Ultrathin polymer films by single molecule deposition

While physical vapour deposition of glass forming materials below their glass transition temperature, Tg, is an exciting route towards glasseswith an extremely high packing density, strong kinetic and thermodynamic stability, vapour deposition of polymer systems is less evident and has so far not been investigated systematically. Here we have successfully prepared ultrathin films of low molecular mass polystyrene (800 g/mol) by thermal evaporation from the melt into a UHV chamber at a maximum deposition rate of 1 nm/h. Samples were studied in situ and in real-time by dielectric spectroscopy, chip-based ac-calorimetry and a quartz crystal microbalance, both during deposition and after reaching the final sample thickness of 5 nm. During film growth well below the bulk-Tg, an initially retarded deposition kinetics was observed along with an accelerated dielectric relaxation dynamics compared to the bulk glass transition. Subsequent temperature cycling above the bulk-Tg revealed continuous changes in the (dielectric) glass transition dynamics and finally lead to desorption of the material at elevated temperatures without restoring the “dielectric” bulk glass transition dynamics. In contrast, simultaneous specific heat spectroscopy revealed bulk dynamics, a striking discrepancy that was discussed in terms of a dominant and accelerated response of PS end-groups in the dielectric spectra in combination with terminal sub-chain dynamics and some degree of end-group segregation.status: publishe