Effect of magnetic field on discotic nematic liquid crystalline polymers under simple shear flow

Abstract

The effect of magnetic field on the discotic nematic liquid crystalline polymers (LCPs) is analyzed with the extended Doi theory, in which the molecular shape parameter (β) is defined at −1.0. The evolution equation for the probability function of the discotic nematic LCP molecules is solved without any closure approximations. The transition among flow‐orientation modes, such as tumbling, wagging, and aligning defined similar to the rodlike LCPs, is strongly affected by the magnetic fields. The new aligning flow‐orientation mode observed for the rodlike LCPs under magnetic fields also can be investigated in the lower shear rate region. On the other hand, the effect of magnetic fields parallel to the x‐ and y‐axis on the time‐averaged first and second normal stress differences ( $\bar N_1^*$, $\bar N_2^*$) are also studied. It can be seen that the shear rate regions of the sign changes of $\bar N_1^*$, $\bar N_2^*$ are completely contrary to those conclusions achieved for the rodlike LCPs. In addition, the absolute values of $\bar \eta ^*$ increase with the magnetic field strength in the lower shear rate range owing to the new aligning flow‐orientation mode. Finally, the flow‐phase diagram versus β is also discussed. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Heat Transfer Organic Materials: Robust Polymer Films with the Outstanding Thermal Conductivity Fabricated by the Photopolymerization of Uniaxially Oriented Reactive Discogens

“…Among the various methods to align DLCs, the incorporation of magnetic field is a promising solution to accomplish the higher degree of bulk orientation with a high uniformity. , It has been well-documented that higher order mesophases can be induced by increasing the strength of the magnetic field. Nevertheless, the anisotropic molecules cannot be aligned even under the strong fields when the magnetic coupling energy (Δχ B 2 ) between the individual molecule and the magnetic field is lower than thermal energy ( kT ); where Δχ and B represent the magnetic anisotropy derived from the difference of diamagnetic susceptibility between two orthogonal molecular axes and the intensity of applied magnetic field, respectively. − The DLC molecules can be aligned under the magnetic field when a macroscopic domain composed of a large number of discogens is formed and acts as a unit; where the N indicates the number of molecules in a domain. As temperature suppresses from the isotropic melting state to the LC state, the discogens coalesce into a mesophase domain.…”

“…As temperature suppresses from the isotropic melting state to the LC state, the discogens coalesce into a mesophase domain. Therefore, the magneto static free energy becomes sufficient to surpass the thermal disordering effects due to an additive effect of anisotropy. − As described in Figure c and Figure S6a, a 50HABET-50THBT mixture is slowly annealed from the isotropic state (140 °C) to the LC state (80 °C) under 1.5 T of magnetic field and then photopolymerized. The aligned samples are prepared in three ways; without any magnetic field, with the static magnetic field, and with the rotating magnetic field (100 rpm).…”