35 research outputs found
Thermodynamic Modelling of Phase Equilibrium in Nanoparticles-Nematic Liquid Crystals Composites
In this work, a theoretical study of phase equilibrium in mixtures of a
calamitic nematic liquid crystal and hard spherical nanoparticles is presented.
A mean-field thermodynamic model is used, where the interactions are considered
to be proportional to the number of contacts, which in turn are proportional to
the areas and area fractions of each component. It is shown that, as the radius
of the particle is increased, the effect of the particles on the
isotropic-nematic transition is less pronounced, and that for large radius the
miscibility increases as the particle radius increases.Comment: This is an Author's Accepted Manuscript of an article published in
"Molecular Crystals and Liquid Crystals" (see reference), available online
at: http://www.tandfonline.com/doi/abs/10.1080/15421406.2011.60944
Multiple interfaces in diffusional phase transitions in binary mesogen-non-mesogen mixtures undergoing metastable phase separations
Theory and simulations of simultaneous chemical demixing and phase ordering
are performed for a mixed order parameter system with an isotropic-isotropic
(I-I) phase separation that is metastable with respect to an isotropic-nematic
(I-N) phase ordering transition. Under certain conditions, the disordered phase
transforms into an ordered phase via the motion of a double front containing a
metastable phase produced by I-I demixing, a thermodynamically driven mechanism
not previously reported. Different kinetic regimes are found depending on the
location of the initial conditions in the thermodynamic phase diagram and the
ratio between diffusional and nematic phase ordering mobilities. For a
diffusional process, depending if the temperature is above or below the
critical co-dissolution point, an inflection point or a phase separation takes
place in the depletion layer. This phase separation leads to the formation of a
second interface where the separation of the two metastable isotropic phases
grows monotonically with time. The observed deviations from the typical Fickian
concentration profiles are associated with strong positive deviations of the
mixture from ideality due to couplings between concentration and nematic
ordering. Although systems of interest include liquid crystalline
nanocomposites, this novel mechanism may apply to any mixture that can undergo
an order-disorder transition and demix.Comment: 24 pages, 8 figures, final accepted authors version, published
version in http://pre.aps.org/abstract/PRE/v86/i1/e01160
Remote activation by green-light irradiation of shape memory epoxies containing gold nanoparticles
Shape memory epoxies (SMEs) that can be remotely activated by the use of green light, are described. An epoxy matrix based on on diglycidylether of bisphenol A (DGEBA cured with a mixture of n-dodecylamine (DA) and m-xylylenediamine (MXDA), exhibits excellent shape memory properties as described in a previous paper (Leonardi et al., 2011). Au NPs with an average diameter close to 5 nm could be uniformly dispersed in this matrix using poly(ethylene oxide) (PEO) chains as stabilizer. These NPs showed a significant photothermal effect even at very low concentrations (0.01 wt% as metallic gold), when irradiated with a 532 nm laser at a power close to 2 W/cm2. Under these conditions, a bended bar (1.4-mm thickness) recovered its initial shape in a few seconds. This formulation may be used to build up devices with the necessary mechanical strength and with the possibility to produce shape recovery by remote activation using green light. A second example was analyzed employing an amphiphilic epoxy matrix to produce a uniform dispersion of Au NPs stabilized with dodecyl chains (average diameter close to 3 nm). A bar (1.4-mm thickness) of the SME with 0.04 wt% Au NPs (as metallic gold) showed a fast recovery of its initial shape by irradiation with a 532 nm laser at a power close to 2 W/cm2. This example shows the feasibility of adapting the epoxy chemistry to disperse Au NPs stabilized with different ligands and obtained through robust synthetic methods.Fil: Leonardi, Agustina Belen. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Puig, Julieta. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Antonacci, Julian. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Departamento de Física; ArgentinaFil: Arenas, Gustavo Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Departamento de Física; ArgentinaFil: Zucchi, Ileana Alicia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Hoppe, Cristina Elena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Reven, Linda. McGill University; CanadáFil: Zhu, Liu. Peking University Shenzhen Graduate School; ChinaFil: Toader, Violeta. McGill University; CanadáFil: Williams, Roberto Juan Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentin
Oxygen-17 NMR relaxation studies of oxide superconductors
In this study, solid state nuclear magnetic resonance (NMR) spectroscopy is used to investigate the structural, electronic, and dynamical properties of a variety of high temperature oxide superconductors and metal carbonyls adsorbed on metal oxide surfaces. The first five chapters describe the \sp{17}O Knight shift, nuclear spin lattice and spin-spin relaxation measurements carried out on a large number of cuprate, bismuthate and plumbate superconductors. After briefly reviewing the basic theory of the NMR parameters of metals in the normal and superconducting states, a detailed \sp{17}O NMR study of YBa\sb2Cu\sb3O\sb{\rm 7-x}, is described in Chapter 2. Chapter 3 contains a comparative study of Tl\sb2Ba\sb2CaCu\sb2O\sb{\rm 8+x}, Bi\sb2Sr\sb2CaCu\sb2O\sb{\rm 8+x}, YBa\sb2Cu\sb3O\sb{\rm 7-x}, and La\sb{1.85}Sr\sb{0.15}CuO\sb4. In this chapter, the \sp{17}O nuclear spin lattice relaxation and Knight shift measurements on La\sb{1.85}Sr\sb{0.15}CuO\sb4 are compared to the theoretical model of Monien, Millis, and Pines of an antiferromagnetic Fermi liquid state. In Chapter 4, it is shown that the normal state nuclear spin lattice relaxation behavior of the "copper-free" superconductors can be accounted for by a simple p conduction band model. The \sp{17}O NMR results for the different families of oxide superconductors are summarized in Chapter 5. The \sp{17}O NMR parameters of the bismuthate and plumbate superconductors show these materials to be weakly metallic oxides with T\sb1T values which are consistent with a BCS-like relationship between the electronic density of states and T\sb{\rm c}. The normal state of the cuprate superconductors display properties consistent with the presence of magnetic interactions and their T\sb1T values show little variation with T\sb{\rm c}. Finally, in Chapter 6 of this thesis, earlier work using \sp{13}C variable temperature magic angle sample spinning (MAS) NMR to study the surface chemistry and fluxional behavior of Fe\sb3(CO)\sb{12} and Ru\sb3(CO)\sb{12} adsorbed on a variety of metal oxide surfaces is described.U of I OnlyETDs are only available to UIUC Users without author permissio