Domain Memory of Mixed Polymer Brushes

Abstract

The nano-phase-separation in mixed polymer brushes consisting of polystyrene and poly(methyl methacrylate) (PS-PMMA) chains attached to a silicon surface is studied. The topographies of the mixed brushes are examined after they have been exposed to solvents which induce or erase nano-phase-separation. It is discussed whether the brush locally forms the same pattern every time the transition from the smooth and featureless to the nanopatterned state occurs ("domain memory") or if the local assembly of the domains emerges in a different arrangement after each cycle of topography switching. A memory measure parameter is introduced, which characterizes quantitatively the domain memory effect in the nanopattern. It is shown that at constant grafting density but with increasing molecular weight of the brush chains the memory measure parameter decreases. In contrast to this, brushes with constant molecular weight, but differing in grafting density, all have a similar domain memory. We discuss a possible origin of the domain memory effect in the mixed brush systems studied and point out its impact on the motion of nanoparticles adsorbed on top of such systems.

Dynamically Reconfigurable Polymer Films: Impact on Nanomotion

“…4) (Movie S2 in Supporting Information). [31] For the system shown here, the maximum length of one step was 200 nm (the third cycle in Fig. 4); the average length of one step made by the sphere was 100 nm.…”

“…For the particular brush described above, a certain area on the brush was selected and its topography was acquired after each cycle of acetone/toluene exposure. [31] When comparing single domains over several switching cycles, it was found that some recover almost completely with only a slight variation in shape, but others turn into a dissimilar pattern (Fig. 3).…”

“…2c and d). [29][30][31] We have carefully checked that the redistribution is not induced by other relevant processes, such as thermal diffusion or capillary forces, the impact of which have been addressed by additional experiments. Diffusion could be excluded by repeating the experiment on homopolymer brushes that do not show changes in topography when being exposed to different solvents: the particles stick to their initial positions.…”

“…The brush shows microphase separation into patterned states of different morphology depending on the selectivity of the solvent. [31] The topographical changes result in redistribution of the polymers PS and PMMA on the top of the brushes. [29,30] It was shown that after treatment with acetone the surface of the brush consists of only PMMA chains, that is, the patterns have a "dimple"-like structure www.advmat.de Figure 2.…”

“…Figure 4 shows three silica spheres 80 nm in diameter on top of the patterned brush surface. [31] Before adsorption the brush was exposed to acetone solvent, so that the surface of the brush consists mostly of PMMA chains. The spheres in the left-upper corner and in the middle were found to be adsorbed in cavities between the patterns, while the third sphere in the bottom-right edge of the microscopic image is situated on top of the underlying pattern (Fig.…”