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Organic–inorganic polymer hybrids based on unsaturated polyester
LETTER TO THE EDITOR
Journal of Non-Crystalline Solids 311 (2002) 195–198 www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Organic–inorganic polymer hybrids based on unsaturated polyester C.-M. Chung
a,*
, S.-J. Lee a, J.-G. Kim b, D.-O. Jang
a
a
b
Department of Chemistry, Yonsei University, Wonju, Kangwon-do 220-710, South Korea Biotechnology Division, Hanwha Chemical R&D Center, 6 Shinsung-dong, Yusung-ku, Taejon 305-345, South Korea Received 4 February 2002; received in revised form 12 July 2002
Abstract Organic–inorganic polymer hybrid materials were first prepared using an unsaturated polyester (UPE) as an organic component. Homogeneous UPE/silica gel hybrids formed via p–p interactions between benzene rings of the two constituents. Photocrosslinking of UPE in the hybrids resulted in the formation of interpenetrating polymer network structure. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Unsaturated polyesters (UPEs) are one of the most important commercial thermosetting resins and are used in a variety of applications due to low cost, ease of handling, and good performance properties. [1]. The unsaturation in the UPE backbone renders sites for crosslinking reaction which is initiated by free radical initiators, thereby resulting in the formation of a three-dimensional network. Approaches to improved UPE materials have been mainly focused on the combination of UPEs with various inorganic powders or fibers. On the other hand, organic–inorganic polymer hybrid materials have received a great deal of at* Corresponding author. Tel.: +82-33 760 2266; fax: +82-33 760 2182. E-mail address: [email protected] (C.-M. Chung).
tention in recent years. The hybrids have organic and inorganic elements that are mixed in a molecular level, and the intimate mixing provides various properties that are different from those of traditional composites [2]. Sol–gel reactions of alkoxysilanes are usually used to prepare the polymer hybrids. Tetramethoxysilane (TMOS), for example, is hydrolyzed and condensed in the presence of an organic polymer, rendering highly cross-linked inorganic networks containing homogeneously dispersed organic polymer molecules. In this work, the preparation of homogeneous polymer hybrids composed of a UPE and an inorganic polymer was first explored. In order to synthesize the hybrids, an attempt has been made to utilize p–p interactions between the two constituents [3]. For the UPE–silica gel hybrids obtained, photocrosslinking of UPE was carried out to obtain an interpenetrating polymer network (IPN) in which both the organic and inorganic
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 1 8 0 0 - 8
LETTER TO THE EDITOR
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polymers have physical interlocking between them [4].
2. Experimental Maleic anhydride (MA) and ethylene glycol (EG) were fed in a molar ratio of 1:1 and condensation polymerization was carried out at 195 °C for 20 h, and then at 215 °C for 2.5 h to obtain PME. Another polyester PMPE was prepared in a similar manner using a molar ratio of MA:phthalic anhydride (PA):EG of 1:1:2. 1 H NMR spectra of the UPEs were taken on a Varian Gemini 300MHz spectrometer in methyl sulfoxide-d6 using tetramethylsilane as an internal standard. An alkoxysilane was added to an acetone solution of a UPE and benzoin methyl ether, and then aqueous HCl was added as catalyst to the resulting solution. The benzoin methyl ether, a typical photoinitiator in free radical photocure systems [5], was employed to initiate photocrosslinking. The mixture was heated at 60 °C for one week. The hybrids obtained were dried and exposed to UV light to initiate crosslinking of UPE. Exposure of the hybrid samples was made on an exposure system of Spectra Energy Co. equipped with a 500 W high-pressure mercury lamp (light intensity: 72 mW/cm2 ). The measurement of crosslinking conversion was carried out with a Genisis FT-IR spectrophotometer (Mattson Instrument Co.) by a KBr-pellet technique. The ratios of calculated areas of the two absorption bands (1644 cm 1 for C@C and 1724 cm 1 for C@O) before and after exposure were compared to determine the degree of conversion of the C@C bonds [6]. The absorption band at 1724 cm 1 was used as an internal standard for the conversion determination. Each hybrid sample, unexposed or exposed, was ground in a mortar, and then was stirred in methanol for preliminary examination of its solvent-resistant property.
