Preparation And Properties Of Polymer Modified Silica Aerogels

Silica aerogels are of great research value and abroadly applied in thermal insulation,adsorption,catalytic,aerospace,high-energy physics,pharmaceutical and environmental because of their exceptional properties,such as low density,high porosity,large specific surface areas,low thermal conductivity and unique pore structure.However,silica aerogels are inherently fragile because of the pearl-necklac-like network structure.The widespread use of silica aerogels has been prevented by their poor mechanical properties.To solve these problems,we focused on polymer reinforced aerogels.A series of polymer modified aerogels were synthesized respectively via submerging and polymerizing method,carbon nanofibers combined polymer method and thermally induced phase separation.And the structures and properties of these modified aerogels were characterized and analysied.While improving the mechanical properties,we also target on simplifying process,reducing consumption of organic solvents and maintaining the low density and high porosity of aerogels.Some progresses in this thesis are briefly summarized as follows.Submerging and polymerizing method was used to synthesise poly?methylmethacrylate?modified silica aerogel.A series of PMMA modified silica aerogels were prepared via sol-gel process followed by ambient pressure drying through hydrolysis and poly condensation of low-cost sodium silicate as a precursor.Subsequently,the effects of various concentrations of MMA solution and silane coupling agent on the structure and properties of aerogels were investigated.Results show that the proper concentration of silane is 37.5vol-%,and the thermal stability temperature of PMMA modified aerogels is 280?.Modified by 50%PMMA,the resulting aerogels have a 13.7-fold increase in Yong's modulus and a 15.1-fold increase in hardness according to the nanoindentation test,CNFs doped silica aerogels and CNFs/PMMA modified aerogels were obtained under ambient conditions based on the carbon nanofibers combined polymer modification method.Absorbency,Zeta potential and surface tension were measured to analyses and compare the dispersion homogeneity of carbon nanofibers?CNFs?suspension which dispersed by sodium dodecyl sulfate?SDS?,sodium dodecyl benzene sulfonate?SDBS?,mixture of polyacrylic acid and triton and D-180 respectively.Optimum concentrations of surfactants and dispersion mechanisms were summarized and discussed.Based on the results,SDS and SDBS was used to prepare surfactant composite aerogels because of their superior dispersion ability.The Frenkel-Halsey-Hill?FHH?equation was applied to quantitatively describe the effects of SDS and SDBS on the surface features of silica aerogels.The structure of surfactant composite aerogels can be described as surface-fractals with fractal dimensions between 2.5 and 2.6.SDS composite aerogels have lower surface roughness and more homogeneous textures than SDBS composite aerogels.Therefore,SDS was selected to disperse CNFs according to the comperhensive analysis of the dispersion ability and aerogel texture.CNFs doped silica aerogels and CNFs/PMMA modified aerogels were prepared under ambient conditions and investigated on microtopography,pore structure and infrared transmission.Thermally induced phase separation?TIPS?of poly?methyl methacrylate??PMMA?from ethanol and water?VEtOH:VH2O=4:1?was used to modify silica aerogels.In this study,cloud point temperature of PMMA?Mw=3 5,000?solutions with polymer contents between 0.65wt-%and 28wt-%was investigated based on turbidity measurements.The dissolution behavior and phase separation mechanism was discussed.Appropriate PMMA concentrations?0.65wt-%?1.1wt-%?1.8wt-%?2.3wt-%?4.6wt-%?8.8wt-%?10.7wt-%?used to modify aerogels were selected based upon the critical temperature of CO₂.Strong and lightweight PMMA modified silica aerogels were synthesized via TIPS method and supercritically dried with carbon dioxide.Effects of polymer molecular weight and concentrations on the texture and properties of aerogels were investigated.The resultant composited aerogels exhibite densities lower than 0.180g/cm3 and keep good thermal stability at 280?.Thermal conductivity is less than 28.61mW/?m K?at room temperature and the porosity is above 90%.The addition of PMMA increased the flexural strength and modulus by 120%and 120%at the same time with the polymer content of 80mg/mL.The compressive strength and modulus was as high as 11.15MPa and 5.05MPa according to uniaxial compression test.Poly?ethylene-co-vinyl alcohol??EVOH?modified silica aerogels were fabricated via TIPS method with EVOH as reinforced polymer and the mixture of isopropanol and water?VIPA:VH2O=0.65:0.35?as solvent.The resultant highly porous?>86.8%?EVOH modified aerogels exhibit low density?<0.202g/cm3?,improved compressive property?2380%increase in strength and 470%in modulus?,good flexibility?450%increase in strength and 210%in modulus?,while maintaining low thermal conductivity(<31.56mW/?m·K?under ambient conditions.The results of TGA show that thermal stability temperature of EVOH modified aerogels is 350? which is 70? higher than that of PMMA modified aerogels.By comparing polymer modified aerogels fabricated via TIPS mothod and chemical cross-linking,we found that aerogels modified by TIPS can not only obtain good mechanical properties but also maximum retain the characters of high posority and low density.The silica network and weak particles "neck" area could be reinforced and strengthened by polymer deposition when cooling the temperature to induce phase separation of polymer solution.Meanwhile,no chemical reactions are needed during the whole process,which means no toxic by-products or environmental pollutions will be produced.Therefore,preparation of roubst aerogel materials via TIPS method has a broader prospect in the field of thermal insulation.