Nematic aligned

The molecular theory of Doi [63,166] has been successfully applied to the description of many nonlinear rheological phenomena in PLCs. This theory assumes an un-textured monodomain and describes the molecular scale orientation of rigid rod molecules subject to the combined influence of hydrodynamic and Brownian torques, along with a potential of interaction (a Maier-Saupe potential is used) to account for the tendency for nematic alignment of the molecules. This theory is able to predict shear thinning viscosity, as well as predictions of the Leslie viscosity coefficients used in the LE theory. The original calculations by Doi for this model employed a preaveraging approximation that was later... 

The gel-sol transition temperature of (PDA) 4BCMU ( 70 C) is independent of polymer concentration. The lowest concentration in toluene where gel formation was observed is 0.06 wt%. Gels of (PDA) 4BCM exhibit optical birefringence which results from a spontaneous nematic alignment in the gel supported by hydrogen bonding [43]. 

If the microscope polarizers are aligned parallel, the transmission decreases by about 3 f-stops (about 8x). Clearly this cell shows twisted nematic alignment, but there are many areas of different orientation delineated by disclination loops. 

Table 34. Twisted Nematic - Alignment (High Tilt Low Tilt) Cell Spacing (6y-12iJ)...

G. P. Bryan-Brown, M. J. Towler, M. S. Bancroft, and D. G. McDonnell, Bistable nematic alignment using bigratings, Proc. Intnl. Display Research Conf. 94, 209 (1994). 

When we now go from the regime of dilute solutions and basically isolated chains to the r me of semidilute and dense solutions of semi-flexible chains we can expect two main effects. Due to the stiffness of the chains and at sufficiently large aspect ratio the chains will prefer a nematic alignment and we expect to see an isotropic to nematic transition. In the limit of almost rigid rods this transition happens at very dilute concentrations. When the chains become more flexible the transition density increases. This behavior was studied for a lyotropic system in [22]. The Hamiltonian employed in this simulation of the bond-fluctuation model was given by... 

When one now introduces an attraction between the monomers leading to a coil-globule transition for the isolated chain, on the one hand the tendency for nematic alignment is increased and on the other hand a liquid-gas critical point is introduced into the model. For flexible chains a Hamiltonian of the form... 

The surface structure of the oblique evaporated film is illustrated schematically as in Fig. 3.3.2. There are many grooves and columns, the features of which vary with evaporation angle and change in the nematic alignment direction and pretilt angle. 

SPs growing simultaneously with the formation of their own nematic alignment. 

A. Spontaneous Nematic Alignment of Polydiacetylene 4BCMU Gels... 

In the absence of a preferred direction in the gel, one would expect nematic alignment only in microscopic domains oriented randomly with respect to one another. Since the principal axes are found to be parallel and perpendicular to the container walls, we believe the walls imposed this preferred direction. Moreover, the striae in Fig. 9 are always more closely spaced when the laser beam passes close to one of the container side walls. As the beam is translated away from the wall, S decreases (by a factor of 2-3 on going 0.5 cm from the wall). Once the beam passes the middle of the container and approaches the opposite side wall, S again increases. 

Wall defects are found, for example, in nematics aligned by magnetic or electric fields, where walls separate domains with different orientations (in a bulk sample, the director can align parallel or perpendicular to a field, both cases being physically equivalent). 

The core structure of cholesteric discli-nations was interpreted by K16man and Frie-del [2, 3]. The rotation vector considered in the Volterra process is normal to the cholesteric axis and is either parallel to the molecules or normal to them, this resulting in a core structure that is either continuous, with a longitudinal nematic alignment of directors in the core (A disclinations), or discontinuous (t disclinations), with a singular line of the type encountered in non-twisted nematic liquids. 

When the rms (root mean square) voltage reaches a critical threshold ( 5 V), a periodic distortion of nematic alignment is... 

Fig. 5 Atomistic and coarse-grained representation of a P3HT chain (left), including coarse-grained angular (0) and dihedral (tf) degrees of freedom (center). Biaxial nematic alignment in a melt of P3HT chains (right). Adapted with permission from Gemiinden et al. [64]. Copyright (2013) American Chemical Society...

In this technique, normally incident plane-polarised light is scattered by thin films ( 20 pm thickness) of nematic aligned with the director either parallel or perpendicular to the incident light direction. The scattered light is detected as a function of angle, and for different polarisations of the incident (i) and scattered (f) beams. The differential scattering cross-section per unit volume is given by [12] ... 

Redecker, M., Bradley, D.D.C., Inbasekaran, M., Woo, E.P. Mobility enhancement through homogeneous nematic alignment of a liquid-crystalline polyfluorene. Appl. Phys. Lett. 74, 1400-1402 (1999)... 

On the other hand, if Uab>Uaa, the miscibility of a SCLCP and a LMMLC is largely increased by the nematic alignment. Reliable data on the effects of change in molecular structure on the SCLCP/LMMLC phase diagrams confirm the conclusions reach about the role of A/A, B/B... 

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