Thermal diffusivity anisotropy measured by a temperature wave method in the homologous series of (p-alkoxybenzylidene)-p′-octylaniline (nO.8)

Abstract

The anisotropy of thermal diffusivity in four homologues of (p-alkoxybenzylidene)-p'-octylaniline (nO.8, n = 4 - 7) was measured using a temperature wave method. The results show that the thermal diffusivity component along the director (α(∥)) is considerably larger than that perpendicular to the director (α(⊥)) in all mesophases, i.e., nematic (N), smectic A (SmA), smectic B (SmB), and smectic G (SmG) phases. Both components of the thermal diffusivity show a dip at the second- or weakly first-order N-SmA phase transition due to the heat capacity anomaly. In contrast, at the first-order SmA-SmB phase transition, thermal diffusivity exhibits a stepwise increase. The x-ray and calorimetric measurements enable a calculation of the thermal conductivity and the study of the effect of the molecular length on the thermal conductivity and diffusivity in the SmA and SmB phases. For the homologues n = 4, 5, and 6, which exhibit the same phase sequence upon cooling, the parallel component of the thermal conductivity k(∥) in the SmA and SmB phases systematically increases with increasing length of the molecular tails, while no such increase is observed in the thermal diffusivity α(∥). We thus conclude that the molecular model [Urbach et al., J. Chem. Phys. 78, 5113 (1983)] is valid for the qualitative prediction of the effect of the molecular length on the magnitude of the thermal conductivity.

Temperature-Dependent Thermoelastic Anisotropy of the Phenyl Pyrimidine Liquid Crystal

“…Abundant research work has been carried out to understand the thermoelastic anisotropy of LCs and how it is related to the molecular structure. − For mechanical anisotropy, it is generally accepted that (1) the elastic constants ( C ) are higher along (∥) the director than perpendicular (⊥) to it, (2) the elastic anisotropies, ( C ∥ – C ⊥ )/ C ⊥ , is on the order of a few percent or less, and (3) the dimensional ratio of rigid cores and side chains determines the elastic anisotropy. − For thermal anisotropy, it has been shown that (1) the thermal diffusivity (α) is higher along the director than perpendicular to it, (2) the thermal anisotropy, (α ∥ – α ⊥ )/α ⊥ , could reach 100% or even more, and (3) the thermal anisotropy stems from structural anisotropy of the LC molecules, and is not dependent on the long-range layer ordering of the LC molecules. − However, previous research typically focused on either the mechanical or the thermal properties in 1–2 phases of the LCs. To the best of our knowledge, no combined study of the anisotropic mechanical and thermal properties of LCs over a wide range of temperatures has been performed.…”

Effect of Smectic Layers on Thermal Diffusivity of Side-Chain Polymer Liquid Crystals

“…Therefore, α // is 2.9 times greater than α ⊥ . This anisotropy is comparable to that of monomeric LCs, but the magnitude of α is more than twice that of monomeric LCs. , Both the anisotropy and magnitude of α are comparable to those of another side-chain LCs, P m BE1, polymethacrylates bearing a different mesogenic moiety of methoxyphenyl benzoate …”

“…This anisotropy is comparable to that of monomeric LCs, but the magnitude of α is more than twice that of monomeric LCs. 11,12 Both the anisotropy and magnitude of α are comparable to those of another side-chain LCs, PmBE1, polymethacrylates bearing a different mesogenic moiety of methoxyphenyl benzoate. 13 The α // and α ⊥ values for PMnMA are listed in the sixth and seventh rows of Table 2, respectively.…”

“…The direction can be macroscopically aligned using external magnetic, electric, and flow fields or surface anchoring via substrate surfaces, affording LC polymer films with greater α values in the out-of-plane direction. − The magnitude and anisotropy of α measured for monomeric LCs increase continuously with increasing LC orientational order parameter independently of the types of LCs, including nematic and smectic. These results have led researchers to conclude that the magnitude and anisotropy of α are primarily determined by the molecular shape and orientational order. − The effects of LC structural orderings other than the orientational order, such as smectic layer structures, have not been thoroughly considered. In contrast, we have found that glassy smectic LCs composed of a series of LC P m BE1 polymethacrylates bearing phenyl benzoate mesogens in the side chains exhibit greater α values for the homologs with a higher layer ordering .…”

Atomistic Mechanism of Anisotropic Heat Conduction in the Liquid Crystal 4-Heptyl-4′-cyanobiphenyl: All-Atom Molecular Dynamics

“…The thermal diffusivity and TC in LC phases are known to be larger in the direction parallel to the nematic director than perpendicular thereto. Thermal anisotropy has been studied both experimentally − and computationally by using the Gay–Berne model. − In experimental studies, Urbach et al discussed the thermal diffusivity behavior by employing a kinetic model based on an extension of the Eyring theory to anisotropic fluids . On the other hand, Sarman Laaksonen calculated the TC of LC phases of the Gay–Berne fluid and suggested that the anisotropy is derived from molecular movement because the movement along the direction perpendicular to the director is impeded by side-by-side collisions .…”