Segmental optical anisotropy

In addition, the optical anisotropy of the statistical segment Aa calculated [50, 51] from the stress-optical coefficient C,... 

It is impossible, apparently, to discuss the phenomena of elasticity, optical anisotropy and dielectric permittivity of polymer without referring to motion of the Kuhn s segments. Indeed, for example, from the time of classical... 

Abstract Macromolecular coils are deformed in flow, while optically anisotropic parts (and segments) of the macromolecules are oriented by flow, so that polymers and their solutions become optically anisotropic. This is true for a macromolecule whether it is in a viscous liquid or is surrounded by other chains. The optical anisotropy of a system appears to be directly connected with the mean orientation of segments and, thus, it provides the most direct observation of the relaxation of the segments, both in dilute and in concentrated solutions of polymers. The results of the theory for dilute solutions provide an instrument for the investigation of the structure and properties of a macromolecule. In application to very concentrated solutions, the optical anisotropy provides the important means for the investigation of slow relaxation processes. The evidence can be decisive for understanding the mechanism of the relaxation. 

Le., it is equal to one third of the optical anisotropy of the Kuhn segment In a system of coordinates of the end element it is twice as small. 

For kinetically flexible chains of relatively high molecular weight M, when the values of (nj/lnl do not vary with M, these values can be used for the determination of the optical anisotropy of a segment aj - 012 = pA according to Kuhn s equation ) equivalent to Eq. (59) if bFo/F, is taken to be unity and x— >... 

Since photoisomerization involves motion of one-half of the probe molecule against the other, it remains unaffected by the overall motions of the organized assemblies, such as the motion of different segments of DNA, or of the peptide chains of a protein or the surfactants in a micelle. In contrast, in optical anisotropy studies, the bending and twisting motion of the macromolecular chains of DNA, proteins, or micelles become superimposed on the orientational motion of the probe. As a result, photoisomerization seems to be a better probe for the microviscosity of complex biological assemblies. 

Noncrystalline chain segments in fibers may also show orientation in the direction of fiber axis. The extent of this amorphous orientation can be estimated from the total orientation in combination with the crystallite orientation as obtained from x-ray analysis. Although spectroscopic, sonic, and NMR methods have been described for the determination of the general orientation, the most facile method is based on optical anisotropy [304]. This fiber property is characterized by the optical birefringence factor An, defined by the following equation ... 

Table 5 lists the number of monomer units in a segment s =A/x, where X is the length of projection of the monomer unit on the axis of the molecule. For polyacrylic chains, X is 2.5 A. In the series presented in the Table, the value of optical anisotropy increases sharply thirty-fold. As shown previously, this reflects the generation of orientational order in the molecule. In this case, attention is drawn to the fact that the increase in... 

An is the difference in the refractive indices n in the plane of the dra i/ing direction and n perpendicular to iti n = (n, + 2n )/3 is the mean refractive index of the sample and Aa the difference of the polarizabilities of the statistical segment (Aa = optical anisotropy) parallel and perpendicular to the axis of the segment. The quotient of orientational birefringence and stress is defined as the stress optical coefficient C. From the simple model of the network chains the product CT (equation 3) should be independent of T if a slight temperature dependence of n is neglected. CT is proportional to the optical anisotropy of the statistical segment. 

Rubber elasticity arises from the orientation of chain segments, and the degree of this orientation underlies the mechanical properties. The most facile way to quantify the orientation is from the (optical) birefringence, defined as the difference in refractive indices for two perpendicular directions. An = ny -tij. This birefringence depends on the optical anisotropy of the chain units, as well as their degree of orientation... 

Optical anisotropy (segmental) ai — Q2) cm Cis-trans conformation (assumed) 160 X 10 (49)... 

The depolarized spectrum is dominated by local fluctuations of the optical anisotropy owing to an overall orientation of stiff molecules, local motions of optically anisotropic segments or internal Ronse-Zimm modes for flexible chain molecnles. The depolarized spectrum thus reflects dynamics of collective molec-nlar orientation when the interaction between translation and rotation can be neglected. 

Relation (156) reflects the fact that both the stresses and the optical anisotropy of a polymeric liquid under motion are determined by the mean orientation of the interacting segments. One can use expression (127) for the stress tensor to write... 

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