Photocontrol of supramolecular azo-containing block copolymer thin films during dip-coating : toward nanoscale patterned coatings

Abstract

Dip-coating allows nanostructured block copolymer (BCP) thin film fabrication in a fast and facile one-step process. It can also be coupled with external controls, such as illumination. Herein, we expose several design principles that enable photocontrol of the nanostructured surface pattern and thickness of supramolecular BCP thin films. This is done using a polystyrene–poly(4-vinylpyridine) (PS-P4VP) BCP and two hydroxy-functionalized small-molecule (SM) azo derivatives that have different photochemical characteristics and that hydrogen bond to the P4VP block. We show how the film preparation concept provides tunability through the chemical structure of the photoactive SM, the relative amount of SM in the dip-coating solution, and the choice of solvent. It was found that the film thickness and SM uptake in the films are increased by illumination when THF is used but are unchanged when toluene is used as solvent, which is attributed to an optical heating effect observable with volatile solvents. The photocontrol of surface patterns is a result of photoinduced changes in the effective volume fraction of the P4VP+SM phase, which is increased by a greater volume of cis isomers, by higher SM uptake (using THF), and by more trans–cis–trans cycling for systems with shorter cis lifetime. The extent of photoinduced change can also be increased by higher molecular mobility due to more flexible SMs, lower BCP molecular weight, and nonmicellar or softer micellar solutions.