BROAD-BAND DIELECTRIC STUDY OF ANOMALOUS DIFFUSION IN A POLY(PROPYLENE GLYCOL) MELT CONFINED TO NANOPORES

The molecular dynamics of an oligomeric poly(propylene glycol) (PPG) m elt confined to nanopores (pore dimensions 2.5, 5.0, and 7.5 nm) is st udied by broadband dielectric spectroscopy. Because poly(propylene gly col) has an in chain dipole moment, the molecular motion of the whole chain (normal mode relaxation) can be measured by dielectric spectrosc opy, in addition to the dynamic glass transition (alpha relaxation). F or the alpha relaxation it was found that near the glass transition te mperature its relaxation rate is greater within the pores compared to the bulk. The possible reasons for this speeding up of the alpha relax ation in the nanopores are discussed. This discussion includes the den sity effect, the influence of surface-melt interactions, as well as th e concept of the hindered glass transition. Besides the alpha relaxati on, a further relaxation process (N-1 process) was found for the PPG m olecules confined to the nanopores. The relaxation rate of the N-1 pro cess is drastically shifted to lower frequencies, compared to both the cu relaxation and to the normal mode process, and shows a pronounced dependence on the pore size. This relaxation process was assigned to t he dynamics of a whole chain or larger parts of it. The slowing down o f 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. (C) 1998 Ameri can Institute of Physics.