(167j) Effect of Dispersity on the Rheological Properties of Polyolefins from Coarse - Grained Molecular Dynamics Simulations

Polyolefins are used in a wide range of commercial applications; tailoring polyolefin properties to reduce the cost of production is constantly underway both at the fundamental and industrial level. Manipulating polyolefin properties for specific applications without altering their chemistry remains a major challenge in polymer science. Dispersity, which is a measure of the width of molecular weight distribution (MWD) and the heterogeneity (or uniformity) of the various chain molecular weights within a polymeric material, can alter the mechanical, rheological and morphological properties of polymers without affecting their chemistry. For instance, monodisperse polymers exhibit non-Newtonian behavior at high shear rates compared to polymers with broader dispersity. Synthetic strategies such as polymer blending and temporal regulation of addition polymerization have been used to control the dispersity of polymers. However, the design space for tuning dispersity is limitless as there are numerous ways by which the distribution can be varied. In this regard, using experiments alone to understand how the shape of the MWD curve can tune polyolefin properties for a range of dispersities can be laborious and expensive.

In this work, we perform coarse-grained molecular dynamics simulations using a simple bead-spring model that has been demonstrated to accurately capture the underlying polymer physics of polyolefin melts. We focus on linear chains that are below the entanglement length, and systematically vary the average molecular weight and dispersity. We compute the radius of gyration and the radial distribution function, followed by mean-squared displacement and chain end-to-end autocorrelation function to understand the structure and dynamics of the different systems. Understanding the molecular phenomena that leads to changes in rheological properties by varying the dispersity is central to polymer design, and we will discuss rheological properties such as viscosity and stress relaxation obtained from equilibrium stress fluctuations. Future work will include the effects of molecular weight distribution shape and or/skewness on mechanical and rheological properties of polyolefins.