pH-induced phase transition control of thermoresponsive nano-micelles possessing outermost surface sulfonamide moieties.

Diblock copolymer comprising thermoresponsive poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) (PIPAAm-co-DMAAm) and hydrophobic poly(benzyl methacrylate) blocks was prepared by reversible addition-fragmentation chain transfer radical polymerization. Terminal functionalization of thermoresponsive blocks with either pH-responsive sulfadimethoxine (SD) or hydroxyl groups was performed through coupling reactions with thiol groups exposed by the aminolysis of dithiobenzoate groups located at P(IPAAm-co-DMAAm) termini. Outermost surface functionalized polymeric micelles were formed through the multi-assemblies of end-functional diblock copolymers with low critical micelle concentration (3.1-3.3 mg/L) regardless of their terminal groups. Variety of outermost surface functional groups had little influence on nano-scale diameters of approximately 19 nm at various pH values. Although the zeta-potentials of nonionic (phenyl and hydroxyl) surface micelles were independent of pH values ranged 8.1-5.4, those of SD-surface polymeric micelles changed from -12 to -4 mV with reducing pH value, which caused by the protonation of surface SD units (pK(a)=6.2). In addition, lower critical solution temperature (LCST) of SD-surface micelles significantly shifted from 38.6 to 22.6 °C with lowering pH from 5.4 to 8.1. These pH-induced lower LCST shifts were caused by extremely increasing surface hydrophobicity through the charge neutralization of SD moieties and the subsequent promoted dehydration of corona-forming polymer chains. These results indicated that the phase transition behavior of thermoresponsive nano-micelles was particularly controlled by modulating the properties of outermost surface chemistry via specific signals (e.g., pH, light, and biomolecular interaction).