity (H2, CO2, O2 and N2) at roomtemperature by the traditional variable volume–constantpressure method [19]. The gas from the cylinder was intro-duced into the upper side of a stainless membrane cell, inwhich the membrane to be tested was tightly sealed withepoxy glue. The flow rate of the permeating gas was mea-sured using a soap film flow meter, which was used tocalculate the gas permeability and selectivity of the mem-branes.

The ‘‘permeability’’, denoted as ‘‘Q’’, can be calculatedusing the following equation:

NNO

SO

Om

f the PPESK precursor.

150 300 450 600 750

0

10

20

30

40

50

b

a

Temperature (oC)

Wei

ght l

oss

(%)

0.025

0.020

0.015

0.010

0.005

0.000 Deriv.w

eight (mg. C o

-1)

Fig. 2. Thermogravimetry profile of PPESK: (a) weight loss, (b) thedifferential weight loss.

2766 B. Zhang et al. / Carbon 44 (2006) 2764–2769

Q ¼ FluxA � DP=l

ð1Þ

where DP is the partial pressure difference of the gasesacross the membrane and l is the thickness of the mem-brane tested.

The ideal separation factor a is calculated based on thepermeation rate of pure gases:

a ¼ Q1

Q2

ð2Þ

where Q1 and Q2 are the permeation rate for gases 1 and 2,respectively.

The gas permeation experiments for each membranewere repeated at least two times, of which the ranges ofgas permeability and permselectivity are shown in Table1. The testing sequence of the pure gases was in an orderof H2, CO2, N2 and O2. When changing the gas from oneto another, the membrane cell was degassed with an aimof removing the trace amount of residual gas in the gaslines quickly. After the permeation tests of all gases werefinished, the permeability of H2 was repeated to ensurethe invariability of permeability of membrane or the preci-sion in permeability to be within ±10%.

3. Results and discussion

3.1. Thermal gravimetric analysis

From the pyrolysis profile of the PPESK precursor,three distinguished stages of thermal degradation can beclearly seen from the weight loss curve (TG) and the differ-ential thermogravimetric curve (DTG) of the PPESK sam-ple, as shown in Fig. 2.

The first stage that starts from 100 �C to 350 �C ismainly due to the evaporation of residual solvent ofNMP (b.p., 202 �C) and some small gases released in thisstage, in which the weight loss is ca. 16.0% of the total loss.In the second stage that starts from 450 �C to 680 �C, amajor thermal degradation of the polymer takes place,resulting in a huge weight loss that accounts for ca. 80%of the total loss, in which the scissions of molecular chainsand the coalescent of the remained chemical groups in thematrix occurs. In the third stage at temperatures over680 �C, the weight loss curve reaches a plateau, in which

Table 1Gas permeance of polymeric membrane and carbon membranes (measured at

Sample Numbers of samples tested Permeability (Barrer)

H2 CO2

Knudsen diffusion – – –PPESK 2 Samples from 2 batches 2.02 0.73CM-500 4 Samples from 3 batches 19.0–20.3 16.8–21.2CM-650 5 Samples from 3 batches 102.9–133.8 83.3–86.1CM-800 6 Samples from 4 batches 44.1–51.7 27.6–34.2CM-950 5 Samples from 3 batches 5.9–6.4 1.2–2.1

1 Barrer = 10�10 cm3 (STP) cm cm�2 s�1 cmHg�1.

some small molecule gases such as H2S, H2 and HCN arereleased, at the same time, the structure rearrangementssuch as tautomerism, cross-linking and aromatic condensa-tion have taken place, leading to a structure consisting ofpoly- and hetero-cyclic aromatic compounds. The totalweight loss of the PPESK sample during the pyrolysis stepup to 800 �C is only ca. 43.0% with the initial weight as ref-erence. The TGA results show clearly that the PPESK pre-cursor used in the present study is thermally stable and hasa high yield of carbon residue, which is a big advantage formaking carbon membranes.

