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Hybrid nanofibrous mats with remarkable solvent and temperature resistance produced by electrospinning technique
Materials Letters 78 (2012) 139–142
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hybrid nanofibrous mats with remarkable solvent and temperature resistance produced by electrospinning technique Yunxia Xu a, Yuquan Wen a,⁎, Yi-nan Wu b, Changxu Lin b, Guangtao Li b a b
State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing, 100081, People's Republic of China Key Lab of Organic Optoelectronic and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing, 100084, People's Republic of China
a r t i c l e
i n f o
Article history: Received 5 October 2011 Accepted 8 March 2012 Available online 16 March 2012 Keywords: Functional Thin films Electrospinning Resistant studies
a b s t r a c t A novel organic/inorganic hybrid copolymer poly[styrene-co-3-(trimethoxysilyl)propyl-methacrylate] [P(St-coTMSPMA)] (denoted as PST) was successfully prepared from styrene and TMSPMA by free-radical polymerization. These composite fiber mats of PST were fabricated via a typical electrospinning process using a single solvent system. As shown in the SEM, the PST composite nanofibers have a smooth and uniform surface and the average diameter is about 0.2± 0.04 μm. After the methoxysilyl groups (Si\O\CH3) were hydrolyzed toward silanol groups (Si\OH), the nanofibrous mats exhibited remarkable solvent and temperature resistance. On account of good chemical and thermal stability, this hybrid mats can provide a useful platform for developing functionalized mat systems. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, electrospinning has been recognized as a simple and efficient approach toward assembling fibrous polymer films composed of fibers with diameters ranging from several micrometers down to tens of nanometers [1]. Nanofibrous membranes have attracted considerable attention due to their remarkable properties (such as very large specific surface area and higher porosity compared to commercial textiles), which make them excellent candidates for separation filters [2], wound dressing materials [3], tissue scaffolds [4], sensors [5], etc. It has been reported that more than 100 different polymers, both synthetic and natural, have been successfully electrospun into nanofibers from polymer solution. Block copolymers can also be electrospun into nanofibers, as shown for example for poly(styrene-b-dimethylsiloxane) diblock copolymers [6] and for styrene–butadiene–styrene triblock copolymers [7]. Recently, to further enhance material properties and thus extend the applicability of such fibers, nanocomposite approaches involving the surface functionalization of electrospun fibers have been attracting considerable attention. However, most electrospun membranes are made from polymers that are soluble in organic or inorganic solvents at certain temperature, for example, PS/DMF-THF, PVP/H2O, PEO/H2O. However, inorganic electrospun membranes derived from SiO2, TiO2, ZnO, etc., are insoluble in most solvents and become fragile after the calcination process. Therefore, the development of a new method for efficient fabrication of electrospun mats with improved solvent and temperature resistance is highly desirable.
