Fundamental studies on polymer and organic-inorganic hybrid nanoparticles reinforced silica aerogels.

摘要

The objective of this research was to reinforce silica aerogels using functional organic-inorganic hybrid nanoparticles, silane end-capped polyurethane oligomer and chain extended polymer, and self-crosslinkable multi-functional polyurethane-urea such that aerogel articles with negligible changes in density and porosity could be produced. The reinforcing entities used in the study were expected to reinforce the aerogel structures by (a) bridging the neighboring secondary particles at the neck region, (b) by forming conformal coatings on several secondary particles, or (c) by forming coating layers on all secondary particles.;The baseline aerogels were synthesized from tetraethoxy silane (TEOS) or a mixture of TEOS and aminopropyltriethoxy silane (APTES). The aerogels were prepared using two-step sol-gel process followed by supercritical drying in liquid carbon dioxide. Polyhedral oligomeric silsesquioxanes (POSS) with different organic side groups and Si-OH or epoxide functionalities were used. Four types of POSS with silanol groups but with different side groups, tri-POSS, cyclo-POSS, i-butyl POSS, and tetra-POSS, were utilized to modify the silica aerogels before the gelation step. Silanol groups from POSS nanoparticles participated in condensation reactions with other silanes, while epoxy-POSS grafted on to the aerogel structures via the reactions with amine groups of APTES after the formation of gel network. The gels were also surface modified with dimethoxy-methyl (3,3,3-trifluoropropyl) silane to render them super-hydrophobic and to compare the performance of such surface modification with those of POSS molecules containing hydrocarbon side groups. The morphology, and mechanical and surface properties of the resultant aerogels were investigated respectively using scanning electron microscopy (SEM), Instron tensile tester, contact angle goniometry, and BET surface area and porosity analyzer. Solid state 13C and 29Si NMR spectra were used to investigate the chemical reactions between the silane monomers and POSS molecules. Small angle X-ray scattering (SAXS) was used to investigate the change of fractal dimension. It was observed that small amounts of POSS significantly enhanced the hydrophobicity and the compressive strength and modulus of the resultant aerogels, with small changes of density. It was found that silanol POSS reacted with other silanols and became part of aerogel structures.;Silica aerogels with polymeric reinforcement such as pre-modified polyurethanes and prepolymers, with functionality greater than or equal to 2, were produced by one-step sol-gel process. The morphology and mechanical and surface properties of the resultant aerogels were investigated respectively using scanning electron microscopy (SEM), Instron tensile tester, contact angle measurement and BET surface area and porosity analyzer. The chemical structures of aerogels were studied by solid state 13C NMR and 29Si NMR. The fractal dimension was studied by SAXS. It was found that the addition of pre-modified polymer alleviated the issues of brittleness and moisture adsorption by either coating the surface of secondary particles or forming reinforced domains at the neck regions between the secondary particles.;Of all the reinforcement methods used, aerogels reinforced with POSS nanoparticles yielded the best results in that compressive modulus showed an improvement by a factor of six compared to TEOS-based aerogels with less than 10 % change in density.