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Change of thermo-morphologic and thermotropic properties in liquid crystals exhibiting multiple phase transitions
Journal of Molecular Liquids 218 (2016) 531–537
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Change of thermo-morphologic and thermotropic properties in liquid crystals exhibiting multiple phase transitions Arif Nesrullajev Mugla Sitki Koçman University, Faculty of Natural Sciences, Department of Physics, Laboratory of Liquid and Solid Crystals, 48000, Mugla, Kotekli, Turkey
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Article history: Received 30 December 2015 Received in revised form 8 February 2016 Accepted 29 February 2016 Available online xxxx Keywords: Liquid crystals Thermotropic properties Surface effects Texture Phase transitions
a b s t r a c t Change of the thermo-morphologic and thermotropic properties of liquid crystals exhibiting multiple phase transitions vs. thickness of liquid crystalline layer have been investigated in this work. The sandwich-cells with 20 μm, 70 μm, 120 μm, 170 μm and 240 μm thicknesses were used. Liquid crystalline materials with smectic C, smectic A and nematic mesophases were objects of investigations. Change of the crystal–smectic C, smectic C–smectic A, smectic A–nematic and nematic–isotropic liquid phase transition temperatures, and transformation of texture from specific types to non-specific types with change of thickness of liquid crystalline layer have been found. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Liquid crystals are materials having sufficiently sensitive and mobile structures, and are important objects for the thermography, optoelectronics, microelectronics and recording systems [1–6]. Availability of various thermotropic phase transitions is an important peculiarity of liquid crystals. By temperature changes at these transitions, transformations of the translational, positional and orientational orders, the change of spatial structure, point-like symmetry and physical properties take place. Accordingly, such changes lead to the change of the microscopic and macroscopic properties of liquid crystalline mesophases [7–12]. Special interest in physics, physical-chemistry and application of liquid crystalline materials have the mesophase–mesophase phase transitions, which occurred between ordered phases and mesophase–isotropic liquid phase transitions, which occurred between ordered and disordered phases. Therefore, topics, which are connected with experimental and theoretical studies of phase transitions in liquid crystals, have attracted the permanent attention of scientists [13–23]. Liquid crystals display the monomorphic and polymorphic properties, and exhibit various types of physically anisotropic optically uniaxial and optically biaxial mesophases. Such mesophases appear at different temperatures, take place within various temperature intervals and exhibit different types of phase transitions [25–29]. Therefore, liquid crystals can be use at different thermal regimes, within various temperature regions and in different climatic conditions. On the other hand, in experimental studies of physical properties of liquid crystalline materials and also in technical devices, which are E-mail address: [email protected].
http://dx.doi.org/10.1016/j.molliq.2016.02.094 0167-7322/© 2016 Elsevier B.V. All rights reserved.
based on these materials, liquid crystals are placed between reference surfaces and must have definite thickness of liquid crystalline layer. But, because of interaction between liquid crystalline molecules and surfaces, character of surfaces and boundary conditions has effect on physical properties of liquid crystals [23–28]. Therefore, it is clear that thickness of liquid crystalline layer (i.e. distance between reference surfaces of the samples) must have an influence on the thermotropic and thermo-morphologic properties of phase transitions, and also on character of the biphasic regions of these transitions. Besides, is well known that the thickness of liquid crystalline displays and multimatrix elements is sufficiently important parameter, which determines the electro-optical, magneto-optical, thermo-optical and acousto-optical effects in liquid crystalline materials. These problems have been partially investigated theoretically in some works [25,29,30]. Unfortunately, influence of boundary conditions on the thermotropic and thermomorphologic properties of liquid crystalline materials have been experimentally investigated insufficiently. We are interested in influence of thickness of liquid crystalline layer, which is placed between reference surfaces of the sandwich-cells, on the thermo-morphologic and thermotropic properties of liquid crystals exhibiting multiple phase transitions. The results of these investigations are presented in this work. 2. Experimental 2.1. Methods Investigations of the thermotropic and thermo-morphologic properties of liquid crystals under investigations have been carried out by the
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Fig. 1. The molecular structure of liquid crystals, examined in this study. LC1: R = C8H17; LC2: R = C9H19.
polarizing optical microscopy (POM) technique using the trinocular polarizing conoscopic/orthoscopic microscope, compensators, optical filters and λ-plates from Olympus Optical Co., Ltd., and also special heaterthermostat with digital temperature control system, multimeters and power supply. Registration of microphotographs and conoscopic images has been carried out by digital microphotographic system from Olympus Optical Co. The thermo-morphologic properties of the heterophase regions of phase transitions have been studied by method of the temperature wedge [31], which was modified by us as the capillary temperature wedge (CTW) device [32,33]. The CTW device was used for study of the thermotropic and thermo-morphologic properties of various liquid crystalline materials and determination of the temperature and linear width of the heterophase regions of phase transitions in [28,34,35]. This device provides the observation of all of the thermal states of liquid crystalline materials in the real scale of time and in a wide temperature range, and provides also the calculation of the phase transition temperatures and the temperature widths of the heterophase regions with an accuracy not less than 10−2 K [28,32–35]. The crystallo-optics and crystallo-physics methods have been also used in this study [36–38]. These methods allow determining number and orientation of the optical axes, value and optical sign of the birefringence, peculiarities of defects and disclinations etc. in materials with optically anisotropic properties.
Fig. 3. Conoscopic images of aligned mesophases in LC1 and LC2. a — SmC mesophase; b — SmA mesophase; c — N mesophase.
120 μm, 170 μm and 240 μm. LC1 and LC2 have been filled into the sandwich-cells by the capillary forces at the isotropic liquid state.
