riv ro u gji U for e 9 larg properties such as large internal surface area, small refractive water separation [9] and self-cleaning [10]. The hydrophobic aerogels are generally obtained by two Perfluoroalkylsilane (PFAS) has fluoroaklyl group which is HF catalyst in a molar ratio of 1:8:0.5:0.1 respectively. The PFAS was used as a coprecursor and the volume ratio of PFAS /E-40 was varied from 0 to 1. After stirring for about 1 h, the homogeneous alcosols were transferred to airtight Materials Science and Engineering C methods: (1) co-precursor method pioneered by Schwertfeger index, low thermal conductivity and high visible transparency [1,2,3]. The aerogels have been used in a variety of applications including Cerenkov radiation dectectors [4,5], thermal insula- tions, heat storage systems [6] and catalyst supports [7]. Most of the aerogels are hydrophilic and become wet in the presence of atmospheric moisture, they get deteriorated with time. There- fore, one of the most important requirements for long-term use of the aerogels is making them hydrophobic [8]. It was also reported that hydrophobic silica aerogels could be used in other practical applications such as efficient absorbers in solvent– hydrophobic and the fluorinated chains can be incorporated into organic or inorganic materials [13]. In this paper, we report and discuss the results on the effect of PFAS as a coprecursor on the hydrophobicity and physical properties of silica aerogels. 2. Experimental Silica aerogels were prepared by mixing polyethoxydisi- loxane (E-40) precursor, ethanol solvent and water containing increase. Fourier transform infrared spectra, relative pore size distribution and thermogravimetric analysis are used to characterize the hydrophobic aerogels. The fluoroaklyl groups are successfully incorporated into the silica aerogels in hydrolysis-condensation reactions. BET shows the surface area of the aerogels is larger than that of the as-prepared pure silica aerogels. And it has the largest surface area when the PFAS/E-40 volume ratio is at 0.6. The weight loss of the aerogels is very small up to 500 °C and the contact angle of water to the surface of the aerogels is about 145°. © 2006 Elsevier B.V. All rights reserved. Keywords: Silica Aerogels; Hydrophobic; Perfluoroalkylsilane 1. Introduction Silica aerogels are porous materials that have unusual [11] and (2) surface derivatization of alcogel followed by Yokogawa and Yokoyama [12]. The first method is simple and less time consuming compared to the second one. perfluoroaklysilane (PFAS) as a coprecursor. With the increase of the and high visible transparency, etc. They have been used in a variety of applications. Most of the aerogels are hydrophilic and become wet in the presence of atmospheric moisture and get deteriorated with time. Therefore, one of the most important requirements for long-term use of the aerogels is making them hydrophobic. In this paper, hydrophobic aerogels are synthesized from polyethoxydisiloxane (E-40) and use volume ratio of PFAS/E-40, the gelation time, shrinkage rate and density Hydrophobic silica aerogels de and perfluo Bin Zhou, Jun Shen ⁎, Yuehua W Pohl Institute of Solid State Physics, Ton Received 6 May 2006; received in revised Available onlin Abstract Silica aerogels are porous materials with unusual properties such as ⁎ Corresponding author. Tel.: +86 21 65982762; fax: +86 21 65986071. E-mail address:
[email protected] (J. Shen). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.06.032 ed from polyethoxydisiloxane alkylsilane , Guangming Wu, Xingyuan Ni niversity, Shanghai 200092, P.R. China m 29 June 2006; accepted 29 June 2006 August 2006 e internal surface area, small refractive index, low thermal conductivity 27 (2007) 1291–1294 www.elsevier.com/locate/msec plastic molds for gelation. After aging, the alcogels were supercritically dried using ethanol as the drying solvent in an autoclave. Bulk density of the aerogel samples was calculated by measuring its weight to volume ratio. The volume shrinkage of the samples was determined by measuring the difference between the volumes of alcogel and the aerogel. The specific surface areas were determined by N2 adorption/desoption measurements (Tristar 3000). Fourier transformation infrared (FT-IR) adsorption spectra were measured by an FT-IR spectrophotometer (FTS-40 Bio-Rad). Contact angles on the aerogels were measured by a contact angle measurement 3.2. Physical properties Fig. 1. Pore size distribution of the silica aerogels: (a) PFAS/E-40=0; (b) PFAS/ E-40=0.2; (c) PFAS/E-40=0.4. 