Association and Aggregation Behavior of Poly(ethylene oxide)-<i>b</i>-Poly (<i>N</i>-isopropylacrylamide) in Aqueous Solution

Abstract

The well-defined block copolymer of poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAm) was synthesized by the reversible addition fragmentation transfer (RAFT) polymerization, and its thermo-induced aggregation process in dilute aqueous solution was studied by laser light scattering. At temperatures below the cloud point of PNIPAm, the PEO-b-PNIPAm chains were associated together to form large and loose structures, which were in a fast equilibrium with the single chains. The association was weakened with increasing concentration, which was against the common ideas about aggregation. Because of such "abnormal" behavior, PEO-b-PNIPAm underwent three stages of transformation during the heating process at 0.1 mg/mL: the disassociation of loose associates at temperatures below 28 °C, the micellization above 42 °C, and the aggregation in between. At each stage, the size and Mw,app exhibited distinct features. On the basis of these observations, possible mechanisms of the association and aggregation were also proposed.