Journal of Membrane Science 226 (2003) 63–73

Characterization of polyethersulfone-polyimidehollow fiber membranes by atomic force microscopy

and contact angle goniometery

K.C. Khulbea,∗, C. Fenga, T. Matsuuraa, G.C. Kapantaidakisb,M. Wesslingb, G.H. Koopsb

a Industrial Membrane Research Institute, Department of Chemical Engineering, University of Ottawa, Ottawa, Canada K1N 6N5b Faculty of Chemical Technology, Membrane Technology Group and EMI, University of Twente, P.O. Box 217,

7500 AE, Enschede, The Netherlands

Received 20 December 2002; received in revised form 1 May 2003; accepted 7 August 2003

Abstract

Asymmetric blend polyethersulfone-polyimide (PES-PI) hollow fiber membranes prepared at different air gap and used forgas separation are characterized by atomic force microscopy (inside and out side surfaces) and by measuring the contact angleof out side surface. The outer surface was entirely different than the inner surface, as expected. On the inner surface noduleaggregates were aligned in rows, may be towards the direction of the bore fluid flow. On the outer surface, alignment of nodularaggregates in one direction was not observed. The average mean roughness parameter of the inner surface increased with theincrease in the air gap used for the preparation of hollow fiber. On the other hand, it was opposite for the outer surface. Fromthe gas separation experiments it was observed that gas permeation rate increased with the increase in air-gap. An attemptwas made to find relationships between the surface morphology observed by AFM and permeability and selectivity of thestudied hollow fiber membranes.

From the contact angle measurement it was observed that the contact angle is directly related to the outside surfaceroughness. A plausible mechanism has been discussed.© 2003 Elsevier B.V. All rights reserved.

Keywords:Hollow fiber PES/PI; Blend polymers; Atomic force microscopy; Contact angle

1. Introduction

Polymer blends are usually classified as homoge-neous or heterogeneous. But in most of the cases anddue to thermo dynamical reasons polymeric blendsbelong to the heterogeneous category. When hollowfibers are prepared from the solution of blended poly-

∗ Corresponding author. Tel.:+1-613-562-5800x6114;fax: +1-613-562-5172.E-mail address:khulbe@eng.uottawa.ca (K.C. Khulbe).

mers and the interaction between the pure polymers isstrong enough, then the miscibility is independent ofbore fluid chemistry.

The blend of polyethersulfone and polyimide wasstudied by Liang et al.[1]. It was shown that the blendwas to be miscible over the whole range of composi-tion. Ekiner [2] patented polyethersulfone-polyimidemembranes for gas separation.

Kapantaidakis and Koops[3] studied the forma-tion and the gas permeation properties of hollow fibermembranes based on PES/PI blends of three different

0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.memsci.2003.08.011

64 K.C. Khulbe et al. / Journal of Membrane Science 226 (2003) 63–73

compositions (80/20, 50/50 and 20/80 wt.%). They re-ported that the air-gap distance in the dry/wet spinningprocess affects both membrane structure and perme-ation properties. Scanning electron microscopy (SEM)was used to investigate the morphological character-istics and the structure of asymmetric hollow fibers.They reported that fibers had porous skin layer, loosesubstructure and high permeation values. Increase inthe air-gap length from 1 to 10 cm during the forma-tion of PES/PI (80/20 wt.%) hollow fibers, resulted inhigher permeation rates for both CO2 and N2. The per-meance of CO2 was increased from 57 to 130 GPUwhile that of N2 increases from 5 to 65 GPU. It wasalso found that the thickness of the hollow fiber skinlayer varied from 1029 Å (air-gap distance of 10 cm) to1198 Å (air-gap distance of 1 cm). The PES-PI hollowfibers were further coated with polydimethyl siloxane(PDMS) and the transport properties of coated hollowfibers were studied.

Kesting [4] suggested four superimposed tiers ofstructure in integrally skinned phase inversion mem-branes: (1) macromolecules (functionally sub macro-molecules, that is displacement of chain segments),(2) nodules (approximately 200 Å in diameter spher-ical macromolecular aggregates, each of which con-tains several tens of macromolecules), (3) nodule ag-gregates (400–1000 Å in diameter spherical clumps ofnodules), and (4) supernodular aggregates (aggregatesof nodule aggregates which constitute the walls of the0.1–2�m in diameter open cells in the membrane sub-structure).

