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Abstract

The first part of the thesis deals with the impact of surface-functionalized nanoparticles (NP) can have on the stability of liquid crystal (LC) phases. Experimental data of NP concentration-driven phase transition between the isotropic and nematic phase in 5OO8 LCs were used to create a minimal mathematical model that would qualitatively describe the observed behavior. The Lebwohl-Lasher-type lattice model was used as a basis, with the NPs acting as a random field within the lattice, and using a constant and Gaussian angular distribution for the random fields. The results of the model are in good qualitative agreement with experimental observations. This work is followed by an experimental study of oleylamine surface-coated Au NPs-induced temperature stabilization of the twist grain boundary A (TGBA) phase in nFBTFO1M7 (n = 9, 10, 11) LCs. Calorimetric and polarized optical microscopy measurements were performed. A significant increase in the temperature range of the TGBA phase was observed only when the lower temperature phase was the smectic A (SmA) phase. In contrast, when the lower temperature phase was the chiral smectic C (SmC*) phase, the increase was negligible. Increasing the NP concentration in 10FBTFO1M7 where all three phases are present caused the SmA phase temperature range to decrease significantly, while for the TGBA and SmC* phases, it increased, shifting a system towards a TGBA-SmA-SmC* triple point. The important mechanisms for stabilizing the TGBA phase were found to be the adaptive defect core targeting (ADCT) mechanism and the mechanism related to the saddle-splay elasticity.
The second part of the thesis deals with caloric effects. Specifically, in solid materials, the impact of doping on the electrocaloric effect (ECE) in barium titanate (BT)-based ceramics was studied. In soft materials, the impact of crosslinker concentration, crosslinking temperature, applied stress during crosslinking, and swelling with LC mesogens on the elastocaloric effect (eCE) in liquid crystalline elastomers (LCE) was investigated.
Doping Ba0.8Ca0.2TiO3 with Zr caused an increase in the ECE, but at still relatively high temperatures (T = 385 K). In contrast, doping Ba0.9Sr0.1TiO3 with Sn caused a decrease in the ECE, but lowered the temperature at which the largest ECE is observed. In the case of the eCE in LCEs, an initial numerical study was performed and compared to the experimental results. It is demonstrated that lower crosslinker concentrations, larger crosslinking temperature, lower applied stresses during crosslinking, and some swelling of LCEs cause an increase in the eCE. A peculiar behavior in eCE response was observed in the case of LCE sample with a 6% mass concentration of crosslinkers which appears to be a consequence of the proximity of the line of critical points, i.e., the crossover effect between the first-order phase transition to supercritical evolution found in relaxor ferroelectrics.