Epoxy-based azo polymers with high chromophore density: Synthesis,characterization and photoinduced birefringence

Three epoxy-based azo polymers (PEP-AZ-Cl, PEP-AZ-CN and PEP-AZ-NT) with high chromophore density were synthesized by using post-polymerization azo-coupling reactions between epoxy-based precursor polymer (PEP-AN) and diazonium salts of 4-chloroaniline, 4-aminobenzonitrile and 4-nitroaniline, respectively. The structures and properties of the azo polymers were characterized by using ¹H-NMR, FT-IR, UV-Vis and thermal analyses. The photoinduced birefringence of the azo polymers was studied by irradiating spin-coated films of the polymers with laser beam at three different wavelengths (488, 532, and 589 nm). The results indicate that the photoinduced birefringence of the azo polymers is related with the electron-withdrawing group on azo chromophores and the excitation wavelength. The excitation wavelength that can cause the efficient responses is determined by the absorption band positions of the azo chromophores, which are mainly affected by the electron-withdrawing group on the chromophores. Therefore, the azo polymers containing chromophores with different electron-withdrawing groups show different responsive behavior to the irradiation light at different wavelengths. When irradiated with 488 nm light, PEP-AZ-Cl shows the shortest time to reach the saturated birefringence but with the lowest saturation birefringence level compared with the other two azo polymers. When irradiated with 532 nm light, PEP-AZ-CN shows the shortest time to reach the saturated birefringence. When irradiated with 532 and 589 nm light, PEP-AZ-NT shows the highest saturation birefringence level.