Solvency and salt addition influence the photoresponsivity and fluorescence in an azobenzene-containing block copolymer

We investigated the effects of solvency and salt addition on the morphology, photoresponsivity, and fluorescence of the block copolymer poly(ethylene glycol)methyl ether-block-poly(6-[4-(4′-methoxyphenylazo)phenoxy]hexyl methacrylate) (PEG-b-PAzo). The morphology of PEG-b-PAzo was manipulated through dissolution in mixed solvents of dimethylformamide (DMF) and H₂O. DMF served as a good solvent for both blocks, whereas H₂O was a PEG-selective solvent. Increasing the concentration of H₂O in the mixed solvent increased the repulsive interaction between H₂O and PAzo chains, resulting in the assembly of PEG-b-PAzo into micelles composed of PEG in the corona and PAzo in the core. The confined geometry forced the mesogens to assemble into H-type aggregates. Adding KCl to the PEG-b-PAzo solution increased PEG segment hydrophobicity, causing an increase in the core radius and a reduction in the corona thickness. The opposite effect was observed when KSCN was added to the PEG-b-PAzo solution. The addition of salts led to slight reductions in H-type aggregate percentage and increases in nonassociated mesogen percentage. The fluorescence behavior of PEG-b-PAzo was closely correlated with a mesogen dispersion state. In the saltless PEG-b-PAzo solution, the increase in the percentage of the H-type aggregates, which was due to the addition of H₂O, caused fluorescence loss. The small changes in mesogen aggregation upon the addition of salts led to only a small change in fluorescence emission.