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Synthesis and characterization of interfacially polymerized polyamide thin films
Desalination 191 (2006) 279–290
Synthesis and characterization of interfacially polymerized polyamide thin films Il Juhn Roha*, Alan R. Greenbergb, Vivek P. Khareb a
Hydranautics, 401 Jones Road, Oceanside, CA 92054, USA Tel. +1 (760) 901-2593; Fax +1 (760) 901-2664; email: [email protected] b Department of Mechanical Engineering, NSF Membrane Applied Science and Technology Center, University of Colorado, Boulder, CO 80309-0427, USA
Received 15 March 2005; accepted 7 June 2005
Abstract A homologous series of thin film composite (TFC) membranes was made by interfacial polymerization (IP) of trimesoyl chloride (TMC) and m-phenylenediamine (MPD). Membrane flux and rejection as well as the chemical and mechanical properties of the unsupported IP film were evaluated as a function of monomer concentration. The surface characteristics of the IP films were determined using electron-spectroscopy-for-chemical-analysis (ESCA) and contact angle measurements, and the mechanical properties were obtained using pendant drop mechanical analysis (PDMA) and surface profilometry. Results indicate that the TMC concentration has a more pronounced effect on the IP film properties than the MPD concentration. Higher TMC concentration leads to a pronounced increase in the IP film thickness and surface hydrophilicity. In contrast, the thickness increase is smaller and surface hydrophilicity decreases when the MPD concentration is increased. Water flux was determined to depend mainly upon the IP film thickness and surface hydrophilicity whereby these two parameters have counterbalancing effects. Consequently, the decrease in water flux with increasing MPD concentration is pronounced, but a corresponding increase in the TMC concentration has a smaller effect. These studies provide an improved basis for understanding the factors that govern TFC membrane performance. Keywords: Thin film composite (TFC) membrane; Properties of polyamide barrier layer; Interfacial polymerization; Diffusion; Structure–property relationships
*Corresponding author.
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.desal.2006.03.004
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1. Introduction Thin film composite (TFC) membranes are extensively used in reverse osmosis (RO) desalination applications. These membranes have a thin and dense active layer that controls membrane performance (permeability and selectivity), and a much thicker porous substrate that provides mechanical support to the active layer. In order to achieve high values of permeability and selectivity, the active layer should be ultrathin and hydrophilic. A variety of polymers and formation schemes have been employed for this purpose [1,2]. Overall, polyamide structures fabricated via interfacial polymerization (IP) have provided the most successful RO membranes [3–5]. The ultrathin polyamide films synthesized via IP can exhibit a range of physicochemical properties based on the polymerization conditions including the monomer concentration and the reactant ratio [6,7]. These properties play a pivotal role in membrane performance. The “solution–diffusion” mechanism proposed by Lonsdale and coworkers [3] is generally accepted to govern species transport through the active layer. According to this mechanism, the solvent and the solute species dissolve in the nonporous active layer and are subsequently transported via diffusion in an uncoupled manner. The water flux (Jw) is given by: Jw =
DwCwVw ( ∆P − ∆Π ) Rg Td
(1)
where Dw and Cw, are the water diffusion coefficient and concentration in the membrane, respectively, Vw is the partial molar volume of water in the external phase, ∆P = Pf – Pp and ∆Π = Πf –Πp are the applied pressure difference and the osmotic pressure difference, respectively, across the membrane, Rg is the gas constant, T is the absolute temperature, and d is the active layer thickness. The f and p subscripts refer to the feed and the permeate sides, respectively.
The salt flux (Js) through the membrane for the case of a large concentration difference is given by:
Js =
Ds K s ∆C d
(2)
where Ds is the diffusion coefficient of salt in the membrane, Ks is the partition coefficient of salt between the membrane and the solution, and ∆Cs = Cs, f – Cs, p is the concentration difference of salt at the two sides of the membrane. Using Eqs. (1) and (2) and the relationship Cs, p = Js/Jw, the salt rejection (R) can be obtained as:
1 ⎪⎧ D K Rg T Cs , f − Cs , p ⎪⎫ ⋅ ⋅ R =1− ⎨ s s ⋅ ⎬ ∆P − ∆Π ⎭⎪ (3) Cs , f ⎩⎪ DwCw Vw Eqs. (1)–(3) indicate that the fluxes and rejection depend upon Dw, Cw, Ds and Ks, which are intrinsic properties of the active layer material. In addition, whereas the fluxes are inversely proportional to the active layer thickness, d, the rejection is independent of d. For the current studies, polyamide TFC membranes were fabricated using trimesoyl chloride (TMC) and m-phenylenediamine (MPD) on a polysulfone support via IP [9,10]. As indicated in Fig. 1, the polyamide contains a crosslinked portion (X) and a more hydrophilic linear moiety (Y) containing free carboxylic acid groups. The number of free carboxylic acid groups as well as the thickness and mechanical properties of the thin film are governed by the monomer concentrations and the concentration ratio. Although many studies of IP TFC membranes have been reported in the literature, relatively few comprehensive investigations have incorporated specific chemical, physical and mechanical properties of the active layer. Consequently, the objective of this study was to evaluate the effect of the active layer surface properties on membrane performance. Our
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NH
NH CO
HN
NH
CO
COOH
CO
NH X
NH
CO
CO Y
Fig. 1. The polyamide structure formed from an IP reaction between MPD and TMC has a crosslinked portion (Xfraction) and linear moiety (Y-fraction), which has an unreacted acid chloride group that subsequently hydrolyzes to a carboxylic acid group.
approach provides a quantitative description of the relationships among the composition, properties, and performance of the polyamide thin film, and yields useful insights regarding the means by which membrane performance can be optimized.
