The sol-gel transition in the hydrolysis of metal alkoxides

Journal of Non-Crystalhne Solids 48 (1982) 31-46 North-Holland Publishing Company

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T H E S O L - G E L T R A N S I T I O N IN T H E H Y D R O L Y S I S OF M E T A L A L K O X I D E S IN R E L A T I O N T O T H E F O R M A T I O N O F G L A S S F I B E R S AND F I L M S S. S A K K A and K. K A M I Y ADepartment of Industrial Chemistry, Faculty of Engineering, Mie University, Tsu, Mie, 514, Japan

The sol-gel transition of S i ( O C 2 H s ) 4 w a s investigated by measuring the volume, the density, the SiO2 content, the viscosity and the specific and intrinsic viscosities of the solution. The transition behavior of Si(OC2Hs)4 solutions with different amounts of water and different catalysts was compared. The measurements indicated that solutions with a small water content were comprised of finear polymer when catalyzed by HCI. Fibers could be easily drawn from such solutions immediately before gelation. Solutions catalyzed by HC1 and containing a large amount of water behaved similarly to solutions catalyzed by NH4OH, suggesting that a high water content causes the formation of non-linear or network colloidal polymers in hydrolysis-polycondensation. Such solutions did not exhibit spinnability but gelled into elastic bulk masses which were easily converted to bulk glasses.

1. Introduction The alkoxide method for preparing oxide glasses in which the gels obtained as a result of the hydrolysis and accompanying polycondensation of metal alkoxides are converted to the corresponding oxide glasses by heating at relatively low temperatures has been developed as a new technique[1-3]. This method provides oxide glasses which are difficult to prepare by the conventional melting technique. SiO 2 glass and m a n y other multicomponent oxide glasses[3-5] have been produced by the alkoxide method as fibers, thin films, coating films and monolithic bodies. The transitions of gels to oxide glasses have been studied by m a n y investigators[3,6] by thermal analysis, X-ray diffraction, infrared spectroscopy, picnometric measurement, electron microscopy etc. However, only a few works have been reported on the sol-gel transition of metal alkoxides. The objective of the present study is to investigate the sol-gel transition occurring in the hydrolysis and polycondensation of silicon tetraethoxide Si(OC2Hs) 4 by measuring the density, the viscosity, the intrinsic viscosity etc., of the solution. The transition behavior is compared between the different alkoxide solutions for SiO 2 glasses; some of them are suitable for the preparation of fibers and others for bulk bodies. The type of polymeric product which might influence the spinnability and the ability to solidify without heavy cracking is also discussed. 0022-3093/82/0000-0000/$02.75© 1982 North-Holland

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S. Sakka, K. Kamiya/ The hydrolysis of metal alkoxides

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S. Sakka, K. Kamiya/ The hydrolysis of metal alkoxides

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2. Experimental2.1. Preparation of alkoxide solutions Silicon tetraethoxide Si(OC2Hs) 4 supplied by Wako Chemicals Co. was used as a starting material. Calculation shows that the SiO 2 content of this chemical corresponds to 28.9 wt%. Five alkoxide-water-ethanol mixture solutions were prepared, the compositions of which are listed in table 1. Water was required for the hydrolysis. Ethanol was used for diluting the alkoxide. Hydrogen chloride HCI and ammonium hydroxide N H 4 O H were used as catalysts for hydrolysis. The water contents were adjusted so that the mole ratios of water to alkoxide were unity for solutions I, II and IV, twenty for solution III and two for solution V, respectively. The mole ratio of water to alkoxide equaling unity corresponds to half the amount of water required for the complete polycondensation of Si(OC2H5) 4 into SiO 2, assuming eq. (1) for hydrolysis, eq. (2) for polycondensation and eq. (3) for the net reaction. nSi(OC2Hs),+ 4 n H 2 0 -, nSi(OH),+ 4 n C 2 H s O H, nSi(On)4 --, nSiO 2+ 2 n n 2 0, n S i ( O C 2 n s ) 4+ 2 n 2 0~ nSiO 2+ 4nC2HsOH. (1) (2) (3)

The amount of ethanol was adjusted so that the concentration of the alkoxide which is expressed as the SiO 2 content in grams per 100 cm 3 of the solution is alike for solutions I, III, IV and V. The concentration of alkoxide in solution II is about two times larger than that in the others. The hydrogen chloride HCl was used as a catalyst for solutions I, II and III, and N H a O H for solutions IV and V. The mole ratio of catalyst to alkoxide was kept at 0.01 for all the solutions. In order to prepare clear solutions without any precipitation, the alkoxide was diluted with half the amount of ethanol listed in table 1, and then the mixed solution consisting of the specified amount of water and another half of the ethanol containing HCI or N H 4 O H was added drop by drop to the alkoxide-ethanol mixture solution from a burette under continued stirring. These operations were carded out at room temperature. The addition of the water-ethanol mixture to the alkoxide had to be made slowly, otherwise white fluocculent precipitates resulted, especially when N H 4 O H was used as a catalyst for hydrolysis. The solutions thus prepared were kept standing at 25°C in the ambient atmosphere without any cover. 2.2. Measurements of volume, density and SiO 2 content of the solutions The volume and weight of the alkoxide solution were measured at certain intervals while it was kept at 25°C until its gelling. The density of the solution was determined by weighing 10 cm 3 of the solution at 25°C. In order to

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S. Sakka, K. Kam(va/ The hydrolysis of metal alkoxides

determine the SiO 2 content, 100 cm3 of the solution was forcibly hydrolyzed and condensed to SiO2 gel by adding concentrated ammonia water. …… 此处隐藏：31111字，全部文档内容请下载后查看。喜欢就下载吧 ……