K. Günther-Schade

Positron Annihilation Spectroscopy in Polymers

During the last decades positron annihilation has become a very powerful tool for the investigation of polymers. In particular, positron annihilation lifetime spectroscopy (PALS) yields valuable information on free volume and other properties. The present invited paper gives examples from recent research of the Kiel group. Generally, the so-called standard model, developed by Tao and Eldrup, is used to determine the size of free volume holes from the ortho-positronium (o-Ps) lifetime o-Ps. Despite its success, the model resorts to several assumptions, including a spherical hole shape. Although the deviations from spherical shape are significant for holes above the size of positronium, average hole sizes Vh, determined by the standard model from o-Ps, show a good correlation with diffusivities D of inert gas molecules when plotted as 1/Vh vs logD, as predicted by the free volume approach. The correlation can further be improved by taking into account the cohesive energy density of the polymers. The o-Ps intensity Io-Ps is often taken as a measure of the hole density. However, Io-Ps is also affected by the Ps formation probability and drops during mechanical milling of polymers due to formation of free radicals by chain scission, for instance. IoPs is also seen to change during phase separation in polymer blends. This can be explored to detect both, the binodal and the spinodal decomposition, already at the initial stage which is not easily accessible by other techniques. PALS was also used to study thermosets. Here we show in-situ results on the cross-linking of an epoxy resin. Finally, we demonstrate the benefits of the positron beam technique which allows investigations of polymer thin films and surfaces. For example, very recent results, obtained at the positron beam in Munich, on the structure and dynamics of epoxy films as function of film thickness will be presented.

Molecular Weight Dependence of the Surface Glass Transition of Polystyrene Films Investigated by the Embedding of Gold Nanoclusters

We probed the glass transition temperature T g at the surface of polystyrene (PS) samples with different molecular weights by embedding of gold nanoclusters. The embedding of the nano-clusters was monitored by x-ray photoelectron spectroscopy (XPS). The onset embedding temperature determined by XPS was taken as a measure for the T g at the surface. We observed a small decrease of T g at the surface relative to the bulk for all molecular weights (Δ T g = 2.5- 8.5 K). This decrease is small for short chains (M W = 3.6 kg/mol) and grows to a saturation for M W ≥ 44 kg/mol (Δ T g ≍ 8 K). This is in accord with the smaller relative increase in the number of degrees of freedom between bulk and surface for shorter chains.

Free Volume Evolution in Bulk Metallic Glass during High Temperature Creep

Metallic glasses lack long-range translational symmetry and have excess volume trapped within their amorphous structure, which has a direct bearing on their physical properties including deformation characteristics. Moreover, the trapped excess free volume is directly correlated to the defect concentration facilitating the possibility to model the temperature and time dependence of the free volume changes during creep as a trade off between defect generation and annihilation. Using differential scanning calorimetry (DSC) analysis the residual free volume of a metallic glass can be characterised based on the glass transition peak height (Δ c p ). In the present work constant strain rate tests were carried out at the ‘onset’ ( T g on = 685 K) and ‘point of inflection’ ( T g p = 705 K) of the calorimetric glass transition to study the time dependent flow behaviour in Zr 55 Cu 30 Al 10 Ni 5 bulk metallic glass. Modelling based on DSC analysis and positron lifetime spectroscopy on samples creep deformed to different plastic strain values corroborate the stress decrease after the peak stress (‘stress overshoot’) occurring in bulk metallic glasses with increasing plastic strain to be associated with a small increase in free volume.