Rheology of strongly segregated poly(styrene-dimethylsiloxane) block copolymers

Strongly segregated poly(styrene-dimethylsiloxane) di- and triblock copolym ers were studied by dynamic rheological measurements at the extreme of the linear regime, below the order-to-disorder transition. The copolymers had l amellar and cylindrical microstructures. Applying the time-temperature supe rposition principle to the high-frequency range of the loss modulus gives a shift factor which enables G' and G " master curves to be plotted at high reduced frequencies, where the effects of the microstructure and the large scale granular morphology are not important. Two critical frequencies can b e determined, w(c)' where the shift factor determined from the loss modulus fail to superimpose the storage modulus, and w(c)" << w(c)' where the loss modulus curves do not superimpose. This is very clear for the diblock and the triblock with the lamellar microstructure. The departure from the time- temperature master curves is more pronounced when going from the cylindrica l diblock to the lamellar diblock, and also to the lamellar triblock. This is explained by a connectivity term which expresses the difficulty of high connectivity structures (like lamellar arrangements or microdomains "anchor ed" by triblock molecules) to accommodate deformation without significant i rreversible distortions (i.e. orientation). (C) 1999 Elsevier Science Ltd. All rights reserved.