pH- and salt-mediated response of layer-by-layer assembled PSS/PAH microcapsules: fusion and polymer exchange

Abstract

We have studied the pH- and salt-mediated response of matrix-type polyelectrolyte microcapsules. The capsules were prepared by layer-by-layer (LbL) adsorption on decomposable CaCO₃ cores using model polyelectrolytes, namely poly-styrenesulfonic acid (PSS) and poly(allylamine hydrochloride) (PAH). Salt-mediated LbL-made microcapsule fusion has been reported recently with a different polycation (R. Zhang, O. Kreft, A. Skirtach, H. Mohwald and G. Sukhorukov, Soft Matter, 2010, 6, 4742–4747) resulting in merging of the capsule's content and formation of anisotropic “Janus-like” capsules indicating no polymer exchange between the capsules. Here we have studied PAH/PSS capsule behavior as a function of pH and salt concentration. Salt (NaCl) does not induce any changes in the capsules up to saturation concentration (6.1 M). In contrast, several sequential processes have been identified for capsules in [HCl] > 0.1 M: (i) shrinkage due to polymer network annealing, (ii) transformation from matrix-type to shell-type capsules due to oscillating inflating–deflating cycles caused by CO₂ formation, and (iii) collapse. The processes depend on acid concentration and the number of layers. If the capsules contact each other, there is an exchange of polymer molecules followed by fusion. The polymer exchange depends on the outermost layer that determines the overall capsule charge. Exchange is enhanced for capsules of the same outermost layer and can be caused by charge redistribution in the capsules at low pH (the negative charge of PSS is reduced (pK_a 1) and the positive charge of PAH is in excess). Control over polymer exchange between the capsules is key in order to design capsules as well as to understand and to trigger fusion. We also show that the observed processes are not reversible and can be stopped at any time by replacement of acid with water. Stable gas-filled capsules can be produced by this method upon transformation from matrix to shell-type capsules.