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Tensile performance of silica-based electrospun fibrous mats
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Tensile performance of silica-based electrospun fibrous mats Jianjun Li⁎, Chujun Nie, Beibei Duan, Xu Zhao, Haibao Lu, Weilong Yin, Yibin Li, Xiaodong He⁎ National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin 150080, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Electrospinning Tensile performance Silica-based
To understand the relationship between mechanical behavior and fiber assembly in electrospun ceramic-based fibrous mats, the tensile performance of silica, silica/polydimethylsiloxane and silica/polymethyl methacrylate hybrid fibrous mats was investigated. Elastic-plastic behavior as a whole was confirmed in the silica-based fibrous mats. The initial isotropy and disordered networks with point contact between the randomly oriented fibers before the tensile test and the highly oriented fibrous bundles with side contact between the aligned fibers after the tensile test were observed using scanning electron microscopy.
1. Introduction
[23,24]. Silica fibers can be electrospun from its sol made from hydrolysis and condensation of tetraethoxysilane (TEOS). Enhanced thermal and mechanical properties from silica and better flexibility due to the polymer provide multifunctional performance to silica-based polymer hybrids [25,26]. Silica sols can be electrospun into fibers without any carrier polymer [21]. Silica fiber has been synthesized on a large-scale with highly ordered mesoporous structure and continuous long fibers by employing electrospinning [27]. Flexible, high-heatresistant, and amphiphobic mats were fabricated by (fluoroalkyl) silane (FAS) modification of electrospun pure silica nanofibrous mats [28]. In the process of preparing silica-based electrospun hybrid products, polymers and silica sol must be dissolved in a suitable organic solvent. The excellent solubility of silica sol in different solvent systems [29,30] makes it possible to prepare silica/polydimethylsiloxane (PDMS) and silica/polymethyl methacrylate (PMMA) electrospun hybrid products with controllable wetting behavior. Mechanical behavior determines the practical applications of electrospun materials, especially ceramic-based products. Most studies on electrospinning have focused on the optimization of processing control parameters and improvement of functional performance. The effects of oriented morphology, structures and tensile properties on electrospun polymer-based nanofibers have been investigated [31–34]. A thorough understanding of the mechanical properties of electrospun ceramicbased fibrous mats is necessary. The elastic-plastic behavior of nonwoven fibrous mats and a constitutive model, which captures the membrane stress-strain behavior as a function of fiber properties and the geometry of the fibrous network, have been investigated from the amorphous polyamide system [35]. Silica hybrid products, including aerogel, film, fiber and particle have been investigated in various applied fields. Our previous re-
Ceramic fibers with intrinsically hard, inert and corrosion resistant features are attractive for their applications in areas such as catalysis, the electronic and photonic industry, environmental science, and energy technology [1]. Electrospinning is a powerful technique that can process viscoelastic solutions into continuous one-dimensional fibers with controllable diameters, compositions, and structures to fabricate non-woven mats [2–4]. Long-range periodic array or structure control in one-dimensional structures can also be realized by modifying the electrospinning setup [5–9]. During the past few years, the electrospinning technique has been developed successfully for ceramic systems to produce micro- and nanofibers. Viscoelastic inorganic precursory solutions [10–12] resulting from the sol-gel process can be used for electrospinning. The combination of sol-gel and electrospinning provides a straightforward route for the preparation of electrospun ceramic fibers. The flexible sol-gel chemical approach in combination with the concept of molecular-level incorporation of nanoscaled components, or organic polymers, can tailor inorganic structures into novel multifunctional hybrid materials, which may be applied in diverse areas, including mechanical improvement, [13–17] and filtration and self-cleaning [18,19]. Electrospinning in combination with the sol-gel technique has directly produced SiO2, Al2O3, TiO2, ZrO2 and other fibers [20–22]. Silica is one of the most typical ceramic materials and is widely used in both academia and industry. Silica and modified silica materials prepared by the sol-gel process are functional materials with a wide range of applications, including controlled release, protective coatings, adsorption, chromatography, separation, biotechnology, energy conservation, cultural heritage restoration and environmental remediation
⁎
Corresponding authors. E-mail addresses: [email protected] (J. Li), [email protected] (X. He).
http://dx.doi.org/10.1016/j.jnoncrysol.2017.05.016 Received 10 December 2016; Received in revised form 25 April 2017; Accepted 15 May 2017 0022-3093/ © 2017 Published by Elsevier B.V.
