W. Kuczynskf a d, J. Pavel b & H. T. Nguyen c a Institute of Molecular Physics, Polish Academy

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1 This article was downloaded by: [Biblioteka Pan W Warszawie] On: 21 February 2012, At: 01:50 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Phase Transitions: A Multinational Journal Publication details, including instructions for authors and subscription information: Observation of edge dislocations in helical smectic liquid crystals W. Kuczynskf a d, J. Pavel b & H. T. Nguyen c a Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, PL , Poznan, Poland b Laboratoire de Physique de la Matière Condensée, Université de Picardie, 33 rue Saint- Leu, 80839, Amiens, Cedex, France c Centre de Recherche Paul Pascal, Avenue A. Schweitzer, 33600, Pessac, Cedex, France d Available online: 19 Aug 2006 To cite this article: W. Kuczynskf, J. Pavel & H. T. Nguyen (1999): Observation of edge dislocations in helical smectic liquid crystals, Phase Transitions: A Multinational Journal, 68:4, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution,

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3 Phu\e Trunsrlion.r, Vol. 68, pp Reprints avdilahle directly from ihc publisher Photocopying pcrmitted by license only.l) 199') OPA (Overseas Publishers Associauon) N.V. Published by license under ihc Gordon and Breach Science Publishers imprint. Printed in Malaysia. OBSERVATION OF EDGE DISLOCATIONS IN HELICAL SMECTIC LIQUID CRYSTALS W. KUCZYNSKI a,*, J. PAVEL and H.T. NGUYEN ahstitute of Molecular Physics, Polish Academy oj Sciences, Smoluchowskiego 17, PL Poznan, Poland; Laboratoire de Physique de la Matiire Condensbe, Universite' de Picardie, 33 rue Saint-Leu, Amiens Cedex, France; 'Centre de Recherche Paul Pascal, Avenue A. Schweitzer, Pessac Cedex, France (Received 3 March 199s) We report the observation of elementary edge dislocations in smectic liquid crystals possessing helical structure. The dislocations were observed in the entire temperature range of helical phases, including ferroelectric, ferrielectric and antiferroelectric phases. The mechanism for visualizing the didocations is based on the phenomenon of selective reflection of circularly polarised light. The performed observations of dislocations deliver not only information on the mechanisms of defect creation in various chiral smectic phases, but also on the structure and properties of the investigated smectics, often inaccessible using standard methods. Keywords; Liquid crystals; Ferroelectric; Antiferroelectric; Dislocations; Smectic layers 1. INTRODUCTION Smectic liquid crystals are characterised by a layer structure. As smectics possess at least one-dimensional positional order, they exhibit screw and edge elementary dislocations. The screw dislocations have been observed in so-called twist grain boundary phases (Goodby et al., 1989). Edge dislocations always exist in smectic liquid crystals but as a rule, they are not visible because any contrast mechanism is lacking. The first direct observation of elementary edge dislocations was performed by * Corresponding author. wkucz@ifmpan.poznan.pl 643

4 644 W. KUCZYNSKI e1 a/. Meyer, Stebler and Lagerwall (MSL) (1978) at the second-order smectic C-smectic A phase transition. In their experiment, the visualisation of dislocations was achieved using a mechanically-induced change of phase transition temperature in a sample of variable thickness. The contrast mechanism was based on the dependence of the phase transition temperature SmA/SmC on mechanical strain. In any dislocation region, where the number of smectic layers increases by 1 due to the increase of sample thickness a mechanical stress must occur. On this side of the dislocations, where the number of layers is lower, a dilatation takes place which will stabilise the smectic A phase with molecules perpendicular to layers, whereas on the other side of the dislocation a decrease of layer thickness must appear to allow the next layer to be introduced. The lower thickness of layers forces molecules to be tilted with respect to the layer normal and in this way stabilises the smectic C phase, In other words, on this side of the dislocation, where the number of smectic layers is less, the transition temperature TcA is lowered and on the other side of dislocation it is enhanced. MSL showed that the shift in the transition temperature is proportional to the strain, which in turn is proportional to (A@)* (where A@ is the tilt angle change in the region of the dislocation). This effect causes, if the temperature is properly chosen, an alternating appearance of C and A phases in a wedge-shaped sample. During observation along the smectic layer normal (homeotropic texture) the stripes of the C phase become visible due to the (small) birefringence as a result of tilting of the local optic axis. This mechanism creates a contrast between C and A phases only close to the C-A transition temperature. Far from this transition the relative change of layer thickness, 1 /N (Nis the number of smectic layers along the sample thickness), is too small to induce the phase transition. The temperature range of visualisation of the dislocations in the MSL method is limited to few tenths of a degree. Apart from smectics A and C, elementary edge dislocations have never been observed in smectic liquid crystals. In this paper we report the observation of edge dislocations in many smectic phases, including ferroelectric, ferrielectric and antiferroelectric. The observations were performed not only at the phase transition, but in the entire temperature range of these phases. All the phases, except for the A phase, possess a helical structure which is of crucial importance in our method. The mechanism of viewing dislocation lines, which is different from that in

