Synthesis And Properties Of Doubly Cross-linked Polyorganosiloxane Aerogels

Silica aerogels with ultra-low thermal conductivity are promising candidates for thermal insulation.However,the practical application of silica aerogels is hardly difficult to be achieved due to the brittleness issue.Aiming at this issue,the organic-inorganic hybridization is an effective method to achieve the mechanical reinforcement of silica aerogels.In this paper,based on the design principle of hybrid silica aerogels,polyorganosiloxane aerogels with doubly cross-linked structure were prepared by radical polymerization and hydrolysis-polycondensation using vinyl-containing silane as the main raw material.Focusing on the flexible and transparent design requirements of aerogel insulation materials,polyorganosiloxane aerogels were given low thermal conductivity,high compressive elasticity,and high optical transparency,respectively.The main conclusions are as follows:（1）In order to obtain a mechanically enhanced aerogel insulation material,based on the double-crosslinking strategy,polyvinylpolymethylsiloxane（PVPMS）aerogels with low thermal conductivity were prepared by optimizing the fabrication parameters of radical polymerization process and sol-gel process.Firstly,the effect of free radical polymerization on the molecular weight of polyvinylmethyldimethoxysilane（PVMDMS）was studied.It was found that the PVMDMS polymer has a high molecular weight when the polymerization time was 72 h and the amount of initiator di-t-butyl peroxide（DTBP）was DTBP/Si=0.100 mol·mol^-1.The weight-average molecular weight was 8563.Secondly,the effect of sol-gel process on the properties of PVPMS aerogels was studied.When the amount of benzyl alcohol（Bz OH）was Bz OH/Si=5.0 mol·mol^-1,the amount of alkali catalyst tetramethylammonium hydroxide（TMAOH）was TMAOH/Si=0.075 mol·mol^-1,and the gel aging temperature was 100℃,PVPMS aerogels would exhibit good performance.The density and linear shrinkage were 0.193g·cm^-3 and 12.94%,specifically.The specific surface area was as high as 1155 m²·g^-1,the water contact angle was 124.0°,and the compressive strength with 50%strain was2.87 MPa,which was much higher than that of silica aerogels.In addition,the total thermal conductivity of PVPMS aerogels with a density of 0.221 g·cm^-3 was 0.02197W·m^-1·K^-1 at room temperature,wherein the vacuum thermal conductivity was 0.01620W·m^-1·K^-1,and the gas thermal conductivity was 0.00577 W·m^-1·K^-1.The thermal insulation mechanism of PVPMS aerogels was that the doubly cross-linked network structure could effectively inhibit solid heat conduction and gaseous heat conduction.（2）For designing flexible heat-insulating materials,a novel strategy of constructing elastic double-crosslinking networks was proposed.And PVMDMS reinforced methyltrimethoxysilane（MTMS）based silica（PRMS）aerogels with high compressive elasticity were prepared by compounding PVMDMS with MTMS.The effect of sol-gel process on the properties of PRMS aerogels was studied.When the amount of PVMDMS polymer was PVMDMS/Si=0.50 mol·mol^-1 and the amount of solvent was Bz OH/Si=5.5 mol·mol^-1,PRMS aerogels would exhibit good performance.The density and linear shrinkage were 0.135 g·cm^-3 and 8.13%,specifically.The specific surface area was as high as 955 m²·g^-1,the water contact angle was 136.9°,and the thermal conductivity was 0.02354 W·m^-1·K^-1 at room temperature.Simultaneously,monolithic PRMS aerogels exhibited a 100%compression resilience and an excellent anti-fatigue property,which could be compressed-decompressed with 20%strain for 20cycles.The high compressive elasticity of PRMS aerogels was primarily related to a reasonable ratio between methyl groups,hydrocarbon polymers and polysiloxanes.（3）For satisfying the application requirements of transparent insulating aerogel glazing,polyvinylpolysilsesquioxane（PVPSQ）aerogels with high optical transparency were prepared by a new fabrication method of adjusting solvent medium based on the double crosslinking strategy.Firstly,the effect of free radical polymerization on the molecular weight of polyvinyltrimethoxysilane（PVTMS）was studied.It was found that the PVTMS polymer has a high molecular weight when the polymerization time was36 h and the amount of DTBP was DTBP/Si=0.025 mol·mol^-1.The weight-average molecular weight was 9298.Secondly,the effect of sol-gel process on the properties of PVPSQ aerogels was studied.When the molar ratio of ethanol（Et OH）to water（H₂O）was Et OH/H₂O=12/15 mol·mol^-1 and the volume ratio of hexamethyldisilazane（HMDS）to Et OH was HMDS/Et OH=10/90 vol·vol^-1,PVPSQ aerogels would exhibit good performance.The density and linear shrinkage were 0.187 g·cm^-3 and 15.49%,specifically.The specific surface area was up to 1030 m²·g^-1,the water contact angle was 123.9°,the thermal conductivity was 0.02576 W·m^-1·K^-1 at room temperature,and the compressive strength with 20%strain was 0.609 MPa.In addition,PVPSQ aerogels showed a high optical transmission in the visible-light region,the typical PVPSQ aerogels with 2 mm thickness had a high light transmittance of 92.3%at 550 nm wavelength.The inhibition of phase separation and light scattering was the key to obtaining a highly optically transparent PVPSQ aerogel.