1292 B. Zhou et al. / Materials Science and E system (JC2000A POWEREACH). The thermal stability was tested using thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) (STA449C). 3. Results and discussion 3.1. Synthesis The reactions relative to PFAS can be written as Eqs. (1), (2) and (3) [13,14]: ð1Þ ð2Þ ð3Þ Rf is the fluoroaklyl group. The reactivity of E-40 with water is much higher than that of PFAS because of the nonreactive Rf groups in PFAS [13]. So the primary silica clusters formed are mainly due to the hydrolysis and condensation reactions of E-40 with water having large number of OH groups on their surface. At later stages the hydrolyzed E-40 and PFAS precede a polycondensation reaction (Eq. (1)) to form aerogels with Rf groups incorporated. Besides, a little hydrolyzed PFAS (Eq. (2)) and hydrolyzed E-40 would also have a polycondensation reaction (Eq. (3)). Table 1 Physical properties of silica aerogels PFAS/E-40 volume ratio Gelation time (h) Volume shrinkage Density (g/mm3) Surface area (m2/g) 20 °C 50 °C 0 2 0.7 0.40 52.55 752.88 0.2 4 1.5 0.66 107.30 964.37 0.4 18 5 0.80 206.70 1089.11 0.6 42 14 0.83 324.80 1093.28 0.8 90 24 0.88 393.42 1024.52 1 158 43 0.89 431.59 971.10 ngineering C 27 (2007) 1291–1294 The effect of PFAS/E-40 volume ratio on the gelation time for silica alcogel at different temperature is shown in Table 1. One el ( 1293d Engineering C 27 (2007) 1291–1294 can see that the gelation time becomes longer with the increase of the PFAS/E-40 volume ratio. This is due to the fact that the Fig. 2. FTIR spectra of modified aerog B. Zhou et al. / Materials Science an hydrolysis and condensation reactions are retarded with the increase of the Si–Rf groups and the relatively decrease of the number of the Si–OC2H5 groups in the sol [15]. Besides, the gelation time decreases dramatically with the rising temperature. Table 1 illustrates the effect the PFAS/E-40 volume ratio on the percentage of relative volume shrinkage of the aerogels. As the PFAS increases the volume shrinkage of the aerogels increases. With the increase of the steric crowding of nonhy- drolysable Rf groups attached to the silica cluster, the cross linkage between the clusters becomes less and the network becomes weak. Therefore, the network shrinks during the supercritical drying process causing an increase in the volume shrinkage. The bulk density of the aerogels also increases because of the increase of the volume shrinkage. 3.3. BET and FT-IR of the aerogel The surface area increases with the increase of the PFAS at first, and has the largest surface area when the PFAS/E-40 volume ratio is at 0.6, then decreases with the increase of the PFAS. All samples have surface areas larger than the unmodified aerogels (Table 1). The pore size distributions of the aerogels are shown in Fig. 1. The unmodified aerogel shows a maximum at about 20 nm. However, the peak of the Rf aerogels is shortened and some pores less than 10 nm emerged, which shifts to progressively lower diameter with the increase of the PFAS. These micro- spores are the main contribution to the increase of the surface areas [16]. The microstructure of the aerogels is influenced by the con- centration of the fluorinated precursor. Some mespores will a) and unmodified silica aerogels (b). appear in aerogels when the fluorinated precursor is at a low concentration [14] and the aerogels may consist of two kinds of pores, so the pore diameter distribution separates in two dis- tributions. With the increasing of the PFAS/E-40 volume ratio, more