cal structures of the polyesters were characterized by 1 H nuclear magnetic resonance (NMR) spectral analysis and infrared (IR) spectroscopy. The number average molecular weight of the polymers was measured to be 700–1400 (Table 1) and the polydispersity index ranged from 2.0 to 3.7. Three kinds of alkoxysilanes were employed as precursors of silica gel (Fig. 2). The sol–gel reactions of the alkoxysilanes were carried out at 60 °C using HCl catalyst in the presence of a UPE. It was confirmed by NMR spectroscopy that the UPEs did not undergo hydrolysis under the reaction conditions. Optical transparency was used as a criterion for the formation of a homogeneous molecular hybrid composed of the organic and inorganic constituents [7,8]. As shown in Table 1, transparent hybrid could not be obtained from the combination of PME and any of the three alkoxysilanes (runs 1–6). In the case of PMPE, the formation of hybrid was not observed when TMOS or MeTMOS was used (runs 7–10). It was considered that the UPEs have a strong self-association tendency due to carboxyl and hydroxyl end groups capable of self-hydrogen bonding [9,10]. The self-aggregation might result in the phase-separated, opaque materials. Actually the IR spectra of PME and PMPE did not show
3. Results and discussion Two UPEs, PME and PMPE, were prepared by condensation polymerization (Fig. 1). The chemi-
Fig. 1. Synthesis of unsaturated polyesters.
LETTER TO THE EDITOR
C.-M. Chung et al. / Journal of Non-Crystalline Solids 311 (2002) 195–198
197
Table 1 Organic–inorganic polymer hybrids from UPEsa Run
UPE
MWb
Alkoxysilane
Ratioc
Appearance
1 2 3 4 5 6
PME PME PME PME PME PME
958 958 958 958 958 958
TMOS TMOS MeTMOS MeTMOS PhTMOS PhTMOS
1/4 1/20 1/4 1/20 1/4 1/20
Turbid Turbid Turbid Turbid Turbid Turbid
7 8 9 10 11 12 13 14
PMPE PMPE PMPE PMPE PMPE PMPE PMPE PMPE
1364 1364 1364 1364 1364 1364 925 713
TMOS TMOS MeTMOS MeTMOS PhTMOS PhTMOS PhTMOS PhTMOS
1/4 1/20 1/4 1/20 1/4 1/20 1/4 1/4
Turbid Turbid Turbid Turbid Transparent Transparent Transparent Transparent
a
Alkoxysilane 0.50 g, acetone 5 ml, aqueous HCl 2 M, water eq. to methoxy groups of alkoxysilane. Number average molecular weight of UPE. c Feed ratio of UPE to alkoxysilane (w/w). b
Fig. 2. Structures of alkoxysilanes.
any carboxyl (or hydroxyl) absorption free from hydrogen bonding: characteristic absorption due to free carboxyl or hydroxyl groups was not observed above 3500 cm 1 [11]. It was also reported that organic–inorganic polymer hybrid could not be prepared with certain organic polymers capable of strong self-hydrogen bonding such as poly(acrylic acid) [7]. However, homogeneous polymer hybrids were successfully prepared when PMPE was combined with PhTMOS (runs 11–14). This implies that p–p interactions between benzene rings of PMPE and those of PhTMOS might play an important role for the homogeneity. It is quite interesting that the hybrid formation was accomplished via p–p interactions despite that the PMPE molecules have a strong self-association tendency due to hydrogen bonding interactions within them. Most organic–inorganic polymer hybrids have been reported to form through hydrogen bonding interactions or covalent bonding between the two constituents. On the other hand, p–p interactions
have been utilized in only a few cases, for example, in order to prepare polystyrene and silica gel polymer hybrids [3]. It was difficult to obtain the spectroscopic evidence for hydrogen bonding interactions between PMPE and silica gel in this study. Also, covalent bond formation by the reaction of carboxyl (or hydroxyl) end groups of PMPE with silanol groups of silica gel is not likely to occur because the resulting bonds are very unstable under the usual sol–gel reaction conditions [12]. Photocrosslinking behavior of the PMPE/silica gel hybrid was investigated by FT-IR spectroscopy. The absorption at 1644 cm 1 representing the C@C stretching vibration gradually decreased with increasing exposure time, indicating the photocrosslinking of PMPE. The double bond conversion was measured using a previously reported method [6]. As shown in Fig. 3, the PMPE hybrids revealed a conversion of 37% when irradiated for 100 s. The photocrosslinking of the hybrid resulted in the formation of an IPN structure that is a combination of PMPE and silica gel in network form. After crosslinking, the PMPE/ silica gel hybrid showed greatly enhanced solvent resistance compared to the semi-IPN hybrid before photoirradiation, supporting the formation of the IPN structure [13]. Mechanical, thermal, and
LETTER TO THE EDITOR
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C.-M. Chung et al. / Journal of Non-Crystalline Solids 311 (2002) 195–198
Acknowledgement This work was supported by Korea Research Foundation Grant (KRF-2000-015-DP0268).
