Preparation and characterization of polyamide-urethane thin-film composite membranes

Preparation and characterization of polyamide-urethane thin-film composite membranes

DESALINATION ELSEVIER Desalination 180 (2005) 189-196 www.elsevier.com/locate/desal Preparation and characterization of polyamide-urethane thin-fi...

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DESALINATION

ELSEVIER

Desalination 180 (2005) 189-196

www.elsevier.com/locate/desal

Preparation and characterization of polyamide-urethane thin-film composite membranes Yong Zhou a, Sanchuan YU b, Meihong Liuc, Congjie

G a o b* aCollege of Material Science and Chemical Engineering, Zhejiang University, Hangzhou 310027, China bTheDevelopment Center of Water Treatment Technology, SOA, Hangzhou 310012, China Tel. +86 (57I) 8893-5329; email: [email protected] CZhejiang University of Sciences, Hangzhou 310028 China Received 15 September 2004; accepted 16 December 2004

Abstract We attempted to produce reverse osmosis membranes with 5-chloroformloxy-isophthaloyl chloride (CFIC) and m-phenylenediamine (MPD) with an interracial polymerization technique on the polysulphone supporting film. The membranes produced were characterized using permeation experiments with salt water. Attenuated total reflectance infrared (ATR-IR), X-ray photoelectronic spectroscopy (XPS), as well as imaging using scanning electronic microscopy (SEM) were used. The results show that the active layer ofa CFIC/MPD TFC membrane is polyamideurethane, including an amide functional group (-CONH-), urethane functional group (-OCONH-) and hydroxyl functional group (-OH). The flux and rejection of a CFIC/MPD TFC membrane are better than a TMC/MPD TFC membrane in the same condition (concentrations ofCFIC = 0.15 wt% and contact time with the organic solution is 20 s, which is the best preparation parameter) and the surface of the CFIC/MPD TFC membrane is a dense, finely dispersed grainy structure.

Keywords:

Polyamide-urethane; Thin-film composite membrane; 5-chloroformloxy-isophthaloyl chloride; m-phenylenediamine

1. Introduction Reverse osmosis (RO) membrane separation plays an important role in commercial water treatment [1-3] and water pollution control [4,5]. The current worldwide expansion and diverse appli*Corresponding author.

cation of RO technology has resulted from the introduction in 1972 of thin-film-composite (TFC) membranes by interfacial polycondensation [6]. Most o f thin skin layers in commercial membranes are aromatic polyamides or their derivatives [7]. Two essential membrane parameters in the RO process are solubility and diffusivity of solutes

0011-9164/05/$- See front matter © 2005 Elsevier B.V. All rights reserved doi: 10.1016/j.desal.2004.12.037

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and solvents to the thin skin layer polymers [8,9] that determine the membrane rejection and flux. The interrelationships between the RO membrane rejection and the polymer/solute/solvent interactions, in conjunction with the chemical structure of the thin skin layer polymers, have been thoroughly studied [10,11]. Hirose et al. [12] studied the relationship between the surface structure of skin layers of crosslinked aromatic polyamide RO membranes and their RO performance. Kwak [ 13] characterized RO permeability in conjunction with macromolecular structures and inherent polymer properties for crosslinked and linear model aromatic polyamides by crosspolarization/magic angle spinning (CP/MAS). Juhn et al. [14] proposed that the mechanical strength of the barrier layer should be an important factor determining its performance. Permeation experiments were then performed to correlate the mechanical strength to the permeation performance of the composite membranes. The experiments indicate that the permeation behavior of composite membranes with a high rupture strength barrier layer agreed well with the solution-diffusion transport mechanism. We synthesized 5-chloroformloxy-iso-phthaloyl chloride (CFIC) with triphosgene as a substitute for phosgene, which is used in some patents [15], to produce RO membranes with CFIC and m-phenylenediaminethrough an interfacial polymerization technique on the polysulphone supporting film. The chemical structure of the active layer, membrane morphology including top surface and cross-section, and RO performance were elucidated.

