Polymer microgels containing nanodiamonds: pH-dependent component interactions and rheology

Carbopols are polyacrylic acid microgels used for controlled drug release applications due to their ability to tune the rheological properties by varying pH. Poor photooxidative stability and limited incorporation of drugs in these gels is overcome by additive incorporation. Understanding the interaction of these additives with the microgel and their effect on gel mechanical properties is of vital importance. Here, we investigate the pH-dependent interactions of microgels with carboxylated nanodiamonds (NDs), a relatively novel carbonaceous material, using rheology. At a lower pH 4.1, increasing the ND concentration significantly increases the yield stress and modulus of the gel network by up to two orders of magnitude compared to the base system. However, addition of NDs at pH 5.5 exhibits minor change in rheological characteristics. These results are interpretated in terms of the underlying colloidal interactions. We hypothesize that in the partially ionized state (pH 4.1), the carboxylic groups on the ND surface and those of the Carbopol interact via hydrogen bonding leading to the formation of a gel-like network whereas in the fully ionized state (pH 5.5) interplay between swelling, electrostatic repulsion and hydrogen bonding result in diminished effect of NDs.