Collective Intermolecular Motions Dominate the Picosecond Dynamics of Short Polymer Chains

Humphrey Morhenn, Sebastian Busch, Hendrik Meyer, Dieter Richter, Winfried Petry, and Tobias Unruh

Phys. Rev. Lett. 111, 173003 – Published 22 October 2013

Abstract

Neutron scattering and extensive molecular dynamics simulations of an all atom $C_{100} H_{202}$ system were performed to address the short-time dynamics leading to center-of-mass self-diffusion. The simulated dynamics are in excellent agreement with resolution resolved time-of-flight quasielastic neutron scattering. The anomalous subdiffusive center-of-mass motion could be modeled by explicitly accounting for viscoelastic hydrodynamic interactions. A model-free analysis of the local reorientations of the molecular backbone revealed three relaxation processes: While two relaxations characterize local bond rotation and global molecular reorientation, the third component on intermediate times could be attributed to transient flowlike motions of atoms on different molecules. The existence of these collective motions, which are clearly visualized in this Letter, strongly contribute to the chain relaxations in molecular liquids.

Received 7 February 2013

DOI:https://doi.org/10.1103/PhysRevLett.111.173003

Authors & Affiliations

Humphrey Morhenn^*

Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) and Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München, 85747 Garching, Germany and Friedrich-Alexander-Universität Erlangen-Nürnberg, Lehrstuhl für Kristallographie und Strukturphysik, Staudtstrasse 3, 91058 Erlangen, Germany

Sebastian Busch

Department of Biochemistry, University of Oxford, South Parks Road, OX1 3QU Oxford, United Kingdom

Hendrik Meyer

Institut Charles Sadron, Université de Strasbourg, CNRS UPR22, 67034 Strasbourg, France

Dieter Richter

Institut für Festkörperforschung, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany

Winfried Petry

Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) and Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München, 85747 Garching, Germany

Tobias Unruh^†

Friedrich-Alexander-Universität Erlangen-Nürnberg, Lehrstuhl für Kristallographie und Strukturphysik, Staudtstrasse 3, 91058 Erlangen, Germany

^*hmorhenn@frm2.tum.de
^†tobias.unruh@fau.de

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Vol. 111, Iss. 17 — 25 October 2013

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Images

Figure 1
Normalized intermediate scattering functions as measured with quasielastic neutron scattering (QENS, symbols) and calculated from the molecular dynamics simulation (MD, solid lines) for different values of momentum transfer $Q$ at $T = 509 K$ . The merged QENS spectra are divided by constant factors (ranging from 1.10 to 1.35) for scaling, as $I (Q, t = 0)$ is not well defined in the experiment due to the limited kinetmatic range of the neutrons. The QENS and MD simulation data are in very good agreement.Reuse & Permissions
Figure 2
Mean-square displacement (MSD) obtained from the simulation at $T = 509 K$ for the center monomers of the $C_{100} H_{202}$ chains (small symbols) and for the center-of-mass coordinates (c.m., big symbols). Additionally plotted are theoretical predictions considering viscoelastic hydrodynamic interactions (dotted line, Eq. (34) in 19) and density fluctuations (dashed line, Eq. (30) in 18) to account for the subdiffusive behavior of the c.m. MSD. The arrows indicate the mean-square chain end-to-end distance $R_{e}^{2}$ , the squared statistical segment length $b^{2}$ , and the segmental $t_{S}$ and Rouse $t_{R}$ relaxation time, to visualize the range of validity of the models. The relaxation times were extracted from a standard Rouse analysis of the normal modes [17]. The solid line represents a $t^{1 / 2}$ power law.Reuse & Permissions
Figure 3
Orientation autocorrelation function of the carbon–carbon bonds at a central position of the molecule (symbols), together with a fit of Eq. (3) (solid line). The resulting distribution of relaxation times (DRT, dashed line, multiplied by 10 for better visualization) shows three distinct relaxation processes.Reuse & Permissions
Figure 4
DRTs for different positions of the carbon–carbon bonds along the molecule (chain end to chain center). The amplitude of the DRTs is presented in color code. Three distinct chain relaxation processes can be identified at central positions of the molecular backbone.Reuse & Permissions
Figure 5
Scalar product of unit displacement vectors of intermolecular neighboring carbon atoms $s (τ$ ) (solid line) and the DRT for the carbon–carbon bond reorientation at a central position of the $C_{100} H_{202}$ chain (dashed line). The maximum of $s (τ)$ corresponds to the onset of the second relaxation process of the carbon–carbon bond vectors.Reuse & Permissions
Figure 6
Displacement vectors of atoms during a time span of $τ = 20 ps$ in a 0.25 nm thick 2D cut ( $23.5 \times 23.5 {nm}^{2}$ ) of the simulation system. The colors highlight the direction of the displacements. Regions of correlated dynamics can easily be identified.Reuse & Permissions

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