Segmental and chain dynamics of polymers: from the bulk to the confined state

Dielectric spectroscopy and temperature modulated DSC are employed to study the molecular dynamics of oligomeric poly(propylene glycol) (PPG) melts of different molecular weights confined to nanoporous glasses (pore sizes 2.5, 5.0, 7.5 and 20 nm). Moreover the results obtained for the polymer are compared with the corresponding monomer. For large pore sizes an acceleration of the segmental dynamics compared to the bulk state is observed which is already known for low molecular-weight 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 of the polymer and of the treatment of internal glass surfaces. The experimental results are discussed in the frame of an interplay of confinement and adsorption effects. Moreover a length scale of about 1.6 nm is estimated as a minimal length scale for the cooperativity for the glass transition. In addition to the α-relaxation the whole chain dynamics (normal mode relaxation) can be measured by dielectric spectroscopy because PPG has a dipole component parallel to the chain. For virgin internal surfaces the relaxation rate of the normal mode relaxation is shifted dramatically to lower relaxation rates. That can be explained by adsorption effects. For treated surfaces this effect is strongly reduced and it is concluded that also in this case the chain dynamics are influenced by geometric (confinement) effects.