摘要:

The work concerns the structures and properties of multilayer smectic phases with complex tilt and dipolar order. The symmetry and thermodynamical classification of multilayer antiferroelectric and ferrielectric phases is given. The main attention is paid to the difference of these phases with respect to classical ferroelectric S*c. A two-layer model of the ferrielectric smectic phase is generalized to describe the sequence of the first order phase transitions ferro-ferri-antiferro-electric and to show the possibility of existence of two isostructural ferrielectric phases, which differ in the value of the helical pitch and in the sense of the helix.

ISSN:0267-8292    

DOI:10.1080/02678299608032034

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A nematic and a cholesteric liquid crystal, both derived fromtrans-1,3,5-cyclohexanetri-carboxylic acid, and a linear side chain polymer analogue were characterized in terms of storage (G″) and loss (G″) moduli as functions of frequency and temperature. It was found that all three model compounds show a shear shinning flow behaviour with zero shear viscosities of the low molar mass systems significantly less than that of the polymer system at the same reduced temperature,T/Tg. With shift factors prescribed by the WLF equation, both theG″ andG″ data of all three compounds are adequately represented by master curves. Furthermore, within the framework of the stretched exponential model, the relaxation behaviours of the low molar mass systems are well described by a single Maxwell element, whereas the polymer system shows a relatively broad distribution of relaxation times. The observed viscoelastic properties suggest a relative ease of material processing of the low molar mass systems compared to the polymer analogue due to a lower zero shear viscosity and a single relaxation time characterizing the dynamics of response to a mechanical or thermal stimulus.

ISSN:0267-8292    

DOI:10.1080/02678299608032035

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Several new liquid crystalline compounds comprising Schiff's-base cinnamate cores and fluorocarbon alkoxy tails of varying lengths have been synthesized and characterized. The thermal properties of these materials are compared to those of their hydrocarbon analogues. The results show that fluorination stabilizes the crystalline phase, quite unexpectedly destabilizes the Scphase, and for two of the compounds produces extraordinarily stable SAphases.

ISSN:0267-8292    

DOI:10.1080/02678299608032036

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The liquid crystalline phases of several rigid-rod, non-polar tolane oligomers are characterized by differential scanning calorimetry and transmitted polarized light microscopy. We determine that a stable nematic phase can be formed at ambient pressure if the molecular axial ratio (length-to-width ratio) is greater than 4.5. A smectic phase forms in addition to the nematic phase if the axial ratio exceeds 6.1. Symmetrical fluorination of the terminal phenyl groups reveals that the liquid crystalline phase behaviour of these rigid rods is highly sensitive to perturbations of the charge distribution along the molecules. Nematic tolane oligomers can exhibit high stregth disclinations (s= ±3/2 and ±2) in their schlieren textures, and we discuss conditions that promote the stability of these defects.

ISSN:0267-8292    

DOI:10.1080/02678299608032037

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

During the macroscopic reorientation of several nematic liquid crystal systems in a magnetic field, an unusual phenomenon, namely the presence of preferred macroscopic alignment at two different directions, was observed. This phenomenon is similar to the visual observation of pattern formation in liquid crystals under non-equilibrium conditions, except that our study was made at the molecular level. The technique used was C-13 NMR spectroscopy combined with rapidly decelerated sample spinning. The deceleration subjects the liquid crystal to a rapid change in the aligning torque. When the deceleration rate is slow, the alignment of the liquid crystals is predicted by their anisotropy of the magnetic susceptibility (Δχ). When the deceleration rate is sufficiently high, the dual alignment is observed for some of the liquid crystal systems studied. In these systems, part of the liquid crystal molecules remain with the director in the initial alignment, but the rest rapidly realign to reach a surface almost perpendicular to the axis of initial alignment, similar to the ‘opening’ of an umbrella. The ratio of the molecules with the two kinds of preferred alignment is determined by the non-equilibrium dynamics as well as Δχof the liquid crystal sample. Several liquid crystal systems were investigated: 4-n-pentyl-4′-cyanobiphenyl (5CB; Δχ>0), 4′-cyanophenyl-l-(trans-4-n-pentylcyclohexane (PCH5; Δχ>0 but smaller than 5CB), and l-(trans-4-n-pentylcyclohexyl)-4′-cyanocyclohexane (CCH5; small Δχ<0). The dual alignment is present in 5CB and CCH5, and it is more pronounced in PCH5. We also prepared several mixtures of 5CB and l-(trans-4-n-ethylcyclohexyl)-4′-cyanocyclohexane (CCH2; small Δχ<0), the Δχof which varies with the composition. The change in alignment became more pronounced as the magnitude of Δχapproached zero. An explanation for these results is suggested.

