Changes in the structural and dynamical properties in glass transition and related processes of poly(vinyl alcohol) analyzed by molecular dynamics simulations.

Abstract

Elucidating how structural and dynamical properties of a polymer chain are varied in a glass transition process is crucial for deepening our understanding of its real nature. Here, an analysis on this point has been conducted for poly(vinyl alcohol) based on molecular dynamics simulations. It is shown that the temperature dependence of the average density has more than one point of change in slope, and by introducing the second derivative method, it is possible to correctly find out those crossover temperatures. It is also shown that the temperature dependence of the dynamics of two structural properties of spatially extended nature, the end-to-end distance and the radius of gyration, has three distinct regions separated also at those crossover temperatures, and considering the relaxation time scales, the low-, intermediate-, and high-temperature regions are assigned to the glassy, (supercooled) rubbery, and liquid (melted) states, respectively. By examining the spectrum of the vibrational density of states in a low-frequency region, which represents the dynamics of spatially more localized nature, it is suggested that inter-basin hopping associated with main-chain C-C torsions is thermally activated as a precursor (from the low-temperature side, β relaxation) to the glass transition. A change in the hydrogen-bonding network manifested by a spectral feature of the vibrational density of states is also discussed.