⁎ Corresponding author. Tel./Fax: + 86 10 62792905. E-mail addresses: [email protected] (Y. Wen), [email protected] (G. Li). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.03.016
In this paper, we introduce a facile method for improving the solvent and temperature resistance of nanofibrous mats. A novel organic/ inorganic hybrid copolymer, poly[styrene-co-3-(trimethoxysilyl) propyl-methacrylate], was synthesized by free-radical copolymerization. One of the monomer, 3-(trimethoxysilyl)propyl-methacrylate with methoxysilyl groups [\Si(OCH3)3], is commercially available and is widely used in the preparation of organic/inorganic hybrid materials [8–11]. Nanofibrous membranes were prepared by electrospinning followed by hydrolysis in acid condition. The prepared membranes can be resistant to different temperatures and the typical solvents. Based on their remarkable solvent and temperature resistance, these hybrid mats can provide a useful platform for developing functionalized membrane systems for applications involving surface chemistry, drug delivery, and multifunctional textiles. 2. Experimental section 2.1. Materials 3-(trimethoxysilyl)propyl-methacrylate (TMSPMA; 97%, Alfa Aesar) was dried over CaH2 overnight and distilled under reduced pressure. Styrene (99%, Alfa Aesar) was purified according to literature [12]. Anhydrous toluene (Beijing Chemical Reagent Co.) was dried by refluxing in the presence of CaH2 and distilled prior to use. The initiator, α,α′,azodiisobutyronitrile (AIBN), was recrystallized from methanol. 2.2. Synthesis of copolymer PST The copolymer, poly[styrene-co-3-(trimethoxysilyl)propyl-methacrylate] [P(St-co-TMSPMA)], PST, with a random polymerization
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degree was synthesized by free-radical polymerization. For a typical preparation, 3.00 g of styrene, 2.5 g of TMSPMA and 0.0068 g of 2,2-azobisisobutyronitrile (AIBN) were dissolved in anhydrous toluene (2.27 g). The obtained solution was added into a 50 ml three-neck round bottom flask equipped with a magnetic stirrer and a refluxing condenser. The flask was placed in an oil bath at 80 °C. After 8 h, the reaction mixture was removed and slowly cooled to room temperature.
characteristic vibration peaks of Si\O\CH3 at 821 and 1087 cm − 1, which disappear after the hydrolysis and cross-linking reactions (Fig. 1b). The weak peak at 1126 cm − 1 can be attributed to the Si\O\Si bonds [14]. The absorbance peaks assigned to the aromatic C\H stretch appeared at ~ 3050 cm − 1, C\H stretch at ~2900 cm − 1, C_O stretch at ~ 1740 cm − 1, aromatic C\C stretch of the polystyrene at ~ 1480 cm − 1, C\H out-of-plane bend at ~750 cm − 1, and aromatic C\C out-of-plane stretch of the polystyrene at ~ 710 cm − 1.
2.3. Preparation of organic/inorganic hybrid thin films
3.2. SEM analysis
A 50 wt.% copolymer PST solution was prepared at room temperature by dissolving PST in a mixture of THF and DMF (3:1 v/v). The copolymer solution was electrospun using a parallel plate setup [13]. During the electrospinning process, the driving high voltage applied was 12 kV, and the distance between the needle and the aluminum foil collector was 15 cm. A constant flow rate of 0.35 ml/h was obtained using a syringe pump. The electrospun nanofibers were dried and annealed at 60 °C for 8 h. After hydrolysis in acid and condensation treatment, the organic/inorganic hybrid thin films were obtained.
3.2.1. Study on the solvent resistance The as-prepared composite nanofibers fabricated by electrospinning consist of solvents and PST. In order to obtain the crosslinkable nanofibers, the as-prepared fibers were immersed in a 0.5 wt.% HCl aqueous solution for 3 days. The purpose is that the methoxysilyl groups can be hydrolyzed toward silanol groups, and then condensed into cross-linkable polysilsesquioxanes [15]. Fig. 2a shows a typical SEM image of originally prepared electrospun PST nanofibers. The average diameter of the nanofibers is about 0.2 ± 0.04 μm at 50 wt.% copolymer. Compared to the as-prepared composite fibers (Fig. 2a), the morphology and structure of the nanofibers were largely maintained after hydrolysis in acid with only small differences (Fig. 2b). Also, this material can be fabricated in nanofibrous membranes in a large area, as revealed in the inset of Fig. 2a. Generally, most electrospun polymer fibers are destroyed upon exposure to typical solvents or at high temperatures, which limit their applicability in uses such as water treatment, and separation filter. However, the above organic/inorganic hybrid copolymer mat exhibited excellent resistance to some typical solvents. In order to study the effect of various solvents on the structure of the copolymer nanofibers, the cross-linked fibers were immersed in various solvents (DMF, H2O, EtOH, THF, MeOH, CHCl3, toluene, n-hexane) at 60 °C for 3 days. The SEM images are shown in Fig. 2c–j. The morphology of the nanofibers was retained with only slight differences compared to the initial state after immersing in typical polar solvents, DMF, H2O, EtOH, THF, MeOH, as shown in Fig. 2c–g. Whereas, as shown in Fig. 2h–j, the fibers were swollen when immersed in non-polar solvents such as CHCl3, toluene, and n-hexane, however despite swelling the structure of the fibers is still retained. At the same time, it is observed that the stability of the fibers in polar solvents is better than that in non-polar solvents. The reason is probably related to the weak interactions between the polar solvents and the copolymers.
