Relaxation of molecular orientation

During steady shear flow at higher rates, a liquid crystalline system is considered to flow as an oriented continuous phase, which is shown schematically in Fig. 5(top). When the flow is ceased, molecular orientation relaxes, while liquid crystalline structure is re-formed under the influence of the wall and dis-clination points. Such structural changes must be reflected in optical data, as shown above. To interpret the optical data for a nematic polymer liquid crystal, a model for structural re-forma-tion including relaxation of molecular orientation will be proposed. 

The melt extmsion temperature is also observed (Fig. 1.16) to have a significant effect on the physical properties as the tear strength in both directions increases with increasing melt temperature. This is probably due to lower levels of orientation as the result of lower stress levels in fhe melt and shorter relaxation times allowing a rapid relaxation of molecular orientation. 

Wilkes, G. L. The Measurement of Molecular Orientation in Polymeric Solids. Vol. 8, pp. 91-136. Williams, G. Molecular Aspects of Multiple Dielectric Relaxation Processes in Solid Polymers. Vol. 33, pp. 59-92. 

Relaxation dispersion data for water on Cab-O-Sil, which is a monodis-perse silica fine particulate, are shown in Fig. 2 (45). The data are analyzed in terms of the model summarized schematically in Fig. 3. The y process characterizes the high frequency local motions of the liquid in the surface phase and defines the high field relaxation dispersion. There is little field dependence because the local motions are rapid. The p process defines the power-law region of the relaxation dispersion in this model and characterizes the molecular reorientations mediated by translational displacements on the length scale of the order of the monomer size, or the particle size. The a process represents averaging of molecular orientations by translational displacements on the order of the particle cluster size, which is limited to the long time or low frequency end by exchange with bulk or free water. This model has been discussed in a number of contexts and extended studies have been conducted (34,41,43). 

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... 

The Doi theory captures the molecular viscoelasticity of LCP, i.e., the relaxation of the orientation distribution under flow. But it completely ignores distortional elasticity and is limited to monodomains. The assumption of spatial uniformity underlies all its key elements the nematic potential, the kinetic equation, and the elastic stress tensor. Therefore, its successes are restricted to situations where distortional elasticity is insignificant. 

Molecular relaxations, and molecular orientation of polymers can be related to tribological performance. Such studies have been conducted with polyimides (11). 

Granicher (1957), which considered only the majority carriers in each region. It is somewhat more difficult to picture the polarization processes in terms of molecular orientations. In the regions where orientational defects provide the relaxation mechanism, their motion in the applied field simply reorients molecules or, if the effective charge associated with the defects is included, then this contributes a polarization in the same direction. In regions where ion states provide the relaxation mechanism, however, motion of these states in the direction of the field tends to orient molecular dipoles antiparallel to the field. The polarizations pro-... 

Defined as the ratio of the material relaxation time to the processing time during flow, Wi characterizes the amount of molecular orientation induced by the flow. For simple shear flow, Wi is given by the product of t times the shear rate. Related to the Deborah number, Wi is used for constant straining, whereas De describes deformations with a varying strain history. 

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