Effect of carbon molecular sieve sizing with poly(vinyl pyrrolidone) K-15 on carbon molecular sieve–polysulfone mixed matrix membrane-main

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Journal of Membrane Science 307 (2008) 5361

Effect of carbon molecular sieve sizing with poly(vinyl pyrrolidone) K-15 oncarbon molecular sievepolysulfone mixed matrix membrane

W.A.W. Rafizah, A.F. Ismail

Membrane Research Unit, Faculty of Chemical and Natural Resources Engineering,

Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia

Received 20 April 2007; received in revised form 29 August 2007; accepted 8 September 2007

Available online 14 September 2007

Abstract

Mixed matrix membranes (MMMs) comprising polysulfone (PSF) Udel P-1700 and 30 wt% carbon molecular sieve (CMS) particles (


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54 W.A.W. Rafizah, A.F. Ismail / Journal of Membrane Science 307 (2008) 5361

achieving almost four times higher selectivity in CO2/N2 and

O2/N2compared to pure Matrimid, the O2and CO2permeabil-

ity was decreased at least fortyfold. Mahajan et al., [7] on the

other hand, proposed to maintain the polymer flexibility during

membrane formation by casting at temperature close to the Tgof the polymer matrix. Several workable MMMs using inter-

mediate Tg polymers have been successfully fabricated using

this approach[1113].Conversely, this approach is not practi-

cal for high Tg polymers because it is very difficult to find a

non-volatile solvent with enough high boiling point to meet the

required temperature during membrane formation [6]. The addi-

tion of plasticizer is also not practical because it could worsen

the intrinsic gasseparation performance of the polymer [1220].

Poly(vinyl pyrrolidone), PVP is a common chemical used as

additive in casting solution for preparation of phase inversion

PSF[2223].It is known as an agent for suppressing macrop-

ore formation in phase inversion membranes. Otherwise, PVP

is an established thermoplastic sizing in composite technology.

The effectiveness of using PVP as a sizing agent to promote

the adhesion between inorganic substrate with polymer matrixhas been extensively reported in fiber reinforced polymer matrix

composites development[24,25].The sizing technique, which

is a surface coating approach deserves a noteworthy considera-

tion and can be adapted in MMMs development. Sizing known

as coating or finishes are widely used to protect fiber surface

from damage, improve the fiber wetting by matrix and pro-

tect fiber surface reactivity [26,27]. Sizing could increase the

strength of the interphase by introducing more chemical reac-

tive site and/or more surface area for adhesion [2831] involving

neither complicated chemical reaction nor grafting process. The

functional groups along sized fibers can react and/or interact

with the matrix, giving rise to strong interactions between thefiber and the matrix[32,33].Furthermore, the entanglement of

polymer sizing and polymer matrix molecules via inter-diffusion

mechanismstrengthens the interphaseadhesion[28]. Inspired by

these successful findings, the application of PVP can be poten-

tially adapted for mixed matrix membrane development with the

intention of improving the compatibility of the inorganic sieving

material with the matrix polymer.

In this study, the modification of CMS particles using siz-

ing technique was explored. Physical deposition of PVP K-15,

sizing agent onto the surface of CMS particles was employed

by treating the CMS particles in sizing bath solution containing

110 wt% PVP K-15 in isopropanol solution prior to embed-

ment into the polymer matrix. In order to analyze the impacts of

CMS sizing with PVP K-15 on the morphology and separation

performance of membrane, MMM films comprising 30wt% of

PVP K-15-sized CMS in polysulfone Udel P-1700 were fab-

ricated and were characterized using TGA, DSC, FESEM and

pure gas permeation test.

2. Experimental

2.1. Raw material

A commercial Udel P-1700 polysulfone, purchased from

Amoco Performance Inc., USA was chosen as the polymer

matrix phase. It has an average molecular weight of 45,000.

