Rheology of diblock copolymer micelles in selective solvents

In diene-selective solvents. styrene-diene (SD) diblock copaiymers form micelles with precipitated S cores and solvated D-coronas. In low molecular weight solvents. the osmotic interaction of the corona blocks forces the micelles to form a cubic lattice and to exhibit elastoplastic behavior. Reflecting the entropy elasticity of these corona blocks. the equilibrium modulus G(c) of the micellar lattice is proportional to the corona number density v. However, G(c) is considerably smaller than G(c)(o)=vk(B)T (k(B)=Boltzmann constant, T=absolute temperature) expected for the case of classical entropy elasticity. This difference between G(e) and G(c)(o) reflects a fact that the osmotically constrained corona blocks have conformational correlation and cannot behave as mutually independent, entropic strands. In homopolymers chemically identical to the corona (i.e. in polymeric solvents), the osmotic interaction is screened and the micelles are randomly dispersed in the system. This micellar dispersion exhibits fast and slow viscoelastic relaxation processes. The fast process corresponds to the orientational relaxation of individual corona blocks, while the slow process corresponds to the micellar diffusion process that results in recovery of random and isotropic distribution of the micelles in the system. Reflecting these relaxation mechanisms, the micelles exhibit characteristic damping/thinning under large strain/fast flow. In particular, the nonlinearities seen for the slow relaxation process are similar to those of Brownian hard spheres, demonstrating a similarity of the slow dynamics in the micellar systems and Brownian suspensions.