Mechanical properties of polyelectrolyte microcapsules filled with a neutral polymer

We study the deformation of "filled" (with a neutral polymer) polyelectrolyte microcapsules under applied load using an atomic force microscope (A-FM)-related force-measuring device. The experimental setup allows combining dynamic force measurements with high-resolution optics. The "filled" capsules are prepared from "hollow" ones (water inside) by changes in their permeability for a high molecular weight polymer by adding organic solvent. At low applied load, capsule deformation was found to be elastic, reversible, and independent of the concentration of the inner polymer, being entirely controlled by the shell properties. Above a certain load capsules deform substantially and partly irreversibly. They start to show variability in the behavior and dependence of their deformation profiles on the driving speed and concentration of the inner polymer solution, which is likely due to the enhanced permeability of the stretched shell. The "filled" capsules were found to be always several times softer than the preformed "hollow" ones, which is shown to be a consequence of the treatment of the shell by organic solvent

Deformation properties of nonadhesive polyelectrolyte microcapsules studied with the atomic force microscope

We study the deformation of nonadhesive polyelectrolyte microcapsules under applied load using an atomic force microscope (AFM)-related force measuring device. Both "hollow" (water inside) and "filled" (water-polyanion solution inside) microcapsules are explored. The "filled" capsules were found to be much stiffer than "hollow" ones. The load-deformation profiles always included two regimes, characterized by different behavior. In the first regime, with a low applied load, capsule deformation is elastic and reversible. Above a certain load, capsules deform substantially and partly irreversibly. In this regime, the "hollow" capsules show variability in the reversibility, as well as in load - deformation profiles, which include different sectors (from substantial deformation at quasiconstant load to noisy regions). The "filled" capsules do not reveal such variability and become stiffer when the load is increased. After substantial deformation the "hollow" capsules enter a third region, in which major damage is caused by higher load. We show that the dramatic changes of the capsule's mechanical properties after filling with polyelectrolyte reflect a combined effect of excess osmotic pressure inside them, changes in the shell stiffness, and possibly a formation inside capsules of an electrostatically stabilized 3D net structure