2.2. Materials and samples
3. Results and discussion
In this work, two homologs 4-octyloxyphenyl esters of 4′buthoxybenzoic acid (LC1) and 4-nonyloxyphenyl esters of 4′buthoxybenzoic acid (LC2) of the 4-alkyloxyphenyl esters of 4′alkyloxybenzoic acid liquid crystalline compound's series, which were purchased from Soyuzkhim Reactive Inc. (Russia), have been investigated. These materials have the calamitic molecular form, are polymorphic liquid crystals, display smectic C (SmC), smectic A (SmA) and nematic (N) mesophases, exhibit the thermotropic phase transitions and are thermal stable and stable to moisture. The structural formula of these materials is given in Fig. 1. As seen in this figure, molecules of LC1 and LC2 have axial symmetry. The purity of LC1 and LC2 was estimated to be 99.70% by the liquid chromatography [39]. The samples used in this study were the sandwich-cells with fixed thickness. The reference surfaces of the sandwich-cells were optical glass slides. The spacer was placed between glass surfaces and fixed the thickness of liquid crystalline layer. The thicknesses of liquid crystalline layer have been examines by the POM technique and the digital micrometer with an accuracy as ± 0.1 μm. The thicknesses of the liquid crystalline layer in the sandwich-cells were fixed as 20 μm, 70 μm,
In LC1 and LC2 the sequence of the phase transitions is as following: solid crystal (Cr) → SmC mesophase → SmA mesophase → N mesophase → isotropic liquid (I). Investigations showed that in regions of the Cr–SmC, SmC–SmA, SmA–N and N–I phase transitions are not sharp boundaries. This fact indicates that for these transitions the heterophase regions take place. Schematic presentation of the phase transition regions in LC1 and LC2 is given in Fig. 2. Investigations showed also that aligned textures of LC1 and LC2 display the conoscopic images, which are presented in Fig. 3. The conoscopic image in Fig. 3a consists of the isogyres, isochromates and two melatopes; the conoscopic images in Fig. 3b and c consist of the isogyres and one apiece melatope. Melatope on the conoscopic image indicates on the place of exist of the optical axis from the sample. As is known, image in Fig. 3a is typical for the biaxial solid crystals and liquid crystals [36,37,40,41]. This image corresponds to SmC mesophase in LC1 and LC2. Images in Fig. 3b and c are typical for the uniaxial solid crystals and liquid crystals [36,37,40,41]. As seen from comparison of Fig. 3b and c, the conoscopic image in Fig. 3b corresponds to more uniform alignment than that in Fig. 3c. This peculiarity is typical for SmA and N mesophases in liquid crystals exhibiting multiple phase transitions
Fig. 2. Schematic representation of the phase transition regions in LC1 and LC2.
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Fig. 4. Textures of SmC mesophase for different thicknesses of liquid crystalline layers. a — 20 μm (LC1); b — 20 μm (LC2); c — 120 μm (LC1); d — 120 μm (LC2); e — 240 μm (LC1); f — 240 μm (LC2). Crossed polarizers; Magnification ×100.
and was observed by us in various liquid crystals. Differences in the conoscopic images for SmA and N mesophases are connected with differences in ordering of liquid crystalline molecules in these mesophases. Peculiarity of smectic mesophases with the rod-like molecules is their layered structure. The mass density function of SmC and SmA mesophases is modulated along the normal to the smectic layer. SmC and SmA mesophases have definite period which is about the length of liquid crystalline molecule. In SmC mesophase, molecules are aligned tilted to the layer; in SmA mesophase, molecules are aligned perpendicularly to the layer. SmC and SmA mesophases have the orientational and translational order. In these mesophases the center of gravity of molecules is in the plane of the smectic layer [7,23,24,42,43]. The mass density function of N mesophase is not modulated along the definite direction. This mesophase has only the long orientational order [7,23, 24,42,43]. Investigations showed that the thermo-morphologic properties of liquid crystalline mesophases under investigations are depended on thickness of liquid crystalline layer, placed between two reference surfaces of the sandwich-cell. Namely, an increase of the thickness of liquid crystalline layer leads to a transformation of textures of SmC, SmA and N mesophases from specific type to non-specific type, and to an appearance of various defects and disclinations in these textures. As an example, in Figs. 4, 5 and 6 the specific and non-specific textures of SmC, SmA and N mesophases in LC1 and LC2 are presented. As seen in these figures, in the sandwich-cells with different thicknesses one and the same mesophase exhibits different types of textures. Investigations showed also that the paramorphism not takes place in textures of LC1 and LC2.
In thin sandwich-cells, textures with the destroyed confocal formations have been observed in SmC mesophase of LC1 and LC2 (Fig. 4a, b). As seen in these textures, the thread-like formations also take place in these textures. Availability of the thread-like formations and destroyed confocal formations is characteristic peculiarity of SmC mesophase [23,44]. An increase of thickness of the sandwich-cells leads to a decrease of sharpness of boundaries between formations in SmC textures of LC1 and LC2 (Fig. 4c, d). In Fig. 4e and f, textures, which have been observed in thick sandwich-cells for SmC mesophase in LC1 and LC2, are presented. Peculiarity of these textures is availability of the destroyed confocal and polygonal formations, which are nonhomogeneously distributed in volume of the sandwich-cell. In thin sandwich-cells, the classic confocal textures have been observed for SmA mesophase in LC1 and LC2 (Fig. 5a, b). Such type of textures is typical and specific for SmA mesophase and are well known [44–46]. In such textures molecules have beam-like or fan-like packing and smectic layers form systems with equidistant surfaces. These surfaces are perpendicular to the director of smectic mesophase. Optical investigations showed that the confocal formations in SmA mesophase of LC1 and LC2 have a positive optical sign. An increase of thickness of the sandwich-cells leads to a change of sizes and distribution of the confocal formations and appearance of the fan-like formations (Fig. 5c, d). In Fig. 5e and f, textures, which have been observed in thick sandwichcells for SmA mesophase in LC1 and LC2, are presented. An increase of thickness of the liquid crystalline layer leads to appearance a lot of destroyed confocal and fan-shaped formations, and various singularities. In thin sandwich-cells for N mesophase in LC1 and LC2 the mosaic texture with sufficiently clear boundaries between uniform regions
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Fig. 5. Textures of SmA mesophase for different thicknesses of liquid crystalline layers. a— 20 μm (LC1); b — 20 μm (LC2); c — 120 μm (LC1); d — 120 μm (LC2); e — 240 μm (LC1); f — 240 μm (LC2). Crossed polarizers; Magnification ×100.
