Wall Slip of Bimodal Polyethylene

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.

Polymer Fractionation at an Interface in Simple Shear with Slip

“…The tube-model parameters calculated for our material are used to calculate the Weissenberg number shown in Table and to define the slip regions in which the shear flow experiment was performed as specified in ref and added as a line to Figure , which serves as a theoretical demarcation between the transition zone and the strong slip region for this material. Experimental studies have shown that strong slip is independent of the MWD. , We note that the experimental point at 110.63 kPa (Figure ) falls exactly at the theoretical boundary between the transition and strong slip zones. In the experimental slip study, this point was identified as the last point in the transition zone, a truly remarkable agreement between theory and experiment considering the very broad MWDs that we are working with.…”

“…The surface energy of the glass slides used in this study is 50 mJ/m 2 , classifying it as high energy . We remark that holes were also observed in the debris even on steel plates, which has a higher surface energy value of 60 mJ/m 2 (see Figure S2.2). The micrographs suggest that these holes could really be bare glass, and FTIR spectroscopy performed at these regions did not contain peaks expected for HDPE (see Figure S2.3).…”

“…On a high-energy surface, tethered or adsorbed chains are believed to preferentially disentangle themselves from the bulk polymer rather than desorb from the wall. The surface energy of the glass slides used in this study is 50 mJ/m 2 , classifying it as high energy . We remark that holes were also observed in the debris even on steel plates, which has a higher surface energy value of 60 mJ/m 2 (see Figure S2.2).…”

“…68 In each of these regions, one or more dominant relaxation mechanisms of the tethered chains could occur. 69,70 The slip behavior of the bulk sample was studied by Sattari et al 62 and is presented in Figure 11. The red circles are interpolated values corresponding to our experiments.…”

“…The manner in which the material slips depends on the shear rate and can be broadly categorized into a weak slip, followed by a transition zone, and then strong slip . In each of these regions, one or more dominant relaxation mechanisms of the tethered chains could occur. , The slip behavior of the bulk sample was studied by Sattari et al and is presented in Figure . The red circles are interpolated values corresponding to our experiments.…”