The solvent,n-methyl-2-pyrrolidone (NMP) supplied by Merck

was used as received. The molecular sieve entities were car-

bon molecular sieve particles synthesized from polyacrilonitrile

(PAN) precursor described elsewhere [34]. The pore sizes ofPAN-based CMS could range from 4 to 6 A, which make them

suitable for use as molecular sieve[35].The CMS particle size

was reduced to less than 25m as verified by FESEM. Poly

(vinylpyrrolidone) kollidone 15 or PVP K-15 with the average

molecular weight of 10,000 was purchased from Merck, Ger-

many. Reagent grade isopropanol (Merck, Germany) was used

as the solvent to dissolved PVP K-15. Prior to any use, PSF

and CMS particles were preconditioned in a vacuum oven at

100 and 250 C, respectively for 12 h to remove trapped mois-

ture.Table 1summarizes the chemical structures and molecular

weights of PSF and PVP used in this study.

2.2. Preparation of PVP K-15-sized CMS

The flaky and white powder of PVP K-15 was dissolved in

isopropanol to produce dilute solution with the concentration of

110 wt% PVP K-15. An intended amount of CMS particleswas

added to the PVP K-15 sizing bath solution and stirred at 30 rpm

for 1 h. Then, the sized CMS were filtered from the excess solu-

tion using Whatman 40 filter paper. The sized CMS cake was

rinsed with isopropanol to remove unadsorbed polymer before

further dryingin a vacuum oven at 50 Cfor24h.Asimilarrange

Table 1

Chemical structures and molecular weights of PSF and PVP

Polymer Chemical structure Molecular weight

PSF (Udel, P-1700) 45,000

PVP K-15 10,000


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W.A.W. Rafizah, A.F. Ismail / Journal of Membrane Science 307 (2008) 5361 55

Fig. 1. Preparation of PSFPVP-sized CMS suspension.

of PVP bath solution has been studied to size carbon and glass

fiber in Refs.[30,36].This working range was chosen because

of several considerations. First, conformation of the bound poly-

mer chains on solid surface may vary with increasing polymer

solution concentration[36].A dilute PVP solution is needed to

wet the particle surface and provide easy access to the polymer

chains to conform. However, a very dilute solution reduces the

possibility of chain attachment. As a result, some part of the

CMS surface will be left unoccupied. The possibility of chainattachment is likely to increase by increasing the concentra-

tion of PVP in bath solution. Therefore, a suitable concentration

of PVP bath solution that can easily wet the particle surface

and provided sufficient chain attachment at the same time was

selected within this bath concentration range. In addition, damp-

ing effect of PVP on carbon surface was encountered when PVP

K-15 bath concentration used exceeding 10 wt%. A thick layer of

PVP formed on the CMS surface and prolonged drying duration

was required. This problem was also reported in Refs.[30,36].

2.3. Fabrication of membrane

There are several approaches to prepare polymersieve sus-

pension. For this study, 25 wt% PSF in NMP solution was first

prepared by dissolving pre-dried PSF pellets in NMP and stirred

for 12 h at temperatures of 6070C. In separate flask, the

intended amount of PVP-sized CMS was wet with small amount

of NMP. Then, the PSF dope solution was gradually added

into the flask containing wet CMS and stirred at 30 rpm. PSF

dope solution addition and stirring process were repeated until

a homogenous suspension solution was obtained. The sequence

for preparation of PSFPVP-sized CMS suspension is summa-

rized inFig. 1. The following casting process was performed

using the protocol outlined in Fig. 2. Forming a fine non-

supported mixed matrix membrane film is a very challengingtask because the membrane film becomes more opaque, brittle

and easy to break during testing upon inclusion of CMS parti-

cles. This phenomenon has been discussed in Refs. [4,17,21]. In

this study, it wasfound that the thickness of workable membrane

films could range from 60 to 70 m and were kept in vacuum

oven at 60 C prior to characterization to avoid contamination

by moisture or impurities.

2.4. Polymer and membrane characterization

Thermogravimetry analysis (TGA) using Mettler Toledo

thermogravimetry analyzer (TGA TSO800GC1) was performed

Fig. 2. Casting procedure of mixed matrix membrane at elevated temperature.

in order to determine the weight of sizing (Wsizing) on the sur-

face of the CMS particles. One gram of sized CMS was heated

in N2 atmosphere from 30 to 800

C with the heating rate of10 Cmin1 and the weight of sizing and estimated sizing thick-

ness was calculated using the following expressions:

Wsizing =Wi Wf

Wf 100 (1)

estimated sizing thickness on CMS =Wi Wf

PVP1000ACMS(2)

whereWirepresents the initial weight of the sized CMS (g), Wfis denoted for the final weight of CMS sample after heating (g),

PVPis the density of PVP (1.1 kg m3) andACMSis the surface

area of CMS (151.53 m2 g1).

