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Oxygen-enriching properties of silicone rubber crosslinked membrane containing cobalt
Journal of Membrane Science 296 (2007) 15–20
Oxygen-enriching properties of silicone rubber crosslinked membrane containing cobalt Hua-Xin Rao, Fan-Na Liu, Zi-Yong Zhang ∗ Department of Material Science and Engineering, Jinan University, Guangzhou 510632, China Received 22 August 2006; received in revised form 25 February 2007; accepted 1 March 2007 Available online 6 March 2007
Abstract A poly(dimethylsiloxane) containing 5% vinyl groups (molar percentage) as a matrix material, 2,4,6,8-tetramethylcyclotetrasiloxane as a crosslinking agent and chlorine platinum acid as a catalyst were used to prepare a silicone rubber crosslinked membrane containing cobalt by an addition reaction between the silicon–hydrogen atom and the vinyl group. The solution casting and undergoing crosslinking reactions were carried out simultaneously at room temperature. The effects of pressure difference, temperature, mechanical property and the contents of vinyl acetic acid cobalt in the membrane on the oxygen-enriching property were discussed. The studies on permselectivities showed that permeability coefficients for oxygen and ideal separation factors for oxygen and nitrogen were increased simultaneously at low pressure difference. For example, at 20 ◦ C and pressure difference of 0.05 MPa, for the crosslinked membrane containing vinyl acetic acid cobalt of 10% (mass fraction), the permeability coefficient for oxygen and ideal separation factors for oxygen and nitrogen were 690 Barrer and 3.42, respectively. The results showed the silicone rubber crosslinked membrane containing cobalt had better oxygen-enriching property than other silicone rubber membranes. © 2007 Elsevier B.V. All rights reserved. Keywords: Gas separation membrane; Silicone rubber; Oxygen-enriching property; Crosslinking
1. Introduction For many years, silicone rubber polymers, and in particular poly(dimethylsiloxane) (PDMS) have received considerable attention as a potential membrane material for gas separation because of its high intrinsic permeability to gases [1]. Therefore, PDMS became one of the most important permselective membranes for oxygen among commercially available polymers. The high permeability of PDMS membrane for oxygen has been attributed to its large free volume, which may be due to the flexibility of the siloxane (–SiO–) linkages of this polymer. However, its separation factor for oxygen and nitrogen is very low, and its membrane-forming ability is so poor that it limits its direct applications [2]. Various modifications of PDMS membranes have focused on enhancement of their membrane-forming ability and permselectivity [3]. Control of the gas permeability and selectivity of PDMS membrane has become a subject of strong research interest because of its
∗
Corresponding author. Tel.: +86 20 85228321. E-mail address: [email protected] (Z.-Y. Zhang).
0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.03.003
importance for the development of new membrane separation processes. To increase its permselectivity, different functional groups and hybrid were introduced into the backbone or side chains of PDMS [4,5]. Permselectivity of modified PDMS membrane were improved as follows: PDMS–polycarbonate [6], PDMS–olyurethane [7] and PDMS–polyurea [8] block copolymerization, PDMS–polyphenyl thioether [9] and PDMS–poly(acrylic acid) [10] graft copolymerization, PDMS–polyethylene blend [11], the introduction of trifluride propyl [4] and t-amino into PDMS [12]. In addition, crosslinking is a simple and effective way to change the membrane structure and to improve selectivity. The crosslinking methods include covalent bond crosslinking and metal ion bond crosslinking, from which an ionomer can be formed [13,14]. Stern suggested in 1994 that a potential method of increasing gas selectivity on silicone rubber polymers, without significantly decreasing their permeability, was to incorporate function groups that induce specific interactions to the desired penetration by gases [15]. Among commercially available polymers, it is very difficult to achieve the ideal material properties, because polymers with high permeability usually have low selectivity, and others with high selectivity have low permeability [16]. Since the “inverse”
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permeability/selectivity relationship is not supported by the any theoretical considerations, one may reasonably ask whether it is possible to synthesize entirely new polymers that exhibit both a higher permeability and a higher selectivity than presently available polymers. In the paper, a silicone rubber crosslinked membrane containing cobalt was prepared by an addition reaction between the silicon–hydrogen atom and the vinyl group. The effects of pressure difference, temperature, mechanical property and the contents of vinyl acetic acid cobalt in the membrane on the oxygen-enriching property were studied. The results showed the silicone rubber crosslinked membrane containing cobalt had better oxygen-enriching property than other silicone rubber membranes.
ent mass percentage (5, 8, 10 and 12%) was dissolved in proper THF and then was added, respectively, into the PDMS solution. The even solutions were obtained after string 1 h. The appropriate 2,4,6,8-tetramethylcyclotetrasiloxane and proper chlorine platinum acid solutions were added slowly into the above solution. After an addition reaction between the silicon–hydrogen atom and the vinyl group for 30 min, a novel crosslinked PDMS membrane were prepared by casting on a PET sheet at room temperature. The resulting membranes were easily stripped and then dried in a vacuum dryer for 24 h to remove any residual solvent. The pink and flexile membranes were approximately 80–100 m thick, average calculated by determining thickness of more than five locations on any membranes. 2.4. Measurement of gas permselectivity
2. Experimental 2.1. Materials Silicone rubber (PDMS) and chlorine platinum acid solution (as a catalyst) were obtained from the Research Center of Organic Silicone of Chengdu, China. The number-average molecular weight of silicone rubber was 500,000, and the vinyl content was 10 mol%. 2,4,6,8-Tetramethylcyclotetrasiloxane (DH 4 ) was obtained from Fine Chemical Institute of Silicone and Fluoride of Guangzhou, China. Vinyl acetic acid was obtained from Aldrich agent. Cobalt hydroxide was obtained from chemical agent of Guangzhou, China. All other chemicals were of analytical grad and used without further purification. Infrared measurements were obtained on KBr compacted power with Bruker EQUINX 55 of German. Physical properties of these membrane materials were studied by using Shimadzu AG-I of Japan. 2.2. Preparation of vinyl acetic acid cobalt Cobalt hydroxide and vinyl acetic acid were mixed by molar ratio of 1 to 2. The mixture was added into proper tetrahydrofuran (THF) and was stirred for 1 h. A fuchsia and transparent solution was obtain. The solution was put into vacuum dryer to eliminate THF and water. The fuchsia vinyl acetic acid cobalt was prepared.