Please cite this article as: Li, J., Journal of Non-Crystalline Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.05.016
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
J. Li et al.
searches reported the fabrication and modification of silica fibrous mats obtained by electrospinning [8,26,29,30]. In this study, we evaluate the mechanical performance of electrospun silica-based (silica, silica/ polymethyl methacrylate and silica/polydimethylsiloxane) fibrous mats under various conditions. The mechanical behavior of the mats was investigated by uniaxial tensile tests. The results provide a basic understanding of the relationship between the mechanical performance and the internal structures of electrospun fibrous mats. 2. Experimental In this study, all the reagents were directly used without further dilution. Tetraethoxysilane (98%) was purchased from the SigmaAldrich Chemical Company. Polymethyl methacrylate (PMMA) was obtained from the Aladdin Company. Polydimethylsiloxane (99.7%), ethanol (99.7%), acetone (99.7%), dichloromethane (99.5%) and hydrochloric acid (37%) were supplied by the Tianjin Fengchuan Chemical Reagent Company. Polyethylene oxide (PEO, Mn = 900,000) was provided by the Changchun Jinghua Company. Silica sol was prepared by the hydrolysis and condensation of tetraethoxysilane and the detailed experimental procedure is reported elsewhere [8]. Polyethylene oxide (PEO) is introduced into silica sols as an electrospinning carrier and it's concentration in silica sol is 1 wt%. Solid polymethyl methacrylate must be dissolved in acetone to become polymethyl methacrylate solution (0.1 g/mL) firstly. Liquid polydimethylsiloxane can be used directly. Silica sol can be mixed with polymethyl methacrylate in acetone or with polydimethylsiloxane in dichloromethane to get the mixtures for electrospinning. Here a needle electrospinning apparatus (Beijing Future Material Sci-tech Company) was used. The flow rate of the mixtures was 2 mL/h and the distance between the needle and collector was 20 cm. The voltage was 24 kV. The scanning electron microscope (SEM) images were performed on the HITACHI SU8000. The tensile behavior of the electrospun mat was tested on tensile testing equipment (Instron 5944) with a tensile speed of 0.5 mm/min at room temperature. Dumbbellshaped tensile specimens were prepared from isotropic electrospun silica-based fibrous mats.
Fig. 1. SEM images of electrospun silica fibers made in acetone solvent with 2:1 (a) and 1:1 (b) Silica sol/acetone volume ratios.
3. Results and discussion Our previous work investigated the influence of solvent on the spinnability and silica-based electrospun fibers. The correct viscosity of the precursory solution is essential for producing a continuous flowing stream of silica from the needle to the collector. The results reveal that acetone can lower the minimum concentration for electrospinning of silica sols, and the diameter of silica fibers decrease as the silica sol/ acetone volume ratio decreases [29]. Experimental studies have demonstrated improvements in the modulus and strength of electrospun polymer nanofibers with reduced diameter [36]. In this manuscript the mechanical behavior of silica fibrous mats that were electrospun from different concentrations of silica sol in acetone is tested. The SEM images of electrospun silica fibers in acetone with 2:1 and 1:1 silica sol/acetone volume ratios are displayed in Fig. 1a) and b), illustrating the effect of silica sol concentration on the diameter of fiber. The average diameter of silica fiber spun with 2:1 silica sol/acetone volume ratio is approximately 1 μm, and the average diameter of silica fiber spun with 1:1 silica sol/acetone volume ratio decreases to 500 nm. The mechanical performance of electrospun silica-based non-woven mats was quantitatively examined using uniaxial tensile tests. All the measured products were cut from the homogeneous region of the collected electrospun mats. To avoid compression damage from the clamp, specimen shaped thin protective cardboard pieces were fabricated to cover the tensile specimen and form a sandwich structure. After mounting the samples onto the instruments we cut off the protective cardboard using scissors (as shown in Fig. 2a)). Fig. 2b) presents the tensile stress-strain curves of electrospun silica fibrous
Fig. 2. (a): Silica fibrous mat with protective cardboard covers; (b): Tensile stress-strain curves of electrospun silica fibrous mats made in acetone solvent with 2:1 and 1:1 silica sol/acetone volume ratios.
