Elasticity of hollow polyelectrolyte capsules prepared by the layer-by-layer technique

Abstract

Osmotically induced deformations (invaginations) of polyelectrolyte capsules were observed in poly(styrene sulfonate, sodium salt) (PSS) solution since PSS of Mw 70 000 is excluded from the capsule interior. It was found that there is a critical osmotic pressure difference at which the initial spherical capsule shape becomes unstable and invaginations are formed. This critical osmotic pressure was obtained as a function of the wall thickness and the capsule size. A theoretical model is provided which describes the relationship between the critical osmotic pressure, the elasticity modulus, the capsule wall thickness, and the capsule radius. The model was verified by measuring the invagination onset as a function of particle radius and wall thickness. The elasticity modulus of the PSS/PAH (polyallylamine hydrochloride) polyelectrolyte multilayer was measured as a function of wall thickness and capsule diameter. The modulus ranges between 500 and 750 MPa, which indicates a relatively strongly interconnected polyelectrolyte multilayer structure. With higher molecular weight PAH the elasticity modulus of the PSS/PAH multilayer was slightly enhanced.PACS. 46.32.+x Static buckling and instability -68.60.Bs Mechanical and acoustical properties

Young's Modulus of Polyelectrolyte Multilayers from Microcapsule Swelling

“…One can conclude that, taking into account realistic values of the osmotic coefficients [15,17] for PSS solutions, Young's modulus E found in our swelling experiment is of the order of 100 Mpa. This value is confined between the values found in the osmotic buckling experiment [5] and recent AFM results [7] and requires further comments.…”

“…It was previously found [3,28] that the thickness of a PSS/PAH bilayer is in the range of 3-5 nm. Here to evaluate the thickness of the shell with a known number of PSS/PAH bilayers we use the average value of 4 nm, as before [5,7]. The radius of the swollen capsules decreases with the shell thickness and is larger for the capsules with higher concentration of the inner polyelectrolyte solution.…”

“…All these effects (also ignored in Ref. [5]) will effectively lead to smaller values of the Young's modulus.…”

“…The permeable state of the capsule shell allows the polymers to penetrate inside (Fig.2b). During the encapsulation process the PSS concentration was increased gradually to avoid an osmotic collapse of the microcapsules [5]. The initial concentration was 1g/L, and was doubled every hour.…”

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

“…For practical applications, understanding the shell permeability and mechanical properties of the PMCs is of importance. The latter can be studied by osmotically induced buckling [889], by osmotic swelling [890] or by direct probing with the AFM.…”