Rheological and Dielectric Behavior of a Styrene−Isoprene−Styrene Triblock Copolymer in Selective Solvents. 2. Contribution of Loop-Type Middle Blocks to Elasticity and Plasticity

Rheological and dielectric behavior was examined for concentrated solutions of a styrene−isoprene−styrene (SIS) triblock copolymer in monomeric and polymeric I-selective solvents, n -tetradecane (C 14 ) and a low- M homopolyisoprene (I-1; M = 1.4K). The I blocks had symmetrically once-inverted dipoles along the block contour, and their midpoint motion was dielectrically detected. The SIS solutions exhibited rubbery, plastic, and viscous behavior at low, intermediate, and high temperatures ( T ). Dielectric and viscoelastic data strongly suggested that the S and I blocks were more or less homogeneously mixed in the viscous regime. In the rubbery and plastic regimes, the S blocks were segregated to form spherical domains, and the I blocks took either the loop or bridge conformation. In these regimes, the inverted dipoles of the I blocks enabled us to dielectrically estimate the loop fraction, φ 1 ≅ 60% in C 14 and I-1. These loops, having osmotically constrained conformations, strongly affected the rheological properties of the SIS solutions. A strong osmotic constraint in C 14 resulted in almost equal contributions of the loops and bridges to the equilibrium modulus. The loop contribution became less significant in I-1 that (partly) screened this constraint. Similarly, the yield stress σ y in C 14 was essentially determined by dangling (noninterdigitated) loops at relatively high T where the S/I mixing barrier was rather small, while the bridges and interdigitated loops had a large contribution when this barrier was enhanced, i.e., at lower T and/or in I-1 (a poorer solvent for the S blocks than C 14 ).