5 EDGE DISLOCATIONS IN SMECTICS 645 the MSL experiment, was based on optical properties of helical structures. The observations of dislocations revealed not only information on the mechanisms of defect creation in various chiral smectic phases, but also on the structure and properties of the investigated smectics, often inaccessible using standard methods. As the method of observation is very simple and provides a lot of information it can be widely used in studies of chiral smectic phases. It does not need any sophisticated and expensive equipment and therefore can be also used in teaching. 2. EXPERIMENT 2.1. Materials We used in our experiments many chiral materials, each possessing a rich set of smectic phases. We paid special attention to materials having antiferroelectric and ferrielectric phases. Among them was the standard antiferroelectric material MHPOBC (Fukuda et al., 1994) with the following scheme of phase transitions: Cr65Ii68Ci 120Cz122C* 123.2C: 125SA151Iso. In the scheme subscript A denotes antiferroelectric order of tilt direction in smectic layers and superscript (*) a helicoidal structure. Letters A and C refer to smectic phases, Cr denotes crystalline phase and Is0 the isotropic phase. The C5 phase displays ferrielectric properties. Most of our investigations have been done for a tolane derivative abbreviated here C8-tolane (Gisse et al., 1993), which has the following chemical formula: Cg H 170-a-C E CH, ) CsH13 where Qr stands for the phenyl ring. C8-tolane has the following phase sequence (transition temperatures are given in T): Cr CA 95 Cry, 96 Cry2 97 C* 104 C: A 135 ISO.

6 646 W. KUCZYNSKI et al. We used also some other well known chiral materials, e.g. DOBAlMPC, which shows phase transitions between smectic I*, C* and A phases Sample Preparation The measuring cells were prepared in the form of a wedge with very small angle (of order of 10p3-10-4rad) and variable thickness (from 0 to few pm). The cells were made of standard microscope glass plates. A small amount of the investigated substance was put on a clean plate, heated to the isotropic phase and covered with a second glass plate. Next, the sample was cooled to the A phase and the plates were pressed together in order to remove the excess material and to diminish the sample thickness. This procedure ensures good homeotropic alignment, necessary for observation of dislocations. The plates used for construction of the cells were not exactly plane so it was usually possible to find a place within the sample where both thickness and wedge angle were convenient for observations Observation Method The cell was placed in a microscope hot stage and investigated using a polarising microscope with small magnification (about 1OOx) between crossed polarizers. Because of small sample thickness and its pseudohomeotropic orientation the intensity of light passing the sample is extremely low and dislocations, if exist, are hardly visible. Much better visibility was achieved in the reflection mode of the microscope, especially for C8-tolane. For this reason most of the experiments described here has been performed for C8-tolane in reflection mode. 3. RESULTS AND DISCUSSION Figures 1, 3 and 5 show dislocations observed in various phases of C8- tolane (ferroelectric C*, ferrielectric C;, and C;2, antiferroelectric Ck and 11). As can be seen, the appearance of dislocations in the various phases is different. The visibility of the lines is quite bad in ferrielectric phases (Fig. l(b) and (c)) and very good in antiferroelectric phases (Figs. 3 and 5). The dislocations were not observable in the C: phase.

