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Quantifying and Controlling the Mechanical properties of polyelectrolyte multilayer capsules: Towards mechanosensitive microcapsules A. Fery MPI f. Colloids and Interfaces Golm Quantifying the mechanical properties of microcapsules is of interest for both a basic science understanding of those systems and for their applications. Provided the mechanical properties can be controlled, new “smart” materials like “mechanosensitive” capsules that change their mechanical properties in reaction to changes in their (solvent) environment could be feasible. In this context, hollow capsules made from polyelectrolyte multilayers are promising systems, because they can be produced with well defined geometry, wall thickness and from a broad variety of materials. Further the interactions that are responsible for the cohesion of the layers are non-covalent and could thus be influenced by changes in the solvent environment. We use a variant of atomic force microscopy to directly probe the deformability of individual microcapsules in solvent environment. We show how elastic constants of the microcapsule wall material can be derived from the measured force-deformation- relations by employing a continuum mechanics model and test the scaling predictions of our model by varying the capsules wall thickness. In a second step, we investigate the response of the capsules towards changes in the salt concentration and pH of the environment. Here we observe a significant impact of both factors on the stiffness for the system PAH (Polyallylamine)/PSS (Polystyrenesulfonate). We discuss the effects in terms of simple molecular pictures. Finally, we show that control over the mechanical properties of the capsules can also be used to fine tune the adhesion properties of the microcapsules.
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