19F MAS NMR quantification of accessible hydroxyl sites on fiberglass surfaces.

Solid-state 19F nuclear magnetic resonance (NMR) spectroscopy is used for the quantitative investigation of accessible hydroxyl sites on low surface area glass fibers. Samples with surface areas as low as 0.2 m2/g are investigated through covalent binding of (3,3,3-trifluoropropyl)dimethylchlorosilane. 19F is an ideal nucleus for solid-state NMR, as it has a nuclear spin of 1/2 and a natural isotopic abundance of 100%. High-speed MAS techniques (with rotor spinning frequencies greater than 15 kHz) sufficiently average the CSA and any strong dipolar couplings to allow for superior resolution, especially from terminal -CF3 groups. Studies of two model silica gels with higher surface area, but different pore sizes, provide chemical shift and spin-lattice relaxation rate parameters for probe molecules bound within different environments: pores approaching the size of the probe molecule and pores much larger than the molecular size where intermolecular interactions are assumed to be at a minimum. Resonances assignable to both types of binding environments are found in the spectra of similarly functionalized low surface area fibers. Accessible hydroxyl coverages in the range of 0.8-1.3 OH/nm2 have been measured, and an initial discussion of fiber surface roughness and microporosity is advanced.