Vianna, Sulivan Dias Borges (2012)
The influence of solid surfaces on the structure and dynamics of polymer melts.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

The structure and dynamics of polymer melts in the proximity of a solid substrate was investigated. This issue is relevant due to its basic scientific aspects as well as for technological applications in the field of interfaces and composite materials. For this, polystyrene of various molecular weights (in the entangled and non-entangled regime) were spin coated on gold surfaces which were created by thermal evaporation. Polystyrene was chosen due to its vast technological application range and for being a well characterized material. The polymer films were characterized using resonance enhanced dynamic light scattering (REDLS) and kinetic surface plasmon resonance spectroscopy (SPR). For this an appropriate REDLS setup adapted for the problem was built. The film thickness ranged from about 4 nm to 440 nm. For different molecular weights, the glass transition temperatures (Tg) were measured by kinetic SPR and compared to the value of Tg of bulk polymer measured by DSC at corresponding heating or cooling rates. Within the experimental errors, no differences in Tg were observed for the entangled and non-entangled polystyrenes samples (350 000 g/mol and 1821 g/mol respectively) measured down to 4 nm for the entangled polymer and 8 nm for the non-entangled molecular weight. The behavior of the dynamics investigated by REDLS revealed two main processes, a slow- and a fast-mode. The slow-mode was shown to be attributed to the free-surface dynamics, specifically the dynamics of thermally activated capillary waves. The signal of the fast-mode originated throughout the entire film thickness and was correlated to the presence of solvent (toluene) residues. Thoroughly annealed films did not show a fast-mode. From the theory of thermally activated capillary waves it is possible to infer physical properties of these supported polymer films, such as e.g. viscosity (eta) and shear modulus (G). The results obtained by REDLS of thin films at a given temperature and capillary wave frequency were compared with rheological measurements of bulk polystyrene at the same temperature and frequency. No differences of eta and G were observed between thin films and the bulk polymer. It was also possible to state that the dynamics of the freesurface is described by a Vogel-Fulcher-Tamman like behavior (VFT) instead of fitting to the Arrhenius equation as proposed by some authors. However, the solid surface with a no-slip boundary condition at the polymer-solid contact leads to the dynamics of the free-surface to slow down while decreasing the film thickness. This should not be confused with a chain confinement effect, but it is simply a hydrodynamic effect governed by the film thickness and the no-slip boundary condition. Even though the film thickness has an impact on the measured capillary wave frequency, it was possible to show that the temperature and frequency dependence of eta and G remain unaltered with film thickness. The relevant factor changing the dynamics is in fact the film thickness itself.

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