Professor Harald Pasch
Title: Professor (Chair of Polymer Characterization)
Office: 215 Polymer Science Building
Phone: +27 (0)21 808-3173
Fax: +27 (0)21 808-4967
MSc (Organic Chemistry), Odessa State University, Ukraine, 1976
PhD (Polymer Science), German Academy of Sciences, Berlin, 1982
DSc (Habilitation, Polymer Science), German Academy of Sciences, Berlin, Germany (1987)
Novel multidimensional techniques for polymer characterization; multidimensional liquid
chromatography; coupling of liquid chromatography and spectroscopy (FTIR, MS, NMR); field flow fractionation; capillary electrophoresis; analytical methods for segmented copolymers, polyolefins, water-soluble polymers, polyelectrolytes; structure-property relationships
Research in the Pasch group is focused on the development of multidimensional analytical techniques for complex polymers. This includes the coupling of different separation methods to each other (two-dimensional chromatography) and the hyphenation of separation methods with information-rich detectors like FTIR, NMR, and mass spectrometry. Separation methods to be used include all types of liquid chromatography (SEC, HPLC, Chromatography at critical conditions), field flow fractionation, electrically driven methods, and fractionation methods based on crystallizability.
Molecular parameters to be addressed are chemical composition distribution, functionality type distribution, and topology type distribution and their correlation with molar mass distribution. Polymer systems of special interest are segmented copolymers (block and graft copolymers), functional polymers, hydrophilic copolymers, and polyolefins.
Hydrophilic Copolymers: Hydrophilic copolymers are of increasing importance in a large number of technical applications. They are used e.g. as surfactants in washing agent formulations, emulsifiers in cosmetic applications, flocculants in water treatment, and as additives in ready-made concrete for construction purposes. Very frequently, these copolymers are complex formulations with three or more monomers or oligomers as the building blocks. In all cases, the application properties of these materials are determined by their molecular structure and heterogeneity. In the Pasch group, multidimensional analytical techniques are developed that provide quantitative information on the molar mass distribution, chemical composition distribution and molecular architecture of such copolymers. Useful separation methods are the different types of liquid chromatography, capillary electrophoresis, and field flow fractionation. The analysis of two samples of a dispersing agent for concrete by two-dimensional chromatography is shown in Fig. 1. The simultaneous separation according to chemical composition and molar mass reveals significant differences in the molecular composition of the samples. These can be correlated with the application properties showing that only the more homogeneous sample exhibits the required dispersing properties.
High Throughput Experimentation: Experimental techniques of combinatorial chemistry have become more and more popular in materials research for the discovery of novel polymeric materials. Parallel and high-throughput methods are applied to synthesize large compositional libraries with minimum amounts of material in order to save time and costs. The preparation of large numbers of target compounds, however, is only the first step in a combinatorial set-up. As important as the fast synthesis is the fast analysis of the prepared sample sets. This, however, is quite complicated for polymers as highly complex materials that very frequently exhibit chemical or functional heterogeneities in addition to the molar mass distribution.
In the Pasch group analytical approaches for high throughput experimentation are developed that focus on liquid chromatography and the combination of liquid chromatography and spectroscopy. While a chromatographic experiment takes 10-30 min with conventional stationary phases, this time can be reduced to a few minutes by using novel materials. Fig. 2 shows the molar mass separation of a styrene-ethyl acrylate copolymer using a conventional three-column set as compared to new highly selective single columns. Within the time for one conventional experiment, 10 experiments can be conducted with the new columns.
Using gradient HPLC the same copolymers can be fractionated by chemical composition. Such fractionations can be conducted in less than 3 min when very small but highly selective HPLC columns are used, see Fig. 3. The photograph shows the small column as compared to a conventional size column. The high reproducibility of the fractionations can be seen from the overlay of five injections.
MALDI-TOF Mass Spectrometry of Complex Polymers: Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is the most powerful mass spectrometric technique for complex polymers. Information on oligomer distributions, functional groups, copolymer compositions, branching etc. can be extracted from MALDI spectra. The advantage of MALDI over other mass spectrometric techniques is that fragmentation of large molecules is significantly reduced and macromolecules can be analysed as intact ions. In the Pasch group MALDI mass spectrometry is used as an information-rich detector for polymer fractionation techniques. Fractions e.g. from the SEC separation of a complex copolymer can be subjected to MALDI analyses to determine the copolymer composition. As can be seen in Fig. 4, the composition of high molar mass fractions (red) is different from the composition of low molar mass fractions (green). The high molar mass fractions can readily be identified as copolymer, while the low molar mass fraction is a homopolymer.
- H. Pasch, B. Trathnigg: HPLC of Polymers. Springer Publishers, 1997/1998
- H. Pasch, W. Schrepp: MALDI-TOF Mass Spectrometry of Synthetic Polymers. Springer Publishers, 2003
- H. Pasch: Characterization of Polymer Heterogeneity by 2D-LC. in "Multiple Detection in Size-Exclusion Chromatography” (A. M. Striegel, editor), ACS Symposium Series 893, American Chemical Society: Washington, DC; 2004
- M. Adler, F. Rittig, S. Becker, H. Pasch: Multidimensional Chromatographic and Hyphenated Techniques for Hydrophilic Copolymers. 1. Analysis of Comb-like Copolymers of Ethylene Oxide and Methacrylic Acid.
Macromol. Chem. Phys. 2005, 206, 2269.
- W. Hiller, H. Pasch, T. Macko, M. Hoffmann, J. Ganz, M. Spraul, U. Braumann, R. Streck, J. Mason, F. van Damme: On-line Coupling of High Temperature SEC and 1H-NMR for the Analysis of Polymers.
J. Magn. Res. 2006, 183, 309
- A. Albrecht, R. Brüll, T. Macko, H. Pasch: Separation of Ethylene-Vinyl Acetate Copolymers by High-Temperature Gradient Liquid Chromatography. Macromolecules 2007, 40, 5545.