Page 611 - Book Hosokawa Nanoparticle Technology Handbook

39 INSTANTANEOUS NANOFOAMING METHOD FOR FABRICATION OF CLOSED-POROSITY SILICA PARTICLE APPLICATIONS
APPLICATION 39

39 INSTANTANEOUS NANOFOAMING METHOD FOR FABRICATION OF
CLOSED-POROSITY SILICA PARTICLE

Most previous methods for fabricating nanoporous foaming in which a dilute phase (gaseous phase) is
solids have focused on formation of open nanopores. enclosed in a closed space in a condensed phase [4].
Those open-porosity nanoporous solids are advanta- Normally, foaming is considered as one of the phase
geous in case where reactants or fluids need to have separation phenomena which take place in a mobile
access to the inner space of those nanopores. From the condensed phase such as liquid. Under such circum-
viewpoint of the characterization of the nanopores, a stances, the size of the unit cell of the foam (bubble)
significant portion of the information on their struc- easily enhances being driven by the interfacial ten-
ture can be obtained from their adsorption behavior. sion. The rate of the growth of the cell is much lower
Bulk properties of nanoporous solids largely depend when the medium for the bubble formation is solid or
on the amount and structure of their nanopores. As a extremely viscous liquid. Therefore, the growth of the
result, the material designing is normally considered cells is expected to be restricted when it occurs in
to be realized by tailoring the nanoporous structure. solid silica matrix.
For example, the electrical inductivity can be varied The experimental procedure for preparing the
as a function of the porosity. In principle, the porosity foamed silica is quite simple as follows. Tetraethyl
as a relevant structural parameter is determined by the orthosilicate (TEOS) was hydrolyzed with ten folds
concentration of the structure-directing additives. larger molar number of water at a weakly acidic con-
Those additives normally form liquid crystalline dition (pH 5) with hydrochloric acid. The hydrolysis
phases at higher concentration. Since these liquid was readily completed in an hour and gave macro-
crystalline phases have highly interconnected struc- scopically homogeneous transparent silica sol. The
ture, the resultant nanopores tend to be open to the silica sol was pretreated by a heat treatment to give
outside. For these reasons, nanoporous solids of the solid silica. In case where the hydrolysis is carried
open-porosity type have been the major object of out at an acidic condition, the obtained pretreated
studies and a quite large number of researchers are silica has many residual hydroxyl functions. At the
engaged in these works. Nevertheless, there are cer- moment of the instantaneous heat treatment above
tain cases where closed nanopores are more desirable 500 C, the residual hydroxyl functions instantly
than open ones. The authors have sought fabrication underwent the condensation to form siloxane
methods of closed-porosity nanoporous silica [1–3]. bondings. At the same time, the silica matrix instan-
For example, nanoscale assemblies of tri-block taneously hardened due to the increase in the density
®
copolymers (Pluronic ) can be immobilized in instan- of siloxane bondings. During the instantaneous con-
taneously solidified silica matrix by rapid drying [2]. densation, the generated water immediately vaporized
In this case, the block copolymers are not given suffi- and formed bubbles (balloons) being enclosed in the
cient time to form complete liquid crystalline phase densely developed siloxane bondings [4]. Fig. 39.1
structure. As a result, the finally obtained solid retains shows a typical transmission electron microscope
the structural features of the dispersed phase of those (TEM) image of the foamed silica. Many spherical
molecular assemblies. Resultantly, the volume frac- balloon-like cells with the diameter of 1–10 nm are
tion of the closed pore exceeds 50% of the total seen. Their spherical shape indicates that they were
nanopores. In the author’s experience, this example is formed as the balloons were blown by the inner pres-
the superior limit of the fraction of the closed pores sure of the enclosed gas (vapor). These bubbles are
that can be attained by the rapid drying method. The quite small compared to those formed in liquids or
complete enclosure of those assemblies of the block normal polymer resins. The hardened silica matrix
copolymer by the rapid drying method is not realistic effectively blocked the bubbles from continuation of
because the drying rate cannot be infinitely enhanced. the growth or coalescence. In the case of foaming in
Actually, the rate of drying process is severely deter- normal plastic resin, the size of the bubble is quite
mined by the heat transfer. It should be noted that hard to restrict below 1 m. This is because these
molecular assemblies normally tend to reach a liquid resins normally need to be softened by increasing
crystalline phase which works as a template for the the temperature up to the vicinity of their softening
formation of open-porosity nanoporous structure. As point. The formed bubbles tend to readily grow or
one of the solutions to avoid the opening of the undergo mutual coalescence. Therefore, the harden-
nanopores toward the outside, the author has ing which occurs simultaneously with the condensa-
introduced a method for the selective formation of tion of the residual hydroxyl functions is considered
closed nanopores. This method is based on so-called to play a crucial role in the realization of the

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