journalof MEMBRANE

S C I E N C E

ELSEVIER Journal of Membrane Science 129 (1997) 55--64

Sulphonated poly(2,6-dimethyl-l,4-phenylene oxide)- polyethersulphone composite membranes

Effects of composition of solvent system, used for preparing casting solution, on membrane-surface

structure and reverse-osmosis performance

Ali Hamza, G. Chowdhury, T. Matsuura*, S. Sourirajan

Industrial Membrane Research Institute, Department of Chemical Engineering, University of Ottawa, Ottawa, Ontario KIN 6N5, Canada

Received 9 July 1996; received in revised form 4 November 1996; accepted 5 November 1996

Abstract

A further investigation on the effect of solvent-system composition on microscopic structure and reverse-osmosis performance of sulphonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO)-polyethersulphone (PES) composite membranes is presented. Charged composite membranes are prepared by coating PES substrate ultrafiltration membranes with dilute solutions of hydrogen-form SPPO (SPPOH) having an ion-exchange capacity of 1.93 meq g-1. Methanol and methanol- chloroform mixtures containing 18, 42 and 66 mass% chloroform are used as solvents for making 1.0 mass% SPPOH coating solutions. Reverse-osmosis performance of the composite membranes is investigated by measuring the membrane permeation rates and rejection for various electrolyte solutions. The effect of the solvent, used in making the coating solution, is also studied through intrinsic-viscosity measurements. The microscopic structure of the SPPOH-PES composite membranes is explored by employing a scanning electron microscope (SEM) and an atomic force microscope operated in the tapping mode (TM AFM). The reverse-osmosis performance is explained in terms of the observed TM AFM skin-layer topographs, which are in turn correlated with the intrinsic-viscosity measurements.

Keywords: Membrane preparation and structure; Composite membranes; Skin layer; Intrinsic viscosity; Atomic force microscopy

I. Introduction

Studies on charged composite membranes for nano- filtration have been reported by a number of workers [1-4]. The selective skin layer of these membranes is usually negatively charged and electrolyte solutes

*Corresponding author. Fax: 1 613 562 5172.

0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. P I I S0376-7388(96)0033 1-6

with higher anionic charge densities and/or lower cationic charge densities are rejected more effectively. The electric feature of these composite membranes, whether cationic or anionic, is manifested by their relative inability to reject neutral solutes and macro- molecules that are orders of magnitude larger than electrolyte solutes. Charged composite membranes are commonly used in nanofiltration applications.

56 A. Hamza et al./Journal of Membrane Science 129 (1997) 5 5 4 4

Asymmetric sulphonated polyphenylene oxide (SPPO) membranes were initially attempted for reverse-osmosis treatment of brackish water [5]. There were, however, several drawbacks, i.e. exces- sive swelling occurred when feed waters contained various mixed ions. Recommendations were also made to fabricate composite membranes with comparatively thinner selective skin layer of SPPO. A number of workers studied preparation and performance of composite membranes employing SPPO as the skin-layer material and polypropylene (PP) or polysulphone (PSF) as the substrate material [6-8].

Further investigations were carried out on the pre- paration and reverse-osmosis performance of thin-film composite membranes with SPPO as the skin layer and polyethersulphone (PES) as the substrate mem- brane [9,10]. The membrane performance was explained in terms of hydration theory, especially in treating salts of the same valence but different ionic radii, as well as in the light of Donnan exclusion which was more dominant when different ions of different valences were involved. Preliminary experiments indi- cated that the solvent used in making the coating solution had a marked effect on the membrane per- formance, affecting the permeation rate significantly, whereas solute separation was altered to a much lesser extent. The change in morphology of the selective skin layer was considered as the possible cause [ 10]. Use of different solvent systems to dissolve a specific poly- mer naturally results in having solutions of different macromolecular orientations due to the change in the competing polymer-polymer and polymer-solvent interactions. Expectedly, this should result in a varia- bility in the microscopic structure of the membrane that, in turn, will affect the membrane performance appreciably.

For a given polymer chain-length, the tightness of the chain and hence its size in various solvent systems can vary with the strength of competing polymer-polymer and polymer-solvent interactions. Since the solution viscosity depends on the size of the coil, the viscosimetric behaviour of polymer solutions can be utilized to deduce solvent power and the role of the different components within the solvent system.