Oxygen and nitrogen permeability coefficients of the membranes were measured according to the variable-volume method of Stern et al. [17]. The operation process had been described elsewhere [18]. Oxygen permeability coefficients (PO2 ) and nitrogen permeability coefficients (PN2 ) and ideal separation factors (αO2 /N2 ) could be calculated using the following equations: (V/t)l P= p A αO2 /N2 =
PO2 PN2
where V and t were the changes in volume for the permeated gas and in time, respectively; A and l were the effective area and thickness of the membrane, respectively; p was the gas pressure difference across the membrane. Unit of permeability coefficient was: cm2 (STP) cm/(cm2 s cmHg). The measurement conditions included: testing temperatures from 20 to 50 ◦ C; gas pressure differences across the membranes from 0.05 to 0.4 MPa; A was 3.8 cm2 ; each value of t was obtained by determining it least three times. The standard deviation was within ca. ±5%. P was usually corrected to standard conditions (STP) of temperature (273 ◦ C) and pressure (76 cmHg). Gas permeation equipment was shown in Fig. 1. 3. Characterization
2.3. Preparation of PDMS crosslinked membrane 3.1. FT-IR spectrum analysis A polydimethylsiloxane (PDMS) containing 5% vinyl groups (molar percentage) was dissolved in proper THF to form a PDMS solution. The above vinyl acetic acid cobalt with differ-
The chemical structure changes during the crossinked process were monitored by Fourier transform infrared (FTIR) and
Fig. 1. Schematic diagram of the apparatus for measurement of gas permeability by the variable-volume method.
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Table 1 The mechanical properties for the crosslinked membrane with different Co(II) salt contents
Fig. 2. FTIR spectrum of vinyl acetic acid (a), vinyl acetic acid cobalt (b), DH 4 (c), silicon rubber (d), crosslinked membrane containing cobalt (e).
shown in Fig. 2(a–e). The characteristic peaks of vinyl acetic acid which contained carboxyl (–COOH) and vinyl groups (–CH CH2 ) were at 2800 cm−1 (υO H ), 1710.1 cm−1 (υC O ) and 1642.1 cm−1 (υC C ) (Fig. 2(a)). Compared with that of vinyl acetic acid, the absorption peak (–COO− ) for vinyl acetic acid cobalt was at 1402.6 cm−1 and 1572.3 cm−1 and the characteristic peak of carboxyl (–COOH) was almost disappeared (Fig. 2(b)). The characteristic peak of silicon–hydrogen groups −1 (Fig. 2(c)). (Si–H) for DH 4 was at 2176.5 cm Structure characteristics of pure silicone rubber and crosslinked membrane were carried out using the FTIR analysis. The results were shown in Fig. 2(d and e). The characteristic peak of vinyl groups (–CH = CH2 ) at 1640.2 cm−1 and 2979.7 cm−1 for PDMS completely disappeared in the FTIR spectra of the crosslinked membrane, which suggested that vinyl groups of PDMS reacted with silicon–hydrogen groups of DH 4. Typically, the most intensive peaks at 1047.2 cm−1 representing Si–O–Si asymmetric stretching were seen in the spectra of the crosslinked membrane. Compared to the characteristic peak of Si–O–Si groups for PDMS and DH 4 , that of Si–O–Si groups for
Co(II) salt content (wt%)
Max-stress (N/mm2 )
Max-strain (%)
Elastic (N/mm2 )
0 5 8 10 12
0.40670 0.48202 0.64312 0.84665 0.50100
9.72508 18.23501 37.54002 56.31511 41.34101
6.707 4.784 3.958 3.046 2.518
the crosslinked membrane was shifted to lower wavenumber, which suggested that the crosslinking reaction was successfully carried out by silicon–hydrogen additive reaction. The silicone rubber crosslinked membrane containing cobalt was prepared by PDMS containing 5% vinyl groups as a matrix material, 2,4,6,8-tetramethylcyclotetrasiloxane as a crosslinking agent and chlorine platinum acid as a catalyst The crosslinking reaction and the chemical structure of the crosslinked membrane were shown in Fig. 3. 3.2. Physicochemical characterization of the crosslinked membrane To gain further insights into the structure–property relationships, we had investigated the physical performance of the PDMS crosslinked membrane with different Co(II) salt content. The results were shown in Table 1. Stain and stress of the crosslinked membranes increased significantly with increasing Co(II) salt content up to 0.847 N/mm2 and 56.31% at 10 wt%, respectively, then decreased sharply to 0.50 N/mm2 and 41.34% at 12 wt%, respectively. Elongation of the crosslinked membrane decreased markedly with increasing Co(II) salt content. With an increase of vinyl acetic acid cobalt content, the dosage of crosslinking agent and crosslinking density of the membrane were increased, resulting in an increase of physical performance of the PDMS crosslinked membrane. However, when crosslink-
Fig. 3. Crosslinking reaction and the structure of the crosslinked membrane.
