The behavior of a low-density polyethylene (LDPE) melt in double-step strain flows is examined. Here emphasis is on double-step shear strains where the second step is reversed, either half way or completely. Data are compared with the "pom-pom" model of McLeish and Larson, which is a molecular theory for the nonlinear rheology of branched polymers. Both integral and differential versions of the pom-pom with molecular drag-strain coupling are used. Semianalytical model predictions are obtained for the stresses in double-step shear flows, and comparison is made with double-step shear strain flows on a LDPE melt. Predictions from the well-known K-BKZ equation are also compared. It is observed that the K-BKZ and the differential version of the pom-pom model give better predictions than the integral version in both types of reversing flows considered. The K-BKZ model performs better than the two versions of the pom-pom in the data-theory comparison of the first normal stress difference. It is concluded that all of the models are in general qualitatively, but not quantitatively, consistent with the experiments and no clear advantage is found among the different models in reversing double step. The better data-theory comparisons for the pom-pom model in reversing flows is explained with the help of a physical picture of backbone stretch and retraction suggesting that the arms preserve the backbone tube orientation.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics
- Mechanics of Materials
- Mechanical Engineering