Rheology of spherical-phase block copolymer melts and solutions

The relation between flow and structure in block copolymer melts and concentrated solutions of polystyrene-polyisoprene and polystyrene-polyethylene- alt-propylene) of varying architecture, molecular weight and composition was examined via rheometry and small-angle x-ray scattering (SAXS). At temperatures below the order-disorder transition temperature (T_ODT) the equilibrium mesophase morphology of these systems consists of spherical microdomains of the minority block arranged onto a body-centered cubic (BCC) lattice in a matrix of majority block.; Linear viscoelastic measurements revealed a power-law dependency of low frequency (ω) BCC plateau modulus ($G0BCC$) on lattice spacing (d₁₁₀) and nearly fluid-like terminal behavior at very low ω. Due to limitations inherent to dynamic oscillatory measurements, steady shear rheology was employed to characterize the terminal flow behavior. Steady shear viscosity (η) was measured, and ex-situ SAXS was used to probe the mesophase structure present after rheological testing. It was determined that for BCC systems the rheology-morphology relation is divided into three regimes with increasing deformation rate: a Newtonian plateau (η₀), a critical shear stress (τ_c) region, and a simple shear-thinning region. In the Newtonian plateau, SAXS indicated that the BCC lattice remains intact, suggesting that flow is due to defect-mediated creeping mechanisms which result in large but finite viscosities (10 7–108 Pa·s). As τ is increased to a critical level, a transition occurs where η abruptly falls several orders of magnitude. At high deformation rates, the rheology is equivalent to that of a block copolymer which is disordered by heating above its T_ODT. SAXS reveals the absence of any BCC lattice order in this regime, indicating that “shear-disordering” occurs at τ_c. After shear-disordering, if τ is dropped below τ_c, the original rheological and morphological state returns with kinetics similar to those following a thermal quench from T > T_ODT, indicating that the shear-disordered state is metastable. Based on models for high temperature creep in metals and ceramics, the dependence of η₀ on temperature and grain size was studied. An η₀ master curve was produced using shift factors based upon the thermodynamic barrier for endblock pullout from a spherical domain. The magnitude of τ_c was found to be a universal function of $G0BCC$ analogous to that found for colloidal crystals.