Polymer Structural Effects on Photo-Orientation

Photo-orientation results in anisotropy and depends on the environment of the photoisomerizable chromophore. To increase the stability of photo- 

1 Fhot merization and Photo-Orientation of High-Temperature Azo-Pol fintides 

For both PI-1 and PX-2, assuming that n e (n, mean refractive index) for optical frequencies, the mean dielectric constant, (e) decreases upon irradiation (A pi = - 0.023, A pi 2 - - 0.016). The dielectric constant is proportional to the chromophore density and the decrease upon irradiation suggests a quasi- 

By means of comparison, in related experiments on PI-3 a and PI-3 b, the mean absorbance returned to its initial value within two minutes after removal of the irradiation light. Clearly, the polymer rigidity intrinsic to the donor-embedded samples must affect the rate of isomerization of the cis azobenzene derivative, suggesting that motion of the chromophore and the polymer backbone are somehow coupled. 

12 TE (A) and TM (B) waveguide modes coupled into PI-1. Because the angular position of theTM modes didn t change appreciably after iitadiation, only TM modes before irradiation are shown. After reference 47, redrawn by permission of OSA. 

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A third aspect of photo-orientation in films of solid polymers is how photo-orientation is influenced by the polymer structure, molecular environment, and film configuration. Polymer structural effects on photo-orientation as well as the effect of intermolecular interaction and free volume are discussed in other chapters of this book. 

The effect of the structure of the polymer backbone on photo-orientation can be seen from the dynamic behavior as well as from the steady-state values of the photoinduced anisotropy in all azo-PURs. The photo-orientation dynamics of PUR-2 resemble but also contrast with those of PUR-1. In PUR-2, AbSi exceeds AbsQ, but not quite, as is the case for PUR-1, and the photostationary-state anisotropy is smaller than that of PUR-1, as can be seen in Figure 4.20. PUR-1 and PUR-2 exhibit exactly the same extinction coefficient at the analysis wavelength because they have the same azo chromophore furthermore, the rate of the cis- trans thermal isomerization is nearly the same in both polymers. The seemingly small difference into the... 

This chapter is organized as follows. Section 4.2 addresses the study of photoisomerization and photoinduced orientation of azobenzene molecules at the molecular level in SAMs of azo-silane molecules. Section 4.3 discusses photoinduced effects in supramolecular assemblies, i.e., LBK multilayer structures containing azobenzene molecules, and compares the photoinduced movement of azobenzenes in these structures to that observed in spin-cast films. Section 4.4 focuses on the isomerization and sub-Tg photoinduced orientation in a series of very high Tg (up to 350°C) nonlinear optical polyimide and thermoplastic donor-embedded polyurethane polymers containing azo dye, especially focusing on polymer structure-Tg-photoinduced molecular movement relationships. Section 4.5 describes pressure effects on photoisomerization and photo-orientation in films of a PMMA polymer containing azo dye. Finally, we make some concluding remarks in Section 4.6. 

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