Structure–performance correlation of polyamide thin film composite membranes: effect of coating conditions on film formation

Journal of Membrane Science 211 (2003) 13–24

Structure–performance correlation of polyamide thin film composite membranes: effect of coating conditions on film formation A. Prakash Rao, S.V. Joshi, J.J. Trivedi, C.V. Devmurari, V.J. Shah∗ Central Salt and Marine Chemicals Research Institute, Gijubhai Badheka Marg, 364002 Bhavnagar, Gujarat, India Received 4 February 2002; received in revised form 24 May 2002; accepted 31 May 2002

Abstract Changes in the structure and chemical composition of polyamide (PA) composite membrane surface were correlated with (a) changes in the coating conditions employed to prepare the membrane; and with (b) resultant changes in performance. Thin film composite (TFC) membranes were formed by the interfacial polymerization of water-soluble difunctional amine with an organic-soluble trifunctional cross-linking agent on top of a porous polysulfone (PS) support. For different sets of film-formation conditions, different performances were exhibited with respect to product water flux and salt rejection. The thickness of the ultra-thin barrier layer of PA which governs the membrane performance characteristics such as permeability and permselectivity, is found to vary under different coating conditions in the range of 0.10–0.30 ␮m. Accordingly, an inverse correlation exists between PA film thickness and membrane flux. The composite membranes were characterized by Attenuated total reflectance infrared (ATR-IR) spectroscopy technique which allowed us to identify the main functional groups of PA barrier layer and thereby estimate its thickness based on the calculated depth of penetration (dp ) of infrared beam into the sample material and absorbance of the carbonyl-stretching characteristic band pertaining to amide linkage. Extensive characterization of a variety of TFC membranes could provide us greater certainty for drawing conclusions about structure–performance relationship. © 2002 Published by Elsevier Science B.V. Keywords: Composite membranes; Flux enhancement; ATR-IR spectra; Skin thickness; Chemical structure

1. Introduction A major breakthrough in the field of membrane separations was the development of composite membranes which are characterized by an ultra-thin separating “barrier” layer supported on a chemically different asymmetric porous substrate wherein the benefits of two separate polymeric layers could be ∗ Corresponding author. Tel.: +91-278-2566511; fax: +91-278-2567562. E-mail address: [email protected] (V.J. Shah).

combined to obtain the desired performance properties for a number of applications, most notably the reverse osmosis (RO) desalination of brackish water, highly saline water and domestic water with low total dissolved solids [1–8]. These membranes also hold promise for other applications such as wastewater treatment [9–13] and separation of organics from aqueous streams by RO [13–20]. Composite membranes have advantages over single-material asymmetric membranes in that, the top-separating layer is formed in situ and hence the chemistry and performance of the top barrier layer and the bottom porous

0376-7388/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 3 7 6 - 7 3 8 8 ( 0 2 ) 0 0 3 0 5 - 8

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substrate can be independently studied and modified to maximize the overall membrane performance. Over the years, improvements in performance of thin film composite (TFC) membranes for aqueous applications have taken place in terms of selectivity (solute rejection) without any appreciable change in membrane productivity (flux). However for such systems, there is an urgent need for developing membranes, which provide higher fluxes or productivities without severely affecting membrane selectivity. In particular, the demand for developing membranes with high water flux is enormous for applications to industrial wastewater treatment and ultra-pure water production [21–27]. A large number of TFC membranes have been successfully developed from different polymers such as polyurea, polyamides (PAs), polyurea-amides, polyether-amides, etc. [28–34], which have shown excellent salt rejection but relatively low water-permeability for RO applications. Studies in the recent past [35,36] have revealed that the membrane performance is related to the molecular/chemical structure of aromatic PAs. In fact, Kim et al. have shown that the incorporation of poly(p-aminostyrene) [35] and poly(m-aminostyrene) [36] to the aromatic PAs for fabricating active skin layer definitely affects membrane performance. However, much of the research that has been done till date to improve membrane performance for aqueous applications has centered on either changing the chemistry or morphology of the barrier layer and measuring its flux and selectivity. Comparatively, little work has been done to change the structure of the barrier (skin) layer in terms of its thickness and chemical composition by changing or optimizing the membrane making conditions and study the effect of such structural changes on overall membrane performance. This type of corroborative study is important because it provides complementary data about membrane structure and performance thereby allowing a more meaningful interpretation of the relationship between structure and performance. In the present paper, we report the formation of several polysulfone (PS)–PA composite membranes under initial and modified conditions of thin film coating. These membranes were subsequently subjected to evaluation and spectral studies. The performance data for each membrane were then correlated to the observed structure (thickness) of that membrane. In this

