Multiple pH-induced morphological changes in aggregates of polystyrene-block-poly(4-vinylpyridine) in DMF/H2O mixtures

Multiple changes in the aggregate morphologies of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblocks have been observed as a function of the apparent pH (pH*) in DMF/H2O mixtures. The pH* changes were induced by adding HCl (in the concentration range 400 nM-20 mM) or NaOH (100 nM-20 mM). On the acid side, as the pH* increases from 7 (20 mM HCl) to 12.3 (the pH* of the original polymer solution without any additional microions), the aggregate morphology changes from large compound micelles (LCMs) to a mixture of spheres, rods, and vesicles (pH* = 8), to spheres (pH* = 8.4), to rods (pH* = 11.8), and then back to spheres (pH* = 12.3). In the presence of NaOH, as the pH* increases from 12.3 to 18 (20 mM NaOH), the morphology changes to rods (pH* = 12.6), then back to spheres again (pH* =17.5), and finally to a mixture of spheres, rods, lamellae, and vesicles (pH* = 18). This level of morphological complexity as a function of pH* is unprecedented. The reasons for the behavior can be ascribed to the amphiprotic nature of P4VP in DMF. The addition of either an acid or a base introduces ionic groups into the corona chains. Thus electrostatic repulsion is introduced and the aggregate morphology changes generally in the direction of bilayers to spheres. However, due to the existence of multiple equilibria, some of the added microions are free, which decreases the steric-solvation interaction and decreases the electrostatic repulsion by shielding. This decrease in the corona repulsion tends to decrease the coil dimensions in the corona. As a result, the morphology is driven in the direction of spheres to bilayers. Therefore, a competition between unshielded electrostatic repulsion and shielding coupled with a decrease of the steric-solvation interaction is induced. At relatively low concentrations. the decrease of the steric-solvation interaction dominates, while at relatively high concentrations, the shielding dominates. In intermediate regions, the unshielded electrostatic repulsion is dominant. The morphological transitions induced by extremely low concentrations of HCl or NaOH (100 nM-1 mu M) are very surprising. The effect of a neutral salt (NaCl) on the neutral copolymer and the effect of pH* on a quaternized copolymer were also explored.