Metastable, ordered solids of rod-like molecules quenched from their liquids at far-from-equilibrium conditions: Phonon spectra of 4-<i>n</i>-octyl- and 4-<i>n</i>-dodecyl-4′-cyanobiphenyl

The results of phase transformation studies carried out in four homologues of 4-n-alkyl-4'cyanobiphenyls, using positron annihilation spectroscopy, are presented. The homologues investigated are, hexyl-, heptyl-, octyl- and decyl-cyanobiphenyls (6CB, 7CB, 8CB and 10CB). In these compounds, the positron lifetime measurements were performed as functions of temperature. The positron annihilation parameters are found to exhibit strong dependence on temperature. It was found that the ortho-positronium pick-off lifetime shows changes which strongly support (i) a gradual disappearance, instead of an abrupt one, of some memory of the more ordered solid phases on passing to the liquid crystalline phases, (ii) the strong tendency for the molecules in the mesophases to undergo anti-parallel bimolecular association and (iii) the formation of cyabotactic groups of a smectic phase in a nematic medium. Changes were also observed in the ortho-positronium formation probability which apparently indicate a systematic transformation of the solid phase from a close-pack structure to an open-pack structure as one goes from a lower to a higher homologue of the compounds investigated. Solid crystalline polymorphism has been observed in 8CB. A change in molecular packing in the solid phase of 10CB has been observed.

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Raman investigation of stable and metastable states of4−octyl−4′-cy

Fehr

Dieudonné

Sauvajol

et al. 2003

Phys. Rev. E

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We study the polymorphism of solid phases of 4-octyl-4(')-cyanobiphenyl (8CB) by Raman spectroscopy. For bulk 8CB, the Raman spectrum of the CN stretch is featured by a single peak, which shifts abruptly at the smectic-A-crystal transition. In confinement, the CN peak splits both at high and low temperatures. In the isotropic and liquid crystal phases, the signature of the liquid crystal bulk (LC) coexists with another peak that is assigned to LC molecules interacting with the matrice interface. We find correlations between the volumic fractions of interfacial liquid and the texture of the matrices. At low temperatures, we assign the splitting of the CN peak to the coexistence of different metastable solid phases. For strong confinements, the temperature dependence of the CN stretching frequency extends to that of the liquid, which suggests the existence of frozen-in smecticlike solid phases. We discuss the structure of these metastable solid phases in the light of neutron diffraction measurements. We also report on the peculiar analogy between the effect of quenched disorder due to the porous matrices and the effect of thermal quenching.

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Metastable, ordered solids of rod-like molecules quenched from their liquids at far-from-equilibrium conditions: Phonon spectra of 4-n-octyl- and 4-n-dodecyl-4′-cyanobiphenyl

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Cited by 3 publications

References 14 publications

The formation mechanism of hollow spherulites and molecular conformation of curcumin and solvate

The formation mechanism of hollow spherulites and molecular conformation of curcumin and solvate

Phase transformations in some homologues of 4-n-alkyl-4'-cyanobiphenyls investigated by positron annihilation spectroscopy

Raman investigation of stable and metastable states of4−octyl−4′-cy

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