details of surface features in aromatic polyamide reverse osmosis membranes...

Details of Surface Features in Aromatic Polyamide ReverseOsmosis Membranes Characterized by Scanning Electronand Atomic Force Microscopy

SEUNG-YEOP KWAK,1 SOO GYUNG JUNG,1 YOUNG SEO YOON,2 DAE WOO IHM2

1 Department of Fiber and Polymer Science, Seoul National University,San 56-1, Shinlim-dong, Kwanak-ku, Seoul 151-742, Korea

2 R & D Center, Saehan Industries, Inc., 14 Nongseo-Ri, Kiheung-Eub, Yongin-City 449-900, Korea

Received 3 June 1998; revised 19 January 1999; accepted 26 January 1999

ABSTRACT: In the present article, some new events on the surface morphology of thearomatic polyamide thin-film-composite (TFC) membranes were demonstrated in con-junction with their inherent chemical nature. In addition, the detailed, quantitativeunderstanding of the microscopic surface features was shown to be essential in con-trolling the water permeability and eventually developing the high performance mem-branes. The surface roughness and the surface area were mainly affected by theexistence or nonexistence of the crosslinking and/or the free amide groups not pertinentto the formation of the hydrogen bonding, which in turn contributed to the waterpermeability. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1429–1440, 1999Keywords: aromatic polyamide thin-film-composite membranes; field-emission scan-ning electron microscopy; atomic force microscopy; chemical nature-surface morphology-water permeability correlation

INTRODUCTION

Reverse osmosis (RO) process has become indus-trially important not only in traditional seawaterand brackish water desalination but also in theproduction of ultrapure water, waste water treat-ment, recovery of valuable substances, and manyother potential applications.1 Most of the com-mercially successful RO membranes are thin-film-composite (TFC) type whose active layers areusually composed of aromatic polyamides andtheir analogues.2 The thin-film active layer per-forms as a permselective medium which allowsthe passage of the solvent (water) and rejects thesolutes. In RO separation, high selectivity of theTFC membranes is a basic requirement and im-

provement of the permeability, while keeping thehigh selective performance is desired from theviewpoint of the process efficiency. The thin-layerpolymers are basically required to have (1) inher-ent rigid (stiff) chemical structure; aromatic com-pounds having high glass transition temperatureshould be an example, (2) thermal and chemicalstability; chemical crosslinking is an effective pro-cess forming network structures with low chainmobility, and (3) proper hydrophilicity; high den-sity of amide linkage and introduction of acidgroups (COOH or SO3H) will be effective for highwater permeability. The presence of intermolecu-lar secondary interaction, such as hydrogen bond-ing, contributes to the mechanical rigidity and thewet strength of a membrane but deteriorates theprocessability and hydrophilicity of the mem-brane. High performance is also dependent on theprocessing aspects, i.e., decrease of the activelayer thickness and increase of the operatingpressure. Morphological structure of the mem-

Correspondence to: S.-Y. Kwak (E-mail: [email protected])Journal of Polymer Science: Part B: Polymer Physics, Vol. 37, 1429–1440 (1999)© 1999 John Wiley & Sons, Inc. CCC 0887-6266/99/131429-12

1429

brane surface is a consequence of all the proper-ties described above which are uniquely displayedby the polymeric material selected for the thin-film formation. In addition, recognizing that theactual separation mostly occurs and the perme-ation just starts on the surface skin of the mem-branes, characterization of the surface morpho-logical structure is of first importance to be per-formed for the fundamental understanding of themembrane performance.

In this study, four model TFC membranes oftwo crosslinked and two linear aromatic poly-amides were prepared via interfacial polymeriza-tion of phenylene diamines with either tri- ordi-functional acid chlorides on the polysulfone mi-croporous supports. In order to investigate thesurface morphology of the four aromatic poly-amide membranes, employed were atomic forcemicroscopy (AFM) and field-emission scanningelectron microscopy (FE-SEM) which delivers a

Table I. Chemical Structures of Aromatic Polyamides

Chemical structure of repeat unit

1430 KWAK ET AL.

brighter and sharper image at a given energythan the conventional SEM. The AFM is based onthe measurement of the force of interaction be-tween the molecules at the surface and the scan-ning tip, mapping submolecular and even atomicresolution images for the nonconducting materi-als.3–5 The AFM is now being widely applied inpolymeric materials involving microfiltration(MF) and ultrafiltration (UF) membranes,6–8 butits application to RO membranes still appears tobe limited. Therefore, it is objective of this studyto report finer details of surface features and an-alyze them quantitatively for the aromatic poly-amide RO membranes from the combined resultsof FE-SEM and AFM, in conjunction with theirchemical structure and transport characteristics,which have never been previously published.