2. Materials and methods TMC (Aldrich, 98%) and MPD (Aldrich, 99%) were distilled under vacuum at 160°C and 170°C, respectively. The purified TMC was stored in a vacuum desiccator containing calcium chloride. The purified MPD was stored in a refrigerator in a dark bottle. The TMC solvent, n-hexane, was dried via exposure to magnesium sulfate and then distilled. The purified n-hexane was stored in a tightly closed bottle with a molecular sieve to absorb any water. RO purified and deionized water was used whenever required. Two series of polyamide thin films were prepared. For series I, the TMC concentration was varied in the range 0.01–1.0 wt/v% (weight per volume) while that of MPD was held constant at 0.5 wt/v%. For series II, the TMC concentration
was fixed at 0.1 wt/v%, and the MPD concentration was systematically varied in the range 0.01– 1.0 wt/v%. The specific compositions and resulting monomer ratios are summarized in Table 1. The TFC membranes were prepared via a twostep process. First, an asymmetric support layer was prepared using standard techniques [11], and then the active layer was formed on its surface. The active layer was prepared via in-situ IP using an unstirred non-dispersion method. The support layer was immersed in a de-ionized water solution containing MPD for 2 h. Excess reagent was squeezed off the surface of the support layer using a soft rubber roller. The support layer, impregnated with the MPD solution, was then reacted with a hexane solution containing TMC by placing the hexane solution on the surface of the support layer for 2 min. After this two-minute IP reaction, the hexane solution was drained off and the polymerized substrate was washed with pure hexane before drying at room temperature. Additional details of the IP methodology as well as the measurement protocols used to characterize RO membrane performance are described in a previous article [12]. For the subject experiments, an RO
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Table 1 Composition and reactant ratios used to synthesize IP films
Series I
Series II
MPD (wt/v%)
TMC (wt/v%)
Reactant Ratio
MPD (wt/v%)
TMC (wt/v%)
Reactant Ratio
0.5 0.5 0.5 0.5 0.5
0.01 0.05 0.1 0.5 1.0
50 10 5 1 0.5
0.01 0.05 0.1 0.5 1.0
0.1 0.1 0.1 0.1 0.1
0.1 0.5 1 5 10
system was operated at 3.03 MPa (440 psi) and used a feed concentration of 2,000 ppm NaCl. Previous work has indicated that the mechanical strength of the active layer is closely associated with TFC performance [12]. Pendant Drop Mechanical Analysis (PDMA) [12–14] was utilized for in-situ mechanical strength measurements of the IP active layer. PDMA is ideally suited to study unsupported thin films so that the measurements are not influenced by the properties of the support layer. The details of the PDMA protocol have been reported previously [12]. The film thickness, a parameter necessary to obtain absolute strength using PDMA, was measured by first preparing the IP films separately on a glass slide. The thin films were washed, then semi-dried in air at room temperature and finally scratched using a sharp knife. The depth of the scratched “valley” was measured using a Tencor P-10 surface profilometer with a 3.0 mg applied force and a scan speed of 0.1 mm/s. The IP film hydrophilicity was estimated through contact angle measurements using an image analyzer system (PAS PX-380) wherein the shape of liquid drops (volume ~3 µL) placed on the IP film surface was recorded. Contact angles (θ) on both sides of the drop were measured after these images had been magnified, and the mean value from six replicates was utilized. The surface tension due to the dispersion and polar forces characteristic of the polymer was obtained from
the following equation [15]: ⎛ γd γd 4⎜ d s L d γ + γL cos θ = ⎝ s γL
⎞ ⎛ γ sP γ LP ⎟ 4⎜ P P ⎠ + ⎝ γs + γL γL
⎞ ⎟ ⎠ −1
(4)
Here, γL, γLd and γLP are the total surface tension, dispersion and polar force components of surface tension for a standard liquid, respectively, and γSd and γSP are the dispersion and polar force components of surface tension for the polymer. The surface tension properties of the reference liquids used in Eq. (4) are shown in Table 2. The surface chemical composition of the IP films was obtained via electron-spectroscopy-for-chemical-analysis (ESCA) (Surface Science Instrument 2803-S), and the concentration of dissolved water in the IP films was obtained using mass-loss measurements. The specific procedures utilized are described in a previous paper [12].
Table 2 Dispersion and polar components of the surface tension of reference liquids Liquids
γL (mN/m)
γdL (mN/m)
γpL (mN/m)
Water Glycerol Formamide
72.8 63.4 58.2
29.1 37.4 35.1
3.7 26.0 23.1
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3. Results and discussion We first determined the polymerization conditions that would result in IP films possessing properties consistent with those of a bulk polymer. Absolute strength measurements were utilized for this purpose. As shown in Fig. 2, the IP films synthesized from low monomer concentrations (<0.01 wt/v%) did not possess adequate strength. Hence, only the IP films synthesized using monomer concentrations greater than 0.01 wt/v% were utilized in subsequent studies. 3.1. Effect of composition on TFC membrane performance The experimental design enabled the effect of systematic changes in the monomer concentration and consequently the reactant ratio on membrane performance to be evaluated. These results are shown in Fig. 3 for series I and II. Overall, the water flux and the salt rejection were more sensitive to changes in the MPD concentration (series II) than the TMC concentration (series I). For the series I membranes, the water flux
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evidenced a decrease of 3% over TMC concentrations from 0.01 to 0.1 wt/v%, but remained almost constant at higher TMC concentrations. Likewise, the salt rejection increased slightly (≈6%) as the TMC concentration increased from 0.01 to 0.1 wt/v%, but remained essentially unchanged at higher TMC concentrations. The behavior of the series II membranes was distinctly different than that of the series I membranes (Fig. 3). The water flux decreased by 24% as the MPD concentration was increased from 0.01 to 0.1 wt/v%. The flux decrease was more modest (≈14%) at higher MPD concentrations, and the flux did not reach an asymptotic value at the maximum MPD concentration tested (1.0 wt/v%). The salt rejections were sensitive at low MPD concentration, increasing by 23% as the MPD concentration increased from 0.01 to 0.1 wt/v%, but reached a constant value for greater MPD concentrations. Overall, these results suggest that the permeability and rejection depend upon the monomer concentrations as well as the concentration ratio. A more complete understanding regarding the
Fig. 2. Absolute strength of the thin film active layers as a function of the MPD and TFC concentrations. For the series I films, the MPD concentration is fixed at 0.5 wt/v%, and for the series II films the TMC concentration is fixed at 0.1 wt/v%.