In the present paper, we are reporting the mor-phology of inner surface and the outer surfaces ofPES/PI hollow fiber membranes in terms of nodulesand nodule aggregates defined by Kesting[4]. Themean roughness parameters of the surfaces were alsoused for surface characterization. The objective of thiswork is to find relationship between the surface char-acterization parameters and the gas permeation prop-erties of hollow fibers.

2. Experimental

Hollow fiber blend membranes were prepared byusing the commercially available materials polyimide,Matrimid 5218 (Ciba-Geigy) and polyethersulfoneSumikaexcel (Sumitomo)[3,5]. The composition of

PES/PI in blend was 80/20 wt.% and blend compo-sition in NMP was 35 wt.%. The spinning dope wasextruded from a spinneret (i.d. = 200�m, o.d. =500�m) at 50◦C. The bore fluid (NMP/water =80/20) was from the central tube of the spinneret tothe center of the fiber at a flow rate of 1.12 ml/min.The fiber traveled through an air gap before enteringthe coagulation bath (water). The air gaps distancewas changed from 1 to 10 cm. The humidity of airwas maintained at 53%.

The technique for the AFM (Tapping Mode) is de-scribed elsewhere[6]. For the inner surface study, thehollow fiber membrane was angle cut by means of asharp razor blade and AFM images were taken fromthe inside surface (spoon shape area). The sample wasfixed on the magnetic disk by using double side ad-hesive tape. The laser beam of the AFM was focusedon the pre-selected spot of the surface prior to the en-gagement of the cantilever. Differences in the mem-brane surface morphology can be expressed in termsof roughness parameters such as the mean roughness.This parameter was measured by AFM itself. Themean roughness is the mean value of surface relativeto the central plane, the plane for which the volumeenclosed by the image above and below this plane areequal.

The roughness parameter depends on the curvatureand size of TM-AFM tip, as well as on the treatment ofthe captured image (plane fitting, flattering, filtering,etc.). Therefore, the roughness parameter should notbe considered as absolute roughness values. However,the same cantilever was used for all AFM images andall the AFM images treated in the same way.

Contact angle measurement was carried out by go-niometric ally determining the angle from observationof the three-phase meniscus system described else-where in detail[7]. For this purpose, a contact an-gle goniometer (Rame Hart, Nodel 100), which wasmodified for hollow fiber was used[7]. Hollow fiberswere partially immersed in the test liquid (distilled wa-ter). Then meniscus was monitored by the goniome-ter equipped with an optical camera. Advancing andreceding contact angles were measured after the equi-librium condition was reached which gradually in-creasing and decreasing the level of the test liquid inthe container in which hollow fiber was partially im-mersed achieved. In all cases, contact angle data isreported in terms of advancing contact angle (θa).

K.C. Khulbe et al. / Journal of Membrane Science 226 (2003) 63–73 65

3. Results

3.1. Atomic force microscopic

3.1.1. Inner surfaceFig. 1a and bshows the AFM images (scan size

3�m) at two different sites of the inner surface ofa hollow fiber, when the air-gap was 1 cm. For bothimages the diameter of nodular aggregates, seen as

Fig. 1. AFM images at four different sites of the inner surface of hollow fiber prepared at 1 cm air-gap: (a and b) at scan 3�m (topsurface) and (c and d) at scan 1�m (3D images).

spheres on the images, is measured. The mean, max-imum and minimum values listed inTable 1indicatethat there is no meaningful difference in the size ofnodular aggregates between two images, although theimages look very different. InFig 1a several nodu-lar aggregates are fused from three to six, while inFig 1bseven to twelve nodular aggregates are alignedin one row. It should be noted that the inside of thehollow fiber was in contact with bore fluid running

66 K.C. Khulbe et al. / Journal of Membrane Science 226 (2003) 63–73

Table 1Mean, maximum and minimum size (diameter) of nodular aggre-gates at the inner and outer surfaces of hollow fibers when the airgap was 1 and 10 cm

Air-gap (cm) Mean(nm)