ISSN:0267-8292    

DOI:10.1080/02678299608032038

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Dielectric dispersion of the ferroelectric liquid crystal (CS-1022) around the SC*-SAtransition and temperature dependence of the spontaneous polarization were measured by varying the cell thickness. Behaviour of the dielectric constant is well described by a formula including the contribution of the soft mode, the Goldstone mode and the ionic conduction. Temperature dependence of the spontaneous polarization is expressed by a formulaPo(TC-T)α.

ISSN:0267-8292    

DOI:10.1080/02678299608032039

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Crystal structures have been determined for 4-[(S)-1-methylpentyloxycarbonyl]phenyl, 4-([S)-1-methylhexyloxycarbonyl]phenyl 4′-octyloxybiphenyl-4-carboxylates (1and2, respectively) and (R)-1-methylheptyl 4-(4′-octyloxybiphenyl-4-yloxymethylene)benzoate (3), which have SCA*, SC* and SCA* phases, respectively. All the crystals have a common packing mode with the previously determined structure of 4-(1-methylheptyloxycarbonyl)phenyl 4′-octyloxybiphenyl-4-carboxylate (MHPOBC). Each crystal forms a smectic-like layer structure composed of largely bent molecules with the long alkyl chain of the chiral group almost perpendicular to the core moiety. Differential scanning calorimetry showed that the crystals of1, 2, and MHPOBC, which have a carbonyloxy group as the central linkage between the biphenyl and phenylene moieties, transform to higher temperature solid phases in a similar temperature range (around 60°C). For3, which has an oxymethylene linkage between the biphenyl and phenylene moieties, three kinds of solids were distinguished.

ISSN:0267-8292    

DOI:10.1080/02678299608032040

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We have undertaken a theoretical study on the structure of planar solitons in nematic and smectic liquid crystals. In nematics we find a soliton solution which can be looked upon as an intertwine between two solitons. In a nematic obtained by unwinding a cholesteric and in a nematic with very high dielectric anisotropy, we have worked out energetics of solitons. A 2π soliton in a ferronematic or a smectic C becomes unstable due to the splay bend elastic anisotropy. The structures of π solitons in smectic A and smectic C in the neighbourhood of a SA–SCphase transition have also been studied.

ISSN:0267-8292    

DOI:10.1080/02678299608032041

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Temperature induced surface transitions in liquid crystal tilted homogeneous cells are experimentally studied. The results are theoretically explained by an elastic theory of surface transitions and the resulting values for the effective splay-bend elastic constantk13for each liquid crystal material are compared with the results from experience with other types of surface anchoring.

ISSN:0267-8292    

DOI:10.1080/02678299608032042

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Various paracyclophane derivatives incorporating 4,4′-biphenyl, 2,5-diphenyl-1,3,4-thiadiazole, phenyl benzoate and 2,6-disubstituted naphthyl rigid cores were synthesized and their mesomorphic behaviour was studied using polarizing microscopy, DSC and X-ray diffraction. Most of these macrocyclic compounds possess liquid crystalline properties with unexpectedly high clearing temperatures compared to those of conventional calamitic mesogens. In this way, the coupling of two appropriate rigid units using flexible chains to form a macrocycle constitutes a new and powerful approach towards mesophase induction and stabilization. The types of mesophase formed by these macrocycles do not depend only on the nature of the bridging chains, but also strongly on the structure of the rigid aromatic system. The smectic A phase and the E phase are formed by polyetherbiphenylophanes. Poly-ethercyclophanes incorporating the 2,5-diphenylthiadiazole rigid core form nematic and smectic C phases. The nematic phase is the only mesophase when the rigid core is the phenyl benzoate unit. No mesomorphic properties could be detected for macrocycles which featured either the benzyl phenyl ether moiety or the 2,6-disubstituted naphthalene unit in their constitution.

ISSN:0267-8292    

DOI:10.1080/02678299608032043

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor