Depth-Resolved Analyses of Water Penetration During Dynamic Wetting in Solvatochromic Dye-Gradient Polymer Brushes by Time-Resolved Fluorescence

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The wide and detailed knowledge of processes that occur during dynamic (de)wetting of surfaces with liquids is of fundamental importance in chemistry, physics, biology, and engineering, in particular for the optimization, development and application of innovative, materials. Despite the progress gained with sophisticated experimental and theoretical approaches, however, even the most advanced models for dynamic (de)wetting rely on simplifying assumptions because of the lack of complete experimental data. Specifically, under dynamic wetting conditions, the main parameters of polymer softening, swelling and structural reorientation are thus far not adequately studied in terms of spatial and temporal resolution. In order to fill this knowledge gap, we propose an approach that monitors the dynamic wetting processes on dye-labeled polymer materials with precisely localized, tailor-made fluorophores, aiming at the delivery of complementary and significantly enhanced input for further development and refinement of models for dynamic wetting of adaptive surfaces.The proposed strategy is based on polymer brushes with controlled thickness and grafting density, that enable a controlled one-dimensional polymer swelling stimulated by solvent and/or temperature. As a key feature, these materials contain fluorosolvatochromic probes at defined depths as well as at the interfaces of the polymer with the supporting substrate and the wetting liquid. For that purpose, precisely tuned dyes will be developed whose emission energy, lifetime and quantum yields change significantly with the varying microscopic environment, thus providing a static and dynamic fluorimetric readout of the physical-chemical changes of the polymer during (de)wetting processes. As an important feature, the polymer brushes contain a dye x-gradient along which the distinct depth of the polymer-attached probes within the material varies linearly.In this set-up, the fluorimetric monitoring of wetting dynamics is facilitated and should enable a depth resolution with up to sub-10 nm precision by spatially resolved microscopy analysis. The confocal microscopic analysis of the emission energy and, in particular, the emission lifetime will reveal the diffusion dynamics of liquid into the polymer layer as well as the triggered polymer chain relaxation. Thereby, the dye-labeled gradient polymer brushes constitute a novel system that enables the fluorimetric analysis of dynamic wetting processes and further provide complementary data sets for critical tests and refinement of existing theoretical models, namely by linking the internal dynamics and relaxations of adaptive polymers, as obtained in this project with high local control, to physical quantities, such contact angles, velocity, drop dimension acquired versus sliding velocity.