Monolithic silsesquioxane materials with well-defined pore structure
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In this article, monolithic porous silsesquioxane materials, which are derived by sol–gel from trialkoxysilanes with substituent groups such as trimethoxysilane (HTMS), methyltrimethoxysilane (MTMS), and vinyltrimethoxysilane (VTMS), are reviewed with a special emphasis on our recent works. Careful controls over fundamental synthetic parameters such as pH, amounts of water and solvent, and kind of solvent and additives play a crucial role in the formation of monolithic gels based on random polysiloxane networks. Crystalline/amorphous precipitation is otherwise observed when the formation of isolated species including polyhedral oligomeric silsesquioxanes dominates or if phase separation of the hydrophobic networks in aqueous media is not adequately controlled. In the successfully controlled system, pore size can be varied from a few tens of nanometers to a few tens of micrometers; porous materials such as transparent aerogels and hierarchically porous monoliths have been explored. In addition, unique properties derived from trialkoxysilanes such as reactivity of the pore surface and flexible mechanical properties are demonstrated. Possibilities in the silsesquioxane materials with controlled pore structures are discussed.
I. INTRODUCTION
The prevailed term “organic–inorganic hybrid materials” is used in myriad aspects in the diverse fields in academic research and industry. The hybrid materials have originally been classified as classes I and II,1 depending on the interaction between organic and inorganic moieties; i.e., class II is based on the covalent bonds between organic and inorganic parts whereas class I is not. The development of hybrid materials has been encouraged by the progress of sol–gel process, which does not require high temperature to fabricate inorganic materials, and other synthetic approaches in liquid media.2,3 As an important member of organic–inorganic hybrids, organopolysiloxanes (so-called silicones) are well known to form a unique class of materials for their physicochemical stability and broad applicability. Sol–gel hybrids called organically modified silicas (ORMOSILs) or organically modified ceramics (ORMOCERs) typically prepared from organically modified alkoxysilanes such as organotrialkoxysilanes (RSi(OR9)3) and bis(trialkoxysilyl)modified organometallic compounds represented as bridged alkoxysilanes ((R9O)3Si–R–Si(OR9)3), have been particularly important from the early times of sol–gel research.2–6 These organically modified alkoxysilanes are widely available in the market, and regarded as the useful precursors for obtaining desired class II
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Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2014.332 J. Mater. Res., Vol. 29, No. 23, Dec 14, 2014
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organic–inorganic hybrid materials. In addition to class I hybrids entrapping organic molecules in the silica gel/glass matrix,7–11 various class II hybrid materials have been reported
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