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Thick & thin: physics of polymers with sticky ends.

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Associative thickeners are linear water soluble polymers with sticky (associating) parts; they are used frequently in formulated dispersions to regulate rheological properties. In their simplest form, they have just two sticky ends; these are the so-called telechelic polymers that we discuss in this lecture. Telechelics are interesting because they can form networks, the behaviour of which (in bulk and at interfaces) is remarkably rich and tunable. We have studied solutions of polyeethylene polymers with alkyl groups at eithet end: a widely studied, almost classical system. Above a certain thershold concentration these solutions become strongly shear thinning visco-elastic fluids; we show that the shear thinning property also leads to some sort of spontaneous self-organisation of the sheared material, called shear banding. Moreover, the self-organisation leads to stress fluctuations with a characteristic statistical pattern reminiscent of that of earthquakes. A simple physical picture of chain pull-out leads to a constitutive equation that can quite well account for the data. At phase boundaries, the self-assembling behaviour leads to adsorbed layers with strong internal cohesion and structure, and where such layers form between two surfaces, surface forces show up with rather unusual features. The physical picture used for the bulk flow properties can be used again to undertand these surface properties. We finally discuss new telechelics with entirely different association mechanisms, either based on charge, or on triple helix formation. These new systems offer rather interesting options for hydrogels with unusual tuning characteristics.

This talk is part of the BSS Formal Seminars series.

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