Journal of Membrane Science 325 (2008) 184–191
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Composite membranes prepared from glutaraldehyde cross-linked sulfonated cardo polyetherketone and its blends for the dehydration of acetic acid by pervaporation Jian Hua Chen a,b , Qing Lin Liu a,∗ , Ying Xiong a , Qiu Gen Zhang a , Ai Mei Zhu a a b
Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China Department of Chemical Engineering, Meizhouwan Vocational Technology College, Putian 351254, China
a r t i c l e
i n f o
Article history: Received 3 June 2008 Received in revised form 8 July 2008 Accepted 9 July 2008 Available online 18 July 2008 Keywords: Cardo polyetherketone Poly(vinyl alcohol) Silicotungstic acid Composite membranes Acetic acid
a b s t r a c t Cardo polyetherketone (PEK-C) composite membranes were prepared by casting glutaraldehyde (GA) cross-linked sulfonated cardo polyetherketone (SPEK-C) or silicotungstic acid (STA) filled SPEK-C and poly(vinyl alcohol) (PVA) blending onto a PEK-C substrate. The compatibility between the active layer and PEK-C substrate is improved by immersing the PEK-C substrate in a GA cross-linked sodium alginate (NaAlg) solution and using water–dimethyl sulfoxide (DMSO) as a co-solvent for preparing the STA-PVA-SPEK-C/GA active layer. The pervaporation (PV) dehydration of acetic acid shows that permeation flux decreased and separation factor increased with increasing GA content in the homogeneous membranes. The permeation flux achieved a minimum and the separation factor a maximum when the GA content increased to a certain amount. Thereafter the permeation flux increased and the separation factor decreased with further increasing the GA content. The PV performance of the composite membranes is superior to that of the homogeneous membranes when the feed water content is below 25 wt%. The permeation activation energy of the composite membranes is lower than that of the homogeneous membranes in the PV dehydration of 10 wt% water in acetic acid. The STA-PVA-SPEK-C-GA/PEK-C composite membrane using water–DMSO as co-solvent has an excellent separation performance with a flux of 592 g m−2 h−1 and a separation factor of 91.2 at a feed water content of 10 wt% at 50 ◦ C. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Acetic acid is one of the top twenty organic intermediates used in the chemical and other allied industries. Acetic acid/water mixtures are encountered in the preparation of several important intermediates, such as vinyl acetate, phthalic anhydride and acetic anhydride, etc. [1–5]. A large number of trays and a high reflux ratio are necessary to obtain glacial acetic acid by distillation due to the closeness of the boiling points of water and acetic acid; hence, acetic acid separation from water is an energy-consuming process [2,6,7]. Pervaporation (PV) processes show great achievements in separating azeotropic mixtures with components with close boiling points and mixtures consisting of heat sensitive compounds [8–12]. From an energy-saving standpoint, PV can be an effective and promising alternative for the separation of acetic acid/water mixtures.
