We report acceleration in the rate of bulk phase gelation of an organoalkoxysilane, 3-methacryloxypropyl- trimethoxysilane (MAPTMS), in the presence of an amphophilic 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-FTIR, 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 = -CH2CH2CH2- O(CO)C(CH 3)=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-OCH 3 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 organoalkox- ysilanes and surfactant-like amino acid additives, we envisage applications in controlling bulk phase gelation of many chain-substituted organoalkoxysilanes.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Materials Chemistry
- Surfaces, Coatings and Films