EXPERIMENTAL

Materials and Membrane Preparation

The diamines used in this study were 1,4-ben-zenediamine (5p-phenylene diamine, PPD) and1,3-benzenediamine (m-phenylene diamine, MPD).The acid chlorides were 1,4-benzenedicarbonylchloride (5terephthaloyl chloride, TPC), 1,3-ben-zenedicarbonyl chloride (5isophthaloyl chloride,IPC), and 1,3,5-benzenetricarbonyl chloride (5tri-mesoyl chloride, TMC). Aqueous solutions (2.0%by weight) of the phenylene diamines were pre-pared using deionized distilled water (conductiv-ity ; 1.0 mmho). Organic solutions (0.1% byweight) of the acid chlorides were prepared in

Figure 1. Field-emission scanning electron micro-graphs of PPD/TMC membrane surface at 20,0003 (a)and at 50,0003 (b).

Figure 2. Field-emission scanning electron micro-graphs of MPD/TMC membrane surface at 20,0003 (a)and at 50,0003 (b).

AROMATIC POLYAMIDE RO MEMBRANE FEATURES 1431

Figure 3. Field-emission scanning electron micro-graphs of PPD/TPC membrane surface at 5,0003 (a),20,0003 (b), and at 50,0003 (c).

Figure 4. Field-emission scanning electron micro-graphs of MPD/IPC membrane surface at 5,0003 (a),20,0003 (b), and at 50,0003 (c).

1432 KWAK ET AL.

n-hexane. Aromatic polyamides were synthesizedvia interfacial polymerization of the aqueous phe-nylene diamines with di-functional acid chlorides(i.e., TPC, IPC) and tri-functional acid chloride(i.e., TMC), which resulted in the linear and thecrosslinked aromatic polyamides, respectively.

The rectangular sheets of proper size were cutand immersed in aqueous solutions (2.0% byweight) of the individual phenylene diamines for1 min. Excess of the diamine solution wassqueezed off from the surface of the PSF supportby passing through the rubber rollers. The im-pregnated PSF substrate was then immediately

immersed in a solution of 0.1% (by weight) acidchloride in n-hexane and reacted for 1 min. Thissubjected an interfacial reaction and a polymericthin film was deposited upon the PSF support sur-face. The resulting TFC membrane was drained,air-dried at room temperature, and then stored indeionized water until usage. The final chemicalstructures of the resulting linear and crosslinkedpolyamides are believed to be as shown in Table I.

Microscopic Observations

The surface features of the aromatic polyamidemembranes were investigated with a Hitachi (To-

Figure 5. Atomic force microscopic images of membrane surface for PPD/TMC (a),and MPD/TMC (b).


kyo, Japan) S-4200 field-emission scanning elec-tron microscope (FE-SEM) and a Park ScientificInstruments (Sunnyvale, CA) AutoProbe M5atomic force microscope (AFM). For the SEM ob-servation, the membrane samples were cut intoappropriate sizes and the surfaces were coatedwith silver by a sputter-coating machine. In theoperation of the AFM, we selected non-contactmode because non-contact mode was appropriatefor imaging the samples of low moduli, such aspolymer that could be easily damaged by the tip.As the tip was scanned over the surface, the vi-bration amplitude of cantilever changed in re-sponse to force gradients that varied with the

tip-to-sample spacing. An image representing sur-face topography was obtained by monitoring thesechanges in vibration amplitude. To obtain the high-est lateral resolution, an image was taken with5123512 pixels. Scanning was performed in 0.6–1Hz rate, where the scan rate of 1 Hz indicated thatone line of data was collected per second. In non-contact mode, scan rates from about 0.5–16 Hzcould be used, depending on the scan conditions.

RESULTS AND DISCUSSION

Figures 1–4 represent FE-SEM micrographs ofsurface morphology for PPD/TMC, MPD/TMC,

Figure 5 (Continued from previous page)

1434 KWAK ET AL.

PPD/TPC, and MPD/IPC membranes, respec-tively. In Figure 1a, the membrane surface ap-pears as a dense, finely dispersed nodular struc-ture, which is typical and similar to macromole-cules in phase inversion membranes.9–12 Thesespherical structures have been reported for thethin-film composite membranes, DESAL3t, of De-salination Systems (Escondido, CA).13 Figure 1b,a higher magnified (350,000) micrograph, indi-cates that some protuberance seems to arise fromthe spherical globules.

