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

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.