The most widely utilized measure of the dilute- solution viscosity of a polymer is its intrinsic viscosity,

[~7] [11]. As [~/] increases, polymer-solvent interaction increases, resulting in a more open structure of the polymer in the solution.

Scanning electron microscopy (SEM) has been widely used to study the structure of membranes. This technique is particularly useful in investigating cross sections of membranes, revealing the change in struc- ture from the skin layer to the microporous support for both asymmetric and composite membranes. How- ever, relying on SEM alone, as a means of character- izing membrane structures, is difficult since the sample preparation procedure required by SEM can affect the structures of membranes under study [12]. Such undesirable effects can be minimized when membrane samples are compared with each other after undergoing the same preparation procedure. Furthermore, SEM is still very powerful in analyzing membrane structures, especially when SEM is com- bined with other microscopic methods and membrane- performance results.

A powerful technique which has been developed more recently is atomic force microscopy (AFM). By this method, one can catch topographical view of membrane surfaces. This tool does not require any preparatory treatment of membranes to be investigated and is capable of providing high- resolution topographical views of membrane-surface structures, down to few nanometres, with relative ease [12-17].

A more recently developed mode of operation for AFM is the tapping mode AFM (TM AFM) [17,18], which has favourable lateral resolution compared to contact and noncontact AFM that were used more frequently in the past.

The objective of this paper is to investigate the effect of the composition of the coating solution on the microscopic structure of thin-film composite mem- branes and their reverse-osmosis performance. Charged composite membranes are prepared by coat- ing porous PES-substrate membranes with dilute solu- tions of SPPOH. Methanol and methanol-chloroform mixtures are used as solvents in making coating solutions. The reverse-osmosis performance of the composite membranes is investigated by using aqu- eous solutions with various solutes. The polymer- solvent interactions in the coating solution are char- acterized through intrinsic-viscosity measurements. The microscopic structure of the SPPOH-PES corn-

A. Hamza et al./Journal of Membrane Science 129 (1997) 551o4 57

posite membranes is studied by employing SEM and TM AFM.

gen-form (SPPOH) and were stored in distilled water until testing.

2. Experimental 2.3. Membrane testing and performance measurement

2.1. Materials

The fiat-sheet membrane used as the substrate for the composite membrane was the ultrafiltration mem- brane, HW17, obtained from Osmonics. HW17 was made from PES material, cast on polyester fibres as backing material [19].

Polyphenylene-oxide (poly(2,6-dimethyl- 1,4- phenylene oxide)) polymer, having an intrinsic viscosity of 0.51 dl g-1 in chloroform at 25°C, was provided by General Electric. Chlorosulfonic acid (C1-SO3H) (general reaction grade), having a density of 1.75 g cm -3 at 20°C and a minimum assay of 97%, was obtained from BDH. The organic solvents used were chloroform and methanol. Both chloroform and methanol were of reagent grade and were obtained from BDH. They were used without further purification. Lithium chloride, sodium chloride and potassium chloride were also obtained from BDH.

2.2. Preparation of composite membranes

Poly(2,6-dimethyl-l,4-phenylene oxide) was sul- phonated following a procedure reported elsewhere [10]. The ion-exchange capacity (IEC) of SPPOH, determined by the titration method, was found to be 1.93 meq g-1 dry SPPOH.

One mass% SPPOH solutions were prepared by dissolving the SPPOH polymer in different solvent systems. The solvent systems were methanol or chloroform-methanol mixtures containing 18, 42, or 66 mass% chloroform. Then, the skin side of the porous PES substrate was brought into contact with an SPPOH solution.

