Study On The Synthesis Of Oligmeric Silsesquioxanes By Co-Hydrolyzation And Properties Of Their Composites

Polysilsesquioxanes are of special structures and excellent property with thermal oxidation stability and electrical property, which are widely utilized in civilian industries and national defence fields. However, complex preparation and molding processes of polysilsesquioxanes restrict their application in in-situ adhesion and advanced composites. In this paper, trialkoxylsilanes were employed to prepare oligomeric silsesquioxanes with various functional groups via co-hydrolyzation. The resultant oligomers are characterized carefully and applied experimentally.Oligomeric silsesquioxanes with two different organic groups were synthesized by the co-hydrolyzation of alkoxylsilane in the present of acid. The ratio of reagents was adjust to obtain various combination of functional groups. The resultant oligomers are characterized by IR and GPC.Oligomeric silsesquioxane obtained from vinylsilane exhibited a lower viscosity than that of oligomers obtained via co-hydrolyzation. The more vinyl content in oligomers resulted in the lower viscosity. Therefore, vinyl content could be adjusted to control the viscosity of oligomers.The gel time and gel content were measured to study the curing behavior of oligomeric silsesquioxane with vinyl group, taking account of catalyst structure, temperature and time. Results showed that catalyst structure is the foremost factor of curing behavior. As far as curing rate is considered, temperature gained more significance than catalyst dosage and time.Quartz fiber and quartz fiber cloth were selected as reinforcing materials to prepare high temperature composite based on oligomeric silsesquioxanes matrixs. Mechanical properties and dielectric property of the composite were tested before and after service at different temperature from250to650C for400seconds. Results showed that the composite exhibited excellent thermal-resistant property. The tensile strength of the composite after heat treatment at550℃was over360MPa,80%of that of untreated composite. The flexural strength remained342MPa,82%of that of untreated composite. The composite showed a steady dielectric property regardless of the heat treatment. Higher treating temperature, bigger dielectric constant, which is below3.30after treatment at550℃. It is found that fiber pretreatment methods are of important influence on the tensile property of fiber and composites. Results showed that regent on the surface of fiber could be removed82-93%by way of dipping in toluene and then heating at350℃for400seconds, meanwhile keeping an acceptable tensile strength of silica fiber, over2.0GPa. A reasonable explanation is as follows:the regent could undergo thermal oxidation and degradation at high temperature, which weakened the abhesion of quartz fiber to matrix resin and thus reduces the physical properties of the composite. Meanwhile, carbonization residual arised from regent strongly affects the dielectric property of the composite.