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Preparation and characterization of polystyrene-based monolith with ordered macroporous structure
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Chinese Chemical Letters 23 (2012) 474–477 www.elsevier.com/locate/cclet
Preparation and characterization of polystyrene-based monolith with ordered macroporous structure Quan Zhou Wu *, Jian Feng He, Ji Ming Ou College of Chinese Materia Medica, Guangzhou University of Chinese Medicine, Guangzhou 510006, China Received 10 November 2011 Available online 3 March 2012
Abstract In this paper, polystyrene-based monoliths with highly ordered macroporous structure were synthesized by using SiO2 colloidal crystal as template. SEM observation shows that the macropores are highly ordered and are interconnected by small windows. The BET surface area of PS monolith is about 36.17 m2/g. The polymer monoliths can resist 5 MPa pressure, showing high mechanical and compressive strength. # 2012 Quan Zhou Wu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Monolith; Polystyrene; Polymer; Ordered macropore
Monoliths have attracted great research interests because of their multimodal porous structure and their potential applications in catalysis, adsorption, chromatographic separation and so on [1]. However, up to now, because the porogens which were used to form pore structure have not been assembled, the monoliths have irregular pore structure. Thus, the substances diffused in the monolith will have complex pathway, and the adsorption and separation efficiency will be limited. Scientists predicted that monoliths with ordered pore structure would have higher column efficiency when monoliths were used as HPLC columns [2,3]. Therefore, to fabricate ordered porous structure monolith is an urgent and important issue whether consideration from theoretical or experimental aspect. However, until recent literatures, monolith with ordered macroporous structure seldom reported except by Fan and Sun et al. [4,5]. Fan et al. prepared pore-connected macroporous polystyrene monolith by using Man/VAc copolymer as template. Recently, Sun et al. prepared hierarchically ordered macro-/mesoporous silica monolith by using poly(ST-co-TMSPM) colloidal crystal as template. In present work, we pay attention to exploit a flexible method to prepare polymer monolith with three-dimensionally ordered macroporous (3DOM) structure by a colloidal crystal templating method. 1. Experimental Monodisperse silica beads were produced according to the literature [6]. Silica monolith template was obtained by centrifugating silica beads in a 1 mL-syringe. After calcined at 800 8C for 8 h, the silica monolith was used as template to fabricate polymer monolith. * Corresponding author. E-mail address: [email protected] (Q.Z. Wu). 1001-8417/$ – see front matter # 2012 Quan Zhou Wu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2012.01.037
Q.Z. Wu et al. / Chinese Chemical Letters 23 (2012) 474–477
475
Styrene (St) and methyl acrylate (MAA) were purified via vacuum distillation. To prepare polystyrene (PS), PS/ DVB and PS/PMAA monoliths, precursor solutions were prepared by mixing St, MAA and divinyl-benzene (DVB) monomers with different molar ratio and dissolving about 1% (wt%) benzoperoxide in it. Then the silica template was put into the precursor solution. Polymerization was carried out at 60–80 8C for 6 h under nitrogen atmosphere. The silica template was removed from composites by etching with aqueous hydrofluoric acid. The obtained 3DOM polymer monolith was then washed by ethanol and water several times, respectively. Finally, it was dried under vacuum. The density (r) of monolith was calculated by the following equation:
r¼
W AL
(1)
where W, A and L are the weight, sectional area and height of the monolith, respectively. To determine the unidirectional compressive strength of the monolith, cylinder-like samples with diameter of about 4 mm were prepared, and the bulk polymer polymerized outside the template was peeled off. The compressive strength was tested by a pressure tester. The compression ratio (e%) was calculated by the following equation:
e% ¼
ðL0 LÞ 100 L0
(2)
where L0 is the initial height of the monolith, and L is the height of the monolith under a certain pressure. Pore structure features of 3DOM polymer monolith were characterized by a Quanta 400 thermal FE environment scanning electron microscope (SEM) or a JSM-6330F field emission scanning electron microscope. A Bruker EQUINOX 55 Fourier transformation infra-red spectrometer (FTIR) was used to analyze the wall composition characteristics. BET surface area was measured by a Micromeritics ASAP 2020 apparatus following the Brunauer– Emmett–Teller (BET) procedure.
