Self-assembly and photo-responsive behavior of novel ABC ₂-type block copolymers containing azobenzene moieties

The self-assembly behavior and photo-responsive properties of novel azobenzene based copolymers poly(ethylene oxide) monomethyl ether-block- polystyrene-block-{poly[6-(4-methoxy-azobenzene-4′-oxy) hexylmethacrylate]} ₂ [MPEO-b-PS-b-(PMMAZO) ₂] with various block lengths of PMMAZO segments in aqueous media were studied by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM), laser light scattering (LLS) and UV-Vis spectrophotometry. It was found that the critical water content for the aggregate formation increases with the PMMAZO weight fraction. By tailoring the molecular structure and copolymer concentration, various self-assembled aggregates were obtained. At a lower concentration, all the copolymers self-assembled into simple vesicles. With increasing the polymer concentration, large compound vesicles were formed. The photo-sensitive properties of the formed aggregates were also studied. Upon irradiation with a polarized light, the aggregates were elongated. Large compound vesicles were found to deform into compact spindles with a very large axial ratio.