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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.

We report the results of atomic force microscopy (AFM) based nanoscale probing of thermal and nanomechanical properties of relatively thick (50−90 nm) polymer brush layers from poly(styrene-co-2,3,4,5,6-pentafluorostyrene) (PSF) and polymethylacrylate (PMA). These layers, with a high density of grafting, are synthesized according to a “grafting-from” approach on a silicon surface modified with a reactive self-assembled monolayer. In the dry state, glassy and rubbery brush layers are found to be homogeneous matters with no signs of lateral chain segregation, which is observed for polymer layers with low to moderate grafting densities. We observed that thermal, mechanical, and thermoelastic properties of these polymer brush layers are virtually identical to that for unconfined polymers obtained concurrently via bulk polymerization. Direct measurement of heat dissipation and the thermoelastic response within the PSF brush layer confirms that the glass−rubber transition occurs between 100 and 110 °C as expected for the high-molecular weight polymer. Surface nanomechanical mapping reveals much lower adhesion of the PSF layer, which is glassy at room temperature and contains fluorine-enriched segments, in comparison with the sticky PMA layer containing polar segments. At room temperature, the PSF layer shows a compression elastic modulus of approximately 1 GPa whereas the rubbery PMA layer has an elastic modulus of 50 MPa, typical for the rubbery state. Heating the glassy PSF layer results in a gradual decrease of the elastic modulus caused by the glass transition, and conversion to the rubbery state is completed above 110 °C with an elastic modulus of 15 MPa.

We report a simple method to synthesize binary polymer brushes from two incompatible polymers of different polarity. The synthetic route is based on a subsequent step-by-step grafting of carboxyl-terminated polystyrene and poly(2-vinylpyridine) to the surface of a Si wafer functionalized with 3-glycidoxypropyl-trimethoxysilane. The end-functional polymers were spin-coated on the substrate, and grafting was carried out at a temperature higher than the glass transition temperature of the polymers. The composition of the binary brushes can be regulated based on grafting kinetics of the first polymer by the change of time or/and temperature of grafting. This method reveals a smooth and homogeneous polymer film on the macroscopic scale, while at the nanoscopic scale the system undergoes phase segregation effecting switching/adaptive properties of the film. Upon exposure to different solvents, the film morphology reversibly switches from “ripple” to “dimple” structures as well as the surface energetic state switches from hydrophobic to hydrophilic. The same switching of hydrophilic/hydrophobic properties was obtained for the different ratios between two grafted polymers in the binary brush.