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Fig. 3. TFC membrane performance as a function of the MPD and TMC concentration. For the series I films the MPD concentration is fixed at 0.5 wt/v%, and for the series II films the TMC concentration is fixed at 0.1 wt/v%.
nature of these effects requires knowledge of the degree to which the intrinsic properties of the thin films are affected by compositional changes. Consequently, the relationships among membrane performance (diffusion of water and salt) and polyamide thin film properties are considered in the next two sections. 3.2. Effect of TMC concentration on thin film properties Fig. 4 shows the relationship between the relative strength of the series I IP films and the water flux and rejection properties of the membrane. The results indicate that the relative strength, which is a function of the stress as well as the polyamide film thickness, is a sensitive function of the TMC concentration. As the TMC concentration is increased from 0.01 to 0.1 wt/v%, the relative strength and the salt rejection (R) increase by 51% and 6%, respectively, whereas the water flux (Jw) decreases by about 3%. Over the concentration range 0.1–1 wt/v% TMC, the relative strength increases dramatically (≈230%) while Jw and R evidence significantly more modest changes of –1% and +2%, respectively. Previous work has
indicated that the water flux and salt rejection do not demonstrate a dependence on the relative strength [12] if the relative strength exceeds a critical value, and the present results are in general agreement with these findings. A similar conclusion may appear warranted as well for the salt rejection, which after an initial small increase, remained relatively constant even though the relative strength continued to increase. On the other hand, the film prepared with 0.01 wt/v% TMC had a relative strength below 5 kPa·mm [12] (the critical value affecting salt rejection) and a corresponding value of salt rejection that was lower than the asymptotic value. This result may imply that the low salt rejection of this membrane is due to a weak active layer that is more prone to local defects. The thickness change of the active layer as a function of TMC concentration is shown in Fig. 5. The thickness increased linearly with TMC concentration over the range from 0.01 to 1.0 wt/v%, with a total increase in thickness of approximately 360%. As previously noted, the water flux decreased modestly over this same concentration range (≈4%). Although a decrease in water flux with increasing film thickness is expected
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Fig. 4. The dependence of the active strength, water flux (Jw) and salt rejection (R) of the series I IP films on the TMC concentration.
Fig. 5. The dependence of the active-layer thickness (d) and the water flux (Jw) of the series I IP films on the TMC concentration.
[Eq. (1)], the magnitude of the former is much smaller than that of the latter. The differences in the response of d and Jw to changes in the TMC concentration thus suggest that the water flux is affected not only by the thickness but also by some other property of the IP polymer film. In order to identify these additional factors, three chemical properties were studied: the water content (Cw), the polar component of surface
tension (γps ) and the Y-fraction (Fig. 6). The polar component of surface tension was used as a measure of the hydrophilicity of the IP film. The chemical composition of the surface of the active layer was analyzed via ESCA. The ESCA spectra provided the C1S, N1S, and O1S core levels and the ratios of the surface composition (%). From the chemical formula of the repeat unit (Fig. 1) and the surface composition (ESCA), the X and Y
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Fig. 6. The dependence of the water permeability coefficient (Cw), Y-fraction and the polar force components of the surface tension (γps ) of the series I IP films on the TMC concentration.
fractions were calculated [16]. The dependence of these chemical properties on the TMC concentration is shown in Fig. 6. Cw, Y-fraction and γps all increased with increasing TMC concentration. The behavior of the Y-fraction implies that the active layer polymerized at low TMC concentration is less hydrophilic than that made at high TMC concentration. This is confirmed by the response of γps , which attains the lowest values at low TMC concentration. Since the water solubility depends upon the hydrophilicity as well as the Y-fraction, the water solubility also increased with increasing TMC concentration. Now, the chemical nature of the active layer should reflect the characteristics of the IP process wherein the condensation reaction of amine and acid chloride occurs on the organic side of the aqueous–organic interface [6,7]. Hence, the reaction should mainly be controlled by the diffusion of MPD through the interfacially polymerized film and the diffusion of TMC in the organic phase. For the series I films, the concentration of MPD was held constant, as the TMC concentration was varied from 0.01 to 1.0 wt/v%. At lower concentrations (< 0.1 wt/v% TMC), the rate-controlling step of the IP process is presumably the diffusion of TMC in the organic phase since the concentra-
tion of MPD at the organic reaction site is relatively high. When the polymerization is controlled by the diffusion of TMC, the TMC concentration should play a major role in determining active layer properties. As expected, the thickness of thin film increases linearly as TMC concentration increases (Fig. 5). The probability of having an unreacted acid chloride group is decreased as the TMC concentration is decreased due to the relatively high MPD concentration. Any unreacted acid chloride groups could easily be converted to the corresponding carboxylic acids through hydration. This could explain the increase in Cw, Y-fraction and γps with TMC over this lower concentration range (Fig. 6). At higher TMC concentrations (> 0.1 wt/v%), the respective magnitudes of these three parameters are even more sensitive to concentration, whereby the maximum rates of increase are observed at the highest TMC concentration. Because of the fixed MPDA concentration (0.5 wt/v%), increasing the TMC concentration may result in a deficiency in the available amine groups at the organic reaction side of the interface. Such a deficiency would lead to an increasing number of unreacted acyl chloride groups, and therefore to an increased Y-fraction containing free carboxylic acid groups at these higher TMC
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concentrations (Fig. 6). The increase in Y-fraction could account for the observed behavior of Cw and γps , i.e. an increase in the slope with increasing TMC concentration. As previously noted, the water flux decreases by only ≈3% over the range of TMC concentrations considered (Fig. 3). This decrease corresponds to an increase in IP film thickness of ≈360% over the same composition range (Fig. 5). Data from Fig. 6 indicate that Cw increases by approximately 300% over this same composition range. Given the inverse relationship between the water solubility and film thickness [Eq. (1)], the relatively constant water flux is most likely due to the counterbalancing trends in Cw and d. These results confirm that the overall performance of TFC membranes depends upon both the intrinsic and extrinsic IP film characteristics. 3.3. Effect of amine concentration on thin film properties Fig. 7 shows the dependence of the IP film relative strength and the TFC membrane flux and rejection on the MPD concentration. Of particular interest is the dramatic decrease in the water flux and corresponding increase in the relative strength
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as the MPD concentration increases from 0.01 to 0.1 wt/v%. This behavior confirms the results of our previous study [12]. The present results also demonstrate that rejection increases over this same concentration range; however, for MPD concentrations >0.1 wt/v% the rejection remains essentially constant whereas the relative strength increases only modestly. Overall, these data indicate that the transport and mechanical properties are closely related and suggest that the unexpectedly high water flux at low MPD concentrations (<0.1 wt/v%) may be due to the low relative strength of the IP film. When the relative strength values of Fig. 7 are taken into account, the value indicates that for MPD concentrations less than 0.1 wt/v% the films have low mechanical strength. Although the salt rejection is an intrinsic property of the active layer material and should not depend upon strength, the rejection also evidences decreased values for such films, consistent with either a “loose” polymer structure or the presence of defects at low MPD concentrations. The active layer thickness for series II films increased with MPD concentration (Fig. 8). Whereas the film thickness increases by ≈27% as the MPD concentration increases from 0.01 to 0.1 wt/v%, further increases in the MPD concen-
Fig. 7. The dependence of the relative strength, water flux (Jw), and salt rejection (R) of the series II IP thin films on the MPD concentration.
