Linear Viscoelastic Behaviour and Relaxation Phenomena of Immiscible Blends: The Application of Creep

Small-amplitude oscillatory shear (SAGS) and incomplete creep/recovery experiments were employed to characterize the rheological behaviour of immiscible blends of polypropylene (PP) with dispersed droplets of polystyrene (PS) at different concentrations of minor phase and compatibilization state. While the linear viscoelastic properties of polymers in the melt state contain valuable information, the plateau and terminal zones of some polymers, especially polymers with very long relaxation time processes, are not accessible with common rheological methods. Furthermore, the interface relaxation phenomena in immiscible blends occur at longer times compared with pure components. A combination of the measurements from dynamic experiments and creep measurements was used to broaden the window of the linear viscoelastic behaviour of neat components as well as the blends. A new experimental creep protocol for immiscible blends was proposed based on Boltzmann Superposition Principle to minimize the morphological modification when sheared. The SAGS results were compared with those obtained from creep/recovery tests and supported by means of morphological investigations and the application of the Palierne model for immiscible blends. The effect of blend composition and compatibilizer, here styrene-ethylene/butylene-styrene (SEBS), on the relaxation spectrum of the blends was studied. The interfacial tension between phases was also determined using the Palierne model. It was found that the proposed creep method measures the linear viscoelastic behavior of the blends and well characterizes their long time behaviour. An extended composite retardation spectrum was constructed by combining the short-time information from SAOS tests with the long-time data from creep/recovery experiments. The obtained spectrum was then used to re-calculate a composite relaxation spectrum as well as other viscoelastic material functions. It was found that the extended composite relaxation spectra predicted the long time relaxation mechanisms and relaxation spectrum portions which are out of the validity limits of the SAOS experiments, and were not seen by the application of dynamic tests only.