Quantitative and convenient protocol for analysis of surface-modified silica nanoparticles using Si-29-NMR and near-infrared diffuse reflection spectroscopy

Near-infrared (NIR) diffuse reflection spectroscopy combined with solid-state Si-29-nuclear magnetic resonance (NMR) was applied to surface-methylated silica nanoparticles for rapid determination of the degree of modification of silanol group. Si-29-NMR with direct polarization method was employed to quantify the amounts of methyl and silanol groups from the areas of the T-2, T-3, Q(2), and Q(3) sites for referential samples modified with different amounts of trimethoxymethylsilane (TMMS). The degree of modification obtained by NMR was correlated with the corresponding NIR spectra by partial least squares (PLS) regression, where C-H overtone and combination bands of the methyl group played a key role. Validity of the regression model was demonstrated for the mixtures of modified and unmodified samples at different ratios and for the samples prepared using excess amount of TMMS. This protocol will enable a high-throughput characterization of surface-modified silica nanoparticles used as a filler for polymer composites and a nucleating agent for polymer foams.