Glass Transition Dynamics and Crystallization Kinetics in the Smectic Liquid Crystal 4-n-Butyloxybenzylidene-4′-n′-octylaniline (BBOA)

Abstract

The molecular dynamics of 4-n-butyloxybenzylidene-4'-n'-octylaniline (BBOA, abbreviated also as 4O.8) was studied by broadband dielectric spectroscopy (BDS) for samples that were exposed to various thermal treatments. Phase transitions between liquid crystalline phases (N, SmA, SmB, and SmB) were evidenced by abrupt changes in the temperature dependence of the dielectric permittivity spectra and dielectric relaxation times. A particularly complex dynamic behavior was revealed for the highly ordered SmB phase that showed clear evidence for cooperative dynamics of a glass-forming liquid as manifested by a Vogel-Fulcher-Tammann (VFT)-type temperature dependence of its structural relaxation time τ(T). At low temperatures, dependence τ(T) again changes from VFT to Arrhenius behavior, a phenomenon commonly observed for supercooled liquids confined to nanometer length scales and occasionally discussed as strong-fragile transition on heating. In this context, our observation supports the idea of partial orientation disorder in a quenched SmB phase, where the length scale of cooperative motions is restricted to the nanometer size of glassy domains causing the deviation from "bulk" VFT-type behavior. Finally, the isothermal crystallization kinetics of the metastable SmB phase was studied in detail. On the basis of structural information about the SmB and SmB phases, determined by X-ray diffraction, the subtle relation between molecular order and relaxation behavior is discussed.

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)

“…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. Recent studies have also examined the vitrification of crystalline phases characterized by both orientational and conformational degrees of freedom (condis type of glass) [31,32].…”

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

“…Sorai and Seki in their pioneering work on calorimetric measurements of n -( o -hydroxy- p -methoxybenzylidene)- p -butylaniline demonstrated that the nematic phase, characterized by a long-range order of molecular alignment, can vitrify. Afterward, the glass-forming ability was found for other nematic liquid crystals (LCs). − A glassy state has also been noticed for smectic crystal phases, in which molecules are arranged within layers, for example, for smectic G (SmG), smectic B (SmB), , and smectic E (SmE). , …”

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

“…The n A parameter (see Table 2), ranging from 2.8 to 2.3 with decreasing temperature, is reduced with respect to the value expected for the homogeneous growth of the spherulitic nuclei (n = 3). Presumably, the reduction of crystal growth dimensionality observed for the non-isothermal experiment (2.24 ≤ n o ≤ 2.67) is caused by a confinement effect, which is related to fact that the factions of the layered SmB cr are restricted by crystalline domains [21]. This effect was also found for crystallization of other materials in nanopores (two-dimensional confinement) [32] and for thin polymeric films (one-dimensional confinement) [33].…”

“…Calorimetric measurements found BBOA to possess a rich mesomorphism on heating [19]: Cr -310.5K -SmB -321.1K -SmA -335.3K -N-351.0K-I. Interestingly, upon cooling, an additional phase transition from the SmB hex to the SmB Cr phase (also called "soft crystal" [20]) was observed at 287 K [21]. X-ray experiments demonstrate that while both phases show a hexagonal arrangement of molecules within the smectic layer, they differ with regard to their molecular correlation length [22].…”