The Microstructure Control And Performance Analysis Of Silicacellulose Nanocrystals Aerogels

Aerogels possess on many unique properties because of its high porosity and special three-dimensional network skeleton structures. Due to their thin and brittle nano-porous networks, however, aerogels are provided with the disadvantages of low strength and poor toughness, which restricts their application and development. Therefore, it is significative to overcome the brittleness of the network structures and increase strength and toughness of aerogels by controlling the pore structures.In this paper, the cellulose was prepared by chemical pretreatment combined with ultrasonic process and methyltrimethoxysilane(MTMS) was chosen as silica precursor. Silica-cellulose composite aerogels were synthesized under different contents of cellulose by sol-gel process and several drying methods. Then, the microstructures and mechanical properties were analyzed.There are some steps to extract the cellulose from wood powder. The mixed solution of benzene and alcohol is used to remove wax and lipid. Then, lignin and hemicellulose are removed by using acidified sodium chlorite and potassium hydroxide solution, respectively. The fibers are further refined by hydrochloric acid. Finally, after ultrasonic treatment, the nano-scale cellulose with a large length to diameter ratio is obtained. In the whole processes, the functional groups and the structures of cellulose are not changed. The cellulose is still I type and the crystallinity is close to 50 %. When cotton pulp was chosen as raw materials, the functional groups and the structures of cellulose also are not influenced by the chemical pretreatment and ultrasonic treatment which only act as refinement and dispersion for the fibers. The crystal forms of cellulose are still I type and the crystallinity is 76.08 %. The appearance of cellulose obtained by acid extracting method assumes a short rob, size uniformity and low length-diameter ratio.Silica-cellulose composite aerogels were prepared by mixing MTMS, DMSO, acetic acid aqueous solution, urea and cetyl trimethyl ammonium chloride(CTAC) via the sol-gel process and atmospheric drying or freeze drying method. The porous characteristics and mechanical property of the composite aerogels for different contents of cellulose were analyzed.The results show that silica-cellulose composite aerogels have threedimensional networks in which cellulose renders the states of aggregation or dispersion. The porous structure of freeze drying aerogels is much better than that of atmospheric drying one. The thermostability of aerogels is influenced by adding cellulose, i.e., the more cellulose is, the more thermal weight loss is. The noncrystalline state and chemical bonding of silica are not influenced by the addition of small amounts of cellulose. Comparing with pure silica aerogels, silica-cellulose aerogels have lower specific surface area, total pore volume and average pore size. The mechanical properties of the composites are changed by the change of cellulosic contents. With the increase of cellulosic contents, the compressive strength of the composite increases, but the toughness decreases. After atmospheric drying, the strength of the aerogels is obviously increased, but their toughness and the amount of deformation are reduced, which is only about 5 %.