has been observed (Fig. 6a, b). Such type of texture is well known and is specific and classic for N mesophase [44,47,48]. The thread-like formations in these textures are the disclinations and are boundaries between different uniform regions of textures. These formations were observed both in the polarized and non-polarized light. Estimations showed that these uniform formations have positive birefringence. Investigations of peculiarities of the mosaics showed that whenever the director of the mosaic is oriented along one of the polarizer or analyzer directions, the mosaics appears black. Maximum intensity in the mosaics has been observed when the optical axis of the mosaics was oriented as θ = 45° between polarizer and analyzer. An increase of thickness of the sandwich-cells leads to an appearance of textures with a lot of disclinations and inversion walls (Fig. 6c, d). In thick sandwich-cells the marble-like textures in LC1 and LC2 have been observed (Fig. 6e, f). The marble-like texture is special type of the mosaic texture. In the marble-like texture boundaries between uniform regions are sufficiently fuzzy. Such peculiarity of this texture is connected with volume distributions of uniform part of the marble-like texture in thick sandwich-cells. Thus, the results, which have been obtained in this work, show that thickness of liquid crystalline layer, which placed between two reference surfaces of the sandwich-cell, has sufficient effect on the morphologic properties of SmC, SmA and N mesophases in liquid crystals exhibiting multiple phase transitions. This effect is connected with change of molecular alignment of SmC, SmA and N mesophases vs. change of thickness of liquid crystalline layer. In Fig. 7, schematic representation of molecular ordering in samples with different thicknesses is
presented. As seen in this figure, alignment and structurization of liquid crystalline molecules in the sandwich-cell with thin liquid crystalline layer is connected with the surface–liquid crystalline molecule interaction. But alignment and structurization of liquid crystalline molecules in the sandwich-cell with thick liquid crystalline layer is connected with interaction between liquid crystalline molecules. Investigations showed that change of thickness of liquid crystalline layer has influence on temperatures of the phase transitions in LC1 and LC2. In Table 1, temperatures of the Cr\\SmC, SmC–SmA, SmA–N and N\\I phase transitions in LC1 and LC2 are presented. As seen in this table, an increase of thickness of liquid crystalline layer leads to the shift of temperatures of all phase transitions to low temperatures. I.e. the thickness of liquid crystalline layer, which placed between the reference surfaces of the sandwich-cell, has influence on the Cr\\SmC, SmC–SmA, SmA\\N, N\\I phase transition temperatures. Namely, differences in phase transition temperatures for the thin (20 μm) and thick (240 μm) samples are as 1.3 K (Cr\\SmC), 1.9 K (SmC–SmA), 3.9 K (SmA\\N) and 5.5 K (N\\I) in LC1 and as 1.5 K (Cr\\SmC), 2.0 K (SmC–SmA), 3.6 K (SmA\\N) and 5.4 K (N\\I) in LC2. Thus, change of thickness of liquid crystalline layer, placed between the reference surfaces of the sandwich-cell, has sufficient effect on the thermomorphologic and thermotropic properties of liquid crystalline materials. Namely, change of thickness of the layer leads to the appearance of non-specific textures and provides the shift of the phase transition temperatures to low temperatures. We would like to emphasize here that dependence of the thermotropic and thermo-morphologic properties on the thickness of liquid crystalline layer for mesophases with
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Fig. 6. Textures of N mesophase for different thicknesses of liquid crystalline layers. a — 20 μm (LC1); b — 20 μm (LC2); c — 120 μm (LC1); d — 120 μm (LC2); e — 240 μm (LC1); f — 240 μm (LC2). Crossed polarizers; Magnification ×100.
different ordering is important peculiarity of liquid crystalline materials. We would like also to note that differences in the morphologic properties and texture defects of liquid crystals on the free surfaces and between two reference surfaces have been presented earlier [25, 26,49–51]. The influence of thickness of liquid crystalline layer on phase transition temperatures is obviously connected with the character of the interaction energy between reference surfaces of the sandwich cell and liquid crystalline molecules in samples with different thicknesses. Two cases of such interactions take place for liquid crystals, which are placed
between two reference surfaces of the sandwich-cells [7,23,50–52]. These interactions are the weak anchoring and the strong anchoring. In the case of the weak anchoring, the energy of interaction between liquid crystalline molecules (WLC) is bigger than that for interaction between liquid crystalline molecules and reference surfaces (WLCS). In this case the WLC N WLCS inequality takes place. In the case of the strong anchoring, the energy of interaction between liquid crystalline molecules is compared with the energy of interaction between liquid crystalline molecules and the reference surfaces. In this case the WLC ≈ WLCS connection takes place.
Table 1 Temperatures of phase transitions in LC1 and LC2 for various thicknesses of liquid crystalline layer. Liquid Crystal
LC1
LC2 Fig. 7. Schematic representation of molecular ordering of liquid crystal in samples with different thicknesses.