way, we could achieve a better understanding of the key physical and constitutional changes that occurred when the formation parameters were changed. 2. Experimental 2.1. Preparation of microporous polysulfone support membrane PS casting solution was prepared by dissolving 15 wt.% Udel P-3500 PS (supplied by the Union Carbide Co., USA) in A.R. grade N,N-dimethylformamide (DMF) at 80–90 ◦ C with continuous stirring. The resultant polymer solution was cast on a non-woven fabric (supplied by Filtration Sciences Corporation, USA; thickness: 80–90 ␮m) and gelled in a water bath consisting of 2% DMF and 0.1 wt.% sodium lauryl sulfate (SLS) surfactant. After 10 min of gelation, the resulting PS membrane was removed from the gelation bath and washed thoroughly with distilled water to remove all DMF and surfactant. The membrane was then subsequently employed as a support medium for TFC membrane development. The entire casting unit was kept in an air-conditioned room and the temperature was maintained between 25 and 30 ◦ C with a relative humidity of 30–35% during the entire period of PS casting. 2.2. Fabrication of thin film composite membranes An indigenous thin film coating machine was developed/fabricated to deposit the active skin layer of PA over the porous PS support membrane by adopting in situ interfacial polymerization technique. Several such composite membranes of 1 m × 15 m size were developed for our studies. A typical TFC membrane was prepared by initially allowing the PS support membrane to immerse in an aqueous solution of 2% (w/v) m-phenylenediamine (MPD, Lancaster Chemical Co.) followed by another dip into the hexane solution of 0.1% (w/v) trimesoyl chloride (TMC, Aldrich) which resulted in in situ formation or lamination of an ultra-thin film of PA over the surface of PS support. The resulting composite film was then subsequently cured in an air-circulation oven at a specific temperature and curing time for attaining the desired stability of the formed structure. The membranes

A. Prakash Rao et al. / Journal of Membrane Science 211 (2003) 13–24 Table 1 Thin film coating conditions for composite membrane development Reaction parameters

Initial membranes

Modified membranes

Coating temperature (◦ C) Interfacial reaction time (s) Curing temperature (◦ C) Curing time (min)

25–30 90 60–65 10

10–15 60 60–62 7

were thoroughly washed with deionized water before carrying out evaluation studies. The said composite membranes were prepared under two different sets of film-coating conditions to examine the effects of critical parameters on membrane structure and performance. Table 1 lists the conditions employed for both sets of membranes.

accessory and a Nicolet 5DX Fourier transform infrared spectrometer. The angle of the ATR accessory (crystal) can be varied from 30 to 60◦ , which causes a change in the depth of penetration of the IR beam into the surface of the membrane. The exact depth of penetration at a particular infrared wave length (λ) could be calculated by knowing the refractive indices of the ATR crystal (n1 ) and membrane sample (n2 ). For ATR-IR analysis of our membrane samples, germanium crystal at 45◦ angle of incidence was employed which gave probing depths of 0.4–0.6 ␮m in the chemical infrared region of interest. Dry specimens of membrane samples were cut to ATR crystal size (2 cm × 5 cm) and mounted on both faces of the germanium crystal, the active layer facing the crystal surface. Spectra are represented in absorbance.