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ing degree was up to a certain value, brittleness of the membrane was increased and flexility of the membrane was decreased, which leaded to a decrease of mechanical property and formingmembrane property of the membrane. The results showed that the crosslinked membrane had highest stain and stress when the content of vinyl acetic acid cobalt was 10 wt%. Based on mechanical property and forming-membrane property of the crosslinked membrane, the membrane containing 10% vinyl acetic acid cobalt (mass%) was used to test the oxygen-enriching properties. 3.3. Swelling behaviour of the crosslinked membrane To attain the physical performance and the degree of chemical crosslinking, the swelling behaviours of the crosslinked membrane and the crosslinked membrane containing 10 wt% vinyl acetic acid cobalt were studied. The mechanical property of the PDMS crosslinked membrane was enhanced due to DH 4 used a crosslinking agent and dual crosslinking of Co(II) ion. The crosslinked membranes were not dissolved, but were swelled slightly in organic agent such as tetrahydrofuran (THF), toluene and chloroform. The crosslinked membrane without cobalt salt and the crosslinked membrane containing 10% vinyl acetic acid cobalt were dipped in THF, toluene and chloroform for 4 h at room temperature, respectively. The swell coefficients of the crosslinked membrane without cobalt salt were 1.011, 1.000 and 1.004, respectively. The swelling coefficients of the crosslinked membrane containing 10 wt% vinyl acetic acid cobalt was 1.012, 1.003 and 1.001. These results showed that the swell was slight and the crosslinking degree of the membrane was higher. The results showed the crosslinked membrane of PDMS and the crosslinked membrane containing cobalt had higher degree of chemical crosslinking and better physical performance. 4. Results and discussion 4.1. The effect of pressure difference on oxygen-enriching properties Fig. 4 showed the influence of pressure difference (p) on permeation coefficient (PO2 , PN2 ) and ideal separation factor (αO2 /N2 ) of the PDMS crosslinked membrane containing 10 wt% vinyl acetic acid cobalt. It revealed that gas difference pressure had affect obviously on permeation coefficient for oxygen (PO2 ) and a gradual decrease for PO2 with the increase of pressure difference was observed at the same temperature. The effect of pressure difference on PO2 was more distinctly at lower pressure difference. For example, when pressure difference was increased from 0.05 to 0.4 MPa, PO2 was decreased from 690 Barrer to 462 Barrer at 20 ◦ C. The permeation coefficient for nitrogen (PN2 ) was also decreased with an increase of difference pressure. However, the effect of pressure difference on PN2 was very slight, to same extent, which indicated that PN2 of the PDMS crosslinked membrane was almost independent on the pressure differences. Similarly, it could be seen that separation factor (αO2 /N2 ) was decreased obviously with an increase of pressure difference. On the one hand, PO2 was decreased obviously
Fig. 4. Effect of pressure differences and temperature on PO2 , PN2 and αO2 /N2 for the PDMS membrane containing 10 wt% cobalt salt at 20 ◦ C (䊉), 30 ◦ C (), 40 ◦ C () and 50 ◦ C (); for the PDMS membrane without cobalt salt at 20 ◦ C ().
and PN2 was decreased slightly with an increase of pressure difference, which resulted in a decrease of αO2 /N2 obviously with an increase of pressure difference. On the other hand, the change was related to transportation mechanism of polymeric membrane for gas. The gas permeation behavior was not only in accord with the general dissolution–diffusion mechanism of Henry’ type dissolution, but also with transportation mechanism of Langmuir adsorption. In fact, a macromolecule cobalt complex, which had reversible adsorption for oxygen molecule and was benefited to permeate through the membrane, was formed between cobalt ion and carboxyl. Apparently, the improvement in the oxygen-enriching properties of the crosslinked membrane should be attributed to ionomer structures formed incorporating carboxylic cobalt groups. The special behavior of oxygen permeation through the crosslinked membrane corresponded to membranes containing cobalt complexes as a fixed oxygen carrier. However, it could be seen form Fig. 4 that PO2 and αO2 /N2 of the crosslinked membrane containing cobalt salt were higher than these of the crosslinked membrane without cobalt salt at 20 ◦ C. The results showed that cobalt ion was benefit for oxygen-enriching property of PDMS membrane. In our previous study [2], a silicone rubber containing cobalt ion used by eleven alkene acid was prepared and oxygenenriching property of the membrane was studied. Under pressure difference of 0.05 MPa and 20 ◦ C, PO2 and αO2 /N2 was 750 Barrer and 3.09, respectively. However, at the same testing condition, for the PDMS crosslinked membrane containing cobalt ion used by vinyl acetic acid (PO2 and αO2 /N2 was 690 Barrer and 3.42, respectively), PO2 was decreased slightly, but αO2 /N2 was increased obviously. In fact, the silicone rubber membranes containing cobaltous ions were turned into an ionomer with a crosslinked structures. There had been some controversies in ionomer investigations about the morphologies of ion aggregates. The FTIR studied of Han and Williams [19] proved that
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ionomers containing carboxylic groups and transitional metal ions tended to form a polymeric complex, which was based on poly (ethylene-co-methacrylic acid). The silicone rubber membranes containing carboxylic cobalt, in another point of view, also validated the above result because, only when polymeric cobalt complexes were formed, could the oxygen-enriching property enhanced. Of course, the molecular chain conformation of the PDMS crosslinked membrane was changed due to an addition of cobalt ion into the membrane, which leaded to increase dissolution permselectivity of gas molecular for the membrane and permselectivity of the crosslinked membrane. In addition, because carbon chain of eleven alkene acid was longer and its space steric hindrance was larger, the capability that eleven alkene acid cobalt dissolved in PDMS membrane containing cobalt was limited and the content of eleven alkene acid cobalt was only 5 wt%. However, the content of vinyl acetic acid cobalt in the PDMS crosslinked membrane containing cobalt was up to 10 wt%. 4.2. The effect of testing temperature on oxygen-enriching properties The effect of testing temperature on PO2 , PN2 and αO2 /N2 of the crosslinked membrane containing 10 wt% vinyl acetic acid cobalt was shown in Fig. 4. Referring to Fig. 4, it could be seen from the figure that PO2 and PN2 were increased with an increase of testing temperature under the same pressure difference. For example, under the same pressure difference of 0.05 MPa, PO2 was increased from 690 Barrer (20 ◦ C) to 744 Barrer (50 ◦ C). The effect of temperature on oxygen-enriching properties of the membrane was in accord with the general role of gas separation membrane. By contrast, αO2 /N2 was decreased with an increase of testing temperature. Under the above condition, αO2 /N2 of the PDMS crosslinked membrane was decreased from 3.42 (20 ◦ C) to 3.02 (50 ◦ C), which were higher than those of general modified silicone rubber membrane (αO2 /N2 was 2.0 or so). Stern et al. [4] demonstrated that the effects temperature on permeability coefficients depended on changes in the diffusion coefficient and the solubility coefficient when they investigated structure–permeability relationships for different silicone polymers. The diffusion coefficient always increased with increasing temperature, whereas for the solubility coefficient it was generally the opposite. The gas permeability enhancement was basically a result of the increase in the diffusion ability of the gases in the PDMS membrane when the temperature was raised. Hence, the solubility behavior of the gas for the membrane became an important control factor for the gas permeation process here. In conclusion, the effect of temperature on oxygen-enriching properties of the membrane was in accord with the general role of gas separation membrane. 4.3. The effect of vinyl acetic acid cobalt content on oxygen-enriching properties The effect of vinyl acetic acid cobalt content in the PDMS crosslinked membrane on oxygen-enriching properties was
19
Fig. 5. Effect of different Co(II) salt contents in the crosslinked membrane on oxygen-enriching properties.
