Effect of molecular weight, branching and temperature on dynamics of polypropylene melts at very high shear rates

Abstract

Dynamics of linear polypropylene (L-PP) and long-chain branched polypropylene (LCB-PP) miscible blends, having weight average molecular weight between 64 and 78 kg/mol, was investigated via high shear rate rheology. Results obtained were compared with the corresponding data for L-PP. High-shear rate secondary Newtonian plateaus, η∞, were identified at three different temperatures for well entangled L-PP/LCB-PP blends above shear rates of 2·106 1/s and their dependence on weight average molecular weight, Mw, was successfully related as η∞(T)=K∞(T)·Mw n with the exponent n = 1.010. Interestingly, the temperature dependant proportionality constant K∞ was found to be about 10–20% lower for the blend in comparison with the pure L-PP whereas the parameter n was found to be the same for both systems. The average values of high-shear rate flow activation energy, E∞, for the blends was found to be slightly lower than for pure L-PP and comparable with low-shear rate flow activation energy of PP like oligomer squalane (C30H62; 2,6,10,15,19,23-hexamethyltetracosane). This suggests that polymer chains are fully disentangled at very high shear rates and chain branching can enhance the flow in this regime due to smaller coil size and higher availability of the free volume (i.e. lower monomeric friction coefficient) in comparison with their linear counterparts. © 2018 Elsevier Ltd