Dynamic Viscoelastic and Steady Flow Properties of High Density Polyethylene Melts

Abstract

Shear storage (G′) and loss moduli (G″), and steady shear (σ) and first normal stress difference (N₁) of a commercial high density polyethylene (HDPE) melts and the fractions have been measured with a Weissenberg Rheogoniometer. Pseudo-equilibrium modulus (G_eN⁰) and average molecular weight between entanglement points (M_e) were found to be 1.32 × 10⁶ Pa and 2.18 × 10³, respectively. The value of M_e is approximately half of the literature value reported for M_c, the critical molecular weight for entanglement coupling estimated from molecular weight dependence of viscosity. The value of zero shear viscosity (η₀) obtained from both dynamic and steady shear measurements are in good agreement and found to be proportional to 3.8 power of the weight- average molecular weight. Cox- Merz rule does not hold so well at high rate of shear for both the whole polymer and its fraction.
Steady state compliance (J_e⁰) of the fractions is much higher than expected value and is remarkably dependent on molecular weight. Storage compliance (J′) obtained from dynamic measurement and steady shear compliance (J_s) from steady flow measurement agree with each other in the value and the behavior for each fraction. It was found that J′ (and J_S) of the fractions at low frequencies (low shear stresses) increase as increasing molecular weight and molecular weight distribution.
On the contrary, J′ at higher angular frequencies decreases with increasing molecular weight and molecular weight distribution. It is suggested that J_s, at high rates of shear decreases with increasing molecular weight and molecular weight distribution supporting Mendelson's results.