Controlling the Critical Behavior of Paranematic to Nematic Transition in Main-Chain Liquid Single-Crystal Elastomers

Abstract

We investigated the paranematic-to-nematic phase transition of nematic main-chain liquid singlecrystal elastomers by means of high-resolution ac calorimetry and deuteron quadrupole-perturbed nuclear magnetic resonance. We determined that the cross-linking density strongly affects the thermodynamic response of the system, driving the transition from first order to supercritical via critical point of liquid-vapor type. This result together with recent findings in side-chain liquid crystal elastomers support the universality of such thermodynamic behavior in liquid crystal elastomers. Furthermore, the impact of frozen-in mechanical field during the preparation of samples has been systematically explored in this work. It is shown that the critical behavior can be controlled by both cross-linkers-induced internal random mechanical fields and frozen-in uniform mechanical field applied externally.

Electrocaloric and elastocaloric effects in soft materials

“…Main-chain LCE materials consisting of a backbone with siloxane-based chain extenders, rod-like mesogenic molecules and isotropic cross-linkers with five cross-linking point were synthesized as described in [19,31]. Accordingly, the second cross-linking step was carried out in the nematic state.…”

Liquid Crystal Polymers 133 5.1 Photochemical Phase Transition 133 5.1.1 Copolymers and Polymer Composites

“…Vemo, da se velika večina snovi pri segrevanju razteza, saj se razdalje med Slika 14: Različne narave faznih prehodov in s tem povezanih termomehanskih odzivov pri različni koncentraciji zamreževalcev [13] Prav tako vpliva na tip prehoda tudi zunanje polje, največje razlike med naravo prehoda pa dobimo, če spreminjamo temperaturo zamreževanja. Če je ta v bližini temperature faznega prehoda v izotropno fazo, je prehod zvezen, če pa je od nje zelo oddaljen, je prehod nezvezen [8].…”

“…Slika 15: Različne narave faznih prehodov in s tem povezanih termomehanskih odzivov pri dveh različnih zunanjih poljih [13] Slika 16: Različne narave faznih prehodov in s tem povezanih termomehanskih odzivov pri dveh različnih temperaturah zamreževanja [12] 5 Raziskave faznega prehoda tekočekristal-nega elastomera iz smektične A v izotropno fazo Slika 18: Prikaz spremembe entalpije in toplotne kapacitete v odvisnosti od temperature za zvezen superkritičen fazni prehod. ∆H L = 0, kar pomeni, da se pri tem prehodu ne sprosti nič latentne toplote.…”

“…14 Različne narave faznih prehodov in s tem povezanih termomehanskih odzivov pri različni koncentraciji zamreževalcev [13] . 36 15 Različne narave faznih prehodov in s tem povezanih termomehanskih odzivov pri dveh različnih zunanjih poljih [13] .…”

“…36 15 Različne narave faznih prehodov in s tem povezanih termomehanskih odzivov pri dveh različnih zunanjih poljih [13] . .…”