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polysulfone composite membrane
Desalination 191 (2006) 291–295
Investigations on the structures and performances of a polypiperazine amide/polysulfone composite membrane Yufeng Zhanga*, Changfa Xiaoa, Enhua Liua, Qiyun Dua, Xiang Wanga, Hongliang Yub a
Key Laboratory of Hollow Fiber Membrane Materials & Membrane Process, Tianjin Polytechnic University, Ministry of Education, Tianjin 300160, PRC Tel. +86 (22) 8127-4195; email: [email protected] b Nanotechnology Industrialization Base of China, Tianjin 300457, PRC Received 1 February 2005; accepted 9 August 2005
Abstract A polypiperazine amide (PA) composite membrane was prepared by interfacial polymerization using a trimesoyl hexane solution as the oil phase and a piperazine aqueous solution as the water phase on a polysulfone ultrafiltration substrate. The surface was inspected by attenuated total reflection–Fourier transform infrared spectroscopy, showing that a polyamide layer was formed. The molecular structure of polypiperazine amide was simulated using Materials Studio as a plane molecule with hexagon rings of 1.5 nm in diameter of the inscribed circle. Incorporating the result of wide angle X-ray diffraction pattern, the supermolecular structure of the PA was predicted as an irregular laminated structure. This model appears good according to the performance of the membrane. Also investigated was the morphology of the membrane by scanning electron microscope, showing a dense layer of 300 to 400 nm on the porous substrate. The composite membrane obtained is a high-performance nanofiltration membrane. Keywords: Interfacial polymerization; Polyamide; Composite membrane; Molecular simulation; FTIR; WAXD; SEM
1. Introduction Over the last three decades following the pioneering development by Cadotte and co*Corresponding author.
workers, thin-film composite (TFC) polyamide (PA) membranes have become the main type of membranes used for reverse osmosis and nanofiltration [1]. Due to the excellent performance and favorable economics, the TFC-based technology is taking the lead in water treatment,
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.2005.08.016
292
Y. Zhang et al. / Desalination 191 (2006) 291–295
particularly in desalination, and great attention has been drawn to it. There are many studies on the mechanism of mass transfer in the membrane separation process, and also a few investigations on the structure of the PA functional barrier layer in TFC [2–4]. It is known that there is a close relationship between the mechanism of mass transfer and the structure of the membrane. A PA composite membrane was prepared by interfacial polymerization using a trimesoyl hexane solution as the oil phase and a piperazine aqueous solution as the water phase on a polysulfone ultrafiltration substrate in this work. The surface was inspected by attenuated total reflection–Fourier transform infrared spectroscopy (ATR–FTIR) showing a PA layer formed. The molecular structure of PA was simulated using Materials Studio and the supermolecular structure was thus predicted as an irregular laminated structure based on the molecular structure and a wide-angle X-ray diffraction (WAXD) pattern. The morphology of the membrane was investigated by scanning electron microscope (SEM), showing a dense layer of 300 to 400 nm on the porous substrate. The composite membrane obtained showed a high performance for nanofiltration applications. 2. Experimental 2.1. Main chemical agents and materials PSf and poly(vinyl pyrrolidinone) (PVP, K-30) was purchased from Shanghai Chemical Reagent. N, N-dimethylacetamide (DMAc), PI, trimesic acid, thionyl chloride, and n-hexane were obtained from Tianjin Medicine. TMC was prepared from trimesic acid and thionyl chloride by reflux and distillation at reduced pressure. 2.2. Methods The PSf substrate membrane was prepared by the phase inversion. Its molecular weight cut-off