Fig. 1. SEM images of SiO2 template and polymer monoliths: (a) SiO2 template, (b) PS, (c) PS/DVB, (d) PS/PMAA.
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Q.Z. Wu et al. / Chinese Chemical Letters 23 (2012) 474–477
2. Results and discussion Fig. 1 shows the SEM image of SiO2 template. It shows that monodisperse silica beads can be assembled into ordered periodic structure by centrifugation. Usually, it has a face-centered cubic (fcc) structure [7]. The diameter of the SiO2 beads measured from the SEM image is about 234 nm. Fig. 2 shows the pictures of PS monolith. Its surface is sealed with a compact and smooth polymer film. However, imperfect monolith was formed if the amount of precursor solution is too low or the polymerization temperature is too high (>80 8C). And the polymer monolith cannot be formed if polymerization temperature is below 60 8C. Besides, for PS/PMMA monolith, if molar ratio of St to MAA is below 1:2, the monolith will be distorted when dissolving the SiO2 template. The density of PS, PS/DVB and PS/PMMA is about 0.45, 0.39 and 0.44 g/cm3, respectively. They are so low that they can float on the water. Fig. 1 shows typical SEM images of PS, PS/DVB and PS/MMA monolith. The SEM observation clearly demonstrates that the materials have highly ordered macroporous structures. We can also see some dark dots in macropores, which are the windows between adjacent macropores, showing the macropores interconnected with each other. Because the SiO2 has an fcc structure, the polymer monoliths obtained from it have fcc structures too. Based on the ideal model of fcc structure, each macropore should be connected to twelve other pores. The macropore diameter measured from the SEM images is about 219 nm. The BET surface area of PS monolith is about 36.17 m2/g. FTIR spectra were used to analyze the wall composition characteristics. For PS monolith, peaks at 540, 696, 753, 902, 1265, 1376, 1450, 1488, 1600, 1720, 1804, 1870, 1943, 2845, 2920, 3022, 3060, and 3083 cm1 are identified as the absorption bands of PS copolymer. For PS/DVB monolith, peaks at 540, 760, 1030, 1072, 1455, 1495, 1600, 2856, 2920, 3062, and 3083 cm1 are identified as the absorption bands of PS/DVB copolymer. And for PS/PMMA monolith, peaks at 538, 760, 1074, 1108, 1200, 1388, 1450, 1495, 1600, 1730, 2850, 2920, and 3029 cm1 are identified as the absorption bands of PS/PMMA copolymer. Strong absorption at 1730 cm1 is attributed to C O bond in ester. The polymer monoliths have high mechanical and compressive strength. Fig. 3 shows the compressive strength of polymer monoliths. The PS/DVB and PS/PMMA monoliths presented in Fig. 3 were produced from precursor solutions with 3:1 molar ratio of styrene to other monomer. All samples were not fractured under the tested pressures. They have higher mechanical strength than inorganic or inorganic/organic hybrid 3DOM materials [8]. 3DOM PS and PS/PMMA monoliths can resist 5 MPa pressure. Above 5 MPa, the monoliths were compressed and the compression ratio was about 30% under 7.5 MPa pressure. For 3DOM PS/DVB monolith, it can resist 2 MPa pressure and it was compressed slightly above 2 MPa, its compression ratio was about 2.8% at 5 MPa. At higher pressure, PS/DVB has lower compression ratio than that of PS and PS/PMMA monoliths. The compression ratio was about 4% and 17% at 7.5 MPa and 17.0 MPa, respectively. This phenomenon can be attributed to the network skeleton structure formed in PS/DVB copolymer because DVB is a cross-linking agent of styrene.
Fig. 2. Pictures of PS monolith.
