Study Of Nanoparticle Liquid Crystal Dispersions Using Optical Microscopy And Solid State Nmr PDF Download

" This thesis concerns the characterization of nanoparticles and liquid crystal nanocomposites. Two types of nanoparticles were prepared: zirconia nanoparticles were functionalized with different ratios of dodecyl- and phenyl- phosphonic acids as a model system for developing a NMR-based characterization method to determine the spatial distribution of the ligands. This study was motivated by the finding that successful dispersions of nanoparticles in liquid crystals often requires mixed ligand shells and it assumed that there is a homogeneous distribution of the two ligands. Secondly, more readily prepared polymer functionalized gold nanoparticles were tested as an alternative to mixed ligand shells of low molar mass mesogenic ligands to achieve stable dispersions in a nematic liquid crystal. Spin diffusion NMR experiments developed for heterogeneous polymeric materials were used as a novel surface characterization method for nanoparticles with mixed ligand shells. 1D solid-state 31P and 1H NMR spectroscopy, respectively used to characterize the surface binding and ligand shell composition, showed that the compositions of the strongly bound ligand shells matched the reaction ratios. The spatial proximities of the dodecyl and phenyl groups were probed by 2D 1H-13C Heteronuclear Correlation (HETCOR) and 1H Double Quantum (DQ) NMR experiments. The HETCOR and 1H DQ NMR experiments indicate a significant population of dodecyl- and phenyl- phosphonic acids are nearest neighbors which would be consistent with molecular level mixing on the nanoparticle surface. However the proton spin diffusion experiments indicate the presence of some type of phase separation. This finding is significant since previous studies of mixed phosphonic acid monolayers in the literature assume a homogeneous distribution. The analysis of the spin diffusion build up curves yielded domain sizes on the order of ~ 4-5 nm which appear to be too large given the nanoparticle dimensions. Control experiments point towards an overestimate of the spin diffusion coefficients since these experiments were originally developed for the extended proton-proton dipolar coupling networks found in polymers. Gold nanoparticles, AuNPs, functionalized with 2000 MW thiolated poly(ethylene oxide), PEO-SH, were dispersed in a common nematic liquid crystal, 4-cyano-4'-pentylbiphenyl (5CB). Dispersions of free PEO, 2000 MW, in 5CB were also prepared. The two types of blends were characterized by polarized optical microscopy (POM) and differential scanning calorimetry (DSC). Relatively high concentrations of 10 wt. %, 20 wt. % and 50 wt. % AuNPs-PEO in 5CB were investigated. The dispersions were all characterized by POM by cooling from the isotropic state to erase all thermal history. Both the AuNP-PEOs and PEO alone are completely miscible in isotropic 5CB to very high concentrations. Whereas the free PEO significantly lowers the isotropic to nematic (I-N) phase transition temperature of 5CB, the AuNP-PEOs only cause a small change, similar to previously studied AuNPs with mesogenic ligands. Below the I-N transition, both the polymer and AuNP dispersions display a biphasic state over a wide temperature range. For higher concentrations (> 10 wt. % PEO), PEO spherulite crystallization nucleates the nematic phase of 5CB. At lower PEO concentrations, the I-N transition of 5CB precedes PEO crystallization which shows a dendritic morphology. The AuNP-PEOs form a cellular network which shares similarities with that formed by AuNPs with mesogenic ligands. Differences in the network morphologies are attributed to the stronger disordering effect of the polymer ligands on the nematic structure as compared to the mesogenic ligands. " --