Embedded donor polyimide

The embedded donor polyimides were prepared by the classical condensation route shown in Scheme 3 28). The flexibility of the polymer could be tuned somewhat by the selection of the dianhydride. Although a wide variety of soluble polyamic acids could be prepared by this method, in practice most of the polyimide derivatives were insoluble in common organic solvents. The two derivatives (PI-5 and... 

Scheme 2. The synthesis of the monomer M-5 used in the preparation of embedded donor type NLO polyimides.

Figure 7. Arrhenius plots for some tethered and embedded donor NLO polyimides (- -) PI-5, (-0-) PI-6, (-V-) PI-2b, (- -) PI-2a.

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. 

To study the polymer structural effects on PAP, including the size of the chromophore and the mode of connection of the chromophore to the main chain, SHG-PAP studies have been performed both on a true side hain polyimide system in which the chromophore is attached to the polymer backbone via flexible tedier (PI-3a in Figure 8.4 Tg = 228 C), and on donor-embedded polyimide syst s (PI-1 and PI-2 in Figure 8.4 Tg = 350 and 252°C, r pectively), in which the azo chromophore is incorporated into the polymer backbone through the donor substituent without any flexible connector or tether. Details of SHG-PAP can be found in reference 14. In both the side-chain and the donor-embedded polyimides, polar order can be generated by thermal poling... 

Similar PID results have been reported in very high Tg (up to SSO C) azo-polyimides. It has been shown previously that irradiation of the donor-embedded polyimide derivatives with polarized light alone at room temperature induces a quasi-permanent nonpolar orientation, which can be thermally erased only by heating the polymer above Tg. While, the lifetimes of the polar order generated by thermal poling of the donor-embedded polyimides were found to be on the order of tens of years to centuries at room temperature, photoisomerization can efficiently depole these polymer films in a matter of minutes at room temperature. Indeed, Figure 8.6 shows the effect of p-polarized irradiation on the SH signal of PI-2, which had been previously thermally... 

NLO chromophore-functionalized polyimides have attracted a lot of interest thanks to their high T., and excellent temporal stability. Verbiest et al. [72,73] and Miller et al. [74] reported several highly stable aromatic polyimides (Fig. 4.3), including the synthesis of the PI-1 polymer, which is the first reported example of a processable donor-embedded side-chain polyimide having a very high Tg of 350°C, and a chemical stability at temperatures as high as 350°C. Poled samples of polymer PI-1 have an EO coefficient of 4-7 pm/V (at 1.3 im) and a much higher orientational... 

FIG. 8.4 Chemical structures of the azo-polyimide polymers. PI-1.Tg = 350°C,and PI-2.Tg = 252°C, are donor-embedded systems. Pt-3a,Tg = 226°C,and PI-3b,Tg = 2IO°C,are side-chain polymers.Along the main chain of PI-2, there is some flexibility, which lowers theTg in the vicinity of PI-3. 

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