Synthesis and viscoelastic properties of model dumbbell copolymers consisting of a polystyrene connector and two 32-arm star polybutadienes

A series of well-defined dumbbell copolymers of (PBd)₃₂PS(PBd)₃₂ type, with the same polystyrene (PS) connector (10 600 g/mol) and 32 polybutadiene (PBd) branches of varying molecular weight (1200, 5000, 15 000, 43 000 g/mol) on each end, were synthesized, and their viscoelastic properties were investigated. For the three larger branches (above the entanglement molecular weight), their relaxation controls the dynamics of the macromolecular dumbbells, which exhibit starlike behavior. For the shorter branch, the backbone fraction (and thus its relaxation) is nonnegligible, and it should be considered in the assessment of the dynamics. A general map of the relaxation of such complex model branched polymers is proposed, where the dumbbell dynamics depend on the branch size for large branches (as in the star case) or on the total molecular size for short branches.