Micellar aggregation and thermogelation of amphiphilic random copolymers in water hierarchically dependent on chain length

ABSTR A C T Micellar aggregates and thermoresponsive gelation of amphiphilic random copolymers in water were examined in detail, focused on effects of the degree of polymerization (DP) on the behavior. For this purpose, we designed random copolymers carrying hydrophilic poly(ethylene glycol) and hydrophobic dodecyl groups, where the DP was varied from 63 to 358. The content of the hydrophobic monomer was set to 60 mol% to facilitate ther-mogelation. All the copolymers formed multichain aggregates in water, while the size distribution depended on DP. Since copolymers with larger DP had less composition distribution, the copolymers with 145 or more of DP formed size-controlled aggregates in water. Their concentrated solutions, above the overlap concentration of the aggregates, gelled entirely upon heating, followed by syneresis. In contrast, the short copolymer with 63 of DP formed aggregates with multimodal size distribution, and the concentrated solution underwent macroscopic phase separation upon heating, followed by gelation of the polymer-rich lower layer. As confirmed by small-angle neutron scattering, the copolymer suitable for thermogelation originally formed aggregates consisting of multiple micelle subunits in the aqueous dilute solution, and the thermogelation of the concentrated solution was driven by the physical packing of the micelle subunits upon heating. Thus, we revealed that the DP hierarchically affected not only the self-assembly of the copolymers into size-controlled micellar aggregates in water but also the thermogelation of the concentrated solutions via intermicellar association.