Steady-shear rheology of block copolymer melts: Zero-shear viscosity and shear disordering in body-centered-cubic systems

The steady-shear rheology of polystyrene-polyisoprene (S/I) block copolymer melts having a body-centered-cubic (bcc) morphology has been characterized as a function of applied shear stress. At temperatures below the order-disorder transition ODT) and at low stresses (τ < 100 Pa), both diblocks and triblocks manifest finite but extremely large (η₀ = 10⁷-10⁸ Pa·s) zero-shear viscosities. Small-angle X-ray scattering (SAXS) measurements indicate that at these low stresses the bcc lattice remains intact. This Newtonian region ends abruptly when a critical shear stress is reached (τ_c ≈ 200 Pa), the steady shear viscosity decreasing by almost 4 orders of magnitude with only a factor of 3 further increase in stress. The viscosity then enters a weakly shear-thinning regime very similar to the behavior of the same block copolymer above its ODT temperature. As shown by SAXS, there is no lattice order in this high-stress regime, indicating that "shear-disordering" has occurred. If the disordered polymer is held fixed or subjected to a low stress (τ < 100 Pa), it gradually regains its original equilibrium state, following the same kinetics as after a thermal quench from above to below the ODT temperature.