Syntheses Of Polyhedral Oligomeric Silsesquioxanes And Their Applications In Room Temperature Vulcanized Silicone Rubbers

Polyhedral oligomeric silsesquioxanes (POSS) are a special class of the oligomeric silsesquioxanes with an three-dimensional structure, the structures of which are between the silicon dioxide (SiO2) and silicone (R2SiO)n, which combined comprehensive properties of inorganic materials and organic materials. Therefore, comprehensive performance of polymeric composites prepared by introducing POSS into polymeric system by chemical bonding/physical blending can be improved to a different extent. In recent years, POSS-modified composites have received much attention to domestic and foreign researchers.Up to the present, the researches on hybrid materials of condensed room temperature vulcanized (RTV) silicone rubbers/POSS are rare. In this dissertation, POSS were mainly introduced into RTV silicone rubbers by chemical bonding, and a series of novel RTV silicone rubbers/POSS hybrid materials were prepared, subsequently, the effects of POSS on the thermal stabilities and mechanical properties of RTV silicone rubbers have been systematically investigated. The investigation of this dissertation has been divided into three parts:(a) syntheses and structure-property characterization of POSS with different substitutentsIn the dissertation, POSS monomers with different substitutents were prepared through hydrolytic condensation reaction, such as octavinyl-POSS (Vinyl-POSS), octamethyl-POSS (Me-POSS), octachloropropyl-POSS (Chloropropyl-POSS), octaphenyl-POSS (Ph-POSS) and octaoctyl-POSS (Octyl-POSS), and their structure and thermal stabilities had been further characterized. Meanwhile, reactive multi-armed POSS derivatives such as octa[(trimethoxysilyl)ethyl]-POSS (OPS) and divinyl-hexa[(trimethoxysilyl)ethyl]-POSS (DVPS) based on Vinyl-POSS were first synthesized by hydrosilylation, and their structure was also determined by corresponding experimental characterizations. By thermal gravity analysis (TGA), it was found that in nitrogen atmosphere, Vinyl-POSS and Me-POSS had the properties that were prone to sublimate at high temperature, and therefore, they had low char yield. But Chloropropyl-POSS and Ph-POSS in higher temperature first melted, then decomposed and charred, which finally resulted in a lot of residues, and so they had good thermal stability, and they were potential fire-retardant materials. But for Octyl-POSS, it tended to evaporate at high temperature, and also had better thermal stability. However, in air atmosphere, all of the organic substitutions of the above POSS monomers were easily oxidized into gas products resulting in weight loss, and the POSS molecules were transformed into a whitish silica powder.(b) the applied researches of POSS in unfilled RTV silicone rubbersIn this part, OPS and DVPS were firstly expectively applied to dealcoholized RTV silicone rubbers as cross-linkers, and different novel dealcoholized RTV silicone rubbers were prepared. And the thermal stabilities and mechanical properties of these novel RTV silicone rubbers were studied.And then, a series of novel OP types of RTV silicone rubbers were prepared by adding different weight percentage of cross-linker OPS. The formation of the cross-linked network in RTV silicone rubbers after they were cured had been confirmed by attenuated total reflection. The distribution of POSS in RTV silicone rubber systems was observed by scanning electronic microscopy (SEM), and the thermal properties and mechanical properties of these novel RTV silicone rubbers have been characterized expectively by TGA and universal testing machine. The results showed that all of these novel RTV silicone rubbers prepared with OPS cross-linker had better thermal stability and mechanical property, compared with the traditional tetra-functional cross-linkers (TEOS and TMOS) and the cross-linker MTMOS used commonly in industrial products. Meanwhile, the thermal decomposition products of the novel RTV silicone rubber were also monitored by real-time FTIR coupled with TGA, at high temperature, gaseous methane was formed in addition to cyclic dimethylsiloxanes (Dn). According to the above analysis, the reinforcing effect of OPS on thermal stabilities and mechanical properties of these RTV silicone rubbers likely originated from the combined effects of increasing dimensionality of effective three-dimensional networks in PDMS matrix resulting from special three-dimensional structure of novel POSS cross-linkers, plasticization of self-cross-linked POSS cross-linkers and uniform distribution of POSS-rich domains.Finally, a series of novel DVP types of RTV silicone rubbers were also prepared further by adding different weight percentage of cross-linker DVPS. The formation of the cross-linked network in RTV silicone rubbers after they were cured had been confirmed by attenuated total reflection. The distribution of POSS in RTV silicone rubber systems was observed by SEM, and the thermal properties and mechanical properties of these novel RTV silicone rubbers have been characterized expectively by TGA and universal testing machine. The experimental results demonstrated that all of these novel RTV silicone rubbers prepared with DVPS cross-linker had better resistance to thermal oxidative stability and mechanical property, compared with the RTV silicone rubbers prepared with the traditional tetra-functional cross-linker TEOS. Meanwhile, the thermal decomposition products of the novel RTV silicone rubber were also monitored by real-time FTIR coupled with TGA. It was found that in addition to cyclic dimethylsiloxanes (Dn), gaseous methane was always formed in the overall thermal degradation of the novel RTV silicone rubber (DVP). According to the above analysis, The reinforcing effect of DVPS on thermal stabilities and mechanical properties of these RTV silicone rubbers likely originated from the synergetic effects of increasing dimensionality of effective three-dimensional networks in PDMS matrix resulting from special three-dimensional structure of novel POSS cross-linkers, plasticization of self-cross-linked POSS cross-linkers and uniform distribution of POSS-rich domains.(c) the applied studies of POSS in the RTV silicone rubber filled with fumed silicaIn this section, based on OPS and DVPS as the cross-linkers, novel reinforced OPF and DVPF series of RTV silicone rubbers were prepared respectively by further adding fumed silica as reinforcing filler. And the distribution of POSS-rich domains and fumed silica in the systems was observed by SEM, and it was found that the sizes of POSS-rich domains and fumed silica in the RTV silicone rubbers are different, ranging from nanometer-scale to several micron. Subsequently, the thermal stabilities and the mechanical properties of these RTV silicone rubbers reinforced by POSS and fumed silica were mainly characterized expectively by TGA and universal testing machine. The experimental results showed that the synergetic effect between fumed silica and POSS cross-linkers (OPS and DVPS) indeed existed, compared with the reference and the blank RTV silicone rubber. The thermal stabilities and the mechanical properties of these RTV silicone rubbers were further enhanced with the increase in weight percentage of fumed silica, which were attributed to the synergetic effect.