2.3. Membrane performance evaluation

3. Results and discussion

Composite membranes were evaluated for permeate flux and selectivity on a batch type RO test kit comprising of six cells in series. Testing was done with NaCl solution of 2000 ppm concentration at a pressure of 250 psig. Circular membrane samples with a diameter of 48 mm were placed in the test cell with the active skin layer facing the incoming feed. The membrane sample was supported on the porous stainless steel sintered disc with a rubber O-ring around it to ensure leak-free operation. The effective membrane area (for each coupon) was around 18.1 cm2 . A standardized digital conductivity meter of Digisun Electronics (model: DI-909) was used to measure the salt (NaCl) concentrations in the feed and product water for determining membrane selectivity. The volume of permeate collected for one hour was used to describe flux in terms of gallons per square feet per day (gfd). Fifty coupons from each flat-sheet membrane were evaluated for flux and salt rejection, results of which have been averaged and depicted.

3.1. Initial membrane performance and characterization of surface layer

2.4. Analytical technique for characterization of skin layer Chemical characterization of the membrane surface was accomplished by attenuated total reflectance infrared (ATR-IR) spectroscopy instrument with a Barnes model 300 continuously variable ATR

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The average performance data with brackish water in terms of flux and selectivity for some of the interfacially formed PA composite membranes under initial thin film coating conditions are listed in Table 2. Although the selectivity of these membranes is consistently good, the flux values are relatively low and definitely need to be improved. In order to understand the reasons behind low membrane flux, the surface-chemistry investigation of these composite membranes was carried out. Accordingly, the chemical species present in the barrier layer could be characterized by ATR-IR analysis of composite membrane Table 2 Reverse osmosis performance of initial composite membranes TFC membrane code

Salt rejection (%)

Product water flux (gfd)

IM-1 IM-2 IM-3 IM-4 IM-5 IM-6 IM-7

94.0 95.0 94.0 94.0 92.0 95.0 95.0

17 19 17 16 11 21 26

Test conditions: 2000 ppm NaCl, 250 psig, 25 ◦ C.

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Fig. 1. ATR-IR spectrum of a porous PS support membrane.

samples wherein the functional groups present in the surface layer, after reaction of the amine species with the cross-linking agent, could be identified. Thus, in case of MPD-TMC in situ reaction on PS support membrane, whose spectrum is given in Fig. 1, the resulting polymeric product, which has formed the barrier layer, is PA. The spectrum of PS–PA is displayed in Fig. 2 which indicates that the interfacial polymerization has occurred since the acid chloride band at 1770 cm−1 is absent and a strong band at 1650 cm−1 (amide I) is present which is characteristic of C=O band of an amide group. In addition to this, other bands characteristic of PA are also seen at 1540 cm−1 (amide II, C–N stretch) and 1610 cm−1 (aromatic ring breathing). Because the ATR-IR technique employed a germanium crystal with a 45◦ angular setting of the crystal, the calculated minimum depth

of penetration was about 0.4–0.5 ␮m in the wavelength region of interest and as a result the IR spectrum of the top 0.4–0.5 ␮m of the membrane surface could be measured. Thus, it is clear that this sampling depth penetrates into the PS region also and as a result the IR spectrum of the composite samples comprised of bands attributed to both PA film and PS support membrane as seen in Fig. 2. It is interesting to note that all ATR-IR spectra are more intense towards longer wavelengths due to the λ dependence on depth of penetration (dp ). At longer wavelengths, the depth of penetration is more, resulting in increased intensity of bands. To get a clear picture about the composition of the barrier layer, a difference spectrum of pure PA was obtained by subtraction of the PS spectrum (Fig. 1) from the PS–PA spectrum (Fig. 2). The IR spectrum of only the PA barrier layer is shown in Fig. 3.

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Fig. 2. ATR-IR spectrum of a PS–PA initial composite membranes. The bands at 1660 and 1546 cm−1 are characteristic of amide functional groups.

Fig. 3. ATR-IR difference spectrum of the resulting PA film for initial low-flux membranes.

