Stabilization of cholesteric blue phases by nanoparticles and nematic caloric effects induced by external fields

Lavrič, Marta (author), Kutnjak, Zdravko (mentor)

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.

• Language: English
• Keywords: Liquid crystals, blue phases, nanoparticles, calorimetry, liquid crystal elastomers, elastocaloric effect.
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2018
• PID: 20.500.12556/RUL-106028
• COBISS.SI-ID: 3275620
• Publication date in RUL: 17.01.2019

Secondary language

• Language: Slovenian
• Title: Stabilizacija holesteričnih modrih faz z nanodelci in nematski kalorični pojavi pod vplivom zunanjih polj

Abstract

Doktorska disertacija je sestavljena iz vsebinsko dveh zaključenih celot, ki so jima skupni tekoči kristali. V obeh delih na različne načine opazujemo urejanje tekočekristalnih molekul, ki dajo v različnih primerih zanimive in aplikativne lastnosti. V prvem delu doktorske disertacije so predstavljeni rezultati meritev opravljenih z različnimi merilinimi tehnikami, ki prikazujejo stabilizacijo holesteričnih modrih faz z različnimi funkcionaliziranimi nanodelci. V drugem delu je predstavljeno nekaj neposrednih meritev elastokaloričnega efekta v glavno-verižnem tekočekristalnem elastomeru. Predstavljen je nematski kalorični pojav, ki je induciran z zunanjim mehanskim poljem. Če povzamem bolj natančno, je v prvem delu predstavljena raziskava stabilizacije različnih holesteričnih modrih faz, kot so modra faza I (BPI- ang. blue phase I), modra faza II (BPII) in modra faza III (BPIII), z dodajanjem različnih nanodelcev v holesterični tekoči kristal (TK). V prvih študijah se je raziskovalo predvsem stabilizacijo modrih faz z mešanicami kiralnih tekočih kristalov s sferičnimi nanodelci in kvantnimi pikami, kot so CdSe, CdSSe in zlati nanodelci. Fazni diagrami temperaturnih prehodov v odvisnosti od koncentracije nanodelcev v TK z dodanimi različnimi tipi sferičnih nanodelcev z različnimi jedri, velikostmi in funkcionalizacijo kažejo stabilizacijo BPIII. To velja vsaj dokler so delci manjše ali podobne velikosti kot jedro disklinacijskih linij. V nadaljni raziskavi pa smo se osredotočili na ploščate in paličaste nanodelce in njihov vpliv na širino območja posameznih modrih faz, saj so bile predstavljene teoretične simulacije, ki so predpostavljale boljšo stabilizacijo z dodajanjem anizotropnih nanodelcev, poleg tega pa taki nanodelci še niso bili uporabljeni v študijah pred tem. Rezultati kalorimetrije visoke ločljivosti in mikroskopije s prekrižanimi polarizatorji so pokazali, da funkcionalizirani anizotropni nanodelci nekoliko drugače stabilizirajo območje modrih faz. V primerjavi s sferičnimi in kvantnimi pikami, ki bolj stabilizirajo amorfno BPIII, nanopalčke in ploščati nanodelci z različnimi jedri, velikostmi in funkcinalizacijo večinoma bolj stabilizirajo bolj urejeno BPI. Predstavljen je teoretični model, ki je osnovan na Landau-de Gennesovem fenomenološkem pristopu, v katerem sta predstavljena dva mehanizma. Prvi DCR (ang. defect core replacement) sloni na zamenjavi tekočega kristala v jedru defekta z nanodelcem, medtem ko je drugi povezan z elastično deformacijo - sedlastim upogibom. Kaže, da ta dva mehanizma igrata veliko vlogo pri stabilizaciji modrih faz. Pomemben, še posebej v primeru večjih anizotropnih nanodelcev, je tudi tretji mehanizem ADCT (ang. adaptive defect core targeting), ki je povezan z energijsko izgubo zaradi motnje, ki jo delec povzroči okrog jedra disklinacijske linije. Ta mehanizem tako omogoči, da delec lažje ostane v jedru disklinacijske linije. Podobno kot je bilo ugotovljeno pri stabilizaciji TGB faz, nastanek težkih anizotropnih skupkov nanodelcev v jedru defektov zmanjša fluktuacije disklinacijskih defektnih linij in tako prispeva k stabilizaciji tako BPI kot tudi BPIII modre faze. V drugem delu so predstavljene neposredne meritve elastokaloričnega (eK) pojava v tekočekristalnih elastomerih. Kalorični pojavi, kot so magnetokalorični, elektrokalorični in mehanokalorični so v zadnjih letih pridobili veliko zanimanje zaradi njihove uporabe v novih, okolju prijaznih grelnih, hladilnih in klimatskih napravah. Med mehanokalorčne spadata barokalorični in elastokalorični. Slednji je najbolj raziskan v kovinah z oblikovnim spominom, v katerih so izmerili spremembo temperature do 40 K. Ker pa je v teh materialih potrebna mehanska napetost večja od 1 GPa, je njihova uporaba v hladilnih napravah povezana z velikimi izzivi. V tej tezi raziskujemo elastokalorični pojav v mehkih materialih, kot so tekočekristalni elastomeri (TKE), pri katerih je dovolj že nekaj redov manjša mehanska napetost (okrog 1 MPa) za vzbujanje eK pojava. Neposredne meritve elastokaloričnega pojava so potrdile, da obstaja merljiv eK pojav v okolici tekočekristalnega faznega prehoda v glavno-verižnih tekočekristalnih elastomerih. Preprost Landau-de Gennesov model opiše večino opaženih lastnosti eK pojava. Raziskave so pokazale, da pričakujemo največji eK pojav v TKE materialih, ki so pripravljeni tako, da imajo pri nematskem prehodu velik termomehanski odziv in veliko latentno toploto, ki dodatno ojača eK odzivnost.

• Keywords: Tekoči kristali, modre faze, nanodelci, kalorimetrija, tekočekristalni elastomeri, elastokalorični pojav.