Maximum(nm)

Minimum(nm)

Inside surface1 Fig. 1a 124 188 93

Fig. 1b 123 143 95

10 Fig. 2a 140 170 117Fig. 2b 165 194 120

Out side surface1 Fig. 4a 124 186 109

Fig. 4b 123 204 94

10 Fig. 5a 137 172 109Fig. 5b 144 148 94

to the longitudinal direction at high speed. The sheerforce working on the surface of the nascent hollowfiber caused nodular aggregates to be aligned to thedirection of the bore fluid. Different images observedinFig 1a and bindicate that the alignment was not com-pleted when the air-gap was as short as 1 cm.Fig. 1cand dshow the 3D images of the two sites at 1�mscan.

Fig. 2a and bshow the AFM images at two differ-ent sites of the inside surface of the hollow fiber (scan4�m) when the air gap was 10 cm. For both images,the diameters of nodular aggregates were measuredand the mean, maximum and minimum values aresummarized inTable 1. Table 1shows that there is nomeaningful difference in the size of nodular aggregatesbetween the two images. The difference in appearancebetweenFig. 2a and bseems to be much less thanthe difference betweenFig. 1a and b. In bothFig. 2aand bnodular aggregates are assembled to a numberof string-like structure, in which they are aligned inone row. This means that the alignment of nodular ag-gregates under the strong sheer force was completedwhen the air-gap was as long as 10 cm. Interestingly,the elongation of the nodular aggregate itself was ob-served, as evidenced by the (mean length/width) ratioof 1.25.Fig. 2c and dshow the 3D images of the twosites at 1�m scan. Comparison ofFig. 2c and dwithFig. 1c and dindicates that the roughness increasedas the air-gap increased. This trend was supported bythe measurement of roughness parameters at different

air-gaps as shown inFig. 3a and b, which correspondto scan size of 1 and 3�m, respectively.

3.1.2. Outer surfaceFig. 4a and bshow the AFM images (scan 4�m) at

two different sites of the outer surface of a hollow fiberwhen the air-gap was 1 cm. For both images the diam-eters of the nodule aggregates were measured and themean, maximum and minimum values are reported inTable 1. Table 1shows there is no meaningful differ-ence in sizes of the nodular aggregates between thetwo images. A noted difference between the inner sur-face (Figs. 1 and 2) and outer surface (Fig. 4) is thatspherical nodules are not aligned to one direction onthe outer surface.Fig. 4c and dshow the 3D imagesof the two sites.Fig. 5a and bshow the AFM images(scan 4�m) at two different sites of the outer surfaceof a hollow fiber when the air-gap was 10 cm. For bothimages the diameters of the nodule aggregates weremeasured and the mean, maximum and minimum val-ues are listed inTable 1. Table shows that there is nomeaningful difference between the two images. Unlikeinner surfaces (Figs. 1 and 2) there was no alignmentof nodule aggregates in one direction, although somepattern is observed in the assemblage of nodular ag-gregates. Moreover, the area of the dark spots, indicat-ing the troughs on the surface increased as the air gapincreased from 1 to 10 cm (fromFigs. 4 to 5). Fig. 5aand bshow the 3D images of the two sites. Compar-ing Fig. 4c and dwith Fig. 5c and d, the roughnessdecreased as the air-gap increased from 1 to 10 cm.The same trend was observed when the roughness pa-rameter was plotted versus air-gap inFig. 6a and bforthe scan sizes of 1 and 4�m, respectively.

3.2. Contact angle

Fig. 7 shows the contact angle of the outer surfaceof the PES/PI hollow fiber membranes against air-gapused for the preparation of membranes. FromFig. 7it seems that the contact angle is decreasing with anincrease in air-gap distance.

3.3. Outer and inner diameters of the hollow fibers

Table 2shows the outside diameters and inside di-ameters of the hollow fibers against the air-gap dis-tance. As the air-gap is increasing both outside and

K.C. Khulbe et al. / Journal of Membrane Science 226 (2003) 63–73 67

Fig. 2. AFM images at four different sites of the inner surface of hollow fiber prepared at 10 cm air-gap: (a and b) at scan 3�m (topsurface) and (c and d) at scan 1�m (3D images).

inside diameters increase while the thickness of thehollow fiber decreases.Fig. 8 shows the trend of de-creasing of hollow fiber thickness against air-gap dis-tance.