Thickness of Liquid Crystalline Layer, μm 20 ± 0.1 70 ± 0.1 120 ± 0.1 170 ± 0.1 240 ± 0.1 20 ± 0.1 70 ± 0.1 120 ± 0.1 170 ± 0.1 240 ± 0.1
Temperatures of Phase Transitions, K Cr–SmC
SmC–SmA
SmA–N
335.5 335.3 335.0 334.7 334.2 333.1 332.7 332.3 332.0 331.6
350.4 350.0 349.6 349.1 348.5 335.6 335.1 334.6 334.2 333.6
356.1 355.2 354.3 353.1 352.2 343.1 342.3 341.5 340.6 339.5
N–I 362.5 361.5 359.7 358.9 357.0 359.5 358.3 357.0 355.9 354.1
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Fig. 8. Temperatures of phase transitions vs. the thickness of liquid crystalline layer in LC1. a — Cr → SmC; b — SmC → SmA; c — SmA → N; d — N → I.
The cases of the weak and strong anchoring can be described by the general correlation for the surface effects as [7,23,50–52]: bL
W LC : W LCS
ð2Þ
Here b is the extrapolation length (the extrapolation length characterizes value of the interaction energy between liquid crystal and surface) and L is the dimension of liquid crystalline molecule. For the weak anchoring the bN L takes place, i.e. the extrapolation length is bigger than dimension of liquid crystalline molecule. For the strong anchoring the b ~ L takes place, i.e. the extrapolation length is comparable to the dimension of liquid crystalline molecule. Thus, the case of smaller values of the extrapolation length (i.e. in thin flat capillaries) takes place sufficient influence of the reference surfaces on the morphologic, alignment and thermotropic properties of liquid crystals. The energy
of interaction between liquid crystalline molecules depends not only on alignment of the director of liquid crystal, but also on distribution of the director in volume of the sample. Just therefore, uniform alignment of liquid crystalline textures has been obtained in thin liquid crystalline layers [26,51–53]. In Figs. 8 and 9, temperatures of phase transitions as a function of the thickness of liquid crystalline layer in LC1 and LC2 are presented. As seen in these figures, the Cr\\SmC and SmC–SmA phase transition temperatures change weakly with an increase of the liquid crystalline layer as compared with the SmA–N and N\\I phase transition temperatures. This effect is connected with the fact that the Cr\\SmC and SmC–SmA phase transitions realize between more ordered phase states than the SmA–N and N\\I phase transitions. Namely, SmC and SmA mesophases have orientational, positional and translational order, and characterize by the D∞ and C2h symmetry groups, accordingly; but N mesophase has only long translational order and characterizes by the D∞h symmetry group. Besides, differences in character temperature dependences vs. thickness of liquid crystalline layer for the Cr\\SmC and SmC–SmA phase transitions on the one hand and for the SmA–N and N\\I phase transitions on the other hand are obviously connected with differences in the interaction energy between liquid crystalline molecules and surfaces for SmC and SmA mesophases, and for N mesophase. Thus, differences in thickness of liquid crystalline layer lead to differences in distribution of liquid crystalline molecules near the surfaces and in volume of the sample. This difference depends on thickness of the flat capillary. I.e. if the thickness of liquid crystalline layer thickness is bigger, the difference in distribution of liquid crystalline molecules is bigger. Boundary layers of liquid crystals near the reference surfaces of the flat capillary determine the morphologic, alignment and thermotropic properties of liquid crystalline mesophase [49,50,52,53]. By elaboration of the reference surfaces of the thin samples, definite uniform alignment of liquid crystal has been obtained by us. In such samples without any elaboration of the reference surfaces, specific textures, which characterize liquid crystalline mesophase, can be obtained. But in the thick samples with elaborated or not elaborated reference surfaces, uniform aligned or specific textures have not been obtained by us. Thus, change of the thickness of the liquid crystalline layer, placed between two reference surfaces of the sandwich-cell, leads to change of the energy of interaction between liquid crystalline molecules and the reference surfaces, and accordingly leads to the change of the thermal energy, which is necessary for carrying out of the phase transitions between liquid crystalline mesophases in LC1 and LC2. I.e., an increase in thickness of liquid crystalline layer between two reference surfaces leads to a decrease of phase transition temperatures. We would like to note that as is known, an increase in thickness of liquid crystalline layer leads also to an decrease of threshold tension of the fields in the electro-optical and magneto-optical transitions for the aligned liquid crystalline textures, in particular by the Frederiks effect [7,23,42,54–56]. 4. Summary The results obtained in this work can be summarized as follows:
Fig. 9. Temperatures of phase transitions vs. the thickness of liquid crystalline layer in LC2. a — Cr → SmC; b — SmC → SmA; c — SmA → N; d — N → I.
– An increase in thickness of liquid crystalline layer, placed between two reference surfaces of the sandwich-cell, leads to a decrease of the Cr\\SmC, SmC–SmA, SmA–N and N\\I phase transition temperatures in LC1 and LC2. – An increase of thicknesses of liquid crystalline layer, placed between two reference surfaces of the sandwich-cells, leads to a transformation specific textures to non-specific textures and appearance various types of defective formations. – The influence of thickness of liquid crystalline layer on the thermomorphologic and thermotropic properties is connected with change of the molecule–molecule and molecule–surface interaction energy, and with change of the extrapolation length. Correction between the molecule–molecule and molecule–surface interactions characterizes
A. Nesrullajev / Journal of Molecular Liquids 218 (2016) 531–537
the energy of coupling between liquid crystalline molecules and reference surfaces. This energy depends on the thickness of liquid crystalline layer, placed between two reference surfaces of the sandwichcell and has influence on temperatures of the phase transitions.
