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Germano S. Iannacchione – One of the best experts on this subject based on the ideXlab platform.

  • High-resolution calorimetric study of the nematic to smectic-A transition in aligned liquid crystal-Aerosil gels.
    Physical Review E, 2009
    Co-Authors: F. Cruceanu, Carl W. Garland, Robert L. Leheny, D. Liang, Germano S. Iannacchione

    Abstract:

    High-resolution ac calorimetry has been used to study the nematic to smectic-A N-SmA phase transition in the liquid crystal octylcyanobiphenyl 8CB confined in aligned colloidal Aerosil gels. A stable and robust nematic alignment was achieved by repeated thermal cycling of the samples in the presence of a strong uniform magnetic field. In some ways such as transition temperature and integrated enthalpy, the dependence of the specific heat peak associated with the N-SmA transition on the Aerosil density for aligned gels is consistent with that observed in unaligned random gel samples. However, a power-law analysis reveals that the behavior of the critical exponent is quite different. For random gels, varies gradually with Aerosil density, whereas we find that for aligned gels shifts abruptly to an XY-like value for the lowest Aerosil density studied and remains essentially constant as the sil density increases. This Aerosil density independence of is consistent with the critical behavior of the smectic correlation lengths obtained from an x-ray scattering study of 8CB in aligned Aerosil gels. The combined calorimetric and x-ray results indicate that the role of quenched randomness in aligned gels of 8CB+sils differs significantly from that in random gels.

  • Role of Aerosil dispersion on the activated kinetics of the LC1-xSilx system.
    Journal of Physical Chemistry B, 2006
    Co-Authors: Dipti Sharma, John C. Macdonald, Germano S. Iannacchione

    Abstract:

    This study explores the role of Aerosil dispersion on activated phase transitions of bulk octylcyanobiphenyl (8CB) liquid crystals by performing heating rate-dependent experiments. Differential scanning calorimetry (DSC) was used at various heating ramp rates in order to probe the activated phase dynamics of the system. The system, LC1-xSilx, was prepared by mixing Aerosil nanoparticles (7 nm in diameter) in the bulk 8CB by the solvent dispersion method (SDM). LC represents bulk 8CB, and Sil represents Aerosil nanoparticles with concentration x in percent. The concentration of the Aerosil nanoparticles (x) varied from 0 to 0.2 g/cm3 in the bulk 8CB. Well-defined, endothermic peaks were found on a heating scan at melting and at the smectic-A to nematic (SmA−N) and nematic to isotropic (N−I) transitions. These peaks show a temperature shift and a change in their shapes and sizes in the presence of Aerosil nanoparticles. In addition, an exothermic peak also appeared before the melting peak during the heating…

  • Induced glassy behavior in the melt of glycerol and Aerosil dispersions
    arXiv: Soft Condensed Matter, 2005
    Co-Authors: Dipti Sharma, Germano S. Iannacchione

    Abstract:

    A high-resolution calorimetric spectroscopy study has been performed on pure glycerol and colloidal dispersions of an Aerosil in glycerol covering a wide range of temperatures from 300 K to 380 K, deep in the liquid phase of glycerol. The colloidal glycerol+Aerosil samples with 0.05, 0.10, and 0.20 mass fraction of Aerosil reveal glassy, activated dynamics at temperatures well above the $T_g$ of the pure glycerol. The onset of glass-like behavior appears to be due to the structural frustration imposed by the silica gel on the glycerol liquid. The Aerosil gel increases the net viscosity of the mixture, placing the sample effectively at a lower temperature thus inducing a glassy state. Given the onset of this behavior at relatively low Aerosil density (large mean-void length compared to the size of a glycerol molecule), this induced glassy behavior is likely due to a collective mode of glycerol molecules. The study of frustrated glass-forming systems may be a unique avenue for illuminating the physics of glasses.

M E Neubert – One of the best experts on this subject based on the ideXlab platform.

