Jiandong Ding

MONTE CARLO SIMULATION OF SELF-AVOIDING LATTICE CHAINS SUBJECT TO SIMPLE SHEAR FLOW. I. MODEL AND SIMULATION ALGORITHM

The efficient on-lattice Monte Carlo (MC) method is extended to simulate, for the first time, the simple shear flow of multiple macromolecular chains with self-avoiding walk (SAW) on a molecular level by introducing a pseudopotential to describe the flow field. This pseudopotential makes sense only for the potential difference associated with each local microrelaxational movement of a bead in the chain strictly defined by the four-site lattice model and bond fluctuation approach. The free-draining bead-spring model is thus investigated at low and high shear rates, and the resultant shear stress and first normal stress difference are obtained by statistics according to the sampled configurational distributions under flow. As the first paper of a corresponding series, the pseudopotential is checked in detail and confirmed by the simulation outputs for both a single SAW chain and multiple SAW chains in two dimensions. The simulated velocity profile is found to greatly satisfy the requirement of the simple sh...

Brownian dynamics simulation of rodlike polymers under shear flow

The highly nonlinear behaviors of rodlike polymers in nematic phase under shear flow are studied with Brownian dynamics simulation. The LebwohlLasher nematogen model is taken as the prototype of the simulation and the mean-field approximation is avoided. By considering the nearest-neighbor intermolecular interaction, the spatial orientational correlation is introduced and therefore the spatial inhomogeneity such as the multiple-domain effect can automatically be incorporated. The transient order parameters, birefringence axes, shear stresses and first normal stress differences are calculated. The important finding of this work is that the director wagging and damped oscillation share the same molecular origin as director tumbling. The only difference is that the system is split into micro-domains which tumble with different phase angles in the wagging and damped oscillation regimes. The tumbling of the director of the whole system is suppressed due to the spatial inhomogeneity of director fields and then the damped oscillation of macroscopic stresses becomes predominant. The negative first normal stress difference exists at moderate shear rates, where both elasticity and viscosity play important role. Our simulation results including some dimensionless scaling parameters find good agreement with experimental observations in literature.

Small Angle Light Scattering from Axial Nematic Droplets

Abstract Small angle light scattering (SALS) from the three dimensional anisotropic spherical nematic droplets with an axial director configuration is investigated theoretically in this paper. The authors have performed an analytical interpretation for the light scattering from a single axial nematic droplet, considering an arbitrary orientation of the principal symmetry axis of the droplet. Series of scattering patterns for the axial nematic droplets are presented graphically and compared with each other as well as with those for the ideal spherulites or the radial nematic droplets. The dependence of V v, scattering on relative polarizabilities between liquid crystals and isotropic matrices is elucidated. The average size of the axial nematic droplets can readily be determined according to the formulae obtained. Hence it is very prospective to apply SALS to distinguish the director configurations of nematic droplets and to make further extensive studies on composite materials containing anisotropic parti...

Restructuring of Polymer-Dispersed Nematic Droplets in External Electric Field

Two types of polymer dispersed liquid crystal (PDLC) films with normal and tangential anchored nematic droplets are prepared, and the restructuring of nematic droplets in external electric fields is monitored by polarized optical microscopy (POM) and small angle light scattering (SALS). The electric field effect in nematic droplets is analysed qualitatively. Some comparisons and discussion are made, and the complication of this effect is indicated.

Brownian dynamics simulation of external magnetic field effect on director tumbling in liquid-crystalline polymers under shear flow

We have made the first investigation of the external magnetic field effect on director dynamics and flow instability in liquid-crystalline polymers subjected to shear flow by Brownian dynamics simulation and Lebwohl-Lasher nematogen model. According to our simulation, the static magnetic field can, depending on the field strength and orientation, enhance or inhibit the flow instability. The corresponding state diagram for director tumbling and shear aligning is presented. The tumbling period in a magnetic field does not obey the conventional scaling relation. In addition, a novel phenomenon about the first normal stress difference vs shear rate has been found in a magnetic field.

