Quantifying and Controlling the Mechanical properties of by liwenting


									Quantifying and Controlling the Mechanical properties of
polyelectrolyte multilayer capsules: Towards mechanosensitive
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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