Kachurak Y. DEVELOPMENT OF LIQUID CRYSTAL SENSITIVE ELEMENTS OF OPTICAL SENSORS FOR DETECTING THE CONCENTRATION OF ACETONE AND ALCOHOL VAPORS

The thesis is devoted to the development of liquid crystal sensitive elements of optical sensors based on nematic LC E7 and cholesteric LC CB15 as a detector of acetone and alcohols, to the study of cholesteric-nematic liquid crystal mixtures for interaction with vapors of harmful organic substances, and manifestations of the blue phase during such interaction. The first chapter is devoted to the study of the latest trends in the field of optical sensors development where liquid crystals are the active medium. Research on the topic of the blue phase (BP) of liquid crystals, methods of its stabilization, and the use of this phenomenon in optical sensors is also considered. Modern approaches to the development of liquid crystal optical sensors using the blue phase effect are considered. In the second chapter of the work, synthesized liquid crystal compounds based on nematic liquid crystal E7 and cholesteric impurity CB15 were investigated. These mixtures demonstrate interaction with vapors of acetone and alcohols, and as a result of such interaction, a change in their own characteristics. The main selection criteria are the temperature and spectral characteristics of the mixtures. Nematic E7 is a mixture of four structurally similar cyanobiphenyls (terphenyls). Its components are 4-cyano-4ʹ -n-pentyl-biphenyl (5CB), 4-cyano-4ʹ-nheptylbiphenyl (7CB), 4-cyano-4ʹ-n-octyloxy-biphenyl (8OCB), and 4-cyano-4ʹʹ -n-pentyl-p-terphenyl (5CT). In this nematic mixture, there is no specific interaction between its components, so we consider it as an almost homogeneous nematic matrix. In addition, E7 is characterized by high refraction (Δn) = 0.2 (for the visible region of the spectrum) and has a wide interval of the nematic phase, - 48oС. The sequence of phase transitions for nematic E7, – Cr (crystal) − 20◦C N (nematic) 70◦C Iso (isotropic liquid). Phase sequence for the optically active cholesteric impurity CB15 - Cr (crystal) - 4◦C N* (chiral nematic) 54◦C - Iso (isotropic liquid). Both components of the developed mixtures are characterized by their own absorption maximum in the ultraviolet range. In total, four mixtures with different weight concentrations of components were synthesized. Specifically, 56% E7 + 44% CB15, 62% E7 + 38% CB15, 80% E7 + 20% CB15, 50% E7 + 50% CB15. Among the synthesized mixtures, two mixtures, namely 62% E7 + 38% CB15 and 56% E7 + 44%, best meet all requirements for use as a sensitive element of an optical sensor. The temperatures of the beginning of the transition process for mixtures of 62% E7 + 38% CB15 are 27 oС, and 56% E7 + 44% - 24 oС. Spectral characteristics of mixtures are characterized by absorption maxima in the visible region of the spectrum. Absorption maxima for the mixture 56% E7 + 44% CB15 – 510-540 nm, for 62% E7 + 38% CB15 – 590-640 nm, and for the mixture 50% E7 + 50% CB15 – 430-480 nm. Possibilities for further modification of the mixture, for improving their parameters and the possibility of detecting other substances are indicated. The third chapter is devoted to the study of the interaction of developed liquid crystal sensitive elements with acetone ((CH₃)₂CO) and alcohols (CH₃OH, C2H5OH, CH3CH(OH)CH3). The interaction with acetone was studied at three concentrations, 60, 120, and 240 ppm. For a mixture with a concentration of the cholesteric impurity CB15 of 38% when interacting with acetone, the transition time to the isotropic state was 230 seconds, while we get a phase transition of the blue phase after 130 seconds from the beginning of the measurements. The duration of the existence of the blue phase in this case was 16.4 seconds. The rate of phase transition when interacting with 120 ppm acetone increases by 25%, and at 240 ppm by 60% relative to the first measurement. The existence of the blue phase during the phase transition decreases linearly, corresponding to an increase in the concentration of acetone. Alcohols show a weaker interaction with the sensitive element, the duration of phase transitions increases by 200-300% at similar concentrations of the substance in the sensor volume. The main mechanism of the effect of acetone and alcohols on the liquid crystal cell was investigated, which consists in reducing the threshold temperature of the transition to the isotropic state. Thus, when alcohols are introduced into the volume of the sensor, the temperatures of the phase transitions decrease by 2.5°C. In the fourth chapter, a version of the concept of an optical sensor based on the developed liquid crystal sensitive element is investigated and proposed. A variant of the spectral converter for use in a pair with a liquid crystal sensitive element was investigated. Based on the photo-receiving module TCS34903, a version of the optical sensor of acetone and alcohols is proposed.