Time-evolution of the structure of organoclay/polypropylene nanocomposites and application of the time/temperature superposition principle

Abstract : We investigated the rheological properties of nanocomposites composed of polypropylene, organoclay, and maleic anhydride grafted polypropylene in small amplitude oscillatory shear. Samples were prepared in two steps: a masterbatch was first obtained by melt extrusion and then diluted into polypropylene using an internal mixer. Three formulations were investigated. The measurement of the storage and loss moduli evolution with time showed that these materials were not stable: the nanostructure obtained after steady shear continuously changed with time, due to the disorientation of the clay platelets and the build-up of a 3D network. The kinetics of the structure build-up (followed via the melt yield stress) showed a two-step process. This feature was found to be valid whatever the nanocomposite formulation. Such evolution of the structure is generally assumed to violate the time-temperature superposition principle. We demonstrate in this paper that the time-temperature equivalence always exists if the same nanostructure is probed. This was achieved by using different annealing times for different temperatures or annealing the samples at the highest temperature before measuring at lower values. The time-temperature equivalence evidences that the temperature does not induce any chemical change within the material, whose properties remain governed by the same physical phenomena.