Core-Shell Monoliths

It was found that a macroporous monolith can be obtained only by adding trifunctional silicon alkoxide such as methyltrimethoxysilane (MTMS) to boehmite nanofibers (AlOOH composition, BNF) dispersion in acetic acid aq. [1]

TEM image of the BNF-PMSQ monolith

In many cases, methods of controlling phase separation using additives such as surfactants and polymers have been adopted for the preparation of macroporous monoliths. These additives become unnecessary after gelation, so they need to be washed and removed. In the case of the large monolith, this becomes time-consuming processes. In the BNF-polymethylsilsesquioxane (PMSQ) gel formation process, the PMSQ coats the BNFs surface and at the same time the nanofiber skeletons adhere to each other, resulting in a macroporous monolith. This process does not require the addition of dispersants or phase separation inducers. The reaction is completed in one step of mixing the two solutions. Normally, to fabricate macroporous monoliths with PMSQ composition, an acid-base two-step reaction was required. In this method, however, the monolith is formed only with an acid catalyst. Because the monolithic gel is flexible to uniaxial compression, it can be dried via evaporative drying.

Evaporative drying of the BNF-PMSQ monolith

This material has potential applications as a thermal insulator. The thermal conductivity was measured to be 30 mW m−1 K-1 at atmospheric pressure. It is not particularly good compared to commercially available high-performance insulation materials, but our monoliths hardly degrade under normal use. Under the low-vacuum condition, the thermal conductivity becomes as low as 12 mW m−1 K−1, which is the same value as silica aerogel. The core-shell material has the potential for special applications utilizing simple vacuum insulation materials and lightweight.

Reference

  1. Gen Hayase, Kazuya Nonomura, Kazuyoshi Kanamori, Ayaka Maeno, Hironori Kaji and Kazuki Nakanishi, “Boehmite Nanofiber–Polymethylsilsesquioxane Core–Shell Porous Monoliths for a Thermal Insulator under Low Vacuum Conditions”, Chem. Mater., 28, 10, 3237–3240 (2016). doi:10.1021/acs.chemmater.6b01010