Morphology and thermal transitions of self-assembled NIPAM-DMA copolymers in aqueous media depend on copolymer composition profile

There is a lack in the understanding of the interplay between the copolymer composition profile and thermal transition observed in aqueous solutions of NIPAM copolymers, as well as the correlation between this transition and the formation and structure of copolymer self-assemblies. For this purpose, we investigated the response of five copolymers with the same molar mass and chemical composition, but with different composition profile in aqueous solution against temperature. Using complementary analytical techniques, we probed structural properties at different length scales, from the molecular scale with Nuclear Magnetic Resonance (NMR) to the colloidal scale with Dynamic Light Scattering (DLS) and Small Angle Neutron Scattering (SANS). NMR and SANS investigations strengthen each other and allow a clear picture of the change of copolymer solubility and related copolymer self-assembly as a function of temperature. At the molecular scale, dehydrating N-isopropyl acrylamide (NIPAM) units drag N,N-dimethyl acrylamide (DMA) moieties with them in a gradual collapse of the copolymer chain; this induces a morphological transition of the self-assemblies from star-like nanostructures to crew-cut micelles. Interestingly, the transition spans a temperature range which depends on the monomer distribution profile in the copolymer chain, with the asymmetric triblock copolymer specimen revealing the broadest one. We show that the broad morphological transitions associated with gradient copolymers can be mimicked and even surpassed by the use of stepwise gradient (asymmetric) copolymers, which can be more easily and reproducibly synthesized than linear gradient copolymers.