investigated further. The results of the previously considerable works [2,4] revealed that cobaltous complex as an oxygen carrier added into the PDMS crosslinked membrane had facilitated transport for oxygen molecule. The above-mentioned “inverse” permeability and permselevitity relationship, which is so often mentioned in the membrane literature, was hold by many investigators to be almost an immutable law of nature, that is, an increase in permeability are attended by losses in permselevitity, vice versa. To enhance the oxygen-enriching property of the crosslinked membrane, larger amounts of the vinyl acetic acid cobalt were incorporated into the membrane. Fig. 5 presented the effect of different vinyl acid cobalt contents in the membrane on oxygen-enriching properties under pressure difference of 0.05 MPa and 20 ◦ C. For the PDMS crosslinked membrane containing cobalt ion, a simultaneous increase in both PO2 and αO2 /N2 with a decrease in pressure difference, exhibited a typical characteristic of the facilitated transport membranes with fixed oxygen carrier. Compared with general modified PDMS membrane, with an increase of vinyl acetic acid cobalt contents in the crosslinked membrane, molecular free space in the membrane was decreased and space hindrance was increased, which leaded to simultaneous increase in both PO2 and αO2 /N2 . Clearly, the PDMS crosslinked membrane with proper vinyl acetic acid cobalt contents exhibited better oxygen-enriching properties. For example, when vinyl acetic acid cobalt content was 10 wt% in the membrane, the appearance was uniform and the intensity of the membrane was highest. Under pressure difference of 0.05 MPa and 20 ◦ C, PO2 and αO2 /N2 of the crosslinked membrane were 690 Barrer and 3.42, respectively, which was higher than general modified PDMS membrane (PO2 and αO2 /N2 were 548 Barrer and 2.04, respectively). The uniformity, flexibility and mechanical property of the crosslinked membrane, however, lessened when vinyl acetic acid cobalt content was increased to more than 10 wt% in the membrane. As mentioned above, the cobalt ions in the crosslinked membrane play two important roles in improving the selectivity of the membrane: the formation of crosslinked networks and the polymeric cobalt complex. The more carboxylic cobalt content
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in the crosslinked membrane, the more polymeric complexes like the oxygen carriers were formed. 5. Conclusions To enhance oxygen-enriching properties of PDMS, 2,4,6,8tetramethylcyclotetrasiloxane with a cyclic structure was used as a crosslinking agent to enlarge the space between molecular chains. The vinyl acetic acid cobalt as an oxygen carrier was incorporated into the crosslinked membrane to enhance the facilitated transport for oxygen. Under pressure difference of 0.05 MPa and 20 ◦ C, when vinyl acetic acid cobalt content was 10 wt% in the membrane, PO2 and αO2 /N2 of the crosslinked membrane were 690 Barrer and 3.42, respectively. PO2 and αO2 /N2 of the crosslinked membrane were increased simultaneously and the oxygen-enriching properties was close to Robeson’s critical “upper bound” [20]. A simultaneous increase in both PO2 and αO2 /N2 with an increase of vinyl acetic acid cobalt contents in the crosslinked membrane and vinyl acetic acid cobalt content was increased to 10 wt% in the membrane. In other words, the above experimental data and physicochemical implications suggested the possibility that many new polymers having both higher permeability and permselevitity could be designed by using crosslinked method and incorporating cobalt ion into the polymers. Acknowledgement This work was partly supported by the natural science fund of Guangdong Province, China (No. 04010430). References [1] T. Aoki, Macromolecular design of permselective membranes, Prog. Polym. Sci. 24 (1999) 951.
