Molecular dynamics in grafted layers of poly(dimethylsiloxane)

Dielectric relaxation spectroscopy (10^-1 Hz to 10⁶_{Hz) is employed to study the molecular dynamics of poly(dimethylsiloxane) (PDMS, Mw=1.7×10⁵g mol^-1 and Mw=9.6×10⁴g mol^-1) as grafted films with thicknesses d below and above the radius of gyration Rg. For the thinnest films the molecular dynamics becomes faster by up to two orders of magnitude with respect to the bulk resulting in a pronounced decrease of the Vogel temperature T0 and hence the calorimetric glass transition temperature Tg. For films with d=41 nm>Rg} the molecular dynamics is comparable to that of the bulk melt. The results are interpreted in terms of a chain confinement effect and compared with findings for low molecular weight glass forming liquids contained in nanoporous materials. Crystallization effects—well known for PDMS—are observed for films of thicknesses above and below R_g.