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.

The preparation of BNF-PMSQ gels can be reproduced by anyone, as they can be prepared simply by mixing BNF aqueous dispersions with MTMS. Few methods for fabricating porous materials using nanofibers themselves for structure control have been reported, and the possibilities are even greater if different combinations of nanofibers and polymers are found. Similar structures can be fabricated using yttria-stabilized zirconia nanocolloid dispersions, which is a promising approach for the future. [2]

Machinability

BNF-PMSQ gel has high mechanical strength and high machinability compared to porous materials of the same bulk density; it is possible to form microstructures of less than a millimeter in size by CNC machining and the processed surface shows high water repellency. We aim to use these properties for applications, for example, as a substrate for spheroid formation and droplet manupulation.

Arrangement of colored water droplets on a CNC-machined water-repellent plate

Reference

  1. Hayase, G.; Nonomura, K.; Kanamori, K.; Maeno, A.; Kaji, H.; Nakanishi, K. Boehmite Nanofiber–Polymethylsilsesquioxane Core–Shell Porous Monoliths for a Thermal Insulator under Low Vacuum Conditions. Chem. Mater. 2016, 28, 3237–3240.
    doi:10.1021/acs.chemmater.6b01010
  2. Hayase, G. Pseudoboehmite Nanorod–polymethylsilsesquioxane Monoliths Formed by Colloidal Gelation. Journal of Asian Ceramic Societies 2019, 7, 469–475.
    doi:10.1080/21870764.2019.1665766 (Open Access)
  3. Hayase, G.; Yoshino, D. CNC-Milled Superhydrophobic Macroporous Monoliths for 3D Cell Culture. ACS Appl. Bio Mater., published online.
    doi:10.1021/acsabm.0c00719 (Preprint: ChemRxiv)