Localized surface plasmon resonance effect on photo-induced alignment of films composed of silver nanoparticles and azopolymers with cyano or methyl substitutes on azobenzene moieties

In this paper, we report the localized surface plasmon resonance of silver nanoparticles on the photo-induced alignment properties of azopolymers. Two series of azopolymer films doped with silver nanoparticles (SNPs) were prepared with different contents of SNPs, in which two side chain azopolymers, with cyano group (AzoCN) and methyl group (AzoCH₃) as substitutes, respectively, were designed and synthesized because of the different interaction between SNPs and each kind of substitute. Tapping-mode atomic force microscope imaging was used to characterize the distribution of SNPs in azopolymer films, from which it was found that in AzoCN film SNPs were almost uniformly distributed, whereas in AzoCH₃ film several decades of single SNP aggregated into a cluster. Photo-induced alignment of azopolymer films doped with SNPs was performed under irradiation of linearly polarized light at 442 nm. The experimental results reveal that there is an obvious difference in photo-induced alignment behavior between two series of azopolymer films. For AzoCN, the alignment rate decreased with the increase of SNPs concentration, and when the SNPs' concentration achieved 0.24% the ratio of alignment rate reached the minimum, about 77% of that of the undoped sample. For AzoCH₃, the alignment rate increased along with the increase of the content of SNPs, showing that the alignment rate, at least for “fast” process, could be enhanced by doping SNPs. This phenomenon resulted from localized surface plasmon resonance of SNPs and was also found to be affected by the chemical structure and the condensed state of azopolymers doped with SNPs.