The MPD/TMC membrane, Figure 2, shows asurface morphology almost identical to that of

previously published, that is, so-called ridge-and-valley structure.14 A closer examination revealsthat underneath the ridge-and-valley layer, thetightly packed globular structure is still presentand the protuberance is clearly shown to origi-nate from the globules. Furthermore, the devel-opment of the protuberance is so prominent (com-parable to that of PPD/TMC) as to produce thestrands of the polymers, which in turn form theridges and valleys.

Compared to the crosslinked aromatic poly-amide membranes, the linear ones (PPD/TPC andMPD/IPC), Figures 3 and 4, exhibit rather differ-

Figure 6. Atomic force microscopic images of membrane surface for PPD/TPC (a), andMPD/IPC (b).


ent surface features in that the surface does notconsist of the globules and the individual polymerstrands become much more widespreading overthe entire surface (compared to Figs. 1 and 2).However, further insights in PPD/TPC at highermagnification (Fig. 3c) indicate that some assem-bly of the nodules still remains, although the sizeof them becomes considerably smaller than thatof the crosslinked counterpart. On the contrary,in MPD/IPC, any evidence and reminiscence ofthe nodular feature can be rarely observable.Therefore, during the interfacial condensation toform the thin-film active layer, the PPD/TMC pairhas most of their macromolecules confined in the

globules but MPD/TMC allows some irregularstrands of polymers developed from the micelles.In the linear cases, it seems that there is notenough driving force to bind the macromoleculesand restrain them from developing their struc-tures. Recognizing that such an interfacial con-densation yields amorphous aromatic polyamidesbecause the chemical reaction rate is faster thanthe crystallization nucleation rate, the fixation ofthe resulting chain conformations are dependenton the presence of the crosslinking and the effi-ciency of the intermolecular hydrogen bonding.As evidenced in Figures 1 and 2 vs. Figures 3 and4, the crosslinking mainly participates in the for-

Figure 6 (Continued from previous page)

1436 KWAK ET AL.

mation of globular structure. In para-oriented ar-omatic polyamides, due to their axial symmetry,the average interchain distance is shorter (in thecase of linear chains, less than 3.1Å)15,16 thanthat of the meta-orientated ones, resulting in rel-atively strong hydrogen bonding. The latter con-taining the irregular structure detracts from thelinear geometry, and the intermolecular distance

is as great as that of the hydrogen bonds not yetformed. The m-phenylene isophthalamide has theNOH. . .O distance of about 3.5Å17 and it hasbeen known that the hydrogen bonding is ineffec-tive if the distance exceeds 3.3Å.18 Therefore, it isconcluded that both the crosslinking and the pos-sible intermolecular hydrogen bonding in PPD/TMC contribute the globular surface characteris-

Figure 7. Vertical height profiles of all four membranes taken from the lines travers-ing the AFM images; PPD/TMC (a), MPD/TMC (b), PPD/TPC (c), and MPD/IPC (d).


tics while lack of the hydrogen bonding in MPD/TMC results in the ridges and valleys of thepolymer strands developed from the sphericalsurface. In linear aromatic polyamides withoutvirtual crosslinks, the hydrogen bonding effectwell explains the existence of the smaller nodulesin PPD/TPC.

Figures 5 and 6 are the atomic force microscopy(AFM) images of the top sides for the surfaces ofthe crosslinked (PPD/TMC, MPD/TMC) and thelinear aromatic polyamides (PPD/TPC, MDP/IPC), respectively. All images represent an areaof 10 mm 3 10 mm; the bar at the left side of theeach image indicates the vertical deviations in thesample with the white regions being the highestand the black regions the lowest. The individualAFM images exhibit similar surface features com-pared to those of FE-SEM. The AFM permits themeasurements of distance variations in the sur-face of the membranes with a line traversing theimage as shown in each figure, which results inthe profile of the morphological properties of thesurface. Figure 7 compares the vertical heightprofiles along the horizontal line of 10 mm for allthe membranes. They are shown to differ in theirsurface line roughness, simply judging from thevertical deviation with respect to the scale of theordinate and the numbers of peaks and valleysalong the abscissa. Quantitative analysis on theroughness of the surface area are possible by theadditional image statistics, where the maximumpeak-to-valley distance, Rp–v, average roughness,Ravg, and root-mean-squared roughness, Rrms areusually found. Rp–v is the difference in heightbetween the highest and the lowest points withinthe selected area. Ravg is the mean roughness of

the surface relative to the center plane that isimaginary, flat floating at the mean height and isdetermined by the average deviation of the datapoints referenced to the average value of the datawithin the area:

