| Abstract
| - We use steady-shear rheology to study the shear-disordering phenomenon in blockcopolymers in the body-centered-cubic (bcc) mesophase. Polystyrene−polyisoprene and polystyrene−poly(ethylene-alt-propylene) diblock, triblock, and starblock systems of varying molecular weight andcomposition were studied in the melt and as concentrated solutions in a selective solvent. The bcc latticeexhibits a low-frequency moduluswhich scales with the cube of the lattice spacing and additionallydepends only on the proximity to the order−disorder transition (ODT). Steady shear destroys the bcclattice at a critical shear stress, which is found to be directly proportional to the lattice modulus: τc =0.038for all block copolymer melts and highly concentrated solutions investigated. The criticalstress is thus uniquely determined by the lattice spacing and the proximity to the ODT. The rheologicalbehavior conforms in a remarkably quantitative manner to the shear-melting processes observed in othersoft crystalline systems, such as colloidal crystals.
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