Influence of shearing history on the rheological properties and processability of branched polymers. I

Abstract

SynopsisThe influence of shearing history on the viscoelastic properties of low-density polyethylene (LDPE) is investigated. Swelling of extrudates from a melt indexer is measured for monitoring the variation in viscoelasticity. Continuous shearing of molten LDPE reduces the swelling ratio. The reduction is not due to molecular degradation, as evidenced by constancy of intrinsic viscosity. The rate of the reduction in the swelling ratio depends on the shearing conditions and characteristics of LDPE, but the swelling ratios finally attain a steady value. The swelling ratio reduced by the continuous shearing is completely recovered by solvent or heat treatment. The ratio of the completely reduced swelling ratio to the completely recovered one is defined as a new index representing the viscoelastic variation, the processing index (PI), and the relationship between PI and the primary molecular parameters of LDPE is investigated. It is concluded that the variation in the viscoelastic properties becomes more remarkable with increase in the weight-average molecular weight. The cause of the viscoelastic variation is also discussed from the rheological and thermodynamic points of view.

Recovery of Shear-Modified Polybutadiene Solutions

“…This allows advantage to be taken of the "shearmodified" state during polymer processing; for example, elastic effects such as die swell are much weaker for some time after the shearing. [8][9][10][11][12][13][14][15] Freeze-drying of dilute polymer solutions has also been investigated as a route to achieving disentangled high-molecular weight polymers, 16 although at least for polystyrene the results are disappointing. [17][18][19] Among the various signatures of a reduced entanglement density, the magnitude of the overshoot in the transient shear stress is especially useful for laboratory studies.…”

Influence of shearing history on the rheological properties and processability of branched polymers. V. Effect of molecular structural parameters on die swell of low‐density polyethylenes

“…This object is readily achieved by comparing the swell ratio of sample A-0 with that of In conclusion, we can say that the swell ratios of the solvent-treated materials increase with increase in M , and MWD but decrease with increase in A. By the way, classifying the swell ratios on the basis of the MI range, i.e., [3][4][5][7][8]22-24, the swell ratios increase with increase in A as shown by the solid lines in Figure 1. This finding seems to be in conflict with the conclusion already mentioned.…”

Rheological properties of LDPE processed by conventional processing machines

“…Batch-type processing machines, such as a 60 cm 3 internal batch mixer (Brabender Plasticoder PL2000) and an 8 in. two-roll mill (Nihon roll), were also employed.…”