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Fig. 3. (a): Tensile stress-strain curve of electrospun silica fibrous mat with 1:1 silica sol/acetone volume ratio; (b), (c), (d) and (e): Digital images of the response of silica fibrous mats at different strains during tensile testing.
Fig. 4. SEM images of the electrospun silica fibrous mat before tensile testing (scale bar: a, 50 μm and b: 10 μm) and after tensile testing (scale bar: c, 100 μm; d: 30 μm and e: 10 μm).
tensile performances of the silica fibrous mats electrospun with 1:1 silica sol/acetone volume ratio at different strains. After reaching the yield strength silica fibrous mats begin to deform plastically (Fig. 3b). Then, necking appears in Fig. 3c). After reaching the ultimate tensile strength damage initiates (Fig. 3d and e) with increasing strain. To observe the kinetics of fiber assembly during stretching, the tensile tests were stopped, and the samples were observed using a scanning electron microscope. SEM images of the electrospun silica fibrous mats before and after uniaxial tensile testing are displayed in
mats spun in acetone solvent with 2:1 and 1:1 silica sol/acetone volume ratios. The strain behavior of the mats as a whole was found to be elastic-plastic. The ultimate tensile strength reached at strain was approximately 0.22 for both samples. The silica fibrous mat spun with a 1:1 silica sol/acetone volume ratio exhibited higher tensile strength than that spun with 2:1 volume ratio, which may be attributed to reduction of the fiber diameter [36]. The response of the mats provides further insight into the deformation mechanisms. Digital images in Fig. 3b), c), d) and e) display the
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Fig. 5. SEM images of the silica/PDMS (a) and silica/PMMA (b) hybrid fibers electrospun with 4:1 volume ratio; Tensile stress-strain curves of silica (1:1 silica sol/acetone volume ratio), silica/PDMS (4:1 silica sol/PDMS volume ratio) and silica/PMMA (4:1 silica sol/PMMA solution volume ratio) fibrous mats (c).
direction of applied stress. Fig. 5a) and b) display the SEM images of the silica/PDMS and silica/PMMA hybrid fibers electrospun with 4:1 silica sol/PDMS (or PMMA solution) volume ratio. The results reveal that the introduction of polymer into silica has no obvious effect on the average diameter of the hybrid fibers. Fig. 5c) displays the tensile stress-strain curves of silica, silica/PDMS and silica/PMMA fibrous mats. In comparison with pure silica fibrous mats, introduction of PDMS or PMMA may increase the tensile strength of the silica-based hybrid fibrous mat. The elasticplastic behavior of silica/PDMS and silica/PMMA hybrid fibrous mats does not change too much. The understanding of tensile performance in electrospun fibers becomes increasingly important for nanotechnology applications. An illustration in Fig. 6 was made in order to discuss the results of the elastic-plastic behavior observed in electrospun silica-based hybrid fibrous mats. Before the tensile test, initial isotropy and a randomly oriented network structure (Fig. 6c) exist in the mat-plane by random point-contact. In the initial stage of stretching, friction among fibers is easily overcome and they orient in the direction of applied stress. Finally, formation of aligned side-contact in the highly oriented fibrous bundles (Fig. 6d) may be useful for the damage resistance of silicabased electrospun mats.
4. Conclusion Fig. 6. Illustration to give insight into the tensile behavior observed in electrospun silicabased hybrid fibrous mats.