∗ Corresponding author. Tel.: +86 592 2183751; fax: +86 592 2184822. E-mail address:
[email protected] (Q.L. Liu). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.07.027
The most widely used hydrophilic membranes, such as poly(vinyl alcohol) (PVA) and polyacrylic acid (PAA) based membranes are good candidates for PV dehydration of organic solvents owing to their water permselectivity and high permeation fluxes [13–18]. However, these hydrophilic membranes used in the PV dehydration of acetic acid usually suffer from poor stability due to the high swelling of membranes in aqueous acetic acid. Therefore, improvements in the membrane stability are necessary. Crosslinked hydrophilic membranes exhibit significantly depressed swelling in aqueous acetic acid, and thus increase the membranes selectivity [2,19]. However, a reduction of the permeation flux is usually found with the cross-linked membranes. To overcome this problem, composite membranes prepared from casting hydrophilic polymers onto porous substrates have been applied [20–24]. The porous substrate provides mechanical strength, and the casting layer provides good separation performance for the membrane. Compared to homogeneous membranes, the separation layer thickness is greatly reduced, so that a significant decrease in the permeation resistance and hence an increase in the permeation flux of composite membranes can be achieved. Polyacrylonitrile (PAN) [25–27] is a widely used substrate in preparing composite
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membranes. However, the PAN substrate is weak for the dehydration of aqueous acetic acid solutions. Cardo polyetherketone (PEK-C) is an excellent solvent resistance polymer with high mechanical properties and thermal stability. In our previous work, SPEK-C/PVA blending membranes were prepared, and we found that the compatibility of SPEK-C and PVA is quite good [28]. In this research, PEK-C composite membranes were prepared by casting glutaraldehyde (GA) cross-linked SPEK-C or STA filled PVA/SPEK-C blending onto the PEK-C substrate. Their PV performances were investigated for the dehydration of acetic acid. The effects of the GA content, the morphology of the interface between the active layer and substrate, the feed water content and the feed temperature on the membrane PV performance were investigated. 2. Experimental 2.1. Materials Poly(vinyl alcohol) (polymerization degree of 1750 ± 50), silicotungstic acid (STA), NaAlg and polyvinylpyrrolidone (PVP) were purchased from Sinophatm Chemical Reagent Co. Ltd. (Shanghai, China). Cardo polyetherketone (polymerization degree of 101) was purchased from Xuzhou Engineering Plastic Factory (Jiangsu, China). Dimethyl sulfoxide (DMSO), glutaraldehyde, Nmethyl-2-pyrrolidone (NMP), concentrated sulfuric acid and glacial acetic acid were purchased from the Shanghai Chemical Reagent Store (Shanghai, China) and were used as received (analytical grade). 2.2. Membrane preparation 2.2.1. Preparation of homogeneous membranes 2.2.1.1. Preparation of SPEK-C/GA homogeneous membranes. SPEK-C (sulfonated degree, SD = 0.75) was dissolved in DMSO to a form 6% (w/v) polymer solution; it was then filtered to remove the insoluble materials. Various amounts of 10 wt% GA and 1.0 M HCl (2.0 mL) were added into the filtered solution and stirred vigorously. Then solutions with GA contents of 0.05, 0.1, 0.15, 0.2 and 0.3 wt% were obtained. In order to get the same thickness of the membranes, the resulting homogeneous solution with the same volume was cast onto a glass plate with a casting knife and then dried in an oven at (60 ± 1) ◦ C with relative humidity (65 ± 2)% for 10 h. The membranes obtained were gently peeled off and were further dried under vacuum at 150 ◦ C for 10 h. The membrane with a GA content of 0.2 wt% was designated as M1 . 2.2.1.2. Preparation of STA-PVA-SPEK-C/GA homogeneous membranes. 0.9 g STA was dissolved in 200 mL of DMSO, into which 6 g SPEK-C and 4 g PVA were added and stirred thoroughly at 90 ◦ C for 5 h. Various amounts of 10 wt% GA and 1.0 M HCl (2.0 mL) were added into the above solution and stirred vigorously, then solutions with GA contents 0.1, 0.2, 0.3, 0.4 and 0.5 wt% were obtained. The procedure described in the previous section was then followed to finish the preparation of the membranes. The membrane with a GA content of 0.4 wt% was designated as M2 . 2.2.2. Preparation of composite membranes 2.2.2.1. Preparation of PEK-C microporous substrates. PEK-C microporous film was prepared onto a glass plate via the wet phase inversion technique using a casting solution containing 18.1 wt% PEK-C, 0.72 wt% PVP and 81.18 wt% NMP. The film was immediately immersed into water for 30 min. Then the resulting film was peeled off, rinsed thoroughly with de-ionized water and dried in an oven at