References
Fig. 3. Plot of conversions of a PMPE/silica gel hybrid (run 11 in Table 1) vs. exposure time.
chemical properties of the PMPE-based IPN polymer hybrids are currently under investigation, and the results will be described elsewhere. 4. Conclusion Organic–inorganic polymer hybrids based on unsaturated polyester were successfully prepared via p–p interactions between benzene rings of PMPE and those of silica gel. The p–p interactions were very effective for the formation of molecular hybrids despite the strong self- hydrogen bonding interactions between PMPE molecules. The IPN structure of PMPE/silica gel was achieved by photocrosslinking of PMPE in the polymer hybrids.
[1] G. Gunduz, in: J.C. Salamone (Ed.), Polymeric Materials Encyclopedia, CRC, Boca Raton, 1996, p. 8469, 8489. [2] B.M. Novak, Adv. Mater. 5 (1993) 422. [3] R. Tamaki, K. Samura, Y. Chujo, Chem. Commun. (1998) 1131. [4] L.H. Sperling, V. Mishra, Polym. Adv. Technol. 7 (1996) 197. [5] A. Ledwith, in: J.M. Kelly, C.B. McArdle, M.J.F. Maunder (Eds.), Photochemistry and Polymeric Systems, Royal Society of Chemistry, Cambridge, 1993, p. 1. [6] C.-M. Chung, J.-G. Kim, J.-H. Choi, J. Appl. Polym. Sci. 77 (2000) 1802. [7] C.J.T. Landry, B.K. Coltrain, J.A. Wesson, N. Zumbulyadis, J.L. Lippert, Polymer 33 (1992) 1496. [8] Q. Hu, E. Marand, Polymer 40 (1999) 4833. [9] H. Zheng, S. Zheng, Q. Guo, J. Polym. Sci. Polym. Chem. 35 (1997) 3169. [10] K. Demmler, G. Gunzler, Makromol. Chem. 90 (1966) 119. [11] E. Pretsch, P. Buhlmann, C. Affolter, Structure Determination of Organic Compounds, Springer, Berlin, 2000. [12] S. Kohjiya, K. Ochiai, S. Yamashita, J. Non-Cryst. Solids 119 (1990) 132. [13] Y. Imai, K. Naka, Y. Chujo, Polym. J. 30 (1998) 990.
Journal of Non-Crystalline Solids 311 (2002) 195–198 www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Organic–inorganic polymer hybrids based on unsaturated polyester C.-M. Chung
a,*
, S.-J. Lee a, J.-G. Kim b, D.-O. Jang
a
a
b
Department of Chemistry, Yonsei University, Wonju, Kangwon-do 220-710, South Korea Biotechnology Division, Hanwha Chemical R&D Center, 6 Shinsung-dong, Yusung-ku, Taejon 305-345, South Korea Received 4 February 2002; received in revised form 12 July 2002
Abstract Organic–inorganic polymer hybrid materials were first prepared using an unsaturated polyester (UPE) as an organic component. Homogeneous UPE/silica gel hybrids formed via p–p interactions between benzene rings of the two constituents. Photocrosslinking of UPE in the hybrids resulted in the formation of interpenetrating polymer network structure. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Unsaturated polyesters (UPEs) are one of the most important commercial thermosetting resins and are used in a variety of applications due to low cost, ease of handling, and good performance properties. [1]. The unsaturation in the UPE backbone renders sites for crosslinking reaction which is initiated by free radical initiators, thereby resulting in the formation of a three-dimensional network. Approaches to improved UPE materials have been mainly focused on the combination of UPEs with various inorganic powders or fibers. On the other hand, organic–inorganic polymer hybrid materials have received a great deal of at* Corresponding author. Tel.: +82-33 760 2266; fax: +82-33 760 2182. E-mail address: [email protected] (C.-M. Chung).
tention in recent years. The hybrids have organic and inorganic elements that are mixed in a molecular level, and the intimate mixing provides various properties that are different from those of traditional composites [2]. Sol–gel reactions of alkoxysilanes are usually used to prepare the polymer hybrids. Tetramethoxysilane (TMOS), for example, is hydrolyzed and condensed in the presence of an organic polymer, rendering highly cross-linked inorganic networks containing homogeneously dispersed organic polymer molecules. In this work, the preparation of homogeneous polymer hybrids composed of a UPE and an inorganic polymer was first explored. In order to synthesize the hybrids, an attempt has been made to utilize p–p interactions between the two constituents [3]. For the UPE–silica gel hybrids obtained, photocrosslinking of UPE was carried out to obtain an interpenetrating polymer network (IPN) in which both the organic and inorganic
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 1 8 0 0 - 8
LETTER TO THE EDITOR
196
C.-M. Chung et al. / Journal of Non-Crystalline Solids 311 (2002) 195–198
polymers have physical interlocking between them [4].
