Investigation of Molecular Weight and Aging Effects on the Stiffness of Polyelectrolyte Multilayer Microcapsules

Abstract

By using a combination of atomic force and confocal microscopy, we explore the effect of molecular weight and aging on the mechanical properties of polyelectrolyte multilayer microcapsules. Within the variability of experimental data, the force vs relative deformation curves and images from confocal scanning of the deformed capsules were found to be the same for all molecular weights used. The freshly prepared polyelectrolyte microcapsules were found to be stiffer than the aged ones. This time softening or aging was shown to be due to faster drainage of the inner solution. Such a rapid drainage is probably caused by an enhanced fragility (or a decrease in the rupture strength) of the multilayer and is observed despite a detected reduction of nanopore size and number. These results are consistent with the concept of a rubbery (elastomeric) state of the multilayer.

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

“…At low pH, permeability of the capsules increases, leading to drainage during compression. No influence of molecular weight on mechanical properties was found [900]. Aging of PMCs for 6 months lead to softening due to a higher drainage rate.…”

Salt softening of polyelectrolyte multilayer microcapsules

“…However, the absence of nanopores in both the capsules exhibited salt post-treatment and then dried under exactly the same conditions indicates that they are not created just by drying or at high vacuum. There is, therefore, a good chance that the nanopores in the dried state of capsules reflect their size and existence in water (see [9] for more details). SEM images also show folds with much sharper edges in case of capsules immersed in water.…”

“…In case of capsules immersed in water, a drop of their water suspension was used [9]. In case of capsules immersed into salt solutions we rinsed them with water 4 times before drying.…”

“…Questions of interest include the elastic and plastic modulus of multilayer shells; whether their finite permeability affects micromechanical experiments; a contribution from encapsulated polymer to the microcapsule stiffness; the microscopic limits to continuum mechanics description of microcapsule behavior; the effects of aging, pH, organic solvent on the mechanical properties; and the nature (cross-linked rubber [4,6] or bulk plastic [1]) of molecularly thin multilayers. A variety of experiments in the atomic force microscope [7][8][9] and other devices [2,[10][11][12] have shed light on some of these questions, as well as raising several new ones.…”

“…As before, we study the capsules with the PSS/PAH shells [1][2][3][4]6,7] by using a combination of atomic force and confocal microscopy [6,8,9]. Several new developments have been made to experiment.…”