Preparation And Modification Of Organosilicon Aerogels

Aerogel is a product of hydrogel or organic gel after drying with liquid in the wet gel replaced by air without much change in the network structure. Aerogels possess high porosity,low density and many other unique properties. The traditional aerogels are prepared by supercritical drying technique. Although the volume shrinkage of the aerogels prepared by this technique is small, the structure of which is not stable and is expensive, its preparation cycle takes a long time. Due to the fact that it is easy to absorb moisture, which results deterioration of the properties of aerogels, it is not suitable for large scale production. Now the main methods used in atmospheric pressure drying process needs the gel network enhancement and surface modification. The preparation of modified organosilicon aerogels usually replaces hydroxyl groups of the surface with hydrophobical organosilicon groups. But the modified surface still faces much strong capillary force in drying process. In this thesis,volatile cyclohexane is added to the gel before the surface modification. At atmospheric pressure, the monolithic organosilicon aerogels with smaller volume shrinkage and high porosity have been prepared. The method is suited for large scale production. In addition, a simple method is proposed for the preparation of raspberry-like polymethylsilsesquioxane particles mediated by tinny calcium carbonate particles. The study of the thesis mainly consists of the following three parts:Firstly, under the condition of using methyltrimethoxysilane as silicon source,superhydrophobic organosilicon aerogel in the form of powder is obtained. Hydrolysis of methyltrimethoxysilane is performed at first, then sodium silicate is used as a catalyst for condensation reaction. After the gel formation, the gel is crushed and additional methyltrimethoxysilanes hydrolysate is added and ammonia is used to catalyze precipitation to occur. Organosilicon aerogel in powder form is obtained after drying the precipitated solid in air. The influence of amounts of methyltrimethoxysilane and sodium silicate solution on the density of the powder aerogel is studied. And its morphology is characterized by SEM(Scanning Electron Microscope). The results show that more methyltrimethoxysilane means a faster gelation process, giving higher hydrophobicity, decreasing the porosity and increasing density. Increasing the amount of sodium silicate solution, gel process becomes slower, hydrophobicity decreases, and the gel become more transparent and the density increases. SEM results show that the gel structure is loose and porous. Under the same condition, water is replaced with ethanol as the solvent. The study finds that compared to water as solvent, the gelation process becomes slower. The SEM image shows that powder becomes finer after drying.Secondly, this thesis discusses the methods of monolithic aerogel preparation usingsodium silicate solution and ammonia as catalyst, respectively. Methyltrimethoxysilane is used as silicon source and cyclohexane as an additive. The hydrolysate of methyltrimethoxysilane with cyclohexane is emulsified, then an alkali is used as catalyst to change it to a gel. Monolithic aerogels are prepared by using the method of atmospheric drying. Through comparison of the products prepared with and without addition of cyclohexane, the following conclusion could be drawn. The addition of cyclohexane can decrease volume shrinkage ratio significantly. At the same time, the aerogel density become lower, and porosity become higher. Due to a large amount of salt resides in the aerogel when it is prepared under catalyzation by sodium silicate. Aerogel preparation is also done by replacement of sodium silicate with ammonia as catalyst. Different proportions of methyltrimethoxysilane and cyclohexane are used to prepare monolithic aerogel, when the dosage of cyclohexane increased to 15 mL, volume shrinkage becomes small, low to 9.3%,and the density becomes low to 0.0594 g/cm~3. The porosity is 96.87%. When the dosage of cyclohexane is over 15 mL, density become increasingly higher, and the volume shrinkage ratio and porosity decreases. By means of SEM and TEM(Transmission Electron Microscope)characterization, it is found that the inner structure has a large number of micron hollow sphere crosslinked to form a porous structure. In order to enhance the network structure of the gel, ethanol and water are mixed as solvent. Ethanol and water in different proportions were studied. The study shows that more water means that the structure of the aerogel is not strong and fragile. More ethanol means a stronger gel structure. In addition, TEM and SEM found that the aerogels consist of hollow balls.Finally, the particle formation behavior of methyltrimethoxysilane is researched in the presence of a small amount of inorganic calcium carbonate particles. Methyltrimethoxysilane is first hydrolyzed in water under the catalyzation by hydrochloric acid to produce hydrolyzate and then neutralized by base sodium hydroxide. Under the neutral condition, the hydrolyzate of methyltrimethoxysilane is mixed with a small amount of newly-prepared precipitation of calcium carbonate. Following this, hydrolytic polycondensation reaction catalyzes by adding ammonia to the mixture was conducted. The results show that polymethylsilsesquioxane(PMSQ) particles with raspberry-like morphology are obtained by this method, while those without addition of calcium carbonate are of smooth surface, which is confirmed by scanning electron microscopy(SEM) micrographs. The static water contact angles changes from 136.2 ± 0.9° of pure PMSQ particles with smooth surface to 146.3 ± 1.1°with raspberry-like morphology, showing a great increment in apparent hydrophobicity.