Isothermal and Non-isothermal Crystallization in Liquid Crystals as Seen by Broadband Dielectric Spectroscopy and Differ
Being composed of shape-anisotropic molecules, liquid crystals (LCs) differ from simple liquids in that they demonstrate a tendency to orientate in specific directions and form various mesophases. Upon cooling, these partially ordered liquid crystalline p
- PDF / 2,232,223 Bytes
- 30 Pages / 439.37 x 666.142 pts Page_size
- 71 Downloads / 251 Views
Abstract Being composed of shape-anisotropic molecules, liquid crystals (LCs) differ from simple liquids in that they demonstrate a tendency to orientate in specific directions and form various mesophases. Upon cooling, these partially ordered liquid crystalline phases can either vitrify or crystallize. Upon heating from a glassy state, LCs, like other low molecular weight systems and polymers, have been found to undergo so-called cold crystallization. This chapter discusses the findings of broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC) studies of the isothermal and non-isothermal crystallization kinetics of the supercooled nematic (N) and chiral nematic (N*) states of, respectively, 2,7-bis(4-pentylphenyl)-9,9-diethyl-9H-fluorene (5P-EtFLEt-P5) and S,S-2,7-bis(4pentylphenyl)-9,9-dimethylbutyl-9H-fluorene (5P-Am*FLAm*P5). The isothermal melt and cold crystallization processes at selected temperatures T c above the glass transition temperature T g (1.07T g ≤ T c ≤ 1.17T g ) are compared in 5P-EtFLEtP5. It was found that 5P-EtFLEt-P5 and 5P-Am*FLAm*P5 display different types of non-isothermal cold crystallization. Finally, the paper discusses the crystallization behavior occurring in the well-ordered smectic B phase (SmB) of 4-nbutyloxybenzylidene-4 -n -octylaniline (BBOA) under various thermal conditions. The DSC analysis revealed two different crystallization mechanisms for fast and slow cooling. Keywords Liquid crystals · Molecular dynamics · Non-isothermal crystallization · Isothermal crystallization · Broadband dielectric spectroscopy · Differential scanning calorimetry
M. Jasiurkowska-Delaporte (B) Institute of Nuclear Physics Polish Academy of Sciences, 31342 Krakow, Poland e-mail: Malgorzata.Jasiurkowska-Delaporte@ifj.edu.pl © Springer Nature Switzerland AG 2020 T. A. Ezquerra and A. Nogales (eds.), Crystallization as Studied by Broadband Dielectric Spectroscopy, Advances in Dielectrics, https://doi.org/10.1007/978-3-030-56186-4_5
119
120
M. Jasiurkowska-Delaporte
Abbreviations 5P-EtFLEt-P5 5P-Am*FLAm*P5 BBOA BDS CONDIS DSC GN GN* HN Is LCs POM N N* ODIC SmB VFT
2,7-Bis(4-pentylphenyl)-9,9-diethyl-9H-fluorene S,S-2,7-bis(4-pentylphenyl)-9,9-dimethylbutyl-9H-fluorene 4-N-butyloxybenzylidene-4 -n -octylaniline Broadband dielectric spectroscopy Conformationally disordered crystals Differential scanning calorimetry Glass of nematic phase Glass of chiral nematic phase Havriliak–Negami Isotropic state Liquid crystals Polarizing optical microscopy Nematic Chiral nematic Orientationally disordered crystal Smectic B Vogel–Fulcher–Tammann
1 Introduction Owing to their remarkable optical and physical properties, liquid crystals (LCs) have attracted great attention from both academia and industry. In addition to their widespread use in displays and electrically driven optical switches, LCs have also found novel applications in a range of areas, including medicine [1–3], pharmacy [4] and sensing [5]. The simplest, and the most commonly used, form of LC is the nematic phase (
