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

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 is scaled as $\omega_\mathrm{BP} \propto \sqrt{G}$. These results provide a positive correlation between the boson peak, shear elasticity, and the glass transition temperature.