Dielectric normal mode relaxation of poly(propylene glycol) melts in confining geometries

Dielectric spectroscopy is employed to study the molecular dynamics of oligomeric poly(propylene glycol) (PPG) liquids (M-w = 1200, 2000 and 4000 g/mol) confined to porous glasses (pore dimensions 2.5, 5.0 and 7.5 nm). In addition to the alpha-relaxation the whole chain dynamics can be measured by that method because PPG has a dipole component parallel to the chain. For the alpha-relaxation it was found that near the glass transition temperature its relaxation rate is lower within the pores than in the bulk. This finding was analyzed from several points of view including density effects, surface effects, and the concept of hindered glass transition. Besides the alpha-relaxation a further relaxation process (N1-process) was found for the PPG molecules confined to the pores. Its relaxation rate is drastic shifted to lower frequencies compared to that of the normal mode process of the bulk material and has a strong pore size dependence. This N1 process was assigned to the dynamics of whole chain or larger parts of it. We argue that the slowing down of its relaxation rate compared to that of the bulk material was mainly controlled by adsorption effects. (C) 1998 Elsevier Science B.V. All rights reserved.