[2] Z.Y. Zhang, Facilitated oxygen transport in a novel silicone polymer membrane containing carboxylic cobalt groups, J. Appl. Polym. Sci. 90 (2003) 1038. [3] F.W. Wu, L. Li, Z.H. Xu, et al., Transport study of pure and mixed gases through PDMS membrane, Chem. Eng. J. 117 (2006) 51. [4] S.A. Stern, V.M. Shan, B.J. Hardy, Structure-permeability relationships in silicone polymers, J. Polym. Sci.: Part B: Polym. Phys. 25 (1987) 1263. [5] B.P. Ho, J.K. Kim, S.Y. Nam, Y.M. Lee, Imide-siloxane block copolymer/silica hybrid membranes: preparation, characterization and gas separation properties, J. Membr. Sci. 220 (2003) 59. [6] W.J. Ward, R. Browall, R.M. Salemme, Ultrathin silicone polycarbonate membranes for gas separation processes, J. Membr. Sci. 1 (1976) 99. [7] Nitto Electric Industrial Co. Ltd, Gas-separating membranes [P], JP Patent: 122907,1982-07-31. [8] Nitto Electric Industrial Co. Ltd, Gas separation membranes [P], JP Patent: 12105,1985-03-29. [9] S. Tsurumaki, M. Shimomura, Gas-separation membranes for oxygen richment [P], JP Patent: 218423,1990-08-31 (2000). [10] J.S. Yang, G.H. Hsiue, Synthesis of acrylic acid grafted silicone rubber via preirradiation graft copolymerization and itsphysicalanddielectric properties, J. Appl. Polym. Sci 61 (1996) 21. [11] M. Yoshida, Y. Sasaki, Manufacture of polyethylene-silicone rubber blend membranes [P], JP Patent: 72932,1990-03-13. [12] Teijin Ltd., Permselective membranes [P], JP Patent: 209608,1984-11-28 (2000). [13] W.J. Chen, C.R. Martin, Gas-transport properties of sulfonated polystyrenes, J. Membr. Sci. 95 (1994) 51. [14] Y.T. Lee, K. Iwamoto, E.J. Seno, Gas permeabilities of perfluorocarboxylate membranes in various ionic forms, J. Membr. Sci. 49 (1990) 85. [15] S.A. Stern, Polymers for gas separations: the next decade, J. Membr. Sci. 94 (1994) 1. [16] T.H. Kim, W.J. Koros, G.R. Husk, et al., Relationship between gas separation properties and chemical structure in a series of aromatic polyimides, J. Membr. Sci. 37 (1988) 46. [17] S.A. Stern, T.F. Sinclair, P. Gareis, An improved permeability apparatus of the variable-volume type, J. Mod. Plast. 42 (1964) 154. [18] J.P. Yang, H.S. Sun, P.C. Huang, The measure of gas separation membrane, Polym. Mater. Sci. Eng. 14 (1998) 41, Chinese. [19] K. Han, H.L. Williams, Ionomers: two formation mechanisms and models, J. Appl. Polym. Sci. 42 (1991) 1845. [20] L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci. 62 (1991) 165.
Oxygen-enriching properties of silicone rubber crosslinked membrane containing cobalt Hua-Xin Rao, Fan-Na Liu, Zi-Yong Zhang ∗ Department of Material Science and Engineering, Jinan University, Guangzhou 510632, China Received 22 August 2006; received in revised form 25 February 2007; accepted 1 March 2007 Available online 6 March 2007
Abstract A poly(dimethylsiloxane) containing 5% vinyl groups (molar percentage) as a matrix material, 2,4,6,8-tetramethylcyclotetrasiloxane as a crosslinking agent and chlorine platinum acid as a catalyst were used to prepare a silicone rubber crosslinked membrane containing cobalt by an addition reaction between the silicon–hydrogen atom and the vinyl group. The solution casting and undergoing crosslinking reactions were carried out simultaneously at room temperature. The effects of pressure difference, temperature, mechanical property and the contents of vinyl acetic acid cobalt in the membrane on the oxygen-enriching property were discussed. The studies on permselectivities showed that permeability coefficients for oxygen and ideal separation factors for oxygen and nitrogen were increased simultaneously at low pressure difference. For example, at 20 ◦ C and pressure difference of 0.05 MPa, for the crosslinked membrane containing vinyl acetic acid cobalt of 10% (mass fraction), the permeability coefficient for oxygen and ideal separation factors for oxygen and nitrogen were 690 Barrer and 3.42, respectively. The results showed the silicone rubber crosslinked membrane containing cobalt had better oxygen-enriching property than other silicone rubber membranes. © 2007 Elsevier B.V. All rights reserved. Keywords: Gas separation membrane; Silicone rubber; Oxygen-enriching property; Crosslinking
1. Introduction For many years, silicone rubber polymers, and in particular poly(dimethylsiloxane) (PDMS) have received considerable attention as a potential membrane material for gas separation because of its high intrinsic permeability to gases [1]. Therefore, PDMS became one of the most important permselective membranes for oxygen among commercially available polymers. The high permeability of PDMS membrane for oxygen has been attributed to its large free volume, which may be due to the flexibility of the siloxane (–SiO–) linkages of this polymer. However, its separation factor for oxygen and nitrogen is very low, and its membrane-forming ability is so poor that it limits its direct applications [2]. Various modifications of PDMS membranes have focused on enhancement of their membrane-forming ability and permselectivity [3]. Control of the gas permeability and selectivity of PDMS membrane has become a subject of strong research interest because of its
∗
Corresponding author. Tel.: +86 20 85228321. E-mail address: [email protected] (Z.-Y. Zhang).
0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.03.003
importance for the development of new membrane separation processes. To increase its permselectivity, different functional groups and hybrid were introduced into the backbone or side chains of PDMS [4,5]. Permselectivity of modified PDMS membrane were improved as follows: PDMS–polycarbonate [6], PDMS–olyurethane [7] and PDMS–polyurea [8] block copolymerization, PDMS–polyphenyl thioether [9] and PDMS–poly(acrylic acid) [10] graft copolymerization, PDMS–polyethylene blend [11], the introduction of trifluride propyl [4] and t-amino into PDMS [12]. In addition, crosslinking is a simple and effective way to change the membrane structure and to improve selectivity. The crosslinking methods include covalent bond crosslinking and metal ion bond crosslinking, from which an ionomer can be formed [13,14]. Stern suggested in 1994 that a potential method of increasing gas selectivity on silicone rubber polymers, without significantly decreasing their permeability, was to incorporate function groups that induce specific interactions to the desired penetration by gases [15]. Among commercially available polymers, it is very difficult to achieve the ideal material properties, because polymers with high permeability usually have low selectivity, and others with high selectivity have low permeability [16]. Since the “inverse”
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H.-X. Rao et al. / Journal of Membrane Science 296 (2007) 15–20
permeability/selectivity relationship is not supported by the any theoretical considerations, one may reasonably ask whether it is possible to synthesize entirely new polymers that exhibit both a higher permeability and a higher selectivity than presently available polymers. In the paper, a silicone rubber crosslinked membrane containing cobalt was prepared by an addition reaction between the silicon–hydrogen atom and the vinyl group. The effects of pressure difference, temperature, mechanical property and the contents of vinyl acetic acid cobalt in the membrane on the oxygen-enriching property were studied. The results showed the silicone rubber crosslinked membrane containing cobalt had better oxygen-enriching property than other silicone rubber membranes.
ent mass percentage (5, 8, 10 and 12%) was dissolved in proper THF and then was added, respectively, into the PDMS solution. The even solutions were obtained after string 1 h. The appropriate 2,4,6,8-tetramethylcyclotetrasiloxane and proper chlorine platinum acid solutions were added slowly into the above solution. After an addition reaction between the silicon–hydrogen atom and the vinyl group for 30 min, a novel crosslinked PDMS membrane were prepared by casting on a PET sheet at room temperature. The resulting membranes were easily stripped and then dried in a vacuum dryer for 24 h to remove any residual solvent. The pink and flexile membranes were approximately 80–100 m thick, average calculated by determining thickness of more than five locations on any membranes. 2.4. Measurement of gas permselectivity
2. Experimental 2.1. Materials Silicone rubber (PDMS) and chlorine platinum acid solution (as a catalyst) were obtained from the Research Center of Organic Silicone of Chengdu, China. The number-average molecular weight of silicone rubber was 500,000, and the vinyl content was 10 mol%. 2,4,6,8-Tetramethylcyclotetrasiloxane (DH 4 ) was obtained from Fine Chemical Institute of Silicone and Fluoride of Guangzhou, China. Vinyl acetic acid was obtained from Aldrich agent. Cobalt hydroxide was obtained from chemical agent of Guangzhou, China. All other chemicals were of analytical grad and used without further purification. Infrared measurements were obtained on KBr compacted power with Bruker EQUINX 55 of German. Physical properties of these membrane materials were studied by using Shimadzu AG-I of Japan. 2.2. Preparation of vinyl acetic acid cobalt Cobalt hydroxide and vinyl acetic acid were mixed by molar ratio of 1 to 2. The mixture was added into proper tetrahydrofuran (THF) and was stirred for 1 h. A fuchsia and transparent solution was obtain. The solution was put into vacuum dryer to eliminate THF and water. The fuchsia vinyl acetic acid cobalt was prepared.
Oxygen and nitrogen permeability coefficients of the membranes were measured according to the variable-volume method of Stern et al. [17]. The operation process had been described elsewhere [18]. Oxygen permeability coefficients (PO2 ) and nitrogen permeability coefficients (PN2 ) and ideal separation factors (αO2 /N2 ) could be calculated using the following equations: (V/t)l P= p A αO2 /N2 =
PO2 PN2
where V and t were the changes in volume for the permeated gas and in time, respectively; A and l were the effective area and thickness of the membrane, respectively; p was the gas pressure difference across the membrane. Unit of permeability coefficient was: cm2 (STP) cm/(cm2 s cmHg). The measurement conditions included: testing temperatures from 20 to 50 ◦ C; gas pressure differences across the membranes from 0.05 to 0.4 MPa; A was 3.8 cm2 ; each value of t was obtained by determining it least three times. The standard deviation was within ca. ±5%. P was usually corrected to standard conditions (STP) of temperature (273 ◦ C) and pressure (76 cmHg). Gas permeation equipment was shown in Fig. 1. 3. Characterization
2.3. Preparation of PDMS crosslinked membrane 3.1. FT-IR spectrum analysis A polydimethylsiloxane (PDMS) containing 5% vinyl groups (molar percentage) was dissolved in proper THF to form a PDMS solution. The above vinyl acetic acid cobalt with differ-
The chemical structure changes during the crossinked process were monitored by Fourier transform infrared (FTIR) and
Fig. 1. Schematic diagram of the apparatus for measurement of gas permeability by the variable-volume method.
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Table 1 The mechanical properties for the crosslinked membrane with different Co(II) salt contents
Fig. 2. FTIR spectrum of vinyl acetic acid (a), vinyl acetic acid cobalt (b), DH 4 (c), silicon rubber (d), crosslinked membrane containing cobalt (e).
shown in Fig. 2(a–e). The characteristic peaks of vinyl acetic acid which contained carboxyl (–COOH) and vinyl groups (–CH CH2 ) were at 2800 cm−1 (υO H ), 1710.1 cm−1 (υC O ) and 1642.1 cm−1 (υC C ) (Fig. 2(a)). Compared with that of vinyl acetic acid, the absorption peak (–COO− ) for vinyl acetic acid cobalt was at 1402.6 cm−1 and 1572.3 cm−1 and the characteristic peak of carboxyl (–COOH) was almost disappeared (Fig. 2(b)). The characteristic peak of silicon–hydrogen groups −1 (Fig. 2(c)). (Si–H) for DH 4 was at 2176.5 cm Structure characteristics of pure silicone rubber and crosslinked membrane were carried out using the FTIR analysis. The results were shown in Fig. 2(d and e). The characteristic peak of vinyl groups (–CH = CH2 ) at 1640.2 cm−1 and 2979.7 cm−1 for PDMS completely disappeared in the FTIR spectra of the crosslinked membrane, which suggested that vinyl groups of PDMS reacted with silicon–hydrogen groups of DH 4. Typically, the most intensive peaks at 1047.2 cm−1 representing Si–O–Si asymmetric stretching were seen in the spectra of the crosslinked membrane. Compared to the characteristic peak of Si–O–Si groups for PDMS and DH 4 , that of Si–O–Si groups for
Co(II) salt content (wt%)
Max-stress (N/mm2 )
Max-strain (%)
Elastic (N/mm2 )
0 5 8 10 12
0.40670 0.48202 0.64312 0.84665 0.50100
9.72508 18.23501 37.54002 56.31511 41.34101
6.707 4.784 3.958 3.046 2.518
the crosslinked membrane was shifted to lower wavenumber, which suggested that the crosslinking reaction was successfully carried out by silicon–hydrogen additive reaction. The silicone rubber crosslinked membrane containing cobalt was prepared by PDMS containing 5% vinyl groups as a matrix material, 2,4,6,8-tetramethylcyclotetrasiloxane as a crosslinking agent and chlorine platinum acid as a catalyst The crosslinking reaction and the chemical structure of the crosslinked membrane were shown in Fig. 3. 3.2. Physicochemical characterization of the crosslinked membrane To gain further insights into the structure–property relationships, we had investigated the physical performance of the PDMS crosslinked membrane with different Co(II) salt content. The results were shown in Table 1. Stain and stress of the crosslinked membranes increased significantly with increasing Co(II) salt content up to 0.847 N/mm2 and 56.31% at 10 wt%, respectively, then decreased sharply to 0.50 N/mm2 and 41.34% at 12 wt%, respectively. Elongation of the crosslinked membrane decreased markedly with increasing Co(II) salt content. With an increase of vinyl acetic acid cobalt content, the dosage of crosslinking agent and crosslinking density of the membrane were increased, resulting in an increase of physical performance of the PDMS crosslinked membrane. However, when crosslink-
Fig. 3. Crosslinking reaction and the structure of the crosslinked membrane.
