Varying systematically the structure of glassy poly(amide imide), poly(ester imide), and polyimide, we have studied the correlation between free volume and transport properties of highly selective polymer membranes. Free volume data were determined by means of positron annihilation lifetime spectroscopy (PALS) while transport properties originate from time-lag measurements of permanent gases. We find a good correlation between PALS average hole size and transport coefficients. The correlation is much better than with free volume data from group contribution methods. It is shown that the permeation properties are controlled not only by free volume fluctuations but also by energy barriers. A modified transport model taking into account the effect of the cohesive energy density on the energy barriers further improves the correlation significantly.

We compare molecular modeling results of two glassy polymer membranes and one rubbery polymer membrane with gas transport parameters and free-volume-related quantities from positronium annihilation. A simple geometric model reveals hole size distributions of asymmetric shape. Among glassy polymers, the distribution parameters show a good correlation with average hole sizes determined by positron annihilation lifetime spectroscopy. Higher permeability is measured in the glassy polymer with the higher mean value of the hole size distribution. The permselectivity of the membranes for permanent gases can be interpreted in terms of the distribution broadness via free-volume-controlled diffusion selectivity. A comparison with the rubbery polymer shows that the permeation behavior is not determined only by the free volume concentration. The thermal fluctuations of the polymer matrix play an important role for gas transport properties.

Chain mobility in a near surface region at a polystyrene/vacuum interface was investigated by embedding of noble metal nanosized clusters. The embedding process was monitored in situ by X-ray photoelectron spectroscopy. The embedding of nanosized clusters needs long-range chain mobility of the polymer. Therefore, the embedding process is a probe for the glass transition in a near surface region. The clusters used in this study are formed by the dewetting of evaporated noble metals onto the polymer surface. First, methodical influences on the embedding process were investigated. An onset embedding temperature T* was defined. T* increases with a heating rate comparable to T g(bulk) determined with a differential scanning calorimeter. Furthermore, T* increases with nominal metal coverage which results in increasing average cluster radius. The cluster size distribution was investigated by transmission electron microscopy. It is shown that T* is an upper limit for T g in a near surface region with a depth of a few nanometers. With optimized probe conditions, embedding processes were performed on monodisperse polystyrene (M w = 3−1000 kg/mol). The T* values fit quite well with the Fox−Flory relation, but with a saturation temperature of approximately 8 K below the bulk value. ΔT = T g(bulk) − T* increases with molecular weight. This molecular weight dependence of T* will be discussed in terms of several models. The chain end segregation model can be ruled out. To investigate the kinetics of the embedding process, isothermal experiments were performed. From these experiments surface viscosities were derived, which are well below bulk values.

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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 V h , determined by the standard model from o-Ps , show a good correlation with diffusivities D of inert gas molecules when plotted as 1/V h 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 I o-Ps is often taken as a measure of the hole density. However, I o-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. I o-Ps 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.