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Svetlana Santer

Publications and source records attributed to Svetlana Santer.

4 recordsLinked to original sources
PubMed2007

Memory of surface patterns in mixed polymer brushes: simulation and experiment.

The correlation between the morphology of mixed polymer brushes and fluctuations of the grafting points is investigated by single-chain-in-mean-field simulations and experiments. The local topography of two types of mixed polystyrene-polymethylmethacrylate (PS-PMMA) brushes that differ in their modes of attachment has been studied during repeated microphase separation into laterally structured and homogeneous morphologies upon changing solvents. In the first type of brush (conventional), each of the surface-attached initiator groups starts the growth of either a PS or a PMMA chain in a random fashion. In the second case (Y-shaped mixed brushes), two chains of different types are attached to the same anchor group on the substrate. Whereas in the first case statistical fluctuations of the chemical composition occur on a local scale, such composition fluctuations are strongly suppressed in the latter case. The microphase-separated morphology is similar in both cases, but Y-shaped brushes exhibit a significantly weaker domain memory than do conventional PS-PMMA mixed brushes. The results of the experiment are compared with simulations, and a simple phenomenological argument and qualitative agreement are found. The observations demonstrate that small fluctuations in the grafting points are amplified by the microphase separation and nucleate the location of the domains in the mixed brush.

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PubMed2006

Domain memory of mixed polymer brushes.

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.

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PubMed2005

On the formation of molecular terraces.

Langmuir-Blodgett-Kuhn films of poly(amic acid) with azobenzene-chromophore side groups (azo-PAA) have been examined using atomic force microscopy (AFM). If films were deposited at specific conditions, i.e., at transfer pressures (pi = 10-26 mN/m) and at transfer rates (nu = 2-10 mm/min), unusual patterns were formed. While the first few transferred layers of azo-PAA exhibited uniform coverage of the underlying surface with a flat topography, successive layers form a pattern of parallel terraces with micrometer width, which are assembled in a pyramid-like structure. The height of the pyramidal steps is 1.6 nm, which corresponds to the thickness of an azo-PAA monolayer. Each even step of such a pyramid has a hydrophobic surface, and each odd step has a hydrophilic surface. The width of the terraces varies significantly from terrace to terrace; however, the width of an individual terrace remains constant over hundreds of micrometers. The orientation of the pyramidal terraces is governed by the film deposition process and is always perpendicular to the transfer direction.

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