Processing, molecular orientation effect

Their low melt viscosity permits molding of thin sections and complex shapes. However, their tendency to form ordered structures causes LC materials to be particularly susceptible to molecular orientation effects during processing. 

In Chapter 8 we will present a detailed discussion of the isotropic phase molecular orientation effects by an applied optical field from a short intense laser pulse. It is shown that both the response time and the induced order Q depend on the temperature vicinity (T-in a critical way they both vary as (r- which becomes very large near T. This near-r critical slowing down behavior of the order parameter Q of the isotropic phase is similar to the slowing down behavior of the order parameter S of the nematic phase discussed in the previous section. Besides the nematic isotropic phase transition, which is the most prominent order disorder transition exhibited by Uquid ciystals, there are other equally interesting phase transition processes among the various mesophases, such as smectic-A smectic-C, which will be discussed in Chapter 4. 

The reason for the activity of the above named classes of liquids is not fully understood but it has been noted that the most active liquids are those which reduce the molecular cohesion to the greatest extent. It is also noticed that the effect is far more serious where biaxial stresses are involved (a condition which invariably causes a greater tendency to brittleness). Such stresses may be frozen in as a result of molecular orientation during processing or may be due to distortion during use. 

In the Z-type deposition film, however, the long spacing of 7.2 nm did not agree with the predicted value of 3.9 nm rather, it was the same value as that of the Y-type deposition film. This result demonstrates that the Z-type film does not possess the Z-type layer structure but the Y-type layer structure. It should be assumed that the molecules were turned over in the deposition process and formed the Y-type layer structure, since the Z-type layer structure in which a hydrophilic group touches on a hydrophobic group is unstable. The conclusion from the examination of long spacings well supports molecular orientations in the LB films determined from the linear Stark effect measurements. From the linear Stark effect and the X-ray diffraction measurements, it is demonstrated that the hetero Y-type deposition method is useful for fabrication of stable noncentrosymmetric LB films. 

Birefringence is a useful property to monitor orientation [1,12,38], Birefringence occurs as the molecular chains are aligned in the process of orientation the magnitude and sign of the effect are determined by the chemical nature of the polymer. 

In terms of location, the primary process may be concentrated at one atom, as in an electron-atom scattering in LEED or in electron emission from core levels. It may also be delocalized over many atoms, as in photoelectron emission from a delocalized valence band level. The latter case is again of value to obtain molecular orientations directly. The polarization of the incident electron may also be used to determine molecular orientations, through its effect on the primary process. 

An important process variable in all types of extrusion is the take-up ratio, defined in the box at the top of the next column. The take-up ratio not only controls product geometry, but it also has a significant effect on molecular orientation in the final product. Whenever polymer melt is stretched, such as an extrudate under a high take-up ratio,... 

Because chromophores orientation is important for creating anisotropy and optical nonlinearities, intensive studies have been performed to understand induced molecular orientation and relaxation processes in polymers. To gain further insight into the physics of thin polymer films and the effects of molecular orientation in solid polymers, studies at high pressure could be beneficial. Pressure as a thermodynamic parameter is widely used to study... 

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