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Nuclear Magnetic Resonance characterization of chiral nematic mesoporous films Manning, Alan P.


Using templation with Nanocrystalline Cellulose (NCC), a mesoporous silica and organosilica film with a tunable chiral nematic pore structure and long, narrow pores has recently been developed. This novel material has interesting optical properties: it selectively reflects left-handed polarized light and has an iridescent appearance, with its perceived colour controlled by tuning the pitch of chiral structure. Its possible applications include enantioselective catalysis and filtering, and optical sensors. In this work, a variety of Nuclear Magnetic Resonance (NMR) spectroscopy experiments were run to characterize the films and composite systems. ¹³C and ²⁹Si Magic Angle Spinning NMR spectra confirmed removal of the NCC template via sulphuric acid and showed the process does not cleave organosilica bonds. NMR cryoporometry, which uses the signal from absorbed liquid water, relates freezing point depression to pore size. This method was found to be non-destructive, accurate, and more sensitive and precise than nitrogen sorption to determine pore sizes. The silica films were found to have a smaller (~3 nm) pore width size distribution than the organosilica films (~6-9 nm). Using Pulsed Field Gradient (PFG) NMR, the diffusion of absorbed water was found to be ~2x as fast perpendicular to the surface normal than parallel to it, with diffusion parallel to the pore axis essentially unrestricted. Silica films had overall slower diffusion than organosilica films. Finally, a composite system was made by functionalizing an organosilica film with n-Octyl, enabling it to absorb ¹⁵N-labelled 8CB liquid crystal. Reversible switching of the reflective properties was seen upon heating absorbed liquid crystals to the isotropic phase. ¹⁵N NMR spectra were taken of the sample with different orientations to the field, showing that at room temperature, the 8CB mesogens are on average aligned down the pores, and after melting, they are isotropic. Large, unexplained magnetic susceptibility effects are seen in the room temperature spectra. Overall, these experiments will enable further development of these materials and other composite systems.

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Attribution 2.5 Canada