Molecular dynamics studies on spatial scale of low energy excitation in a simple polymer system

TL;DR

This study uses molecular dynamics simulations to analyze the spatial scale of low energy excitations in a simple polymer system, revealing temperature-dependent behavior and a typical group size of six monomers involved in these excitations.

Contribution

It provides new insights into the spatial characteristics and temperature dependence of low energy excitations in polymers through detailed molecular dynamics analysis.

Findings

Identification of a broad peak at specific frequency indicating LEE

Spatial scale of LEE varies with temperature as T^{0.52} below T_g and T^{0.97} above T_g

Approximately six monomers are involved in the motion related to LEE

Abstract

A molecular dynamics simulation is performed to investigate spatial scale of low energy excitation (LEE) in a single linear chain of united atoms. The self part of the dynamic structure function, $S_\mathrm{S}(q,\omega)$, is obtained in a wide range in frequency space ($\omega$) and reciprocal space ($q$). A broad peak corresponding to the LEE is detected at $\omega/2\pi=2.5 \times 10^{11} \mathrm{s^{-1}}$ ($\equiv \omega_{\mathrm{LEE}}/2\pi$) on the contour maps of $S_\mathrm{S}(q,\omega)$, near and below the glass transition temperature ($T_{\mathrm{g}}$=230 K). The $S_\mathrm{S}(q,\omega_{\mathrm{LEE}})$ is symmetric around a maximum along the logarithm of $q$. The inverse of $q_{\mathrm{max}}$, giving the maximum position of $S_\mathrm{S}(q,\omega_{\mathrm{LEE}})$, depends on temperature as $2\pi/q_{\mathrm{max}}\sim T^{0.52}$ for $60 \mathrm{K}<T<T_{\mathrm{g}}$ and $2\pi/q_{\mathrm{max}}\sim T^{0.97}$ for $T_{\mathrm{g}}<T<600 \mathrm{K}$, which is the spatial scale of the motion corresponding to the LEE at low temperatures. Based on a Gaussian approximation for the displacements of monomer groups which give rise to the motion relevant to the LEE, it is found that the number of monomers contained in a group is about 6.