(345e) On the Prediction of the Viscoelastic Behavior of Highly-Branched Polymer Chains: A Novel Topological - Rheological Approach

One of the most important issues in the production of polyolefins is the accurate control of the molecular architecture. In general, the rheological behaviour of branched polymers differs from that of linear polymers since the flow properties of polymers are strongly affected by the presence of long-chain branches on the polymer chains. Low-density polyethylene (LDPE), which is known to contain multiple long-chain branches, is a typical example of a polymer with rheological behaviour of profound industrial interest. However, its complex kinetic mechanism and extreme process conditions do not facilitate a detailed study that could reveal information on its molecular architecture.

To address this problem, an advanced novel combined kinetic/topology stochastic Monte Carlo (MC) algorithm has been developed for the prediction of the exact topological architectures of a large ensemble of highly-branched polymer chains. This MC algorithm is applied to a high-pressure LDPE tubular reactor and a number of average and distributed polymer chain properties (i.e., SCBs and LCBs per molecule, SCB and LCB length distribution, branching order distribution (BOD), SCBs and LCBs per 1000 carbon atoms distribution) are accurately predicted at different points along the reactor length. In addition, important topological properties of the polymer chains (e.g., seniority-priority branching distributions, radius of gyration, branching factor distribution, etc.) are directly calculated, on the basis of the calculated topological characteristics of the LDPE polymer chains. Subsequently, the molecular information provided by the kinetic/topology MC algorithm is further employed for the prediction of the viscoelastic behaviour of the polymer melt.

A state-of-the-art rheology model, the so called ?branch on branch? model, is used to accurately predict the rheological properties of LDPE. An ensemble of fully characterized - in terms of their topological characteristics - branched polymer chains, generated by the kinetic/topology MC simulator, is fed to the rheology model and several important viscoelastic properties (e.g., viscosity-shear rate (angular frequency) curve, storage and loss moduli, tan delta, etc) are accurately predicted. Based on the calculated relaxation modulus, other useful properties can be calculated as well (e.g., discrete and continuous relaxation spectra, steady-state recoverable compliance, etc.). Through the combined use of both simulators (i.e., the stochastic MC and the rheological), the effects of the process conditions (e.g., peak temperatures, pressure, solvent concentrations, etc.) on the final rheological properties of LDPE grades can be assessed. The validity of the above computational approach is verified through a direct comparison of model predictions with available experimental measurements on the molecular weight distribution, long chain branching and linear viscoelastic data. Through this comparison it is shown that the theoretical predictions are in excellent agreement with available industrial data on several LDPE grades.

References

Das, C., Inkson N. J., Read D. J., Kelmanson M. A., and McLeish T. C. B., (2006) ?Computational linear rheology of general branch-on-branch polymer,?J. Rheol., 50, 207.

Meimaroglou D. and Kiparissides C., (2010), ?A Novel Stochastic Approach for the Prediction of the Exact Topological Characteristics and Rheological Properties of Highly-Branched Polymer Chains?, submitted to Macromolecules

Pladis P., Meimaroglou D., Baltsas A. and Kiparissides C., (2010), ?Prediction of the Linear Viscoelastic Behavior of Low-density Polyethylene in High-Pressure Reactors? submitted to Journal of Rheology.