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 ?-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.
Dielectric spectroscopy in combination with temperature modulated differential scanning calorimetry and quasielastic/inelastic neutron scattering are employed to investigate the molecular (glassy) dynamics of poly(dimethyl siloxane) (PDMS) and poly(methyl phenyl siloxane) (PMPS) confined to random nanoporous glasses with nominal pore sizes between 2.5 nm and 20 nm. Inside the pores PDMS and PMPS have faster molecular dynamics than in the bulk state. Down to a pore size of 7.5 nm the temperature dependence of the relaxation times (or rates) obeys the Vogel/Fulcher/Tammann (VFT) equation where the data obtained from dielectric and thermal spectroscopy agree quantitatively. At a pore size of 5 nm this VFT-like temperature dependence changes to an Arrhenius behavior. At the same confining length scale the increment of the specific heat capacity at Tg normalized to the weight of the confined polymer vanishes. The results indicate that a minimal length scale seems to be relevant for glassy dynamics in both polymers although the estimated length scale of about 5 nm seems to a bit too large in comparison to other experimental results and theoretical approaches. Neutron scattering is employed to investigate methyl group reorientation and the fast segmental dynamics of both polymers in confinement. Although the methyl group rotation is a localized process these experiments show that a part of the methyl groups is immobilized by the confinement whereas the effects for PDMS are much more pronounced than for PMPS. With regard to the segmental dynamics, neutron scattering reveals a big difference in the behavior of both polymers. Whereas the data obtained for PMPS are in accord with a boundary layer formed at the surfaces of the nanopores, for PDMS a considerable amount of elastic scattering is observed. To explain this result it is assumed that some structure formation of PDMS takes place in the nanopores, although the thermal data show no crystallization or melting effects.
Specific heat spectroscopy (SHS) employing AC nanochip calorimetry was used to investigate the glassy dynamics of ultra-thin films (thicknesses: 10 nm–340 nm) of a polymer blend, which is miscible in the bulk. In detail, a Poly(vinyl methyl ether) (PVME)/Polystyrene (PS) blend with the composition of 25/75 wt. % was studied. The film thickness was controlled by ellipsometry while the film topography was checked by atomic force microscopy. The results are discussed in the framework of the balance between an adsorbed and a free surface layer on the glassy dynamics. By a self-assembling process, a layer with a reduced mobility is irreversibly adsorbed at the polymer/substrate interface. This layer is discussed employing two different scenarios. In the first approach, it is assumed that a PS-rich layer is adsorbed at the substrate. Whereas in the second approach, a PVME-rich layer is suggested to be formed at the SiO2 substrate. Further, due to the lower surface tension of PVME, with respect to air, a nanometer thick PVME-rich surface layer, with higher molecular mobility, is formed at the polymer/air interface. By measuring the glassy dynamics of the thin films of PVME/PS in dependence on the film thickness, it was shown that down to 30 nm thicknesses, the dynamic Tg of the whole film was strongly influenced by the adsorbed layer yielding a systematic increase in the dynamic Tg with decreasing the film thickness. However, at a thickness of ca. 30 nm, the influence of the mobile surface layer becomes more pronounced. This results in a systematic decrease in Tg with the further decrease of the film thickness, below 30 nm. These results were discussed with respect to thin films of PVME/PS blend with a composition of 50/50 wt.%as well as literature results.