Tensile performance has been experimentally characterized in randomly oriented non-woven electrospun silica, silica/polydimethylsiloxane and silica/polymethyl methacrylate hybrid fibrous mats. Formation of aligned side-contact in the highly oriented fibrous bundles in the direction of applied stress was clearly confirmed in SEM images, which may be useful for the damage resistance of electrospun mats. This study may increases the understanding of the relationship between mechanical performance and fiber assembly of electrospun fibrous mats.
Fig. 4, and reveal the kinetics of fiber assembly. Without strain, a randomly oriented network structure was observed in Fig. 4a) and b), which indicates initial isotropy in the mat-plane. Most silica fibers distribute individually by random point-contact. Such contact points represent the friction between the fibers. Fig. 4c), d) and e) show the SEM images of silica fibers after tensile testing. The results clearly confirm that the majority of the fibers align with the direction of applied stress. Additionally, highly oriented fibrous bundles form in the 4
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[17] M.D. Morales-Acosta, C.G. Alvarado-Beltrán, M.A. Quevedo-López, B.E. Gnade, A. Mendoza-Galván, R. Ramírez-Bon, J. Non-Cryst. Solids 362 (2013) 124–135. [18] J.P. Zhang, S. Seeger, Adv. Funct. Mater. 21 (2011) 4699–4704. [19] C. Zeng, H. Wang, H. Zhou, T. Lin, RSC Adv. 5 (2015) 61044–61050. [20] X. Mao, Y. Bai, J.Y. Yu, B. Ding, J. Am. Ceram. Soc. 99 (2016) 2760–2768. [21] S.S. Choi, S.G. Lee, S.S. Im, S.H. Kim, J. Mater. Sci. Lett. 22 (2003) 891–893. [22] C.L. Shao, H.Y. Guan, Y.C. Liu, J. Gong, N. Yu, X.H. Yang, J. Cryst. Growth 267 (2004) 380–384. [23] R. Ciriminna, A. Fidalgo, V. Pandarus, F. Beland, L.M. IIharco, M. Pagliaro, Chem. Rev. 113 (2013) 6592–6620. [24] M. Pagliaro, Silica-based Materials for Advanced Chemical Applications, RSC Publishing, 2009. [25] T. Pirzada, S.A. Arvidson, C.D. Saquing, S.S. Shah, S.A. Khan, Langmuir 28 (2012) 5834–5844. [26] J.J. Li, J.G. Cao, Z.L. Wei, M. Yang, W.L. Yin, K. Yu, Y.T. Yao, H.B. Lu, X.D. He, J.S. Leng, J. Mater. Chem. A 2 (2014) 16569–16576. [27] G.T. Zhu, X.S. Li, X.M. Fu, J.Y. Wu, B.F. Yuan, Y.Q. Feng, Chem. Commun. 48 (2012) 9980–9982. [28] M. Guo, B. Ding, X.H. Li, X.L. Wang, J.Y. Yu, M.R. Wang, J. Phys. Chem. C 114 (2010) 916–921. [29] Y.T. Yao, H.B. Lu, J.J. Li, Q.Y. Peng, Q. Wang, Y.B. Li, X.D. He, J.S. Leng, RSC Adv. 5 (2015) 89113–89120. [30] Z.L. Wei, J.J. Li, C. Wang, J.G. Cao, Y.T. Yao, H.B. Lu, Y.B. Li, X.D. He, Appl. Surf. Sci. 392 (2017) 260–267. [31] L.Q. Liu, D. Tasis, M. Prato, H.D. Wagner, Adv. Mater. 19 (2007) 1228–1233. [32] A. Baji, Y.W. Mai, S.C. Wong, M. Abtahi, P. Chen, Compos. Sci. Technol. 70 (2010) 703–718. [33] E. Zussman, M. Burman, A.L. Yarin, R. Khalfin, Y. Cohen, J. Polym. Sci. B Polym. Phys. 44 (2006) 1482–1489. [34] C.B. Huang, S.L. Chen, D.H. Reneker, C.L. Lai, H.Q. Hou, Adv. Mater. 18 (2006) 668–671. [35] M.N. Silberstein, C.L. Pai, G.C. Rutledge, M.C. Boyce, J. Mech. Phys. Solids 60 (2012) 295–318. [36] D. Papkov, Y. Zou, M.N. Andalib, A. Goponenko, S.D. Cheng, Y.A. Dzenis, ACS Nano 7 (2013) 3324–3331.
