Depolarised Rayleigh scattering and textures in the nematic phase of some 4,n-alkyloxybenzoic acids

Petrov, M.; Simova, P.

doi:10.1088/0022-3727/18/2/011

Cited by 37 publications

(13 citation statements)

References 17 publications

(10 reference statements)

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“…around 120°C for OOBA), the monomers disappear and only cyclic and open dimers remain, the cyclic predominating. In a set of articles [23][24][25][26] we have demonstrated very sharp anomalies exhibited by the temperature dependences of some typical liquid crystal constants at T = T*. We also found that this temperature divides the nematic phase interval for OOBA into two subphases: a hightemperature one N, with the properties of a conventional nematic and a low-temperature one N, with markedly quasi-smectic character.…”

Section: Wolccular Stvuctzrve and Two Nematic Subphnses For Oobamentioning

confidence: 69%

“…We also found that this temperature divides the nematic phase interval for OOBA into two subphases: a hightemperature one N, with the properties of a conventional nematic and a low-temperature one N, with markedly quasi-smectic character. The transition N, ++N, is accompanied by a strong texture transformation [25]. Above T* the coexistence of the dimers and monomers having different molecular lengths is not favourable for smectic-like fluctuations and, as a result, a classic nematic is established, while below T*, the monomers disappear [ 221, the dimers are more compact molecular forms and this stimulates the sniectic order fluctuations as well as the formation of quasi-smectic complexes.…”

Section: Wolccular Stvuctzrve and Two Nematic Subphnses For Oobamentioning

confidence: 99%

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Surface memory effect in nematics with smectic C short-range order and hydrogen bonded molecules

Petrov

Tsonev

1996

Liquid Crystals

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To cite this article: Minko P. Petrov & Lyubomir V. Tsonev (1996) Surface memory effect in nematics with smectic C shortrange order and hydrogen bonded molecules, Liquid Crystals, 21:4, 543-555, A surface memory effect (SME) is found in the dimerized nematic with short range smectic C order of 4-n-octyloxybenzoic acid (OOBA) in a cell whose walls have been covered with an obliquely evaporated SiO or I T 0 layer. The recording and erasure processes are discus'sed and erasure in the nematic phase is realized. A mechanism for the surface memorization is presented which takes into account the specific character of dimerized nematics preceding cooling into a smectic C phase.

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Section: Wolccular Stvuctzrve and Two Nematic Subphnses For Oobamentioning

confidence: 69%

Section: Wolccular Stvuctzrve and Two Nematic Subphnses For Oobamentioning

confidence: 99%

Surface memory effect in nematics with smectic C short-range order and hydrogen bonded molecules

Petrov

Tsonev

1996

Liquid Crystals

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“…infrared technique; see Petrov and Simova [ 23], and According to the suggestion of these last three studies, references therein. Some anomalies in the temperature the phenomenon of the transition between two nematic dependence of the elastic constants had already been`s ubphases' should be thought of as a particular type of observed by Rondelez [ 24] and con® rmed in the dielecanchoring transition driven by the temperature.…”

Section: State-of-the-artmentioning

confidence: 99%

Spontaneous chiral domains in the nematic phase of achiral trans-4-alkylcyclohexanecarboxylic acids