2.4. Stability studies Cross-linked thin films were immersed into different solvents, such as DMF, H2O, EtOH, THF, MeOH, CHCl3, toluene, n-hexane, and at different temperatures, room temperature (denoted as RT), 60 °C, 80 °C, 100 °C, and 120 °C. 2.5. Characterization The morphologies of the fabricated nanofibers at various temperatures and solvents were observed using a field emission scanning electron microscopy (FESEM) on a JEOL JSM-5400 system. The molecular structure of the nanofibers was characterized by Fourier transform infrared (FT-IR, Nicolet 6700). 3. Results and discussions 3.1. IR analysis The FT-IR spectra of electrospun PST fibrous mats are shown in Fig. 1. The copolymer mats before hydrolysis (Fig. 1a) show the
Fig. 1. FT-IR spectra of copolymer mats (a), after hydrolysis and cross-linked in HCl (b).
Y. Xu et al. / Materials Letters 78 (2012) 139–142
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Fig. 2. SEM images of electrospun nanofiber mats: (a) before, (b) after being immersed in HCl for 3 days; immersed in different solvents at 60 °C for 3 days (c) DMF, (d) H2O, (e) EtOH, (f) THF, (g) MeOH, (h) CHCl3, (i) toluene, (j) n-hexane. (All the pictures are in the same scale bar).
The non-polar solvents have stronger interactions and a larger effect on the copolymer, or may even react with a small part of the copolymer, however, this kind of interactions was evidently insufficient to destroy its structure. As shown in Fig. 2c–j, the fibers immersed in these solvents are not destroyed and the morphology and structure were kept intact. 3.2.2. Effect of temperature on the morphology of fibers Furthermore, measurement of the nanofibers at different temperatures (RT, 60 °C, 80 °C, 100 °C, 120 °C) was also carried out. DMF and toluene were used as two representative solvents due to their high boiling points (DMF, 152 °C; toluene, 111 °C). SEM images of the nanofibers after exposure under different temperatures (RT, 60 °C, 80 °C, 100 °C, 110 °C) in toluene are shown in Fig. 3a–e. As expected, with increasing toluene temperature, the fibers are swollen more. However, the structure of the fibers is still kept and not damaged. Compared to toluene, when immersed into DMF at different temperatures (RT, 60 °C, 80 °C, 100 °C, 120 °C), the fibers have only a slight change in morphology, as revealed in Fig. 3f–j. The obtained results illustrate that the structure of the electrospun nanofiber mats made
from this hybrid copolymer was stable in typical solvents and also at elevated temperatures. 4. Conclusions In conclusion, a novel hybrid copolymer nanofibrous mat was successfully prepared by electrospinning followed by acid treatment. The IR patterns showed that the methoxysilyl groups of the hydrolyzed PST composite nanofibers can be converted into the silanol groups. As shown by SEM, nanofibrous mats were stable in various solvents at different temperatures. Based on good chemical and thermal stability, this novel class of hybrid nanofibrous mats may actually provide a useful platform for the development of new functional membrane systems. Acknowledgments We gratefully acknowledge the financial support from the National Science Foundation of China (20533050, 50873051 and 50673048), MOST (2007AA03Z07) and the Transregional Project (TRR61).
