Dielectric characterization of different mesogenic substances and a mixture with non-conventional gold nanoparticles

Abstract

The study of liquid-crystalline matter and nano-structured materials is an important and vast field of research with potential implications in the development of new technologies, like sensors and displays. In this work we have analyzed and characterized different systems by dielectric spectroscopy. The first part of this thesis is devoted to the characterization of an orthoconic liquid-crystalline mixture, W-129, with ferroelectric properties. The analysis of the dielectric response of this material has revealed a plurality of ferroelectric smectic C* subphases; they represent smectic intermediate variants situated between the ferroelectric phase and the antiferroelettric one, known like antiferrielectric phases (SmCFI*, SmCFII*, ... ), whose study is still open and is collecting a lot of interest in academic environments. The same liquid crystalline mixture was then doped with unconventional gold nanoparticles. These nanoparticles are functionalized with a hydrophilic polymer which becomes hydrophobic exceeded 40°C. The nano-composite material obtained by the dispersion of the gold nanoparticles presents interesting characteristics, such as an enhancement of dielectric increments (or strengths) probably due to the molecular interactions between the ferroelectric liquid crystal and the gold nanoparticles, which translates into an increase of the order of the liquid crystal host, in a stabilization of the smectic subphases and in an enhanced memory effect already seen in the pure liquid-crystalline mixture. The third and last part is devoted to the characterization of a new "banana-shaped" liquid crystal, which exhibits some unusual physical properties during the nematic phase, in particular, the presence of regions of more ordered molecules, organized in a smectic C phase inside a nematic one. The dielectric spectra acquired during the nematic phase show the presence of a relaxation response between 10 and 20 Hz which, with the addition of relatively large values of permittivity, may suggest the presence of a ferroelectric response due to the existence of cybotactic clusters.