Alignment and alignment transition of bent core nematics Omaima Elamain, Gurumurthy Hegde, and Lachezar Komitov Citation: Appl. Phys. Lett. 103, 023301 (2013); doi: 10.1063/1.4813443 View online: http://dx.doi.org/10.1063/1.4813443 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v103/i2 Published by the AIP Publishing LLC.
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APPLIED PHYSICS LETTERS 103, 023301 (2013)
Alignment and alignment transition of bent core nematics Omaima Elamain,1 Gurumurthy Hegde,2 and Lachezar Komitov1,a) 1
Department of Physics, Gothenburg University, SE-412 96 Gothenburg, Sweden Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang, 26300, Kuantan, Malaysia
2
(Received 20 May 2013; accepted 23 June 2013; published online 8 July 2013) We report on the alignment of nematics consisting of bimesogen bent core molecules of chlorine substituent of benzene derivative and their binary mixture with rod like nematics. It was found that the alignment layer made from polyimide material, which is usually used for promoting vertical (homeotropic) alignment of rod like nematics, promotes instead a planar alignment of the bent core nematic and its nematic mixtures. At higher concentration of the rod like nematic component in these mixtures, a temperature driven transition from vertical to planar alignment was found near the C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4813443] transition to isotropic phase. V The alignment of liquid crystals is of vital importance for their device applications such as liquid crystal displays. From many liquid crystal phases known so far (more than 40) only the thermotropic nematic phase of calamitic liquid crystals, with rod like (RL) molecules, is the one which found application in the majority of liquid crystal devices present today. Therefore, the existing knowledge about the alignment liquid crystals is related exclusively to RL nematics, and very little is known about the alignment of other kind of nematics, namely, those which molecules has bent shape. During the last decade the biaxial properties of nematics consisting of bent core (BC) molecules and possible ferroelectric behavior of such materials have attracted a lot of scientific as well as application interest. Biaxiality1 and ferroelectricity2 of these materials are expected to result in fast switching of the BC molecules which in turn will give rise to an ultrafast electro-optical response. Even though there are a number of studies pointing to existence of such physical properties, unambiguous prove is still lacking. Further study on these materials is required in order to prove or reject the existence of biaxiality and ferroelectricity. Important issues in these studies are the preparation of the samples, in general, and the field-off alignment of the BC nematics, in particular. As known, the alignment of the calamitic nematic liquid crystals is a result of the interactions between the liquid crystal and the surface of the confining sample substrates. These interactions have quite complicate character and are still not well understood. Generally speaking, the liquid crystal/surface interaction results in two major kinds of alignment of the RL nematic liquid crystals: planar and vertical alignment, i.e., PA and VA, respectively. An intermediate alignment between those two is the tilted alignment (TA). Usually, planar alignment of RL nematic liquid crystals is achieved by means of alignment layers made from polyimides which after deposition on the substrates and completing the imidization process are unidirectionally rubbed or, in the case of photoalignment materials, illuminated by a linearly polarized UV light. The rubbing as well as the illumination process generates anisotropic physical properties of the contact surface of the alignment layer with the liquid a)
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crystal molecules. When alignment layer and the liquid crystal are brought in contact, liquid crystal molecules at the surface adopt alignment promoted by the surface anisotropy of the alignment layer. This alignment is transmitted further to the molecules in the liquid crystal bulk via elastic forces. When the alignment layer is made from material with low surface tension with respect to the liquid crystal material and if, in addition, has alkyl side group oriented vertically to the substrate surface, then the molecules of the RL nematics adopt orientation vertical to the substrate due to the physicochemical and sterical interactions between the surface alkyl side groups and the terminal alkyl groups of the liquid crystal molecules. Concerning the alignment of BC nematics, the situation is more complicated. In some publications is reported the achievement of VA with BC nematics3 in others is shown that such alignment was not possible to be achieved4 by means of alignment layers made from materials known to promote VA of RL nematics. We present in this work the results of our study on the alignment of BC nematic material by alignment layers made from materials known to promote PA and VA of RL nematic liquid crystals, respectively. We also report on the temperature driven transition from PA to VA and visa versa in the mixtures of BC with RL nematic, which takes place near the transition to isotropic phase, when the RL nematic components concentration exceeds 20 wt. %. This alignment transition was found to exhibit a certain hysteresis. The nematic liquid crystals studied in the present work was the BC nematic 4-chloro-1,3 Phenylene bis{4-(9 decenyloxy)-1,1-biphenyl-4 carboxylate} (ClPbis10BB) (work name F493) and its binary mixtures with different concentrations of the RL nematic liquid crystal component which was MLC6608 (De < 0) and MLC6873-100 (De > 0), respectively, both from Merck Ltd. The molecular structure, phase sequence, and transition temperatures of the BC nematic material studied in this work is shown in Fig. 1, and the synthesis of the material is described elsewhere.5 Conventional sandwich cells with cell gap of about 2 lm were used in the experiments. The cells consisted of two parallel glass substrates. The cells were filled with the liquid crystal material in the isotropic phase of the material by means of capillary forces. The surface of the cell
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FIG. 1. The chemical structure of the BC liquid crystal material and phase transition temperatures (in degree centigrade) according to Ref. 5.
