Dielectric and Viscoelastic Behavior of Styrene-Isoprene Block Copolymers: Dielectric Detection of Isoprene Block Dynamics

Dielectric and viscoelastic behavior was examined for a styrene-isoprene diblock copolymer, SI 11-60, and its head-to-head dimer, SI 2 S 11-(60) 2 -11 triblock copolymer, with the sample code numbers representing the block molecular weights M S, M I, and M I2 in the unit of kg mol –1 . SAXS data showed that the copolymers formed a bcc lattice of spherical S domains and patches of those lattices were randomly oriented to form a polygrain structure. The I and/or I 2 blocks were well entangled in their matrix phase embedding those spherical S domains. The I block had the type-A dipole parallel along its backbone, whereas the I 2 block had the type-A dipole once inverted at the block midpoint. Thus, the slow dielectric relaxation of SI 11-60 and SI 2 S 11-(60) 2 -11 exclusively detected the end-to-end vector fluctuation of the I block and the end-to-center vector fluctuation of the I 2 block, respectively, without being contributed from the viscoelastically detected (but dielectrically inert) slow dynamics of the polygrain structure. Comparison with a reference material, 6-arm star homopolyisoprene 6(I-59) having the arm molecular weight almost identical to M I of the I block, revealed that SI 11-60 and 6(I-59) exhibited very similar dielectric relaxation except for the relaxation intensity moderately smaller for SI 11-60 by a factor of 0.65. In contrast, SI 2 S 11-(60) 2 -11 showed the dielectric relaxation much weaker, by a factor of 0.1, compared to 6(I-59). These results suggested that the motion of the I block anchored on the S domain was similar to the arm retraction of the 6(I-59) star in a dilating entanglement network (as considered in the standard tube model) but was constrained by a thermodynamic requirement of preserving a constant mass density without the motion of the S–I junction. In contrast, the motion of the I 2 block having its two ends fixed on the S domains appeared to occur just through the Rouse fluctuation along an array of the trapped entanglements under the requirement of preserving a constant mass density, and thus much less significant compared to the motion of the I block.