Macromolecules, Vol.29, No.1, 104-112, 1996
Viscoelastic Relaxation of Styrene-Butadiene Diblock Copolymer Micellar Systems .1. Behavior in a Nonentangling, Short Polybutadiene Matrix
For blends of styrene-butadiene (SE) diblock copolymers in a nonentangling homopolybutadiene (hB) matrix, viscoelastic data obtained in a previous study (Watanabe, H.; Kotaka, T. Macromolecules 1983, 16, 769) were reexamined to elucidate features of relaxation of individual B blocks. The blends contained spherical micelles with S cores and B corona, and two-step relaxation composed of fast and slow processes was observed. This study mainly examined features of the fast process that were not specified in the previous study. When the neighboring micelles were entangled through their corona blocks, a nearly universal relationship was found between reduced moduli for the fast process, G(tau)* = [M(bB)/c(bB)RT]G*, and reduced frequencies, omega tau*, with c(bB) and M(bB) being the concentration of the B blocks in the hB matrix and B block molecular weight, respectively, and tau* being the relaxation time for the fast process. In addition, tau* was found to increase exponentially with M(bB)/M(e), with M(c), being the entanglement spacing. These features of the fast process mere qualitatively the same as those for relaxation of entangled star chains, strongly suggesting that the fast process corresponded to star-like relaxation (arm-retraction) of entangled 13 blocks being tethered on the S cores. Quantiutatively, tau* for the B blocks were 3-4 orders of magnitude longer than the relaxation times of corresponding star hB chains. This difference was related to effects of the S cores that worked as an impenetrable wall and constrained the B block relaxation. For the slow relaxation process of the SE micellar blends, the relaxation times tau(s) were compared with characteristic times for Stokes-Einstein (SE) diffusion of the micelles tau(SE) = pi R(m),eta(eff)delta(2)/kT, with R(m) and 6 being the micelle radius and diffusion distance, respectively, and eta(eff) being an effective viscosity for micelle diffusion. For concentrated micelles being entangled through their B blocks, tau(s) were close to tau(SE) with delta and eta(eff) being taken as the micelle diameter (2R(m)) and a viscosity eta(fast) for the fast process of the micelles (relaxation ofindividual B blocks). This result strongly suggested that the slow process for those entangled micelles corresponded to the SE diffusion of micelles, being in harmony with the previous assignment. However, for dilute, nonentangled micelles, c,were significantly shorter than tau(SE). This result suggested changes in the molecular mechanism for the slow process with the extent of entanglements between the micelles.