Considering that the strength of interfacial adhesion is alsoinfluenced by the compatibility of sizing polymer with polymer

matrix, it is vital to verify the miscibility of these two polymers

during fabrication [37]. In order to determine the miscibility

of PSF/PVP blend, the glass transition temperature (Tg) of the

polymers was analyzed with differential scanning calorimetry

(Mettler Toledo DSC 822e). Sample was cut into small pieces,

weighed and placed into pre-weighed aluminum crucible. Then,

the sample was heated from 30 to 300 C with a heating rate of

10 Cmin1 in the first cycle to remove the thermal history. The

sample was cooled from 250 to 30 C at the rateof 10Cmin1.

The same heating procedure was repeated in the second heat-

ing cycle. Tg of the sample was determined as the midpoint

temperature of the transition region in the second heatingcycle.

Attenuated total reflection fourier transformed infrared

(ATR-FTIR) was used to correlate the changes in chemical

environment on the CMS surface before and after sizing pro-

cess. The pre-dried sample of either unsized CMS or sized CMS

was pressed against a 45 incidence germanium element. The

IR-spectra were recorded on Thermo Nicolet 5700 ATR-FTIR

spectroscopy, which is supplied by Thermo Nicolet Corporation

and Spectra Tech, USA. Based on the IR-spectra, a qualitative

difference in the distribution of functional groups on the surface

of sized CMS and unsized CMS could be made. In addition, the

miscibility of PVP K-15 in PSF matrix was confirmed by ana-


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lyzing the spectra shifts or the intensity changes of characteristic

groups of PSF and PVP K-15 in PSF/PVP blend.

Gas permeationproperties for mixed matrix membranes were

determined using variable-volume constant-pressure method

with a permeation cell described elsewhere[38].Each pure gas

(99.97% purity) was tested in the sequence of N2 and O2 and

measured three times for each membrane. The measurement

was performed at 30 C at 1.5 bar. The pure gas permeability

was determined using the following expression:

Pi =Vil

Atp(3)

where i represents the gas penetrant i, Vi is the volume of gas

permeated through the membrane (cm3, STP),l the membrane

thickness (cm),A the effective membrane area (cm2),tthe per-

meation time (s) and p is the transmembrane pressure drop

(cmHg). The selectivity was obtained using Eq.(4):

i/j=Pi

Pj

(4)

wherei/jis the selectivity of gaspenetrant i to gaspenetrantj, PiandPjare the permeability of gas penetrantiandj, respectively.

Field emission scanning electron microscope (FESEM) was

used to qualitatively analyze the morphology of the fabricated

membrane films and observed the compatibility between the

sieves and the polymer matrix. Prior to testing, the film samples

were fractured in liquid nitrogen in order to achieve a clean

break. The samples were then mounted on a stainless steel stand

with carbon tape and coated with 15 nm of gold using a sputter

coater.

3. Results

3.1. Effects of carbon molecular sieve sizing with

poly(vinyl pyrrolidone) K-15 on polysulfonecarbon

molecular sieve interphase

From the thermal gravimetry analysis, the estimated sizing

level on the CMS particles after sizing in 10 wt% PVP K-15

in isopropanol solution bath was about 10.2 wt% and the siz-

ing thickness was approximately 7 A. ATR-FTIR analysis was

also performed in order to correlate the changes in the chemi-

cal environment on the CMS surface before and after the sizing

process. This technique, which probes approximately the first

2m of the sample surface was used to verify the deposition ofPVP K-15 on the CMS surface. The analytical evaluations of the

ATR-FTIR spectra of the unsized CMS and PVP-sized CMS are

presented inFig. 3.They are consistent with the earlier reports

[36,39,40].A qualitative difference in the distribution of func-

tional group on the sized CMS and the unsized CMS can be

noted.

Based on the FTIR analysis presented in Fig. 3, the

highlighted regions represent the presence of two important

characteristic bands for PVP K-15. The infrared absorption at

1654 and 1289 cm1, respectively corresponds to the amide I

carbonyl (-C O) band and amide III (CN stretching) band of

PVP[41,42].The surface of unsized CMS on the other hand is

Fig. 3. Infrared spectra forPVP K-15, unsized CMS andPVP K-15-sized CMS.

almost inert with no apparent appearance of characteristic peak.