Acknowledgements This research is supported by “National Natural Science Foundation of China” (No. 21301040), “Innovative Research Group of National Natural Science Foundation of China (No. 11421091) and Funding of Science and Technology on Advanced Composites in Special Environment Laboratory. References [1] Y.Q. Dai, W.Y. Liu, E. Formo, Y.M. Sun, Y.N. Xia, Polym. Adv. Technol. 22 (2011) 22326–22338. [2] D. Li, Y.N. Xia, Adv. Mater. 16 (2004) 1151–1170. [3] D.H. Reneker, I. Chun, Nanotechnology 7 (1996) 216–223. [4] A. Greiner, J.H. Wendorff, Angew. Chem. Int. Ed. 46 (2007) 5670–5703. [5] G.Q. Chang, G.X. Song, J. Yang, R.S. Huang, A. Kozinda, J.Y. Shen, Appl. Phys. Lett. 101 (2012) 263505. [6] J. Yu, Y.J. Qiu, X.X. Zha, M. Yu, J.L. Yu, J. Rafique, J. Yin, Eur. Polym. J. 44 (2008) 2838–2844. [7] R. Kessick, G. Tepper, Appl. Phys. Lett. 84 (2004) 4807–4809. [8] Y.T. Yao, J.G. Cao, M. Yang, J.J. Li, S.Y. Zhao, W.L. Yin, Y.B. Li, X.D. He, J.S. Leng, RSC Adv. 3 (2013) 12026–12030. [9] D. Li, Y.L. Wang, Y.N. Xia, Nano Lett. 3 (2003) 1167–1171. [10] W. Sigmund, J. Yuh, H. Park, V. Maneeratana, G. Pyrgiotakis, A. Gaga, J. Taylor, J.C. Nino, J. Am. Ceram. Soc. 89 (2006) 395–407. [11] D. Li, Y.N. Xia, Nano Lett. 4 (2004) 933–938. [12] A. Biswas, H. Park, W.M. Sigmund, Ceram. Int. 38 (2012) 883–886. [13] Dylan J. Boday, Beatrice Muriithi, Robert J. Stover, Douglas A. Loy, J. Non-Cryst. Solids 358 (2012) 1575–1580. [14] B. Wang, L.Z. Wu, L. Ma, J.C. Feng, Compos. Part B 42 (2011) 891–897. [15] H. Maleki, L. Durães, A. Portugal, J. Non-Cryst. Solids 385 (2014) 55–74. [16] B. Wang, L.Z. Wu, L. Ma, Y.G. Sun, S.Y. Du, Mater. Des. 31 (2010) 2659–2663.
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Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Tensile performance of silica-based electrospun fibrous mats Jianjun Li⁎, Chujun Nie, Beibei Duan, Xu Zhao, Haibao Lu, Weilong Yin, Yibin Li, Xiaodong He⁎ National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin 150080, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Electrospinning Tensile performance Silica-based
To understand the relationship between mechanical behavior and fiber assembly in electrospun ceramic-based fibrous mats, the tensile performance of silica, silica/polydimethylsiloxane and silica/polymethyl methacrylate hybrid fibrous mats was investigated. Elastic-plastic behavior as a whole was confirmed in the silica-based fibrous mats. The initial isotropy and disordered networks with point contact between the randomly oriented fibers before the tensile test and the highly oriented fibrous bundles with side contact between the aligned fibers after the tensile test were observed using scanning electron microscopy.
