Polymer brushes in competing solvents: phase transitions and switching.

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The mixture of two good solvents which are competing in wetting the polymer can lead to collapse of polymer chains and to phase transitions in elastic polymer materials. This phenomenon is called ''co-non-solvenc'' in the literature, and can open novel options for the switching of polymer materials such as gels and polymer layers under a small variation of the solvent composition. On the other hand, complex multi-component solvent mixtures are a characteristic property of living systems too. In this project we want to study and to understand the properties of densely grafted polymer brushes under the influence of co-non-solvents both using computer simulations and analytical models. Besides their technological potential polymer brushes being an excellent and well defined model system to study collective properties and phase transition in polymer physics. We start to analyze and rationalize the conditions of discontinuous phase transitions of a single polymer brush in the presence of a co-non-solvent. After that polymer brush double-layers consisting of two oppositely oriented brushes shall be studied. This is related with the possibility to investigate switching of this nano-chanel under equilibrium conditions. Eventually, we want to study the non-equilibrium properties of such channels under the condition of simple flow of the solvent mixture. Here, it is of particular interest to investigate the possible switching of the channel by changing the flow conditions, i.e. using the channel as a possible stimuli-responsive valve. Molecular dynamics simulations based on a coarse-grained polymer model will be used and further developed for the given problem. Essential elements of the implementation of this model have been developed in preliminary work, as well as a simplified theoretical model for the description of the collapse transition in a homogeneous polymer brush. The ultimate goal of the project is to develop concepts for switching of tailored nano-channels using solutions of competing solvents and to achieve a universal understanding of ''co-non-solvency" in polymer brushes.