Stephanie L. Vivod

Structure−Property Relationships in Porous 3D Nanostructures: Epoxy-Cross-Linked Silica Aerogels Produced Using Ethanol as the Solvent

Cross-linking silica aerogels with organic groups has been shown to improve the strength over un-cross-linked aerogels by as much as 2 orders of magnitude. Previous cross-linking chemistry has been developed using solvents specifically chosen to dissolve the monomers and accommodate the reaction temperature. Because the process of making the aerogels requires so much solvent, it is of interest to consider less toxic solvents such as ethanol to increase safety and enhance scale up. To this end, two different epoxy precursors with suitable solubility in ethanol were evaluated as cross-linkers for silica gels prepared from (3-aminopropyl)triethoxysilane and tetraethylorthosilicate. In addition, 1,6-bis(trimethoxysilyl)hexane (BTMSH) was used as an additive in the underlying silica structure to add flexibility to the aerogels. It was found that the ethanol-derived aerogels exhibited more shrinkage than those prepared from other solvents but that including BTMSH in the aerogels significantly reduced this shrinkage. Inclusion of BTMSH also imparted the ability of the aerogel monoliths to recover elastically when compressed up to 50% strain. In addition, optimized cross-linked aerogels prepared in this study have mechanical properties comparable to those using other more undesirable solvents and cross-linkers.

Reinforcing polymer cross-linked aerogels with carbon nanofibers

We have previously reported cross-linking the mesoporous silica structure of aerogels with di-isocyanates, styrenes or epoxies reacted with amine decorated silica surfaces. These approaches have been shown to significantly increase the strength of aerogels with only a small effect on density or porosity. Herein, we examine the effect of including up to 5% (w/w) carbon nanofibers in the silica backbone before cross-linking. The addition of 5% carbon nanofibers to the lowest density aerogels studied triples the compressive modulus and the tensile stress at break is increased five-fold with no density penalty. The carbon fiber also improves the strength of the initial hydrogels before cross-linking, which may have implications in manufacturing.

Synthesis and Characterization of Hyperbranched Polyazomethine Ablators for Space Exploration Applications

A novel series of ablative composites containing a hyperbranched polyazomethine synthesized inside a carbon fiber preform (HyPAZA) were prepared, which have similar density to phenolic impregnated carbon ablators (∼0.3  g/cc). A novel method of synthesizing strong hyperbranched polyazomethine thermosets has been developed, enabling polyazomethines to be studied in ablators for the first time. Several formulations of HyPAZA perform better than the phenolic impregnated carbon ablator in terms of polymer char yield, composite mechanical strength, CO2 laser ablation tests at heat fluxes of 550 and 1100  W/cm2, and small-scale arcjet testing at a heat flux of 400  W/cm2. Char yields of hyperbranched polyazomethines were as high as 79% at 1000°C by thermogravimetric analysis. This is one of the highest char yields ever reported for a fully organic polymer. Some HyPAZA composites are over 10 times stronger than the carbon fiber preform, as determined by compression tests. Specimens were also tested in an arcjet ...