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ing degree was up to a certain value, brittleness of the membrane was increased and flexility of the membrane was decreased, which leaded to a decrease of mechanical property and formingmembrane property of the membrane. The results showed that the crosslinked membrane had highest stain and stress when the content of vinyl acetic acid cobalt was 10 wt%. Based on mechanical property and forming-membrane property of the crosslinked membrane, the membrane containing 10% vinyl acetic acid cobalt (mass%) was used to test the oxygen-enriching properties. 3.3. Swelling behaviour of the crosslinked membrane To attain the physical performance and the degree of chemical crosslinking, the swelling behaviours of the crosslinked membrane and the crosslinked membrane containing 10 wt% vinyl acetic acid cobalt were studied. The mechanical property of the PDMS crosslinked membrane was enhanced due to DH 4 used a crosslinking agent and dual crosslinking of Co(II) ion. The crosslinked membranes were not dissolved, but were swelled slightly in organic agent such as tetrahydrofuran (THF), toluene and chloroform. The crosslinked membrane without cobalt salt and the crosslinked membrane containing 10% vinyl acetic acid cobalt were dipped in THF, toluene and chloroform for 4 h at room temperature, respectively. The swell coefficients of the crosslinked membrane without cobalt salt were 1.011, 1.000 and 1.004, respectively. The swelling coefficients of the crosslinked membrane containing 10 wt% vinyl acetic acid cobalt was 1.012, 1.003 and 1.001. These results showed that the swell was slight and the crosslinking degree of the membrane was higher. The results showed the crosslinked membrane of PDMS and the crosslinked membrane containing cobalt had higher degree of chemical crosslinking and better physical performance. 4. Results and discussion 4.1. The effect of pressure difference on oxygen-enriching properties Fig. 4 showed the influence of pressure difference (p) on permeation coefficient (PO2 , PN2 ) and ideal separation factor (αO2 /N2 ) of the PDMS crosslinked membrane containing 10 wt% vinyl acetic acid cobalt. It revealed that gas difference pressure had affect obviously on permeation coefficient for oxygen (PO2 ) and a gradual decrease for PO2 with the increase of pressure difference was observed at the same temperature. The effect of pressure difference on PO2 was more distinctly at lower pressure difference. For example, when pressure difference was increased from 0.05 to 0.4 MPa, PO2 was decreased from 690 Barrer to 462 Barrer at 20 ◦ C. The permeation coefficient for nitrogen (PN2 ) was also decreased with an increase of difference pressure. However, the effect of pressure difference on PN2 was very slight, to same extent, which indicated that PN2 of the PDMS crosslinked membrane was almost independent on the pressure differences. Similarly, it could be seen that separation factor (αO2 /N2 ) was decreased obviously with an increase of pressure difference. On the one hand, PO2 was decreased obviously
Fig. 4. Effect of pressure differences and temperature on PO2 , PN2 and αO2 /N2 for the PDMS membrane containing 10 wt% cobalt salt at 20 ◦ C (䊉), 30 ◦ C (), 40 ◦ C () and 50 ◦ C (); for the PDMS membrane without cobalt salt at 20 ◦ C ().
and PN2 was decreased slightly with an increase of pressure difference, which resulted in a decrease of αO2 /N2 obviously with an increase of pressure difference. On the other hand, the change was related to transportation mechanism of polymeric membrane for gas. The gas permeation behavior was not only in accord with the general dissolution–diffusion mechanism of Henry’ type dissolution, but also with transportation mechanism of Langmuir adsorption. In fact, a macromolecule cobalt complex, which had reversible adsorption for oxygen molecule and was benefited to permeate through the membrane, was formed between cobalt ion and carboxyl. Apparently, the improvement in the oxygen-enriching properties of the crosslinked membrane should be attributed to ionomer structures formed incorporating carboxylic cobalt groups. The special behavior of oxygen permeation through the crosslinked membrane corresponded to membranes containing cobalt complexes as a fixed oxygen carrier. However, it could be seen form Fig. 4 that PO2 and αO2 /N2 of the crosslinked membrane containing cobalt salt were higher than these of the crosslinked membrane without cobalt salt at 20 ◦ C. The results showed that cobalt ion was benefit for oxygen-enriching property of PDMS membrane. In our previous study [2], a silicone rubber containing cobalt ion used by eleven alkene acid was prepared and oxygenenriching property of the membrane was studied. Under pressure difference of 0.05 MPa and 20 ◦ C, PO2 and αO2 /N2 was 750 Barrer and 3.09, respectively. However, at the same testing condition, for the PDMS crosslinked membrane containing cobalt ion used by vinyl acetic acid (PO2 and αO2 /N2 was 690 Barrer and 3.42, respectively), PO2 was decreased slightly, but αO2 /N2 was increased obviously. In fact, the silicone rubber membranes containing cobaltous ions were turned into an ionomer with a crosslinked structures. There had been some controversies in ionomer investigations about the morphologies of ion aggregates. The FTIR studied of Han and Williams [19] proved that
