Pritish Sinha

Two‐dimensional chromatography of complex polymers, 8. Separation of fatty alcohol ethoxylates simultaneously by end group and chain length

A comprehensive two-dimensional liquid chromatography system was developed to precisely describe the molecular heterogeneity of fatty alcohol ethoxylates. The end-group functionality was analyzed by gradient HPLC while ethylene oxide oligomer distributions were characterized by liquid adsorption chromatography. A baseline separation of all functionality fractions irrespective of the ethylene oxide oligomer chain length was achieved on nonpolar X-Terra C(18) with a methanol-water gradient, whereas an isocratic flow of isopropanol-water on a polar Chromolith Si column gave a separation according to the oligomer chain length without interference of the end-group distribution. The combination of these two methods to conduct online two-dimensional liquid chromatography experiments resulted in a comprehensive two-dimensional picture on the molecular heterogeneity of the sample.

Analysis of polystyrene-b-polyisoprene copolymers by coupling of liquid chromatography at critical conditions to NMR at critical conditions of polystyrene and polyisoprene.

For the investigation of the molecular heterogeneity of polystyrene-b-polyisoprene block copolymers, a chromatographic separation method, namely liquid chromatography at critical conditions was developed. This method was coupled on-line with (1)H-NMR(where NMR stands for nuclear magnetic resonance) for the comprehensive analysis of the polystyrene-b-polyisoprene copolymers. The copolymers were synthesized by two different methods: sequential living anionic polymerization and coupling of living precursor blocks. While (1)H-NMR allows just for the analysis of the bulk chemical composition of the block copolymers, the coupling with liquid chromatography at critical conditions provides selective molar mass information on the polystyrene and polyisoprene blocks within the copolymers. The polyisoprene block molar mass is determined by operating at chromatographic conditions corresponding to the critical point of adsorption of polystyrene and size exclusion chromatography mode for polyisoprene. The molar mass of the polystyrene block is determined by operating at the critical conditions of polyisoprene. In addition to the molar mass of each block of the copolymers, the chemical composition distribution of the block copolymers was determined. By using the coupling of liquid chromatography at critical conditions to (1)H-NMR, one can also detect the homopolymers formed during synthesis. Finally the microstructure of the polyisoprene block in the copolymers was evaluated as a function of molar mass.

Characterisation of blends of polyisoprene and polystyrene by on-line hyphenation of HPLC and 1H-NMR: LC-CC-NMR at critical conditions of both homopolymers

Blends of polystyrene (PS) and polyisoprene (PI) were analysed by on-line hyphenation of LC at critical conditions and (1) H-NMR. Chromatography at critical conditions was established for both PS and PI. At both critical conditions, a perfect separation into the blend components was achieved. By operating at critical conditions of one blend component and size exclusion mode for the other it is possible to determine the molar mass using a suitable calibration. By using NMR as a detector, the microstructure of PI can be identified, quantified and the chemical composition of the blends can be calculated by monitoring the signal intensities of the olefinic protons of isoprene and the aromatic protons of PS.

Comprehensive two-dimensional liquid chromatography for the separation of protonated and deuterated polystyrene.

Liquid chromatography at critical conditions (LCCC) has been shown to be a powerful method for the separation of complex polymers regarding chemical composition, functionality, or molecular topology. LCCC has never been used, however, to separate polymers according to the degree of deuteration. This is a very challenging task since polymers shall be separated that are identical regarding molar mass, endgroups and chemical composition. In the present work, critical conditions were established in such a way that one component of a complex mixture elutes at critical conditions, whereas the other component shows size exclusion chromatography (SEC) behaviour. Blends of protonated (h) and deuterated (d) polystyrene (PS) were separated by LCCC at critical conditions of both h-PS and d-PS. Depending on the molar masses of the blend components, baseline separation could be achieved. In order to improve the separation further, comprehensive two-dimensional liquid chromatography was carried out on a number of model blends. In the first dimension LCCC was used, which separated the blends according to isotopic effects whereas in the second dimension the separation took place with respect to hydrodynamic volume. In order to further improve the separation of a number of blends a separation protocol was used where one component shows SEC conditions whereas the other component shows liquid adsorption chromatography (LAC) conditions. This separation protocol was achieved by varying the column temperature.

Onflow liquid chromatography at critical conditions coupled to 1H and 2H nuclear magnetic resonance as powerful tools for the separation of poly(methylmethacrylate) according to isotopic composition

The present work addresses a major challenge in polymer chromatography by developing a method to separate and analyze polymers with identical molar masses, chemical structures and tacticities that is solely based on differences in isotope composition. For the first time, liquid chromatography at critical conditions (LCCC) was used to separate PMMA regarding the H and D isotopes. At critical conditions of H-PMMA, D-PMMA eluted in the adsorption mode and vice versa. By online onflow LCCC-NMR, both PMMA species were clearly identified. Different from other detectors, NMR can distinguish between H and D. Onflow LCCC-H/NMR and LCCC-D/NMR measurements were carried out and the H/D-blend components were detected. (1)H and (13)C NMR provided the tacticity of protonated PMMA. Double resonance (13)C{H} and triple resonance (13)C{H,D} provided the tacticity of the deuterated samples. Samples with similar tacticities were used to ensure that separation occurs solely regarding the isotope labeling.