The solution at the membrane surface was drained by holding the membrane vertically, leaving a thin layer of the SPPOH solution. The coated layer was then dried for 0.5 h at room temperature. This coating procedure was repeated three times and was followed by drying overnight under ambient conditions. The membranes so prepared were maintained in the hydro-

The composite membranes were tested simulta- neously in a typical reverse-osmosis testing system, with an effective membrane area of 10.2cm 2, described in detail in the literature [20]. The perfor- mance of the composite membranes was evaluated in terms of permeation rate across the membrane as well as membrane-solute separation. All membranes were pressurized under pure-water flow at 1723 kPa gauge (250 psig) until the permeation rate became constant before any solute-separation testing took place. The experiments were conducted at the operating pressure of 1379 kPa gauge (200 psig) and at room tempera- ture. The solution concentration was 0.05 mol 1-1 for all electrolyte solutes. For each experiment, pure- water permeation rate (PWP), product permeation rate (PR) in the presence of the solute (both in m s-1), and solute separation, defined as

Separation

Feed concentration - Permeate concentration]

= L F--d~ed c o n ~ ]

were determined. The permeation rate was converted to that at 25°C by using the density and viscosity data of water [20]. The concentration of the electrolyte solutes was determined using a conductivity meter (Radiometer Copenhagen CDM 80).

2.4. Intrinsic-viscosity measurement

The intrinsic viscosities of SPPOH in different solvent systems were measured at 25°C. A Cannon- Fenske routine viscometer of ASTM size 50 and a constant of 0.003635cSts -1, from International Research Glassware, was used for this purpose. The flow times (t) of 1.0, 0.5, and 0.25 mass% polymer in each solvent system, as well as that of the solvent without polymer (to), were measured. The solvent systems were methanol and chloroform-methanol mixtures of 18, 42, and 66 mass% chloroform content. A plot of (t-to)/(toC) vs. C was then made. The intrinsic viscosity was then obtained by linear extra- polation of (t-to)/(toC) to zero concentration.

58 A. Hamza et aL /Journal of Membrane Science 129 (1997) 5 5 4 4

2.5. Microscopic structure of the membranes

Micrographs of the cross sections of composite membranes, as well as that of the uncoated substrate, HW 17, were obtained employing SEM. TM AFM was employed to obtain three-dimensional topographical micrographs of the skin layers of the substrate and composite membranes.

SEM pictures of these membranes were obtained using a Nanolab 7 SEM, operating at 20 keV. The samples were prepared by immersing membranes in liquid nitrogen, followed by fracturing them. The fractured surfaces were then mounted for viewing the cross sections.

The three-dimensional topographical micrographs were obtained employing TM AFM of a multimode scanning probe microscope (MMSPM), equipped with NanoScope III and a 1553D scanner (Digital Instru- ments, Santa Barbara, CA). The membranes for the AFM were readily used without any preparatory treat- ment. A piezoelectric scanner performed a high-reso- lution raster scan in the X - Y plane, parallel to the surface of the membranes, while a sensing and feed- back system operated in the Z direction reporting and controlling the variation in height across the skin layer. Calibration of the microscope using standard samples resulted in less than 2% deviation in the X-Yplane and less than 10% deviation in the Z direction. The AFM was equipped with a NanoScope III system. Section analysis, provided through the NanoScope III, was used to estimate the size of at least 15 SPPOH nodules on the surface of the substrate membrane and of skin layer of each of the composite membranes.

3. Results and discussion

SEM was utilized in investigating the cross sections of the four different composite membranes, as well as the uncoated substrate ultrafiltration membrane, HWl7. An SEM micrograph of the cross section of the substrate membrane is shown in Fig. 1. SEM micrographs of the cross sections of two composite membranes, one made using methanol as the solvent for making the coating solution and the other using 66% chloroform and 34% methanol as the solvent mixture, are shown in Fig. 2. The following conclu- sions can be drawn from these micrographs. First, the

1"

Fig. 1. SEM micrograph of the cross section of polyethersulphone substrate ultrafiltration membrane, HW17.

skin-layer thickness of the composite membranes is not clearly visible, which makes the evaluation of the coated layer impossible. Second, the coating proce- dure employing the various chloroform-methanol mixtures has no apparent effect on the structure of the porous substrate membrane.

Reverse-osmosis experiments were performed using four composite membranes and three electrolyte solutions. The results for salt separation are shown in Fig. 3, whereas those for product rate are shown in Fig. 4. The charged nature of the effective skin layer of the composite membranes is obvious. The product rate (PR) for each membrane increases with increase in the ionic radius of the metal cation (Me+), 0.6× 10 -1°, 0.95× 10 - l ° and 1.33 x 10 -1° m for Li +, Na + and K ÷, respectively. Salt separation, on the other hand, decreases with increase in the cation radius. This is an expected trend for membranes having the sul- phonate anionic group (SO3) in their active skins, as explained by the hydration theory in previous inves- tigations [9,10].

