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 para llel 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 rel axation process (N1-process) was found for the PPG molecules confined to the pores. Its relaxation rate is drastic shifted to lower frequenc ies compared to that of the normal mode process of the bulk material a nd 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 s lowing down of its relaxation rate compared to that of the bulk materi al was mainly controlled by adsorption effects. (C) 1998 Elsevier Scie nce B.V. All rights reserved.