Liquid Crystals, 1997, Vol. 23, No. 2, 241±246 Memory eect in ®lled nematic liquid crystals by A. GLUSHCHENKO*², H, KRESSE, V. RESHETNYAK§¶, YU. REZNIKOV² and O. YAROSHCHUK² ²Institute of Physics, National Acad. Sci., prospect Nauki 46, Kyiv 252650, Ukraine Martin-Luther-Universitu Èt, Halle-Wittenberg, Institut fur Physikalische Chemie, Mu È hlpforte 1, D-06108 Halle, Germany § Institute of Surface Chemistry, National Acad. Sci., prospect Nauki 31, Kyiv 252650, Ukraine ¶Faculty of Mathematical Studies, University of Southampton, Southampton SO17 1BJ, U.K. ( Received 19 November 1996; in ®nal form 14 March 1997; accepted 15 March 1997 ) The irreversible electro-optical response (memory eect) of `liquid crystal±aerosil’ suspensions under the in¯uence of an electric ®eld is investigated. Thereby, the in¯uence of the aerosil concentration on the eciency of the memory eect is studied. The experimental results are discussed in the frame of a model where the ®lled nematic is considered as a system of aerosil agglomerates in a liquid crystal matrix stabilized by the forces of liquid crystal orientational elasticity. 1. Introduction Liquid crystals 5CB and ZhKM-1285 contain molecules The physics of heterogeneous liquid crystals (LCs) is with cyano groups, which can form hydrogen bonds. of interest for both applied and fundamental physics Such a special interaction is not possible for ZLI-4801. [1]. One version of such systems is provided by ®lled In order to prove the importance of hydrogen bonds on nematic LCs consisting of suspensions of aerosil particles the memory eect, 0´5 weight per cent of a proton accept, in a LC matrix. Thereby the solid particles generate di- n- nonyl 4-(4-cyanobenzoyloxy)benzylidenemalonate defects in the liquid crystalline phase. As a result, such (HA) was added to ZLI-4801. a medium scatters light intensively in the initial state. Aerosils A300 and R812 (pyrogenic silica of By applying an electric ®eld, the system clears due to DEGUSSA, Germany) were used as solid components both reorientation of the LC molecules and transforma- [8]. The surface of aerosil A300 contains about 5 OH tions of defects. The system can remain in the cleared groups nmÕ 2 , whereas for aerosil R812 which is modi®ed state (memory eect) [2±4] or relax back very quickly with hydrophobic polymethylsiloxane fragments, a den- ( t < 1 ms) [5±6] to the initial state on removing the sity of only 0´29 OH groups nmÕ 2 was measured [9]. ®eld. As noted before [5, 7], the characteristics of the Suspensions with various aerosil concentrations c = memory eect depend upon the chemical nature of both 2 ± 25 wt% ( c = m a /( m a + m m ), where m a and m m are the the aerosil surface and the LC molecules. The aim of weight of the aerosil and the mixture, respectively) were this work was to study the in¯uence of chemical modi- prepared by mixing the components in an ultrasonic ®cation of the aerosil surface, of aerosil concentration mixer. Films of the suspensions of 10 mm thickness placed and of electric ®eld parameters on the memory eect. between two glass substrates equipped with transparent ITO electrodes, were investigated. Observations of the 2. Experimental suspensions by polarized light microscopy showed the 2.1. Samples and measurements chaotically disordered texture of the LC containing Samples of 4-pentyl-4 ¾-cyanobiphenyl (5CB) aerosil agglomerates with sizes up to several microns. (NIOPIK, Russia) with a dielectric anisotropy D e = 7, The experimental set-up for our investigation has been and of nematic liquid crystalline mixtures ZhKM-1285 described in previous papers [5±7]. The intensity of the (NIOPIK, Russia) with D e = 11´8 and ZLI-4801 (Merk, HeNe-laser light beam transmitted through the sample, Germany) with D e = 5´1 were used in the experiments. I out , is measured within a cone of 2ß. From these data, the sample transmittance, T = I out /I in , is calculated ( I in is the intensity of the incoming beam). Based on these *Author for correspondence. 0267±8292/97 $12´00 Ñ 1997 Taylor & Francis Ltd.