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«ace Alignment, Anchoring Transitions, Optical Properties, and Topological Defects in the Thermotropic Nematic Phase of an OrganoSiloxane Tetrapodes ...»

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Surface Alignment, Anchoring Transitions, Optical Properties, and

Topological Defects in the Thermotropic Nematic Phase of an OrganoSiloxane Tetrapodes

Young-Ki Kim,a Bohdan Senyuk,†a Sung-Tae Shin,b Alexandra Kohlmeier,c Georg H. Mehl,c and

a

5 Oleg D. Lavrentovich*

a

Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent, OH 44242, USA; *E-mail: olavrent@kent.edu

b

Liquid Crystal Display Research and Development Center, Samsung Electronics Corporation, Korea c Department of Chemistry, The University of Hull, Cottigham Road, Hull HU6 7RX, United Kingdom † Present address: Department of Physics, University of Colorado, Boulder, Colorado 80309, USA We perform optical, surface anchoring, and textural studies of an organo-siloxane “tetrapode” material in the broad temperature range of the nematic phase. The optical, structural, and topological features are compatible with the uniaxial nematic order rather than with the biaxial nematic order, in the entire nematic temperature range -25˚C T 46˚C studied. For homeotropic alignment, the material experiences surface anchoring transition, but the director can be realigned into an optically uniaxial texture by applying a sufficiently strong electric field. The topological features of textures in cylindrical capillaries, in spherical droplets and around colloidal inclusions are consistent with the uniaxial character of the long-range nematic order. In particular, we observe isolated surface point defectboojums and bulk point defects-hedgehog that can exist only in the uniaxial nematic.

1. Introduction A uniaxial nematic (Nu) is a liquid crystal (LC) showing uniaxial anisotropy of physical properties with a single director n  n being ˆ ˆ the symmetry axis and the optic axis.1 Since the theoretical prediction in 1970,2 there is a growing interest to the biaxial nematic (Nb) phase with three directors n  n, m  m, and ˆ  ˆ. The Nb phase was first observed experimentally by Yu and Saupe3 in a ˆˆ ˆ ˆ l l lyotropic mixture potassium laurate-decanol-water system. Although it is generally assumed that the biaxial phase is well documented in this case, there are reports that the biaxial optical features are only transient and when it is left intact, the samples eventually relax into the uniaxial state.4, 5 A more recent example of Nb in a lyotropic LC has been presented in dispersions of board-like platelets.6 The presence of N b is documented for thermotropic polymer LCs by solid state NMR spectroscopy, see Ref. 7 and references therein.

The low-molecular weight thermotropic version of Nb is of a special interest, as it would allow one in principle to construct fastswitching displays and other electro-optic devices.8-10 Design and synthesis of appropriate molecules have proven to be difficult. Simple rod-like mesogens explored so far do not yield a spontaneous biaxial order, although some of them do show a field-induced biaxiality with very fast (nanoseconds) electro-optic response.11 Significant synthetic efforts thus extended to novel and more complex molecular architectures, such as cyclic mesogenic oligomer12 to find the evidence of Nb phase behaviour.13 Very promising candidates for potential spontaneous N b behaviour were discussed for the so-called bent-core molecules.14-22 However, re-examination of their properties lead to a conclusion that some of these materials are in fact uniaxial nematics and that their apparent biaxial appearance is caused by factors other than the true long-range biaxial orientational order.23-31 One of these mimicking factors is a surface anchoring transition (reorientation of a uniaxial director n at a bounding substrate) which results in optical biaxial appearance of the cell.25, 27, 28 The second ˆ mechanism is rooted in the standard protocol of determining the phase diagram by cooling and heating the samples; it turns out that the flow and bidirectional director tilt during the thermal expansion/contraction of the material also lead to biaxial optical features.30 Last but not least, formation of cybotactic smectic-C short-range clusters in a uniaxial nematic environment31-34 of bent-core materials is one of the most often discussed mechanisms of how the material can acquire an appearance of the biaxial nematic in X-ray and other characterization techniques.26, 35-39 The smectic clustering might, on the other hand, facilitate a formation of the electric and magnetic field-induced biaxiality.40-42 In this regard, it is of interest to note an extraordinary strong effect of the electric43 and magnetic44 fields on the phase diagram of the bent-core nematics, resulting in a shift of phase transitions by 4-12˚C.