1. Introduction
[23,24]. Silica fibers can be electrospun from its sol made from hydrolysis and condensation of tetraethoxysilane (TEOS). Enhanced thermal and mechanical properties from silica and better flexibility due to the polymer provide multifunctional performance to silica-based polymer hybrids [25,26]. Silica sols can be electrospun into fibers without any carrier polymer [21]. Silica fiber has been synthesized on a large-scale with highly ordered mesoporous structure and continuous long fibers by employing electrospinning [27]. Flexible, high-heatresistant, and amphiphobic mats were fabricated by (fluoroalkyl) silane (FAS) modification of electrospun pure silica nanofibrous mats [28]. In the process of preparing silica-based electrospun hybrid products, polymers and silica sol must be dissolved in a suitable organic solvent. The excellent solubility of silica sol in different solvent systems [29,30] makes it possible to prepare silica/polydimethylsiloxane (PDMS) and silica/polymethyl methacrylate (PMMA) electrospun hybrid products with controllable wetting behavior. Mechanical behavior determines the practical applications of electrospun materials, especially ceramic-based products. Most studies on electrospinning have focused on the optimization of processing control parameters and improvement of functional performance. The effects of oriented morphology, structures and tensile properties on electrospun polymer-based nanofibers have been investigated [31–34]. A thorough understanding of the mechanical properties of electrospun ceramicbased fibrous mats is necessary. The elastic-plastic behavior of nonwoven fibrous mats and a constitutive model, which captures the membrane stress-strain behavior as a function of fiber properties and the geometry of the fibrous network, have been investigated from the amorphous polyamide system [35]. Silica hybrid products, including aerogel, film, fiber and particle have been investigated in various applied fields. Our previous re-
Ceramic fibers with intrinsically hard, inert and corrosion resistant features are attractive for their applications in areas such as catalysis, the electronic and photonic industry, environmental science, and energy technology [1]. Electrospinning is a powerful technique that can process viscoelastic solutions into continuous one-dimensional fibers with controllable diameters, compositions, and structures to fabricate non-woven mats [2–4]. Long-range periodic array or structure control in one-dimensional structures can also be realized by modifying the electrospinning setup [5–9]. During the past few years, the electrospinning technique has been developed successfully for ceramic systems to produce micro- and nanofibers. Viscoelastic inorganic precursory solutions [10–12] resulting from the sol-gel process can be used for electrospinning. The combination of sol-gel and electrospinning provides a straightforward route for the preparation of electrospun ceramic fibers. The flexible sol-gel chemical approach in combination with the concept of molecular-level incorporation of nanoscaled components, or organic polymers, can tailor inorganic structures into novel multifunctional hybrid materials, which may be applied in diverse areas, including mechanical improvement, [13–17] and filtration and self-cleaning [18,19]. Electrospinning in combination with the sol-gel technique has directly produced SiO2, Al2O3, TiO2, ZrO2 and other fibers [20–22]. Silica is one of the most typical ceramic materials and is widely used in both academia and industry. Silica and modified silica materials prepared by the sol-gel process are functional materials with a wide range of applications, including controlled release, protective coatings, adsorption, chromatography, separation, biotechnology, energy conservation, cultural heritage restoration and environmental remediation
⁎
Corresponding authors. E-mail addresses: [email protected] (J. Li), [email protected] (X. He).
http://dx.doi.org/10.1016/j.jnoncrysol.2017.05.016 Received 10 December 2016; Received in revised form 25 April 2017; Accepted 15 May 2017 0022-3093/ © 2017 Published by Elsevier B.V.
