Highly structured pH-responsive honeycomb films by a combination of a breath figure process and in situ thermolysis of a polystyrene-block-poly(ethoxy ethyl acrylate) precursor

Abstract

In the present work, we show that Cu(0)-mediated controlled radical polymerization is a suitable method to synthesize high molar mass polystyrene-b-poly(ethoxy ethyl acrylate) PS-b-PEEA diblock copolymers. This method, applied at room temperature, is mandatory for complete preservation of ethoxy ethyl protecting groups during the course of polymerization. The synthesized PS-b-PEEA diblock copolymers were subsequently used for the elaboration of pH sensitive hierarchically structured honeycomb (HC) films through the Breath Figure (BF) process. The PS-b-PEEA hydrophobic honeycomb films were characterized by optical microscopy and atomic force microscopy (AFM) to reveal the hexagonal array of pores at the micrometer length scale, together with the phase segregation of the diblock copolymer. Similar to highly structured natural materials, the biomimetic honeycomb polymer films displayed intense iridescence. Moreover, the increase of surface roughness by peeling off the top layer of the PS-b-PEEA HC films produced superhydrophobic surfaces exhibiting a water contact angle of 155°. Subsequent deprotection of PEEA into pH-responsive poly(acrylic acid) (PAA) was performed in situ from the PS-b-PEEA honeycomb film by a simple thermolysis step carried out at 90 °C. The resulting PS-b-PAA honeycomb films showed a clear pH-responsive behavior with a water contact angle gap of 65° between a pH of 3 and 10.