The solvent system employed in making the coating solutions has a marked effect on the composite mem- brane performance. It can be seen that electrolyte separation increases slightly with increase in the chloroform content, with the exception for the mem- brane made from 66 mass% chloroform in the coating- solvent system. Product rate, on the other hand, shows a steady drop with increase of chloroform in the coating-solvent system.

A. Hamza et al./Journal of Membrane Science 129 (1997) 55-64 59

1"

0.9

0 .8

0.7-

0.6-

0.5.

0.4-

0.3-

0.2

0.1-

0 / /

[ [ ~ 0%CHCl3 ~ II~CHCI3 [ ~ 42%CHCl3 m 66%CHCl3 ]

Fig. 3. Effect of solvent system in coating solutions on the separation of different electrolytes. Operating conditions: pressure, 1379 kPa gauge (200 psig); temperature, 25°C.

Fig. 2. SEM micrographs of cross sections of SPPOH-PES composite membranes made using different solvents for coating solution. Top: methanol; bottom: 66 mass% chloroform and 34 mass% methanol.

Four porous PES-substrate membranes, which were same as the one used for the preparation of the composite membranes, were contacted with methanol and chloroform-methanol mixtures of 18, 42, 66 mass% chloroform separately, and were dried after draining the solvent from the membrane surface. The contacting and the drying were done in a way similar to that for SPPOH-PES composite membrane pre- paration. Then, pure-water permeation rate (PWP) was determined for each membrane after 1 h of com- paction at 2069 kPa gauge (300 psig) under water. PWP measurements at 1379 kPa gauge (200 psig) were 18.9×10 -6, 17.1×10 -6, 18.2×10 -6 and 12.5x

16-

141 ~ 1 2

10 o

• 9'

i s.

4

O:

[ ~ 0%CHCI3 ~ 18%CHCI3 [ ~ 42%CHCI3 m 66%CHCl3 ]

Fig. 4. Effect of solvent system in coating solutions on the permeation rate of different electrolyte solutions. Operating conditions: pressure, 1379kPa gauge (200psig); temperature, 25°C.

1 0 - 6 m s -1 for the first, second, third and fourth membrane, respectively. The compaction trend was very similar for the four membranes. These results indicate that the permeability of the substrate mem- brane did not significantly change for the different solvents, except for the mixture with 66 mass% chloroform. The effect of the last solvent system on the substrate might be the cause of the decrease in solute separation, indicated in Fig. 3. The permeabil-

60 A. Hamza et al./Journal of Membrane Science 129 (1997) 5 5 ~ 4

Table 1 Intrinsic viscosity of SPPOH polymer having IEC value of 1.93 meq g-i in solvent systems of various chloroform-methanol proportions

Solvent composition Intrinsic viscosity, [~], at 25°C (mass%) (dl g 1)

CH3OH 1.99+0.12 18% CHC13, 82% CH3OH 1.51-4-0.07 42% CHC13, 58% C H a O H 1.27-t-0.06 66% CHCI3, 34% CH3OH 0.925:0.07

ity of the SPPOH-coated membranes, on the other hand, decreased steadily with increase in the chloro- form content in the solvent system. Therefore, the

effect of the composit ion of the solvent system on the performance of the SPPOH-coated membranes is primarily due to the change in the polymer morphol- ogy in the coated layer.

Apparently, the use of a more hydrophilic solvent with a higher methanol content created a more open structure in the charged polymer and such an open structure was retained even after drying and subse- quent contact of the coated layer with the feed aqueous solution. Interestingly, this open structure affected the membrane product rate more substantially than the solute separation.

SPPOH intrinsic viscosities in the solvent systems were measured in order to quantify the p o l y m e r -

!

1

"-x,.

y \ .

\ \

,~..J ...... 300

--~"' 200 ×

..... z I00

i j . < f < "

4OO

I'IN

X and Y 100.000 nm / division

Z 15.000 n m / division

Fig. 5. AFM micrograph of the skin layer of polyethersulphone substrate ultrafiltration membrane, HW17.

