Nonlinear Relaxation Behavior of Diblock Copolymer Micellar Dispersions:Effects of Corona-Matrix and Corona-Corona Entanglements

摘要

Nonlinear relaxation modulus was examined for styrene-isoprene (SI) diblock copolymer micelles having rigid S cores and flexible I corona blocks. These micelles, being dispersed randomly in entangling and/or nonentangling matrices of homopolyisoprene (hI), exhibited fast and slow relaxation processes. The fast process reflected the orientational relaxation of the corona I blocks and the slow process detected the relaxation of the Brownian stress σ_B. This σ_B reflected the anisotropy of micelle distribution and relaxed through the micelle diffusion. The fast and slow processes exhibited the nonlinear damping of the modulus under large step strains (γ), and the time-strain separability was observed for both processes either in the entangling or nonentangling matrices. For the fast process, the damping function h_f(γ) hardly changed with the SI micelle concentration C_(SI) and matrix molecular weight M_(matrix) and was close, in magnitude, to h(γ) of homopolymers. The nonlinearity of the corona relaxation characterized with this h_f(γ) was attributed to retraction of the strain-elongated corona blocks occurring in prior to their rotational motion. The damping function h_s(γ) of the slow process, being smaller than h_f(γ), decreased with increasing C_(SI). This nonlinearity of the slow process, similar to that of Brownian suspensions, reflected the γ-insensitivity of the micelle distribution anisotropy for large γ, and the strong C_(SI) dependence of h_s(γ) was attributed to relative rotation of concentrated micelles colliding with each other under large strains. Interestingly, the damping of the slow process was weaker in the entangling matrices than in the nonentangling matrices. This effect of the corona-matrix entanglement, observed for both of the concentrated and dilute micelles with and without corona-corona entanglement, was attributed to the elasticity of the entangling matrices that disturbed the relative rotation of the mutually colliding micelles.