Structure and Dynamics of a Colloidal Silica−Poly(methyl methacrylate) Composite by ¹³C and ²⁹Si MAS NMR Spectroscopy

Abstract

A silica−poly(methyl methacrylate) (PMMA) composite was prepared by condensation polymerization of a 2 nm shell of the silane coupling agent 3-(trimethoxysilyl)propyl methacrylate (TPM) on the surface of 10.5 nm diameter sol−gel colloidal silica particles followed by free-radical polymerization of a 50 wt % dispersion of the TPM−silica in methyl methacrylate. Cross-polarization combined with magic angle spinning and high-power decoupling (CP/MAS) and single-pulse 29Si NMR spectra together with quasi-adiabatic cross-polarization (QACP) 13C NMR spectra provided quantitative analyses of the structural components of the parent silica, the TPM−silica, and the composite. The parent silica contained one ethoxy group and eleven hydroxy groups per ten silicon atoms. The TPM−silica contained one residual methoxy group per TPM group and no residual hydroxy groups. Polymerization with MMA consumed 85% of the methacrylate groups of the TPM. Time constants T1ρH for proton spin−lattice relaxation in the rotating frame detected via 13C and 29Si CPMAS spectra showed rapid spin diffusion between all CH protons in the samples, but not between the CH protons and the OH protons that cross-polarize 29Si atoms in the parent silica. Time constants T1ρC for carbon spin−lattice relaxation in the rotating frame showed that the TPM−silica has substantial motion at kilohertz frequencies leading to fast relaxation, whereas the PMMA composite is more rigid and the ethoxy groups in the parent silica are more mobile. Measurements of 1H−1H dipolar transverse relaxation times via 13C and 29Si detection showed decreasing strengths of homonuclear dipolar interactions due to increasing molecular motion in the order composite > TPM−silica > OH groups in parent silica.