Effects of magnetic field on shear stress of nematic liquid crystalline polymer under simple shear flow

Abstract

The effect of magnetic fields on the shear stress of liquid crystalline polymers (LCPs) under shear flows is numerically analyzed using the Doi theory. The evolution equation for the probability density of LCP molecules is directly solved without any closure approximations. When the magnetic field is imposed on the LCPs along the shear flow, the low-shear rate plateau of the time-averaged generalized viscosity g à ð¼ r à xy = _ c Ã Þ disappears, which is due to the manner of the new aligning mode observed by Fu et al. The negative first normal stress difference N à 1 becomes opposite in all the shear rate regimes with the disappearance of periodic oscillations. For the magnetic field parallel to the velocity gradient direction, the more complicated sign changes of N à 1 is investigated because of the moment caused by the magnetic field vertical to the one caused by the shear flow. The ratio N à 2 = N Ã1 increases with the increasing magnetic field strength. Finally, when the molecular shape parameter is considered as 0.9, the similar discussions are achieved according to the phase diagram among tumbling, wagging, and aligning modes versus the magnetic field strength.

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

“…The sign of $\bar N_2^*$ becomes positive in all the shear regions, once the effect caused by the shear flow is swamped by the magnetic fields. Compared with Ref 19. in which the rodlike LCPs is affected by the magnetic fields, $\bar N_1^*$ , $\bar N_2^*$ are almost opposite to the results when the magnetic fields are along the y ‐axis.…”

“…The sign changes of the time‐averaged $\bar N_1^* \,$ and $\bar N_2^*$ are completely opposite with those results obtained for rodlike LCPs by the Refs 18. and19. In addition, as the mirror images of rodlike LCPs the values of $\bar \eta ^*$ are negative in the entire shear rate regions.…”

“…To capture the flow characteristic of discotic nematic LCPs, we have to consider the fact that the shortest molecular dimension aligns along the director. Through comparing with the results of rodlike LCPs,19 the conclusion that the negative viscosity $\bar \eta ^*$ is a mirror image of that corresponding to rods has been made. In the low shear region, the absolute values of $\bar \eta ^*$ increase with the magnetic field strength which can be expected if the phase formed by the discotic LCPs under magnetic fields is taken into account.…”

“…If one can freely control the molecular orientation configuration, the processing optimization may become much simpler. It is well known that the electric or magnetic field imposed on the rodlike LCPs results in the improvement of performance for LCP products 18, 19. Some experiments,20, 21 such as the uses of surface effect and shear flow, were made to obtain alignment phase of discotic LCs.…”

Practical Method for Modeling Directionality of Liquid Crystalline Polymers

“…Different researchers have introduced different methods to make more isotropic films. Among these are the introduction of counter rotating dies by Lusignea (2004) insertion of a porous media in the die by Boles et al (2008) and addition of a magnetic field by Fu and Wang (2010). During almost all manufacturing processes of LCPs, these materials are formed in the molten state.…”