substrates facing the liquid crystal was precoated with thin layer promoting PA and VA alignment, respectively. The layer promoting PA was made from SE 7992, and the one promoting VA was made from SE 1121, both from Nissan Chemical Industries, Japan. These alignment materials were dissolved in N-Methyl-2-Pyrrolidone (NMP) with concentration of 5 wt. %. The alignment material was deposited onto the glass substrate surface by spin coating at 3000 rpm, and then it was dried first at 100 C for 3 min and after that imidized at 190 C for 30 min. This procedure is giving a layer thickness of about 50–100 nm. Before assembling the cell, the PA layer was unidirectionally rubbed in order to obtain uniform field-off alignment of the BC nematic material and its mixtures, respectively. For the same reason, the VA layer was gently unidirectionally rubbed. In the BC nematic were dissolved the RL nematics MLC6608 and MLC6873-100, respectively, in concentration 5, 10, 20, and 30 wt. %, and the alignment of the mixtures as well as of the pure BC nematic material was studied at planar and vertical anchoring conditions, respectively. The nematic liquid crystal materials under investigation were injected into the cell in isotropic phase (120 C) by capillary action. The optical and electro-optical characteristics of the cells were investigated by means of optical polarizing microscope equipped by a photodiode connected to Tektronix TDS 540 digital storage oscilloscope. The position of the slow optic axis (i.e., the sample optic axis) was detected by inserting a k-red optical plate between the sample and the analyzer of the polarizing microscope.6 All the samples, containing alignment layer made from SE 7992, were found to possess uniform PA with optic axis, i.e., with the average long molecular axis, oriented parallel to the rubbing direction. This was confirmed by means of the k-red optical plate inserted between the sample and the analyzer of the microscope.6,7 Hence, we may conclude that the BC nematic and its mixtures under investigation are interacting with the unidirectionally rubbed alignment layer made from SE 7992 in similar way as the conventional RL nematics do. In Fig. 2, schematically presented are the planar alignment of RL nematic (Fig. 2(a)) and BC nematic (Fig. 2(b)), respectively. Likewise the RL nematics, the BC molecules lie with their average long molecular axis along the rubbing direction of the alignment layer. However, a substantial difference between the alignment behavior of the BC nematic and its mixtures and the one of RL nematics was observed in the case of vertical anchoring condition, i.e., when the alignment layer in the cell was promoting VA. While the alignment layer made from SE 1112 was promoting VA of the RL nematics, such as MLC6608 and MLC6873-100 used in this work, it was found that this
FIG. 2. Alignment of RL molecules and BC molecules promoted by unidirectionally rubbed polyimide layer for PA. The rubbing direction of the alignment layer is indicated by arrow.
alignment layer is promoting PA of the BC nematic material instead of VA. The origin of such a behavior seems to be the molecular shape of the BC molecules. Let us now analyze the alignment of RL and BC nematics promoted by alignment layer made of material for VA, such as SE 1112, which contains alkyl side chains. After deposition of the alignment layer onto the substrate surface these chains adopt perpendicular orientation with respect to the substrate. In Fig. 3, schematically illustrated is the interaction between the alkyl side chains of the alignment layer with the terminal molecular chains of the RL and BC nematic material, respectively. The case of VA of RL nematics
FIG. 3. Alignment of RL molecules (a) and BC molecules (b) by polyimide alignment layer for promoting VA.
