The re-entrant nematic and the compounds of Gray

Cladis, P. E.

doi:10.1080/026782998207532

Cited by 16 publications

(6 citation statements)

References 21 publications

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“…The molecules with an imine group have greater aspect ratio and shape anisotropy than these values of 8OCB. The calculated molecular lengths are in agreement with the experimental results [44]. As seen from Table 1, ⟨L x ⟩ and ⟨L y ⟩ are unequal and the shape biaxialities for nematic 8OCB, CBOOA, and HBAB are calculated to be 0.122, 0.102, and 0.075, respectively, being in the order of the biaxiality parameters measured by Madsen et al [45].…”

Section: Resultssupporting

confidence: 86%

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Comparison of structural properties of some liquid crystals

Çapar¹

2017

Turk J Phys

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Abstract:Molecular dynamics simulations were performed for three liquid crystalline molecules, 4-cyano-4 ′ -octyloxybiphenyl (8OCB), n-p-cyanobenzylidene-p-octyloxyaniline (CBOOA), and p-n-hexyloxybenzylidene-p ′ -aminobenzonitrile (HBAB). Simulation data were obtained for three liquid crystals in the nematic and isotropic phases to analyze their structural properties. The imine spacer group in the Schiff bases induces a stepped core structure in which the linearity is maintained. The distributions of the angles between some defined vectors imply that the core segments of CBOOA and HBAB are more floppy than the 8OCB core. The biaxialities, molecular dimensions, and molecular anisotropies were calculated. The results were compared for three mesogens in the nematic and isotropic phases.

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Section: Resultssupporting

confidence: 86%

“…Average molecular dimensions (Å), aspect ratio ⟨Lz⟩/⟨w⟩ , anisotropy ⟨∆L⟩ , and biaxiality ⟨λL⟩ for 8OCB, CBOOA, and HBAB in the nematic and isotropic phases. The experimental values in parentheses were predicted by Cladis[44]. *: Isotropic phase.…”

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confidence: 99%

Comparison of structural properties of some liquid crystals

Çapar¹

2017

Turk J Phys

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“…Initially the focus of research centered on re-entrant nematics, where two nematic phases, usually separated by a smectic phase, can be detected on varying the temperature [2,3]. This phase behavior is typically found for molecules with strong polar groups, where on cooling due to antiparallel correlations in a type of supramolecular organization entities with incommensurate length scales are formed.…”

Section: Introductionmentioning

confidence: 99%

Nematic-nematic phase transition in the liquid crystal dimer CBC9CB and its mixtures with 5CB: A high-resolution adiabatic scanning calorimetric study

et al. 2011

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The phase transition behavior of the liquid crystal dimer α,ω-bis(4,4'-cyanobiphenyl)nonane (CBC9CB), which has been reported to exhibit a nematic-nematic phase transition, has been investigated by means of high-resolution adiabatic scanning calorimetry. This nematic-nematic phase transition is weakly first-order with a latent heat of 0.24±0.01 kJ kg(-1). Mixtures up to 40 wt % with 4-pentyl-4'-cyanobiphenyl (5CB) liquid crystals have also been investigated, which also show this nematic to nematic phase transition. The transition stays weakly first-order with a decreasing latent heat with increasing concentration of 5CB. For mixtures with more than 40 wt % uniaxial nematic-unknown nematic phase transition was not observed.

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“…The preceding experimental results clearly provide evidence that compound DL-10,9 , analogous to that of DL-8,9 , exhibits a re-entrant nematic phase in a N–SmA d –N re phase sequence. This is noteworthy given the fact that the re-entrant phenomenon, where a LC phase characterized by higher symmetry reappears in sequence of phase transitions to the states attaining lower symmetry progressively, rarely occurs. ,,− That is, in comparison with the high rate of recurrence of other LC phases, re-entrant phases rarely exist in mixtures ,,− or single component systems composed of monomers, such as calamitics and discotics, oligomers, ,− and polymers., − In recent times, this behavior has also been observed in banana-shaped LCs, supramolecular complexes, − and metal-containing LCs . Indeed, the probability and the incidence of finding re-entrant behavior seem to be more in mixtures of materials; ,,− as is well-known, the re-entrant phenomenon, in a phase sequence, viz., N–SmA–N re , was first observed by Cladis in mixtures of cyano compounds .…”

Section: Resultsmentioning

confidence: 99%

Structure–Property Correlations in Cyanobiphenyl-Based Dimer-Like Mesogens

Prabhu

Yelamaggad

2015

J. Phys. Chem. B

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In this contribution, we present the results of our extensive investigations on the synthesis and phase behavior of five series of dimer-like compounds formed by covalently linking a promesogenic, cyanobiphenyl core, with a nonmesogenic N-(n-alkyl)-salicylaldimine segment via an oxy(oligomethylene)oxy spacer of varying length and parity. This work is a continuation of our previous short study where the occurrence of re-entrant nematic (Nre) phase seemed to be vitally dependent on the parity of the spacer and the length of the terminal chain. Thus, working along these lines the effects on the thermal (Nre phase) behavior of the various spacers and the terminal tails have been investigated comprehensively. In particular, with the aim of exploring the fundamental relationships between molecular structural features and thermal properties, five spacers, namely, oxy(hexyl)oxy, oxy(heptyl)oxy, oxy(octyl)oxy, oxy(nonyl)oxy, and oxy(decyl)oxy spacers, have been used. Each series consists of six mesogens, as the terminal tail attached to the salicylaldimine core has been varied from n-pentyl to n-decyl. For the sake of comparison and completeness, previously reported materials have also been studied in detail, and data are included here. The characterization results derived from the optical and calorimetric studies clearly illustrate a pronounced odd-even effect; however, the effect attenuates as the length of the spacer is elongated. The even-members show higher clearing temperatures and enthalpies of transition than those of adjoining odd-members. Three series of mesogens possessing an even-parity spacer show thermodynamically stable nematic (N) and/or smectic A (SmA) phase(s); the lower homologues display only the N phase, and the middle ones exhibit both N and SmA phases while the higher members show the SmA phase solely. In contrast, the vast majority of the odd-spaced compounds of two series show a monotropic N phase. X-ray diffraction study confirms the partial bilayer order of the SmA (SmAd) phase with L < d < 2L where d is the layer spacing and L is the molecular length. Contrary to what was previously noted, none of the new compounds synthesized favor the stabilization of the Nre phase. At the present time, it is extremely difficult to point out the exact cause for the origin of re-entrant behavior in some compounds exclusively, as they hardly differ in their structure. It was also found that the phase transitional properties of even-members are comparable to that observed for 4'-n-alkoxy-4-cyanobiphenyls (nOCBs) and 4-n-alkyl-4-cyanobiphenyls (nCBs). In essence, our study validates the point that dimer-like compounds resemble liquid crystal (LC) dimers, as well as monomers to some degree, in the context of their thermal behavior; thus, they can be regarded as the bridging structures between LC monomers and dimers.

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The re-entrant nematic and the compounds of Gray

Cited by 16 publications

References 21 publications

Comparison of structural properties of some liquid crystals

Comparison of structural properties of some liquid crystals

Nematic-nematic phase transition in the liquid crystal dimer CBC9CB and its mixtures with 5CB: A high-resolution adiabatic scanning calorimetric study

Structure–Property Correlations in Cyanobiphenyl-Based Dimer-Like Mesogens

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