However, the presence of characteristic peaks of PVP could beobserved in the sized CMS spectra. The emergence of these

peaks becomes more prominent in the spectrum of CMS surface,

which was sized in 10 wt% PVP K-15 bath solution. This find-

ing supports that PVP was successfully deposited on the carbon

surface. The most sufficient and stable sizing level was achieved

by sizing the CMS particles in 10 wt% PVP K-15 bath solution

comparedto sizing using 1 and 5 wt% PVP K-15 bath concentra-

tion. This is because as the concentration of PVP in bath solution

increases, the possibility of PVP chains to successfully adsorb

and conform onto the CMS surface was also increased. Adsorp-

tion of PVP chains per area of CMS increased, thus forming a

stronger adherence to the surface[43].The peak at 2360 cm1

in FTIR spectra presented inFig. 3could be attributed to CO2peak. CO2peaks arecommon in IR spectra (2349 cm

1) because

of presence in air. There is a possibility that the background

correction run was not often enough to compensate for environ-

mental conditions and can be easily detected within such a low

absorbance range (


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Table 2

Glass transition temperature of PSF, PVP and PSF/PVP blend

Polymer Composition Tg (C)

PSF 1 183.01

PVP 1 188.04

PSF/PVP 32:1 183.93

analysis to study the miscibility of their polymer blend. Tg of

pure PSF, pure PVPand PSF/PVP blend arepresented in Table 2.

DSC curves for these polymers are also depicted in Fig. 4.The

DSC scan for PSF/PVP blend exhibited a single Tg located at

183.93 C, which is consistent with the miscibility character of

PSF/PVP K-15 mixture. As suggested by Walsh and Roston

[46],the Tg of a miscible blend can be predicted using simple

Fig. 4. DSC curves of (a) pure PSF, (b) pure PVP and (c) PSF/PVP blend.

Table 3

Peak assignments of polysulfone

Wavenumber (cm1) Probable assignment

1585 Benzene ring stretch

1504, 1488 Aromatic carbon groups (skeletal vibration)

1323, 1294 Sulfonate groups vibration

1241,1014 Antisymmetric COC stretch

1151 RSO2R

series, parallel and logarithms models.

Series model : 1

Tgb=

x1

Tg1+

x2

Tg2(5)

Parallel model : Tgb = x1Tg1 + x2Tg2 (6)

Logarithms model : lnTgb = x1ln Tg1 + x2ln Tg2 (7)

wherex1 andx2 is the composition of polymer 1 and polymer

2, respectively,Tgb

the glass transition temperature of polymer

blend, Tg1 the glass transition temperature for polymer 1 and

Tg2 is the glass transition temperature for polymer 2. The Tgof

PSF/PVP blend obtained from DSC results is in a good agree-

ment with predicted value proposed by these three models. This

finding further confirmed the miscibility of PVP in PSF.

In order to further understand the nature of the PSF/PVP K-

15 mixture at molecular level, the blend was studied by means of

ATR-FTIR spectroscopy. Although ATR-FTIR probes approx-

imately the first 2m of the sample surface, in this study it

was assumed that the resultant spectrum could represent the

whole part of the blend film. Studies of molecular interaction in

polymer blending system using FTIR has been performed and

discussed in Refs.[4245].The spectra of immiscible polymersare basically the sum of the spectra components in pure poly-

mers. In contrast, anyspecific interactions occurring between the

characteristic groups of pure polymers in miscible blend were

indicated by frequency shifts or absorption intensity changes

[4245].FTIR spectra for pure PSF, PVP K-15 and PSF/PVP

K-15 blend are presented in Fig. 5 for comparison. The probable

peak assignment for pure PSF and PVP K-15 are summarized

inTables 3 and 4, respectively. These results are consistent with

[38,4245].