is 6000, and the water flux is about 400– 500 L/m2/h at a pressure of 0.6 MPa. The top active layer was obtained through interfacial polymerization between TMC in hexane and an aqueous phase containing PI. The substrate was first immersed into the PI aqueous phase for minutes, and then taken out to removed the aggregate aqueous solution on the surface of the substrate. Subsequently, it went through a TMC/n-hexane organic phase, and interfacial polymerization occurred at the W/O interface (i.e., the surface of the substrate), and a cured polyamide layer (i.e., the ultra-thin desalination functional layer) was thus formed on the surface of the substrate. Finally, it was heat-treated in hot air for a certain time. 2.2.1. Molecular simulation The molecular formula of poly(piperazine trimesoyl amide) was simulated using Materials Studio through optimization. 2.2.2. Measurements The ATR–FTIR spectra were recorded on a Bruker spectrometer using a ZnSe crystal (25 mm ×5 mm × 2 mm) at a nominal incident angle of 24E yielding about 12 internal reflections at the sample surface at 25EC. WAXD patterns of a PA film were obtained at an angle from 10 to 30E with Rigaku-Denki x-ray diffraction using nickelfiltered CuKα radiation. The structure of the membrane cross sections, after being gold-coated, was investigated with a Philips XL30ESME SEM. The permeate flux and salt rejection of the composite membrane were tested by 1‰ MgSO4, 1‰ Na2SO4, 1‰MgCl and 0.5‰ NaCl aqueous solution under the conditions of 22 ± 1EC and 0.6 MPa, and the concentrations of solutions were fixed by a MC226 (Mettler Toledo). The permeate flux (F) can be calculated by (1)
Y. Zhang et al. / Desalination 191 (2006) 291–295
where J is the volume of the permeate flux, A is the surface area of the membrane, and T is time. Salt rejection (R) is given by (2) where Cf and Cp are the concentrations of the feed and permeate, respectively. 3. Results and discussion Fig. 1 presents the ATR–FTIR spectra of the surface of the composite membranes made by interfacial polymerization from trimesoyl and
293
piperazine. It is clearly seen that the characteristic peak of 1614 cm!1 for amide I appears, which means a polyamide layer formed on the PSf substrate. Fig. 2 shows a computer-simulated model structure of polypiperazine trimesoyl amide. The model molecule consists of hexagon rings. The diameter of the inscribed circle of the hexagon ring is around 1.5 nm. This model formula is one with relatively low energy, and the one with the lowest energy is similar but with unclosed rings. Although the rings are open, the two ends of each ring are very close. For simplification, we built up such a model with closed hexagon rings, as shown in Fig. 2. Further, the barrier layer can be thought of as laminates consisting of laminate molecules.
Fig. 1. FTIR–ATR spectra of the polyamide.
(a)
(b)
Fig. 2. Model structure of polypiperazine amide molecule. (a) One ring. (b) Polyring.
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Fig. 3. Membrane performance as a function of testing time.
Since the diameter of the inscribed circle of the hexagon ring is much larger than that of ions, the sieve effect only by the molecular rings or holes cannot reject any ion. The space between two neighbor laminate molecules thus determines the rejection performance of the polyamide layer only if the space is much smaller than the diameter of the molecular ring and the holes in different molecule laminates do not align with one another. Of course, the electro-static effect is another factor to retard ions. Nevertheless, it is easy to understand that the smaller the space between the laminate molecules, the stronger both the sieve effect and the electro-static effect and thus the higher the rejection of the layer. A film sample of polyamide was examined by WAXD. Since no peaks appear on WAXD patterns, it can be concluded that the polyamide layer on the composite membrane is of an amorphous supermolecular structure. That means the space between the molecular laminates may vary and the positions of the molecular laminates are not in regular order. The rejection behavior of the membrane is largely dependent on the space. Fig. 3 shows the performance of the composite membrane with a nascent polyamide layer. With time, the rejection of the membrane increases
Table 1 Performance of polypiperazine trimesoylamide/polysulfone composite membrane Aqueous solutions
MgSO4 NaSO4 MgCl2 NaCl
Membrane performance Salt rejection, %
Water permeation flux, L/m2/h/MPa
94 96 78 31
60 62 57 66
gradually to a plateau and the flux decreases. This indicates that the space between the molecular laminates get shorter and shorter under pressure, and eventually a balanced densified polyamide layer is reached. After prepressing, the membrane showed an excellent nanofiltration performance with a high rejection for MgSO4 and Na2SO4 but low for NaCl, as shown in Table 1. Figs. 4 and 5 are SEMs of the surfaces of the PSf substrate and the composite membrane, respectively. It is obvious that the surface of the composite membrane is smoother than that of the substrate.
