Photoinduced Alignment under Solvent Vapor Annealing (PA-SVA): Enhanced Ordering and Patterning in Block Copolymer Films

In the past few decades, alignment control and directed self-assembly of block copolymer thin films have been widely investigated. Most approaches, however, require chemically or topographically prepatterned substrates, increasing the processing complexity. In this work, we present a facile strategy to form well-aligned block copolymer thin films by photoinduced alignment under solvent vapor annealing (PA-SVA) without chemically or topographically prepatterned substrates. Polystyrene-block-poly(ethylene oxide) (PS-b-PEO) consisting of two polymer blocks with distinct hydrophilicities and mechanical properties is incorporated with 4-butyl-4′-hydroxyazobenzene (4AzOH) to achieve the light-assisted self-assembly. Under solvent vapor annealing, the PS-b-PEO chains are swollen and possess mobilities. Upon polarized light irradiations, light-responsive azobenzene molecules undergo trans–cis–trans isomerization and achieve photoinduced alignment. The arrangement of the azobenzene molecules can further induce the alignment of the PS-b-PEO chains through supramolecular interactions between the phenol groups of the 4AzOH and the ether groups of the PEO chains. AFM-IR measurements are applied to characterize the selective blending of the 4AzOH molecules in the PEO segments. With different PA-SVA times, the transitions of the PS-b-PEO phase separation can be observed; the alignment of the PS-b-PEO/4AzOH films is further characterized by orientation analysis. The facile design of the PA-SVA strategy not only enables the manipulation of light-responsive molecules in block copolymer chains but also provides an alternative way for obtaining well-ordered block copolymer thin films.