Boson peak, elasticity, and glass transition temperature in polymer glasses: Effects of the rigidity of chain bending

Naoya Tomoshige,Nobuyuki Matubayasi,Kang Kim,Hideyuki Mizuno,Tatsuya Mori

doi:10.1038/s41598-019-55564-2

Abstract

The excess low-frequency vibrational spectrum, called boson peak, and non-affine elastic response are the most important particularities of glasses. Herein, the vibrational and mechanical properties of polymeric glasses are examined by using coarse-grained molecular dynamics simulations, with particular attention to the effects of the bending rigidity of the polymer chains. As the rigidity increases, the system undergoes a glass transition at a higher temperature (under a constant pressure), which decreases the density of the glass phase. The elastic moduli, which are controlled by the decrease of the density and the increase of the rigidity, show a non-monotonic dependence on the rigidity of the polymer chain that arises from the non-affine component. Moreover, a clear boson peak is observed in the vibrational density of states, which depends on the macroscopic shear modulus G. In particular, the boson peak frequency ωBP is proportional to sqrt{G}. These results provide a positive correlation between the boson peak, shear elasticity, and the glass transition temperature.

Highlights

The excess low-frequency vibrational spectrum, called boson peak, and non-affine elastic response are the most important particularities of glasses
It is crucial for Molecular dynamics (MD) simulations to solve finite-dimensional effects that are not captured by the mean-field treatments[45,46,47]
We demonstrate that polymeric glasses can exhibit extremely-large non-affine elastic response, whereas the boson peak (BP) is scaled by the behavior of macroscopic shear modulus

Summary

Introduction

The excess low-frequency vibrational spectrum, called boson peak, and non-affine elastic response are the most important particularities of glasses. The excess vibrational modes at low frequencies and the excess heat capacity at low temperatures exceeding the Debye predictions, which describe the corresponding crystalline values, have been observed universally in various glassy materials This phenomenon, which is referred to as the boson peak (BP), has been widely studied. Relevant systems to experiments and applications have been simulated, including covalent-bonding[48,49,50,51,52,53], metallic[54,55,56,57], polymeric[58,59,60,61,62] glasses These simulation studies complete theoretical understandings based on simple systems and experimental observations of more complex systems. Another experiment[20] has shown that the polymerization effects on the BP is explained by the change in macroscopic elasticity as the frequency and intensity variations of the BP are both scaled by the Debye values

Methods

Results

Conclusion