Table 2Outside and inside diameter and thickness of the PES/PI hollow fiber membrane

Air-gap (cm) Outer diameter(�m)

Inner diameter(�m)

Thickness of themembrane (�m)

1 406 221 92.52 406 223 91.54 408 229 89.56 415 239 88.0

10 423 244 89.5

4. Discussion

The effect of air-gap on the performance of the hol-low fiber membrane and on its morphology is quite

68 K.C. Khulbe et al. / Journal of Membrane Science 226 (2003) 63–73

Inside surface mean roughness (1µm) Inside surface mean roughness (3µm)

(a) (b) 0 2 4 6 8 10 12Air gap (cm)

0

20

40

60

80

Ave

rage

mea

n ro

ughn

ess

(nm

)

0 2 4 6 8 10 12Air gap (cm)

0

10

20

30

40

50

Ave

rage

mea

n ro

ughn

ess

(nm

)Fig. 3. Mean roughness of the inner surface of hollow fibers against air-gap used for the preparation of hollow fiber membrane: (a) atscan 1 �m and (b) at scan 3 �m.

complex. It depends on the bore fluid rate, polymerconcentration in the solution, properties of the solventused, spin rate, etc. In the present study all the param-eters were kept constant except air-gap.

Kapantaidakis and Koops [3] and Kapantaidakiset al. [5] reported that the permeation rate of CO2 andN2 were directly related to the air-gap for these hollowfibers. Khulbe et al. [8] reported that the roughnessof the top surface is directly related to the permeationof gases through poly(phenylene oxide) (PPO) mem-branes. However, this study was focused on flat mem-branes. Wood [9] studied gas permeation through PPOhollow fibers and reported that the trend for the per-meation rate of both methane and CO2 was increasewith increasing air-gap distance.

4.1. Inner surface

In the earlier study on CO2 and N2 permeationthrough PES/PI blended hollow fiber membranescoated with PDMS [3], a slight increase in permeationrate was observed with an increase in air-gap. Thiswas attributed to protracted coagulation from the boreside, resulting in a more open structure on the boreside of membrane. It was also postulated that solventNMP diffused from the bore liquid into the dope,since the bore liquid contained NMP as high as 80%.The increase in air gap resulted in a longer contacttime between the nascent fiber and the bore liquid,

leading to intrusion of a larger quantity of NMP intothe membrane and hence looser structure of polymeron the bore side.

The AFM images support the above view. The nodu-lar aggregates were aligned more to the direction ofthe bore fluid flow as the air-gap increased. In thisprocess, initially fused nodular aggregates are isolatedfrom each other before being aligned in a row. Whenthe nodular aggregates are reassembled (in a row), theyare no longer fused as tightly as before. Hence, thesurface roughness increases as nodular aggregates arebetter aligned to the flow direction with an increase inair-gap. In other words, the nodular aggregates tend tobe more loosely packed as the air-gap increases. Thisis an agreement with a conclusion in the earlier work[3].

4.2. Outer surface

Based on a sharp decrease in CO2/N2 selectivityof uncoated hollow fibers when the air-gap was in-creased, it was concluded in the earlier study [3] thatthe skin layer (on the outer surface of the hollowfiber) contained a small fraction of tiny pores andthe fraction increased with the air gap. It was furtherpostulated that the increase in porosity was due to theincrease in the water content at the skin layer, as thecontact time with humid air increased with an air-gapdistance.

K.C. Khulbe et al. / Journal of Membrane Science 226 (2003) 63–73 69

Fig. 4. AFM images at four different sites of the outer surface of hollow fiber prepared at 1 cm air-gap: (a and b) at scan 4 �m (topsurface) and (c and d) at scan 1 �m (3D images).

The above view is also supported by the AFM im-ages. The casting solution is under stress inside thespinneret. As soon as it comes out of the spinneret, thestress perpendicular to the fiber axis will be released.However, the stress parallel to the axis will remain un-til the fiber reaches the gelation bath. The release ofthe stress perpendicular to the axis will result in theexpansion of the fiber diameter, while the stress paral-lel to the axis will elongate the fiber and decrease thefiber diameter.