Ravg 5 1/S E0

a E0

b

uf~x, y! 2 z0u dxdy (1)

Where S is the specified area, f(x, y) is the heightin the specified area, a and b are the length of twosides of the area, and zo is the mean height (i.e.,average value of the heights) in the area. zo isgiven by

zo 5 1/S E0

a E0

b

f~x, y! dxdy (2)

Rrms is defined by

Rrms 5 F1/S E0

a E0

b

$f~x, y! 2 zo%2 dxdyG 1/2

(3)

It is noted that because Rrms contains squaredterms, large deviations from the mean height areweighted more heavily than they are in Ravg, andvice versa. The quantitative analyses of the sur-face roughness were performed with three to fivereplication images for the individual membranes,and their arithmetic means are summarized inTable II and Figure 8. The membrane surfacearea within the selected area can be also mea-

Table II. Maximum Peak-to-Valley Distance, Surface Roughness, Surface Area, and RO Performances ofAromatic Polyamide RO Membranes*

Membranes

MaximumPeak-to-Valley

Distance,Rp2v (mm)

Surface Roughness (mm)Surface

Area(mm2)

Water Flux(gfd)

SaltRejection

(%)Average, Ravg Root-mean-square, Rrms

PPD/TMC 0.30 0.019 0.026 136.9 7a 11d 67.3MPD/TMC 0.46 0.042 0.053 171.2 17 ; 25b 23d 96PPD/TPC 1.39 0.140 0.178 235.6 53c 49d 30.2MPD/IPC 1.84 0.207 0.261 325.7 — 56d 27

* All the thin-film-composite membranes were prepared by interfacial polymerization of 1.0 wt % of diamine and 1.0 wt % of acidchloride. All the results were obtained with 2,000 ppm synthetic seawater and at the operating pressure of 50 kg/cm2.

a Ref. 19.b Ref. 20.c Ref. 21.d Tested in the laboratory.

1438 KWAK ET AL.

sured approximately by adding all the area of thetriangles formed through connecting a data pointto two of its nearest corresponding neighbors. Theresults are also summarized in Table II and Fig-ure 8.

From Table II and Figure 8, the surface of thecrosslinked aromatic polyamides is smootherthan that of the linear analogues by an order ofmagnitude. The meta-positioned polyamides arefound to have rougher surface compared to thepara-positioned polyamides. The membrane sur-face area increases accordingly with the increas-ing surface roughness. Moreover, there is an ob-vious correlation of the surface roughness andsurface area with the water permeability for thearomatic polyamide membranes (refer to Table II

and Fig. 8). MPD/IPC membrane, which consistsof the roughest surface and hence possesses thelargest surface area to contact possibly with wa-ter molecules, yields the highest water flux. Thenonexistence of the crosslinking and the presenceof the free amide groups basically due to themeta-orientation of the aromatic rings played animportant role in enhancing the surface rough-ness and surface skin area, which in turn in-creased the water flux of the membrane. On thecontrary, the presence of the crosslinking and thelack of the free amide groups, i.e., PPD/TMC,participated in reducing the surface roughness,the surface area, and hence the water flux.

Considering that most of RO applications re-quired the membranes with relatively higher re-

Figure 8. Comparison of maximum peak-to-valley distance, surface roughness, sur-face area, and water flux of PPD/TMC, MPD/TMC, PPD/TPC, and MPD/IPC membranes.


jection and flux at the same time, the MPD/TMCwas the most useful among the four membranes.