3.2. XPS analysis

XPS is a powerful tool to study the surface elementalcompositions of materials. Fig. 3 shows the XPS spectraof the original PPESK membrane and the carbon mem-branes obtained at different pyrolysis temperatures, reveal-ing that for all membranes, carbon, oxygen, nitrogen andsulfur are the main elements on the membrane’s surface.The variation trends of the content of these four elementswith the pyrolysis temperature are plotted in Fig. 4, fromwhich it can be seen that the carbon content increases asthe pyrolysis temperature increases from 500 �C to650 �C, then passes through a minimum at 800 �C as thetemperature further increases from 650 �C to 950 �C. It isbelieved that in this pyrolysis step, rearrangement reactionsbetween the poly- and hetero-cyclic aromatic nitrogen-con-taining compounds in carbon structure take place, whichresults in big loss of oxygen and sulfur at temperatures over800 �C. As the pyrolysis temperature increases, the oxygen

30 �C)

Selectivity

O2 N2 H2/N2 CO2/N2 O2/N2

– – 3.74 0.80 0.940.49 0.38 5.30 1.90 1.303.1–4.4 0.43–0.47 41.5–45.1 37.3–38.4 6.9–7.9

10.2–12.5 0.59–0.67 156.8–172.0 127.5–141.2 14.1–18.54.4–4.5 0.16–0.18 275.6–278.5 150.4–213.8 23.8–27.51.3–2.4 0.55–0.65 9.9–10.1 2.1–3.7 2.3–2.9

700 600 500 400 300 200

O1s N1s

Inte

nsity

(a.u

.)

Binding energy (eV)

Original

500oC

650 oC800oC

950oC

C1s

S2p

Fig. 3. XPS spectra of the original PPESK membrane and as-preparedcarbon membranes.

0

3

6

9

12

85

90

95

950800650500

C N O S

Elem

enta

l con

tent

(%)

Pyrolytic temperature (oC)

Fig. 4. Variation trend of element compositions of carbon membraneswith pyrolysis temperature, obtained from XPS measurement.

Tran

smitt

ance

(%)

500100020003000

a

b

c

d

e

Wavenumber (cm-1)

Fig. 5. FT-IR spectra of original polymer membrane (a), and carbonmembranes obtained at 500 �C (b), 650 �C (c), 800 �C (d) and 950 �C (e).

B. Zhang et al. / Carbon 44 (2006) 2764–2769 2767

content in the membrane goes through a maximum at800 �C. In the case of sulfur element, its variation trendis similar to that of oxygen, though the degree varies.For the nitrogen element, its content decreases monotoni-cally to some degree as the temperature increases.

3.3. FT-IR analysis

Fig. 5 shows the FT-IR spectra of the precursor mem-brane and the pyrolytic carbon membranes prepared at dif-ferent temperatures between 500 �C and 950 �C.

For the starting PPESK precursor membrane, theabsorption peaks can be assigned to bonds such as Ar–O–Ar bond (3010 cm�1, 1242 cm�1, 1488 cm�1), C@O bond(1668 cm�1) in diphenyl ketone, C–H bond stretching(3068 cm�1) in benzene ring, skeleton vibration of benzenering (1590 cm�1 and 1507 cm�1), C–N bond stretching(1325 cm�1), and O@S@O bond (1152 cm�1 and 1167 cm�1),which clearly reflect the chemical structure shown inFig. 1. After heat treatment, the intensity of these absorp-tion peaks decreases markedly and some even disappearcompletely, which is obviously due to the decompositionand removal of the chemical groups mentioned above thatresults in the release of gases such as CO, SO2 and CO2.

For the membrane obtained at 500 �C, a new peak appearsat 3436 cm�1, which can be assigned to a new phenol-likegroup (PhOH) formed due to the cleavage of the etherbonds. Another obvious peak at 2227 cm�1 may be due tothe formation of cyanobenzene (PhCN) structure [20].When the pyrolysis temperature further goes up to650 �C, only three peaks over 1000 cm�1 can be seen, thepeaks at 3432 cm�1 and 1562 cm�1 indicate the presenceof nitrogen heterocyclic compounds, while the peak at1561 cm�1 could be due to the combination effect of N–Hbond bending vibration and the C–N bond stretching inaromatic acryl amine. These results lead one to speculatethat a phenyl nitril structure (PhNH2) might be formed,which is further transformed into an aromatic heterocyclicstructure. When the pyrolysis temperature is increased to800 �C and 950 �C, a large quantity of the poly- andhetero-cyclic aromatic compounds would be formed, whichis in parallel to the polymerization of aromatic core radicalsand to the release of aromatic and heterocyclic hydrogen[21]. These reactions mentioned above are responsible, inone way or another, for a thermally stable carbon structureand a graphite-like hexagonal planar structure.