Please cite this article as: Li, J., Journal of Non-Crystalline Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.05.016
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
J. Li et al.
searches reported the fabrication and modification of silica fibrous mats obtained by electrospinning [8,26,29,30]. In this study, we evaluate the mechanical performance of electrospun silica-based (silica, silica/ polymethyl methacrylate and silica/polydimethylsiloxane) fibrous mats under various conditions. The mechanical behavior of the mats was investigated by uniaxial tensile tests. The results provide a basic understanding of the relationship between the mechanical performance and the internal structures of electrospun fibrous mats. 2. Experimental In this study, all the reagents were directly used without further dilution. Tetraethoxysilane (98%) was purchased from the SigmaAldrich Chemical Company. Polymethyl methacrylate (PMMA) was obtained from the Aladdin Company. Polydimethylsiloxane (99.7%), ethanol (99.7%), acetone (99.7%), dichloromethane (99.5%) and hydrochloric acid (37%) were supplied by the Tianjin Fengchuan Chemical Reagent Company. Polyethylene oxide (PEO, Mn = 900,000) was provided by the Changchun Jinghua Company. Silica sol was prepared by the hydrolysis and condensation of tetraethoxysilane and the detailed experimental procedure is reported elsewhere [8]. Polyethylene oxide (PEO) is introduced into silica sols as an electrospinning carrier and it's concentration in silica sol is 1 wt%. Solid polymethyl methacrylate must be dissolved in acetone to become polymethyl methacrylate solution (0.1 g/mL) firstly. Liquid polydimethylsiloxane can be used directly. Silica sol can be mixed with polymethyl methacrylate in acetone or with polydimethylsiloxane in dichloromethane to get the mixtures for electrospinning. Here a needle electrospinning apparatus (Beijing Future Material Sci-tech Company) was used. The flow rate of the mixtures was 2 mL/h and the distance between the needle and collector was 20 cm. The voltage was 24 kV. The scanning electron microscope (SEM) images were performed on the HITACHI SU8000. The tensile behavior of the electrospun mat was tested on tensile testing equipment (Instron 5944) with a tensile speed of 0.5 mm/min at room temperature. Dumbbellshaped tensile specimens were prepared from isotropic electrospun silica-based fibrous mats.
Fig. 1. SEM images of electrospun silica fibers made in acetone solvent with 2:1 (a) and 1:1 (b) Silica sol/acetone volume ratios.
3. Results and discussion Our previous work investigated the influence of solvent on the spinnability and silica-based electrospun fibers. The correct viscosity of the precursory solution is essential for producing a continuous flowing stream of silica from the needle to the collector. The results reveal that acetone can lower the minimum concentration for electrospinning of silica sols, and the diameter of silica fibers decrease as the silica sol/ acetone volume ratio decreases [29]. Experimental studies have demonstrated improvements in the modulus and strength of electrospun polymer nanofibers with reduced diameter [36]. In this manuscript the mechanical behavior of silica fibrous mats that were electrospun from different concentrations of silica sol in acetone is tested. The SEM images of electrospun silica fibers in acetone with 2:1 and 1:1 silica sol/acetone volume ratios are displayed in Fig. 1a) and b), illustrating the effect of silica sol concentration on the diameter of fiber. The average diameter of silica fiber spun with 2:1 silica sol/acetone volume ratio is approximately 1 μm, and the average diameter of silica fiber spun with 1:1 silica sol/acetone volume ratio decreases to 500 nm. The mechanical performance of electrospun silica-based non-woven mats was quantitatively examined using uniaxial tensile tests. All the measured products were cut from the homogeneous region of the collected electrospun mats. To avoid compression damage from the clamp, specimen shaped thin protective cardboard pieces were fabricated to cover the tensile specimen and form a sandwich structure. After mounting the samples onto the instruments we cut off the protective cardboard using scissors (as shown in Fig. 2a)). Fig. 2b) presents the tensile stress-strain curves of electrospun silica fibrous
Fig. 2. (a): Silica fibrous mat with protective cardboard covers; (b): Tensile stress-strain curves of electrospun silica fibrous mats made in acetone solvent with 2:1 and 1:1 silica sol/acetone volume ratios.
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Fig. 3. (a): Tensile stress-strain curve of electrospun silica fibrous mat with 1:1 silica sol/acetone volume ratio; (b), (c), (d) and (e): Digital images of the response of silica fibrous mats at different strains during tensile testing.
