Christos Grigoriadis

University of Ioannina, Yannina, Epirus, Greece

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Publications (10)48.55 Total impact

  • Macromolecules 08/2014; 47(16):5691-5702. · 5.93 Impact Factor
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    ABSTRACT: The thermodynamic, optical, structural, and dynamic properties of the semifluorinated (E)-1-(4-octylphenyl)-2-(4-(perfluorooctyl)phenyl)diazene (4) and the corresponding (E)-1,2-bis(4-octylphenyl)diazene (5) have been studied with differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction, and dielectric spectroscopy. 4 combines the azobenzene properties with the fluorophobic effect and gives rise to a responsive material with a temperature and dc-bias-driven switchable dielectric permittivity within the narrower nematic phase. This is caused by the nematic potential that inevitably brings some fluorocarbon chains in proximity to the hydrocarbon chains from adjacent molecules. Frustration is alleviated by reducing the nematic-to-isotropic transition temperature and by increasing the crystalline-to-nematic transition temperature, thus limiting the stability of the nematic phase. Unlike the normal isotropic phase of compound 5, the isotropic phase of compound 4 contains dipoles with short-range orientation correlations. Optimizing the type of interactions may result in materials with applications as molecular switches and electrooptic devices.
    Colloid and Polymer Science 08/2014; 292(8):1939-1948. · 2.41 Impact Factor
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    ABSTRACT: The morphology, the viscoelastic, the dielectric properties and the dynamics of phase transformation are studied in symmetrically and asymmetrically substituted alkyl[1]benzothieno[3,2-b][1]benzothiophenes (C8-BTBT) by X-ray scattering, rheology and dielectric spectroscopy. The interlayer spacing reflects the molecular and supramolecular ordering, respectively, in the symmetrically and asymmetrically substituted BTBTs. In the asymmetric BTBT, the core layer is double in size with a broader network of intermolecular interactions though the increased S-S contacts that is prerequisite for the development of high performance OFET devices. Two crystal states with elastic and viscoelastic responses were identified in the symmetric compound. In contrast, the SmA phase in the asymmetric compound is a viscoelastic solid. A path-dependent dielectric environment with a switchable dielectric permittivity was found in both compounds by cooling below 0 oC with possible implications to charge transport. The kinetics of phase transformation to the crystalline and SmA phases revealed a nucleation and growth mechanism with rates dominated by the low activation barriers.
    The Journal of Physical Chemistry B 01/2014; · 3.38 Impact Factor
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    ABSTRACT: The self-organization and dynamics of bottlebrush polymers consisting of poly(2-bromoisobutyryloxyethyl methacrylate) (PBiBEM) backbone and grafted n-butyl acrylate (PBA) chains are studied as a function of the side-chain length by X-ray scattering, dielectric spectroscopy, and rheology in relation to the respective homopolymers. All short-range correlations are dominated by the PBA side chains, whereas long-range correlations reflect the screening of the backbones by the side chains. Such correlations show up also in the segmental dynamics. The PBA segmental dynamics are slowed-down, whereas the backbone dynamics are plasticized by the side chains. Overall the bottle-brush architecture imparts dynamic homogeneity to the backbone and side-chain dynamics.
    Macromolecular Chemistry and Physics 07/2012; 213(13). · 2.45 Impact Factor
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    ABSTRACT: The nematic-to-isotropic, crystal-to-nematic, and supercooled liquid-to-glass temperatures are studied in the liquid crystal 4-pentyl-4'-cyanobiphenyl (5CB) confined in self-ordered nanoporous alumina. The nematic-to-isotropic and the crystal-to-nematic transition temperatures are reduced linearly with the inverse pore diameter. The finding that the crystalline phase is completely suppressed in pores having diameters of 35 nm and below yields an estimate of the critical nucleus size. The liquid-to-glass temperature is reduced in confinement as anticipated by the model of rotational diffusion within a cavity. These results provide the pertinent phase diagram for a confined liquid crystal and are of technological relevance for the design of liquid crystal-based devices with tunable optical, thermal, and dielectric properties.
    ACS Nano 11/2011; 5(11):9208-15. · 12.03 Impact Factor
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    ABSTRACT: The dynamics of the herringbone structure formation have been studied in a monobromo hexa-peri-hexabenzocoronene derivative by infrared spectroscopy and complementary techniques. Selective probing of the vibration modes corresponding to the aromatic core and the alkyl chains, allowed investigation of their role in the phase transformation dynamics over an extraordinarily broad time-window (1-10(5) s). Identical kinetics were found suggesting that both the core and the alkyl chains simultaneously drive the system from the undercooled liquid crystalline to the crystalline phase with the herringbone structure.
    The Journal of Physical Chemistry B 11/2011; 115(50):14919-27. · 3.38 Impact Factor
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    ABSTRACT: The effect of dipole substitution on the self-assembly, thermodynamics, and dynamics has been studied in a series of hexa-peri-hexabenzocoronenes (HBCs). The HBCs bear the same number and type of aliphatic chains, but different dipoles directly attached to the cores ranging from ∼0 to ∼3.4 D. Dipole substitution alters the energetics and reduces the transition temperature favoring the columnar hexagonal liquid crystalline phase at the expense of the crystalline phase. The equation of state was obtained by independent pressure-volume-temperature measurements in both phases that resulted in the equilibrium phase diagram. According to the latter, increasing pressure imparts stability to the crystalline phase. The molecular and supramolecular dynamics investigated, respectively, by dielectric spectroscopy and rheology, identified a hierarchy of motions comprising a fast axial motion, a slower process that completely relaxes the dipole moment, and an even slower soliton-like relaxation of structural defects.
    The Journal of Physical Chemistry B 05/2011; 115(19):5807-14. · 3.38 Impact Factor
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    ABSTRACT: The distinctly different unit cells, dipolar dynamics and viscoelastic properties of the two columnar phases in a dipole-functionalized discotic liquid crystal (mono-iodine hexa-peri-hexabenzocoronene) were employed as fingerprints and allowed investigating the kinetic pathways towards formation of the crystalline phase. X-Ray scattering, dielectric spectroscopy and rheology revealed a nucleation and growth process. The transformation involved coexisting unit cells composed from columns with either tilted or non-tilted disks and the absence of intermediate states. The transition can be described as a transformation from a structurally weakly ordered but dipolar well-ordered liquid crystalline phase to a structurally well-ordered but dipolar disordered crystalline phase.
    Soft Matter 05/2011; 7(10):4680-4689. · 4.15 Impact Factor
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    ABSTRACT: The role of alkyl chain substitution on the phase formation and core dynamics is studied in a series of diphenylamine functionalized perylenemonoimides (PMIs), by X-ray scattering, calorimetry and site-specific solid-state NMR techniques. In addition, the strong dipole associated with the donor-acceptor character of the molecules allow an investigation of the dynamics with dielectric spectroscopy. The self-assembly revealed an ordered phase only in PMIs with branched alkyl chains. This phase comprises a helical stacking of molecules with a molecular twist angle of 60 degrees. Results from solid-state NMR further pointed out the importance of intramolecular hydrogen bonding in stabilizing the intracolumnar packing within the ordered phase. Moreover, the core dynamics are frozen as revealed by the value of the dynamic order parameters and the reduced strength of dipolar relaxation. The kinetics of phase transformation from the isotropic to the ordered phase proceeds via a nucleation and growth mechanism, and the rates are dominated by the nucleation barrier. Within the isotropic phase the core dynamics display strong temperature dependence with rates that depend on the number of methylene units in the alkyl chains.
    Journal of the American Chemical Society 06/2010; 132(21):7478-87. · 11.44 Impact Factor