Morphological Evolution of Poly(solketal methacrylate)-<i>block</i>-polystyrene in Thin Films

Driving the size scale of block copolymer (BCP) microdomains to the nanoscopic level poses numerous challenges in obtaining thin films with highly oriented, laterally aligned structures of large areas. Here, we used the morphological evolution of the lamellar microdomains in thin films of symmetric poly(solketal methacrylate-b-styrene) (PSM-b-PS) BCPs that are converted into poly(glycerol mono-methacrylate-b-styrene) (PGM-b-PS) BCPs by the use of a photoacid generator dissolved in the thin film. This simple hydrolysis, performed in the solid state, causes a marked increase in the segmental interaction parameter, converting a hydrophobic-hydrophobic BCP into a hydrophobic-hydrophilic BCP, wherein the segmental interaction parameter (χ) increased from 0.035 to 0.438 at 25 °C. To orient the microdomains normal to the film surface a hydroxyl-terminated random copolymer (PSM-r-PS) where the fraction of the mers can be varied to tune the interfacial interactions with the substrate and where the (PSM-r-PS) is simultaneously transformed into PGM-r-PS along with the BCPs. The use of the photoacid generator alleviates the need to use toxic vapors and enables a transformation of the BCP in a gradient manner across the substrate that can be used to promote long-range lateral ordering. As the BCP period decreases, the film thickness also must be decreased. Consequently, the ability to use a fully hydrophobic significantly enhances the ability to prepare uniform thin films over large areas.