From ellipsometry measurements, Pye and Roth [Phys. Rev. Lett. 107, 235701 (2011)] presented evidence of the presence of two glass transitions originating from two distinctly different and simultaneous mechanisms to reduce the glass transition temperature within freestanding polystyrene films with thickness less than 70 nm. The upper transition temperature Tug(h) is higher than the lower transition temperature Tlg(h) in the ultrathin films. After comparing their data with the findings of others, using the same or different techniques, they concluded that new theoretical interpretation is needed to explain the two transitions and the different dependences of Tug(h) and Tlg(h) on film thickness and molecular weight. We address the problem based on advance in delineating the different viscoelastic mechanisms in the glass-rubber transition zone of polymers. Theoretical considerations as well as experiments have shown in time-scales immediately following the segmental alpha-relaxation are the sub-Rouse modes with longer length scale but shorter than that of the Rouse modes. The existence of the sub-Rouse modes in various polymers including polystyrene has been repeatedly confirmed by experiments. We show that the sub-Rouse modes can account for the upper transition and the properties observed. The segmental alpha-relaxation is responsible for the lower transition. This is supported by the fact that the segmental alpha-relaxation in ultrathin freestanding PS films had been observed by dielectric relaxation measurements and photon correlation spectroscopy. Utilizing the temperature dependence of the segmental relaxation times from these experiments, the glass transition temperature T_alpha_g associated with the segmental relaxation in the ultrathin film is determined. It turns out that T_alpha_g is nearly the same as Tlg(h) of the lower transition, and hence definitely segmental alpha-relaxation is the mechanism for the lower transition. Since it is unlikely that the segmental alpha-relaxation can give rise to two very different transitions simultaneously, a new mechanism for the upper transition is needed, and the sub-Rouse modes provide the mechanism.
|Autori:||Daniele Prevosto; Capaccioli S ;K. L. Ngai|
|Titolo:||Origins of the two simultaneous mechanisms causing glass transition temperature reductions in high molecular weight freestanding polymer films|
|Anno del prodotto:||2014|
|Digital Object Identifier (DOI):||10.1063/1.4865752|
|Appare nelle tipologie:||1.1 Articolo in rivista|