Texture transitions in the liquid crystalline alkyloxybenzoic acid 6OBAC

Abstract

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

Some Features of Liquid Crystalline Oxadiazoles

“…Texture transitions have been previously observed inside the nematic phase of some mesomorphic thermotropic materials belonging to the families of the alkyloxybenzoic and cyclohexane acids [14][15][16]. In some cases, peaks in DSC scanning and dielectric spectroscopy investigations accompany the evidence of the texture transition observed by polarized light microscopy.…”

Growth of toric domains in mesophases of oxadiazoles

“…Phase transition approaching, the local smectic order experienced by the cybotactic clusters becomes the order of all the material. Smectic plane aggregations in very small regions, as we imagine the cybotactic clusters, can be more persistent near the cell walls and in particular at defects on the wall [7]. We can imagine the defects on the walls to be responsible of the position of the boundaries between domains in the nematic phase.…”

“…Above the transition temperature, the nematic assumes the usual schlieren appearance. Texture transitions have been previously observed inside the nematic range of some alkyloxybenzoic and cyclohexane acids [6][7][8][9][10][11][12][13]. The common explanation for the presence of a transition in the nematic range of alkyloxybenzoic acids is based on the existence of cybotactic clusters favouring a local smectic order in the nematic melt [14,15].…”

“…A strong paramorphosis is existing between the low temperature nematic phase and the smectic phase [6,7,10]. Probably, it is the role of the anchoring at the cell walls of cybotactic clusters, which is relevant for the paramorphosis.…”

The GIMP Retinex Filter Applied to the Fabric Fault Detection

“…In addition, an approach based on an image processing, developed for the study of liquid crystals [22], had been proposed by one of the authors of this article in [23][24][25].…”