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Table 1 Chemical stability of PEK-C substrate Strong acid/alkali
Content (wt%)
Before immersion (g)
After immersion (g)
Color changed or not
NaOH HCl H2 SO4 H2 SO4 H2 SO4
20 35 30 50 98
0.1021 0.9812 0.9789 0.9798 0.9903
0.1018 0.9814 0.9785 0.9789 –
No No No No Garnet
50 ◦ C for 6 h. The PEK-C substrate obtained was immersed sequentially in a strong acid and/or a strong alkali solution for a week to test their chemical stability. A very high chemical stability was observed, as shown in Table 1. NaAlg was dissolved in de-ionized water to form 4 wt% polymer solution at ambient condition, and then the insoluble materials were removed by filtering the solution, into which a certain amount of 10 wt% GA and 1.0 M HCl (2.0 mL) were added. A solution with 0.2 wt% GA content was thus obtained. In order to enhance the hydrophilicity of the substrate and hence to improve the compatibility between the active layer and the substrate, the PEK-C microporous substrate was immersed in the GA cross-linked NaAlg solution for two days, rinsed with deionized water and dried in an oven at 50 ◦ C for 5 h. 2.2.2.2. Preparation of PEK-C composite membranes. (1) Preparation of SPEK-C-GA/PEK-C composite membranes The PEK-C substrate was soaked with ethanol and fixed onto a glass plate. Then the SPEK-C/GA homogeneous solution (0.2 wt% GA content) was cast onto it, and dried in an oven at (50 ± 1) ◦ C with a relative humidity of (65 ± 2)% for 5 h. The membrane thus obtained was peeled off and further dried under vacuum at 150 ◦ C for 10 h; this membrane was designated as M3 . (2) Preparation of STA-PVA-SPEK-C-GA/PEK-C composite membranes The same procedure, as that for preparing the M3 membrane, was followed to prepare the STA-PVA-SPEK-C-GA/PEK-C composite membrane (GA content 0.4 wt%); these membranes were designated as M4 . In order to investigate the effect of the morphology of the interface between the active layer and substrate on the PV performance, the procedure for preparing the M4 was followed by replacing DMSO (solvent used for M4 ) with water–DMSO a co-solvent to prepare the STA-PVA-SPEK-C-GA/PEK-C composite membrane; these membranes were designated as M5 . 2.3. Membrane characterization 2.3.1. Scanning electron microscope (SEM) study The surface and cross-sectional morphologies of the membranes were measured using SEM (XL30, Oxford Instruments) operating at EHT = 20 kV. 2.3.2. Contact angle measurement The static contact angles between the membranes and water were measured by the pendant drop method using contact angle meter (SL200B, SOLON TECH, Shanghai, China) at 28 ± 1 ◦ C in a relative humidity of (65 ± 2)%. All the reported values are the average of ten measurements taken at different locations on the same membrane surface. The errors are less than 6%.
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2.4. Sorption experiments
where MD and MW denote the masses of the dried and swollen membranes, respectively.
The prepared membranes were weighed and immersed in water/acetic acid mixtures at a temperature of 50 ◦ C for 48 h to reach swelling equilibrium. The samples were taken out at appropriate intervals, wiped with tissue paper carefully to remove the surface solvent, and weighed as quickly as possible. They were then dipped again into the mixtures. The experiments continued until the weight of the samples remained approximately constant. All experiments were repeated at least for three times, and the results were averaged. The errors were less than 2.5%. The degree of swelling (DS) is calculated by DS (%) =
MW − MD × 100% MD
(1)
3. Results and discussion 3.1. Membrane characterization 3.1.1. SEM images The PV performance of PEK-C composite membranes is inherently related to the morphology of the substrate. Fig. 1a, b and c shows the morphology of the cross-section and the surface of the PEK-C substrate. Fingerlike voids are observed in the bulk of the substrate. Near the top surface, there are microvoids in the honeycomb structure layer. This suggests that the porous PEK-C
Fig. 1. The SEM images (a) cross-section of PEK-C; (b) cross-section of PEK-C (local magnification); (c) the surface of PEK-C;(d) the cross-section of M3 ; (e) the cross-section of M4 ; (f) the cross-section of M5 ; (g) the surface of M3 ; (h) the surface of M4 ; (i) the surface of M5 .
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Fig. 1. (Continued).
substrate is suitable for preparing composite membranes. Fig. 1d, e and f shows the morphology of the cross-sections of the M3 , M4 and M5 membranes, respectively. These figures show that the compatibility between the active layer and the substrate of the M5 membrane is better than that of the M3 and M4 membranes. This may be due to the PEK-C substrate with low hydrophilicity and the active layer with high hydrophilicity. However, by immersing the PEK-C substrate into the GA cross-linked NaAlg solution and using the water–DMSO co-solvent to prepare the M5 membrane, the hydrophilicity of the PEK-C surface and the lamination of the active on the substrate are greatly improved. Water is the solvent of NaAlg, hence the NaAlg can be dissolved in the water–DMSO co-solvent. All these increase the compatibility between the active layer and the substrate of the M5 membrane. We have performed
the PV experiment for about 72 h without delamination of the active layer from the substrate. Fig. 1g, h and i are the surface morphologies of the M3 , M4 and M5 membranes, respectively. This indicates that the surface of the composite membranes is smooth, and there are no voids generated. 3.1.2. Contact angles between water and the membrane surface The contact angle is usually used as an indicative of membrane hydrophilicity. The smaller the contact angle, the higher is the hydrophilicity of a membrane. The effects of the GA content on the contact angle of the SPEK-C/GA and STA-PVA-SPEK-C/GA homogeneous membranes are displayed in Fig. 2. This figure shows that the contact angle increases steadily with increasing GA content, indicating a decrease in the hydrophilicity of the membrane with
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Fig. 2. Effects of GA content on the contact angle of the SPEK-C/GA and STA-PVASPEK-C/GA homogeneous membranes.
the introduction of more GA into the cross-linked membranes. This can be attributed to a decrease of the number of –OH groups in the membranes resulting from the esterification reaction between –OH and aldehyde groups. 3.2. Swelling properties The PV transport mechanism can generally be interpreted by the solution diffusion model. Thus, the preferential sorption characteristics of the membranes were explored. Water sorbed into a membrane is important for PV dehydration because this affects the membrane permselectivity. Water sorbed by the hydrophilic –SO3 H groups (SPEK-C) or –OH groups (PVA) results in the membrane swelling, assisting penetrants in diffusing through the membrane. However, too much water uptake by the membrane results in excessive swelling, mechanical fragility and morphological instability of the membrane. Crosslinking is believed to be an effective way to control the excessive swelling of membranes. Fig. 3 shows the effect of GA content and feed water content on the DS of the SPEKC/GA and STA-PVA-SPEK-C/GA homogeneous membranes at 50 ◦ C. One can observe from Fig. 3 that adding GA can effectively depress the DS of the membrane. The DS of the SPEK-C/GA homogeneous membrane decreased with increasing GA content, and arrived at a minimum when the GA content is 0.2 wt%; therafter it increased with further increases in the GA content. This can be attributed to the fact that the esterification reaction between the –OH of SPEK-C and aldehyde groups of GA takes place making the SPEK-C/GA membrane become more compact. However, with further GA addition, the aldehyde groups of the GA is excessive, hence more acetic acid molecules are sorbed by the excessive aldehyde groups resulting in more swelling of the SPEK-C/GA membrane. Ruckenstein and Sun [29] investigated poly(vinyl acetal) membrane swelling behavior in acetic acid aqueous, with the result that the aldehyde groups prefer to absorb acetic acid molecules. From Fig. 3, one can also find that the STA-PVA-SPEK-C/GA and SPEK-C/GA homogeneous membranes share a similar trend of the DS change.
The permeation flux decreased with increasing GA content, and achieved a minimum at a GA content of 0.2 wt%; it then increased with further increases in the GA content. From Fig. 4a, one can find that the separation factor shows the opposite trend. This is due to the fact that the crosslinking between GA and SPEK-C makes the SPEK-C/GA homogeneous membrane become more compact and hence decreases its DS. Therefore, the permeation flux decreases and the separation factor increases with increasing GA content. However, when the GA content is more than 0.2 wt%, the excessive aldehyde groups can absorb acetic acid molecules, which causes the SPEK-C/GA homogeneous membrane to become more swollen; hence the permeation flux increases and the separation factor decreases. The pervaporation separation index (PSI) is the product of the permeation flux and the separation factor, which determines the separation capability of a membrane. It can be used as a guide for designing and selecting a new membrane in PV separation with an optimal integration of permeation flux and permselectivity. Fig. 4b shows the PSI of the SPEK-C/GA homogeneous membrane, indicating an optimal performance at 0.2 wt% GA content. Herewith these components were used for the preparation of the SPEK-C-GA/PEK-C composite membranes. 3.3.1.2. Effects of GA content on PV performance of STA-PVA-SPEKC/GA homogeneous membrane. The effects of the GA content on the PV performance of the STA-PVA-SPEK-C/GA homogeneous membranes are shown in Fig. 5. It is found that the permeation flux, separation factor and PSI share a similar trend with that of the SPEK-C/GA homogeneous membranes mentioned above. The PV performance of the STA-PVA-SPEK-C/GA homogeneous membranes achieved an optimal value at 0.4 wt% GA content, which is higher than that of the SPEK-C/GA homogeneous membrane (0.2 wt% GA content). This is due to the fact that the density of –OH groups in the STA-PVA-SPEK-C membranes is higher than that of SPEK-C membranes. Fig. 2 shows that the contact angle of the STA-PVA-SPEK-C/GA membranes is always greater than that of the SPEK-C/GA membranes, while the water permeation flux of the STA-PVA-SPEK-C/GA membranes is always higher than that of the SPEK-C/GA membranes. This is because the contact angle of membrane can only provide information on the hydrophilicity of the membrane. The PV performance of the membrane is determined not only by the hydrophilicity of the membrane, but also by the microstructure of the amorphous region (the average intermolec-
3.3. Pervaporation experiment 3.3.1. Effect of GA content on PV performance 3.3.1.1. Effects of GA content on PV performance of SPEK-C/GA homogeneous membranes. The effects of GA content on the PV performance of the SPEK-C/GA homogeneous membranes are shown in Fig. 4.
Fig. 3. Effects of GA content and feed water content on the swelling degree of the SPEK-C/GA and STA-PVA-SPEK-C/GA homogeneous membranes.
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Fig. 4. Effects of GA content on the PV performance of the SPEK-C/GA homogeneous membranes (a) permeation flux and separation factor; (b) PSI.
ular distance of the polymer chains of amorphous region and the area of the amorphous region) of the membrane. So the experimental results on the water permeation performance may not be contradictable. 3.3.2. Effects of feed water content on PV performance of homogeneous and composite membranes The effects of the feed water content on the PV performance of the homogeneous membranes (the M1 and M2 ) and the composite membranes (M3 , M4 and M5 ) are shown in Fig. 6. The permeation flux increased and the separation factor decreased with increasing water content, which is attributed to the inherent hydrophilicity of the membranes. The increasing water content makes the membranes become more swollen; hence, this increases the permeation flux and decreases the separation factor. From Fig. 6, one also finds that when water content is below 25 wt%, the permeation flux of the M3 membrane is higher than that of the M1 membrane, and the separation factor of the M3 and M1 membranes remains almost constant. When water content is above 25 wt%, the permeation flux of the M3 membrane is less than that of the M1 membrane, and the separation factor of the M3 membrane is higher than that of the M1 membrane. Both the permeation flux and separation factor of a membrane are determined by the swelling and thickness of the membrane. When water content is below 25 wt%, membrane thickness is the dominant factor because the swelling is not significant at lower water content values. Therefore, the flux for the M3 membrane was larger than that of the M1 membrane and the separation factor of the M3 and M1 membranes remained almost constant.
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Fig. 5. Effects of GA content on the PV performance of the STA-PVA-SPEK-C/GA homogeneous membranes (a) permeation flux and separation factor; (b) PSI.
However, when the water content was above 25 wt%, the swelling of the membrane was significant and was found to be the dominant factor for the M1 membrane. On the other hand, the swelling of the active layer of the M3 membrane is greatly depressed because it is anchored to the substrate. Hence, the flux of the M3 membrane is less than that of the M1 membrane and the separation factor of the M3 membrane is higher than that of the M1 membrane.
Fig. 6. Effects of feed water content on the PV performance of the homogeneous membranes (M1 and M3) and composite membranes (M3 , M4 and M5 ).
