Manipulating the structures and photoresponsive properties of an azobenzene-containing triblock copolymer by changing solvency and adding salts

Abstract

We examined the structure–property relationship of a triblock copolymer comprising azobenzene sequences, namely, poly(6-[4-(4′-ethoxyphenylazo)phenoxy]hexyl methacrylate)-block-poly(ethylene glycol)-block-poly(6-[4-(4′-ethoxyphenylazo)phenoxy]hexyl methacrylate) (PMMAzo-b-PEG-b-PMMAzo), in mixed dimethylformamide (DMF)/H2O solvents, with the effects of solvency and salts being the main focus. When H2O (a PEG-selective solvent) was added to DMF (a neutral solvent), the poor compatibility between H2O and PMMAzo molecules led to the formation of micelles comprising PMMAzo and PEG in the core and corona, respectively. Increases in the H2O content in the mixed solvent decreased the solvency, which caused increases in the micelle association number and hydrophobic core radius. The azobenzene moieties confined in the micellar core inhibited their photoisomerization, which caused decreases in the photoisomerization kinetics and cis isomer content. The addition of KCl to the PMMAzo-b-PEG-b-PMMAzo solution increased the hydrophobicity of PEG chains and the core radius and decreased the corona thickness. Moreover, the KSCN addition to the aforementioned solution increased the hydrophilicity of PMMAzo segments, which caused the core radius and corona thickness of micelles to decrease and increase, respectively. Salt addition disturbed the compact packing of mesogens and reduced the mesogen association. Consequently, with an increase in the salt concentration, the content of H-type mesogen aggregates increased, whereas that of nonassociated mesogens decreased.