Comment on Justin E. Pye and Connie B. Roth, J. Polym. Sci., Part B Polym.Phys. 2015, 53, 64-75

TL;DR

This paper critically examines previous claims about transitions in polystyrene films, using dielectric data to clarify the nature of these transitions and their relation to molecular motions and aging.

Contribution

It provides experimental evidence that sub-Rouse modes, not segmental relaxations, explain the observed transitions in polystyrene films.

Findings

Sub-Rouse modes are coupled to density and respond to aging.

The upper transition is not from segmental a-relaxation, contrary to previous suggestions.

Dielectric data clarifies the origin of transitions in high molecular weight PS films.

Abstract

Two transitions were found by ellipsometry in freestanding high molecular weight PS films by Pye and Roth (PR). PR suggested that the upper and the lower transitions both came from the segmental a-relaxation, and the upper transition occurs in 90% of the material. In this paper we use dielectric relaxation data of freestanding films to rule out their suggestion. Furthermore, we demonstrate by experimental evidences that the sub-Rouse modes are coupled to density, and respond to physical aging to validate our interpretation and can explain the upper transition.