Self-Assembled Micelles from Thermoresponsive Poly(methyl methacrylate)- b-poly(N-isopropylacrylamide) Diblock Copolymers in Aqueous Solution

The temperature-dependent phase behavior and self-assembly behavior in aqueous solution of the thermoresponsive amphiphilic diblock copolymer PMMA21-b-PNIPAM283 made of a short permanently hydrophobic poly(methyl methacrylate) block and a long thermoresponsive poly(N-isopropylacrylamide) are studied. Turbidimetry, dynamic light scattering (DLS), and synchrotron small-angle X-ray scattering (SAXS) provide temperature-dependent structure information. A lower critical solution temperature (LCST) behavior with a cloud point TCP = 31.1 °C in dilute solution is observed. Below TCP, spherical core-shell micelles are present, having a small PMMA core and a hydrated PNIPAM shell with a radial gradient of water content. Below TCP, the micelles are interpenetrated and show a weak correlation with each other. As the temperature approaches TCP, the micellar core shrinks and thus, the aggregation number decreases, revealing that the PMMA blocks are still mobile. Moreover, the micellar shell dehydrates above TCP and the micelles notably contract. They form clusters, which grow and transform into large compact aggregates as temperature is raised far above TCP.