Self-structured surface patterns on epoxy-based azo polymer films induced by laser light irradiation

Article abstract of DOI:10.1021/ma200832b

In this study, two series of epoxy-based azo polymers with high chromophore density were synthesized and self-structured surface pattern formation on the polymer films was studied by laser light irradiation under different conditions. To synthesize the azo polymers, two epoxy-based precursor polymers (PEP-AN and PEP-35AN) were prepared by step polymerizations of N,N-di(epoxypropyl)aniline and N,N-di(epoxypropyl)-3,5-dimethylaniline with aniline and 3,5-dimethylaniline, respectively. The azo polymers were obtained through postpolymerization azo-coupling reactions between the precursor polymers and diazonium salts of 4-chloroaniline, 4-aminobenzonitrile, 4-nitroaniline, and 2-methyl-4-nitroaniline. The epoxy-based precursor polymers and azo polymers were characterized by using ¹H NMR, FT-IR, UV-vis, and DSC analyses. The self-structured surface pattern formation on films of the two series of azo polymers was studied by irradiating the polymer films with a normal-incident laser beam at two different wavelengths (488 and 532 nm). The results show that the photoinduced surface pattern formation is closely related with the structure of azo chormophores, excitation wavelength, and light polarization condition. The efficient excitation wavelength is closely related with the absorption band position, which is mainly determined by the electron-withdrawing groups on the azo chromophores. The methyl substituents on the azo chromophores can enhance the photoinduced surface pattern formation ability in some cases. For comparison, the surface-relief-grating (SRG) formation was studied by irradiating the polymer films with interfering laser light. The SRG formation rate is also dependent on the azo chromophore structure and wavelength of the incident laser light. The self-structured surface pattern formation needs a higher energy input and shows stricter wavelength requirement compared with those of the SRG formation. These observations could lead to the deeper understanding of the mechanism of the self-structured surface pattern formation and development of materials with better performance.