Interplay between Core and Interfacial Mobility and Its Impact on the Measured Glass Transition: Dielectric and Calorimetric Studies

Abstract

The dynamics and thermodynamics of confined triphenyl phosphite (TPP) were studied using broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). Geometric confinement in channels having length scales commensurate with the molecular size of TPP causes bifurcation of the dynamics: two populations are observed, distinguished by their reorientational mobilities and glass transition temperatures. Upon cooling, significant changes in the relaxation process and temperature dependence occur due to the slow vitrification of the molecules in close proximity to the interface. Such a kinetic aspect of glass formation is unusual. This surface interaction alleviates constraints on the molecules, allowing their glass transition to shift to lower temperatures. Simultaneously, it was observed that the structural relaxation process shifts to lower frequencies, and the distribution of the relaxation times becomes narrower upon annealing. This effect is especially visible at lower frequencies, indicating the decreasing contribution of those molecules characterized by slower dynamics. In addition, it was found that structural relaxation times, as well as the glass transition temperatures, can be significantly modified by annealing samples over a particular range of temperatures. This work facilitated the understanding of the interplay between different kinds of mobility and its impact on changes in the glass transition temperature for two-dimensional confined materials.