Torgova¹,

Komitov²,

Strigazzi³

1998

Liquid Crystals

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We have discovered a spontaneous chiral ordering in thermotropic nematic liquid crystals forming hydrogen bonds, namely in trans-4-alkylcyclohexan ecarboxylic acids, which do not contain any chiral group in the molecular structure, and have suggested a possible explanation of this phenomenon. The investigation was performed by polarizing light microscopy, using cells with glass substrates treated to give a uniform planar alignment in the nematic phase. On cooling from the isotropic phase, two types of nematic domains with diVerent alignment were found to grow: some possessing a unidirectional planar orientation as expected, and others with a spontaneous twisted molecular arrangement. The appearance of the twisted domains and the sense of their twist were random, i.e. they always emerged in various regions of the cell with diVerent twist sense: thus, they cannot be regarded as induced by stable defects in the surface aligning ® lm. With the coalescence of the nematic droplets, the number of the domains with the spontaneous twist very quickly decreased, since two chiral domains characterized by diVerent handedness transformed into a racemic domain with uniform planar alignment. Here the explanation of the macroscopic texture is given in terms of molecular structure, based upon the ability of the carboxylic acids to organize through hydrogen bonds into not only dimers, but also trimers and oligomers. The microscopic structure in¯uences the macroscopic twisted organization of the director inside the domains, and the surface lies normally to the helix axis. A brief review is given of work done in the ® elds of hydrogen bonded liquid crystals and of order transitions, together with a Landau± de Gennes model describing the mechanism of the spontaneous chiral ordering.

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“…This was thought as due to the presence of cybotactic clusters [6] having short-range smectic order in the nematic phase under a certain temperature. In the nematic melt, if the temperature is low enough, dimers aggregate in clusters with smectic C ordering [6][7][8][9][10][11]. The member with 6 carbon atoms in the tail, 6OBAC (monomer C 6 H 13 OC 6 H 10 COOH) is not able to display a smectic phase.…”

Section: Introductionmentioning

confidence: 99%

Texture transitions in the liquid crystalline alkyloxybenzoic acid 6OBAC

2006

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The 4,n-alkyloxybenzoic acid 6OBAC has a very rich variety of crystalline structures and two nematic sub-phases, characterised by different textures. It is a material belonging to a family of liquid crystals formed by hydrogen bonded molecules, the 4,n-alkyloxybenzoic acids n indicates the homologue number). The homologues with 7 ≤ n ≤ 13 display both smectic C and N phases. In spite of the absence of a smectic phase, 6OBAC exhibits two sub-phases with different textures, as it happens in other materials of the homologue series which possess the smectic phase. This is the first material that exhibits a texture transition in a nematic phase directly originated from a crystal phase. Here we present the results of an image processing assisted optical investigation to characterise the textures and the transitions between textures. This processing is necessary to discriminate between crystal modifications and nematic subphases. IntroductionThe mesogenic features of the alkyloxybenzoic acids are due to the presence of hydrogenbonded dimers, where the strength and the orientation of the hydrogen bonds provide the conditions for the presence of smectic and nematic phases. Peculiar behaviours, like textures transitions in the nematic phase and the appearance of domains of spontaneous twist and dendrite structures [1][2][3][4][5], makes liquid crystals with hydrogen bonded dimers, such as 4,nalkyloxybenzoic acids, very attracting materials for the research of chiral behaviour in nonchiral compounds. In the nematic phase, the material is a mixture of monomers and dimers (closed and open, Fig. 1), with concentrations depending on the temperature. Near the nematic-isotropic phase transition, as reported in [4], a process of oligomerization, with the formation of trimers, quadromers and so on, is possible: these oligomers produce a chiral nematic domain in a material with achiral molecules. In the 4,n-alkyloxybenzoic acids (nOBAC), the monomers are composed of two sterically distinct molecular parts, the oxybenzoic acid residue and the aliphatic chain. The number n of carbon atoms in the aliphatic tail gives origin to the homologue nOBAC series. The molecule unit determining the behaviour of the member in the series is the acid residue when the chain is short: no mesophases are present in the first and in the second member of the series 1OBAC and 2OBAC, where the melting point is above the temperature range of mesophase stability. From 3 to 6 carbon atoms in the tail, nematic but no smectic phase is

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Depolarised Rayleigh scattering and textures in the nematic phase of some 4,n-alkyloxybenzoic acids

Cited by 37 publications

References 17 publications

Surface memory effect in nematics with smectic C short-range order and hydrogen bonded molecules

Surface memory effect in nematics with smectic C short-range order and hydrogen bonded molecules

Spontaneous chiral domains in the nematic phase of achiral trans-4-alkylcyclohexanecarboxylic acids

Texture transitions in the liquid crystalline alkyloxybenzoic acid 6OBAC

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