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Fig. 8. The dependence of the active-layer thickness (d) and the water flux (Jw) of the series II IP films on the MDP concentration.
Fig. 9. The dependence of the water permeability coefficient (Cw), Y-fraction and the polar force components of the surface tension (γsp) of the series II IP films on the MPD concentration.
tration (from 0.1 to 1.0 wt/v%) produce an additional increase in the film thickness that is more modest ≈7%). Correspondingly, the water flux decreases by ≈24% as the MPD concentration increases from 0.01 to 0.1 wt/v% and by an additional 14% as the MPD concentration reaches 1.0 wt/v%. Given that the membrane flux is inversely proportional to the film thickness [Eq. (1)], the difference in the magnitude of the response of
d and Jw to changes in the MPD concentration implies that the water flux was also affected by other factors. Fig. 9 shows the Y-fraction, γps and Cw, as a function of MPD concentration. In contrast with the effect of increasing TMC concentration, the Y-fraction is observed to decrease with increasing MPD concentration. This dependence suggests that an active layer polymerized at low MPD
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concentration would be more hydrophilic than that made at high MPD concentration. Indeed, this conjecture is supported by the behavior of γps , which also decreases with increasing MPD concentration. In addition, the concentration of water dissolved in the active layer, Cw, depends upon the hydrophilicity. Overall, this relationship is evidenced in Fig. 9 at low MPD concentrations (< 0.1 wt/v%) where the water solubility evidences a monotonic decrease with increasing MPD concentration in a manner similar to that observed for γ sp and the Y-fraction. At higher MPD concentrations, Cw, γps and the Y-fraction continue to decrease but at a rate lower than that observed at lower concentrations. For series II films with MPD concentrations <0.1 wt/v%, the rate-controlling step is most likely the diffusion of MPD through the interfacially polymerized film. If this is indeed the case, the diffusion rate of MPD should play a major role in determining thin film properties. As expected, the thickness of the thin film increases significantly with MPD concentration over this range (Fig. 8). In addition, within this composition range, the number of available acid chloride groups (from TMC) is larger than the number of amine groups (from MPD). This mechanism could account for the fact that at low MPD concentrations, the Yfraction having free carboxylic acids is greater (Fig. 9). For MPD concentrations >0.1 wt/v%, the ratecontrolling step could well be the diffusion of TMC in the organic phase. If the TMC concentration at the interface were deficient with respect to the MPD concentration, film thickness would remain constant despite an increasing MPD concentration. Such behavior is apparent in Fig. 8. In addition, the deficiency of TMC monomer at the IP reaction site reduces the probability of having unreacted acid chloride groups, which in turn reduces the Y-fraction having free carboxylic acid groups at high MPD concentrations (Fig. 9). As previously noted, the water flux decreases by approximately 34% over the range of MPD
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concentrations considered (Fig. 3). This decrease corresponds to an increase in IP film thickness of approximately 37% over the same composition range (Fig. 8). In addition, data from Fig. 9 show that the water solubility decreases by approximately 25% as the MPD concentration is increased from 0.01 to 1 wt/v%. Consequently, it would appear as though the observed dramatic decrease in water flux can be explained by the observed increase in film thickness as well as decrease in water solubility, in good agreement with the relationships expressed in Eq. (1). These results indicate that although the overall effect of any changes in the intrinsic film characteristics as a function of MPD concentration is small, the change in permeation performance is surprisingly large.
4. Conclusions These experiments demonstrate that variation in the acid chloride concentration has a more pronounced effect on the thin film material properties than corresponding variations in the amine concentration. Increasing the TMC concentration from 0.1 to 1.0 wt/v% led to a marked increase in the IP film thickness as well as surface hydrophilicity. In contrast, the thickness and surface hydrophilicity changes were relatively small when the MPD concentration was varied from 0.1 to 1.0 wt/v%. However, because the water flux mainly depends upon the IP film thickness and surface hydrophilicity (through Cw) and these two parameters have counterbalancing effects, the decrease in water flux with increasing MPD concentration was pronounced but a corresponding increase in the TMC concentration had a relatively smaller effect. Overall, these experiments provide an improved basis for understanding the relationship between the specific polymer properties of the active thin layer and IP TFC membrane performance.
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Acknowledgment The authors gratefully acknowledge the support of the original PDMA studies by the Membrane Applied Science and Technology Center at the University of Colorado at Boulder.
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[7] G.-Y. Chai and W.B. Krantz, J. Membr. Sci., 93 (1994) 175. [8] H.K. Lonsdal, U. Merten and R.L. Riley, J. Appl. Polym. Sci., 9 (1965) 1341. [9] L.T. Rozelle, J.E. Cadotte, K.E. Cobian and C.V. Kopp, Jr., in S. Sourirajan, ed., NS-100 Membranes for Reverse Osmosis and Synthetic Membranes, National Research Council Canada, Ottawa, Canada, 1977, pp. 249. [10] J.E. Cadotte, Interfacially Synthesized Reverse Osmosis Membrane, US Patent 4,277,344, 1981. [11] I.J. Roh, S.Y. Park, J.J. Kim and C.K. Kim, J. Polym. Sci.: Part B, 36(11) (1988) 1821. [12] I.J. Roh, J.J. Kim and S.Y. Park, J. Membr. Sci., 197 (2002) 199. [13] A.R. Greenberg, V.P. Khare and W.B. Krantz, Mat. Res. Soc. Symp. Proc., Material Research Society, 356 (1995) 541. [14] V.P. Khare, A.R. Greenberg and W.B. Krantz, J. Appl. Polym. Sci., 90 (2003) 2618–2628. [15] S. Wu, Polymer Interface and Adhesion, Marcel Dekker, Inc., New York and Basel, 1982. [16] J.-Y. Koo, R.J. Petersen and J.E. Cadotte, Polymer Prepr., 27(2) (1986) 391.
Synthesis and characterization of interfacially polymerized polyamide thin films Il Juhn Roha*, Alan R. Greenbergb, Vivek P. Khareb a
Hydranautics, 401 Jones Road, Oceanside, CA 92054, USA Tel. +1 (760) 901-2593; Fax +1 (760) 901-2664; email: [email protected] b Department of Mechanical Engineering, NSF Membrane Applied Science and Technology Center, University of Colorado, Boulder, CO 80309-0427, USA
Received 15 March 2005; accepted 7 June 2005
Abstract A homologous series of thin film composite (TFC) membranes was made by interfacial polymerization (IP) of trimesoyl chloride (TMC) and m-phenylenediamine (MPD). Membrane flux and rejection as well as the chemical and mechanical properties of the unsupported IP film were evaluated as a function of monomer concentration. The surface characteristics of the IP films were determined using electron-spectroscopy-for-chemical-analysis (ESCA) and contact angle measurements, and the mechanical properties were obtained using pendant drop mechanical analysis (PDMA) and surface profilometry. Results indicate that the TMC concentration has a more pronounced effect on the IP film properties than the MPD concentration. Higher TMC concentration leads to a pronounced increase in the IP film thickness and surface hydrophilicity. In contrast, the thickness increase is smaller and surface hydrophilicity decreases when the MPD concentration is increased. Water flux was determined to depend mainly upon the IP film thickness and surface hydrophilicity whereby these two parameters have counterbalancing effects. Consequently, the decrease in water flux with increasing MPD concentration is pronounced, but a corresponding increase in the TMC concentration has a smaller effect. These studies provide an improved basis for understanding the factors that govern TFC membrane performance. Keywords: Thin film composite (TFC) membrane; Properties of polyamide barrier layer; Interfacial polymerization; Diffusion; Structure–property relationships
*Corresponding author.
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.desal.2006.03.004
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1. Introduction Thin film composite (TFC) membranes are extensively used in reverse osmosis (RO) desalination applications. These membranes have a thin and dense active layer that controls membrane performance (permeability and selectivity), and a much thicker porous substrate that provides mechanical support to the active layer. In order to achieve high values of permeability and selectivity, the active layer should be ultrathin and hydrophilic. A variety of polymers and formation schemes have been employed for this purpose [1,2]. Overall, polyamide structures fabricated via interfacial polymerization (IP) have provided the most successful RO membranes [3–5]. The ultrathin polyamide films synthesized via IP can exhibit a range of physicochemical properties based on the polymerization conditions including the monomer concentration and the reactant ratio [6,7]. These properties play a pivotal role in membrane performance. The “solution–diffusion” mechanism proposed by Lonsdale and coworkers [3] is generally accepted to govern species transport through the active layer. According to this mechanism, the solvent and the solute species dissolve in the nonporous active layer and are subsequently transported via diffusion in an uncoupled manner. The water flux (Jw) is given by: Jw =
DwCwVw ( ∆P − ∆Π ) Rg Td
(1)
where Dw and Cw, are the water diffusion coefficient and concentration in the membrane, respectively, Vw is the partial molar volume of water in the external phase, ∆P = Pf – Pp and ∆Π = Πf –Πp are the applied pressure difference and the osmotic pressure difference, respectively, across the membrane, Rg is the gas constant, T is the absolute temperature, and d is the active layer thickness. The f and p subscripts refer to the feed and the permeate sides, respectively.
The salt flux (Js) through the membrane for the case of a large concentration difference is given by:
Js =
Ds K s ∆C d
(2)
where Ds is the diffusion coefficient of salt in the membrane, Ks is the partition coefficient of salt between the membrane and the solution, and ∆Cs = Cs, f – Cs, p is the concentration difference of salt at the two sides of the membrane. Using Eqs. (1) and (2) and the relationship Cs, p = Js/Jw, the salt rejection (R) can be obtained as:
1 ⎪⎧ D K Rg T Cs , f − Cs , p ⎪⎫ ⋅ ⋅ R =1− ⎨ s s ⋅ ⎬ ∆P − ∆Π ⎭⎪ (3) Cs , f ⎩⎪ DwCw Vw Eqs. (1)–(3) indicate that the fluxes and rejection depend upon Dw, Cw, Ds and Ks, which are intrinsic properties of the active layer material. In addition, whereas the fluxes are inversely proportional to the active layer thickness, d, the rejection is independent of d. For the current studies, polyamide TFC membranes were fabricated using trimesoyl chloride (TMC) and m-phenylenediamine (MPD) on a polysulfone support via IP [9,10]. As indicated in Fig. 1, the polyamide contains a crosslinked portion (X) and a more hydrophilic linear moiety (Y) containing free carboxylic acid groups. The number of free carboxylic acid groups as well as the thickness and mechanical properties of the thin film are governed by the monomer concentrations and the concentration ratio. Although many studies of IP TFC membranes have been reported in the literature, relatively few comprehensive investigations have incorporated specific chemical, physical and mechanical properties of the active layer. Consequently, the objective of this study was to evaluate the effect of the active layer surface properties on membrane performance. Our
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NH
NH CO
HN
NH
CO
COOH
CO
NH X
NH
CO
CO Y
Fig. 1. The polyamide structure formed from an IP reaction between MPD and TMC has a crosslinked portion (Xfraction) and linear moiety (Y-fraction), which has an unreacted acid chloride group that subsequently hydrolyzes to a carboxylic acid group.
approach provides a quantitative description of the relationships among the composition, properties, and performance of the polyamide thin film, and yields useful insights regarding the means by which membrane performance can be optimized.
2. Materials and methods TMC (Aldrich, 98%) and MPD (Aldrich, 99%) were distilled under vacuum at 160°C and 170°C, respectively. The purified TMC was stored in a vacuum desiccator containing calcium chloride. The purified MPD was stored in a refrigerator in a dark bottle. The TMC solvent, n-hexane, was dried via exposure to magnesium sulfate and then distilled. The purified n-hexane was stored in a tightly closed bottle with a molecular sieve to absorb any water. RO purified and deionized water was used whenever required. Two series of polyamide thin films were prepared. For series I, the TMC concentration was varied in the range 0.01–1.0 wt/v% (weight per volume) while that of MPD was held constant at 0.5 wt/v%. For series II, the TMC concentration
was fixed at 0.1 wt/v%, and the MPD concentration was systematically varied in the range 0.01– 1.0 wt/v%. The specific compositions and resulting monomer ratios are summarized in Table 1. The TFC membranes were prepared via a twostep process. First, an asymmetric support layer was prepared using standard techniques [11], and then the active layer was formed on its surface. The active layer was prepared via in-situ IP using an unstirred non-dispersion method. The support layer was immersed in a de-ionized water solution containing MPD for 2 h. Excess reagent was squeezed off the surface of the support layer using a soft rubber roller. The support layer, impregnated with the MPD solution, was then reacted with a hexane solution containing TMC by placing the hexane solution on the surface of the support layer for 2 min. After this two-minute IP reaction, the hexane solution was drained off and the polymerized substrate was washed with pure hexane before drying at room temperature. Additional details of the IP methodology as well as the measurement protocols used to characterize RO membrane performance are described in a previous article [12]. For the subject experiments, an RO
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Table 1 Composition and reactant ratios used to synthesize IP films
Series I
Series II
MPD (wt/v%)
TMC (wt/v%)
Reactant Ratio
MPD (wt/v%)
TMC (wt/v%)
Reactant Ratio
0.5 0.5 0.5 0.5 0.5
0.01 0.05 0.1 0.5 1.0
50 10 5 1 0.5
0.01 0.05 0.1 0.5 1.0
0.1 0.1 0.1 0.1 0.1
0.1 0.5 1 5 10
system was operated at 3.03 MPa (440 psi) and used a feed concentration of 2,000 ppm NaCl. Previous work has indicated that the mechanical strength of the active layer is closely associated with TFC performance [12]. Pendant Drop Mechanical Analysis (PDMA) [12–14] was utilized for in-situ mechanical strength measurements of the IP active layer. PDMA is ideally suited to study unsupported thin films so that the measurements are not influenced by the properties of the support layer. The details of the PDMA protocol have been reported previously [12]. The film thickness, a parameter necessary to obtain absolute strength using PDMA, was measured by first preparing the IP films separately on a glass slide. The thin films were washed, then semi-dried in air at room temperature and finally scratched using a sharp knife. The depth of the scratched “valley” was measured using a Tencor P-10 surface profilometer with a 3.0 mg applied force and a scan speed of 0.1 mm/s. The IP film hydrophilicity was estimated through contact angle measurements using an image analyzer system (PAS PX-380) wherein the shape of liquid drops (volume ~3 µL) placed on the IP film surface was recorded. Contact angles (θ) on both sides of the drop were measured after these images had been magnified, and the mean value from six replicates was utilized. The surface tension due to the dispersion and polar forces characteristic of the polymer was obtained from
the following equation [15]: ⎛ γd γd 4⎜ d s L d γ + γL cos θ = ⎝ s γL
⎞ ⎛ γ sP γ LP ⎟ 4⎜ P P ⎠ + ⎝ γs + γL γL
⎞ ⎟ ⎠ −1
(4)
Here, γL, γLd and γLP are the total surface tension, dispersion and polar force components of surface tension for a standard liquid, respectively, and γSd and γSP are the dispersion and polar force components of surface tension for the polymer. The surface tension properties of the reference liquids used in Eq. (4) are shown in Table 2. The surface chemical composition of the IP films was obtained via electron-spectroscopy-for-chemical-analysis (ESCA) (Surface Science Instrument 2803-S), and the concentration of dissolved water in the IP films was obtained using mass-loss measurements. The specific procedures utilized are described in a previous paper [12].
Table 2 Dispersion and polar components of the surface tension of reference liquids Liquids
γL (mN/m)
γdL (mN/m)
γpL (mN/m)
Water Glycerol Formamide
72.8 63.4 58.2
29.1 37.4 35.1
3.7 26.0 23.1
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3. Results and discussion We first determined the polymerization conditions that would result in IP films possessing properties consistent with those of a bulk polymer. Absolute strength measurements were utilized for this purpose. As shown in Fig. 2, the IP films synthesized from low monomer concentrations (<0.01 wt/v%) did not possess adequate strength. Hence, only the IP films synthesized using monomer concentrations greater than 0.01 wt/v% were utilized in subsequent studies. 3.1. Effect of composition on TFC membrane performance The experimental design enabled the effect of systematic changes in the monomer concentration and consequently the reactant ratio on membrane performance to be evaluated. These results are shown in Fig. 3 for series I and II. Overall, the water flux and the salt rejection were more sensitive to changes in the MPD concentration (series II) than the TMC concentration (series I). For the series I membranes, the water flux
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evidenced a decrease of 3% over TMC concentrations from 0.01 to 0.1 wt/v%, but remained almost constant at higher TMC concentrations. Likewise, the salt rejection increased slightly (≈6%) as the TMC concentration increased from 0.01 to 0.1 wt/v%, but remained essentially unchanged at higher TMC concentrations. The behavior of the series II membranes was distinctly different than that of the series I membranes (Fig. 3). The water flux decreased by 24% as the MPD concentration was increased from 0.01 to 0.1 wt/v%. The flux decrease was more modest (≈14%) at higher MPD concentrations, and the flux did not reach an asymptotic value at the maximum MPD concentration tested (1.0 wt/v%). The salt rejections were sensitive at low MPD concentration, increasing by 23% as the MPD concentration increased from 0.01 to 0.1 wt/v%, but reached a constant value for greater MPD concentrations. Overall, these results suggest that the permeability and rejection depend upon the monomer concentrations as well as the concentration ratio. A more complete understanding regarding the
Fig. 2. Absolute strength of the thin film active layers as a function of the MPD and TFC concentrations. For the series I films, the MPD concentration is fixed at 0.5 wt/v%, and for the series II films the TMC concentration is fixed at 0.1 wt/v%.