BTC (59.3 g, 0.2 mol) in anhydrous tetrahydrofuran (130 mL) and solid 5-hydrooxyisophtalic acid (21 g, 0.1 mol) cooled at 2°C with an ice bath. During the addition, the temperature was maintained at 2-10°C. After the addition was complete the reaction mixture was stirred at room temperature until it became homogenous. Then the reaction mixture was transferred into a funnel to evaporate the solvent to afford a slightly yellow thick oil (28.5 g).The product (7.1 g) was obtained by distillation in vacuum at 145°C and 1 mm Hg pressure. IR spectra were recorded on a Nicolet IR200 spectrometer with samples prepared by KBr pellets. The product is 5-chloroformloxyisophthaloyl chloride with a characteristic of C=O bands of the acyl chloride band at 1762 cm -~ and a chloroformloxy band at 1788 cm- ~,as well as 1610 cm(aromatic ring breathing).

2.2. Preparation of TFC membrane To fabricate the TFC membrane, a support substance composed of microporous polysulphone was first prepared by the following procedures. A solution of 16.5% (by weight) with a Udel P-3500PS (Union Carbide, USA), 0.3% water, 0.3% SAA in n-methyl-2-pyrrolidone (Sigma-Aldrich Chemical) was cast onto a glass plate with a polyester nonwoven fabric using a 0.45 mm knife gape. The plate was immediately

2. Experimental 2.1. Synthesis and characterization of 5-chloroformloxyisophthaloyl chloride A solution ofTEA/imidazole (mol ratio is 5:1, 1.5 g) in anhydrous tetrahydrofuran (20 mL) was added slowly to a magnetically stirred solution of

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2000

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1500

Wavenumbers(cm"~) Fig. 1 IR spectrogram of CFIC.

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immersed in a water bath at room temperature in one smooth motion. Within 30 s the PS gelled into a white microporous sheet; the top face (in contact with air) was used as a support surface for the TFC membranes. Then a layer of aqueous solution was formed on a microporous polysulphone supporting film by putting into contact the solution with the supporting film for 120 s and removing any extra solution. The solution contained 2.0% (w/v) of m-phenylenediamine, 3.0% of triethylamine, and 4.0% of camphor sulfonic acid. The TFC membrane was obtained by making contact with IP1016 solution (isoparaphin-type hydrocarbon oil, Idemitsu Chemical) containing 5-chloroformloxyisophthaloyl chloride or TMC with the surface area of the supporting film that was previously coated with the aqueous solution, and holding the film at 110°C in a hot air dryer for 10 min so that a skin layer was formed on the supporting film. The composite RO membrane was washed in water and dried. All membrane samples were prepared and tested in at least duplicate with a total of six membrane tests for RO performance, results of which have been averaged (Tables 1 and 2). The membrane tests for the chemical structure and morphology analysis of skin layer were conducted at 0.5 MPa using pure water (<2 N/cm) for 4 h in order to remove the MPD in the membranes (Fig. 2).

flux was determined by direct measurement of the permeate flow:

CFIC concentration (% w/v)

Contact time with organic solution(s)

Flux Salt (L/m2.h) rejection (%)

2.3. RO performance of TFC membranes

0.05

20 60 20 20 20 20 60 60 20 20 20

40 36 25 20 22 19 15 17 12 15 14

All tests for RO performance were conducted at 1-3 MPa using 500-8000 mg/L NaC1 solution (pH 6.5) at 25°C in cross-flow cells. The water COCI

NH2

Flux (L/m2.h) = permeate (L) / membrane area (m2).time (h) The salt rejection rate was measured by the salt concentration in the permeate obtained through

Table 1 Parameters for the TFC membranes based on the reaction of m-PDA with TMC (aqueous phase: 2.0% MPD; organic phase: 0.15% (w/v) of TMC dissolved in IP1016 solution TMC concentration % (w/v)

Contact time with organic solution(s)

Flux Salt (L/m2.h) rejection (%)