Looking into Table 2, both outer and inner diam-eters of the hollow fiber are increasing with the airgap. Therefore, polymer relaxation due to stress re-lease (perpendicular to the axis) seems to dominatewhile the fiber is traveling through the air-gap. Thelarger the air-gap, the more relaxed the polymer, andthe macromolecules tend to coil and entangle witheach other, enhancing the fusion of the nodular aggre-gates. This is reflected in the decrease of the rough-ness parameters on the outer surface with an increase

70 K.C. Khulbe et al. / Journal of Membrane Science 226 (2003) 63–73

Fig. 5. AFM images at four different sites of the outer surface of hollow fiber prepared at 10 cm air-gap: (a and b) at scan 4 �m (topsurface) and (c and d) at scan 1 �m (3D images).

in the air gap. It is hence expected that the selectivityof the membrane will increase in air gap [10]. The ex-perimental results were, however, exactly the opposite.These unexpected results are probably attributable tothe increase in porosity, which was caused by an in-crease in water content in the skin layer, as postulatedin the earlier work. This interpretation is supported bythe images given in Fig. 5a and b, where relativelylarge areas of dark spots are observed. Some of thesedark spots may be penetrating through the skin layer

into the porous sub layer underneath, forming poresthat are responsible for the gas leakage.

The present results are also consistent with the re-sults observed by Feng et al. [11] in the study of struc-ture and performance of microporous polyetherimidehollow fiber membranes made by solvent-spinningmethod. Instead of air-gap they studied the effectof bore liquid flow rate varied between 0.1 and0.4 ml/min, while keeping the air-gap constant, i.e.80 cm. The morphology of the surfaces was depending

K.C. Khulbe et al. / Journal of Membrane Science 226 (2003) 63–73 71

0 2 4 6 8 10Air gap (cm)

0

10

20

30

40

50

Ave

rage

mea

n ro

ughn

ess

(nm

)

0 2 4 6 8 10 12Air gap (cm)

0

4

8

12

Ave

rage

mea

n ro

ugh

ness

(nm

)

12(b) (a)

Outside surface mean roughness (1µm) Outside surface mean roughness (4µm)

Fig. 6. Mean roughness of outer surface of the hollow fibers against air-gap used for the preparation of hollow fiber membrane: (a) atscan 1 �m and (b) at scan 4 �m.

on the bore fluid rate. Nodular aggregates werestrongly aligned in the direction of the bore liquidflow at the inside surface of the hollow fiber, whilenodule aggregates were more weakly aligned at theoutside surface. Elongation of the nodular aggregate

303234363840424446485052545658606264666870

0 1 2 3 4 5 6 7 8 9 10 11 12

Air-gap (cm)

Con

tact

Ang

le (

Deg

ree)

Fig. 7. Effect of air-gap distance on contact angle of the outer surface of the hollow fiber membrane.

was observed. The roughness of the inside surface in-creased with an increase in the bore liquid flow rate,while an opposite trend was observed at the outsidesurface. The surface porosity increased as the boreliquid flow rate increased.

72 K.C. Khulbe et al. / Journal of Membrane Science 226 (2003) 63–73

82

84

86

88

90

92

94

96

0 1 2 3 4 5 6 7 8 9 10 11 12

Air-gap (cm)

Thi

ckne

ss o

f ho

llow

fib

er (

mic

rom

eter

)

Fig. 8. Effect of air-gap distance on the thickness of the hollow fiber membrane.

Table 1 indicates that on the outer surface, whenthe air gap distance is 10 cm, the mean size of nodu-lar aggregates is slightly higher than the mean sizeof nodular aggregates when the air gap is 1 cm. It isreported that the thickness of the active layer of theprevious one was less than the second one [3]. Kapan-taidakis and Koops [3] found that the thickness of theskin layer of the hollow fiber membranes were 1029and 1198 Å for the 10 and 1 cm air-gap distance, re-spectively. At present due to lack of data no relationcould be found between nodular aggregates and thethickness of the active layer.

