NMR Relaxometry for the Thermal Stability and Phase Transition Mechanism of Flower-like Micelles from Linear and Cyclic Amphiphilic Block Copolymers

Linear and cyclic amphiphilic block copolymers consisting of poly(ethylene oxide) (PEO) as the hydrophilic segment and poly(methyl acrylate) or poly(ethyl acrylate) as the hydrophobic segments were synthesized and self-assembled to form flower-like micelles. The micelles from linear (methyl acrylate)(12)(ethylene oxide)(73)(methyl acrylate)(12) (M-n = 1000-3200-1000, l-MOM) showed a cloud point (T-c) at 46 degrees C by the transmittance of the micellar solution, whereas that of cyclic (methyl acrylate)(30)(ethylene oxide)(79) (M-n = 2600-3500, c-MO) increased to 72 degrees C, as previously reported. DLS showed comparable diameters (l-MOM, 14 nm; c-MO, 12 nm) and T-c values (/-MOM, 48 degrees C; c-MO, 75 degrees C). For the investigation of the difference in T-c and the phase transition mechanism based on the polymer topology, NMR relaxometry was performed to determine the spin lattice (T-1) and spin spin (T-2) relaxation times. A decrease in T-2 of the PEO segment in both l-MOM and c-MO was observed above T, suggesting that slow large-scale motions, such as the detachment of a chain end from the core, bridging, and interpenetration of the micelles, were inhibited. T-1 of the PEO segment in l-MOM continuously increased in the experimental temperature range, indicating that the segment is hydrated even above its T-c. On the other hand, that of c-MO reached a ceiling above its T-c, likely due to the prevention of the rotation of the PEO main chain bonds caused by dehydration. Similar results were obtained for linear (ethyl acrylate)(8)(ethylene oxide)(79)(ethyl acrylate)(8) (M-n = 800-3500-800, l-EOE) and its cyclic (ethyl acrylate)(15)(ethylene oxide)(78) (M-n = 1500-3400, c-EO).