3.4. XRD analysis

X-ray diffraction (XRD) is a useful tool for studying thearrangement of the carbon atoms at the molecular level. Itis well known that the inter-atomic distance of carbonatoms on the same plane is 1.41 A, and it has been wellestablished that the d spacing, i.e., the inter-planar dis-tance, and its variation, which can be monitored usingXRD, can serve as indicative of the graphitization degreeof the examined carbonaceous materials.

Fig. 6 shows the XRD patterns of the original polymermembrane and carbon membranes obtained at differenttemperatures, from which it can be seen that the originalPPESK membrane has a symmetric diffraction peak at19.3� that is due to the diffraction of the (002) plane,indicating that the PPESK polymer has a well-orderedstructure. With the increasing of pyrolysis temperature, this(002) peak shifts gradually from low angles to high angles.

15 30 45 60

d002 =3.65Å

d002 =4.60Å

d002 =4.71Å

d002 =3.90Å

d002 =3.84Å

2 theta ( o)

e

d

c

ba

Inte

nsity

(a.u

.)

Fig. 6. XRD patterns of original polymer membrane (a), and carbonmembranes obtained at 500 �C (b), 650 �C (c), 800 �C (d) and 950 �C (e).

0.1 1 10 100 10001

10

O2/N

2 Sel

eciti

vity

O2 Permeability (Barrer)

Robeson's Upper Bound

PPESK polymeric membrane

CM-500

CM-650

CM-800

CM-950

Commercially attractive region

Fig. 7. Permeation properties and separation factor for O2/N2 at 30 �C.For comparison, some reference data of carbon membranes made frompolyimide are also shown (� data from Ref. [4] and m data from Ref. [6]).

2768 B. Zhang et al. / Carbon 44 (2006) 2764–2769

At 500 �C, the (002) diffraction peak appears at 18.5� witha wide and non-asymmetric shape, for which the corre-sponding interlayer spacing (d0 0 2) is 4.71 A, implying thatthe original regular structure in PPESK had been destroyedand replaced by a chaotic structure. At the same time, theappearance of a new peak at 42.1� due to the (100) reflec-tion suggests that a rudimentary three-dimensionallyordered structure starts to be formed. This also implies thata graphite-like structure has been formed in the turbost-ratic carbon matrix [22,23]. When the pyrolysis tempera-ture further increases from 500 �C to 950 �C, the shape ofthe (002) diffraction peak becomes more symmetric andsharper, of which the corresponding value of d0 0 2

decreases gradually from 4.71 A to 3.65 A that approachesto the magic value of the interlayer distance of graphite(d0 0 2 = 3.35 A). In addition, the peak at 42.1� alsobecomes sharper, indicating that further graphitizationhas taken place. The results presented here clearly showthat the polymeric precursor used in this work, i.e., PPESKis a graphitizable carbonaceous material.

3.5. Test of gas permeation performance

The permeation performance of the membranes wasmeasured at room temperature. The upper side of mem-brane cell was controlled to be at 2.0 atm through a pres-sure regulator, while the lower side was maintained at1.0 atm. Table 1 shows the permeation results of pure gasesin polymeric membrane and the pyrolytic carbon mem-branes, from which it can be seen that in the case of carbonmembranes, the permeability and permselectivity for allgases are much higher than the starting polymeric mem-brane, which is several times higher or even one or twoorders of magnitude higher. As the pyrolysis temperatureincreases from 500 �C to 950 �C, the permeability for allsingle gases passes through a maximum at 650 �C, andthe variation of the selectivity follows the similar trend.In the case of the CM-800 membrane, the maximum idealselectivity for H2/N2, CO2/N2 and O2/N2 pairs reaches278.5, 213.8 and 27.5, respectively, while the permeabilitiesof H2, CO2 and O2 are 44.1, 34.2 and 4.4 Barrer, respec-

