Strategies to Incorporate Mesoporosity - Mesoporous Organic-Inorganic Non-Siliceous Hybrid Materials - page 48

Fig. 3.13 Adjustment of the mesoporous size based on surfactant molecules with different
hydrophobic volumes
make it possible to control the pore size. Using soft templates with low molecular
weights (Fig. 3.13 ), such as C 16 TABr and oligomeric surfactants (e.g., C 16 EO 10
and C 16 EO 20 ), the pore diameters are usually distributed in the range of 2-4 nm
[ 46 - 48 , 52 , 55 , 56 ]. In general, the block-copolymer micelles are larger than those
aggregated by low molecular weight surfactants. For instance, the pore width
could be largely expanded to 6-10 nm if triblock copolymers (EO 80 PO 30 EO 80 ,
EO 106 PO 70 EO 106 , and EO 20 PO 70 EO 20 ) were employed [ 48 , 73 , 74 ]. On the other
side, diblock-copolymers always direct larger pore sizes compared to triblock-
copolymers with similar molecular weights or PPO chains because the latter tends
to bending aggregation. High molecular weight block-copolymers are of great
interest owing to the relatively large mesopores in the resultant mesoporous mate-
rials. Colloidal templating of polystyrene-block-poly(oxyethylene) (PS- b -PEO)
was newly developed for the fabrication of porous aluminum phosphonates with
large spherical pores [ 75 ]. PS- b -PEO was dissolved in a mixture of THF and etha-
nol in the presence of water to form spherical aggregates and then mixed with pre-
cursor solutions before preparation. Then, the mixed solutions were spray-dried at
different temperatures, from 110 to 230 °C, and calcined at an appropriate temper-
ature to eliminate the surfactants. Interestingly, the size of the spherical PS- b -PEO
aggregates (the number of PS- b -PEO molecules in the aggregates) was variable
on the basis of the amount of water present. Therefore, the pore diameter could be
controlled from large mesopores (30 nm) to macropores (200 nm), as it was related
to the size of the colloidal PS- b -PEO template.
The addition of organic swelling agents is another significant way to expand
the pore sizes. The hydrophobic organic species can be solubilized inside