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Acknowledgment
[31]
This work has been partially supported by the Research Foundation of Mugla Sitki Kocman University, Grant No. 15/124.
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Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Change of thermo-morphologic and thermotropic properties in liquid crystals exhibiting multiple phase transitions Arif Nesrullajev Mugla Sitki Koçman University, Faculty of Natural Sciences, Department of Physics, Laboratory of Liquid and Solid Crystals, 48000, Mugla, Kotekli, Turkey
a r t i c l e
i n f o
Article history: Received 30 December 2015 Received in revised form 8 February 2016 Accepted 29 February 2016 Available online xxxx Keywords: Liquid crystals Thermotropic properties Surface effects Texture Phase transitions
a b s t r a c t Change of the thermo-morphologic and thermotropic properties of liquid crystals exhibiting multiple phase transitions vs. thickness of liquid crystalline layer have been investigated in this work. The sandwich-cells with 20 μm, 70 μm, 120 μm, 170 μm and 240 μm thicknesses were used. Liquid crystalline materials with smectic C, smectic A and nematic mesophases were objects of investigations. Change of the crystal–smectic C, smectic C–smectic A, smectic A–nematic and nematic–isotropic liquid phase transition temperatures, and transformation of texture from specific types to non-specific types with change of thickness of liquid crystalline layer have been found. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Liquid crystals are materials having sufficiently sensitive and mobile structures, and are important objects for the thermography, optoelectronics, microelectronics and recording systems [1–6]. Availability of various thermotropic phase transitions is an important peculiarity of liquid crystals. By temperature changes at these transitions, transformations of the translational, positional and orientational orders, the change of spatial structure, point-like symmetry and physical properties take place. Accordingly, such changes lead to the change of the microscopic and macroscopic properties of liquid crystalline mesophases [7–12]. Special interest in physics, physical-chemistry and application of liquid crystalline materials have the mesophase–mesophase phase transitions, which occurred between ordered phases and mesophase–isotropic liquid phase transitions, which occurred between ordered and disordered phases. Therefore, topics, which are connected with experimental and theoretical studies of phase transitions in liquid crystals, have attracted the permanent attention of scientists [13–23]. Liquid crystals display the monomorphic and polymorphic properties, and exhibit various types of physically anisotropic optically uniaxial and optically biaxial mesophases. Such mesophases appear at different temperatures, take place within various temperature intervals and exhibit different types of phase transitions [25–29]. Therefore, liquid crystals can be use at different thermal regimes, within various temperature regions and in different climatic conditions. On the other hand, in experimental studies of physical properties of liquid crystalline materials and also in technical devices, which are E-mail address: [email protected].
http://dx.doi.org/10.1016/j.molliq.2016.02.094 0167-7322/© 2016 Elsevier B.V. All rights reserved.
based on these materials, liquid crystals are placed between reference surfaces and must have definite thickness of liquid crystalline layer. But, because of interaction between liquid crystalline molecules and surfaces, character of surfaces and boundary conditions has effect on physical properties of liquid crystals [23–28]. Therefore, it is clear that thickness of liquid crystalline layer (i.e. distance between reference surfaces of the samples) must have an influence on the thermotropic and thermo-morphologic properties of phase transitions, and also on character of the biphasic regions of these transitions. Besides, is well known that the thickness of liquid crystalline displays and multimatrix elements is sufficiently important parameter, which determines the electro-optical, magneto-optical, thermo-optical and acousto-optical effects in liquid crystalline materials. These problems have been partially investigated theoretically in some works [25,29,30]. Unfortunately, influence of boundary conditions on the thermotropic and thermomorphologic properties of liquid crystalline materials have been experimentally investigated insufficiently. We are interested in influence of thickness of liquid crystalline layer, which is placed between reference surfaces of the sandwich-cells, on the thermo-morphologic and thermotropic properties of liquid crystals exhibiting multiple phase transitions. The results of these investigations are presented in this work. 2. Experimental 2.1. Methods Investigations of the thermotropic and thermo-morphologic properties of liquid crystals under investigations have been carried out by the
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Fig. 1. The molecular structure of liquid crystals, examined in this study. LC1: R = C8H17; LC2: R = C9H19.
polarizing optical microscopy (POM) technique using the trinocular polarizing conoscopic/orthoscopic microscope, compensators, optical filters and λ-plates from Olympus Optical Co., Ltd., and also special heaterthermostat with digital temperature control system, multimeters and power supply. Registration of microphotographs and conoscopic images has been carried out by digital microphotographic system from Olympus Optical Co. The thermo-morphologic properties of the heterophase regions of phase transitions have been studied by method of the temperature wedge [31], which was modified by us as the capillary temperature wedge (CTW) device [32,33]. The CTW device was used for study of the thermotropic and thermo-morphologic properties of various liquid crystalline materials and determination of the temperature and linear width of the heterophase regions of phase transitions in [28,34,35]. This device provides the observation of all of the thermal states of liquid crystalline materials in the real scale of time and in a wide temperature range, and provides also the calculation of the phase transition temperatures and the temperature widths of the heterophase regions with an accuracy not less than 10−2 K [28,32–35]. The crystallo-optics and crystallo-physics methods have been also used in this study [36–38]. These methods allow determining number and orientation of the optical axes, value and optical sign of the birefringence, peculiarities of defects and disclinations etc. in materials with optically anisotropic properties.
Fig. 3. Conoscopic images of aligned mesophases in LC1 and LC2. a — SmC mesophase; b — SmA mesophase; c — N mesophase.
120 μm, 170 μm and 240 μm. LC1 and LC2 have been filled into the sandwich-cells by the capillary forces at the isotropic liquid state.
