Creep and Stress Relaxation Behavior of Polypropylene, Metallocene-Prepared Polyethylene and their Blends

Creep and stress relaxation of a polypropylene (PP)-based copolymer, a metallocene-prepared linear low density polyethylene (m-LLDPE) and their m-LLDPE/PP blends have been investigated. Struik and Nutting relationships were used for fitting the data obtained from the creep experiments. A relatively good agreement was found between the Struik model and the experimental data obtained from PP and the 50/50 blend, however, the results showed that the validity of the model is less for the m-LLDPE itself. A good correlation was also found for the Nutting relationship and the experimental data. Maxwell and Kohlrausch-Williams-Watts (KWW) equations were used to fit the data obtained from stress relaxation experiments. The results indicated that while the Maxwell model was not good enough to predict the stress relaxation time, KWW model could fit the data much better. Analysis of the data with KWW equation revealed that the relaxation time increased with m-LLDPE content, but not significantly. However, the factor that describes the width of relaxation time distribution reduced with m-LLDPE, showing that m-LLDPE had the broadest relaxation time distribution compared to that of PP and the blends.