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 solu tions of a styrene-isoprene-styrene (SIS) triblock copolymer in monome ric and polymeric I-selective solvents, n-tetradecane (C-14) and a low -M homopolyisoprene (I-1; M = 1.4K). The I blocks had symmetrically on ce-inverted dipoles along the block contour, and their midpoint motion was dielectrically detected. The SIS solutions exhibited rubbery, pla stic, and viscous behavior at low, intermediate, and high temperatures (T). Dielectric and viscoelastic data strongly suggested that the S a nd I blocks were more or less homogeneously mixed in the Viscous regim e. In the rubbery and plastic regimes, the S blocks were segregated to form spherical domains, and the I blocks took either the loop or brid ge conformation. In these regimes, the inverted dipoles of the I block s enabled us to dielectrically estimate the loop fraction, phi(1) cong ruent to 60% in C-14 and I-1. These loops, having osmotically constrai ned conformations, strongly affected the rheological properties of the SIS solutions. A strong osmotic constraint in C-14 resulted in almost equal contributions of this loops and bridges to the equilibrium modu lus. The loop contribution became less significant in I-1 that (partly ) screened this constraint. Similarly, the yield stress sigma(y) in C- 14 was essentially determined by dangling (noninterdigitated) loops at relatively high T where the S/I mixing barrier was rather small, whil e the bridges and interdigitated loops had a large contribution when t his barrier was enhanced, i.e., at lower T and/or in I-1 (a poorer sol vent for the S blocks than C-14).