Optical anisotropy development

The optical anisotropies y and molar Kerr constants mK of model compounds, and of polylp-chlorostyrene) and polyfp-bromostyrene), are determined. Averages and over all conformations of the polymer chains are calculated as functions of the fraction wm of the meso diads using the RIS model originally developed for PS. 

The use of photon correlation spectroscopy to study the dynamics of concentration fluctuations in polymer solutions and gels is now well established. In bulk polymers near the glass transition there will be slowly relaxing fluctuations in density and optical anisotropy which can also be studied by this technique. In this article we review the development of the field of photon correlation spectroscopy from bulk polymers. The theory of dynamic light scattering from pure liquids is presented and applied to polymers. The important experimented considerations involved in the collection and analysis of this type of data are discussed. Most of the article focuses on the dynamics of fluctuations near the glass transition in polymers. All the published work in this area is reviewed and the results are critically discussed. The current state of the field is summarized and many suggestions for further work are presented. 

The optical measurements presented in the previous chapters can be used to either characterize local, microstractural properties or as probes of bulk responses to orientation processes. In either case, it is normally desirable to make the connection between experimental observables and their molecular or microstractural origins. The particular molecular properties that are probed will naturally depend on the physical interaction between the light and the material. This chapter explores molecular models and theories that describe these interactions and identifies the properties of complex materials that can be extracted from measurements of optical anisotropies. The presentation begins with a discussion of molecular models that are applied to polymeric materials. Using these models, optical phenomena such as birefringence, dichroism, and Rayleigh and Raman scattering are predicted. Models appropriate for particulate systems are also developed. 

The technology of mesophase-pitch-based carbon fiber has stimulated the rapid development of the chemistry of mesophase behavior and preparation. The carbonization schemes and mechanisms leading to optical anisotropy via the mesophase, the control of carbonization with emphasis on the preparation of spinnable mesophase, and the mesophase transition and reactivity in relation to the structure of its constituent molecules are summarized in this paper. 

The present authors found another pyrolysis mechanism leading to optical anisotropy (10), in which no definite liquid phase was observable during the carbonization. In some carbonaceous substances, such as semi-anthracite coal, optical anisotropy over broad regions develops promptly at certain temperatures from the highly viscous stage. The component layers appear to be stacked, and are rearranged by heat-treatment to show optical anisotropy. Such a mechanism can be called the "preordered layer-transformation mechanism". 

Figure 1. Three steps in the development of optical anisotropy via the mesophase. (a) Nucleation of mesophase spherules. (b) Growth and coalescence of mesophase. (c) Flow anisotropy.

Figure 3 describes reaction schemes for naphthalene carbonization catalyzed by metallic potassium or by aluminum chloride (13,14) these catalysts produce contrasting isotropic and anisotropic carbons, respectively. The intermediate structures are similar except for more naphthenic structure induced in the AlCl -catalyzed carbonization. The role of naphthenic structures leading to optical anisotropy has been recognized in many examples, and their introduction can improve the anisotropic development, as described later. Higher fusibility, lower melting temperature, and higher solubility of the intermediate molecules may be obtained by the formation of partially naphthenic structures (15). 

Figure 4. Schematic diagram to illustrate three possible effects of cocarbonization on the development of optical anisotropy. A, additive B, principal carbonizing substance.

It will be shown that the level of uniaxial orientation and the variation of local mechanical properties generated by controlling the injection temperature can be conveniently characterized by microhardness measurement in combination with the measurement of optical birefringence An and DSC. In Section 2.7 we saw that microhardness is a very useful mechanical property, which can provide direct information about the anisotropy developed within highly oriented polymers. 

Moreover, the mean polarizability approximation can yield highly accurate results for dispersion and optical anisotropy of crystalline solutions outside the absorption band. This is due to the fact that the concentration broadening in crystals of this type (with only van der Waals interactions between the molecules) does not affect the integral oscillator strength of a transition. The mean polarizability approximation served as the basis for the procedure developed by Obreimov for the analysis of the composition of multicomponent systems as applied to a wide variety of isotopic mixtures, both liquid and crystalline (for the details, see (20)). 

An important phenomenon observed in amorphous plastics (also observed in optical glass) is the development of optical anisotropy due to stress. The stress-optical characteristic of transparent plastics is the basis of the important technique of photoelasticity by which stress and strain in complicated shapes, for which no analytical solution is readily available, can be determined experimentally and simply. 

The development of H-bonded complexes is now considered. An influence of UV irradiation on optical properties of low-molecular-weight azobenzene-con-taining material (Fig. 2.4b) has been investigated (Aoki et ah, 2000) on the basis of such interactions. The first observation of photoinduced optical anisotropy in H-bonded complexes of azobenzene dyes and copolymers (Fig. 2.4b) has been recently demonstrated (Medvedev et ah, 2005). In this case, the induced anisotropy was stable, and the maximum dichroic ratio of 2 has been observed. A kinetics of the induction of birefringence (maximum value of ca. 0.01) in one of these complexes is shown in Fig. 2.5. An influence of H-bonding on the mesomorphic and photoorientation properties was recently demonstrated (Cui and Zhao, 2004). In this approach, the amorphous azopyridine side-chain polymer was converted into liquid crystalline polymers through self-assembly with a series of commercially available, aliphatic, and aromatic carboxylic acids (Fig. 2.4d). 

Fig. 11. Polarized light micrograph showing the bicontinuous pcrcolaing structure developed by SD for the PP/EPR mixture (50/50 wi/wl) unmixed at 200 C for 20 min. and subsequently crystallized by quenching the mixture in an ice-water bath (polarizer and analyzer l ing set in vertical and horizontal directions). Note that the percolating PP-richdomains contain the volume-filling spheruliltes and have high optical anisotropy. From N. Inaba, T. Yamada, S. Suzuki, and T. Hashimoto (1988).

Today the electrooptical properties of liquid crystals form well-developed branches both in the physics and technology of liquid crystals. In addition, electrooptical measurements are the basis of a number of precise methods for determining the physical parameters of a material, such as its elastic and viscosity coefficients, optical anisotropy, spontaneous polarization, flexoelectric coefficients, anchoring energies at interfaces, etc. 

The first observation of natural optical anisotropy was made in 1669 by Bartolinius in calcite crystals, in which light travels at different velocities depending on the direction of propagation relative to the crystal structure. The electrooptic effect, electric-field-induced anisotropy, was first observed in glass in 1875 by J. Kerr. Kerr found a nonlinear dependence of refractive index on applied electric field. The term Kerr effect is used to describe the quadratic electrooptic effect observed in isotropic materials. The linear electrooptic effect was first observed in quartz crystals in 1883 by W. Rontgen and A. Kundt. Pockels broadened the analysis of this relationship in quartz and other crystals, which led to the term Pockels effect to describe linear behavior. In the 1960s several developments... 

---