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Fig. 3. TFC membrane performance as a function of the MPD and TMC concentration. For the series I films the MPD concentration is fixed at 0.5 wt/v%, and for the series II films the TMC concentration is fixed at 0.1 wt/v%.
nature of these effects requires knowledge of the degree to which the intrinsic properties of the thin films are affected by compositional changes. Consequently, the relationships among membrane performance (diffusion of water and salt) and polyamide thin film properties are considered in the next two sections. 3.2. Effect of TMC concentration on thin film properties Fig. 4 shows the relationship between the relative strength of the series I IP films and the water flux and rejection properties of the membrane. The results indicate that the relative strength, which is a function of the stress as well as the polyamide film thickness, is a sensitive function of the TMC concentration. As the TMC concentration is increased from 0.01 to 0.1 wt/v%, the relative strength and the salt rejection (R) increase by 51% and 6%, respectively, whereas the water flux (Jw) decreases by about 3%. Over the concentration range 0.1–1 wt/v% TMC, the relative strength increases dramatically (≈230%) while Jw and R evidence significantly more modest changes of –1% and +2%, respectively. Previous work has
indicated that the water flux and salt rejection do not demonstrate a dependence on the relative strength [12] if the relative strength exceeds a critical value, and the present results are in general agreement with these findings. A similar conclusion may appear warranted as well for the salt rejection, which after an initial small increase, remained relatively constant even though the relative strength continued to increase. On the other hand, the film prepared with 0.01 wt/v% TMC had a relative strength below 5 kPa·mm [12] (the critical value affecting salt rejection) and a corresponding value of salt rejection that was lower than the asymptotic value. This result may imply that the low salt rejection of this membrane is due to a weak active layer that is more prone to local defects. The thickness change of the active layer as a function of TMC concentration is shown in Fig. 5. The thickness increased linearly with TMC concentration over the range from 0.01 to 1.0 wt/v%, with a total increase in thickness of approximately 360%. As previously noted, the water flux decreased modestly over this same concentration range (≈4%). Although a decrease in water flux with increasing film thickness is expected
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Fig. 4. The dependence of the active strength, water flux (Jw) and salt rejection (R) of the series I IP films on the TMC concentration.
Fig. 5. The dependence of the active-layer thickness (d) and the water flux (Jw) of the series I IP films on the TMC concentration.
[Eq. (1)], the magnitude of the former is much smaller than that of the latter. The differences in the response of d and Jw to changes in the TMC concentration thus suggest that the water flux is affected not only by the thickness but also by some other property of the IP polymer film. In order to identify these additional factors, three chemical properties were studied: the water content (Cw), the polar component of surface
tension (γps ) and the Y-fraction (Fig. 6). The polar component of surface tension was used as a measure of the hydrophilicity of the IP film. The chemical composition of the surface of the active layer was analyzed via ESCA. The ESCA spectra provided the C1S, N1S, and O1S core levels and the ratios of the surface composition (%). From the chemical formula of the repeat unit (Fig. 1) and the surface composition (ESCA), the X and Y
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Fig. 6. The dependence of the water permeability coefficient (Cw), Y-fraction and the polar force components of the surface tension (γps ) of the series I IP films on the TMC concentration.
fractions were calculated [16]. The dependence of these chemical properties on the TMC concentration is shown in Fig. 6. Cw, Y-fraction and γps all increased with increasing TMC concentration. The behavior of the Y-fraction implies that the active layer polymerized at low TMC concentration is less hydrophilic than that made at high TMC concentration. This is confirmed by the response of γps , which attains the lowest values at low TMC concentration. Since the water solubility depends upon the hydrophilicity as well as the Y-fraction, the water solubility also increased with increasing TMC concentration. Now, the chemical nature of the active layer should reflect the characteristics of the IP process wherein the condensation reaction of amine and acid chloride occurs on the organic side of the aqueous–organic interface [6,7]. Hence, the reaction should mainly be controlled by the diffusion of MPD through the interfacially polymerized film and the diffusion of TMC in the organic phase. For the series I films, the concentration of MPD was held constant, as the TMC concentration was varied from 0.01 to 1.0 wt/v%. At lower concentrations (< 0.1 wt/v% TMC), the rate-controlling step of the IP process is presumably the diffusion of TMC in the organic phase since the concentra-
tion of MPD at the organic reaction site is relatively high. When the polymerization is controlled by the diffusion of TMC, the TMC concentration should play a major role in determining active layer properties. As expected, the thickness of thin film increases linearly as TMC concentration increases (Fig. 5). The probability of having an unreacted acid chloride group is decreased as the TMC concentration is decreased due to the relatively high MPD concentration. Any unreacted acid chloride groups could easily be converted to the corresponding carboxylic acids through hydration. This could explain the increase in Cw, Y-fraction and γps with TMC over this lower concentration range (Fig. 6). At higher TMC concentrations (> 0.1 wt/v%), the respective magnitudes of these three parameters are even more sensitive to concentration, whereby the maximum rates of increase are observed at the highest TMC concentration. Because of the fixed MPDA concentration (0.5 wt/v%), increasing the TMC concentration may result in a deficiency in the available amine groups at the organic reaction side of the interface. Such a deficiency would lead to an increasing number of unreacted acyl chloride groups, and therefore to an increased Y-fraction containing free carboxylic acid groups at these higher TMC
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concentrations (Fig. 6). The increase in Y-fraction could account for the observed behavior of Cw and γps , i.e. an increase in the slope with increasing TMC concentration. As previously noted, the water flux decreases by only ≈3% over the range of TMC concentrations considered (Fig. 3). This decrease corresponds to an increase in IP film thickness of ≈360% over the same composition range (Fig. 5). Data from Fig. 6 indicate that Cw increases by approximately 300% over this same composition range. Given the inverse relationship between the water solubility and film thickness [Eq. (1)], the relatively constant water flux is most likely due to the counterbalancing trends in Cw and d. These results confirm that the overall performance of TFC membranes depends upon both the intrinsic and extrinsic IP film characteristics. 3.3. Effect of amine concentration on thin film properties Fig. 7 shows the dependence of the IP film relative strength and the TFC membrane flux and rejection on the MPD concentration. Of particular interest is the dramatic decrease in the water flux and corresponding increase in the relative strength
287
as the MPD concentration increases from 0.01 to 0.1 wt/v%. This behavior confirms the results of our previous study [12]. The present results also demonstrate that rejection increases over this same concentration range; however, for MPD concentrations >0.1 wt/v% the rejection remains essentially constant whereas the relative strength increases only modestly. Overall, these data indicate that the transport and mechanical properties are closely related and suggest that the unexpectedly high water flux at low MPD concentrations (<0.1 wt/v%) may be due to the low relative strength of the IP film. When the relative strength values of Fig. 7 are taken into account, the value indicates that for MPD concentrations less than 0.1 wt/v% the films have low mechanical strength. Although the salt rejection is an intrinsic property of the active layer material and should not depend upon strength, the rejection also evidences decreased values for such films, consistent with either a “loose” polymer structure or the presence of defects at low MPD concentrations. The active layer thickness for series II films increased with MPD concentration (Fig. 8). Whereas the film thickness increases by ≈27% as the MPD concentration increases from 0.01 to 0.1 wt/v%, further increases in the MPD concen-
Fig. 7. The dependence of the relative strength, water flux (Jw), and salt rejection (R) of the series II IP thin films on the MPD concentration.
