Thermoreversible physical gelation of block copolymers in a selective solvent

Solidlike and liquidlike linear viscoelastic responses were observed at low and high temperatures (T) for polystyrene-polyisoprene-polystyrene (SIS) triblock copolymers dissolved in an I-selective solvent, n-tetradecane (C14). These SIS systems behaved as thermoreversible physical gels, with the microphase-separated S domains working as the cross-links at low T. Linear responses of those systems were examined in the vicinity of the sol-gel transition temperature T-gel. For the systems fully equilibrated in an isothermal state at T-gel, the power-law behavior, G'(omega) similar to G "(omega) similar to omega(n), was observed only at high frequencies to, and a separate slow relaxation mode was observed at low omega. For the system quenched from high T(>T-gel) to low T(<T-gel), no power-law behavior was found during the transient gelation process. The power-law behavior should have been observed if the system had a self-similar, fractal network structure at T-gel. Thus, the observed lack of this behavior indicated that the structural self-similarity vanished over large length scales, possibly due to dynamic compositional fluctuation of the copolymer chains that dominated slow viscoelastic responses at T-gel (congruent to T-ODT) Similar features were observed also for a SI diblock copolymer in C14.