Organosiloxane Transparent Aerogels and Hierarchically Porous Monoliths
- PDF / 487,229 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 76 Downloads / 228 Views
Organosiloxane Transparent Aerogels and Hierarchically Porous Monoliths Yasunori Kodera, Gen Hayase, Kazuyoshi Kanamori, Kazuki Nakanishi and Teiichi Hanada Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan ABSTRACT A transition of porous structures in monolithic poly(methylsilsesquioxane) (PMSQ, CH3SiO1.5) gels from uniform mesopores to hierarchical pore structures consisting of macro- and mesopores, has been investigated using a sol-gel system containing surfactant Pluronic F127. A broad variation of porous morphology is controlled by changing the concentration of F127. Sufficient concentrations of F127 inhibit the occurrence of macroscopic phase separation of hydrophobic PMSQ condensates and lead to well-defined mesoporous transparent aerogels with high specific pore volume. Mesopores are developed through microscopic phase separation of PMSQ colloid-surfactant complexes in the solvent. Macroscopic phase separation regulates well-defined macropores in the micrometer range on decreasing concentrations of F127, in which microscopic phase separation concurrently takes place in the PMSQ-rich gelling phase after the onset of macroscopic phase separation. Monolithic PMSQ gels with hierarchical macro- and mesopore structures are consequently obtained. INTRODUCTION Recent progress in porous materials through sol-gel chemistry has unlimitedly extended the available chemical compositions and pore characteristics. The representative one is mesoporous materials with ordered pore alignments such as so-called mesoporous silica [1]. Also, porous materials with well-defined macropores are prepared by inducing phase separation parallel to the sol-gel transition [2]. Despite of the world-wide active research in porous materials through sol-gel chemistry, those derived from organotrialkoxysilanes with hydrophobic substituent groups such as methyl group have not been intensely explored regardless of the expected priority in mechanical properties and hydrophobicity. This mainly is due to the difficulty in controlling hydrolysis and polycondensation reactions of these precursors, which are stabilized by forming cyclic and cage species and do not cause homogeneous gelation especially in dilute and/or acidic conditions [3,4]. Moreover, the too-high phase separation tendency in aqueous polar media makes the control of the sol-gel reaction of these precursors further difficult. We have investigated various macroporous and mesoporous poly(methylsilsesquioxane) (PMSQ) monoliths by adequately controlling the sol-gel reactions and phase separation of methyltrimethoxysilane (MTMS) for years [5,6]. Various factors such as pH, water/Si ratio, and type and amount of solvent, are found to be particularly critical. Various types of porous PMSQ materials are obtained in monoliths, films, and particles. Among them, we found that a copresence of adequate surfactant in a modified acid/base two-step sol-gel process allows a drastic suppression of phase separation in a large amount of po
Data Loading...
