Abstract
Macromolecular liquids display short-time anomalous behaviors in disagreement with conventional single-molecule mean-field theories. In this study, we analyze the behavior of the simplest but most realistic macromolecular system that displays anomalous dynamics, i.e., a melt of short homopolymer chains, starting from molecular dynamics simulation trajectories. Our study sheds some light on the microscopic molecular mechanisms responsible for the observed anomalous behavior. The relevance of the correlation hole, a unique property of polymer liquids, in relation to the observed subdiffusive dynamics, naturally emerges from the analysis of the van Hove distribution functions and other properties.
Highlights
The dynamics of synthetic and natural macromolecular fluids is described conventionally by mean-field theories of single-molecule motion
The Rouse model provides a simple description of chain dynamics for long polymer chains, while predicting the scaling exponents of chain dynamics in remarkable agreement with experiments [1,14]
neutron spin echo (NSE) experiments show that the dynamics at times shorter than the Rouse longest relaxation time are subdiffusive
Summary
The dynamics of synthetic and natural macromolecular fluids (e.g., polymer melts [1], proteins [2,3,4,5], DNAs [6,7,8], and cellular microfilaments [9]) is described conventionally by mean-field theories of single-molecule motion. The underlying assumption in these approaches is that the relaxation of the surrounding fluid occurs on a different timescale compared to the dynamics of the tagged molecule. When this hypothesis holds, it is possible to derive a single-chain equation of motion by projecting, through Mori–Zwanzig techniques [10], the dynamics of the entire fluid onto a set of slow relevant variables (here, the coordinates of the tagged chain). Summarizing, the simple mean-field formalism of the Rouse model provides a useful general description of the polymer dynamics, which one can improve by including a more realistic molecular description than the typical chain of beads connected by harmonic springs. The fundamental hypothesis of the separation of timescales that motivates the Rouse formalism, i.e., a Langevin equation in the lab-frame for the monomer (beads) coordinates, becomes questionable when describing systems where the “solute” and the “solvent” relax on the same timescale [22]
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