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From this difference spectrum, it is concluded that the amine groups have completely reacted with acid chloride groups to form the PA over a PS support. Based on this observation, the characteristic band at 1650 cm−1 was eventually used for the quantitative determination of surface film thickness by measurements of peak intensities of the carbonyl-containing amide layer of MPD-TMC reaction product. Incidentally, the absorbance value is proportional to the thickness of the PA layer since the IR beam penetrates through the surface layer and enters the porous support. The results of film thickness measurements calculated by the ATR-IR method along with the spectral band positions of PA for initial composite membranes are given in Table 3. Two things can be observed: (1) all the initial composite membranes are characterized by high skin layer thickness of approximately 0.24–0.25 ␮m, associated with a rigid cross-linked PA structure which explains for high salt rejection and low flux performance; (2) the thickness values of these PA films are definitely greater than the standard estimated 2000 Å (0.20 ␮m) thickness of the FT-30 high-flux membrane. The physical significance of these observations was obvious in that to achieve a higher membrane flux for the same selectivity, one must decrease the PA film thickness. 3.2. Performance and characterization of modified composite membranes Based on the above findings and observations, reaction conditions of thin film coating were modified which were conducive for generating very thin films Table 3 Thickness and spectral data of PA barrier layer for initial composite membranes based on ATR-IR measurements TFC code

Thickness (t, ␮m)

Infrared band positions (cm−1 )

IM-1 IM-2 IM-3 IM-4 IM-5 IM-6 IM-7

0.25 0.24 0.25 0.25 0.28 0.23 0.21

1660, 1650, 1664, 1665, 1660, 1650, 1650,

1610, 1612, 1608, 1610, 1610, 1612, 1612,

1546, 1540, 1545, 1545, 1546, 1540, 1540,

783 785 780 783 783 785 1454, 785

ATR-IR conditions: germanium crystal, 45◦ angle. Based on several membrane samples.

Table 4 Performance of modified composite membranes prepared by the interfacial polymerization of MPD and TMC TFC membrane code

Salt rejection (%)

Product water flux (gfd)

MM-8 MM-9 MM-10 MM-11 MM-12 MM-13 MM-14

96.0 95.0 95.0 95.0 95.0 94.0 95.0

40 33 39 36 34 31 36

RO test conditions: 2000 ppm NaCl, 250 psig, 25 ◦ C.

of PA on to the porous PS support so as to get a desired membrane performance. Membranes were thus prepared under modified thin film coating conditions and subsequently tested with 0.2% NaCl solution, performance results of which in terms of selectivity and flux are listed in Table 4. Although there is not much change in the selectivity of these membranes, the modified conditions have definitely helped to improve the product water flux to a greater extent which goes to show that the conditions adopted by us for in situ formation of PA, in the aftermath of initial studies, are highly favorable for developing high-flux membranes. Samples of these composite membranes were characterized by ATR-IR for surface analysis, identification of functional groups and determination of barrier layer thickness based on the absorbance measurements of the characteristic amide band at 1650 cm−1 . Interestingly, the peak intensities of this band and subsequently the PA film thickness for the modified membranes were found to be relatively much low as compared to the initial membranes which explains for the superior flux performance of the former over the latter. The PA film thickness data for the modified composite membranes as calculated by the ATR-IR method are given in Table 5 which is in good agreement with the performance data given in Table 3. Thus, it can be said with certainty that the optimized conditions of film coating have definitely helped to produce very thin films with enhanced water flux rates. The spectra of the modified high-flux membranes (Fig. 4) also showed peaks, characteristic of PA, at 1540 cm−1 (C–N stretch) and 1610 cm−1 (aromatic ring) over and above the carbonyl-containing 1650 cm−1 band

A. Prakash Rao et al. / Journal of Membrane Science 211 (2003) 13–24 Table 5 Spectral data and thickness of PA barrier layer for modified composite membranes based on ATR-IR measurements TFC code

Thickness (t, ␮m )

Infrared band frequency ( cm−1 )

MM-8 MM-9 MM-10 MM-11 MM-12 MM-13 MM-14

0.16 0.18 0.16 0.17 0.18 0.19 0.17

1664, 1662, 1664, 1662, 1660, 1650, 1660,

1610, 1608, 1608, 1608, 1610, 1612, 1608,

1540, 1548, 1540, 1540, 1545, 1548, 1545,

1465, 1450, 1460, 1455, 1450, 1450, 1460,

780 781 780 783 780 785 780

ATR-IR conditions: germanium crystal, 45◦ angle. Taken for several TFC samples.