Fig. 3. SEM images of electrospun nanofiber mats: immersed at different temperatures for 3 days in toluene (a) RT, (b) 60 °C, (c) 80 °C, (d) 100 °C, (e) 110 °C; in DMF (f) RT, (g) 60 °C, (h) 80 °C, (i) 100 °C, (j) 120 °C. (All the pictures are in the same scale bar).
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References [1] [2] [3] [4] [5]
Park JC, Ito TT, Kim KW, Kim BS, Kim IS. Polym J 2010;42:273–6. Reneker DH, Yarin AL. Polymer 2008;49:2387–425. Park JH, Braun PV. Adv Mater 2010;22:496–9. Neamnark A, Rujiravanit R, Supaphol P. Carbohydr Polym 2006;66:298–305. Kenawy ER, Abdel-Hay FI, El-Newehy MH, Wnek GE. Mater Chem Phys 2009;113: 296–302. [6] Ma ML, Hill RM, Lowery JL, Fridrikh SV, Rutledge GC. Langmuir 2005;21:5549–54. [7] Fong H, Reneker DH. J Polym Sci, Part B: Polym Phys 1999;37:3488–93.
[8] Du JZ, Chen YM, Zhang YH, Han CC, Fischer K, Schmidt M. J Am Chem Soc 2003;125:14710–1. [9] Du JZ, Chen YM. Macromolecules 2004;37:5710–6. [10] Ji XL, Hampsey JE, Hu QY, He JB, Yang ZZ, Lu YF. Chem Mater 2003;15:3656–62. [11] Park JW, Thomas EL. J Am Chem Soc 2002;124:514–5. [12] Holland BT, Blanford CF, Do T, Stein A. Chem Mater 1999;11:795–805. [13] Shin YM, Hohman MM, Brenner MP, Rutledge GC. Polymer 2001;42:9955. [14] Zhang XZ, Zhuo RX. Langmuir 2001;17:12–6. [15] Cao Z, Du BY, Chen TY, Li HT, Xu JT, Fan ZQ. Langmuir 2008;24:5543–51.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hybrid nanofibrous mats with remarkable solvent and temperature resistance produced by electrospinning technique Yunxia Xu a, Yuquan Wen a,⁎, Yi-nan Wu b, Changxu Lin b, Guangtao Li b a b
State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing, 100081, People's Republic of China Key Lab of Organic Optoelectronic and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing, 100084, People's Republic of China
a r t i c l e
i n f o
Article history: Received 5 October 2011 Accepted 8 March 2012 Available online 16 March 2012 Keywords: Functional Thin films Electrospinning Resistant studies
a b s t r a c t A novel organic/inorganic hybrid copolymer poly[styrene-co-3-(trimethoxysilyl)propyl-methacrylate] [P(St-coTMSPMA)] (denoted as PST) was successfully prepared from styrene and TMSPMA by free-radical polymerization. These composite fiber mats of PST were fabricated via a typical electrospinning process using a single solvent system. As shown in the SEM, the PST composite nanofibers have a smooth and uniform surface and the average diameter is about 0.2± 0.04 μm. After the methoxysilyl groups (Si\O\CH3) were hydrolyzed toward silanol groups (Si\OH), the nanofibrous mats exhibited remarkable solvent and temperature resistance. On account of good chemical and thermal stability, this hybrid mats can provide a useful platform for developing functionalized mat systems. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, electrospinning has been recognized as a simple and efficient approach toward assembling fibrous polymer films composed of fibers with diameters ranging from several micrometers down to tens of nanometers [1]. Nanofibrous membranes have attracted considerable attention due to their remarkable properties (such as very large specific surface area and higher porosity compared to commercial textiles), which make them excellent candidates for separation filters [2], wound dressing materials [3], tissue scaffolds [4], sensors [5], etc. It has been reported that more than 100 different polymers, both synthetic and natural, have been successfully electrospun into nanofibers from polymer solution. Block copolymers can also be electrospun into nanofibers, as shown for example for poly(styrene-b-dimethylsiloxane) diblock copolymers [6] and for styrene–butadiene–styrene triblock copolymers [7]. Recently, to further enhance material properties and thus extend the applicability of such fibers, nanocomposite approaches involving the surface functionalization of electrospun fibers have been attracting considerable attention. However, most electrospun membranes are made from polymers that are soluble in organic or inorganic solvents at certain temperature, for example, PS/DMF-THF, PVP/H2O, PEO/H2O. However, inorganic electrospun membranes derived from SiO2, TiO2, ZnO, etc., are insoluble in most solvents and become fragile after the calcination process. Therefore, the development of a new method for efficient fabrication of electrospun mats with improved solvent and temperature resistance is highly desirable.
