Photoresponsive behavior and self-organization of azobenzene-containing block copolymers

Abstract

Photoresponsive behavior and self-organization of poly(tert-butyl acrylate)-block-poly(6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate) (PtBA-PAzoMA) diblock copolymers in solvents and thin films were investigated using UV-vis spectroscopy, transmission electron microscopy, and small angle X-ray scattering. When PtBA-PAzoMA is dissolved in a solvent, the photoresponsive behavior strongly depends on the solvency. In a PtBA-selective solvent, PtBA-PAzoMA formed a micellar structure because of the unfavorable interaction between solvent molecules and PAzoMA blocks. The confinement of PAzoMA in the core of micelles reduced the kinetics of photoisomerization when compared with the PtBA-PAzoMA in a neutral solvent. Additionally, the confinement promoted the organization of azobenzene moieties into the H- and J-type aggregates. By contrast, PtBA-PAzoMA in neutral solvent adopted a fully extended conformation, resulting in a reduced association of azobenzene mesogens. When PtBA-PAzoMA was prepared in a thin-film architecture, the PtBA-PAzoMA phase separated into both lamellar and cylindrical structures, depending on the volume fraction of the azobenzenes. The phase-separated microdomains provided a confined geometry that led to a substantial reduction of both the photoisomerization rate and the cis isomer content in the photostationary state.