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Fig. 8. The dependence of the active-layer thickness (d) and the water flux (Jw) of the series II IP films on the MDP concentration.
Fig. 9. The dependence of the water permeability coefficient (Cw), Y-fraction and the polar force components of the surface tension (γsp) of the series II IP films on the MPD concentration.
tration (from 0.1 to 1.0 wt/v%) produce an additional increase in the film thickness that is more modest ≈7%). Correspondingly, the water flux decreases by ≈24% as the MPD concentration increases from 0.01 to 0.1 wt/v% and by an additional 14% as the MPD concentration reaches 1.0 wt/v%. Given that the membrane flux is inversely proportional to the film thickness [Eq. (1)], the difference in the magnitude of the response of
d and Jw to changes in the MPD concentration implies that the water flux was also affected by other factors. Fig. 9 shows the Y-fraction, γps and Cw, as a function of MPD concentration. In contrast with the effect of increasing TMC concentration, the Y-fraction is observed to decrease with increasing MPD concentration. This dependence suggests that an active layer polymerized at low MPD
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concentration would be more hydrophilic than that made at high MPD concentration. Indeed, this conjecture is supported by the behavior of γps , which also decreases with increasing MPD concentration. In addition, the concentration of water dissolved in the active layer, Cw, depends upon the hydrophilicity. Overall, this relationship is evidenced in Fig. 9 at low MPD concentrations (< 0.1 wt/v%) where the water solubility evidences a monotonic decrease with increasing MPD concentration in a manner similar to that observed for γ sp and the Y-fraction. At higher MPD concentrations, Cw, γps and the Y-fraction continue to decrease but at a rate lower than that observed at lower concentrations. For series II films with MPD concentrations <0.1 wt/v%, the rate-controlling step is most likely the diffusion of MPD through the interfacially polymerized film. If this is indeed the case, the diffusion rate of MPD should play a major role in determining thin film properties. As expected, the thickness of the thin film increases significantly with MPD concentration over this range (Fig. 8). In addition, within this composition range, the number of available acid chloride groups (from TMC) is larger than the number of amine groups (from MPD). This mechanism could account for the fact that at low MPD concentrations, the Yfraction having free carboxylic acids is greater (Fig. 9). For MPD concentrations >0.1 wt/v%, the ratecontrolling step could well be the diffusion of TMC in the organic phase. If the TMC concentration at the interface were deficient with respect to the MPD concentration, film thickness would remain constant despite an increasing MPD concentration. Such behavior is apparent in Fig. 8. In addition, the deficiency of TMC monomer at the IP reaction site reduces the probability of having unreacted acid chloride groups, which in turn reduces the Y-fraction having free carboxylic acid groups at high MPD concentrations (Fig. 9). As previously noted, the water flux decreases by approximately 34% over the range of MPD
289
concentrations considered (Fig. 3). This decrease corresponds to an increase in IP film thickness of approximately 37% over the same composition range (Fig. 8). In addition, data from Fig. 9 show that the water solubility decreases by approximately 25% as the MPD concentration is increased from 0.01 to 1 wt/v%. Consequently, it would appear as though the observed dramatic decrease in water flux can be explained by the observed increase in film thickness as well as decrease in water solubility, in good agreement with the relationships expressed in Eq. (1). These results indicate that although the overall effect of any changes in the intrinsic film characteristics as a function of MPD concentration is small, the change in permeation performance is surprisingly large.
4. Conclusions These experiments demonstrate that variation in the acid chloride concentration has a more pronounced effect on the thin film material properties than corresponding variations in the amine concentration. Increasing the TMC concentration from 0.1 to 1.0 wt/v% led to a marked increase in the IP film thickness as well as surface hydrophilicity. In contrast, the thickness and surface hydrophilicity changes were relatively small when the MPD concentration was varied from 0.1 to 1.0 wt/v%. However, because the water flux mainly depends upon the IP film thickness and surface hydrophilicity (through Cw) and these two parameters have counterbalancing effects, the decrease in water flux with increasing MPD concentration was pronounced but a corresponding increase in the TMC concentration had a relatively smaller effect. Overall, these experiments provide an improved basis for understanding the relationship between the specific polymer properties of the active thin layer and IP TFC membrane performance.
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Acknowledgment The authors gratefully acknowledge the support of the original PDMA studies by the Membrane Applied Science and Technology Center at the University of Colorado at Boulder.
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