In this work, we explore an organo-siloxane material (Fig. 1) with a molecular structure that resembles a tetrapode.45-52 Originally,45 the phase sequence for a material with this structure was determined as Nb (37oC) Nu (47oC)I, where I stands for the isotropic phase. Later on, Figueirinhas et al.46 presented the phase sequence of a mixture with a nematic deuterated probe as Tg(-30oC)Nb (0oC) Nu (47oC)I, here Tg is the glass transition temperature, based on extensive solid state NMR investigations, with the results being in line with those reported in Ref. 7. The difference in the transition temperatures, compared to the pure sample45 was attributed to the admixing of the probe, indicating that the stability of the N b phase is strongly affected by external stimuli. A combination of XRD studies and fast field cycling experiments suggest that a local C2h symmetry of the material in the nematic state, supporting the view that local clustering may be crucial for the explanation of the formation of a biaxial nematic phase.49 Polineli et al.52 present an additional evidence of a Nu  Nb transition of the tetrapode material shown in Fig.1 at 37o C, as originally reported;45 the authors find that though the transition is observed in conoscopic and optical tests, it is not easily identifiable in the elastic and electric measurements. Tallavaara et al.50 report that 129Xe NMR studies indicate a phase transition at around 16o C  18o C but it is not clear whether the low temperature nematic phase is uniaxial or biaxial. Finally, Cordoyiannis et al.48 state that high-resolution adiabatic scanning calorimetry do not show a discernible Nu  Nb transition.





In this work, we use electro-optical and optical microscopy (polarized light, polarization-sensitive fluorescence, conoscopy) techniques to explore the nature of the nematic phase of the tetrapode material shown in Fig.1. The samples represent (i) flat layers of thickness between 4 μm to 50 μm confined between two solid plates (ii) round capillaries of diameter 50 μm and 150 μm ; (iii) spherical or nearly spherical freely suspended droplets of diameter between 5 μm and 20 μm. The observed electro-optical, surface and topological features of these samples show that the material has a uniaxial order in the whole nematic temperature range studied, 25o C  T  46o C.

2. Material and Techniques

Figure 1 shows the chemical structure of the tetrapodic material. Four mesogens are connected to the siloxane core through four siloxane spacers. We confirmed the phase transition as Tg (27o C) N (46o C) I in the regime of cooling with the rate 0.1o C / min ; the transition temperatures agree well, within 1  3o C, with the previous studies,46, 49 if one does not discriminate between the two versions of the nematic (N) phase.

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The flat cells were assembled from parallel glass plates with transparent indium tin oxide (ITO) electrodes. For planar alignment, the ITO glass substrates were spin coated with polyimide PI2555 (HD Microsystems); the polymer was rubbed unidirectionally. For ˆ homeotropic alignment (the director n is perpendicular to bounding plates), the glass plates were treated with a weak solution of lecithin in hexane.

The cell thickness d was set by spacers mixed with UV-curable glue NOA 65 (Norland Products, INC.) that was also used for sealing the cells. The actual cell thickness was measured by a light interference method. The material was filled into the cells in the isotropic phase. To prevent a possible memory effect, we performed most of the experiments for the temperatures above 25o C, which is slightly higher than the glass transition temperature Tg  27o C. The temperature was controlled by a hot stage LTS350 with a controller TMS94 (both Linkam Instruments) with 0.01o C accuracy. A typical rate of temperature change was   0.1o C / min to minimize the effects caused by thermal expansion.30, 53 Cooling was assisted by a circulation of liquid nitrogen.

