Sol–Gel Process of Oxides Accompanied by Phase Separation

Abstract

A versatile sol-gel method for fabricating porous oxide materials with well-defined co-continuous macropores has been reviewed. The chemical instability, in many cases induced by polymerization of the network-forming components, triggers the formation of biphasic morphologies, followed by an irreversible freezing of the transient morphology by the sol-gel transition of the gelling phase. Upon removal of the non-gelling phase, an oxide framework comprising of controlled macropores can be obtained. The mesopore system of such macroporous materials can be further modified either by a physico-chemical treatment or a supramolecular templating technique. Pure silica and siloxane-based organicinorganic hybrids with a hierarchical pore system in monolithic form have been successfully applied to the novel type of separation medium for high performance liquid chromatography, HPLC. Additional topics are also described including recent advances in the 3D-analysis of the interfacial properties of macroporous systems, extended compositional variations in the network-forming phase, and emerging applications in areas of biochemistry.In the preparation of amorphous materials, the unstable or metastable state plays an important role in forming multi-phasic structures in the length scale longer than several nanometers. While in metastable states one often observes a formation of dispersed phases associated with thermally activated diffusion, unstable states are the starting points of various spontaneous morphology formation processes represented by the spinodal decomposition. In melt-quenching fabrication processes of oxide glasses, a pseudo one-phase material is initially formed by rapidly cooling a multi-component melt, and partially crystallized or phase-separated microstructures are controlled by subsequent reheating processes. The metastable glassy state is conveniently used to bring a system into a thermodynamically unstable region as well as to physically freeze the transient heterogeneous state at will. In a chemically polymerizing system, however, the polymerization and crosslinking reactions take place in the presence of a solvent. It is virtually impossible to physically freeze the reaction without influencing all the chemical interactions among the constituents. A continuous polymerization process is chemically equivalent to the continuous physical cooling with respect to the chemical interaction (mutual solubility) and material transport (mobility of the solutes). That is, the polymerizing system resulting in a sol-gel transition is a chemical analogue to a continuously cooled system resulting in a glass transition. If a process of developing heterogeneity in the system superposes a structurefreezing process, a competition will take place between the two processes. Various kinds of transient heterogeneous structures will be frozen in the resultant solidified materials.Phase-separation phenomena related to the heterogeneity formation in materials have been investigated first in metallic alloys and oxide glasses, follow...

Organosilica Monoliths with Multiscale Porosity: Detailed Investigation of the Influence of the Surfactant on Structure Formation

“…Thus, the slower the condensation reactions and the longer the gelation time, the coarser the solid framework. Detailed studies on the influence of ethylene oxide moieties on the structure formation have already been performed in pure silica systems [32][33][34]. For monolithic silsesquioxane materials starting from bridged organosilane precursors, significant differences in the final structure are expected due to the completely changed polarity of the precursor.…”

Hierarchically porous monolithic silica with varying porosity using bis(trimethoxysilyl)arenes as precursors

“…The obtained compounds were utilized for the preparation of monolithic silica hybrids in an adapted sol-gel-route resulting in the formation of a macroporous skeleton by phase separation induced by poly(ethylene glycol), according to the synthesis developed by K. Nakanishi. 14 The amount of polymer was varied spanning a quite wide range to adjust defined macropore morphology comparable to pure silica monoliths. The porosity was characterized by N 2 and dibromomethane sorption, Hg-intrusion and SEM.…”

“…13,14 Such insights are of significant relevance with respect to the crucial role of the combined meso-and macroporosity of this type of monolithic silica, which endows the material with both a high surface area (mesoporosity) and a high pore volume (macroporosity). Evidently, the prepared hybrid materials may have potential applications as chromatographic supports in separation sciences (monolithic HPLC columns), as they provide, compared to pure SiO 2 materials, high specific surface areas and a high permeability in combination with a hydrophobic surface resulting from the chemical functionality.…”

Development of Non-Siliceous Porous Materials and Emerging Applications

“…15,20,23 In this section, the main features of macropore formation in SiO 2 solgel systems are briefly stated, underlining the major advantages in the view of producing monolithic materials.…”