2.2. Materials and samples
3. Results and discussion
In this work, two homologs 4-octyloxyphenyl esters of 4′buthoxybenzoic acid (LC1) and 4-nonyloxyphenyl esters of 4′buthoxybenzoic acid (LC2) of the 4-alkyloxyphenyl esters of 4′alkyloxybenzoic acid liquid crystalline compound's series, which were purchased from Soyuzkhim Reactive Inc. (Russia), have been investigated. These materials have the calamitic molecular form, are polymorphic liquid crystals, display smectic C (SmC), smectic A (SmA) and nematic (N) mesophases, exhibit the thermotropic phase transitions and are thermal stable and stable to moisture. The structural formula of these materials is given in Fig. 1. As seen in this figure, molecules of LC1 and LC2 have axial symmetry. The purity of LC1 and LC2 was estimated to be 99.70% by the liquid chromatography [39]. The samples used in this study were the sandwich-cells with fixed thickness. The reference surfaces of the sandwich-cells were optical glass slides. The spacer was placed between glass surfaces and fixed the thickness of liquid crystalline layer. The thicknesses of liquid crystalline layer have been examines by the POM technique and the digital micrometer with an accuracy as ± 0.1 μm. The thicknesses of the liquid crystalline layer in the sandwich-cells were fixed as 20 μm, 70 μm,
In LC1 and LC2 the sequence of the phase transitions is as following: solid crystal (Cr) → SmC mesophase → SmA mesophase → N mesophase → isotropic liquid (I). Investigations showed that in regions of the Cr–SmC, SmC–SmA, SmA–N and N–I phase transitions are not sharp boundaries. This fact indicates that for these transitions the heterophase regions take place. Schematic presentation of the phase transition regions in LC1 and LC2 is given in Fig. 2. Investigations showed also that aligned textures of LC1 and LC2 display the conoscopic images, which are presented in Fig. 3. The conoscopic image in Fig. 3a consists of the isogyres, isochromates and two melatopes; the conoscopic images in Fig. 3b and c consist of the isogyres and one apiece melatope. Melatope on the conoscopic image indicates on the place of exist of the optical axis from the sample. As is known, image in Fig. 3a is typical for the biaxial solid crystals and liquid crystals [36,37,40,41]. This image corresponds to SmC mesophase in LC1 and LC2. Images in Fig. 3b and c are typical for the uniaxial solid crystals and liquid crystals [36,37,40,41]. As seen from comparison of Fig. 3b and c, the conoscopic image in Fig. 3b corresponds to more uniform alignment than that in Fig. 3c. This peculiarity is typical for SmA and N mesophases in liquid crystals exhibiting multiple phase transitions
Fig. 2. Schematic representation of the phase transition regions in LC1 and LC2.
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Fig. 4. Textures of SmC mesophase for different thicknesses of liquid crystalline layers. a — 20 μm (LC1); b — 20 μm (LC2); c — 120 μm (LC1); d — 120 μm (LC2); e — 240 μm (LC1); f — 240 μm (LC2). Crossed polarizers; Magnification ×100.
and was observed by us in various liquid crystals. Differences in the conoscopic images for SmA and N mesophases are connected with differences in ordering of liquid crystalline molecules in these mesophases. Peculiarity of smectic mesophases with the rod-like molecules is their layered structure. The mass density function of SmC and SmA mesophases is modulated along the normal to the smectic layer. SmC and SmA mesophases have definite period which is about the length of liquid crystalline molecule. In SmC mesophase, molecules are aligned tilted to the layer; in SmA mesophase, molecules are aligned perpendicularly to the layer. SmC and SmA mesophases have the orientational and translational order. In these mesophases the center of gravity of molecules is in the plane of the smectic layer [7,23,24,42,43]. The mass density function of N mesophase is not modulated along the definite direction. This mesophase has only the long orientational order [7,23, 24,42,43]. Investigations showed that the thermo-morphologic properties of liquid crystalline mesophases under investigations are depended on thickness of liquid crystalline layer, placed between two reference surfaces of the sandwich-cell. Namely, an increase of the thickness of liquid crystalline layer leads to a transformation of textures of SmC, SmA and N mesophases from specific type to non-specific type, and to an appearance of various defects and disclinations in these textures. As an example, in Figs. 4, 5 and 6 the specific and non-specific textures of SmC, SmA and N mesophases in LC1 and LC2 are presented. As seen in these figures, in the sandwich-cells with different thicknesses one and the same mesophase exhibits different types of textures. Investigations showed also that the paramorphism not takes place in textures of LC1 and LC2.
In thin sandwich-cells, textures with the destroyed confocal formations have been observed in SmC mesophase of LC1 and LC2 (Fig. 4a, b). As seen in these textures, the thread-like formations also take place in these textures. Availability of the thread-like formations and destroyed confocal formations is characteristic peculiarity of SmC mesophase [23,44]. An increase of thickness of the sandwich-cells leads to a decrease of sharpness of boundaries between formations in SmC textures of LC1 and LC2 (Fig. 4c, d). In Fig. 4e and f, textures, which have been observed in thick sandwich-cells for SmC mesophase in LC1 and LC2, are presented. Peculiarity of these textures is availability of the destroyed confocal and polygonal formations, which are nonhomogeneously distributed in volume of the sandwich-cell. In thin sandwich-cells, the classic confocal textures have been observed for SmA mesophase in LC1 and LC2 (Fig. 5a, b). Such type of textures is typical and specific for SmA mesophase and are well known [44–46]. In such textures molecules have beam-like or fan-like packing and smectic layers form systems with equidistant surfaces. These surfaces are perpendicular to the director of smectic mesophase. Optical investigations showed that the confocal formations in SmA mesophase of LC1 and LC2 have a positive optical sign. An increase of thickness of the sandwich-cells leads to a change of sizes and distribution of the confocal formations and appearance of the fan-like formations (Fig. 5c, d). In Fig. 5e and f, textures, which have been observed in thick sandwichcells for SmA mesophase in LC1 and LC2, are presented. An increase of thickness of the liquid crystalline layer leads to appearance a lot of destroyed confocal and fan-shaped formations, and various singularities. In thin sandwich-cells for N mesophase in LC1 and LC2 the mosaic texture with sufficiently clear boundaries between uniform regions
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Fig. 5. Textures of SmA mesophase for different thicknesses of liquid crystalline layers. a— 20 μm (LC1); b — 20 μm (LC2); c — 120 μm (LC1); d — 120 μm (LC2); e — 240 μm (LC1); f — 240 μm (LC2). Crossed polarizers; Magnification ×100.