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ionomers containing carboxylic groups and transitional metal ions tended to form a polymeric complex, which was based on poly (ethylene-co-methacrylic acid). The silicone rubber membranes containing carboxylic cobalt, in another point of view, also validated the above result because, only when polymeric cobalt complexes were formed, could the oxygen-enriching property enhanced. Of course, the molecular chain conformation of the PDMS crosslinked membrane was changed due to an addition of cobalt ion into the membrane, which leaded to increase dissolution permselectivity of gas molecular for the membrane and permselectivity of the crosslinked membrane. In addition, because carbon chain of eleven alkene acid was longer and its space steric hindrance was larger, the capability that eleven alkene acid cobalt dissolved in PDMS membrane containing cobalt was limited and the content of eleven alkene acid cobalt was only 5 wt%. However, the content of vinyl acetic acid cobalt in the PDMS crosslinked membrane containing cobalt was up to 10 wt%. 4.2. The effect of testing temperature on oxygen-enriching properties The effect of testing temperature on PO2 , PN2 and αO2 /N2 of the crosslinked membrane containing 10 wt% vinyl acetic acid cobalt was shown in Fig. 4. Referring to Fig. 4, it could be seen from the figure that PO2 and PN2 were increased with an increase of testing temperature under the same pressure difference. For example, under the same pressure difference of 0.05 MPa, PO2 was increased from 690 Barrer (20 ◦ C) to 744 Barrer (50 ◦ C). The effect of temperature on oxygen-enriching properties of the membrane was in accord with the general role of gas separation membrane. By contrast, αO2 /N2 was decreased with an increase of testing temperature. Under the above condition, αO2 /N2 of the PDMS crosslinked membrane was decreased from 3.42 (20 ◦ C) to 3.02 (50 ◦ C), which were higher than those of general modified silicone rubber membrane (αO2 /N2 was 2.0 or so). Stern et al. [4] demonstrated that the effects temperature on permeability coefficients depended on changes in the diffusion coefficient and the solubility coefficient when they investigated structure–permeability relationships for different silicone polymers. The diffusion coefficient always increased with increasing temperature, whereas for the solubility coefficient it was generally the opposite. The gas permeability enhancement was basically a result of the increase in the diffusion ability of the gases in the PDMS membrane when the temperature was raised. Hence, the solubility behavior of the gas for the membrane became an important control factor for the gas permeation process here. In conclusion, the effect of temperature on oxygen-enriching properties of the membrane was in accord with the general role of gas separation membrane. 4.3. The effect of vinyl acetic acid cobalt content on oxygen-enriching properties The effect of vinyl acetic acid cobalt content in the PDMS crosslinked membrane on oxygen-enriching properties was
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Fig. 5. Effect of different Co(II) salt contents in the crosslinked membrane on oxygen-enriching properties.
investigated further. The results of the previously considerable works [2,4] revealed that cobaltous complex as an oxygen carrier added into the PDMS crosslinked membrane had facilitated transport for oxygen molecule. The above-mentioned “inverse” permeability and permselevitity relationship, which is so often mentioned in the membrane literature, was hold by many investigators to be almost an immutable law of nature, that is, an increase in permeability are attended by losses in permselevitity, vice versa. To enhance the oxygen-enriching property of the crosslinked membrane, larger amounts of the vinyl acetic acid cobalt were incorporated into the membrane. Fig. 5 presented the effect of different vinyl acid cobalt contents in the membrane on oxygen-enriching properties under pressure difference of 0.05 MPa and 20 ◦ C. For the PDMS crosslinked membrane containing cobalt ion, a simultaneous increase in both PO2 and αO2 /N2 with a decrease in pressure difference, exhibited a typical characteristic of the facilitated transport membranes with fixed oxygen carrier. Compared with general modified PDMS membrane, with an increase of vinyl acetic acid cobalt contents in the crosslinked membrane, molecular free space in the membrane was decreased and space hindrance was increased, which leaded to simultaneous increase in both PO2 and αO2 /N2 . Clearly, the PDMS crosslinked membrane with proper vinyl acetic acid cobalt contents exhibited better oxygen-enriching properties. For example, when vinyl acetic acid cobalt content was 10 wt% in the membrane, the appearance was uniform and the intensity of the membrane was highest. Under pressure difference of 0.05 MPa and 20 ◦ C, PO2 and αO2 /N2 of the crosslinked membrane were 690 Barrer and 3.42, respectively, which was higher than general modified PDMS membrane (PO2 and αO2 /N2 were 548 Barrer and 2.04, respectively). The uniformity, flexibility and mechanical property of the crosslinked membrane, however, lessened when vinyl acetic acid cobalt content was increased to more than 10 wt% in the membrane. As mentioned above, the cobalt ions in the crosslinked membrane play two important roles in improving the selectivity of the membrane: the formation of crosslinked networks and the polymeric cobalt complex. The more carboxylic cobalt content
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in the crosslinked membrane, the more polymeric complexes like the oxygen carriers were formed. 5. Conclusions To enhance oxygen-enriching properties of PDMS, 2,4,6,8tetramethylcyclotetrasiloxane with a cyclic structure was used as a crosslinking agent to enlarge the space between molecular chains. The vinyl acetic acid cobalt as an oxygen carrier was incorporated into the crosslinked membrane to enhance the facilitated transport for oxygen. Under pressure difference of 0.05 MPa and 20 ◦ C, when vinyl acetic acid cobalt content was 10 wt% in the membrane, PO2 and αO2 /N2 of the crosslinked membrane were 690 Barrer and 3.42, respectively. PO2 and αO2 /N2 of the crosslinked membrane were increased simultaneously and the oxygen-enriching properties was close to Robeson’s critical “upper bound” [20]. A simultaneous increase in both PO2 and αO2 /N2 with an increase of vinyl acetic acid cobalt contents in the crosslinked membrane and vinyl acetic acid cobalt content was increased to 10 wt% in the membrane. In other words, the above experimental data and physicochemical implications suggested the possibility that many new polymers having both higher permeability and permselevitity could be designed by using crosslinked method and incorporating cobalt ion into the polymers. Acknowledgement This work was partly supported by the natural science fund of Guangdong Province, China (No. 04010430). References [1] T. Aoki, Macromolecular design of permselective membranes, Prog. Polym. Sci. 24 (1999) 951.
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