Segmental Mobility and Glass Transition Temperature of Freely Suspended Ultrathin Polymer Membranes

A new experimental method taking advantage of the sensitivity of dielectric spectroscopy (DS) and being able to probe the segmental dynamics of freely standing ultrathin polymer layers over a broad frequency range without altering or eliminating their two free surfaces, was studied. The polymer films were suspended over interdigitated comb electrodes, IDE. The electric signal was measured by applying an ac voltage to elevated metallic electrodes made up by the two interdigitated comb structures deposited on a highly insulating substrate. Atactic polystyrene of three different molecular weights(Mw) was used. Exact amounts of the polymer and the probe were mixed together and dissolved in chloroform. Film thicknesses were determined by ellipsometry on reference samples prepared under the same spincoating conditions. The new finding proves that the tremendous Tg reductions in freely standing films of PS are observed on the basis of discontinuities in the volume thermal expansivity are related to a correspondingly huge shift in the time scale of the structural relaxation.