Wall Slip of Bimodal Polyethylene
Mohammadali Sattari, Yongwoo Inn, Paula M. Wood‐Adams
Abstract
Cohesive slip occurs in the flow of linear polyethylene melts over high-surface-energy walls, changing both the velocity profile and the stress. The slip behavior of bimodal molecular weight distribution (MWD) polymers is different from that of unimodal MWD polymers and is not yet fully understood. Here, we study a set of four polyethylenes with a systematic change in MWD, making it possible to examine the effect of bimodal molecular weight distribution on the slip behavior. The results show that slip in simple shear occurs over a broad range of stress. Higher short-chain content increases slip velocity at all stresses within the transition region before strong slip is fully engaged. At stresses where strong slip is fully engaged, the slip behavior is independent of molecular weight distribution. The stress at which the slip velocity is independent of MWD lowers as the short-chain content increases. Disentanglement between the adsorbed and mobile chains is evaluated via the change in friction coefficient with stress. This analysis reveals that the degree of entanglement at the cohesive failure interface drops sharply in the transition region. This region is only accessible in simple shear and not in capillary flows. Finally, melt rupture was observed to occur after the onset of slip for certain conditions.