7 EDGE DISLOCATIONS IN SMECTICS FIGURE l(a) and (b)

8 648 W. KUCZYNSKI et a/. FIGURE I(c) FIGURE 1 Edge dislocations observed in thin wedge-shaped sample of C8-tolane: (a) in the helical smectic C* phase (T= 99.8"C), (b) at the transition ferroelectric smectic C'/ferrielectric smectic C' (T= 96.8"C), (c) at the transition ferrielectric C5> phase- antiferroelectric smectic Ccbhase (94.7"C). (See Color Plate I.) It is worth noticing that at the clearing point we observed also a system of lines, which was, however, of a completely different nature. The lines had nearly the same periodicity and the same shape as the dislocation lines observed at lower temperatures. They were also visible in non-polarised light, both in reflection and in transmission. 3.1, Mechanisms of Dislocations' Visualisation It is clear, that the mechanisms for viewing dislocations are different in various smectic phases because of different structures and properties of these phases. This statement is confirmed by the observation that the appearance of the dislocation lines is different. However, all the investigated samples were chiral and possessed a helical structure in the tilted phases (if the helical pitch was short enough). Therefore, we think that

9 EDGE DISLOCATIONS IN SMECTICS 649 the most important reason for seeing them is common to all the investigated phases: the phenomenon of circular dichroism, closely related to the helical structure. If selective reflection occurs in the infrared region (as in MHPOBC), the intensity of the reflected light is low, the contrast is nearly the same as in transmission and the visibility of the dislocations is still poor. However, if selective reflection occurs in the visible region, as in C8-tolane, then the intensity of the reflected light is high and the visibility of dislocations is good. This is confirmed by the following observations: (1) The reflected light is elliptically polarised. (2) We did not observe the dislocations in reflection in non-chiral materials (smectics C and 0), which do not show selective reflection. (3) Light reflected from Ci and C" phases of MHPOBC is quite weak (because the selective reflection line lies in the infrared region). (4) The lines are not observable in the Ci phase, probably because of a very short helix period (Cluzeau et al., 1995) which gives very weak reflection in the visible region of the spectrum Classification of Lines Smectic C* Phase As can be seen in Fig. l(a), in the ferroelectric C* phase the observed lines (especially close to the C, phase) resemble those observed by MSL at the transition C/A. The most striking differences are that in our case they are coloured and exist in the whole temperature range of the C* phase. Close to the C, phase some discontinuity is visible, probably for the same reasons as in the MSL lines. In the whole temperature range of the C* phase a modulation of the colour of the reflected light (from blue to green) is distinct. With increase of temperature the colours shift towards violet (see Fig. 2). At a certain temperature the selective reflection vanishes and the microscopic field of view becomes black. This corresponds to when the pitch of the helix is so short that the reflection takes place in the ultraviolet region of the spectrum. During further increase of temperature suddenly red stripes appear, similar to those observed in the low-temperature region of the C* phase and located at the same places. However, the intensity of the reflected light is much weaker now. All these observations

10 650 W. KUCZYNSKI et al. 0,7 5. E\ 5. 50!6 JZ CO,5.- yq.' 0,4 x m 2 ' 0,3 Q X Colour of reflection 0,8 0 Selective reflection 1. Helical pitch T /oc FIGURE 2 Scheme of the selective reflection wavelength XR dependence on temperature in the helical smectic C* phase. The helical pitch p was estimated using the form p = XR/(~ n), where n is refractive index (n x 1.5) and m the order of interference (indicated in the figure). suggest that now a second-order selective reflection occurs. Rising the temperature further, a hypsochromic shift of selective reflection is observed again and after a small fraction of a kelvin the reflection disappears. The observed behaviour of the selective reflection proved that the helical pitch decreases quickly with rising temperature. This is confirmed by the following observation: assuming that the weak reflection at higher temperatures is caused by second-order interference, then the calculated pitch values p lie on the same curve of the p(t) dependence as the values calculated for first-order reflection at lower temperatures and agree with results of measurements performed by Cluzeau et al. (1995) Fevvietectric Phases The system of dislocation lines is much less visible in ferrielectric phases C1;l and Cry2 (see Fig. l(b)). In some places their location is the same as it was in the C' phase. However, in other places they become invisible and

