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

Abstract

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 (C14) 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, φ1 ≅ 60% in C14 and I-1. These loops, having osmotically constrained conformations, strongly affected the rheological properties of the SIS solutions. A strong osmotic constraint in C14 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 σy in C14 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 C14).