Linear viscoelastic properties of hyperbranched polyisobutylene

Creep and dynamic mechanical measurements were carried out on a series of hyperbranched polyisobutylenes (PIBs), having a range of molecular weights (greater than or equal to 10(6) daltons) and branching frequencies (3-57 branches/molecule). For all samples, the molecular weight of the branches was higher than the entanglement molecular weight of linear PIE, by as much as a factor of 10: nevertheless, only for molecular weights of approximately half-million daltons does the zero-shear viscosity exceed that of linear PIE. Both the viscosity and the length of the entanglement plateau are governed primarily by the branching frequency, rather than by the length of the branches. Such behavior is quite distinct from star-branched polymers. However, the magnitude of the plateau modulus and the temperature dependence of the terminal zone shift factors are the same for hyperbranched, star-branched, and linear PIE. (C) 2001 The Society of Rheology.