A. Hamza et al./Journal of Membrane Science 129 (1997) 55~54 61

polymer interactions in different solvent systems. Table 1 shows intrinsic viscosities of SPPOH having an IEC value of 1.93 meq g - ] in the different chloro- form-methanol mixtures. The table indicates that the polymer-polymer interaction decreases with increase in methanol content, since intrinsic viscosity increases with increase in methanol content. These results also support the idea that the polymer has a more open structure in the top skin when the methanol content in the coating solution becomes higher. Conversely, as the chloroform content in the solvent system is increased, the SPPOH intra-macromolecular interac-

tions become more favourable over the SPPOH-sol- vent interactions in the dilute solutions, leading to tighter SPPOH coils. Once these solutions are used in coating, the PES substrate, their macromolecular orientations, are reflected in the formed skin layers of the composite membranes.

Three-dimensional topographic AFM micrograph of the substrate ultrafiltration membrane is shown in Fig. 5, whereas AFM micrographs of the skin layers of two of the composite membranes are shown in Fig. 6 (solvent: methanol) and Fig. 7 (66% chloroform: 34% methanol as the solvent system). It should be noted

Z

jf~

~ "w,, . 1.2 -..,..,,, ./.-~~ 1 . 0

~"- . ~ / O . 8

×~, / ,"" 0 • 6

-~"" 0 , 4 Y "~'~. / " X "'~ " / 0 . 2

X and Y 200.000 nm / division

Z 15.000 nrn / division

Fig. 6. AFM micrograph of the skin layer of SPPOH-PES composite membrane made using methanol as the solvent for the coating solution.

62 A. Hamza et al./Journal o f Membrane Science 129 (1997) 55-64

Z

400 nM

Fig. 7. AFM micrograph of the skin layer of SPPOH-PES composite membrane made using 66 mass% chloroform ÷ 34 mass% methanol as the solvent system for the coating solution.

that the scales used are not always the same. Each A F M micrograph shows that the membrane surface is not smooth and consists of many nodules. These nodules are considered to be aggregates of polymer aggregates defined by Kesting [11]. Similar A F M pictures were obtained for composite membranes prepared by using other solvent systems. Justifiably, the effects of substrate surface characteristics on the skin structure of the composite membranes are the same for all membranes. Thus, the differences in surface characteristics of the composite membranes can be attributed to variations in the coating solutions. The composite membrane micrographs were further subjected to section analysis, provided through the NanoScope III software, to estimate the size range of the nodules on the surface of the four composite membranes. Sizes of nodules (diameters) for the

composite membrane skin layers are estimated as follows:

• substrate membrane, nodule size is of the range 71-153 nm

• solvent methanol, nodule size is of the range 85-125 nm

• 18% chloroform, nodule size is of the range 54-70 nm

• 42% chloroform, nodule size is of the range 37-51 nm

• 66% chloroform, nodule size is of the range 20-32 nm

These results also suggest that the SPPOH polymer has a more open structure in a coated skin layer formed by using a higher methanol content in the solvent system, resulting in a larger nodule size.

A. Hamza et al./Journal of Membrane Science 129 (1997) 55~54 63

Hence, reverse-osmosis performance experiments, intrinsic-viscosity measurements and AFM microgra- phy, all resulted in the same set of conclusions.

4. Conclusions

1. An increase in the methanol content in the methanol-chloroform solvent mixture increases the intrinsic viscosity of the SPPOH solution due to increase in the polymer-solvent interac- tions.

2. An increase in the methanol content in the metha- nol-chloroform solvent mixture increases the diameter of the polymer nodules observed on the surface of the selective skin layer, due to a more open structure of the polymer.

3. An increase in the methanol content in the methanol-chloroform solvent mixture increases the product rate in reverse-osmosis experiments, without significant decrease in solute separation.

4. The selection of a proper solvent system is impor- tant in reverse-osmosis membrane design.

Acknowledgements

One of the authors, A. Hamza, is indebted to the Secretariat of Scientific Research of Libya for support. Thanks are also extended to K.C. Khulbe and B. Kruczek for their efforts with the AFM method.

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