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is well described and discussed in the literature.8,9 The terminal groups of the RL mesogens, which usually are also alkyl groups, interact physico-chemically as well as sterically with the surface alkyl groups by penetrating in between them (interdigitating), as shown in Fig. 3(a). Thus, the RL molecules of the nematic in contact with the surface anchored alkyl chains will adopt VA with respect to the cell substrate. Then the alignment of the RL molecules at the substrate surface will be transferred to the bulk liquid crystal molecules by the elastic forces. For the same reason one of the BC molecules two shoulders, which is in a direct contact with the solid surface alkyl chains, will adopt orientation perpendicular to the substrate. However, the other molecular shoulder, due to the large angle (about 137 ) between the molecular shoulders, do not transfer the vertical alignment to the bulk BC molecules,10 as shown in Fig. 3(b). Hence, the surface alkyl chains are not able to produce vertical alignment of the BC nematic. Instead, the layer promoting VA of RL nematics is now promoting PA of BC nematics. Our study on the alignment of BC nematics were extended to their binary mixtures with RL nematics. It was found also that these mixtures of the BC nematic with RL nematics such as MLC6608(De < 0) and MLC6873-100 (De > 0), respectively, showed only PA within their entire nematic interval for concentration of the RL nematic component in the mixture up to 20 wt. %. However, for concentration of the RL nematic over 20% it was found that in a narrow temperature interval, just below the isotropic transition temperature was observed VA of the mixture, which quickly was transformed into PA on cooling. The temperature interval of existence of the VA in these mixtures was found to increase with the RL nematic concentration and the appearance of VA on cooling and heating to take place at different temperatures, i.e., there is certain hysteresis in this temperature-induced PA/ VA transition. The VA alignment of the BC/RL nematic mixtures was distinguished from the isotropic phase by applying a mechanical pressure on the cell so that the mechanically induced birefringence in the VA state could be seen, in the case of MLC6873-100 (De > 0), or by applying an electric field across the cell so that field-induced birefringence in the VA state will take place, in the case of MLC6608(De < 0). There seems to be two reasons for the temperature induced alignment transition: (1) The concentration of the RL molecules absorbed onto the substrate surface increases with their concentration in the BC/RL mixture which in turn causes weakening of the effect of the surface absorbed BC molecules on the alignment of the BC/RL mixture. (2) At high temperatures and high concentrations of RL molecules, the BC/RL nematic mixture behaves as uniaxial, i.e., adopt VA in the presence of homeotropic anchoring condition. The observed hysteresis behavior of the temperature induced VA/PA transition indicates that the transition from biaxial to uniaxial state of the nematic mixture is, as expected, of first order. Here, we should underline that the motivation of studying the mixtures of BC with RL nematics is not only to understand the mechanism of their alignment promoted by different anchoring conditions but also to find out how much
Appl. Phys. Lett. 103, 023301 (2013)
we can extend the temperature range of the BC/RL nematic mixtures in which this mixture are able to show fieldinduced optically isotropic state.6 This is at present ongoing investigation. In conclusion, our study reported on the alignment of BC nematic as well as of its binary mixtures with RL nematics MLC6608 (De < 0) and MLC6873-100 (De > 0), respectively. This study showed that these liquid crystal materials behaved likewise the RL nematics what concerns the promoted alignment by planar anchoring condition, i.e., they adopted PA in the presence of such anchoring condition with average molecular axis oriented along the rubbing direction of the PA layer. However, a substantial difference in the alignment behavior of the RL nematics and BC nematic and its mixtures with RL nematics was observed in the presence of vertical anchoring conditions. While the RL nematics at such anchoring condition usually exhibited VA within their entire nematic temperature interval, the BC nematic and its mixtures with RL nematics show only PA at the same anchoring condition. This difference we attributed to the specific molecular shape of the BC molecules. Once they are anchored at the solid surface, capable of promoting vertical alignment of RL nematics, the BC molecules screen the effect of this condition from the rest of liquid crystal molecules, and although the mixtures contain RL molecules no VA was obtained until the concentration of the RL molecules exceeded certain concentration. It seems that there is such a critical concentration of RL molecules in the BC/LC mixtures since only at higher concentration of the RL nematic component, e.g., higher than 20 wt. % in our case, it was found VA of the nematic mixtures in narrow temperature interval near the isotropic transition temperature. However, further study on the specific features of the alignment of BC nematics and their mixtures with RL nematics is required in order to shed more light on the peculiarities of BC nematic/solid surface interactions. Another important issue, which is a subject of ongoing study, is to find out whether any correlation between the alignment behavior of the BC/RL mixtures as a function of the concentration of the RL nematic component and the appearance of field-induced optically isotropic state, which is an indication of the fieldinduced optical biaxiality of the nematic, does exist. 1
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