Q.Z. Wu et al. / Chinese Chemical Letters 23 (2012) 474–477
477
PS/DVB PS PS/PMMA
30 25 20 15 10 5 0 0
2
4
6
8
10
12
14
p/MPa Fig. 3. Curves of compressive strength of polymer monoliths.
3. Conclusion Polystyrene-based monoliths with highly ordered macroporous structure can be synthesized by using SiO2 colloidal crystal monolith as template. SiO2 colloidal crystal monolith can be assembled by centrifugation of monodisperse silica beads in a tube. After filtration of the monomers into the SiO2 monolith, polymers can be produced at 60–80 8C for 6 h under nitrogen atmosphere. Finally, the polystyrene-based monoliths can be obtained after removal of the SiO2 template by HF acid. The obtained monoliths have highly ordered macroporous structure, low density, and high mechanical and compressive strength. This study presents a flexible method to prepare polymer monolith with ordered structure. Acknowledgment This work was financially supported by the Foundation for Distinguished Young Talents in Higher Education of Guangdong, China (No. K5090004). References [1] [2] [3] [4] [5] [6] [7] [8]
O.G. Potter, E.F. Hilder, J. Sep. Sci. 31 (2008) 1881. F. Gritti, G. Guiochon, J. Chromatogr. A 1216 (2009) 4752. N. Vervoort, P. Gzill, G.V. Baron, et al. J. Chromatogr. A 1030 (2001) 177. J.W. Fan, J.Y. Deng, C.M. Xing, et al. J. Polym. Sci. A: Polym. Chem. 44 (2006) 653. Z. Sun, Y. Deng, J. Wei, et al. Chem. Mater. 23 (2011) 2176. S.M. Chang, M. Lee, W. Kim, J. Colloid Interface Sci. 286 (2005) 536. B.T. Holland, C.F. Blanford, T. Do, et al. Chem. Mater. 11 (1999) 795. Q. Wu, J. Liao, Q. Yin, et al. Mater. Res. Bull. 43 (2008) 1209.
Chinese Chemical Letters 23 (2012) 474–477 www.elsevier.com/locate/cclet
Preparation and characterization of polystyrene-based monolith with ordered macroporous structure Quan Zhou Wu *, Jian Feng He, Ji Ming Ou College of Chinese Materia Medica, Guangzhou University of Chinese Medicine, Guangzhou 510006, China Received 10 November 2011 Available online 3 March 2012
Abstract In this paper, polystyrene-based monoliths with highly ordered macroporous structure were synthesized by using SiO2 colloidal crystal as template. SEM observation shows that the macropores are highly ordered and are interconnected by small windows. The BET surface area of PS monolith is about 36.17 m2/g. The polymer monoliths can resist 5 MPa pressure, showing high mechanical and compressive strength. # 2012 Quan Zhou Wu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Monolith; Polystyrene; Polymer; Ordered macropore
Monoliths have attracted great research interests because of their multimodal porous structure and their potential applications in catalysis, adsorption, chromatographic separation and so on [1]. However, up to now, because the porogens which were used to form pore structure have not been assembled, the monoliths have irregular pore structure. Thus, the substances diffused in the monolith will have complex pathway, and the adsorption and separation efficiency will be limited. Scientists predicted that monoliths with ordered pore structure would have higher column efficiency when monoliths were used as HPLC columns [2,3]. Therefore, to fabricate ordered porous structure monolith is an urgent and important issue whether consideration from theoretical or experimental aspect. However, until recent literatures, monolith with ordered macroporous structure seldom reported except by Fan and Sun et al. [4,5]. Fan et al. prepared pore-connected macroporous polystyrene monolith by using Man/VAc copolymer as template. Recently, Sun et al. prepared hierarchically ordered macro-/mesoporous silica monolith by using poly(ST-co-TMSPM) colloidal crystal as template. In present work, we pay attention to exploit a flexible method to prepare polymer monolith with three-dimensionally ordered macroporous (3DOM) structure by a colloidal crystal templating method. 1. Experimental Monodisperse silica beads were produced according to the literature [6]. Silica monolith template was obtained by centrifugating silica beads in a 1 mL-syringe. After calcined at 800 8C for 8 h, the silica monolith was used as template to fabricate polymer monolith. * Corresponding author. E-mail address: [email protected] (Q.Z. Wu). 1001-8417/$ – see front matter # 2012 Quan Zhou Wu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2012.01.037
Q.Z. Wu et al. / Chinese Chemical Letters 23 (2012) 474–477
475
Styrene (St) and methyl acrylate (MAA) were purified via vacuum distillation. To prepare polystyrene (PS), PS/ DVB and PS/PMAA monoliths, precursor solutions were prepared by mixing St, MAA and divinyl-benzene (DVB) monomers with different molar ratio and dissolving about 1% (wt%) benzoperoxide in it. Then the silica template was put into the precursor solution. Polymerization was carried out at 60–80 8C for 6 h under nitrogen atmosphere. The silica template was removed from composites by etching with aqueous hydrofluoric acid. The obtained 3DOM polymer monolith was then washed by ethanol and water several times, respectively. Finally, it was dried under vacuum. The density (r) of monolith was calculated by the following equation:
r¼
W AL
(1)
where W, A and L are the weight, sectional area and height of the monolith, respectively. To determine the unidirectional compressive strength of the monolith, cylinder-like samples with diameter of about 4 mm were prepared, and the bulk polymer polymerized outside the template was peeled off. The compressive strength was tested by a pressure tester. The compression ratio (e%) was calculated by the following equation:
e% ¼
ðL0 LÞ 100 L0
(2)
where L0 is the initial height of the monolith, and L is the height of the monolith under a certain pressure. Pore structure features of 3DOM polymer monolith were characterized by a Quanta 400 thermal FE environment scanning electron microscope (SEM) or a JSM-6330F field emission scanning electron microscope. A Bruker EQUINOX 55 Fourier transformation infra-red spectrometer (FTIR) was used to analyze the wall composition characteristics. BET surface area was measured by a Micromeritics ASAP 2020 apparatus following the Brunauer– Emmett–Teller (BET) procedure.
Fig. 1. SEM images of SiO2 template and polymer monoliths: (a) SiO2 template, (b) PS, (c) PS/DVB, (d) PS/PMAA.
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Q.Z. Wu et al. / Chinese Chemical Letters 23 (2012) 474–477
2. Results and discussion Fig. 1 shows the SEM image of SiO2 template. It shows that monodisperse silica beads can be assembled into ordered periodic structure by centrifugation. Usually, it has a face-centered cubic (fcc) structure [7]. The diameter of the SiO2 beads measured from the SEM image is about 234 nm. Fig. 2 shows the pictures of PS monolith. Its surface is sealed with a compact and smooth polymer film. However, imperfect monolith was formed if the amount of precursor solution is too low or the polymerization temperature is too high (>80 8C). And the polymer monolith cannot be formed if polymerization temperature is below 60 8C. Besides, for PS/PMMA monolith, if molar ratio of St to MAA is below 1:2, the monolith will be distorted when dissolving the SiO2 template. The density of PS, PS/DVB and PS/PMMA is about 0.45, 0.39 and 0.44 g/cm3, respectively. They are so low that they can float on the water. Fig. 1 shows typical SEM images of PS, PS/DVB and PS/MMA monolith. The SEM observation clearly demonstrates that the materials have highly ordered macroporous structures. We can also see some dark dots in macropores, which are the windows between adjacent