0.15 0.15 0.15

20 20 20

13.5 19.3 16.8

Table 2 Parameters for the TFC membranes based on the reaction of m-PDA with TMC (aqueous phase: 2.0% MPD; organic phase: 0.05-0.15%(w/v) of CFIC dissolved in IP1016 solution

0.10 0.15

oCOCI

0.15 0.20 TMC

MPD

Fig. 2. Structure of the monomers used.

tCIC

94.5 98.1 97.0

50.2 47.2 97.5 96.0 99.3 98.0 98.1 97.1 97.5 96.8 98.0

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measurements of the electrical conductance of the permeate and the feed using a conductance meter (Cany Precision Instruments, China): Rejection (%) = (1-permeate conductance/ feed conductance)

2.4. Characterization of TFC membrane Scanning electron microscopy (SEM) was performed with a JSM-5610LV instrument. Cross-sectional viewing was done by first taking a thin strip of membrane (1 "× 1.8"), holding the ends to form a hoop, immersing the strip in liquid nitrogen, then removing it and flattening the hoop with forceps. Magnifications up to 20,000 were obtained. ATR-IR characterization of the TFC membrane surface was made with a Model 300 Barnes continuously variable ATR accessory, using a Nicolet 560 Fourier transform infrared spectrometer. Surface chemical characterization was carried out by X-ray photoelectron spectra (XPS), with a Perkin Elmer PHI 5000C ESCA system with a Mg/A1 dual-anode Hel/Hell ultraviolet source (400 W, 15 kV, 1253.6 eV). The spectra were taken with the electron emission angle at 54 ° to give a sampling depth of 10 nm by a concentric hemispherical energy electron analyzer operating in the constant pass energy mode of 29.35 eV, using a 720-#m diameter analysis area. Membranes were mounted on a sample holder without adhesive tape and kept overnight at high vacuum in the preparation chamber before they were transferred to the analysis chamber of the spectrometer for their analysis. Each spectral region was scanned with several sweeps until a good signal-to-noise ratio was observed. For each membrane XPS analysis was carried out three times using different samples. Membranes were irradiated separately and for a maximum of 20 min to minimize X-ray-induced sample damage.

3. Results and discussion

3.1. Functional group composition of the nearsurface region The CFIC-MPD in situ reaction on a PS support membrane, whose spectrum is given in Fig. 3--the resulting polymeric product, which has formed the barrier layer--is polyamideurethane. The spectrum of PS-polyamide-urethane is displayed in Fig. 4. The skin layer is so thin (less than 0.5 #m) that the underlying PS also contributes to the spectrum, so a difference spectrum (Fig. 5) of resulting the membrane is obtained, which indicates that the interracial polymerization has occurred since the acyl chloride band at 1762 cm-~ and chloroformloxy band at 1788 cm-1 are absent and two bands at 1653 cm-1 (amide I), 1728 cm-~ are present, which are characteristic of C=O bands of an amide group and a urethane group. In addition to this, other bands characteristic of polyamide-urethane are also seen at 1541 cm -] (amide II, C-N stretch) and 1610 cm -~ (aromatic ring breathing). 3.2. XPS result The composite membranes consisted of a thin layer of polyamide (about 0.2 #m) deposited on

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1000

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Fig. 3. ATR-IR spectrumof a porous PS membrane.

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Table 3 Surface composition by XPS analysis of the membrane barrier layer

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71.35 17.98 10.67 0.25 0.15 1.7

73.86 16.09 10.05 0.22 0.14 1.6

Wavenumbers(cm "1) Fig. 4. A T R - I R spectrum o f a polyamide-urethane T F C

membrane.

