Deformation Properties of Nonadhesive Polyelectrolyte Microcapsules Studied with the Atomic Force Microscope

Abstract

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" (waterpolyanion 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.

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…Three different regimes within the deformation profiles for PMCs in water were found for hollow microcapsules [892,896,897] (see Fig. 44) for small loads (j < 0.2-0.3), PMCs showed an elastic response.…”

“…In general, filled PMCs are stiffer than hollow ones, as was shown for PAH/PSS capsules filled with PSS that was precipitated as PSS/Y 3+ on MF [896,898] or MnCO 3 templates [898]. This is expected from the osmotic pressure acting across the shell.…”

“…In the second regime drainage of the liquid from the capsule occurs and hysteresis in the force curve is observed. The inset shows a loading/unloading (upper/lower trace) cycle in the first regime, where the response of the capsule is elastic (reprinted from [896] with kind permission from Vinogradova).…”

“…This was attributed to an increased water permeability of the capsules after the treatment with acetone/water mixtures necessary to fill them. Lebedeva et al [902] found that PAH/PSS (MF templated) filled by PSS using permeability increase with acetone were 1-2 times stiffer than the hollow ones, values were 10-100 times smaller than for those filled by precipitation [896,898]. For MF templated PMCs, filled with PSS with the precipitation method, an additional effect occurs that increases the stiffness and changes the shape of the load-deformation curve.…”

Young's Modulus of Polyelectrolyte Multilayers from Microcapsule Swelling

“…First, by studying osmotically induced buckling of "hollow" (water inside) capsules immersed in a polyelectrolyte solution [5], the Young's modulus was found to be above 1000 MPa, close to the elasticity of the bulk plastics [6]. The second, more recent, approach is based on measuring the deformation of microcapsules under applied load using an atomic force microscope (AFM) [7,8,9]. This method yielded an estimate of the lower limit for Young's modulus of the order of 1-10 MPa which corresponds to an elastomer [6].…”