(33e) Structure, Chain Dynamics, and Viscoelastic Response of Semiflexible Disperse Polymer Melts: Insights from Molecular Dynamics Simulations

All synthetic polymers have a certain degree of non-uniformity of chain lengths, characterized by dispersity Ð, owing to their polymerization route. It has been demonstrated that both high and moderate Ð can exhibit complementary properties and ultimately influence processability and final application of the polymer. The effects of chain length dispersity on viscoelastic response remains an open question in polymer science. However, the design space for manipulating this parameter is large and performing experiments can be time consuming. To address this issue, we use molecular dynamics simulations to span a large dispersity and weight average molecular weight range. We model semiflexible linear polymer chains that follow Schulz – Zimm molecular weight distribution, with dispersities between 1–2. While linear polymer chains have the simplest polymer architecture, they display complicated microstructure and rich dynamic behavior. We use the Kremer Grest bead – spring model to study their static conformational structures, dynamics, and viscoelastic response. To control the degree of flexibility of the chains, which in turn affects observable macroscopic properties, we employ a three-body bending potential. First, we investigate the structure by computing their pair correlation function, end-to-end distance and radius of gyration and benchmark these systems with corresponding uniform melt to check any deviation from ideal chain behaviors as we vary Ð and chain stiffness. We also perform entanglement analysis by computing the entanglement length N_e, and mean number of kinks (entanglements) from their primitive paths. Dynamics is studied through the monomer and center of mass mean squared displacement. Due to the heterogeneity in chain length, we analyze the mobility of specific chains in the melt in order to gain insights into molecular mechanisms that may contribute to additional relaxation processes. Lastly, we compute the zero-shear viscosity from the stress autocorrelation function using the Green – Kubo relations and unravel molecular origin of their viscoelastic responses. We also calculate the storage and loss moduli from the stress relaxation behavior. Using the plateau modulus, we estimate the entanglement length and make comparisons with the N_e from the primitive path analysis. In summary, this work provides the necessary foundation to accelerate and guide the predictive design of semiflexible disperse polymer melts.

Keywords: Polymer Melts, Dispersity, Semiflexible, Dynamics, Primitive Paths, Viscoelasticity