has been observed (Fig. 6a, b). Such type of texture is well known and is specific and classic for N mesophase [44,47,48]. The thread-like formations in these textures are the disclinations and are boundaries between different uniform regions of textures. These formations were observed both in the polarized and non-polarized light. Estimations showed that these uniform formations have positive birefringence. Investigations of peculiarities of the mosaics showed that whenever the director of the mosaic is oriented along one of the polarizer or analyzer directions, the mosaics appears black. Maximum intensity in the mosaics has been observed when the optical axis of the mosaics was oriented as θ = 45° between polarizer and analyzer. An increase of thickness of the sandwich-cells leads to an appearance of textures with a lot of disclinations and inversion walls (Fig. 6c, d). In thick sandwich-cells the marble-like textures in LC1 and LC2 have been observed (Fig. 6e, f). The marble-like texture is special type of the mosaic texture. In the marble-like texture boundaries between uniform regions are sufficiently fuzzy. Such peculiarity of this texture is connected with volume distributions of uniform part of the marble-like texture in thick sandwich-cells. Thus, the results, which have been obtained in this work, show that thickness of liquid crystalline layer, which placed between two reference surfaces of the sandwich-cell, has sufficient effect on the morphologic properties of SmC, SmA and N mesophases in liquid crystals exhibiting multiple phase transitions. This effect is connected with change of molecular alignment of SmC, SmA and N mesophases vs. change of thickness of liquid crystalline layer. In Fig. 7, schematic representation of molecular ordering in samples with different thicknesses is
presented. As seen in this figure, alignment and structurization of liquid crystalline molecules in the sandwich-cell with thin liquid crystalline layer is connected with the surface–liquid crystalline molecule interaction. But alignment and structurization of liquid crystalline molecules in the sandwich-cell with thick liquid crystalline layer is connected with interaction between liquid crystalline molecules. Investigations showed that change of thickness of liquid crystalline layer has influence on temperatures of the phase transitions in LC1 and LC2. In Table 1, temperatures of the Cr\\SmC, SmC–SmA, SmA–N and N\\I phase transitions in LC1 and LC2 are presented. As seen in this table, an increase of thickness of liquid crystalline layer leads to the shift of temperatures of all phase transitions to low temperatures. I.e. the thickness of liquid crystalline layer, which placed between the reference surfaces of the sandwich-cell, has influence on the Cr\\SmC, SmC–SmA, SmA\\N, N\\I phase transition temperatures. Namely, differences in phase transition temperatures for the thin (20 μm) and thick (240 μm) samples are as 1.3 K (Cr\\SmC), 1.9 K (SmC–SmA), 3.9 K (SmA\\N) and 5.5 K (N\\I) in LC1 and as 1.5 K (Cr\\SmC), 2.0 K (SmC–SmA), 3.6 K (SmA\\N) and 5.4 K (N\\I) in LC2. Thus, change of thickness of liquid crystalline layer, placed between the reference surfaces of the sandwich-cell, has sufficient effect on the thermomorphologic and thermotropic properties of liquid crystalline materials. Namely, change of thickness of the layer leads to the appearance of non-specific textures and provides the shift of the phase transition temperatures to low temperatures. We would like to emphasize here that dependence of the thermotropic and thermo-morphologic properties on the thickness of liquid crystalline layer for mesophases with
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Fig. 6. Textures of N mesophase for different thicknesses of liquid crystalline layers. a — 20 μm (LC1); b — 20 μm (LC2); c — 120 μm (LC1); d — 120 μm (LC2); e — 240 μm (LC1); f — 240 μm (LC2). Crossed polarizers; Magnification ×100.
different ordering is important peculiarity of liquid crystalline materials. We would like also to note that differences in the morphologic properties and texture defects of liquid crystals on the free surfaces and between two reference surfaces have been presented earlier [25, 26,49–51]. The influence of thickness of liquid crystalline layer on phase transition temperatures is obviously connected with the character of the interaction energy between reference surfaces of the sandwich cell and liquid crystalline molecules in samples with different thicknesses. Two cases of such interactions take place for liquid crystals, which are placed
between two reference surfaces of the sandwich-cells [7,23,50–52]. These interactions are the weak anchoring and the strong anchoring. In the case of the weak anchoring, the energy of interaction between liquid crystalline molecules (WLC) is bigger than that for interaction between liquid crystalline molecules and reference surfaces (WLCS). In this case the WLC N WLCS inequality takes place. In the case of the strong anchoring, the energy of interaction between liquid crystalline molecules is compared with the energy of interaction between liquid crystalline molecules and the reference surfaces. In this case the WLC ≈ WLCS connection takes place.
Table 1 Temperatures of phase transitions in LC1 and LC2 for various thicknesses of liquid crystalline layer. Liquid Crystal
LC1
LC2 Fig. 7. Schematic representation of molecular ordering of liquid crystal in samples with different thicknesses.