tively. These results seem to be in agreement with thetrade-off relationship between gas permeability and perm-selectivity of membranes that is not uncommon [2,4,24].But this is not the case for membranes CM-500 and CM-950. The CM-500 membrane with low permeability wasmade at relatively low temperature (500 �C), and shouldnot be regarded as real ‘‘carbon’’ membrane because ofthe incomplete carbonization. For the CM-500 membrane,one advantage is its flexibility, i.e., it is easy to handle itwithout worrying about being broken that is quite commonwhen handling carbon membranes made at high tempera-tures. For the CM-950 membrane made at a high temper-ature of 950 �C, the permeability and permselectivity dropsubstantially in comparison to the membranes such as CM-650 and CM-850. According to the literature [25], this isalso the case for carbon membranes made from polypyrro-lone. This may be due to a number of factors such as thecollapse, coalescence and rearrangement of pore structureand the densification of carbon matrix in membranes madeat high temperatures. The gas permeability of carbon mem-branes is in an order of H2 > CO2 > O2 > N2, which exactlyfollows the order of kinetic diameter of these four mole-cules [24]. This also indicates unambiguously that for car-bon membranes made in the present study, a molecularsieving mechanism is involved in the gas permeationprocess.

Fig. 7 compares the gas permeance of the PPESK-derived carbon membranes for the O2/N2 system, in whichthe O2/N2 selectivity is correlated to the O2 permeability.For clarity and simplicity reasons, the Robeson’s upperbound and four sets of literature data for polyimide-derived carbon membranes for O2/N2 separation are alsoshown in Fig. 7. It can be seen clearly from Fig. 7 that sim-ilar to three polyimide-derived carbon membranes, themembranes CM-650 and CM-800 prepared in the presentwork fall in the shadow region that is the scope or require-ment for membranes that are competitive and feasible com-mercially. This also clearly shows that the PPESK-derivedcarbon membranes can compete with those well-acceptedcarbon membranes made from polyimides in terms of O2/N2 separation. This leads one to believe that PPESK is of

B. Zhang et al. / Carbon 44 (2006) 2764–2769 2769

great potential as precursor for making carbon membraneswith high performance for air separation. To further estab-lish this, extensive exploration is necessary, and the work isnow in progress.

4. Conclusions

The preliminary results presented here have demon-strated that the PPESK polymer with good thermal stabil-ity and high carbon yield after carbonization is a verypromising precursor for making high performance carbonmembranes for O2/N2 separation. The chemical composi-tion, microstructure and gas separation capability of thePPESK-derived carbon membranes could be tuned byadjusting the preparation conditions such as the pyrolysistemperature. For the PPESK-derived carbon membranesprepared at 800 �C, the maximum ideal permselectivitiesfor H2/N2, CO2/N2 and O2/N2 pairs could reach 278.5,213.8 and 27.5, respectively, while the permeabilities ofH2, CO2 and O2 are 44.1, 34.2 and 4.4 Barrer, respectively.The separation performance of the PPESK-derived carbonmembranes is comparable to polyimides-based carbonmembranes reported in literature.

Acknowledgements

The work was partly supported by the National NaturalScience Foundation of China (No. 20276008), the NationalBasic Research Program of China (No. G2003CB615806),and the Program for New Century Excellent Talents inUniversities supported by the Education Ministry of China(No.NCET-04-0274).

References

[1] Ismail AF, David LIB. A review on the latest development of carbonmembranes for gas separation. J Membr Sci 2001;193(1):1–18.

[2] Geiszler VC, Koros WJ. Effects of polyimide pyrolysis conditions oncarbon molecular sieve membrane properties. Ind Eng Chem Res1996;35(9):2999–3003.

[3] Park HB, Kim YK, Lee JM, Lee SY, Lee YM. Relationship betweenchemical structure of aromatic polyimides and gas permeationproperties of their carbon molecular sieve membranes. J Membr Sci2004;229(1–2):117–27.

[4] Suda H, Haraya K. Gas permeation through micropores of carbonmolecular sieve membranes derived from Kapton polyimide. J PhysChem B 1997;101(20):3988–94.

[5] Jones CW, Koros WJ. Carbon molecular sieve gas separationmembranes-I. Preparation and characterization based on polyimideprecursors. Carbon 1994;32(8):1419–25.

[6] Ghosal AS, Koros WJ. Air separation properties of flat sheethomogeneous pyrolytic carbon membranes. J Membr Sci 2000;174(2):177–88.

[7] Shiflett MB, Foley HC. Ultrasonic deposition of high-selectivitynanoporous carbon membranes. Science 1999;285(5435):1902–5.

[8] Chen YD, Yang RT. Preparation of carbon molecular sieve mem-brane and diffusion of binary mixtures in the membrane. Ind EngChem Res 1994;33(12):3146–53.

[9] Wang H, Zhang L, Gavalas GR. Preparation of supported carbonmembranes from furfuryl alcohol by vapor deposition polymeriza-tion. J Membr Sci 2000;177(1–2):25–31.

[10] Centeno TA, Fuertes AB. Carbon molecular sieve membranes derivedfrom a phenolic resin supported on porous ceramic tubes. Sep PurifTech 2001;25(1–3):379–84.

[11] Centeno TA, Fuertes AB. Carbon molecular sieve gas separationmembranes based on poly(vinylidene chloride–co-vinyl chloride).Carbon 2000;38(7):1067–73.

[12] David LIB, Ismail AF. Influence of the thermastabilization processand soak time during pyrolysis process on the polyacrylonitrilecarbon membranes for O2/N2 separation. J Membr Sci 2003;213(1–2):285–91.

[13] Gilron J, Soffer A. Knudsen diffusion in microporous carbonmembranes with molecular sieving character. J Membr Sci 2002;209(2):339–52.

[14] Ismail AF, David LIB. Future direction of R and D in carbonmembranes for gas separation. Membr Technol 2003(4):4–7.

[15] Jian XG, Meng YZ, Zheng HB. Preparation of poly(phthalazinoneether sulfone). China patent 93109180.2, 1993.

[16] Jian XG, Meng YZ, Zheng HB. Preparation of poly(phthalazinoneether ketone). China patent 93109179.9, 1993.

[17] Jian XG, Da Y, Zeng L, Xu RX. Application of poly(phthalazinoneether sulfone ketone)s to gas membrane separation. J Appl Polym Sci1999;71(14):2385–90.

[18] Meng YZ, Hay AS, Jian XG, Tjong SC. Synthesis of novelpoly(phthalazinone ether sulfone ketone)s and improvement of theirmelt flow properties. J Appl Polym Sci 1997;66(8):1425–32.

[19] Stern SA, Gareis PJ, Sinclair TF, Mohr PH. Performance of aversatile variable-volume permeability cell: comparison of gaspermeability measurements by the variable-volume and variable-pressure methods. J Appl Polym Sci 1963;7(6):2035–51.

[20] Hatori H, Yamada Y, Shiraishi M, Yoshihara M, Kimura T. Themechanism of polyimide pyrolysis in the early stage. Carbon 1996;34(2):201–8.

[21] Painter PC, Yamada Y, Jenkins RG, Coleman MM, Philip LJW.Investigation of retrogressive reactions leading to the carbonizationof solvent-refined coal. Fuel 1979;58(4):293–7.

[22] Manocha LM, Bhatt H, Manocha SM. Development of carbon/carbon composites by co-carbonization of phenolic resin and oxidisedPAN fibers. Carbon 1996;34(7):841–9.

[23] Ci LJ, Zhu HW, Wei BQ, Xu CL, Liang J, Wu DH. Graphitizationbehavior of carbon nanofibers prepared by the floating catalystmethod. Mater Lett 2000;43(5–6):291–4.

[24] Steel KM, Koros WJ. An investigation of the effects of pyrolysisparameters on gas separation properties of carbon materials. Carbon2005;43(9):1843–56.

[25] Kita H, Yoshino M, Tanaka K, Okamoto K-i. Gas permselectivity ofcarbonized polypyrrolone membrane. Chem Commun 1997(11):1051–2.