2. Experimental Maleic anhydride (MA) and ethylene glycol (EG) were fed in a molar ratio of 1:1 and condensation polymerization was carried out at 195 °C for 20 h, and then at 215 °C for 2.5 h to obtain PME. Another polyester PMPE was prepared in a similar manner using a molar ratio of MA:phthalic anhydride (PA):EG of 1:1:2. 1 H NMR spectra of the UPEs were taken on a Varian Gemini 300MHz spectrometer in methyl sulfoxide-d6 using tetramethylsilane as an internal standard. An alkoxysilane was added to an acetone solution of a UPE and benzoin methyl ether, and then aqueous HCl was added as catalyst to the resulting solution. The benzoin methyl ether, a typical photoinitiator in free radical photocure systems [5], was employed to initiate photocrosslinking. The mixture was heated at 60 °C for one week. The hybrids obtained were dried and exposed to UV light to initiate crosslinking of UPE. Exposure of the hybrid samples was made on an exposure system of Spectra Energy Co. equipped with a 500 W high-pressure mercury lamp (light intensity: 72 mW/cm2 ). The measurement of crosslinking conversion was carried out with a Genisis FT-IR spectrophotometer (Mattson Instrument Co.) by a KBr-pellet technique. The ratios of calculated areas of the two absorption bands (1644 cm 1 for C@C and 1724 cm 1 for C@O) before and after exposure were compared to determine the degree of conversion of the C@C bonds [6]. The absorption band at 1724 cm 1 was used as an internal standard for the conversion determination. Each hybrid sample, unexposed or exposed, was ground in a mortar, and then was stirred in methanol for preliminary examination of its solvent-resistant property.
cal structures of the polyesters were characterized by 1 H nuclear magnetic resonance (NMR) spectral analysis and infrared (IR) spectroscopy. The number average molecular weight of the polymers was measured to be 700–1400 (Table 1) and the polydispersity index ranged from 2.0 to 3.7. Three kinds of alkoxysilanes were employed as precursors of silica gel (Fig. 2). The sol–gel reactions of the alkoxysilanes were carried out at 60 °C using HCl catalyst in the presence of a UPE. It was confirmed by NMR spectroscopy that the UPEs did not undergo hydrolysis under the reaction conditions. Optical transparency was used as a criterion for the formation of a homogeneous molecular hybrid composed of the organic and inorganic constituents [7,8]. As shown in Table 1, transparent hybrid could not be obtained from the combination of PME and any of the three alkoxysilanes (runs 1–6). In the case of PMPE, the formation of hybrid was not observed when TMOS or MeTMOS was used (runs 7–10). It was considered that the UPEs have a strong self-association tendency due to carboxyl and hydroxyl end groups capable of self-hydrogen bonding [9,10]. The self-aggregation might result in the phase-separated, opaque materials. Actually the IR spectra of PME and PMPE did not show
3. Results and discussion Two UPEs, PME and PMPE, were prepared by condensation polymerization (Fig. 1). The chemi-
Fig. 1. Synthesis of unsaturated polyesters.
LETTER TO THE EDITOR
C.-M. Chung et al. / Journal of Non-Crystalline Solids 311 (2002) 195–198
197
Table 1 Organic–inorganic polymer hybrids from UPEsa Run
UPE
MWb
Alkoxysilane
Ratioc
Appearance
1 2 3 4 5 6
PME PME PME PME PME PME
958 958 958 958 958 958
TMOS TMOS MeTMOS MeTMOS PhTMOS PhTMOS
1/4 1/20 1/4 1/20 1/4 1/20
Turbid Turbid Turbid Turbid Turbid Turbid
7 8 9 10 11 12 13 14
PMPE PMPE PMPE PMPE PMPE PMPE PMPE PMPE
1364 1364 1364 1364 1364 1364 925 713
TMOS TMOS MeTMOS MeTMOS PhTMOS PhTMOS PhTMOS PhTMOS
1/4 1/20 1/4 1/20 1/4 1/20 1/4 1/4
Turbid Turbid Turbid Turbid Transparent Transparent Transparent Transparent
a
Alkoxysilane 0.50 g, acetone 5 ml, aqueous HCl 2 M, water eq. to methoxy groups of alkoxysilane. Number average molecular weight of UPE. c Feed ratio of UPE to alkoxysilane (w/w). b
Fig. 2. Structures of alkoxysilanes.