⁎ Corresponding author. Tel./Fax: + 86 10 62792905. E-mail addresses: [email protected] (Y. Wen), [email protected] (G. Li). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.03.016
In this paper, we introduce a facile method for improving the solvent and temperature resistance of nanofibrous mats. A novel organic/ inorganic hybrid copolymer, poly[styrene-co-3-(trimethoxysilyl) propyl-methacrylate], was synthesized by free-radical copolymerization. One of the monomer, 3-(trimethoxysilyl)propyl-methacrylate with methoxysilyl groups [\Si(OCH3)3], is commercially available and is widely used in the preparation of organic/inorganic hybrid materials [8–11]. Nanofibrous membranes were prepared by electrospinning followed by hydrolysis in acid condition. The prepared membranes can be resistant to different temperatures and the typical solvents. Based on their remarkable solvent and temperature resistance, these hybrid mats can provide a useful platform for developing functionalized membrane systems for applications involving surface chemistry, drug delivery, and multifunctional textiles. 2. Experimental section 2.1. Materials 3-(trimethoxysilyl)propyl-methacrylate (TMSPMA; 97%, Alfa Aesar) was dried over CaH2 overnight and distilled under reduced pressure. Styrene (99%, Alfa Aesar) was purified according to literature [12]. Anhydrous toluene (Beijing Chemical Reagent Co.) was dried by refluxing in the presence of CaH2 and distilled prior to use. The initiator, α,α′,azodiisobutyronitrile (AIBN), was recrystallized from methanol. 2.2. Synthesis of copolymer PST The copolymer, poly[styrene-co-3-(trimethoxysilyl)propyl-methacrylate] [P(St-co-TMSPMA)], PST, with a random polymerization
140
Y. Xu et al. / Materials Letters 78 (2012) 139–142
degree was synthesized by free-radical polymerization. For a typical preparation, 3.00 g of styrene, 2.5 g of TMSPMA and 0.0068 g of 2,2-azobisisobutyronitrile (AIBN) were dissolved in anhydrous toluene (2.27 g). The obtained solution was added into a 50 ml three-neck round bottom flask equipped with a magnetic stirrer and a refluxing condenser. The flask was placed in an oil bath at 80 °C. After 8 h, the reaction mixture was removed and slowly cooled to room temperature.
characteristic vibration peaks of Si\O\CH3 at 821 and 1087 cm − 1, which disappear after the hydrolysis and cross-linking reactions (Fig. 1b). The weak peak at 1126 cm − 1 can be attributed to the Si\O\Si bonds [14]. The absorbance peaks assigned to the aromatic C\H stretch appeared at ~ 3050 cm − 1, C\H stretch at ~2900 cm − 1, C_O stretch at ~ 1740 cm − 1, aromatic C\C stretch of the polystyrene at ~ 1480 cm − 1, C\H out-of-plane bend at ~750 cm − 1, and aromatic C\C out-of-plane stretch of the polystyrene at ~ 710 cm − 1.
