(597b) Investigating the Effect of Chain Length Dispersity on Dynamics of Homopolymer Melts from Coarse Grained Molecular Dynamics

Polymeric materials are used in a wide range of commercial applications, from packaging to consumer products and more recently in electronic devices with an annual production volume of over 100 million metric tons. Efforts towards enhancing their performance and processability to expand their scope of application are currently underway both at the fundamental and industrial level. Polymer physical properties are closely linked to molecular weight dispersity (Đ) and shape of the molecular weight distribution (MWD). Đ is a measure of the variability in chain sizes in the polymer sample. In an ideal case, a polymer sample with dispersity of unity is termed monodisperse (uniform) and samples greater than unity are disperse, with a non-uniform chain size. These parameters can significantly impact mechanical and rheological response of the polymer melts. For instance, monodisperse melts exhibit non-Newtonian behavior at high shear rates compared to melts with broader dispersity. However, the design space for controlling these parameters is huge and current synthetic strategies such as polymer blending and temporal regulation of addition polymerization maybe expensive and time consuming. Specifically, the mobility of chains in a melt can provide insights into unique viscoelastic response of disperse polymer melts. Fundamental assessment of small scale dynamics of different melts is not easily accessible using experiments alone. Here, we use coarse grained molecular dynamics (CGMD) to capture the molecular mechanisms that brings about the observable macroscopic changes of polydispersed melts that follows Schulz-Zimm and Flory-Schulz molecular weight distribution.

Specifically, we focus on systems that range from short, and moderately entangled chains, by systematically varying the dispersity for melts for a specific MWD. Dispersity of up to 4 was achieved by varying specific distribution parameter, the limit of shortest and longest chain length and total number of chains in the melt. All systems were benchmarked with monodisperse melts with the same average molecular weight. We analyze the structure using radial distribution function, radius of gyration, and structure factor. We then probe dynamics from the mean square displacement (MSD) and end-to-end autocorrelation function. We find that with an increase in dispersity, there is a considerable difference in the average mobility of chains vs. the monodisperse system. The shortest chains in the disperse melt move significantly faster than the longest chains, as a result of this there is some form of constraint release pathways offered by the faster motion of the shorter chains in the melts, thereby increasing the longer chains mobility. We also find that the global and internal structures are dependent on the Đ and molecular weight distribution as well. These findings indicate that polymer properties may be tuned using Đ and MWD.

Keywords: Polymer melts, viscoelastic, constraint release, dispersity, Schulz-Zimm distribution, Flory - Schulz distribution, coarse - graining