with reduced intensities (absorbance) as compared to the initial membranes due to reduction in surface film thickness. Additionally, the ATR-IR difference spectra of PA surface layer for modified membranes showed a new medium-intense band around 1450 cm−1 (not observed in initial membranes), which is indicative of the formation of reaction side products under optimized film formation conditions. This additional band is most likely associated with a carboxylic acid group (C–O stretching/O–H bending) which could

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result from an acid chloride group of TMC reacting with water instead of the amine group of MPD. From the absorption coefficients (intensities) of the two bands, namely that of amide and carboxylic acid, it can be calculated that two-thirds of the acid chloride groups have formed amide groups and that one-third have formed carboxylic acid groups. The formation of this latter species is assumed to be favorable in terms of flux performance of the composite membrane. In other words, carboxylic acid formation may also contribute partially to the increased flux of modified composite membranes. Generally speaking, a higher ratio of hydrophilicity to hydrophobicity in the molecular chain will lead to an increase in water flux of membrane from the said polymer material. 3.3. Determination of structure–performance relationship Table 6 gives the summary of membrane performance, film thickness and PA spectral data for both the initial and modified composite membranes. The dependence of flux on PA film thickness is graphically represented in Fig. 5 which shows an inverse

Fig. 4. ATR-IR spectrum of PS–PA modified high-flux membrane. The peaks for PA are relatively suppressed due to reduction in film thickness.

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Table 6 Summary and comparison of initial and modified composite membranes Membrane codea

PA film thickness (t, ␮m)

Water flux (gfd)

Salt rejection (%)

PA characteristic bands (cm−1 )

IM-1 IM-2 IM-3 IM-4 IM-5 IM-6 IM-7 MM-8 MM-9 MM-10 MM-11 MM-12 MM-13 MM-14

0.25 0.24 0.25 0.25 0.28 0.23 0.21 0.16 0.18 0.16 0.17 0.18 0.19 0.17

17 19 17 16 11 21 26 40 33 39 36 34 31 36

94.0 95.0 94.0 94.0 92.0 95.0 95.0 96.0 95.0 95.0 95.0 95.0 94.0 95.0

1660, 1650, 1664, 1665, 1660, 1650, 1650, 1664, 1662, 1664, 1662, 1660, 1650, 1660,

a

IM-1–IM7 represent initial membranes; MM-8–MM14 represent modified membranes.

Fig. 5. Graphical representation of correlation of flux with PA film thickness.

1546 1540 1545 1545 1546 1540 1540 1540, 1548, 1540, 1540, 1545, 1548, 1545,

1465 1450 1460 1455 1450 1450 1460

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correlation trend/behaviour. The following points are obvious from the summary table and graphical representation: (a) membrane selectivity is independent of film thickness; (b) membrane productivity is governed by PA film thickness which in turn is controlled by the reaction conditions; (c) flux is inversely proportional to the thickness of the PA barrier layer; (d) for the same salt rejection membranes, the permeate flux is more for those membranes which have a relatively reduced surface film thickness; (e) the interfacial reaction product, namely PA, spans the entire thickness of the barrier layer which is responsible for the membrane performance; and (f) low-flux initial membranes are characterized by the presence of amide groups in the barrier layer while high-flux modified membranes are distinguished by the presence of amide and car-

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boxylic acid groups in the barrier layer, as witnessed from the spectral data. It is thus clear that the critical parameters for thin film coating such as reaction time, relative humidity and coating temperature play an important role in determining the structure of the interfacially polymerized surface film and subsequently the membrane performance. Also, an introduction of hydrophilicity into the surface polymer will render high water permeability to the composite membrane formed from it. 3.4. Elucidation of polyamide chemical structure based on membrane performance The active skin layer of TFC membranes namely PA, which showed a selective flux performance in the

Fig. 6. Different representations of PA chemical structure: (a) cross-linked network structure; (b) linear hydrophilic structure; (c) cross-linked hydrophilic structure.