2.3. Preparation of organic/inorganic hybrid thin films
3.2. SEM analysis
A 50 wt.% copolymer PST solution was prepared at room temperature by dissolving PST in a mixture of THF and DMF (3:1 v/v). The copolymer solution was electrospun using a parallel plate setup [13]. During the electrospinning process, the driving high voltage applied was 12 kV, and the distance between the needle and the aluminum foil collector was 15 cm. A constant flow rate of 0.35 ml/h was obtained using a syringe pump. The electrospun nanofibers were dried and annealed at 60 °C for 8 h. After hydrolysis in acid and condensation treatment, the organic/inorganic hybrid thin films were obtained.
3.2.1. Study on the solvent resistance The as-prepared composite nanofibers fabricated by electrospinning consist of solvents and PST. In order to obtain the crosslinkable nanofibers, the as-prepared fibers were immersed in a 0.5 wt.% HCl aqueous solution for 3 days. The purpose is that the methoxysilyl groups can be hydrolyzed toward silanol groups, and then condensed into cross-linkable polysilsesquioxanes [15]. Fig. 2a shows a typical SEM image of originally prepared electrospun PST nanofibers. The average diameter of the nanofibers is about 0.2 ± 0.04 μm at 50 wt.% copolymer. Compared to the as-prepared composite fibers (Fig. 2a), the morphology and structure of the nanofibers were largely maintained after hydrolysis in acid with only small differences (Fig. 2b). Also, this material can be fabricated in nanofibrous membranes in a large area, as revealed in the inset of Fig. 2a. Generally, most electrospun polymer fibers are destroyed upon exposure to typical solvents or at high temperatures, which limit their applicability in uses such as water treatment, and separation filter. However, the above organic/inorganic hybrid copolymer mat exhibited excellent resistance to some typical solvents. In order to study the effect of various solvents on the structure of the copolymer nanofibers, the cross-linked fibers were immersed in various solvents (DMF, H2O, EtOH, THF, MeOH, CHCl3, toluene, n-hexane) at 60 °C for 3 days. The SEM images are shown in Fig. 2c–j. The morphology of the nanofibers was retained with only slight differences compared to the initial state after immersing in typical polar solvents, DMF, H2O, EtOH, THF, MeOH, as shown in Fig. 2c–g. Whereas, as shown in Fig. 2h–j, the fibers were swollen when immersed in non-polar solvents such as CHCl3, toluene, and n-hexane, however despite swelling the structure of the fibers is still retained. At the same time, it is observed that the stability of the fibers in polar solvents is better than that in non-polar solvents. The reason is probably related to the weak interactions between the polar solvents and the copolymers.
2.4. Stability studies Cross-linked thin films were immersed into different solvents, such as DMF, H2O, EtOH, THF, MeOH, CHCl3, toluene, n-hexane, and at different temperatures, room temperature (denoted as RT), 60 °C, 80 °C, 100 °C, and 120 °C. 2.5. Characterization The morphologies of the fabricated nanofibers at various temperatures and solvents were observed using a field emission scanning electron microscopy (FESEM) on a JEOL JSM-5400 system. The molecular structure of the nanofibers was characterized by Fourier transform infrared (FT-IR, Nicolet 6700). 3. Results and discussions 3.1. IR analysis The FT-IR spectra of electrospun PST fibrous mats are shown in Fig. 1. The copolymer mats before hydrolysis (Fig. 1a) show the
Fig. 1. FT-IR spectra of copolymer mats (a), after hydrolysis and cross-linked in HCl (b).
Y. Xu et al. / Materials Letters 78 (2012) 139–142
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Fig. 2. SEM images of electrospun nanofiber mats: (a) before, (b) after being immersed in HCl for 3 days; immersed in different solvents at 60 °C for 3 days (c) DMF, (d) H2O, (e) EtOH, (f) THF, (g) MeOH, (h) CHCl3, (i) toluene, (j) n-hexane. (All the pictures are in the same scale bar).
