R-Silsesquioxane-Based Network Polymers by Fluoride Catalyzed Synthesis: An Investigation of Cross-Linker Structure and Its Influence on Porosity.

Nai-Hsuan Hu,Joseph C Furgal

doi:10.3390/ma13081849

Abstract

Silsesquioxane-based networks are an important class of materials that have many applications where high thermal/oxidative stability and porosity are needed simultaneously. However, there is a great desire to be able to design these materials for specialized applications in environmental remediation and medicine. To do so requires a simple synthesis method to make materials with expanded functionalities. In this article, we explore the synthesis of R-silsesquioxane-based porous networks by fluoride catalysis containing methyl, phenyl and vinyl corners (R-Si(OEt)3) combined with four different bis-triethoxysilyl cross-linkers (ethyl, ethylene, acetylene and hexyl). Synthesized materials were then analyzed for their porosity, surface area, thermal stability and general structure. We found that when a specified cage corner (i.e., methyl) is compared across all cross-linkers in two different solvent systems (dichloromethane and acetonitrile), pore size distributions are consistent with cross-linker length, pore sizes tended to be larger and π-bond-containing cross-linkers reduced overall microporosity. Changing to larger cage corners for each of the cross-linkers tended to show decreases in overall surface area, except when both corners and cross-linkers contained π-bonds. These studies will enable further understanding of post-synthesis modifiable silsesquioxane networks.

Highlights

Sol-gel reactions of R- and bis-R-alkoxysilanes have been used over the years to synthesize a wide variety of highly porous materials containing hybrid inorganic/organic moieties [1,2,3,4,5,6]
Though many reports speculate that the pore sizes obtained in sol-gel reactions are directly related to the bridging groups used, especially for rigid spacers [2,26,30,31,32], we found that multiple pore size distributions (0.5 to 100 nm) could be obtained by changing the reaction solvent for the same bridge
Cross-linkers with various electron density, rigidity and length were reacted with R-Si(OEt)3 to determine how they impact structure and properties such as surface area, porosity, reactivity and sol-gel reaction completion, where R = Me, phenyl, vinyl (Scheme 1)

Summary

Introduction

Sol-gel reactions of R- and bis-R-alkoxysilanes have been used over the years to synthesize a wide variety of highly porous materials containing hybrid inorganic/organic moieties [1,2,3,4,5,6] Historically these types of porous materials have focused on using bridged alkoxysilanes synthesized by acid or base catalysis in a random structural formation, there are many advances in controlling the reactivity that give more focused pore sizes and overall reaction control [7,8,9,10,11,12,13,14]. These improvements are due to the rapid equilibration processes that occur, which allow for the facile generation of more uniform porosity in materials as well as controlled swellability [13,15,24,25,26,27,28,29]

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Results

Conclusion