Evaluation of viscoelastic master curves of filled elastomers and applications to fracture mechanics

The viscoelastic response of filler-reinforced elastomers has been investigated by dielectric- and dynamic-mechanical spectroscopy. Horizontal and vertical shifting factors are evaluated, which are used for the construction of viscoelastic master curves. They are discussed in the framework of filler network effects and the slowed-down dynamics of a polymer layer close to the filler surface. The observed shifting behaviour is shown to be related to the superposition of two relaxation processes, i.e. that of the polymer matrix and the filler network, leading to a failure of the time-temperature superposition principle. While the matrix transforms according to the Vogel-Fulcher equation, the filler network exhibits an Arrhenius dependence, which results from the thermal activation of filler-filler bonds, i.e. glassy-like polymer bridges between adjacent filler particles. Based on the viscoelastic master curves relaxation time spectra are evaluated. By referring to a recently developed theory of crack propagation in viscoelastic solids it is demonstrated that the behaviour of the scaling exponent of the relaxation time spectra correlates fairly well with that of the crack propagation rates measured under moderate severity conditions.