CONCLUSIONS

From the field-emission scanning electron mi-crographs (especially at 50,0003 magnifications),it is conjectured that the interfacial polymeriza-tion of the crosslinked aromatic polyamides (PPD/TMC, MPD/TMC) formed the spherical globulesof polymer network. Some protuberance origi-nated and developed from the globular surfaces,which formed the knot-like structure (PPD/TMC)or the ridge-valley structure (MPD/TMC) depend-ing on the extent of development. The differencein the degree of growth was explained by consid-ering the inherent chemical structure and inter-molecular secondary bonding. The linear aro-matic polyamides (PPD/TPC, MPD/IPC) gave thesurface morphology of fully developed, much morewidely spread polymer strands, although therewas some reminiscence of the nodular structurein the PPD/TPC. The discrepancy in the morphol-ogy between the crosslinked and the linear wasascribed to the crosslinking. Furthermore, thesymmetry of the chemical structure and hencehydrogen bonding was a crucial factor for thepreference of keeping globular structure in thepara-oriented aromatic polyamides relative to themeta-oriented.

The atomic force microscopy (AFM) was an ef-fective means to quantitatively analyze the mor-phological structure of the membrane surface.Among the four aromatic polyamide membranes,the linear meta-positioned exhibited the highestvalues of Rp–v, Ravg, and Rrms (see Table II) andthus was the most favorable for the formation ofthe rough and large surface. In contrast, thecrosslinked para-oriented was the smoothest andsmallest in the surface among the four. Further-more, the surface roughness and the surface areawere found to be directly correlated to the waterflux of the membranes; the rougher the surfaceand the larger the skin area, the higher the waterflux, and vice versa. The lack of virtual crosslinksand hydrogen bonding have reflected the roughersurface with larger area and in turn higher fluxwhile the presence of the crosslinking and inter-molecular bonding played a reverse role.

The authors are grateful to Saehan Industries, Inc. andResearch Institute of Engineering Science, Seoul Na-tional University for the financial support of this work.

REFERENCES AND NOTES

1. Williams, M. E.; Bhattacharya, D., Ray, R. J.; Mc-Cray, S. B. In Membrane Handbook; Sirkar, K. K.;Ho, W., Eds.; Van Nonstrand Reinhold: New York,1992.

2. Lonsdale, H. K. J Membr Sci 1982, 10, 81.3. Meyer, G.; Amer, N. M. Appl Phys Lett 1988, 53,

2400.4. Alexander, S.; Hellemans, L.; Marti, O.; Schneir, J.;

Elings, V.; Hansma, P. K.; Longmire, M.; Gurley, J.J Appl Phys 1989, 65, 164.

5. Rugar, D.; Hansma, P Phys Today 1995, 65, 23.6. Fritzsche, A. K.; Arevalo, A. R.; Connolly, A. F.;

Moore, M. D.; Elings, V.; Wu, C. M. J Appl PolymSci 1992, 45, 1945.

7. Dietz, P.; Hansma, P. K.; Inacker, O.; Lehmann,H.-D.; Hermann, K.-H. J Membr Sci 1992, 68, 101.

8. Grutter, P.; Zimmermann-Edling, W.; Brodbeck, D.Appl Phys Lett 1991, 60, 2201.

9. Kesting, R. J Appl Polym Sci 1973, 17, 1771.10. Nguyen, T. D.; Matsuura, K.;. Sourirajan, S. Ind

Eng Chem Prod Res Dev 1991, 60, 2201.11. Nguyen, T. D.; Matsuura, K.; Sourirajan, S. Chem

Eng Commun 1987, 57, 351.12. Wood, H.; Sourirajan, S. J Coll Int Sci 1993, 160,

93.13. Kulkarni, A.; Mukherjee, D.; Gill, W. N. J Appl

Polym Sci 1996, 60, 483.14. Petersen, R. J.; Cadotte, J. E. In Handbook of In-

dustrial Membrane Technology; Porter, M. E., Ed.;Noyes Publications: Park Ridge, NJ, 1990.

15. Inoue, K.; Hoshino, S. J Polym Sci B: Polym Phys1973, 11, 1077.

16. Northolt, M. Eur Polym J 1974, 10, 799.17. Kakida, H.; Chatani, Y.; Tadokoro, H. J Polym Sci,

Polym Phys 1976, 14, 427.18. Hughes, E. W.; Moore, W. J. J Am Chem Soc 1949,

71, 2618.19. McCray, S. B.; Friesen, D. T.; Ray, R. Presented at

5th Annual Meeting North American MembraneSociety; Lexington, MA, May 17–20, 1992.

20. Cadotte, J. E.; Rozelle, L T. NTIS Report No. PB-229337 (Nov. 1972).

21. Redondo, J. A.; Frank, K. F. Desalination, 1991, 82,31.

1440 KWAK ET AL.

details of surface features in aromatic polyamide reverse osmosis membranes...

Documents