Macromolecules, Vol.29, No.1, 113-118, 1996
Viscoelastic Relaxation of Styrene-Butadiene Diblock Copolymer Micellar Systems .2. Behavior in Entangling, Long Polybutadiene Matrices
For blends of styrene-butadiene (SE) diblock copolymers in entangling, high-M homo-polybutadiene (hB) matrices, two-step viscoelastic relaxation behavior found in a previous study (Watanabe, H.; Kotaka. T. Macromolecules 1984, 17, 342) was reexamined to elucidate features of the fast process on the basis of recently accumulated knowledge for relaxation of homopolymer blends. The SB/hB blends contained spherical micelles with S cores and B corona. For the fast process, the micelles exhibited nearly universal dependence of reduced moduli, G(r)* = [M(bB)/c(bB)RT] G*(SB), On reduced frequencies, omega tau*, with c(bB) and M(bB) being the concentration of the B blocks in the hB matrix and the B block molecular weight, respectively, and tau* being the relaxation time of the fast process. Behavior of r* changed with M(bB), c(bB), and the matrix molecular weight M(hB). For dilute micelles with C-bB < C-e (entanglement threshold for the B blocks), the B blocks were entangled only with the hB matrix and tau* were found to be close to constraint release (CR) relaxation times tau(CR) evaluated from data for star-hB blends. This result led us to attribute the fast process of the dilute micelles to CR relaxation of the B blocks due to the matrix motion. On the other hand, for concentrated micelles with C-bB > c(e), the B blocks were entangled with both hB matrix chains and B blocks of neighboring micelles. For M(bB) much greater than MhB, tau*(conc) of the concentrated micelles were longer than tau(CR) and increased exponentially with c(bB). For those micelles, tau*(conc) agreed well with those in a nonentangling matrix. However, with increasing MhB --> MhB, tau*(conc) became insensitive to C-bB and approached tau(CR). These results for the concentrated micelles were similar to those for homopolymer blends, suggesting that the CR mechanism affected the fast process of those micelles in the following way : For M(bB) much greater than MhB, the CR relaxation was rapid and entanglement between the matrix and the B blocks did not survive at time scales of tau*(conc). Thus, at those time scales, the hB matrix behaved just as a diluent and the B blocks relaxed with-a starlike (arm retraction) mechanism similar to that in the nonentangling matrix. On the other hand, for large M(hB), the CR relaxation was retarded and the entanglement with the matrix still survived at time scales of tau*(conc). For this case, the CR relaxation became a rate-determining step for the B block relaxation and led to the c(bB)-insensitive tau* (congruent to tau(CR)).