In order to use the electric field as the means of director reorientation, we measured the dielectric properties of tetrapode with a precision LCR meter 4284A (Hewlett Packard) in the cell of thickness d  10 μm. We measured the dielectric permittivity for the directions parallel ( ε ) and perpendicular ( ε  ) to the director n in homeotropic and planar cells, respectively, and thus determined the ˆ dielectric anisotropy ε = ε  ε . We did not notice any biaxiality of the dielectric tensor when measuring the permittivity in the planar cells (which would have resulted in two different values of ε  ). To some extent, this result correlates with the previous studies by Merkel et al.51 in which the biaxial dielectric anisotropy in the direction perpendicular to the main director was determined to be very weak, only about (-0.011). Such a small value of dielectric anisotropy does not provide a clear evidence of N b behaviour, as compared to other possible mechanisms, such as surface tilt, surface inhomogeneities or even surface anchoring transition that we observe in the tetrapode nematic. In the homeotropic cells, the director experiences a surface reorientation (an anchoring transition) when the temperature is lowered, as we discuss in a greater detail later. To reinforce the homeotropic alignment during the dielectric measurement of ε, the cells were kept in the magnetic field of 1.4 T, directed normally to the cell. It allowed us to keep n perpendicular to the glass ˆ plates in the range of temperatures 20o C  T  TNI (46o C), where TNI is the N-I transition temperature; the diamagnetic anisotropy of tetrapode material is positive. The dielectric anisotropy ε changes its sign depending on temperature and frequency, Fig. 2a. Figure 2b shows the crossover frequency f c that separates the regions of different sign of ε, as the function of temperature being in line with those reported earlier.52 The solid line is a tentative extrapolation of the dependency to lower temperatures. The condition ε 0 can be used to distinguish the surface anchoring transition of a uniaxial N u phase from a hypothetical appearance of two optic axes if the material was a N b phase.

3. Results and Discussion

3.1 Homeotropic Alignment and Its Temperature Dependence Optical tests of homeotropic cells offer a straightforward approach to determine whether the phase is N u or N b of orthorhombic

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Fig. 4 Transition from (a) dark uniaxial texture to (b-d) birefringent textures in the homeotropic cell (d = 4.5 μm) at (a) T = 45oC, (b) 40oC(= T*), (c) 35oC, and (d) 20oC; inset in (a) is a conoscopic image of the cell.

Fig. 5 (a-d) Retardation maps and (f) local retardation as a function of temperature in a homeotropic cell (d = 4.5 μm) at (a) T = 45oC, (b) 40oC(= T*), (c) 35oC, (d) 30oC, and (e) 20oC.

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polarized light propagation. The value of T * varies in a wide range 31o C  T*  44o C, depending on the cell thickness d ; T * decreases as d increases (Fig. 3). Below T *, the dark homeotropic texture (Fig. 4a) becomes birefringent (Fig. 4b-d) with brightness increasing when the temperature decreases.

Using LC-Polscope (Abrio Imaging System), we mapped the optical retardance across the cells (Fig. 5a-e) and also measured the change of retardation  (Fig. 5f) in a preselected location as the function of temperature for the homeotropic cell of thickness d  4.5 μm.

The transition between the textures is reversible; the birefringent texture becomes dark when the temperature increases above T *. The

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to the field, the frequency f should be below ~10 Hz at T  5o C, Fig. 2b. Note that for low frequencies, the measured dielectric anisotropy ε is influenced by finite electric conductivity; the condition ε 0 means that the combined action of the dielectric and ˆ electric current torques leads to the alignment of n along the field. Another complication is that a high electric field might cause a dielectric breakdown.57, 58 We indeed observed such a breakdown in our system for voltages higher than 200-250 V, which is close to or smaller than the voltage needed to realign the director into a homeotropic state when the temperature is very low, T  5o C. To avoid dielectric breakdown, we performed the electric field experiments only in the range 5o C T 46o C.

Fig. 6 Change of PM texture and conoscopic pattern (a, b, d, f, h) without and (c, e, g, i) with a vertical field in a homeotropic cell (d = 6.9 μm) at (a) T = 45oC, (b, c) 40oC(= T*), (d, e) 20oC, (f, g) 0oC, and (h, i) -5oC; scale bar is 100 μm. (j) Transmittance vs. voltage curves for (b-i).



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