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We report here the first AFM single capsule mechanical measurements on hydrogen-bonded polymeric multilayer microcapsules made of poly(vinylpyrrolidone)/poly(methacrylic acid) (PVPON/PMAA) and of poly(-vinylpyrrolidone-co-NH-20)/poly(methacrylic acid) (PVPON-co-NH-20/PMAA), as well as of capsules derived from these systems chemical crosslinking. The stiffness of the non-crosslinked hydrogen-bonded capsules was found to be proportional to the square of the wall thickness which is in agreement with previous observations on other multilayer capsules and continuum mechanical theory. The found elastic modulus of 610 MPa for low pH (= 2) is typical for a highly stable, glass-like structure similar to electrostatically bound multilayers. At pH > 6, (PMAA) capsules obtained through chemical crosslinking of hydrogen-bonded (PVPON/PMAA) multilayers, or crosslinked (PVPON-co-NH-20/PMAA) capsules showed a sharp hundreds-fold stiffness decrease to ∼1 mN/m which was orders of magnitude lower than those reported earlier for polymeric multilayer systems. pH-Triggered softening was reversible and highly reproducible. Softening of both (PMAA) and (PVPON-co-NH-20/PMAA) crosslinked capsules resulted from increased PMAA ionization, and additional dissociation of intermolecular hydrogen bonds occurred in the case of (PVPON-co-NH-20/PMAA) crosslinked system.

This paper uses the plane-wave expansion and finite-difference time-domain methods to study tunable 3D photonic crystals for use in display applications. The paper calculates particle diameter and refractive index for operation across the visible spectrum and estimates the reflectivity achievable for one to five layers of particles. The effects of disorder in particle position and diameter are then studied and the results show the system to be robust against such effects. Finally, the paper discusses the potential performance of this display technology in terms of reflectivity, color gamut, and contrast ratio. This paper uses the plane-wave expansion and finite-difference time-domain methods to study tunable 3D photonic crystals for use in display applications. The paper calculates particle diameter and refractive index for operation across the visible spectrum and estimates the reflectivity achievable for one to five layers of particles. The effects of disorder in particle position and diameter are then studied and the results show the system to be robust against such effects. Finally, the paper discusses the potential performance of this display technology in terms of reflectivity, color gamut, and contrast ratio.

We compare the behavior of a new two-dimensional aqueous colloidal model system with a simple numerical treatment. To the first order the attractive interaction between the colloids induced by an in-plane rotating ac electric field is dipolar, while the charge stabilization leads to a shorter ranged, Yukawa-like repulsion. In the crystal-like 'rafts' formed at sufficient field strengths, we find quantitative agreement between experiment and Monte Carlo simulation, except in the case of strongly interacting systems, where the well depth of the effective potential exceeds 250 times the thermal energy. The 'lattice constant' of the crystal-like raft is located approximately at the minimum of the effective potential, resulting from the sum of the Yukawa and dipolar interactions.The experimental system has display applications, owing to the possibility of tuning the lattice spacing with the external electric field. Limitations in the applied field strength and relative range of the electrostatic interactions of the particles result in a reduction in tunable lattice spacing for small and large particles, respectively. The optimal particle size for maximizing the lattice spacing tunability was found to be around 1000 nm., Comment: 25 pages 13 figures

In this paper we report on AFM force spectroscopy measurements on hollow polymeric spheres of colloidal dimensions made from polyelectrolyte multilayers of polyallylamine and polystyrenesulfonate in water. We find that the shells show a linear force-deformation characteristic for deformations of the order of the shell wall thickness. This experimental outcome is discussed in terms of analytical results of continuum mechanics, in particular the scaling behaviour of the shell spring constant with wall thickness, shell radius and speed of the deformation is analysed. The experimental results agree well with the predictions of Reissner for thin shells and allow us to rescale our stiffness data such that a master curve of shell stiffness is obtained. The result of Reissner is strictly valid only for point like loading situations, while in our experiments a more extended plate like load is applied. Experimentally we find indeed little influence of the probe geometry on the shell spring constant. This result agrees well with finite element (FE) calculations that show that the Reissner result is a good approximation also for non point like loading situations, as long as small deformations are considered.

Mechanical properties of polyelectrolyte multilayer capsules were studied using a new method combining atomic-force microscopy and reflection interference contrast microscopy. By measuring the force vs. deformation for poly(styrene sulfonate)/poly(allylamine) capsules the existence of different deformation regimes depending on the applied deformation was shown. The present paper focuses on the small-deformation regime. The elastic response of the deformed capsule was studied as a function of the wall thickness and the capsule size, and showed the theoretically expected variations. The Young modulus obtained from the experiments ranges between 1.3 and 1.9 GPa.

The adhesion area and topology of spherical hollow polyelectrolyte shells adhering to flat glass or polyelectrolyte-covered glass surfaces are studied. Strong adhesion is found for anionic poly(sodium 4-styrenesulfonate)-terminated shells on cationic poly(ethylene imine)-covered glass, while no adhesion of those shells on uncoated glass is found. The adhering shells are deformed and obtain a truncated sphere topology with a circular adhesion disk. The radius of the adhesion disks can reach up to 50% of the shell radius for shells of several tens of microns. The dependency of the size of adhesion areas on the capsule radius and capsule wall thickness is also investigated. Remarkably, the size of the adhesion areas is found to depend strongly on the wall thickness, which offers interesting perspectives for controlling capsule adhesion properties. A model based on the energy balance of deformation and wetting energies is presented that explains the observed trends.