Y. Zhang et al. / Desalination 191 (2006) 291–295
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Fig. 6 is an SEM of the cross section of the composite membrane. It can be seen that there is a dense layer of 300 to 400 nm on the top of the porous substrate. 4. Conclusions
Fig. 4. SEM photograph of the substrate surface.
Fig. 5. SEM photograph of the surface of the composite membrane.
The PA barrier layer of the composite membrane formed by the interfacial polymerization using a trimesoyl hexane solution as the oil phase and a piperazine aqueous solution as the water phase is verified from the result of ATR–FTIR. A simulated molecule model of PA by Material Studio was studied and the model was found to agree with performance of the membrane. The supermolecular structure was thus predicted as an irregularly laminated structure based on the molecular structure and the WAXD pattern. The PA/PSf composite membrane herewith prepared showed excellent nanofiltration performance with a high rejection of 94% for MgSO4 and 96% for Na2SO4 but a low rejection of only 31% to NaCl. The morphology of the membrane was investigated by SEM, showing that the surface of the composite membrane is smoother than that of the substrate with a dense layer of 300 to 400 nm on the porous substrate. Acknowledgement The study was supported by the China HighTech R&D Program (863 Program) No. 2002AA302619. References
Fig. 6. SEM photograph of the cross section of the composite membrane.
[1] R.J. Petersen, J. Membr. Sci., 83 (1993) 81–150. [2] V. Freger, J. Gilron and S. Belfer, J. Membr. Sci., 209 (2002) 283–292. [3] A.M. Mika, R.F. Childs and J.M. Dickson, Desalination, 121 (1999) 149–158. [4] W.R. Bowen, N. Hilal, R.W. Lovitt and C.J. Wright, Coll. Surfaces A: Physicochem. Eng. Aspects, 157 (1999) 117–125.
Investigations on the structures and performances of a polypiperazine amide/polysulfone composite membrane Yufeng Zhanga*, Changfa Xiaoa, Enhua Liua, Qiyun Dua, Xiang Wanga, Hongliang Yub a
Key Laboratory of Hollow Fiber Membrane Materials & Membrane Process, Tianjin Polytechnic University, Ministry of Education, Tianjin 300160, PRC Tel. +86 (22) 8127-4195; email: [email protected] b Nanotechnology Industrialization Base of China, Tianjin 300457, PRC Received 1 February 2005; accepted 9 August 2005
Abstract A polypiperazine amide (PA) composite membrane was prepared by interfacial polymerization using a trimesoyl hexane solution as the oil phase and a piperazine aqueous solution as the water phase on a polysulfone ultrafiltration substrate. The surface was inspected by attenuated total reflection–Fourier transform infrared spectroscopy, showing that a polyamide layer was formed. The molecular structure of polypiperazine amide was simulated using Materials Studio as a plane molecule with hexagon rings of 1.5 nm in diameter of the inscribed circle. Incorporating the result of wide angle X-ray diffraction pattern, the supermolecular structure of the PA was predicted as an irregular laminated structure. This model appears good according to the performance of the membrane. Also investigated was the morphology of the membrane by scanning electron microscope, showing a dense layer of 300 to 400 nm on the porous substrate. The composite membrane obtained is a high-performance nanofiltration membrane. Keywords: Interfacial polymerization; Polyamide; Composite membrane; Molecular simulation; FTIR; WAXD; SEM
1. Introduction Over the last three decades following the pioneering development by Cadotte and co*Corresponding author.
workers, thin-film composite (TFC) polyamide (PA) membranes have become the main type of membranes used for reverse osmosis and nanofiltration [1]. Due to the excellent performance and favorable economics, the TFC-based technology is taking the lead in water treatment,
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.2005.08.016
292
Y. Zhang et al. / Desalination 191 (2006) 291–295
particularly in desalination, and great attention has been drawn to it. There are many studies on the mechanism of mass transfer in the membrane separation process, and also a few investigations on the structure of the PA functional barrier layer in TFC [2–4]. It is known that there is a close relationship between the mechanism of mass transfer and the structure of the membrane. A PA composite membrane was prepared by interfacial polymerization using a trimesoyl hexane solution as the oil phase and a piperazine aqueous solution as the water phase on a polysulfone ultrafiltration substrate in this work. The surface was inspected by attenuated total reflection–Fourier transform infrared spectroscopy (ATR–FTIR) showing a PA layer formed. The molecular structure of PA was simulated using Materials Studio and the supermolecular structure was thus predicted as an irregular laminated structure based on the molecular structure and a wide-angle X-ray diffraction (WAXD) pattern. The morphology of the membrane was investigated by scanning electron microscope (SEM), showing a dense layer of 300 to 400 nm on the porous substrate. The composite membrane obtained showed a high performance for nanofiltration applications. 2. Experimental 2.1. Main chemical agents and materials PSf and poly(vinyl pyrrolidinone) (PVP, K-30) was purchased from Shanghai Chemical Reagent. N, N-dimethylacetamide (DMAc), PI, trimesic acid, thionyl chloride, and n-hexane were obtained from Tianjin Medicine. TMC was prepared from trimesic acid and thionyl chloride by reflux and distillation at reduced pressure. 2.2. Methods The PSf substrate membrane was prepared by the phase inversion. Its molecular weight cut-off
is 6000, and the water flux is about 400– 500 L/m2/h at a pressure of 0.6 MPa. The top active layer was obtained through interfacial polymerization between TMC in hexane and an aqueous phase containing PI. The substrate was first immersed into the PI aqueous phase for minutes, and then taken out to removed the aggregate aqueous solution on the surface of the substrate. Subsequently, it went through a TMC/n-hexane organic phase, and interfacial polymerization occurred at the W/O interface (i.e., the surface of the substrate), and a cured polyamide layer (i.e., the ultra-thin desalination functional layer) was thus formed on the surface of the substrate. Finally, it was heat-treated in hot air for a certain time. 2.2.1. Molecular simulation The molecular formula of poly(piperazine trimesoyl amide) was simulated using Materials Studio through optimization. 2.2.2. Measurements The ATR–FTIR spectra were recorded on a Bruker spectrometer using a ZnSe crystal (25 mm ×5 mm × 2 mm) at a nominal incident angle of 24E yielding about 12 internal reflections at the sample surface at 25EC. WAXD patterns of a PA film were obtained at an angle from 10 to 30E with Rigaku-Denki x-ray diffraction using nickelfiltered CuKα radiation. The structure of the membrane cross sections, after being gold-coated, was investigated with a Philips XL30ESME SEM. The permeate flux and salt rejection of the composite membrane were tested by 1‰ MgSO4, 1‰ Na2SO4, 1‰MgCl and 0.5‰ NaCl aqueous solution under the conditions of 22 ± 1EC and 0.6 MPa, and the concentrations of solutions were fixed by a MC226 (Mettler Toledo). The permeate flux (F) can be calculated by (1)
Y. Zhang et al. / Desalination 191 (2006) 291–295
where J is the volume of the permeate flux, A is the surface area of the membrane, and T is time. Salt rejection (R) is given by (2) where Cf and Cp are the concentrations of the feed and permeate, respectively. 3. Results and discussion Fig. 1 presents the ATR–FTIR spectra of the surface of the composite membranes made by interfacial polymerization from trimesoyl and
293
piperazine. It is clearly seen that the characteristic peak of 1614 cm!1 for amide I appears, which means a polyamide layer formed on the PSf substrate. Fig. 2 shows a computer-simulated model structure of polypiperazine trimesoyl amide. The model molecule consists of hexagon rings. The diameter of the inscribed circle of the hexagon ring is around 1.5 nm. This model formula is one with relatively low energy, and the one with the lowest energy is similar but with unclosed rings. Although the rings are open, the two ends of each ring are very close. For simplification, we built up such a model with closed hexagon rings, as shown in Fig. 2. Further, the barrier layer can be thought of as laminates consisting of laminate molecules.
