Research On Regulation Of Crosslinking Degree And Properties Of Addition-Cure Liquid Silicone Rubber Reinforced By Vi-POSS

Addition-cure liquid silicone rubber（ALSR）is a kind of higher grade silicone elastomers.It has not only structural stability,but also excellent properties such as non-toxic,good biocompatibility,no by-products,small dimensional shrinkage,synchronous deep vulcanization inside and outside,excellent flow performance,and low vulcanization temperature,etc.So far,the application of ALSR has extended to many fields,such as medical orthopedics,electronics and electronics,defense and military industry,automotive industry,and aerospace and so on.Pure ALSR matrix has low intermolecular force,low cohesion energy,and poor comprehensive mechanical properties.In practical applications,reinforcing agents are often used to regulate the crosslinking degree of ALSR,optimize the interaction between components,and thereby improve the performance of ALSR.Polyhedral oligomeric silsesquioxanes（POSS）have a unique nanoscale cage structure and excellent solubility,which can be used as a potential building block for organic inorganic hybrid materials.Using POSS as a reinforcing filler to prepare POSS/silicone rubber composites can further improve the properties of silicone rubber in terms of heat resistance and mechanical strength.In this study,two new liquid polyfunctional POSS modified by siloxane oligomers were designed and synthesized,and used as reinforcing fillers to prepare POSS/Addition-cure liquid silicone rubber composites.The effects of POSS on the comprehensive properties of silicone rubber composites were systematically investigated.This work is mainly carried out from the following aspects:1.Using hexamethylcyclotrisiloxane,anhydrous methanol,and vinyl chlorosilane as the main raw materials,the heteroterminal oligosiloxane ^{Me O}MD₂M^Viwas prepared by a two-step method,and its structure was characterized by instrumental analysis.The siloxane was characterized in that it is composed of four-Si（CH₃）₂O-segments,and the two terminal groups at the end of the molecular chain are dimethylmethoxysilicyl and dimethylvinylsilyl,respectively.2.Synthesis of novel polyhedral oligosilsesquioxane derivatives through classical Piers-Rubinsztajn reaction（P-R reaction）.Using Q type H-POSS,Q type Me H-POSS,and ^{Me O}MD₂M^Vi as starting materials,liquid methyl vinyl POSS（Me Vi-POSS）and liquid vinyl POSS（Vi-POSS）were produced by using tris（pentafluorophenyl）borane to catalyze the coupling reaction of silyl hydrogen with siloxy groups,respectively and their structures and thermal stability properties were analyzed for relevant characterization.In thermogravimetric analysis（TGA）,it was showed that Vi-POSS and Me Vi-POSS have high thermal stability and are not prone to sublimation;Comparing above two POSSs,Vi-POSS has a higher thermal degradation temperature and better thermal stability due to different substituents.Using Me Vi-POSS and Vi-POSS as reinforcing fillers,the effects of POSS addition on the control of crosslinking degree,thermodynamics,and optical properties of addition molded liquid silicone rubber were investigated.For the addition-cure liquid silicone rubber reinforced with Me Vi-POSS,it was found that when the addition amount of Me Vi-POSS was 4wt%,the tensile strength,elongation at break,and Young’s modulus of the silicone rubber increased from 0.34MPa,57.65%,and 0.74 MPa of pure silicone rubber to 1.18 MPa,165.40%,and 0.81MPa,respectively;The storage modulus increased from 3.68 MPa to 6.34 MPa,and the glass transition temperature increased from-88.08℃to-83.26℃;It was also found that the thermal stability of silicone rubber under nitrogen was improved after the introduction of Me Vi-POSS,and the UV transmittance of silicone rubber remained above 90%when Me Vi-POSS was added to 6wt%;It was demonstrated by scanning electron microscopy that Me Vi-POSS was uniformly dispersed in silicone rubber with no obvious aggregation.For the addition-cure liquid silicone rubber reinforced with Vi-POSS,it was found that when the addition amount of Vi-POSS was 4wt%,the tensile strength,elongation at break,and Young’s modulus of the silicone rubber increased from 0.34MPa,57.65%,and 0.74 MPa of the pure silicone rubber to 2.52 MPa,243.15%,and1.16 MPa,respectively;The storage modulus increased from 3.68 MPa to 7.04 MPa,and the glass transition temperature increased from-88.08℃to-82.71℃;At the same time,it was found that the heat resistance of silicone rubber under nitrogen was improved after the introduction of Vi-POSS,and when Vi-POSS was added to 6wt%,the silicone rubber still had excellent transparency.Through scanning electron microscopy,it was proved that Vi-POSS can be uniformly dispersed.By comprehensively comparing the effects of Me Vi-POSS and Vi-POSS on the properties of silicone rubber,it is found that Vi-POSS has a better regulation effect on the crosslinking degree of silicone rubber and a better reinforcement effect.