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of the membranes becoming larger. The interaction between acetic acid and water molecules decreases with increasing temperature, so it will be easy for both acetic acid and water molecules to diffuse through the membrane. This results in a decrease of the selectivity and an increase of the permeation flux. The permeation flux of the composite membranes is much higher than that of the homogeneous membranes; however, the separation factor of the former is somewhat higher than that of the latter. See Section 3.3.2 for the explanation. 3.3.4. Permeation activation energy In order to estimate the permeation activation energy, which may further enhance an understanding of the utility of the prepared membranes for the separation of water/acetic acid mixtures, the following Arrhenius type relation is employed.
J = J0 exp − Fig. 7. Effects of feed temperature on the PV performance of the homogeneous membranes (M1 and M2 ) and composite membranes (M3 , M4 and M5 ).
The effects of feed water content on the PV performance of the M2 , M4 and M5 membranes are similar to that of the M1 and M3 membranes (Fig. 6). Fig. 6 also indicates that there is little difference between the M4 and M5 membranes for their PV performance. Under the same operating conditions, the permeation flux of the M4 membrane is higher than that of the M5 membrane; however, the separation factor of the former is less than that of the latter. This can be attributed to the fact that water–DMSO was used as the co-solvent in preparing the M5 membranes. Therefore, NaAlg can be dissolved into the water–DMSO co-solvent, which makes the interface of the M5 membrane more compact than that of the M4 membrane. 3.3.3. Effects of feed temperature on PV performance of homogeneous and composite membranes Temperature plays an important role in the PV process. The effects of temperature on the PV performance of the homogeneous and composite membranes were investigated by separating 10 wt% water in the feed at 30, 40, 50 60 and 70 ◦ C. As shown in Fig. 7, just as for many other hydrophilic membranes, the separation factor decreased and the permeation flux increased with increasing operating temperature. According to the free volume theory, the frequency and amplitude of chain jumping (i.e., thermal agitation) increases with increasing temperature, resulting in the free volume
E RT
(2)
The parameters E, J0 and J are the activation energy, the preexponential factor and the flux of a PV process, respectively. Accordingly, the logarithm of the total flux of the membranes (M1 , M2 , M3 , M4 and M5 ) in the PV of 10 wt% water in acetic acid is plotted against the reciprocal of the absolute temperatures (Fig. 8). A typical linear relationship is observed. The total permeation activation energies of the homogeneous membrane M1 and composite membrane M3 are 17.97 and 14.09 kJ mol−1 , respectively. Those of the homogeneous membrane M2 and the composite membranes M4 and M5 are 13.46, 10.76 and 11.12 kJ mol−1 , respectively. Hence one may draw a conclusion that the permeants pass through the composite membranes easier than that for the homogeneous membranes. Meantime, the morphology of the interface between the active layer and substrate affects the permeation activation energy. The more compact the interface between the active layer and substrate, a larger permeation activation energy is required. 3.4. Conclusions The GA cross-linked SPEK-C/GA and STA-PVA-SPEK-C/GA homogeneous membranes, as well as SPEK-C-GA/PEK-C and STAPVA-SPEK-C/PEK-C composite membranes were prepared. The permeation flux decreased and separation factor increased with increasing GA content. The permeation flux achieved a minimum and the separation factor achieved a maximum with increasing GA content up to a certain amount. Thereafter, the permeation flux increased and the separation factor decreased with further increase in GA content. The PV experiments show that the permeation flux of the composite membranes is much higher than that of the homogeneous membranes, and there is almost no difference in the separation factor between the composite membranes and the homogeneous membranes, at low feed water content. The morphology of the interface between the active layer and substrate affects the PV performance of the composite membranes. The more compatible are the active layer and substrate, the better is the PV performance of the composite membrane. The permeation activation energy of the composite membranes is less than that of the homogeneous membranes. Acknowledgements
Fig. 8. Effects of temperature on the ln J of M1 , M2 , M3 , M4 and M5 membranes.
The support of National Nature Science Foundation of China Grant no. 50573063, the Program for New Century Excellent Talents in University and the research fund for the Doctoral Program of Higher Education (no. 2005038401) in preparation of this article is gratefully acknowledged. The authors are grateful to Prof. James
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