An experimental and computational approach to pH-dependent self-aggregation of poly(2-isopropyl-2-oxazoline)

Besides temperature, self-aggregation of poly(2-isopropyl-2-oxazoline) (PIPOX) can also be triggered via pH in aqueous solution (25 degrees C, pH > 5). Lowest energy structures and interaction energies of PIPOX with H3O+, OH-, and H2O were calculated by DFT methods showed that, in addition to their ability to protonate PIPOX, H3O+ ions had strong interaction with both water and PIPOX in acidic conditions. H3O+ ions acted as compatibilizer between PIPOX and water and increased the solubility of PIPOX. OH- ions were found to have stronger interaction with water compared to PIPOX resulting in desorption of water molecules from PIPOX phase and decreased solubility, leading to enhanced hydrophobic interactions among isopropyl groups of PIPOX and formation of aggregates at high pH. Results concerning the effect of end-groups on aggregate size were in good agreement with statistical mechanics calculations. Moreover, the effect of polymer concentration on the aggregate size was examined. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 210-22