Interplay between Crystallization and Glass Transition in Nematic Liquid Crystal 2,7-Bis(4-pentylphenyl)-9,9-diethyl-9H-fluorene

Abstract

This article presents the crystallization behavior and molecular dynamics of the supercooled nematic state of the newly synthesized liquid crystal 2,7-bis­(4-pentylphenyl)-9,9-diethyl-9H-fluorene (5P-EtFLEt-P5) studied by means of broadband dielectric spectroscopy (BDS). 5P-EtFLEt-P5 is a fragile glass-forming system with a high fragility parameter (m f ≈ 121). The study compares the isothermal melt- and cold-crystallization processes at several selected temperatures T c in the vicinity of the glass-transition temperature T g (1.07T g ≤ T ≤ 1.17T g). Our findings reveal that at low temperatures, the crystallization of the Cr1 phase from the nematic melt state occurs more quickly than the cold crystallization. The dimensionality of the growing crystallites (n) was found to be slightly higher for the cold- than for the melt-crystallization process, varying from n ∼ 5 to n ∼ 3 with increasing temperature. Our experimental results are discussed in terms of dynamic and thermodynamic properties of the material. The study also uses polarized optical microscopy to investigate the isothermal secondary cold crystallization (the formation of Cr2 from Cr1 upon heating), which is inaccessible by BDS measurements because of its very fast crystallization rates.

Relaxation dynamics and crystallization study of glass-forming chiral-nematic liquid crystal S,S-2,7-bis(4-pentylphenyl)-9,9-dimethylbutyl 9H-fluorene (5P-Am*FLAm*-P5)

“…Finally, further conclusions can be drawn by comparing our present findings for 5P-Am * FLAm * -P5 with those obtained previously for a non-chiral 5P-EtFLEt-P5 compound [23] composed of molecules with the same core. i) 5P-EtFLEt-P5 shows a very strong tendency to vitrification in the N phase, even upon cooling at rates as low as 0.1 K/min; it then follows the sequence GN-N-Cr1-Cr2-Is upon heating.…”

“…The local and long-range correlations of 5P-Am * FLAm * -P5 molecular reorientations described by n = a HN b HN ∼ 0.45 and m = a HN ∼ 0.9 are very strong. Two secondary β and γ processes are attributed, respectively, to the smallangle rotational diffusion of molecules around the short axis and to librations around the long axis, analogously to non-chiral 5P-EtFLEt-P5 liquid crystal with the same molecular core [23].…”

“…Glasses were initially understood as amorphous solids with some degrees of freedom frozen-in [19]. Gradually, the definition of a vitreous state expanded to embrace a partially ordered glass formed by the cooling of an orientationally disordered crystal (ODIC) [20], characterized by a substance whose molecules are disorientated despite having their centers distributed regularly through the crystalline lattice, and nematic liquid crystals [21][22][23], consisting of randomly distributed molecules with long molecular axes tending to align parallel to each other. Also, some smectic phases, such as smectic E [24][25][26][27], smectic B [28,29] and smectic G [30], form partially ordered glasses.…”

“…The opposite scenario was revealed for the non-isothermal melt crystallization in the smectic-B phase of the BBOA liquid crystal [9]. Our recent paper comparing the melt and cold isothermal crystallization in a glass-forming non-chiral 5P-EtFLEt-P5 liquid crystal found that the melt crystallization processes occur more quickly than the cold ones; this was attributed to the fact that the molecular order in the nematic state (N) upon cooling was higher than that of the metastable nematic state obtained after softening of the glass [23]. However, further systematic studies of other substances are required to understand the relationship between the structure of LC phases and the kinetics of its crystallization.…”

Kinetics of Non-Isothermal and Isothermal Crystallization in a Liquid Crystal with Highly Ordered Smectic Phase as Reflected by Differential Scanning Calorimetry, Polarized Optical Microscopy and Broadband Dielectric Spectroscopy

“…It was found that the crystallization process in LCs is influenced by structure of the initial phase and thermal history of the sample. Although there are several scientific studies concerning the formation of three-dimensional (3D) crystalline solids from the nematic phase (N) [13][14][15][16][17][18], little attention is paid to crystallization from highly-ordered crystal-like smectic phases. An interesting example in this regard is the crystallization of the smectic B (SmB) phase in 4-n-butyloxybenzylidene-4 -n -octylaniline (BBOA, also abbreviated as 4O.8).…”