Oriented films uniformity

Relationship between elasticity and orientational order As remarked in chapter I, a uniformly oriented film of nematic liquid crystal may be prepared by prior treatment of the surfaces with which it is in contact. If the preferred orientation imposed by the surfaces is perturbed, let us say by a magnetic field, a curvature strain will be introduced in the medium. The theory of such a deformation will be discussed at length in 3.2 for the present it will suffice to state some of the important results. The free energy per unit volume of the deformed medium relative to the state of uniform orientation is... 

The LSCEs represent a new class of macromolecular systems distinguished by macroscopically uniform anisotropy. The concept of LSCE can be furthermore extended to other synthesis routes and to the densely crosslinked systems. Macroscopic uniaxially oriented films can be formed by mechanical force, alignment surface [30], magnetic and electric fields [31], polarized light [32], etc., and then crosslinked to form an anisotropic network (LC network is abbreviated as LCN) if the mixture components contain polymerizable or crosslink-able bifunctional monomers (Figure 9.13). Alternatively, amorphous or liquid crystalline side-group and/or main-chain polymers incorporating additional... 

Flat film and sheet extrusion produce uniaxially oriented film and sheet with improved clarity, better thickness uniformity, and a wide range of thicknesses. While the dies are very expensive and difficult to... 

A similar ordered structure was obtained also by shearing the mesophase of the 20/80 copolyester composition which was heated at 330 °C but in contrast to the 40/60 composition, it was not uniformly distributed in the sample. The viscosity of the mesophase of the 20/80 copolyester composition at 275 °C was at least one order of magnitude higher than the viscosity of the mesophase of the 40/60 composition at the same temperature and was reduced significantly by heating the polymer at 330 °C, as shown in Fig. 4. The mechanical properties of the oriented films from both copolyester compositions are summarized in Table 1 and suggest... 

The development of ordered bands perpendicular to the shear direction in the oriented film of both copolyester compositions is an optical effect observed with liquid-crystalline behaving polymers under mechanical stress conditions and appears to relate to the electro-optical effects observed in nematic liquid crystals. The non-uniform distribution of shear bands in the oriented film of the 20/80 copolyester composition is due to the inhomogeneity of its mesophase which is composed of rich PET and PHBA areas because no shear bands are observed readily in oriented film which is prepared below 310-320 °C, that is when the PHBA domains are not fused. ... 

When drawdown is high, the film may be uniaxially oriented and the properties of the final film isotropic. In the manufacture of strapping tape this effect is accentuated. If the cast or quenched film is to be used to feed an orientation tine, additional attention must be given to the amorphous—crystalline nature of the film ia the draw processes so that maximum strength can be achieved and uniform gauge and optical quality maintained. Slot casting is used for the orientation of these resins, polyesters, polyamides, and a variety of others. 

Tubular film may be biaxially oriented by passing eooled film through a pair of nip rolls, reheating it to a uniform temperature and then passing it between a further pair of rolls. Between the two pairs of rolls a mandrel of air is introduced in order to expand the film. The proeess is similar to that shown for poly(vinylidene ehloride) in Figure 17.4. 

Example 4.3 A plastic shrink wrapping with a thickness of 0.05 mm is to be produced using an annular die with a die gap of 0.8 mm. Assuming that the inflation of the bubble dominates the orientation in the film, determine the blow-up ratio required to give uniform biaxial orientation. 

---