Anisotropy polymer orientation, dichroism

Thus, the process of PAN transformation under the effect of IR radiation proceeds with considerable self-acceleration. The irradiation of uniaxially oriented PAN films gives a polymer with a distinct anisotropy of optical properties, dichroism in the visible spectral region in particular. Figure 8 presents dichroism curves [D =/(X)] at various angles (ip) between the polarization plane and the orientation axis. The same figure shows the dependence D =f(uniaxially oriented film. 

The resulting monoliths displayed dichroism, with a maximum coinciding with that of the maximum of the absorption band of the (unreacted) template molecule. The observed anisotropy of the monoliths (Ah. = h — h, ca. 0.02 AU) was comparable with that of common oriented polymer films. 

The definitions given above for An and Ca are in terms of the anisotropies in the optical properties over the wavelength range of visible light. Similar anisotropies can also exist or can be induced in other wavelength ranges. For example, infrared dichroism (the absorption of different amounts of polarized infrared radiation in the directions parallel and perpendicular to the direction of orientation) is useful in characterizing polymers by vibrational spectroscopy. 

An independent verification of this procedure was possible for lb [82]. The four n-propyl substituents ensure that this molecule can be oriented to a high degree in stretched polymer sheets. Linear dichroism (LD) measurements on the aligned samples resulted in the determination of transition moment directions. A pattern similar to that of parent 1 was obtained. Both LD and emission anisotropy procedures yielded similar values, which, additionally, confirmed that the assignment of fluorescence depolarization to excited state tautomerization was correct. 

One of the most interesting photoresponsive properties of azo polymers is the photoinduced birefringence and dichroism (Xie et al., 1993). The photoinduced anisotropy is caused by the disparity of the repeated trans-cis isomerization of azo chromophores under linear polarized light irradiation. The most efficient excitation occurs in the polarization direction, which can force the chromophores to continually change their orientation and to be eventually stabilized at the direction perpendicular to the polarization (Natansohn and Rochon, 2002). The effect shows potential applications in areas such as reversible optical data storage, optical switching and sensors. 

The optical anisotropy, as characterized by the difference between the absorption of IR light polarized in the directions parallel and perpendicular to the reference axis (i.e., the direction of applied strain), is known as the IR linear dichroism of the system. For a uniaxially oriented polymer system [10, 28-30], the dichroic difference, A/4(v) = y4 (v) - Ax v), is proportional to the average orientation, i.e., the second moment of the orientation distribution function, of transition dipoles (or electric-dipole transition moments) associated with the molecular vibration occurring at frequency v. If the average orientation of the transition dipoles absorbing light at frequency is in the direction parallel to the applied strain, the dichroic difference AA takes a positive value on the other hand, the IR dichroism becomes negative if the transition dipoles are perpendicularly oriented. 

In another type of application a low molecular fluorescent probe is added to a system containing macromolecules. As would be expected, the rotation of a small species is insensitive to the molecular weight of high polymers, but depends on the "microscopic viscosity" which is a function of free volume. For instance, Nishijima has shown that the microscopic viscosity of liquid paraffin hydrocarbons levels off for molecular weights above 1000 and that the microscopic viscosity of polystyrene containing 10 volume"/ benzene is only 200 times as high as that of benzene (15). Nishijima also showed that the emission anisotropy is a useful index of molecular orientation. Since both the excitation and the emission are anisotropic, the method yields the fourth moment of the distribution function of orientations, while other optical properties (dichroism, birefringence) depend on the second moment (15). 

However, for a certain polymer, the signs and patterns of the CD spectra were unchanged, hi addition, changing the direction and orientation of the film sample did not significantly affect the spectral intensity and pattern. Furthermore, the films did not show birefringence in polarized optical microscopy analysis. These observations rule out the possibility that the CD spectra are due to linear dichroism based on film anisotropy and support that the film spectra reflect molecular chirality. CD absorptions similar to those indicated in film were observed also for a decalin suspension of THF-insoluble polymer (fine power ground with a mortar) synthesized using (-)-Sp-FILi. This further supports that the CD spectra in film (Fig. 34) do not arise from sample anisotropy. 

Measruement of IR dichroism (i.e., a directionally preferred absorption of optical energy) has been well established as a convenient spectroscopic technique for detecting anisotropy of a sample created by preferential molecular orientations. " The specificity of IR absorption bands to individual submolecular groups makes this technique especially suitable for the detailed study of segmental orientations of polymer chains and their dynamics. Absorption of IR radiation is caused by the interaction of the electric field vector of the incident light with the electric dipole transition moment associated with a... 

---