Structural and dynamic properties of a new type of discotic nematic compounds

Ebert, Martina; Jungbauer, D.; Kleppinger, Ralf; Wendorff, Joachim H.; Kohne, B.; Praefcke, Klaüs

doi:10.1080/02678298908028958

Cited by 69 publications

(25 citation statements)

References 11 publications

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“…23 The dynamics of nematogens in the isotropic phase are strongly influenced by local structures ͑pseudonematic domains͒ with a correlation length increasing dramatically as the phase transition is approached. 23 This pretransitional effect has been confirmed many times for rodlike nematogens using different techniques 23 and for a discotic nematogen 24 as well. A recent computer simulation study 25 performed on a variety of model systems including rodlike and disklike nematogens revealed universal power law regimes on short to intermediate time scales as these systems cross N-I phase transition.…”

Section: Introductionmentioning

confidence: 59%

Dynamics of a discotic liquid crystal in the isotropic phase

Fruchey

Fayer

2006

The Journal of Chemical Physics

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Optically heterodyne-detected optical Kerr effect (OHD-OKE) experiments are conducted to study the orientational dynamics of a discotic liquid crystal 2,3,6,7,10,11-hexakis(pentyloxy)triphenylene (HPT) in the isotropic phase near the columnar-isotropic (C-I) phase transition. The OHD-OKE signal of HPT is characterized by an intermediate power law t(-0.76+/-0.02) at short times (a few picoseconds), a von Schweidler power law t(-0.26+/-0.01) at intermediate times (hundreds of picoseconds), and an exponential decay at long times (tens of nanoseconds). The exponential decay has Arrhenius temperature dependence. The functional form of the total time dependent decay is identical to the one observed previously for a large number of molecular supercooled liquids. The mode coupling theory schematic model based on the Sjogren [Phys. Rev. A 33, 1254 (1986)] model is able to reproduce the HPT data over a wide range of times from <1 ps to tens of nanoseconds. The studies indicate that the HPT C-I phase transition is a strong first order transition, and the dynamics in the isotropic phase display a complex time dependent profile that is common to other molecular liquids that lack mesoscopic structure.

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Section: Introductionmentioning

confidence: 59%

Dynamics of a discotic liquid crystal in the isotropic phase

Fruchey

Fayer

2006

The Journal of Chemical Physics

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“…4). However, inverse (exothermic) DSC peaks are reported for the lower homologue of this compound between the glass and crystal melting transitions on heating and ascribed to structural variations (recrystallization) taking place in these compounds [20]. Even the calamitic nematic liquid crystal 5CB is reported to exhibit an exothermic peak on heating, due to the occurrence of a more stable, additional crystalline phase, which is not observed while cooling the sample [21].…”

Section: Vijayaraghavan and S Kumarmentioning

confidence: 94%

Effect of Slow Cooling on a Discotic Nematic Liquid Crystal: Evidences for Nematic–Nematic Transitions

Vijayaraghavan

Kumar

2006

Molecular Crystals and Liquid Crystals

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There is considerable interest in discotic liquid crystals because of their possible applications in liquid crystal displays and in one-dimensional conductors. In view of these applications, knowledge and control of their phase behavior and transition temperatures are crucial. We have studied a discotic nematic liquid crystal, namely, hexakis(4-nonylphenylethynyl)benzene. The phase sequence reported for this sample on cooling is isotropic (I) 81.8 C discotic nematic (N D ) and 39 C crystal (Cr). The phase sequences were obtained using differential scanning calorimetry (DSC) and optical polarizing microscopy (OPM) studies while varying the sample temperature rapidly (5 C=min). However, on slowly cooling the sample (0.36 C=min), we observed anomalous optical textures and DSC peaks in the nematic region at 75 C and 52 C. We also find discontinuous changes in the optical transmission intensity and diamagnetic anisotropy in the vicinity of these temperatures on slow cooling. Further, the XRD pattern showed small but distinct shifts in the lateral (100) peaks in the vicinity of these transitions. We infer that the variations in the lateral distance between the discotic molecules may be responsible for these observed nematic-nematic (N-N) transitions.

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“…These rod-like polyamides do Porod-Kratky plot for a macromolecule with limited number of kuhn steps 9 6 11. and exit slits (6,9) and total reflection prisms (7,10) onto the detector which is a photomultiplier tube (12 Table 3.…”

mentioning

confidence: 99%

Kerr Effect and Wide Angle Light-Scattering Studies of a Para-Aromatic Polyamide in Dilute Solution

Shere¹,

Sethumadhavan²,

Stein³

et al. 1996

Macromolecules

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The geometric and optical properties of a para-aromatic polyamide are characterized using wide angle light scattering and electric birefringence. The polydispersity correction was applied theoretically by assuming the most probable distribution. The aromatic polyamide studied can be satisfactorily modeled as a Kratky-Porod wormlike chain with a presistence length of 220 ( 50 Å and an optical unit anisotropy ratio of 2.2 ( 0.3 in tetrahydrofuran. The refractive index of 1.67 ( 0.03 is comparable to that of similar polyamides. The high persistence length of the polymer chain and the high inherent optical anisotropy of the repeat unit are found to be responsible for the exceptionally high birefringence observed in the oriented films. It is also conclusively established that there is no aggregation in the absence of moisture.

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Structural and dynamic properties of a new type of discotic nematic compounds

Cited by 69 publications

References 11 publications

Dynamics of a discotic liquid crystal in the isotropic phase

Dynamics of a discotic liquid crystal in the isotropic phase

Effect of Slow Cooling on a Discotic Nematic Liquid Crystal: Evidences for Nematic–Nematic Transitions

Kerr Effect and Wide Angle Light-Scattering Studies of a Para-Aromatic Polyamide in Dilute Solution

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