Linear viscoelastic properties of hyperbranched polyisobutylene

Creep and dynamic mechanical measurements were carried out on a series of h yperbranched polyisobutylenes (PIBs), having a range of molecular weights ( greater than or equal to 10(6) daltons) and branching frequencies (3-57 bra nches/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 h alf-million daltons does the zero-shear viscosity exceed that of linear PIE . Both the viscosity and the length of the entanglement plateau are governe d primarily by the branching frequency, rather than by the length of the br anches. Such behavior is quite distinct from star-branched polymers. Howeve r, the magnitude of the plateau modulus and the temperature dependence of t he terminal zone shift factors are the same for hyperbranched, star-branche d, and linear PIE. (C) 2001 The Society of Rheology.