Structure-property relationships of polymeric materials

The mechanical properties of polymeric materials are markedly determined by morphological characteristics, supramolecular structure and molecular orientation. These parameters are of particular significance in fibers and films, which are uniaxially or biaxially anisotropic due to their manufacturing process. Influencing structure formation at will, e.g. in thermoplastic processing, allows for a broad variation of properties of such materials and an adaptation according to particular demands.

The correlation between structure or morphology and macroscopic properties and its understanding on a fundamental level is a central theme in physical chemistry. In this regard we study e.g. the course of phase separation processes and its influence on the generation of specific morphologies. This comprises crystallization of predominantly thermoplastic polymers, which often occurs from an oriented state in many technically relevant processes and whose kinetics are markedly controlled by the degree of orientation. Likewise, liquid-liquid phase separation is an important primary step in structure formation with block-copolymers or blends and plays an important role in membrane formation as well. To study such phenomena we apply methods providing structural information on different lengths scales with sufficient temporal resolution. These include spectroscopy, x-ray diffraction and light scattering as well as confocal laser scanning microscopy combined with fluorescence techniques.

Polyelectrolytes as functional materials

Polyelectrolytes are macromolecular compounds carrying a large number of ionizable groups. The combination of high molar mass and high charge density results in properties differing significantly from those of simple electrolytes as well as neutral polymers.

Polyelectrolytes are widespread in biological and technical environments. Numerous surface finishing processes (paper, textiles) need polyelectrolytes to perform satisfactorily. They also play an important role as flocculants in water treatment, as stabilizers in dispersions, and as additives in laundry detergents. Crosslinked polyelectrolytes absorb and bind extremely large quantities of water; that's why they are used as superabsorbants in hygiene articles such as diapers, or in geotextiles.

Our research on polyelectrolytes aims at a fundamental physico-chemical understanding of such systems. We investigate dilute solutions via analysis of the sedimentation equilibrium (analytical ultracentrifugation) and by electric birefringene. Polyelectrolyte networks are studied with respect to mechanical and swelling properties. Static light scattering is used to investigate network heterogeneity, which depends on the conditions and chemistry used for synthesis. Dynamic phenomena in networks are followed by birefringence measurements and also by fluorescence correlation spectroscopy and fluorescence recovery after photobleaching (planned).

As an important prerequisite, all these investigations require well-characterized, tailor-made polyelectrolytes. Besides studying samples from cooperating groups, therefore, an appreciable part of work is devoted to synthesis.

Dynamics in concentrated polymer systems

For a few years we have been investigating the dynamics of linear polymer chains enclosed in networks. Such systems are of interest because they allow to systematically study the model assumptions and general conditions underlying the reptation theory. This theory is of widespread importance for concentrated polymer systems; it deals with molecular dynamics in melts and concentrated solutions.
So far our experimental approach has been based on recording the orientation relaxation after a stepwise deformation by birefringence measurements. Alternatively, we would like to make use of the potentials of confocal laser scanning microscopy. This requires employing polymers labeled by some fluorescence dye. One way is to follow the fluorescence recovery after photobleaching (FRAP): The fluorophor present in a microscopic volume element (typical length scale: 1 mm) is irreversibly bleached by an intense laser pulse and the diffusion of unbleached molecules into this volume element is then analyzed. Contrary to the birefringence method, this allows for a spatial resolution so that scanning of the sample will give information on heterogeneities or gradients.
Another way to study the dynamics with the same equipment is by fluorescence correlation spectroscopy. This technique is methodically related to dynamic light scattering, which is also established in our group.

• Studies in chemistry at the Technichal University Clausthal (1969-1979).
• Ph.D. studies with Prof. Dr. G. Rehage on "Statische und dynamisch-mechanische Untersuchungen über die Elastizität und Struktur von vernetzten Polymeren" (static and dynamic-mechanic investigations on the elasticity and structure of cross-linked polymers) (1979).
• Post-Doctoral Fellow at IBM Research Laboratory in San Jose, California (1979-1980).
• "Akademischer Rat" at the Institute of Physical Chemistry, Technical University Clausthal (1980-1992).
• Habilitation in "Physical Chemistry" at the Technical University Clausthal, topic of the habilitation thesis: "Untersuchungen zur molekularen Gestalt gelöster Polyelektrolyte mit der Methode der elektrischen Doppelbrechung" (Investigations on the molecular shape of dissolved polyelectrolytes with the method of electric birefringence) (1987).
• Research fellow at BASF Corporation, Fibers Division, in Williamsburg, Virgina (1988-1989).
• Research fellow at Textile Research Institute (TRI) in Princeton, New Jersey (1989).
• Professor of Textile and Fiber Chemistry at Stuttgart University and head of the Institutes for Man-Made fibers and textile Chemistry of the DITF in Denkendorf (1992-2002)
• Professor of Physical Chemistry at the Clausthal University of Technology (since 2002)

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