Amino Acid Catalyzed Bulk-Phase Gelation of Organoalkoxysilanes via a Transient Co-operative Self-Assembly

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• 作者：Shukun Shen ; Daodao Hu ; Peipei Sun ; Xiaoru Zhang ; Atul N. Parikh
• 刊名：Journal of Physical Chemistry B
• 出版年：2009
• 出版时间：October 15, 2009
• 年：2009
• 卷：113
• 期：41
• 页码：13491-13498
• 全文大小：266K
• 年卷期：v.113,no.41(October 15, 2009)
• ISSN：1520-5207

文摘

We report acceleration in the rate of bulk phase gelation of an organoalkoxysilane, 3-methacryloxypropyltrimethoxysilane (MAPTMS), in the presence of an amphiphilic additive, N-phenyl glycine (NPG). The MAPTMS gelation occurs within 30 min in the presence of 0.5 wt % NPG, which took several months in the absence of NPG. Using a combination of ATR-FT IR, ²⁹Si NMR, ¹H NMR, viscosity analysis, SEM, UV−vis, and π−A isotherm measurements, we elucidate the molecular-level details of the structural changes during NPG-catalyzed MPTMS gelation rate. On the basis of these results, we propose a gelation mechanism in which a transient cooperative self-assembly process fosters hydrolysis and retards early condensation thereby promoting the formation of extended three-dimensionally cross-linked gels. Specifically, the amphiphilic character of the hydrolysis product of MAPTMS, consisting of a hydrophobic tail R = −CH₂CH₂CH₂O(CO)C(CH₃)═CH₂ and a hydrophilic Si−OH headgroup, promotes micelle formation at high MAPTMS/water ratio. NPG readily inserts within these micelles thus retarding the topotactic condensation of silanols at the micellar surface. This in turn allows for a more complete hydrolysis of Si−OCH₃ groups prior to condensation in MAPTMS. With increased silanol concentration at the micellar periphery, a delayed condensation phase initiates. This formation of a covalently bonded Si−O−Si framework (and possibly also the formation of the methanol byproduct) likely destabilizes the micellar motif thus promoting its transformation into condensed mesophases (e.g., lamellar microstructure) upon gelation. Because of the generality of this transient and co-operative organic−inorganic self-assembly between hydrolyzed amphiphilic organoalkoxysilanes and surfactant-like amino acid additives, we envisage applications in controlling bulk phase gelation of many chain-substituted organoalkoxysilanes.