mespores are formed; therefore the distribution separates in two even more narrow distributions. Meanwhile, during the hydrolysis and condensation process, the Rf molecular chain will be introduced into silica clusters and the pores inside silica network will be partially occupied. The pores with smaller size are formed and the diameter generally decreases with the in- crease of the concentration of the fluorinated precursor. The FT-IR adsorption spectra for the silica aerogels are shown in Fig. 2. The peak at 1230 cm−1, 900 cm−1 and the band between 1330 cm−1 and 1470 cm−1 (Fig. 2(a)) are assigned to Fig. 3. Photograph of awater droplet on the surface of the hydrophobic silica aerogel. The thermal stability of the hydrophilic and hydrophobic aerogels was tested using TGA-DSC (Fig. 4).The percentage of 1294 B. Zhou et al. / Materials Science and Engineering C 27 (2007) 1291–1294 the C–F bonds in the aerogel [17]. The band at 2900 cm−1 is about the stretching of C–H bonds [18], which become much greater in Fig. 2(a) due to the C–H bond of the fluoroaklyl groups in the silica aerogel. The adsorption peak at 1100, 800 and 470 cm−1 are the characteristic FT-IR features of Si–O–Si bonds [19]. The bands at 3400, 1640 and 960 cm−1 are corresponding to the O–H groups or absorbed water [19]. 3.4. Hydrophobicity of the aerogel and the thermal analysis In order to test hydrophobicity, we have added a drop of water on top of an Rf aerogel. The water droplet was photo- graphed and the contact angle was calculated using the formula θ=2tan−1(2h/d), where h is the height of the water droplet and d is the width of the droplet touching the aerogel surface. The measured contact angle for the hydrophobic aeroge is estimated up to 145° (Fig. 3). Fig. 4. TGA analyses of hydrophilic aerogel (a) and hydrophobic aerogel (b). weight loss in the as-prepared pure silica aerogels observed in two steps around 100 °C and 450 °C is due to physically adsorbed ethanol and water in pores, and the oxidation of CH3 groups or other residual organic groups in the silica aerogel. For the hydrophobic aerogels, the percentage of weight loss is quite small up to a temperature of 500 °C, which is due to the decomposition of the fluoroaklyl groups. 4. Conclusions The use of PFAS as a coprecursor with E-40 resulted in hydrophobic silica aerogels. With the increase of the PFAS, the gelation time, the shrinkage rate and density increase, which are due to the steric effects of fluoroaklyl groups. The surface area of the fluorinated aerogels is larger than that of the as-prepared pure silica aerogels. And Rf aerogel has the largest surface area when the PFAS/E-40 volume ratio is at 0.6. The contact angle of water to the surface of hydrophobic aeogels can be up to 145°. The weight loss of the hydrophobic aerogel is very small up to 500 °C. References [1] A.R. Bizulaev, A.F. Danilyuk, S.F. Ganzhur, E.A. Kravchenko, A.P. Onuchin, Nucl. Instrum. Methods Phys. Res., A 433 (1999) 396. [2] J. Fricke, T. Tillotson, Thin Solid Films 297 (1997) 212. [3] Z.S. Deng, J. Wang, J.D. Wei, J. Shen, B. Zhou, L.Y. Chen, J. Sol–Gel Sci. Technol. 19 (2000) 677. [4] P.J. Carlson, K.E. Johansson, J.K. Norrby, O. Pingot, S. Tavernier, F. Van Den Bogert, L. Van Lancker, Nucl. Instrum. Methods 160 (1979) 407. [5] J. Pinto da Cunha, F. Neves, M.I. 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Deng, J.D. Wei, A.M. Wu, Y.P. Bao, J. Wang, J. Shen, B. Zhou, L.Y. Chen, J. Inorg. Mater. 15 (2000) 381. [17] M. Tillotson Thomas, G. Foster Kenneth, John G. Reynolds, J. Non-Cryst. Solids 350 (2004) 202. [18] K. Awazu, H. Onuki, Appl. Phys. Lett. 69 (1996) 482. [19] G.M. Wu, J. Wang, J. Shen, Q.Y. Zhang, B. Zhou, Z.S. Deng, B. Fan, D.P. Zhou, F.S. Zhang, J. Shen, J. Phys., D, Appl. Phys. 34 (2001) 1301. Hydrophobic silica aerogels derived from polyethoxydisiloxane �and perfluoroalkylsilane Introduction Experimental Results and discussion Synthesis Physical properties BET and FT-IR of the aerogel Hydrophobicity of the aerogel and the thermal analysis Conclusions References