  • calorimetric study of the nematic to smectic a and smectic a to smectic c phase transitions in liquid crystal Aerosil dispersions
    Physical Review E, 2005
    Co-Authors: A. Roshi, Germano S. Iannacchione, Paul S. Clegg, R J Birgeneau, M E Neubert

    Abstract:

    A high-resolution calorimetric study has been carried out on nanocolloidal dispersions of Aerosils in the liquid crystal 4-n-pentylphenylthiol-4′-n-octyloxybenzoate (8S5) as a function of Aerosil concentration and temperature spanning the smectic-C to nematic phases. Over this temperature range, this liquid crystal possesses two continuous XY phase transitions: a fluctuation-dominated nematic to smectic-A transition with alpha approximately alphaXY=-0.013 and a mean-field smectic-A to smectic-C transition. The effective critical character of the N-SmA transition remains unchanged over the entire range of the introduced quenched random disorder while the peak height and enthalpy can be well described by considering a cutoff length scale to the quasicritical fluctuations. The robust nature of the N-SmA transition in this system contrasts with cyanobiphenyl-Aerosil systems and may be due to the mesogens being nonpolar and having a long nematic range. The character of the SmA-SmC transition changes gradually with increasing disorder but remains mean field like. The heat capacity maximum at the SmA-SmC transition scales as rho with an apparent evolution from tricritical to a simple mean-field step behavior. These results may be generally understood as a stiffening of the liquid crystal (both the nematic elasticity as well as the smectic layer compression modulus B) with silica density.

  • first order isotropic smectic a transition in liquid crystal Aerosil gels
    Physical Review E, 2004
    Co-Authors: M Ramazanoglu, Paul S. Clegg, R J Birgeneau, C W Garland, M E Neubert, Julie M Kim

    Abstract:

    The short-range order which remains when the isotropic to smectic-$A$ transition is perturbed by a gel of silica nanoparticles (Aerosils) has been studied using high-resolution synchrotron x-ray diffraction. The gels have been created in situ in decylcyanobiphenyl, which has a strongly first-order isotropic to smectic-$A$ transition. The effects are determined by detailed analysis of the temperature and gel density dependence of the smectic structure factor. In previous studies of the continuous nematic to smectic-$A$ transition in a variety of thermotropic liquid crystals the Aerosil gel appeared to pin, at random, the phase of the smectic density modulation. For the isotropic to smectic-$A$ transition the same gel perturbation yields different results. The smectic correlation length decreases more slowly with increasing random-field variance in good quantitative agreement with the effect of a random pinning field at a transition from a uniform phase directly to a phase with one-dimensional translational order. We thus compare the influence of random fields on a freezing transition with and without an intervening orientationally ordered phase.

  • high resolution x ray study of the nematic smectic a and smectic a smectic c transitions in liquid crystal Aerosil gels
    Physical Review E, 2003
    Co-Authors: Paul S. Clegg, Germano S. Iannacchione, R J Birgeneau, C W Garland, Robert L. Leheny, Sei Jin Park, M E Neubert

    Abstract:

    The effects of dispersed Aerosil nanoparticles on two of the phase transitions of the thermotropic liquidcrystal material 4-n-pentylphenylthiol-4 8-n-octyloxybenzoate (8 ¯ S5) have been studied using high-resolution x-ray diffraction techniques. The Aerosils hydrogen bond together to form a gel which imposes a weak quenched disorder on the liquid crystal. The smectic-A fluctuations are well characterized by a two-component line shape representing thermal and random-field contributions. An elaboration on this line shape is required to describe the fluctuations in the smectic- C phase; specifically the effect of the tilt on the wave-vector dependence of the thermal fluctuations must be explicitly taken into account. Both the magnitude and the temperature dependence of the smectic-C tilt order parameter are observed to be unaffected by the disorder. This may be a consequence of the large bare smectic correlation length in the direction of modulation for this transition. These results show that the understanding developed for the nematic to smectic-A transition for octylcyanobiphenyl and octyloxycyanobiphenyl liquid crystals with quenched disorder can be extended to quite different materials and transitions.

Paul S. Clegg – One of the best experts on this subject based on the ideXlab platform.

  • calorimetric study of the nematic to smectic a and smectic a to smectic c phase transitions in liquid crystal Aerosil dispersions
    Physical Review E, 2005
    Co-Authors: A. Roshi, Germano S. Iannacchione, Paul S. Clegg, R J Birgeneau, M E Neubert

    Abstract:

    A high-resolution calorimetric study has been carried out on nanocolloidal dispersions of Aerosils in the liquid crystal 4-n-pentylphenylthiol-4′-n-octyloxybenzoate (8S5) as a function of Aerosil concentration and temperature spanning the smectic-C to nematic phases. Over this temperature range, this liquid crystal possesses two continuous XY phase transitions: a fluctuation-dominated nematic to smectic-A transition with alpha approximately alphaXY=-0.013 and a mean-field smectic-A to smectic-C transition. The effective critical character of the N-SmA transition remains unchanged over the entire range of the introduced quenched random disorder while the peak height and enthalpy can be well described by considering a cutoff length scale to the quasicritical fluctuations. The robust nature of the N-SmA transition in this system contrasts with cyanobiphenyl-Aerosil systems and may be due to the mesogens being nonpolar and having a long nematic range. The character of the SmA-SmC transition changes gradually with increasing disorder but remains mean field like. The heat capacity maximum at the SmA-SmC transition scales as rho with an apparent evolution from tricritical to a simple mean-field step behavior. These results may be generally understood as a stiffening of the liquid crystal (both the nematic elasticity as well as the smectic layer compression modulus B) with silica density.

  • first order isotropic smectic a transition in liquid crystal Aerosil gels
    Physical Review E, 2004
    Co-Authors: M Ramazanoglu, Paul S. Clegg, R J Birgeneau, C W Garland, M E Neubert, Julie M Kim

    Abstract:

    The short-range order which remains when the isotropic to smectic-$A$ transition is perturbed by a gel of silica nanoparticles (Aerosils) has been studied using high-resolution synchrotron x-ray diffraction. The gels have been created in situ in decylcyanobiphenyl, which has a strongly first-order isotropic to smectic-$A$ transition. The effects are determined by detailed analysis of the temperature and gel density dependence of the smectic structure factor. In previous studies of the continuous nematic to smectic-$A$ transition in a variety of thermotropic liquid crystals the Aerosil gel appeared to pin, at random, the phase of the smectic density modulation. For the isotropic to smectic-$A$ transition the same gel perturbation yields different results. The smectic correlation length decreases more slowly with increasing random-field variance in good quantitative agreement with the effect of a random pinning field at a transition from a uniform phase directly to a phase with one-dimensional translational order. We thus compare the influence of random fields on a freezing transition with and without an intervening orientationally ordered phase.

  • Evolution of the isotropic-to-nematic phase transition in octyloxycyanobiphenyl+Aerosil dispersions.
    Physical Review E, 2004
    Co-Authors: A. Roshi, Germano S. Iannacchione, Paul S. Clegg, Robert J. Birgeneau

    Abstract:

    High-resolution ac calorimetry has been carried out on dispersions of Aerosils in the liquid crystal octyloxycyanobiphenyl (8OCB) as a function of Aerosil concentration and temperature spanning the crystal to isotropic phases. The liquid crystal 8OCB is elastically stiffer than the previously well studied octylcyanobiphenyl (8CB)+Aerosil system and so general quenched random-disorder effects and liquid crystal specific effects can be distinguished. A double heat capacity feature is observed at the isotropic to nematic phase transition with an Aerosil independent overlap of the heat capacity wings far from the transition and having a nonmonotonic variation of the transition temperature. A crossover between low and high Aerosil density behavior is observed for 8OCB+Aerosil. These features are generally consistent with those on the 8CB+Aerosil system. Differences between these two systems in the magnitude of the transition temperature shifts, heat capacity suppression, and crossover Aerosil density between the two regimes of behavior indicate a liquid crystal specific effect. The low Aerosil density regime is apparently more orientationally disordered than the high Aerosil density regime, which is more translationally disordered. An interpretation of these results based on a temperature dependent disorder strength is discussed. Finally, a detailed thermal hysteresis study has found that crystallization of a well homogenized sample perturbs and increases the disorder for low Aerosil density samples but does not influence high-density samples.