Stabilization of cholesteric blue phases by nanoparticles and nematic caloric effects induced by external fieldsLavrič, Marta (Avtor) Kutnjak, Zdravko (Mentor) Liquid crystalsblue phasesnanoparticlescalorimetryliquid crystal elastomerselastocaloric effect.The doctoral dissertation is composed of two contently completed parts, with one common thing - liquid crystals. In both parts we are observing the order o liquid crystal molecules, leading to interesting and applicative properties. In the first part, the experimental results from various techniques are demonstrating the stabilization of cholesteric liquid crystal blue phases in liquid crystals doped with various functionalized nanoparticles. The second part presents some first direct experimental studies of the elastocaloric effect in side-chin and main-chain liquid crystal elastomers. In this part the nemato- and smecto-caloric effects, induced by external mechanical field, are presented. Specifically, in the first part the stabilization of different blue phases such as Blue Phase I (BPI), II (BPII) and III (BPIII) is studied by addition of the various types of nanoparticles (NPs) to the cholesteric liquid crystal (LC). Initial studies exploiting the spherical CdSe, CdSSe and gold nanoparticles demonstrate the stabilization of the disordered BPIII phase. The concentration-temperature phase diagram demonstrates widening of the BPIII with increasing concentration of spherical nanoparticles. It is found that different types of spherical nanoparticles with different core, size and functionalization always stabilize better the disordered BPIII phase as long the size of nanoparticles remains smaller or proportional to the disclination defect-line core size. In our research the platelet and rod nanoparticles were used to stabilize blue phase temperature range, since the theoretical prediction of better stabilization of blue phases with anisotropic nanoparticles were made and only spherical nanoparticles were used before. Polarized optical microscopy (POM) and high-resolution calorimetry experiments exploring the addition of platelet nanoparticles show a completely different picture than in the case of spherical nanoparticles. In contrast to spherical nanoparticles and quantum dots, which broaden the range of amorphous BPIII, the strongly anisotropic nanoparticles such as the platelet NPs and nanorods of different core, size and functionalization stabilize almost exclusively the cubic BPI. A theoretical model describing the BPs stabilization mechanisms is presented, based on the Landau-de Gennes phenomenological approach. In the case of spherical NPs, two mechanisms, the Defect Core Replacement (DCR) and the saddle-splay elasticity mechanism seem to play major role in stabilizing the disordered BPIII phase. In the case of anisotropic NPs, the third Adaptive Defect Core Targeting (ADCT) mechanism related to the energy penalty, due to disruption of the disclination core-surrounding order, represents an additional driving force of NPs into the cores of disclination lines. Similarly to what was found in the stabilization of the TGB$_A$ phase, the reduction of disclination defect-lines’ fluctuations caused by the formation of heavy anisotropic NPs’ clusters in their cores can also play an important role in stabilizing both BPI and BPIII. In the second part of the doctoral thesis the elastocaloric effect in liquid crystal elastomers is studied by a direct experimental technique. In recent years caloric effects, such as magnetocaloric effect, electrocaloric effect and mechanocaloric effect, attracted significant attention due to their applications in new, environmental friendly heat-management devices such as air conditioning devices, coolers and heat pumps. In the group of mechanocaloric effects belong barocaloric effect and elastocaloric effect (eCE). So far, the best elastocaloric response, exceeding a temperature change of 40 K, was achieved in shape memory alloy wires. However, the significant stress field required in those experiments, ~1 GPa, presents significant challenge for cooling applications. In this thesis, the elastocaloric effect in soft materials is explored. Such soft materials that need several order of magnitude lower stress field (closer to 1 MPa) are liquid crystal elastomers (LCEs). Direct measurements of the elastocaloric effect presented in this thesis demonstrate the existence of significant eCE in main-chain LCEs near the nematic transition. The simple Landau-de Gennes phenomenological model is presented, capturing most of the observed features of eCE. It is deduced that best eCE is achieved in LCEs engineered in such a way to sustain large thermomechanical response and to have large latent heat at the nematic phase transition, which greatly enhances the eCE.20182019-01-17 07:45:07Doktorsko delo/naloga106028VisID: 95612COBISS_ID: 3275620sl