Table 4 Elemental compositions of polymer model structures Structure

O ¢-

.=_ E

Totally crosslinked

71.4 19.0 9.5 0.27 0.13 2.0

75.0 12.5 12.5 0.17 0.17 1.0

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72.0 16.0 12.0 0.22 0.17 1.33

Linear with pendant OH

TMC/MPI)

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Fig. 5. ATR-IR difference spectrum of a polyamideurethane TFC membrane

polysulfone porous support. Thus, the chemical information obtained through XPS analysis is derived only from the near surface o f the top layer, and the polysulfone layer is not probed; the main chemical elements studied by XPS were C, N, and O. Table 3 shows the atomic concentration percentages of the two membranes. There is a significant excess o f oxygen for the TMC/MPD TFC membrane with respect to the corresponding theoretical ratio for amide (O/N=I), which indicates a high content o f - C O O H on the surface of

C% 0% N% O/C N/C O/N

m n m

CFIC/MPD

C% 0% N% O/C N/C O/N

73.7 15.8 10.5 0.21 0.14 1.50

this membrane which come from the hydrolysis of the acyl chloride. The XPS result for the CFIC/ MPD TFC membrane is near to the elemental compositions o f the linear polymer structure with pendant OH given in Table 4. The near-surface region of the CFIC/MPD membrane contains urethane (-NHCOO-) functional groups, amide (-CONH-) groups and aromatic groups according to the A T R - I R result.

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194

....... ,:. 1

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• Z:-COOH:

Fig. 6. Chemical structures of aromatic polyamide-urethane.

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Fig. 8. Effect of operating pressure on RO rejection and flux (2000 mg/L NaC1, 25°C).

The chemical structures of the CFIC/MPD membrane skin layer are shown in Fig. 6 by considering the hydrolysis of the acyl chloride and chloroformloxy.

The best preparation parameters for the CF1C/ MPD composite membrane are: concentrations of CFIC = 0.15 wt% and contact time with organic solution = 20 s (Table 2). The TMC/MPD membrane was prepared by interfacial polymerization of MPD with TMC under the same conditions as for the production of the CFIC/MPD membranes (Table 1). Fig. 7 compares the RO rejection and flux characteristics between TMC/MPD and CFIC/ MPD membranes at 1.5 MPa using 2000 mg/L NaC1 solution (pH 6.5) at 25°C in cross-flow cells. It should, however, be noted that the value of the RO characteristics of our CFIC/MPD membrane cannot directly be compared to that value because the preparation parameters and test conditions were not the same. The rejection and

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Fig. 10. SEM imagesofa TFC membranemade fromCFIC/MPD,membrane(x20,000). (a) Top surface;(b) cross-section.

flux of the CFIC/MPD membrane are higher than the TMC/MPD membrane. Furthermore, the effect of the operating pressure and NaC1 concentration on RO rejection and flux are also considered. Fig. 8 shows that the flux and rejection of the CFIC/MPD TFC membrane increase with an increase of operating pressure; the relationship between the flux and the operating pressure is nearly linear. This agrees with the Kimura-Sourirajan analysis of RO membrane transport [ 16]. In Fig. 9, the flux decreased with increasing NaC1 concentration, while the rejection increased with increasing NaC1 concentration when the NaC1 concentration was less than 1000 mg/L and then kept steady when the NaC1 concentration was up to 8000 mg/L. 3.4. TFC membrane morphology

The skin layer structures of CFIC/MPD TFC membrane were observed by SEM. SEM photographs of the TFC membrane skin layer surface and cross-section are shown in Fig. 10. From the SEM photographs we can see that the surface of

the skin layer is dense but not smooth. The white part is the peak and the black part is the valley. The skin layer is very thin (about 0.2/zm), and the support membrane is sponge-structured, which can support high pressure.

4. Conclusions

The polyamide-urethane TFC RO membranes were successfully prepared with CFIC and MPD through an interracial polymerization technique on the polysulphone supporting film. The active layer of the CFIC/MPD TFC membrane is polyamide-urethane, including the amide functional group -CONH-, urethane functional group -OCONH- and the hydroxyl functional group -OH. The flux and rejection of the CFIC/MPD TFC membrane is better than the TMC/MPD TFC membrane under the same conditions (concentrations of CFIC = 0.15 wt% and contact time with organic solution of 20 s, which are the best preparation parameters); the surface of the CFIC/ MPD TFC membrane has a dense, finely dispersed, grainy structure.

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Acknowledgment This work was supported b y the National 863 project (No. 2002AA328010) and 973 project No. 2003CB615700 in China.

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