Shear and extensional rheology of sparsely branched metallocene-catalyzed polyethylenes

The purpose of this study was to identify any rheological effects that are consistent with the presence of sparse levels of long chain branching (LCB) in three commercial metallocene-catalyzed polyethylenes (MCPE) all of the same melt flow index of 1.0. Two Dow INSITE MCPEs with apparently varying l evels of LCB of approximately 0.17 and 0.57/10 000 carbon atoms and one Exx on EXXPOL MCPE with no LCB were studied. The breadth of distribution as det ermined by M (w) / M (n) of the three samples was 2.11 for the Exxon and on e of the Dow samples, and 2.42 for the other Dow sample that had the highes t degree of LCB. The MCPE with the highest branching seemed to have a sligh tly higher molecular weight tail in the distribution. Both the Dow samples had significantly higher how activation energies than the Exxon sample, con sistent with the presence of LCB, but this method could not distinguish bet ween the two branched polymers. The differences in M (w) could also not acc ount for the appreciably higher zero-shear viscosities of the branched samp les relative to the linear sample. Despite the differences in M (w) and LCB content in the two Dow samples, they exhibited almost identical shear how curves at temperatures between 120 and 170 degrees C. They also exhibited v ery similar shear stress growth behavior. Under constant extension rate def ormation, the two samples with LCB showed a significant degree of strain ha rdening relative to the linear sample. Comparison between the two Dow sampl es revealed that the sample with the higher degree of LCB showed a greater degree of strain-hardening behavior. The extensional behavior is consistent with the LCB content determined by means of solution light scattering. (C) 2000 The Society of Rheology. [S0148-6055(00)00905-6].