Comparison of PSF/PVP K-15 blend spectrum with the

spectra of PSF and PVP K-15 reveals the occurrence of

interactions between the characteristic groups of PSF and

PVP K-15 in the miscible blend. These interactions are

indicated by frequency shifts involving several characteristic

groups and absorption intensity changes. The strongest shift

which is about 24 cm1 has been detected for amide car-

Table 4

Peak assignments of poly(vinyl pyrrolidone)

Wavenumber (cm1) Probable assignment

1654 Amide C O and CN stretch vibration

1493/1461/1423/1374 CH deformation of cyclic CH2groups

1289 Amide III band (CN stretch)

1071 CO


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bonyl group (C O) (16541679 cm1), the secondary shift

which is about 12 cm1 can be observed for sulfonate groups

(13231322 cm1 and 12941293 cm1) and COC stretch-

ing mode (12411240 cm1). Obvious changes in absorption

intensity are detected for sulfonate group (stretching vibration

at 1150 cm1) and aromatic carbon group (skeletal vibration

assigned at 1322 and 1293 cm

1

). In addition, a peak locatedat 1289 cm1 which assigned for amide III band (CN stretch-

ing mode) is no longer observable in the PSF/PVP K-15 blend

spectrum. Probably the frequency of this peak has shifted or

its absorption intensity has been reduced and overlapped with

the sulfonate group vibration of PSF at 1294 cm1. As a result

a single peak at 1293 cm1 appears in PSF/PVP K-15 blend

spectrum. Kapantaidakis et al.[44]reported that a similar trend

of the absorption intensity changes were also observed for sul-

fonate groups and aromatic carbon groups in their PI/PSF blend

polymer.

Thisanalysis suggests the occurrenceof interactions and mix-

ing of PSF and PVP K-15 at the molecular level. The spectra

shifts and intensity changes of characteristic groups of PSF andPVP K-15 could be attributed to the inter-molecular interaction

within PSF/PVP K-15 blend. In agreement with Sionkowska

[42]and Zeng et al.[45],the spectra shift of bands such as the

significant shifting of amide carbonyl group band in PVP and the

sulfonate group band in PSF suggests the interactions between

PSF and PVP have occurred. The good compatibility between

these two polymers can be rationalized by interaction mainly

contributed by C O group in PVP chain and sulfonate group

in PSF chain. This result was supported by Kapantaidakis et

al.[44]who studied the miscibility between PI/PSF blend. The

occurrence of interaction at molecular level due to C O group

in PI and sulfonate group in PSF has been observed.

3.2. Effects of carbon molecular sieve sizing with

poly(vinyl pyrrolidone) K-15 on polysulfonecarbon

molecular sieve mixed matrix membrane morphology

A qualitative assessment was conducted by using FESEM

images in order to compare the morphology of the fabri-

cated MMM containing PVP-sized CMS and MMM containing

untreated CMS. Fig. 6 reveals the comparison of the cross-

sectionalimages of theunmodified MMMand MMM containing

PVP-sized CMS. Both of these membranes were loaded with

30 wt% of CMS. In all FESEM images, CMS particles (


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Fig. 6. Comparison of FESEM micrographs for the cross-section of PSFCMS mixed matrix membrane with 30 wt% CMS loading: (a) containing unmodified CMS

under 1000magnification, the white bar indicates 10m; (b) containing unmodified CMS under 2500magnification, the white bar indicates 1m; (c) containing

PVP-sized CMS under 1000 magnification, the white bar indicates 10 m; (d) containing PVP-sized CMS under 2500 magnification, the white bar indicates1m.

interaction with the surrounded PSF matrix by introducing more

reactive side groups such asC O that can form specific inter-

action with sulfonate group of PSF. The interaction between

PVP K-15 and PSF matrix has been confirmed via ATR-FTIR

results. This finding indicates the occurrence of intimate mixing

at molecular level between the PVP K-15 sizing layer and PSF

matrix. In addition, it is envisioned that as PSFPVP interphases

are in contact, some of the PVP chains possibly migrate into the

PSF region via inter-diffusion mechanism. The inter-diffusion

mechanism between polymer interphases was also discussed

by Sperling[43]and Laot[37].These chains finally entangledwithin the PSF network and strengthen the interphase region.

3.3. Effects of carbon molecular sieve sizing on gas

permeation of mixed matrix membranes

The gas permeation results for unmodified MMMs and

MMMs containing PVP-sized CMS are listed in Table 5.The

O2and N2permeability for PSFPVP-sized CMS MMMs were

higher than those of pure PSF membranes and lower than those

of the unmodified PSF-CMS30 MMMs. These MMMs exhib-

ited the highest O2/N2 selectivity, which was almost 1.7 times

of the selectivity in unmodified MMMs.

