John M. Sebastian

Steady-shear rheology of block copolymer melts and concentrated solutions: Defect-mediated flow at low stresses in body-centered-cubic systems

We investigate the low-deformation-rate behavior of sphere-forming block copolymers in the microphase-separated state by both creep and dynamic viscoelastic measurements. Polystyrene-polyisoprene and polystyrene-poly(ethylene-alt-propylene) diblock, triblock, and starblock systems of varying molecular weights and compositions, all of which form a polycrystalline body-centered-cubic (bcc) mesophase, are studied as melts and as concentrated solutions in a matrix-selective solvent. At low stresses, the bcc lattice is preserved, and steady-shear measurements reveal a Newtonian viscosity behavior. Following models for high-temperature creep in metals and ceramics, and incorporating the thermodynamic barrier for endblock pullout from spherical microdomains, we have been able to collapse the zero-shear viscosity versus temperature data for diblock melts of a given composition onto a master curve. By analogy with the behavior of polycrystalline metals and ceramics, we anticipated a substantial effect of thermal history on viscosity, through its impact on the grain size. However, only a modest thermal history effect was observed.