pH-Triggered softening of crosslinked hydrogen-bonded capsules
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(N-vinylpyrrolidone-co-NH2-20)/poly(methacrylic acid) (PVPON-co-NH2-20/PMAA), as well as of capsules derived from these systems via 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-NH2-20/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-NH2-20/PMAA) crosslinked capsules resulted from increased PMAA ionization, and additional dissociation of intermolecular hydrogen bonds occurred in the case of (PVPON-co-NH2-20/PMAA) crosslinked system.