Uniaxial film

T Damerau and M Hennecke, Determination of orientational order parameters of uniaxial films with a commercial 90°C-angle fluorescence spectrometer, J. Chem. Phys., 103 6232-6240, 1995. 

At least one patent has issued dealing with Langmuir-Blodgett (L-B) films containing polarizable side groups. Uniaxial films of 7,7-bis(hexadecylamino)-8,8-dicyanoquinodimethane can be cast which exhibit SHG.(101) Orientation is assured by alternating successive transferred layers of the polar substrate with arachidic acid layers. A P value of 500xl0 30 esu is claimed for the quinodimethane, but no information about SHG of the L-B films is provided. 

Figure 5.14. Diagram of a-parameter curves, uniaxial film orientation, and morphology.

Dielectric constant e - Film, uniaxial Film, biaxial (qucisi-isotropic) 2.8 2.8 (5)... 

Films can be divided into three classes—isonopic, uniaxial, and biaxial—depending on the form of their permittivity tensor s (1.1.2°). For isotropic films, the three principal values of the permittivity tensor are equal, i = 62 = s-, and there is no preferred orientation within the film. The permittivity tensor of a uniaxial film has two different principal values, one that describes the propagation of radiation in the film plane and another that is perpendicular to the film plane, so that = Sy s. For a biaxial film all of the principal values of the permittivity tensor are different, so that Ex By For crystalline films with triclinic... 

A typical procedure of converting the DR measured from ATR spectra or iRRAs/ tr into angles of orientation requires the MSEF components within the layer. For example, the DR measured from the ATR spectrum of an uniaxial film for a mode perpendicular to a long-chain molecule is expressed as [517]... 

Nuzzo et al. [509] observed that with increasing temperature from 80 to 400 K, the intensity of the VasCH2 band of alkyl thiol SAMs on Au(lll) decreases while that of the VsCH2 band is almost the same, which was attributed to the intrinsic temperature dependences of these modes. However, this effect can also be due to reducing the film symmetry. As follows from Eqs. (3.50) and (3.52), the AasMg value obtained from the -polarized spectrum of a uniaxial film is the same as for the isotropically distributed film material (if intrinsic spectral perturbations, such as the splitting due to the crystal field effect [504], do not redistribute the intensities). On the other hand, biaxial symmetry should change this ratio. 

Uniaxial Film Splits Under Transverse Load... 

Figure 11.4 shows the type of orientation in two films, one a uniaxial film extruded through a slot die and the other with biaxial orientation [8-11]. Table 11.1 shows the tensile properties of LCP film made by conventional processing for 0° orientation. We note that MD tensile properties are about 10 times higher than TD properties. 

We saw previously in Figures 11.2 and 11.3 how a uniaxial film is produced by extruding LCP through a fixed slot die. Unlike random coil polymers, if the uniaxial LCP film is heated above Tg and below Tm, then transverse stress is applied, and the LCP film will tear, as in Figure 11.2(a). The reason for this is that the rigid rod molecular segments remain aligned, even near the melt temperature, and, therefore, the LCP film remains weak in the transverse direction... 

To suppress the light leakage at oblique angles and further widen the viewing angle, several phase compensation schemes using uniaxial films [20-22] and biaxial films [23-25] have been proposed. Computer simulation and experimental results have been reported. 

In this section, we focus on the analytical solutions for the uniaxial film-compensated wide view LCDs. With the analytical solutions, the interdependency between the LC cell and film parameters is clearly revealed. More importantly, analytical solutions provide a clear physical description of the compensation mechanisms. 

Table 8.1 lists some commercially available compensation films, classified by their refractive indices. Different LC modes need different types of compensation films in order to obtain satisfactory compensation effect. For example, the IPS mode may require a biaxial compensation film with nx>n >riy [26], while the VA mode needs a compensation film with nx>riy> [27]. Theoretical analyses of biaxial film-compensated LCDs are rather difficult. Here, we focus on the uniaxial film-compensated wide view LCDs. 

Uniaxial film is an anisotropic birefringent film with only one optical axis. For simplicity, let us limit our discussions to non-absorption uniaxial films. From the viewpoint of optical axis orientation, uniaxial films can be classified into a film and c film. An a film s optical axis is parallel to the film surface, while a c film s optical axis is perpendicular to the film surface. 

Both a film and c film can be further divided into positive or negative films depending on the relative values of the extraordinary refractive index tie and the ordinary refractive index Table 8.1 lists aU the types of compensation films and their refractive index relationship. In our analyses, we focus on the uniaxial films. As a general rale, a positive uniaxial film means Tie > no, otherwise, rig < tig for a negative uniaxial film. 

Both uniaxial compensation film and nematic LC layer can be treated as uniaxial media. When a light propagates into a uniaxial film, generally two forward eigenwaves (one ordinary wave... 

In the uniaxial-film-compensated LCDs, both a and c films are commonly used. In these two special cases. Equation (8.16) can be further simplified. 

For simplicity but without losing generality, let us assume that in eaeh pixel the LC direetors form a four-domain orientation profile, as Figure 8.21(a) shows. Figure 8.21(b) depicts the calculated voltage-dependent transmittance curve of a typical MVA-LCD using Merck MLC-6608 LC material whose parameters are listed in Table 8.3. Here, the absorption loss of polarizers has been taken into consideration. In the film-compensated MVA eells, the refractive indices of the uniaxial films and polarizers are still the same as those listed in Table 8.2. 

In the following, we use the four-domain MVA-LCD as an example to demonstrate some uniaxial-film compensation schemes and provide each scheme with a comprehensive analytical solution. All of these compensation schemes are equally applicable to PVA mode LCDs. 

Figure 8.30 Voltage-dependent transmittance curves of a r cell. dAn=436nm, uniaxial film rfAn = 53.3 nm and its optic axis is perpendicular to that of LC cell.

X. Zhu, Z. Ge, and S. T. Wu, Analytical solutions for uniaxial film-compensated wide-view liquid crystal displays, J. Display Technology, 2, 3 (March, 2006). 

H. Yoshida, K. Nakamura, H. Tsuda, et al.. Direct-view polymer-dispersed LCD with crossed Nicols and uniaxial film, J. Soc. Inf. Display, 2, 135 (1994). 

Orientation is a continuous operation and occurs in the direction of the film motion or the machine direction (MD). Usually, a cast sheet is transported on heated rollers (Fig. 9) to permit the sheet to reach a uniform temperature at which the polymer molecules are sufficiently mobile. Having reached this temperature, the sheet is abruptly accelerated between two rollers of different speeds. This point is defined as the draw point. In some instances, additional heat may be required to boost the temperature, or nip rolls are needed to regulate tension and to prevent film slippage (Fig. 9). Uniaxial film can be obtained by orienting perpendicular (transverse) to the MD using tenter frames or melt inflation, where the melt removal rate is equal to the melt velocity at the die. However, these two methods are seldom employed. The properties required by the user determine the orientation required to develop the desired physical properties. 

We consider an elliptically polarized plane wave of frequency normal incidence onto an inhomogeneous, nonabsorbing, locally uniaxial film of thickness d. The dielectric tensor of the medium has the form... 

The number of molecules per unit area is denoted by N. For an uniaxial film with its optical axis parallel to the surface normal the tangential t and normal n component of the... 

Yalcin B and Cakmak M (2005) Molecular orientation behavior of poly(vinyl chloride) as influenced by the nanoparticles and plasticizer during uniaxial film stretching in the rubbery stage, J Polym Sci Polym Phys 43 724-742. 

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