The non-polar solvents have stronger interactions and a larger effect on the copolymer, or may even react with a small part of the copolymer, however, this kind of interactions was evidently insufficient to destroy its structure. As shown in Fig. 2c–j, the fibers immersed in these solvents are not destroyed and the morphology and structure were kept intact. 3.2.2. Effect of temperature on the morphology of fibers Furthermore, measurement of the nanofibers at different temperatures (RT, 60 °C, 80 °C, 100 °C, 120 °C) was also carried out. DMF and toluene were used as two representative solvents due to their high boiling points (DMF, 152 °C; toluene, 111 °C). SEM images of the nanofibers after exposure under different temperatures (RT, 60 °C, 80 °C, 100 °C, 110 °C) in toluene are shown in Fig. 3a–e. As expected, with increasing toluene temperature, the fibers are swollen more. However, the structure of the fibers is still kept and not damaged. Compared to toluene, when immersed into DMF at different temperatures (RT, 60 °C, 80 °C, 100 °C, 120 °C), the fibers have only a slight change in morphology, as revealed in Fig. 3f–j. The obtained results illustrate that the structure of the electrospun nanofiber mats made
from this hybrid copolymer was stable in typical solvents and also at elevated temperatures. 4. Conclusions In conclusion, a novel hybrid copolymer nanofibrous mat was successfully prepared by electrospinning followed by acid treatment. The IR patterns showed that the methoxysilyl groups of the hydrolyzed PST composite nanofibers can be converted into the silanol groups. As shown by SEM, nanofibrous mats were stable in various solvents at different temperatures. Based on good chemical and thermal stability, this novel class of hybrid nanofibrous mats may actually provide a useful platform for the development of new functional membrane systems. Acknowledgments We gratefully acknowledge the financial support from the National Science Foundation of China (20533050, 50873051 and 50673048), MOST (2007AA03Z07) and the Transregional Project (TRR61).
Fig. 3. SEM images of electrospun nanofiber mats: immersed at different temperatures for 3 days in toluene (a) RT, (b) 60 °C, (c) 80 °C, (d) 100 °C, (e) 110 °C; in DMF (f) RT, (g) 60 °C, (h) 80 °C, (i) 100 °C, (j) 120 °C. (All the pictures are in the same scale bar).
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References [1] [2] [3] [4] [5]
Park JC, Ito TT, Kim KW, Kim BS, Kim IS. Polym J 2010;42:273–6. Reneker DH, Yarin AL. Polymer 2008;49:2387–425. Park JH, Braun PV. Adv Mater 2010;22:496–9. Neamnark A, Rujiravanit R, Supaphol P. Carbohydr Polym 2006;66:298–305. Kenawy ER, Abdel-Hay FI, El-Newehy MH, Wnek GE. Mater Chem Phys 2009;113: 296–302. [6] Ma ML, Hill RM, Lowery JL, Fridrikh SV, Rutledge GC. Langmuir 2005;21:5549–54. [7] Fong H, Reneker DH. J Polym Sci, Part B: Polym Phys 1999;37:3488–93.
[8] Du JZ, Chen YM, Zhang YH, Han CC, Fischer K, Schmidt M. J Am Chem Soc 2003;125:14710–1. [9] Du JZ, Chen YM. Macromolecules 2004;37:5710–6. [10] Ji XL, Hampsey JE, Hu QY, He JB, Yang ZZ, Lu YF. Chem Mater 2003;15:3656–62. [11] Park JW, Thomas EL. J Am Chem Soc 2002;124:514–5. [12] Holland BT, Blanford CF, Do T, Stein A. Chem Mater 1999;11:795–805. [13] Shin YM, Hohman MM, Brenner MP, Rutledge GC. Polymer 2001;42:9955. [14] Zhang XZ, Zhuo RX. Langmuir 2001;17:12–6. [15] Cao Z, Du BY, Chen TY, Li HT, Xu JT, Fan ZQ. Langmuir 2008;24:5543–51.