Photoresponsive nanostructures of azobenzene-containing block copolymers at solid surfaces

The self-assembly properties and photoresponsive behaviours of an azobenzene-containing block copolymer poly(acrylic acid-block-6-(4-(p-tolyldiazenyl)phenoxy)hexyl acrylate) (PAA-b-PAzo) are reported. As azobenzene has reversible trans–cis photoisomerization abilities, trans PAA-b-PAzo solutions were converted into cis PAA-b-PAzo solutions via UV irradiation. Thus, trans or cis PAA-b-PAzo films were prepared after spin-coating the associated solutions on freshly cleaned silicon wafers. The trans PAA-b-PAzo films self-assembled to produce a hemimicelle morphology with the PAA blocks as the matrix and PAzo blocks as the dispersed phase, which was driven by microphase separation. In contrast, the cis PAA-b-PAzo films generated nanostructures with smaller sizes than the trans PAA-b-PAzo thin films. Moreover, when under UV illumination for 10 min, the island-like hemimicelle assemblies in the trans PAA-b-PAzo films fused into a chain-like morphology based on the photoinduced solid-to-liquid transitions at the nanoscale. Our findings suggest a new method to control the microphase separation and morphologies of block copolymers.