Temperature-induced intermicellization of "hairy" and "crew-cut" micelles in an aqueous solution of a thermoresponsive copolymer.

Temperature-induced intermicellar structures in aqueous solutions of the thermoresponsive methoxypoly(ethylene glycol)-block-poly(N-isopropylacrylamide) (MPEGn-b-NIPAAM71) copolymer that exhibit a lower critical solution temperature were studied by means of turbidimetry, dynamic light scattering (DLS), shear viscosity, and rheo small-angle light scattering (rheo-SALS) methods. The length of the hydrophilic chains (MPEG) of the copolymer varies from n=0 to n=114. It is shown that this change has a major impact on the temperature-induced association behavior of the polymer in solution. The turbidity results at quiescent conditions revealed a transition peak in the turbidity curve at intermediate temperatures, and this peak as well as the cloud point is shifted toward higher temperatures with increasing length of the hydrophilic chains of the copolymer. The DLS measurements disclosed a fast and a slow relaxation mode, which both are diffusive. From the fast and slow relaxation times the sizes of unimers/micelles and intermicellar clusters, respectively, can be determined. The temperature-induced aggregation is less pronounced in solutions of copolymers with long hydrophilic chains, and the intermicellar structures exhibit an interesting transition at intermediate temperatures. In the shear viscosity measurements large association complexes are formed at high temperatures and at low shear flow for the polymers with short hydrophilic chains, whereas at high shear rates breakup of interaggregate chains was observed. For the copolymer with the highest number of hydrophilic chains (n=114), a novel transition peak was found in the viscosity data. The rheo-SALS results divulged shear-induced structural changes of the association complexes at elevated temperatures. For copolymers with short hydrophilic chains, shear-induced disruption of association complexes was found at higher temperatures, whereas for hairy micelles augmented shear flow promoted the growth of complexes.