The gas transport through mixed matrix membrane can occur

through three main pathways, which are through dense PSF

matrix, highly selective CMS and/or through non-selective gaps

or voids between the matrix wall and sieve particles. Dense PSF

matrix provides a very selective but highly resistive pathway to

the gas flow. Gas transport through CMS particles is less resis-

tive than that of dense PSF matrix and offers the most selective

pathway because CMS is capable of discriminating between size

and shape differences of the gas penetrants. The gaps or voids

conversely allow the bypassing of gas through its unselective

and non-resistive pathways. The gas transport through the gaps

Table 5

Comparison of gas permeation between PSFCMS30 and PSFPVP K-15-sized

CMS mixed matrix membranes for O2and N2

Membrane CMS (wt%) PO2 (barrers) PN2 (barrers) O2/N2

PSF 0 1.58 (0.28)a 0.29 (0.04) 5.50

PSF-CMS30 30 6.77 (0.01) 1.82 (0.06) 3.69

PSFPVP-sized

CMS

30 6.52 (0.23) 1.08 (0.04) 6.05

a Values in parenthesis is standard deviation; 1 barrer = 1 1010 (cm3

STP cm (cm2 s cmHg)).


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is assumed to be the Knudsen diffusion. As supported by the

FESEM images inFig. 6(a) and (b), a gross bypassing of gas

could occur through the submicron gaps between the polymer

matrix wall and the CMS particles in PSF-CMS30. Since the

gas transport through those unselective gaps has been assumed

to be the Knudsen diffusion, the degree of increment in N 2per-

meability became larger and exceeded the degree of increment

of O2permeability due to gas flow through CMS pathway. As a

result, these membranes exhibited higher O2and N2permeabil-

ity with poor selectivity compared to pure PSF membrane. On

the other hand, the FESEM images in Fig. 6(c) and (d) reveal

that the interfacial gaps were almost invisible in PSFPVP-sized

CMS MMMs structure indicating that the gap size between the

polymer matrix wall and CMS particles has been extensively

reduced. The transport resistance of this pathway increased and

suppressed the permeation of gas via Knudsen diffusion mech-

anism. More penetrant gases were directed to flow through

CMS pathway. This was supported by a substantial reduction in

N2 permeability for PSFPVP-sized CMS MMMs, which was

about 41% from the N2 permeability in PSF-CMS30. A con-siderable improvement in O2/N2 selectivity was also achieved

by using PSFPVP-sized CMS whereby the selectivity was

enhanced from 3.69 to 6.05. The findings of this study prove

that the PVP K-15 sizing layer not only capable of inducing the

interfacial adhesion of PSF matrix and CMS particles but also

allowing the gas transport through these two phases to proceed

without creating additional non-selective and resistive layer in

the interphase region.

4. Conclusions

In this study, MMMs were successfully fabricated by com-bining polysulfone, Udel P-1700 and PVP K-15-sized CMS.

CMS sizing with PVP K-15 has brought a dramatic impact on

the adhesion of the CMS and PSF matrix. ATR-FTIR analysis

demonstratedthat the sizingagent (PVP K-15) hasbeen success-

fully deposited onto the external surface of CMS and intimate

interactions at molecular level between miscible blend of PSF

matrix and PVP K-15 sizing polymer has also been established.

The FESEM images revealed a considerable improvement in

the interfacial adhesion between PSF matrix and CMS particles

was achieved in PSFPVP K-15-sized CMS MMMs compared

to that of unmodified PSFCMS MMMs. The voids or gaps

surrounding the CMS was reduced to a great extend suggest-ing that the PVP K-15 sizing layer has successfully bridged the

matrix and sieve phases by physically inducing the molecular

interactions with both PSF matrix and CMS particles. With the

absence of interfacial voids, a substantial recovery of separation

performance can be achieved without adversely worsen the gas

permeability or sacrificing selectivity.

Acknowledgements

Theauthor would like to express sincere gratitudeto National

Science Fellowship (NSF) from Ministry of Science, Technol-

ogy and Innovation Malaysia (MOSTI) for the financial support.

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Documents

Effect of carbon molecular sieve sizing with poly(vinyl pyrrolidone) K-15 on carbon molecular sieve–polysulfone mixed matrix membrane-main