Interplay between Crystallization and Glass Transition in Nematic Liquid Crystal 2,7-Bis(4-pentylphenyl)-9,9-diethyl-9 H-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-9 H-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 ( mf ≈ 121). The study compares the isothermal melt- and cold-crystallization processes at several selected temperatures Tc in the vicinity of the glass-transition temperature Tg (1.07 Tg ≤ T ≤ 1.17 Tg). 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.