Influence of molecular parameters on the stress dependence of viscous and elastic properties of polypropylene melts in shear

The stress dependencies of the steady-state viscosity η and, particularly, that of the steady-state elastic compliance J e of various linear isotactic polypropylenes (PP) and one long-chain branched PP are investigated using creep-recovery tests. The creep stresses applied range from 2 to 10,000 Pa. In order to discuss the stress-dependent viscosity η and elastic compliance J e with respect to the influence of the weight average molar mass M w and the polydispersity factor M w / M n the PP are characterized by SEC–MALLS. For the linear PP, linear steady-state elastic compliances in the range of 10 − 5 –10 − 3 Pa − 1 are obtained depending on the molar mass distribution. of the LCB-PP is distinctly higher and comes to lie at around 10 − 2 Pa − 1 . is found to be independent of M w but strongly dependent on polydispersity. η and J e decrease with increasing stress. For the linear PP, J e as a function of the stress τ is temperature independent. The higher M w / M n the stronger is the shear thinning of η and the more pronounced is the stress dependence of J e . For the LCB-PP, the strongest stress dependence of η and J e is observed. Furthermore, for all PP J e reacts more sensitively to an increasing stress than η . A qualitative explanation for the stronger stress dependence of J e compared to η is given by analyzing the contribution of long relaxation times to the viscosity and elasticity.