Molecular dynamics of poly(propylene) glycol in nanometer confinements

Dielectric spectroscopy and temperature modulated DSC are employed to study the molecular dynamics (dynamic glass transition) of oligomeric poly(propylene) glycol (PPG) melts (M-W = 1000, 2000, 3000 and 5000) confined to nanoporous glasses (pore sizes 2.5, 5.0, 7.5 and 20 nm). For large pore sizes a speeding up of the molecular dynamic compared to the bulk state is observed which is already known for low molecular glass forming liquids. For smaller pore sizes the molecular dynamics is slower than in the bulk. The observed behavior is nearly independent of the molar mass and of the treatment of internal glass surfaces. The experimental results are discussed in the frame of an interplay of confinement and packing effects. Moreover a length scale of 1.5 nm is estimated as lower bound for the glass transition.