Concentration Dependence of Loop Fraction in Styrene−Isoprene−Styrene Triblock Copolymer Solutions and Corresponding Changes in Equilibrium Elasticity

Dielectric behavior was examined for solutions of a styrene-isoprene-styrene (SIIS) triblock and SI diblock copolymers in an I-selective solvent, n -tetradecane (C14). The SI copolymer had noninverted type-A dipoles in the I block while the SIIS copolymer, a head-to-head coupled dimer of SI, had once-inverted dipoles in the I block. At a low temperature (5 °C) where the S blocks formed glassy, spherical microdomains, the I blocks of SIIS had either the bridge or loop configurations. The loop fraction φ l in these I blocks was estimated from the dielectric losses of the SIIS and SI solutions at low frequencies. The φ l was found to increase (from 0.6 to 0.8) with decreasing SIIS concentration (from 50 to 20 wt %). This increase of φ l was attributed to stretching and destabilization of the bridge configuration on dilution. The equilibrium elasticities of the bridge and dangling loop, estimated from the φ l values and rheological data of the concentrated SIIS/C14 solutions, were comparable in magnitude. This significant elasticity of the dangling loops reflected the strong osmotic constraint on the I block conformation in the concentrated I/C14 matrix phase.