Thickness of Liquid Crystalline Layer, μm 20 ± 0.1 70 ± 0.1 120 ± 0.1 170 ± 0.1 240 ± 0.1 20 ± 0.1 70 ± 0.1 120 ± 0.1 170 ± 0.1 240 ± 0.1
Temperatures of Phase Transitions, K Cr–SmC
SmC–SmA
SmA–N
335.5 335.3 335.0 334.7 334.2 333.1 332.7 332.3 332.0 331.6
350.4 350.0 349.6 349.1 348.5 335.6 335.1 334.6 334.2 333.6
356.1 355.2 354.3 353.1 352.2 343.1 342.3 341.5 340.6 339.5
N–I 362.5 361.5 359.7 358.9 357.0 359.5 358.3 357.0 355.9 354.1
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Fig. 8. Temperatures of phase transitions vs. the thickness of liquid crystalline layer in LC1. a — Cr → SmC; b — SmC → SmA; c — SmA → N; d — N → I.
The cases of the weak and strong anchoring can be described by the general correlation for the surface effects as [7,23,50–52]: bL
W LC : W LCS
ð2Þ
Here b is the extrapolation length (the extrapolation length characterizes value of the interaction energy between liquid crystal and surface) and L is the dimension of liquid crystalline molecule. For the weak anchoring the bN L takes place, i.e. the extrapolation length is bigger than dimension of liquid crystalline molecule. For the strong anchoring the b ~ L takes place, i.e. the extrapolation length is comparable to the dimension of liquid crystalline molecule. Thus, the case of smaller values of the extrapolation length (i.e. in thin flat capillaries) takes place sufficient influence of the reference surfaces on the morphologic, alignment and thermotropic properties of liquid crystals. The energy
of interaction between liquid crystalline molecules depends not only on alignment of the director of liquid crystal, but also on distribution of the director in volume of the sample. Just therefore, uniform alignment of liquid crystalline textures has been obtained in thin liquid crystalline layers [26,51–53]. In Figs. 8 and 9, temperatures of phase transitions as a function of the thickness of liquid crystalline layer in LC1 and LC2 are presented. As seen in these figures, the Cr\\SmC and SmC–SmA phase transition temperatures change weakly with an increase of the liquid crystalline layer as compared with the SmA–N and N\\I phase transition temperatures. This effect is connected with the fact that the Cr\\SmC and SmC–SmA phase transitions realize between more ordered phase states than the SmA–N and N\\I phase transitions. Namely, SmC and SmA mesophases have orientational, positional and translational order, and characterize by the D∞ and C2h symmetry groups, accordingly; but N mesophase has only long translational order and characterizes by the D∞h symmetry group. Besides, differences in character temperature dependences vs. thickness of liquid crystalline layer for the Cr\\SmC and SmC–SmA phase transitions on the one hand and for the SmA–N and N\\I phase transitions on the other hand are obviously connected with differences in the interaction energy between liquid crystalline molecules and surfaces for SmC and SmA mesophases, and for N mesophase. Thus, differences in thickness of liquid crystalline layer lead to differences in distribution of liquid crystalline molecules near the surfaces and in volume of the sample. This difference depends on thickness of the flat capillary. I.e. if the thickness of liquid crystalline layer thickness is bigger, the difference in distribution of liquid crystalline molecules is bigger. Boundary layers of liquid crystals near the reference surfaces of the flat capillary determine the morphologic, alignment and thermotropic properties of liquid crystalline mesophase [49,50,52,53]. By elaboration of the reference surfaces of the thin samples, definite uniform alignment of liquid crystal has been obtained by us. In such samples without any elaboration of the reference surfaces, specific textures, which characterize liquid crystalline mesophase, can be obtained. But in the thick samples with elaborated or not elaborated reference surfaces, uniform aligned or specific textures have not been obtained by us. Thus, change of the thickness of the liquid crystalline layer, placed between two reference surfaces of the sandwich-cell, leads to change of the energy of interaction between liquid crystalline molecules and the reference surfaces, and accordingly leads to the change of the thermal energy, which is necessary for carrying out of the phase transitions between liquid crystalline mesophases in LC1 and LC2. I.e., an increase in thickness of liquid crystalline layer between two reference surfaces leads to a decrease of phase transition temperatures. We would like to note that as is known, an increase in thickness of liquid crystalline layer leads also to an decrease of threshold tension of the fields in the electro-optical and magneto-optical transitions for the aligned liquid crystalline textures, in particular by the Frederiks effect [7,23,42,54–56]. 4. Summary The results obtained in this work can be summarized as follows:
Fig. 9. Temperatures of phase transitions vs. the thickness of liquid crystalline layer in LC2. a — Cr → SmC; b — SmC → SmA; c — SmA → N; d — N → I.
– An increase in thickness of liquid crystalline layer, placed between two reference surfaces of the sandwich-cell, leads to a decrease of the Cr\\SmC, SmC–SmA, SmA–N and N\\I phase transition temperatures in LC1 and LC2. – An increase of thicknesses of liquid crystalline layer, placed between two reference surfaces of the sandwich-cells, leads to a transformation specific textures to non-specific textures and appearance various types of defective formations. – The influence of thickness of liquid crystalline layer on the thermomorphologic and thermotropic properties is connected with change of the molecule–molecule and molecule–surface interaction energy, and with change of the extrapolation length. Correction between the molecule–molecule and molecule–surface interactions characterizes
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the energy of coupling between liquid crystalline molecules and reference surfaces. This energy depends on the thickness of liquid crystalline layer, placed between two reference surfaces of the sandwichcell and has influence on temperatures of the phase transitions.
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Acknowledgment
[31]
This work has been partially supported by the Research Foundation of Mugla Sitki Kocman University, Grant No. 15/124.
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