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The Physics of Stimuli-Responsive Polymer Brushes Probed with Optics
Advanced functional substrates designed from polymer brushes have known a rapidly growing interest over the past fiteen years, with a particular attention paid to the elaboration of brushes made of stimuli-responsive polymers, in order to design smart surfaces whose properties can be altered on demand via the control of an external parameter. This has fostered research focusing on the use of stimuli-responsive polymer brushes to control the interactions between a surface and biological objects. Many of these biological applications rely on the fact that adsorption of proteins on a brush-grafted surface depends strongly on the conformation of the tethered chains, and can thus be modulated through appropriate variations of the external stimulus.The rational design of such brushes thus requires to address these central questions : how does the conformation of the grafted chains depend on the parameters of the brush (e.g. its grafting density), and how do such parameters influence the response of the brush to the applied stimulus, its overall structure, and in fine its usage properties for the target application ? This calls in particular for the development of experimental techniques that allow to combine the quantitative outputs of e.g. neutron reflectometry with the ease of access and implementation of laboratory-scale setups, in order to quickly and efficiently screen the effect of the molecular parameters of a brush on its response. In this context, I will present a recent study performed in collaboration with Delphine Débarre at LIPhy. I will describe an original optical setup, based on the well-known Reflection Interference Contrast Microscopy, that allows us to perform space- and time-resolved measurements of the reflectivity of brush-coated surfaces, over the visible spectrum. I will show that such measurements can give quantitative access to the density profile of a brush, and how we have applied this technique in order to investigate the thermal response of brushes made of PNIPAM, one of the most studied thermoresponsive polymer. We are thus able to probe in details how the conformation of PNIPAM brushes varies as the temperature is change across the polymer Lower Critical Solution Temperature, and we provide one of the very few experimental evidence for a phenomenon called “vertical phase separation” within the brushes, which has been theoretically predicted several years ago.