X-aerogels

This project has been supported by the NASA Glenn Research Center Science Advisory Board, the University of Missouri Research Board, the US National Science Foundation under CMMl-0653919/0653970 and CHE-0809562, and the Army Research office under W91INF-10-1-0476. We also wish to thank our many collaborators at the Missouri University of Science and Technology, NASA GRC, Oklahoma State University, and the University of North Texas who have made crossUnked aerogels (X-Aerogels) possible Antonella Alunni, Prof. Massimo Bertino, Dr. Lyim Capadona, Alex Capecelatro, Naveen Chandrasekaran,... 

Figure 14.5. Typical SEM and N2 sorption data for the RF-MOx systems in the native aerogel, xerogel, and X-aerogel forms. Case shown the RF-CoO system. Open circles, adsorption dark circles, desorption. Insets BJH curves from the desorption branches of the isotherms.

Overall, based on SEM, TEM, and N2 sorption porosimetry, the prevalent model for interpenetrating RF-MOx networks is one consisting of random distributions of two independent networks of nanoparticles, which in the native and the X-aerogel forms are looser, with less points of contact, while xerogels are more compact, as expected. 

Figure 14.7. Typical XRD and EDAX data as a function of pyrolysis temperature (under Ar) for the smeltable RF-MOx systems in the native aerogel (top), xerogel (middle), and X-aerogel (bottom) forms. Case shown RF-SnO to.

Figure 14.9. XRD data of RF-CrO - as a function of the pyrolysis (Ar) temperature. Top native aerogel middle xerogel bottom X-aerogel.

Ji [8] and Hu [7] also studied the NMR spectra and found that the role of chitosan in the aerogels is somewhat modified from the pure material by its interaction with the silica framework. They also analyzed the Xe NMR spectra of Xe in Si02-X aerogels as a function of temperature and pressure, and they showed that the Xe behavior in pores is consistent with the pore size measurements. 

These physical data, together with measured densities in the 0.23-0.32 g/cm range, averaging 0.27 g/cm for a representative set of clear monoliths, and the stoichiometric data, were used to construct a model for a Si02-X aerogel. This model, shown in Figure 18.6, is consistent with all of the data, but it is not unique since the material is not crystalline. 

Ayers and Hunt [2] found that the cytotoxicity of their Si02-X aerogels was measured to be 1 on a 0-4 scale, where 4 means severe cell damage and 0 is no cell damage. 

A hemolysis screening gave a result of 19.7% hemolysis, which is high, but the authors ascribed the result to residual fluoride ion, because HF was used in their synthesis, or to a physical absorptive effect toward certain blood components. It is clear that further study on clean Si02-X aerogels is required to characterize their biocompatibility more reliably. 

The chemical properties of Si02-X aerogels have been studied extensively. These studies fall into several categories. One involves chemistry of metallic species, while another is directed toward the interaction of aerogel particles with the external environment. 

In the case of reactions of HMDI (bis(4-isocyanatocyclohexyl)methane) with Si02-X aerogels, they reported that one HMDI isocyanate reacts with an amine on chitosan but the other one does not. The assumption was that this was due to steric (distance) requirements. That made the aerogel -NCO composite able to attach to other entities in a controlled manner. 

Compressive creep tests allow measurement of strain as a function of time when a constant stress is applied. These can be conducted at several stress levels for aerogel of various densities. Loads are removed at the end of the creep test, and strains as a function of time are monitored to determine the recovery behavior. Compressive relaxation tests can be conducted at different strain levels. The relaxation functions determined at the same strain level at different temperatures can be shifted horizontally to determine whether a master curve can be formed for use to determine the long-term behavior. Recovery behavior after relaxation can also be characterized by monitoring the stress as a function of time after removing partially the step strain. For aerogels that contain polymers such as X-aerogels... 

Three-point bending on an X-aerogel beam specimen allows measurements of flexural modulus and strength. The ASTM standard D790 (Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials) specifies a length/width ratio of 16. The stress [Pg.509]

Three-point bending tests on X-aerogel beam specimens with nominal dimensions of 8.6 X 8.6 X 48 mm were conducted on an Instron material testing system at room temperature (21°C) and at —196°C. Figure 22.4 shows the stress-strain curves obtained through three-point bending tests of X-SiOx and X-VOx at room temperature and cryogenic... 

Compression tests were conducted at room temperature (23°C) under 35% relative humidity. The effect of polymer crosslinking was investigated by comparing results from native and X-aerogels. The strain recovery behavior on X-aerogel specimens was... 

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