Viscoelastic behavior of cubic phases in block copolymer melts

The viscoelastic behavior of block copolymer melts that exhibit a cubic pha se has been examined by oscillatory shear experiments. A low frequency plat eau in the measured storage modulus that is absent in the disordered phase is found for both gyroid and body-centered cubic sphere phases of diblock a nd triblock copolymer melts. The magnitude of the apparent plateau modulus is found to be insensitive to strain amplitude for strains of 0.05%-5%, and so is believed to be characteristic of the true linear response. The value of the apparent terminal relaxation frequency, beneath which G"(omega) > G '(omega) is, however, sensitive to strain amplitude in the same range of st rains and decreases steadily with decreasing strain, indicating that the te rminal regime is highly sensitive to nonlinear effects. The presence of one or more entangled blocks is found to decrease the terminal frequency, and thus extend the range of linear behavior. Experimental results for the depe ndence of the plateau modulus and the unit cell size upon molecular volume yield effective power law exponents that are close, but not identical, to t hose predicted by self-consistent field theory. (C) 1999 The Society of Rhe ology. [S0148-6055(99)00701-4].