11 EDGE DISLOCATIONS IN SMECTICS 65 1 are replaced by some other lines, which are not always perpendicular to the direction of the thickness gradient. The lines are visible on a grey background which proves lack of selective reflection in the visible region. Similar textures are now observable in transmission also. The grey background with a large number of defects resembling the schlieren texture of non-chiral smectics C seems to indicate a random distribution of tilt directions in the smectic layers The Antifervoelectric Ci Phase The dislocations are very visible in the entire temperature range of the Ca phase (see Figs. l(c) and 3). The distance of the dislocations is nearly the same as in the C* phase and supposedly corresponds to the single distance of the smectic layers. Direct proof of this assumption is delivered by the measurement of the layer thickness. For this measurement we used wedge-shaped cells of known wedge angle. The angle was measured in a diffraction experiment using a He-Ne laser. Figure 4 presents typical results obtained with a cell of wedge angle 0.15 mrad filled with C8-tolane. The smectic layer periodicity, calculated from the slope of the straight line amounts to 34& in good agreement with FIGURE 3 Edge dislocations in the antiferroelectric smectic C:, phase, few degrees below the transition C;,-Ci (89.6 C). (See Color Plate 11.)

12 652 W. KUCZYNSKI et al \ c rn 0 Q CI m rn i Dislocation number FIGURE 4 Typical plot of dislocation position as function of its successive number. Wedge angle is 0.15 mrad, average distance of successive dislocation lines is 22.8 pm. results obtained for similar molecules from X-ray experiments (Fukuda et al., 1994). A few lines of different appearance can be distinguished in the Ci-phase. Certain kinds appear with a probability dependent on temperature. Two main kinds of stripes can be easily specified: (a) The stripes are single, orange coloured (due to selective reflection) and separated by black lines. These lines are broad at the transition to the C;* phase but sharp at a few degrees lower temperatures (Fig. l(c)). This type of lines exists mostly in the upper temperature range of the antiferroelectric Ca phase. (b) The stripes, separated by sharp black lines appear in pairs; each stripe in a pair is differently coloured (Fig. 3). This type of stripes exists mostly at lower temperatures (several degrees below the transition from ferrielectric phase). The different colour of selectively reflected light in neighbouring stripes proves that the helical pitch in them is different, but in every second layer it is the same. The system of double stripes must be related to the double-layer antiferroelectric order of tilt directions.

13 Antiferroelectric 1; Phase EDGE DISLOCATIONS IN SMECTICS 653 The system of double stripes does not change position at the phase transition CA-IA, but changes colours (Fig. 5). In C8-tolane the orange-greenish coloured double stripes adopt two grey scale steps. On the other hand, in MHPOBC, the stripes, which were hardly visible in the Ci phase, show striking colours in the I: phase. In both cases the reason is probably the same - the change in the value of the pitch of the helix. The structure of the dislocations, their environment and the visualisation mechanism are probably the same in both phases Transition Region Smectic A- Isotropic Phase At the transition SmA-isotropic phase a system of different lines was observed in place of the described dislocations. These lines, however, are completely different from those of the dislocation lines described above. They are hardly visible, but have nearly the same positions and the same distances as the dislocations observed in the helical phases. They could be observed in both chiral and non-chiral materials, in polarised or nonpolarised light, in transmission or in reflection. However, we never observed them in materials possessing a nematic-isotropic or cholesteric-isotropic phase transition. We suppose that the visibility of these lines is connected with phase equilibrium at the clearing point. FIGURE 5 Edge dislocations in the antiferroelectric smectic Ii phase (63.8"C).

14 654 W. KUCZYNSKI et al. There are at least two hypotheses possible which can explain the appearance of these lines, both assuming spatial inhomogeneity of the clearing temperature: (i) impurities gather in the dislocation regions (in the same way as defects gather in the vicinity of dislocations in solid crystals (Kuczynski, 1975)), - and the clearing temperature is lower there, (ii) dislocations introduce disorder into the smectic A structure and therefore the clearing temperature is lowered in their surroundings (similarly the transition temperature for smectic A-smectic C is changed in the MSL experiment) Structure of Defects As already mentioned, the dislocation lines become visible in our experiments due to the phenomenon of circular dichroism. This means, there must exist a coupling between the helical structure and the dislocation structure. Such a coupling seems to be hardly probable, as it concerns two incommensurate periodicities which differ very strongly, by two orders of magnitude (the periods are about 3 nm for the smectic layers and about 300 nm for the helical pitch). The above-mentioned coupling is, however, an experimental fact and the only question is, what is the mechanism of that coupling. Already Lagerwall and Stebler (1980) pointed out that the tilt direction of molecules in smectic layers is not random at the dislocations - molecules are tilted in the plane defined by the dislocation line and the normal to the layers. They argued that such a tilting direction ensures constant density around the dislocation. Thus, the azimuthal angle of the tilt direction, which rotates in chiral smectics creating the helical structure, is fixed at dislocations (located probably in the middle of the sample). If this direction is also fixed at the sample walls due to the surface interactions, then the effective helical pitch is determined by the local sample thickness. Depending on the phase structure and the kind of surface interactions, the helical pitch adopts a value, that allows the rotation of the c-director by a multiple of 7r or 27r on the sample thickness. Detailed models of the structure of various smectic phases fulfilling such conditions will be published elsewhere (Kuczynski et al., 1998).

