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6759808 
Journal Article 
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 
Watanabe, H; Sato, T; Osaki, K; Yao, ML; Yamagishi, A; , 
1997 
1 
Macromolecules
ISSN: 0024-9297
EISSN: 1520-5835 
AMER CHEMICAL SOC 
WASHINGTON 
30 
19 
5877-5892 
English 
WOS:A1997XY56700044 
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, phi(1) congruent to 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 this 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 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, 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).