Reorganization of binary polymer brushes: Reversible switching of surface microstructures and nanomechanical properties

For a binary polymer brush layer, we investigated the morphological state, the structure reordering, and the nanomechanical properties as a function of treatment with selective solvents. Two incompatible polymers, poly(methyl acrylate) (PMA) and poly(styrene-co-2,3,4,5,6-pentafluorostyrene) (PSF), were randomly grafted one after another onto a silicon wafer via the "grafting from" method producing thick (20-150 nm) dense mixed brush layers. The resulting layers possessed a nanostructured surface exhibiting either complete vertical or a combination of vertical and lateral microphase segregation of the two components. The lateral and vertical reorganization of the mixed brush layer was observed to be quick (on the order of a few minutes) and reversible for at least 100 "switches" between good and bad solvent states for each component. Atomic force microscopy (AFM) images revealed different surface structure states upon exposure to different solvents. Since PSF and PMA are mechanically dissimilar (glassy and rubbery, respectively) at room temperature, phase imaging was used to roughly verify the resulting structure. However, to determine vertical segregation in addition to truly authenticating the lateral ordering, surface nanomechanical mapping was conducted, which also allowed, for the first time, to directly determine the elastic modulus and adhesion. Results show the bimodal response of the mechanically heterogeneous surface, with elastic modulus and adhesion distributions very different for the "glassy state" and the "rubbery state". Furthermore, depth profiling of the elastic modulus conducted for binary brushes confirmed the vertical segregation in the mixed brush. Results demonstrated the dramatic mechanical contrast of the surface as a function of solvent conditions and decisively revealed the modes of phase segregation in a binary polymer brush.