15 EDGE DISLOCATIONS IN SMECTICS 655 CONCLUSIONS The method of investigating dislocations described in this paper is based on the optical properties of helical structures. Hence, it can be used for the investigation of the structure and defects in many helical smectic phases, e.g. ferroelectric, ferrielectric and antiferroelectric. The observations can be performed not only at the phase transitions, but in the entire temperature range of the phases. The applied method of observation is very simple and provides a lot of information on the mechanisms of defect creation in various chiral smectic phases and on the structure and properties of investigated smectics, often inaccessible using standard methods. Therefore, it can be widely used in studies of helical smectic phases. It can be also useful for detection of some phase transitions, hardly observable in standard preparations, e.g. transitions between C* and ferrielectric phases (see Fig. l(b) and (c)). Because the method does not need any sophisticated and expensive equipment it can be also used in teaching. References Cluzeau, P., H.T. Nguyen, C. Destrade, N. Isaert, P. Barois and A. Babeau (1995). New chiral tolane series with antiferroelectric properties. Mol. Cryst. Liq. Cryst., 260, 69. Fukuda, A., Y. Takanishi, T. Isozaki, K. Ishikawa and H. Takezoe (1994). Antiferroelectric chiral smectic liquid crystals. J. Muter. Chem., 4(7), 997. Gisse, P., J. Pavel, H.T. Nguyen and V.L. Lorman (1993). Dielectric permittivity of antiferroelectric liquid crystals. Ferroelectrics, 147, 27. Goodby, J.W., M.A. Waugh, S.M. Stein, E. Chin, R. Pindak and J.S. Patel (1989). Characterization of a new helical smectic liquid crystal. Nature, 337, Kuczynski, W. (1975). Effect of the defects on the nonlinear optical properties of ADPcrystal. Acta Phys. Polon. A, 48(3), 409. Kuczynski, W., J. Pavel, M. Boschmans, P. Gisse, B. Crepy, V.L. Lorman and H.T. Nguyen (1998). To be published. Lagerwall, S.T., R.B. Meyer and B. Stebler (1978). Direct observation of dislocations in smectics. Ann. Phys., 3, 249. Lagerwall, S.T. and B. Stebler (1980). Optical techniques for the analysis of defects in smectic liquid crystals. Proc. XXXVSession Physics of Defects, Les Houches, p Meyer, R.B., B. Stebler and S.T. Lagerwall (1978). Observation of edge dislocations in smectic liquid crystals. Phys. Rev. Lett., 41(20), 1393.

16 Color Plate 1 (See Fig. 1. Observation of Edge Dislocations in Helical Smectic Liquid Crystals by W. Kuczynski et al., pp. 647&648) FIGURE 1 Edge dislocations observed in thin wedge-shaped sample of C8-tolane: (a) in the helical smectic C phase (T= 99.8 C), (b) at the transition ferroelectric smectic C*/ferrielectric smectic C* (T= 96.8 C), (c) at the transition ferrielectric C;? phase-antiferroelectric sinectic Cf Lhase (94.7 C). Color Plate I1 (See Fig. 3. Observation of Edge Dislocations in Helical Smectic Liquid Crystals by W. Kuczynski et a/., p. 651) FIGURE 3 Edge dislocations in the antiferroelectric smectic Ca phase, few degrees below the transition C:,-Ci (89.6 C).

17 Figure l(a) and (b) continued