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RO process, could be interfacially generated by reacting, in situ, m-MPD solution with TMC solution. Now the reaction of diamine with acid chloride can lead to two types of side reactions over and above the usual linear PA chain formation. In the first type, a network chain-branching structure is formed (Fig. 6(a)) by the cross-linking reaction of another diamine molecule with terminal acid chloride group of TMC via amide linkage (–CO-NH). In the second type, hydrolysis of the third or terminal acid chloride group of TMC to a carboxylic acid results in a linear hydrophilic structure (Fig. 6(b)). Generally speaking, both the types of structure are essential for producing high performance RO membranes. While the network structure formed by the cross-linking reaction increases salt rejection at the expense of some flux, the carboxylic acid structure formed by the hydrolysis reaction increases water flux due to its hydrophilic properties. Hence the idea is to maintain a 1:1 ratio of cross-linking reaction to hydrolysis reaction so that the resultant membranes formed from the above will have an equal ratio of hydrophilic and network structure (Fig. 6(c)) which will give higher water flux with a reasonably good salt rejection. As far as our studies are concerned, the performance and spectra of initial membranes indicates the formation of a cross-linked structure, such as shown in Fig. 6(a), in the absence of the hydrolysis of the terminal acid chloride group which is explained not only by high salt rejection and low water flux but also by the absence of the carboxylic acid characteristic band in the ATR-IR spectra. Also, the increased PA film thickness for the membranes formed under initial reaction conditions, which results in a low flux performance, might arise due to the formation of a rigid network polymeric structure. However, for modified membranes, wherein the reaction conditions were changed for attaining improved performance, the high flux values with similar (unchanged) membrane selectivities as also the presence of a carboxylic acid band in the ATR-IR spectra are all indicative of a cross-linked hydrophilic structure such as depicted in Fig. 6(c) which reflects a 1:1 ratio of cross-linking reaction to hydrolysis reaction that plays an important role in determining both water flux and membrane selectivity. It is interesting to note that this type of hybrid structure results in a decreased thickness of the formed PA film. This might be explained due to

an increase of the hydrophilicity with a corresponding decrease of the network or chain-branching structure resulting in the formation of a partially rigid structure as against a fully rigid network structure with increased PA film thickness observed for initial low-flux membranes. Although, on one hand it may be possible that the structural or chemical or conformational changes might not be contributing to film thickness, on the other hand it would be equally hard to explain otherwise the coincidence of carboxylic acid presence in low thickness modified membranes. Notwithstanding the above, the combined effect of hydrophilicity (carboxylic acid generation) and reduced film thickness contributes to an overall increase in permeate flux. Finally, it can be concluded that the composite membranes prepared under modified reaction conditions exhibited better performance in terms of flux over the initial membranes, selectivity however remaining the same for both, which clearly indicates that the terminal acid chloride group in TMC is essential and responsible for high membrane performance, be it flux or selectivity.

4. Summary RO composite membranes were prepared under two different sets of thin film coating conditions for exploring the performance, thickness and chemical structure of the resulting PA barrier layer. These studies are clearly indicative of the changes in membrane performance with interfacial reaction parameters and thickness of the active skin layer. Membranes made under modified reaction conditions exhibited a superior water flux with reduced PA film thickness than those made under initial conditions. In either of the cases, however, the salt rejection remained more or less constant irrespective of the flux. It was concluded that PA films possessing a cross-linked hydrophilic structure are a prerequisite to obtain membranes having both high selectivity and high water flux. Based on structure–performance correlation studies, the most probable chemical structures of PA were elucidated for initial low-flux high selectivity and modified high-flux high selectivity composite membranes. Thus, the complementary surface investigations involving performance and structure are very useful and informative

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for the study and analysis of composite films which are formed in situ by interfacial polymerization.

Acknowledgements The authors would like to thank Dr. P.K. Ghosh, Director of Central Salt and Marine Chemicals Research Institute, Bhavnagar, for his constant encouragement, constructive suggestions and fruitful scientific discussion.

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