Fig. 4. SEM images of the electrospun silica fibrous mat before tensile testing (scale bar: a, 50 μm and b: 10 μm) and after tensile testing (scale bar: c, 100 μm; d: 30 μm and e: 10 μm).
tensile performances of the silica fibrous mats electrospun with 1:1 silica sol/acetone volume ratio at different strains. After reaching the yield strength silica fibrous mats begin to deform plastically (Fig. 3b). Then, necking appears in Fig. 3c). After reaching the ultimate tensile strength damage initiates (Fig. 3d and e) with increasing strain. To observe the kinetics of fiber assembly during stretching, the tensile tests were stopped, and the samples were observed using a scanning electron microscope. SEM images of the electrospun silica fibrous mats before and after uniaxial tensile testing are displayed in
mats spun in acetone solvent with 2:1 and 1:1 silica sol/acetone volume ratios. The strain behavior of the mats as a whole was found to be elastic-plastic. The ultimate tensile strength reached at strain was approximately 0.22 for both samples. The silica fibrous mat spun with a 1:1 silica sol/acetone volume ratio exhibited higher tensile strength than that spun with 2:1 volume ratio, which may be attributed to reduction of the fiber diameter [36]. The response of the mats provides further insight into the deformation mechanisms. Digital images in Fig. 3b), c), d) and e) display the
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Fig. 5. SEM images of the silica/PDMS (a) and silica/PMMA (b) hybrid fibers electrospun with 4:1 volume ratio; Tensile stress-strain curves of silica (1:1 silica sol/acetone volume ratio), silica/PDMS (4:1 silica sol/PDMS volume ratio) and silica/PMMA (4:1 silica sol/PMMA solution volume ratio) fibrous mats (c).
direction of applied stress. Fig. 5a) and b) display the SEM images of the silica/PDMS and silica/PMMA hybrid fibers electrospun with 4:1 silica sol/PDMS (or PMMA solution) volume ratio. The results reveal that the introduction of polymer into silica has no obvious effect on the average diameter of the hybrid fibers. Fig. 5c) displays the tensile stress-strain curves of silica, silica/PDMS and silica/PMMA fibrous mats. In comparison with pure silica fibrous mats, introduction of PDMS or PMMA may increase the tensile strength of the silica-based hybrid fibrous mat. The elasticplastic behavior of silica/PDMS and silica/PMMA hybrid fibrous mats does not change too much. The understanding of tensile performance in electrospun fibers becomes increasingly important for nanotechnology applications. An illustration in Fig. 6 was made in order to discuss the results of the elastic-plastic behavior observed in electrospun silica-based hybrid fibrous mats. Before the tensile test, initial isotropy and a randomly oriented network structure (Fig. 6c) exist in the mat-plane by random point-contact. In the initial stage of stretching, friction among fibers is easily overcome and they orient in the direction of applied stress. Finally, formation of aligned side-contact in the highly oriented fibrous bundles (Fig. 6d) may be useful for the damage resistance of silicabased electrospun mats.
4. Conclusion Fig. 6. Illustration to give insight into the tensile behavior observed in electrospun silicabased hybrid fibrous mats.
Tensile performance has been experimentally characterized in randomly oriented non-woven electrospun silica, silica/polydimethylsiloxane and silica/polymethyl methacrylate hybrid fibrous mats. Formation of aligned side-contact in the highly oriented fibrous bundles in the direction of applied stress was clearly confirmed in SEM images, which may be useful for the damage resistance of electrospun mats. This study may increases the understanding of the relationship between mechanical performance and fiber assembly of electrospun fibrous mats.
Fig. 4, and reveal the kinetics of fiber assembly. Without strain, a randomly oriented network structure was observed in Fig. 4a) and b), which indicates initial isotropy in the mat-plane. Most silica fibers distribute individually by random point-contact. Such contact points represent the friction between the fibers. Fig. 4c), d) and e) show the SEM images of silica fibers after tensile testing. The results clearly confirm that the majority of the fibers align with the direction of applied stress. Additionally, highly oriented fibrous bundles form in the 4
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