Viscoelastic Properties and Determination of Plateau Modulus for Ziegler‐Natta Catalyzed Ethylene‐Propylene Random Copolymer

Abstract

The viscoelastic behavior of ethylene‐propylene random copolymer with ultra‐high molecular weight (UHPPR) and broad molecular weight distribution (MWD), produced with Ziegler‐Natta catalyst, was investigated by means of oscillatory rheometry at 180°C, 200°C, and 220°C. The loss modulus (G″) curves of 180°C and 200°C present a pronounced peak at 38.10 rad/s and 84.70 rad/s, respectively, while the peak of G″ curve at 220°C locates beyond 100 rad/s. Compared with UHPPR, G″(ω) curves at 180°C and 200°C for Ziegler‐Natta catalyzed ethylene‐propylene random copolymer (PPR) with conventional molecular weight and broad MWD, did not show a peak at 0.01–100 rad/s, respectively. This fact indicates that high molecular weight is responsible for a peak of G″ curves for UHPPR. On the other hand, the activation energy of crossover relaxation time τ c (ΔH a,c) and terminal relaxation time τm (ΔH a,m) is 57.4 kJ/mol and 57.7 kJ/mol, respectively, and shows the same dependence of temperature, which also gives another evidence of the presence of a peak in the G″(ω) curves at 180°C and 200°C. For UHPPR, the plateau modulus (G N 0) which was determined to be 4.51×105 Pa and 3.67×105 Pa at 180°C and 200°C, respectively, decreases with increasing temperature and is independent of molecular weight and MWD. The entanglement points of each molecular chain for UHPPR are much higher than those for PPR with conventional molecular weight, which is a partial reason why the melt viscosity increases with increase of molecular weight.