Relaxation of Spherical Micellar Systems of Styrene−Isoprene Diblock Copolymers. 1. Linear Viscoelastic and Dielectric Behavior

Abstract

Linear viscoelastic and dielectric relaxation behavior was examined for blends of styrene−isoprene (SI) diblock copolymers in a nonentangling homopolyisoprene (hI) matrix. The copolymers formed spherical micelles with S cores and I corona. The I blocks had type-A dipoles, and their global motion induced dielectric relaxation. The micelles exhibited fast and slow relaxation processes for both dielectric loss ε‘‘ and viscoelastic moduli G*. For the fast process, a nearly universal relationship was found for reduced moduli G r * = [ M bI / c bI RT ] G * SI and reduced frequencies ωτ*, where G * SI was the contribution of the SI micelles to G *, M bI and c bI were the molecular weight and concentration for the I block, and τ* was a relaxation time that increased exponentially with c bI for large c bI . Similar behavior was found also for ε‘‘ SI (SI contribution to ε‘‘). These features were qualitatively the same as those for the relaxation of star chains, indicating that the fast process corresponded to starlike relaxation of individual I blocks tethered on the S cores. Effects of the S cores on τ* were discussed within the content of the tube model. For the slow process of the concentrated micelles entangled through their I blocks, a relaxation time τ s was close to the Stokes−Einstein (SE) diffusion time τ SE evaluated from the viscosity associated with the fast process. Thus, the slow process of those concentrated micelles was attributed to their SE diffusion governed by the relaxation of individual corona I blocks. On the other hand, for dilute micelles in the nonentangling matrix, τ s was significantly shorter than τ SE but close to the SE diffusion time evaluated from the matrix viscosity. This result suggested that the slow process of those micelles corresponded to the SE diffusion in the pure matrix.