Broadband dielectric study of anomalous diffusion in a poly(propylene glycol) melt confined to nanopores

Abstract

The molecular dynamics of an oligomeric poly(propylene glycol) (PPG) melt confined to nanopores (pore dimensions 2.5, 5.0, and 7.5 nm) is studied by broadband dielectric spectroscopy. Because poly(propylene glycol) has an in chain dipole moment, the molecular motion of the whole chain (normal mode relaxation) can be measured by dielectric spectroscopy, in addition to the dynamic glass transition (α relaxation). For the α relaxation it was found that near the glass transition temperature its relaxation rate is greater within the pores compared to the bulk. The possible reasons for this speeding up of the α relaxation in the nanopores are discussed. This discussion includes the density effect, the influence of surface–melt interactions, as well as the concept of the hindered glass transition. Besides the α relaxation, a further relaxation process (N1 process) was found for the PPG molecules confined to the nanopores. The relaxation rate of the N1 process is drastically shifted to lower frequencies, compared to both the α relaxation and to the normal mode process, and shows a pronounced dependence on the pore size. This relaxation process was assigned to the dynamics of a whole chain or larger parts of it. The slowing down of its relaxation rate compared to that of the bulk material was mainly controlled by adsorption effects and the dependence on the pore size could be understood in the framework of a simple model.