any carboxyl (or hydroxyl) absorption free from hydrogen bonding: characteristic absorption due to free carboxyl or hydroxyl groups was not observed above 3500 cm 1 [11]. It was also reported that organic–inorganic polymer hybrid could not be prepared with certain organic polymers capable of strong self-hydrogen bonding such as poly(acrylic acid) [7]. However, homogeneous polymer hybrids were successfully prepared when PMPE was combined with PhTMOS (runs 11–14). This implies that p–p interactions between benzene rings of PMPE and those of PhTMOS might play an important role for the homogeneity. It is quite interesting that the hybrid formation was accomplished via p–p interactions despite that the PMPE molecules have a strong self-association tendency due to hydrogen bonding interactions within them. Most organic–inorganic polymer hybrids have been reported to form through hydrogen bonding interactions or covalent bonding between the two constituents. On the other hand, p–p interactions
have been utilized in only a few cases, for example, in order to prepare polystyrene and silica gel polymer hybrids [3]. It was difficult to obtain the spectroscopic evidence for hydrogen bonding interactions between PMPE and silica gel in this study. Also, covalent bond formation by the reaction of carboxyl (or hydroxyl) end groups of PMPE with silanol groups of silica gel is not likely to occur because the resulting bonds are very unstable under the usual sol–gel reaction conditions [12]. Photocrosslinking behavior of the PMPE/silica gel hybrid was investigated by FT-IR spectroscopy. The absorption at 1644 cm 1 representing the C@C stretching vibration gradually decreased with increasing exposure time, indicating the photocrosslinking of PMPE. The double bond conversion was measured using a previously reported method [6]. As shown in Fig. 3, the PMPE hybrids revealed a conversion of 37% when irradiated for 100 s. The photocrosslinking of the hybrid resulted in the formation of an IPN structure that is a combination of PMPE and silica gel in network form. After crosslinking, the PMPE/ silica gel hybrid showed greatly enhanced solvent resistance compared to the semi-IPN hybrid before photoirradiation, supporting the formation of the IPN structure [13]. Mechanical, thermal, and
LETTER TO THE EDITOR
198
C.-M. Chung et al. / Journal of Non-Crystalline Solids 311 (2002) 195–198
Acknowledgement This work was supported by Korea Research Foundation Grant (KRF-2000-015-DP0268).
References
Fig. 3. Plot of conversions of a PMPE/silica gel hybrid (run 11 in Table 1) vs. exposure time.
chemical properties of the PMPE-based IPN polymer hybrids are currently under investigation, and the results will be described elsewhere. 4. Conclusion Organic–inorganic polymer hybrids based on unsaturated polyester were successfully prepared via p–p interactions between benzene rings of PMPE and those of silica gel. The p–p interactions were very effective for the formation of molecular hybrids despite the strong self- hydrogen bonding interactions between PMPE molecules. The IPN structure of PMPE/silica gel was achieved by photocrosslinking of PMPE in the polymer hybrids.
[1] G. Gunduz, in: J.C. Salamone (Ed.), Polymeric Materials Encyclopedia, CRC, Boca Raton, 1996, p. 8469, 8489. [2] B.M. Novak, Adv. Mater. 5 (1993) 422. [3] R. Tamaki, K. Samura, Y. Chujo, Chem. Commun. (1998) 1131. [4] L.H. Sperling, V. Mishra, Polym. Adv. Technol. 7 (1996) 197. [5] A. Ledwith, in: J.M. Kelly, C.B. McArdle, M.J.F. Maunder (Eds.), Photochemistry and Polymeric Systems, Royal Society of Chemistry, Cambridge, 1993, p. 1. [6] C.-M. Chung, J.-G. Kim, J.-H. Choi, J. Appl. Polym. Sci. 77 (2000) 1802. [7] C.J.T. Landry, B.K. Coltrain, J.A. Wesson, N. Zumbulyadis, J.L. Lippert, Polymer 33 (1992) 1496. [8] Q. Hu, E. Marand, Polymer 40 (1999) 4833. [9] H. Zheng, S. Zheng, Q. Guo, J. Polym. Sci. Polym. Chem. 35 (1997) 3169. [10] K. Demmler, G. Gunzler, Makromol. Chem. 90 (1966) 119. [11] E. Pretsch, P. Buhlmann, C. Affolter, Structure Determination of Organic Compounds, Springer, Berlin, 2000. [12] S. Kohjiya, K. Ochiai, S. Yamashita, J. Non-Cryst. Solids 119 (1990) 132. [13] Y. Imai, K. Naka, Y. Chujo, Polym. J. 30 (1998) 990.