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 °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 °C, as previously reported. DLS showed comparable diameters ( l-MOM, 14 nm; c-MO, 12 nm) and T c values ( l-MOM, 48 °C; c-MO, 75 °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 c, 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).