macropores, showing the macropores interconnected with each other. Because the SiO2 has an fcc structure, the polymer monoliths obtained from it have fcc structures too. Based on the ideal model of fcc structure, each macropore should be connected to twelve other pores. The macropore diameter measured from the SEM images is about 219 nm. The BET surface area of PS monolith is about 36.17 m2/g. FTIR spectra were used to analyze the wall composition characteristics. For PS monolith, peaks at 540, 696, 753, 902, 1265, 1376, 1450, 1488, 1600, 1720, 1804, 1870, 1943, 2845, 2920, 3022, 3060, and 3083 cm1 are identified as the absorption bands of PS copolymer. For PS/DVB monolith, peaks at 540, 760, 1030, 1072, 1455, 1495, 1600, 2856, 2920, 3062, and 3083 cm1 are identified as the absorption bands of PS/DVB copolymer. And for PS/PMMA monolith, peaks at 538, 760, 1074, 1108, 1200, 1388, 1450, 1495, 1600, 1730, 2850, 2920, and 3029 cm1 are identified as the absorption bands of PS/PMMA copolymer. Strong absorption at 1730 cm1 is attributed to C O bond in ester. The polymer monoliths have high mechanical and compressive strength. Fig. 3 shows the compressive strength of polymer monoliths. The PS/DVB and PS/PMMA monoliths presented in Fig. 3 were produced from precursor solutions with 3:1 molar ratio of styrene to other monomer. All samples were not fractured under the tested pressures. They have higher mechanical strength than inorganic or inorganic/organic hybrid 3DOM materials [8]. 3DOM PS and PS/PMMA monoliths can resist 5 MPa pressure. Above 5 MPa, the monoliths were compressed and the compression ratio was about 30% under 7.5 MPa pressure. For 3DOM PS/DVB monolith, it can resist 2 MPa pressure and it was compressed slightly above 2 MPa, its compression ratio was about 2.8% at 5 MPa. At higher pressure, PS/DVB has lower compression ratio than that of PS and PS/PMMA monoliths. The compression ratio was about 4% and 17% at 7.5 MPa and 17.0 MPa, respectively. This phenomenon can be attributed to the network skeleton structure formed in PS/DVB copolymer because DVB is a cross-linking agent of styrene.
Fig. 2. Pictures of PS monolith.
Q.Z. Wu et al. / Chinese Chemical Letters 23 (2012) 474–477
477
PS/DVB PS PS/PMMA
30 25 20 15 10 5 0 0
2
4
6
8
10
12
14
p/MPa Fig. 3. Curves of compressive strength of polymer monoliths.
3. Conclusion Polystyrene-based monoliths with highly ordered macroporous structure can be synthesized by using SiO2 colloidal crystal monolith as template. SiO2 colloidal crystal monolith can be assembled by centrifugation of monodisperse silica beads in a tube. After filtration of the monomers into the SiO2 monolith, polymers can be produced at 60–80 8C for 6 h under nitrogen atmosphere. Finally, the polystyrene-based monoliths can be obtained after removal of the SiO2 template by HF acid. The obtained monoliths have highly ordered macroporous structure, low density, and high mechanical and compressive strength. This study presents a flexible method to prepare polymer monolith with ordered structure. Acknowledgment This work was financially supported by the Foundation for Distinguished Young Talents in Higher Education of Guangdong, China (No. K5090004). References [1] [2] [3] [4] [5] [6] [7] [8]
O.G. Potter, E.F. Hilder, J. Sep. Sci. 31 (2008) 1881. F. Gritti, G. Guiochon, J. Chromatogr. A 1216 (2009) 4752. N. Vervoort, P. Gzill, G.V. Baron, et al. J. Chromatogr. A 1030 (2001) 177. J.W. Fan, J.Y. Deng, C.M. Xing, et al. J. Polym. Sci. A: Polym. Chem. 44 (2006) 653. Z. Sun, Y. Deng, J. Wei, et al. Chem. Mater. 23 (2011) 2176. S.M. Chang, M. Lee, W. Kim, J. Colloid Interface Sci. 286 (2005) 536. B.T. Holland, C.F. Blanford, T. Do, et al. Chem. Mater. 11 (1999) 795. Q. Wu, J. Liao, Q. Yin, et al. Mater. Res. Bull. 43 (2008) 1209.