Regensburg 2004 – wissenschaftliches Programm
CPP 14: SYMPOSIUM: Understanding and Controlling Complex Structures: From Synthetic Polymers to Biomaterials II
CPP 14.7: Vortrag
Dienstag, 9. März 2004, 16:15–16:30, H 37
Single Molecule Studies of Surface Melting in Free Standing Polymer Films — •Frank Cichos1 and Jörg Schuster2 — 1Institut für Physik 123705, TU Chemnitz, 09107 Chemnitz — 2Institut für Physik 121501, TU Chemnitz, 09107 Chemnitz
Thin polymer films show a drastic decrease of the glass transition temperature with decreasing film thickness. While this effect depends on the chain length if the radius of gyration of the polymer chain is on the order of the film thickness, it is independent of it for small molecular weights and thus points to a very general effect involved in the glass transition. We present for the first time results of single molecule studies in free standing polymer films far below the bulk glass transition temperature. By tracking individual fluorescent molecules we are able to follow the local dynamics in the film as a function of temperature. We show that already at temperatures of about 40 K below the bulk glass transition temperature a small highly mobile fraction of dye molecules coexists with a large number of completely immobile molecules. The mobile fraction shows translational diffusion which only weakly depends on temperature. However, the number of molecules released from the film strongly increases with increasing temperature. This increase is explained by the desorption of dye molecules adsorbed at the polymer surface and by the release of molecules from a growing No-dqmoltenNo-dq layer. We interpret these findings in terms of an inhibition of the glass transition in the film by the dynamics of the surface. This inhibition becomes more effective for smaller film thicknesses and therefore effectively decreases the glass transition temperature of the film compared to the bulk value.