Theoretical and Experimental Studies on Depth-Induced Light Extinction in Nematic Droplets

The effect of depth-induced light extinction (DILE) is proposed and investigated both in theory and in experiment. This phenomenon may occur by adjusting the effective depth of an anisotropic entity observed orthoscopically in a polarizing microscope while maintaining the internal structure. The analytical interpretations are made in both monochromatic light and multichromatic light and reveal that the DILE effect is much more obvious in monochromatic light. The DILE in the radial nematic droplet as well as that in the normal-oriented axial one is then calculated and analysed in detail. The experimental observations are in satisfactory agreement with corresponding theoretical analyses and prove that the nematic droplets in the poly(methyl acrylate)-based polymer dispersed liquid crystal (PDLC) film have the radial director configuration. The advantage of DILE is that it can, by two-dimensional polarizing micrographs, reflect the third dimension parallel to the incident light. Some prospective applications of DILE are pointed out. Finally, extinction effects besides DILE are classified and discussed.

Small angle light scattering from bipolar nematic droplets

Abstract Theoretical and experimental studies are made of small angle light scattering (SALS) from the nematic droplets with a bipolar director configuration. Serial expressions for the scattered amplitudes from a single bipolar anisotropic sphere such as a bipolar nematic droplet are obtained using Rayleigh-Gans (RG) theory. Analytical solutions are also derived under reasonable approximations. All of the formulae are suitable for an arbitrary orientation of the droplet symmetry axis. Series of calculated Hv and Vv scattering patterns for the bipolar nematic droplets with typical orientations of droplet axes are presented. Some theoretical patterns have been confirmed in experiment with polymer dispersed liquid crystal (PDLC) films. As a result, SALS is proven useful to distinguish the director configurations of nematic droplets, and further to discriminate the orientations of the droplet axes. Moreover, the average size of the bipolar nematic droplets can be estimated by Hv scattering with the formulae ...

A preliminary morphological evidence for the existence of back-flow effect associated with the formation of band texture after cessation of shear for a polymeric liquid crystal

SummaryThe band texture of the hydroxypropylcellulose (HPC) film is observed after cessation of shear by optical microscopy and scanning electron microscopy (SEM). The sinusoidal supramolecular structure associated with the band texture of HPC is verified by polarized optical microscopy, so the band texture corresponds to the spatial periodicity of local director or molecular orientation. On the other hand, however, the pleated morphology, a depth periodicity, is confirmed by SEM and the pleat period is consistent with the band period. What is more, even without polarizer and analyser, the bands can still be found in an optical microscope with the same periodicity as those under cross polars, and these bands might be attributed to a density periodicity. Both the depth and density periodicities result from the mass flow after cessation of shear and must be coupled to the orientation periodicity resulting from director rotation. As a consequence, we propose that the back-flow effect is striking in the formation of band textures and should be taken into consideration in order to give a plausible and explicit mechanism of band formation.

Compensation Effects of Crystals and Liquid Crystals in Polarizing Optical Microscope

This paper investigated the effect of a tilting compensator on the anisotropic objects with the radial director configuration in a polarizing optical microscope under orthoscopic observation. The polymer spherulites of poly (ethylene oxide) (PEO) and the radial liquid crystal (LC) droplets in a polymer dispersed liquid crystal (PDLC) film were observed, on behalf of two-dimensional (2D) and three-dimensional (3D) anisotropic entities, respectively. Calculated birefringence patterns found good agreement with the experimental polarizing micrographs. Besides the well-known Maltese cross, the so-called depth-induced light extinction (DILE) bands and corresponding compensation behaviors were investigated in the LC droplets. According to the theoretical and experimental studies, we have revealed plausibly the detailed characteristics of the compensation effects in the 2D and 3D anisotropic objects and the striking differences between them. Some applications of the compensation effect were further pointed out.