Available online at www.sciencedirect.com Materials Chemistry and Physics 110 (2008) 140–144 Optical properties of some mixed nematic liquid crystals in electric field Suleyman Yilmaz ∗ Harran University, Department of Physics, Osmanbey Campus, 63400 Sanliurfa, Turkey Received 1 October 2007; received in revised form 4 January 2008; accepted 18 January 2008 Abstract In this study, optical transmittance of mixed nematic liquid crystals (MNLCs) was investigated in terms of temperature variations and rotational angle of polarizer through electrooptic methods under dc electric field. The optical transmittances of the mixed structures, which were prepared at certain proportions, were measured in their phase transition region as contained molecular anisotropy. In the results of measurements, while the optical transmittance is in low level in the nematic phase, the optical transmittance was observed in the highest level by the nematic-isotropic phase transition. It was also observed that the domain structure of the material was affected considerably by the applied electric field as the temperature changes and phase transition region of the mixed structures had broader range than by the pure crystalline structure and its phase transition temperature was changed by the molecular anisotropy. © 2008 Elsevier B.V. All rights reserved. Keywords: Nematic liquid crystals; Optical transmittance; Electrooptic effects; Phase transitions 1. Introduction As it is well known, nematic liquid crystals (NLCs) are com- monly used in optical processing systems and photonic devices. Optical properties and electrooptic effects provide the basis for liquid crystal display technology. Therefore, this treatise of the optical properties of NLCs has substantial significance in tech- nological applications. At the same time, NLCs are preferred in many different applications [1–4] due to their anisotropic struc- ture, which relates to molecular orientation and temperature variation. When dc electric field is applied on these materi- als, electrooptical effects come into existence with variation on the molecular orientations of the LCs. This dc electric field changes the anisotropy of the material and this situation leads to important effects on the processing of the optical materials. This study presents a complete examination of all known phenomena occurring in liquid crystals under the effect of electric fields. The investigation of optical properties of NLCs, which have single optical axis and stable structure, will benefit consider- ably to physics of phase transition in these materials [5]. By taking into account the mentioned properties of NLCs, the opti- ∗ Tel.: +90 414 3440020x1304; fax: +90 414 3440051. E-mail address: syilmaz@harran.edu.tr. cal and the electrooptical properties of pure and mixed NLCs are examined in the temperature range including phase transi- tions. 2. Experimental setup In this work, commercial NLCs 4 ′ -(octyloxy)-4-biphenylcarbonitrile (C 21 H 25 NO) and 4 ′ -isothiocyanatophenyl-4-pentylbicyclo[2.2.2.]octane-1- carboxylate (C 21 H 27 NO 2 S) were obtained from Sigma–Aldrich and were used as pure and mixed structures (Fig. 1). Firstly, these two materials were used at cell preparation as purely and secondly, proportionally mixed structures of these two materials were obtained by chemical procedures (Table 1). Morphologic texture and phase transition interval in all of the NLC samples were obtained by using Leica 180 DM LP polarization microscope with a ther- mal table and a CCD camera. The optical measurement system consists of Phywe 1 mW HeNe laser with 632.8 nm, SR540 optical chopper, New Focus large area UV photoreceiver, SR530 Lock in Amplifier, our own designed sample holder providing heating and electric field, and optical apparatus such as polarizer and analyzer. Experimental system is comprised of three units such as the heating unit, the electric field unit and the optical measurement unit (Fig. 2). The reg- ular increments of the temperature were provided by the temperature control unit to include all of the phases for the six samples. In progressing system, dc electric field was provided by the electric field unit to affect the molecu- lar orientation. For determining the molecular orientation and anisotropy, the angle of polarizer was changed in direction with the clockwise and the counter- clockwise. The data obtained by the experimental system were transferred to a computer via RS-232 serial interface and compiled with the LabVIEW 8 program. 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.01.027