Numerical Simulation And Experimental Study On Viscoelastic Rheological Behavior Of Polyethylene Melts

With the rapid development of computer technology, computer aided design andfinite element method have been used to extensively study the complex die runnerdesign and viscoelastic rheological behavior of polymer melts. Therefore, by usingthe powerful tool of numerical simulation, the complex viscoelastic rheologicalbehaviors of polyethylene melts in different channels are studied in this investigation.Consequently, the analysis of viscoelastic flow problems has two significantconsiderations, namely,(i) verifying and developing the constitutive models, and (ii)providing the guidance for polymer processing. In addition, the flow instabilities ofpolyethylene melts are investigated by using the double capillary rheometer, so as toexplore the mechanism.On one hand, the complex viscoelastic rheological behaviors of polyethylenemelts in three different channels, such as planar contraction, planar flow around aconfined cylinder, and a cross-slot channel, are characterized by two differentialconstitutive models derived from POM–POM molecular theory, namely, the doubleconvected Pom–Pom model (DCPP) and the single/simplified modified DCPP(S–MDCPP) model. The predictive results of DCPP model are obtained by using thePolyflow software, and the S–MDCPP model is calculated by using our in-housecode. During the numerical simulation, the equal low-order elements are applied tosolve the velocity–pressure–stress variables by using the stabilized iterativefractional step algorithm based on the modified version of finite incremental calculus(FIC) process. Meanwhile, the discrete elastic viscous stress splitting (DEVSS)method in combination with the streamline upwind (SU) algorithm is adopted to dealwith the convection dominated problem of the constitutive equations.Firstly, the numerical results are compared with Verbeeten’s experimental results.Results show that DCPP and S–MDCPP models are capable of capturing the rheological behavior of polyethylene melts. The S–MDCPP model, however, isbetter than DCPP model. Moreover, the effect of Weissenberg number andconstitutive parameters (q, r and ξ) of the S–MDCPP model upon the rheologicalbehavior of LDPE melts is discussed.Secondly, during the analysis of the viscoelastic cross-slot flow problems, aquantitative mathematical model is established between the backbone stretch ofbranched polymer and the macroscopic flow time. The aim is to construct theconnection of three physical quantities, namely, the microscopic backbone stretch,the macroscopic flow time and the intrinsic stretch relaxation time of molecule,which is helpful to quantitatively analysis the stretch behavior and relaxationmechanism of the macromolecule.On the other hand, the flow instabilities not only affect the quality of finishedproducts, but also reduce the production efficiency. As we know, the performance ofpolymers is determined by its structure; accordingly different melts exhibit distrinctflow instabilities. For example, the apparent morphology of LLDPE during theextrusion demonstrates smooth zone, sharkskin, whorled, stick-slip transition, thesecond smooth zone and gross melt fracture. However, the apparent morphology ofLDPE exhibits smooth zone, sharkskin and gross melt fracture. In brief, comparedwith the LLDPE melts, the ‘sharkskin’phenomenon and the second smooth zone arenot found during the extrusion of LDPE melts.All in all, the flow of polymer melts results in the various conformations ofmolecular chain, and than exhibits different complex rheological behavior. Therefore,investigating the relationship between the topological structure and complexrheological behavior of polymer melts not only contributes to the development ofnew constitutive model and exploring the flow instabilities mechanism, but also hasgreat scientific significance on the optimizing process conditions, mould design andincreasing production efficiency.