Nonlinear rheology and structural changes of (BS)_n multiblock copolymers under shear flow

For a series of 1,4-butadiene (B)-styrene (S) symmetrical multiblock copolymers, BSB triblock, BSBSB pentablock, and BSBSBSB heptablock copolymers having the same block molecular weights and B/S composition, the rheological behavior and structure were examined in dibutyl phthalate (DBF) at 25°C. DBF is an S-selective solvent, i.e., a good solvent for the S blocks and a poor solvent for the B blocks. The copolymer concentrations C (= 22-25 wt %) were chosen to be just above (by 1 wt %) respective microphase separation concentrations. In a well-equilibrated state, the copolymer/DBP solutions formed a bcc lattice of the spherical domains of unsolved B blocks bridged by the S blocks (lattice-type network), thereby exhibiting elastic behavior under small strains. This lattice-type network was disrupted under steady shear to lose its elasticity, and the heaviest disruption occurred at an intermediate shear rate close to the B/S concentration fluctuation frequency. The elasticity was recovered after cessation of the preshear, and the time t_r ^∞ required for full recovery was insensitive to the preshear rate γ̇_pre, i.e., to the magnitude of the network disruption. This behavior suggested that the bridge-type S blocks crossing the flow planes were converted to loops under the preshear, and the re-formation of the bridges, requiring the B blocks to be thermally pulled out from their domain, was the rate-determining step for the recovery. In other words, t _r^∞ of the multiblock copolymers appeared to be primarily determined by the ypre-insensitive thermodynamic barrier for transient mixing of the pulled out B block into the S/DBP matrix. For such multiblock copolymers, /" was found to increase strongly with increasing copolymer molecular weight, and a ratio of t_r^∞ of the triblock, pentablock, and heptablock copolymers was close to 1:5:25. This ratio was consistent with the Rouse/reptation motion of the multiblock chain retarded by the B/S mixing barrier.