Rheology of strongly segregated poly(styrene–dimethylsiloxane) block copolymers
Résumé
Strongly segregated poly(styrene–dimethylsiloxane) di- and triblock copolymers were studied by dynamic rheological measurements at the extreme of the linear regime, below the order-to-disorder transition. The copolymers had lamellar and cylindrical microstructures. Applying the time–temperature superposition 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 be determined, ω′c where the shift factor determined from the loss modulus fail to superimpose the storage modulus, and ω″cmuch less-thanω′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 cylindrical 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 “anchored” by triblock molecules) to accommodate deformation without significant irreversible distortions (i.e. orientation).