Effect of Processing Conditions on Chemical Makeup of Di-isocyanate Crosslinked Silica Aerogels
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Sol-gel derived silica aerogels are attractive candidates for many unique thermal, optical, catalytic, and chemical applications1 because of their low density and high mesoporosity. However, their inherent fragility has restricted their use to, for example, insulation in extreme temperature environments such as Mars. We have previously reported crosslinking the mesoporous silica structure of an aerogel with di-isocyanates reacted with silanols on the surface,2,3 or epoxies reacting with an amine decorated silica surface.4 Either approach has been shown to significantly increase the strength of the aerogel with only a small effect on density or porosity. Thus, these hybrid materials may be enabling for future space exploration missions as well as advanced aeropropulsion systems which demand lighter weight, robust, dual purpose materials for insulation, radiation protection and/or structural elements of habitats, rovers, astronaut suits and cryotanks. Utilizing amine-decorated silica particles to react with di-isocyanate oligomers analogous to the epoxies will produce polyurea crosslinks, in addition to carbamates produced from reaction with silanols on the surface as shown in Scheme 1. Since it is suggested in the literature that polyureas are mechanically more robust in general than are polyurethanes5, this approach might result in yet stronger materials. Herein, we have examined the effects of four processing parameters for producing this type of polymer crosslinked aerogel on properties of the resulting monoliths. Concentration of total silane (total APTES plus TMOS in a 1 to 3 v/v ratio) from 7 to 30% by volume in acetonitrile (CH3CN) and the amount of water (7 to 25% by volume) used to catalyze gellation should determine the density of the underlying silica. The number of washes (from 0 to 4) to remove water and by-products of gellation, and concentration of diisocyanate crosslinker (7 to 34% by weight in CH3CN) used for soaking the silica gels should determine the amount/length of polymer forming the crosslinks. A statistical experimental design methodology was employed to reduce the number of experiments and to allow computation of empirical models describing the relationship between the variables and the measured responses. In all, 30 different runs using different combinations of the four variables plus 5 repeats were utilized to produce a total of 35 separate crosslinked aerogels. These were evaluated by NMR, microscopy, surface analysis, mechanical testing and skeletal and bulk density. Herein, we will focus on the results of CP-MAS NMR, giving insight to the amount of polymer crosslink present in the monoliths and relate this to microstructure.