Fig. 1. FTIR–ATR spectra of the polyamide.
(a)
(b)
Fig. 2. Model structure of polypiperazine amide molecule. (a) One ring. (b) Polyring.
294
Y. Zhang et al. / Desalination 191 (2006) 291–295
Fig. 3. Membrane performance as a function of testing time.
Since the diameter of the inscribed circle of the hexagon ring is much larger than that of ions, the sieve effect only by the molecular rings or holes cannot reject any ion. The space between two neighbor laminate molecules thus determines the rejection performance of the polyamide layer only if the space is much smaller than the diameter of the molecular ring and the holes in different molecule laminates do not align with one another. Of course, the electro-static effect is another factor to retard ions. Nevertheless, it is easy to understand that the smaller the space between the laminate molecules, the stronger both the sieve effect and the electro-static effect and thus the higher the rejection of the layer. A film sample of polyamide was examined by WAXD. Since no peaks appear on WAXD patterns, it can be concluded that the polyamide layer on the composite membrane is of an amorphous supermolecular structure. That means the space between the molecular laminates may vary and the positions of the molecular laminates are not in regular order. The rejection behavior of the membrane is largely dependent on the space. Fig. 3 shows the performance of the composite membrane with a nascent polyamide layer. With time, the rejection of the membrane increases
Table 1 Performance of polypiperazine trimesoylamide/polysulfone composite membrane Aqueous solutions
MgSO4 NaSO4 MgCl2 NaCl
Membrane performance Salt rejection, %
Water permeation flux, L/m2/h/MPa
94 96 78 31
60 62 57 66
gradually to a plateau and the flux decreases. This indicates that the space between the molecular laminates get shorter and shorter under pressure, and eventually a balanced densified polyamide layer is reached. After prepressing, the membrane showed an excellent nanofiltration performance with a high rejection for MgSO4 and Na2SO4 but low for NaCl, as shown in Table 1. Figs. 4 and 5 are SEMs of the surfaces of the PSf substrate and the composite membrane, respectively. It is obvious that the surface of the composite membrane is smoother than that of the substrate.
Y. Zhang et al. / Desalination 191 (2006) 291–295
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Fig. 6 is an SEM of the cross section of the composite membrane. It can be seen that there is a dense layer of 300 to 400 nm on the top of the porous substrate. 4. Conclusions
Fig. 4. SEM photograph of the substrate surface.
Fig. 5. SEM photograph of the surface of the composite membrane.
The PA barrier layer of the composite membrane formed by the interfacial polymerization using a trimesoyl hexane solution as the oil phase and a piperazine aqueous solution as the water phase is verified from the result of ATR–FTIR. A simulated molecule model of PA by Material Studio was studied and the model was found to agree with performance of the membrane. The supermolecular structure was thus predicted as an irregularly laminated structure based on the molecular structure and the WAXD pattern. The PA/PSf composite membrane herewith prepared showed excellent nanofiltration performance with a high rejection of 94% for MgSO4 and 96% for Na2SO4 but a low rejection of only 31% to NaCl. The morphology of the membrane was investigated by SEM, showing that the surface of the composite membrane is smoother than that of the substrate with a dense layer of 300 to 400 nm on the porous substrate. Acknowledgement The study was supported by the China HighTech R&D Program (863 Program) No. 2002AA302619. References
Fig. 6. SEM photograph of the cross section of the composite membrane.
[1] R.J. Petersen, J. Membr. Sci., 83 (1993) 81–150. [2] V. Freger, J. Gilron and S. Belfer, J. Membr. Sci., 209 (2002) 283–292. [3] A.M. Mika, R.F. Childs and J.M. Dickson, Desalination, 121 (1999) 149–158. [4] W.R. Bowen, N. Hilal, R.W. Lovitt and C.J. Wright, Coll. Surfaces A: Physicochem. Eng. Aspects, 157 (1999) 117–125.