Doubly Thermosensitive Self-Assembly of Diblock Copolymers in Ionic Liquids

We describe the doubly thermosensitive micellization-unimer-inverse micellization self-assembly of diblock copolymers in ionic liquids (ILs). Well-defined diblock copolymers (PBnMA(11)-b-PNIPAm and PBnMA(22)-b-P(NIPAm-r-AAm)) were prepared by reversible addition-fragmentation transfer (RAFT) polymerization, where BnMA, NIPAm, and AAm refer to benzyl methacrylate, N-isopropyl acrylamide, and acrylamide, respectively. The typical hydrophobic ILs 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide ([C(2)mim][NTf2]), 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim]PF6), and a 1:1 mixture were selected as solvents. Both PNIPAm and P(NIPAm-r-AAm) exhibit an upper critical solution temperature (UCST) phase diagram in these ILs, whereas PBnMA has a lower critical solution temperature (LCST). Consequently, at low temperature, the copolymers formed micelles with PNIPAm cores, whereas above the LCST of the PBnMA, inverse micelles with PBnMA cores were formed. Single polymer chains (unimers) were observed at intermediate temperatures. This doubly thermosensitive aggregation behavior was monitored by light scattering intensity and via the hydrodynamic radius obtained by dynamic light scattering.