Fig. 7 shows the contact angle of the outer sur-face of PES/PI hollow fiber membranes against air gapdistance. From Fig. 7 it seems that the contact angleis decrease with the increase in air gap distance. Inother words, the contact angle is directly related to theroughness of the surface. Asmanraft [7] reported sim-ilar observation in the study of hollow fibers used fordialysis.

Fig. 8 shows the relation between the thicknessesof the PES/PI hollow fiber membranes and air-gapdistances. On comparing Figs. 7 and 8, it is interestingto note that the trend of decreasing in both cases, i.e.

thickness and contact angle, against air-gap distanceis similar.

5. Conclusions

From the present study, the following conclusionscan be drawn.

1. The morphology of the surfaces (inside and out-side) of hollow fibers depends on the air gap duringhollow fiber formation.

2. At inner surface, the nodular aggregates werealigned, may be to the direction of bore fluid andformed rows of nodular aggregates. This visualeffect was more significant when the air-gap in-creased. The rows were formed at least by sevento thirteen super nodular aggregate.

3. The mean roughness parameters of the both sur-faces depend on the air-gap used to make fiber. Themean roughness parameter of the inner surface in-creased with the increase of air-gap, while the trendwas opposite for the outer surface.

4. Contact angle of the outer surface is directly relatedto the roughness of the surface.

K.C. Khulbe et al. / Journal of Membrane Science 226 (2003) 63–73 73

5. The inner and outer diameter of the hollow fibersdepend on the air-gap.

6. The trend of decreasing of contact angle and thethickness of the hollow fiber membranes againstair-gap distances is similar.

References

[1] K. Liang, J. Grebowicz, E. Valles, F.E. Karasz, W.J.MacKnight, Thermal and rheological properties of misciblepolyethersulphone/polyimide blends, J. Polym. Sci., Part B;Pol. Phys. 30 (1992) 465–476.

[2] O.M. Ekiner, Blends of polyethersulfone with aromaticpolyimides, polyamides or polyamide-imides and gasseparation membranes made there from, European PatentApplication 0,648,812 A2 (1994).

[3] G.C. Kapantaidakis, G.H. Koops, High flux polyethersul-fone-polyimide blend hollow fiber membranes for gasseparation, J. Membr. Sci. 204 (2002) 153–171.

[4] R.E. Kesting, The four tiers of structure in integrally skinnedphase inversion membranes and their relevance to the variousseparation regimes, J. Appl. Polym. Sci. 41 (1990) 2739–2752.

[5] G.C. Kapantaidakis, G.H. Koops, M. Wessling, Preparationand characterization of gas separation hollow fiber membranes

based on polyethersulphone-polyimde miscible blends,Desalination 145 (2002) 353–357.

[6] K.C. Khulbe, B. Kruczek, G. Chowdhury, S. Gagne,T. Matsuura, S.P. Verma, Characterization of membranesprepared from PPO by Raman Scattering and atomic forcemicroscopy, J. Membr. Sci. 111 (1996) 57–70.

[7] M. Asmanrafat, Surface characterization of hollow fibermembranes used in artificial kidney, M.Sc. thesis, Universityof Ottawa, Ottawa, Canada, January 2002.

[8] K.C. Khulbe, T. Matsuura, G. Lamarche, H.J. Kim, Themorphology characterization and performance of dense PPOmembranes for gas separation, J. Membr. Sci. 135 (1997)221–223.

[9] M. Wood, Poly(2,6-dimethyl-11,4-phenylene oxide) hollowfibers for gas separation. Ph.D. thesis, University of Ottawa,Ottawa, Canada, 2002.

[10] K.C. Khulbe, C.Y. Feng, J.M.A. Tan, Polyphenylene Oxideand Modified Poly Polyphenylene Oxide: Gas, Vapor andLiquid Separation, in: G. Chowdhury, B. Kruczek, T.Matsuura, (Eds.), Kulwer Academic Press, London, 2001,p 231.

[11] C.Y. Feng, K.C. Khulbe, G. Chowdhury, T. Matsuura,V.C. Sapkal, Structural and performance study ofmicroporous polyethrimide hollow fiber membranes made bysolvent-spinning method, J. Membr. Sci. 189 (2001) 193–203.