title: Development of New Liquid Chromatographic Techniques for the Characterization of Poly(Bisphenol A Carbonate)
Creators: name: Apel, Nico
Publisher: name: UNSPECIFIED
Year: 2021

description: The processing and application properties of polymers in general and of Poly(bisphenol A carbonate) (PC) in particular strongly depend on their composition (molar mass distribution, architecture, chemical moiety). Thus, a comprehensive knowledge of the molecular parameters is required for a rational material development. However, the overall structure of polymer chains is a difficult-to-access function with regard to molar mass distribution (MMD) on the one hand and the complex chain architecture on the other hand. Unfortunately, there are no comprehensive characterization methods available that can extract information on the structure of branched PCs so far. Therefore, the aim of this work was the development of a liquid chromatographic method that allows a PC separation according to chain structure types and quantification with regard to molar masses as well as proportions of these species. In the first part of this thesis, the critical conditions of PC on a PGC stationary phase were established applying the eluent systems CHCl3/TCB and CHCl3/DCB. At these conditions a separation of PC according to the number of hydroxyl end-groups was observed, which was confirmed by MALDI-TOF-MS analysis of the eluting fractions. Analysis of a branched PC sample revealed that a separation of branched structures based on the hydroxyl end-groups could not be achieved with the developed method. Furthermore, a gradual change in the separation selectivity was observed over time, which was explained by the formation of polymer monolayers on the stationary phase and thus necessitated a continuous adjustment of the mobile phase composition. However, in contrast to methods already known in the literature, the presented method was suitable for the identification of linear PC structures with two hydroxyl end-groups. Thus, the developed method can be used to investigate PC degradation products, which might be formed upon hydrolysis or oxidation during the application period of the material. Furthermore, the method may be highly valuable to monitor the end-capping during the PC synthesis, which is used to control the molar masses on the one hand and on the other hand protects the polymer chains from degradation. Taking into account the drawbacks of porous graphitic carbon as stationary phase for the separation of PC, a normal-phase silica stationary phase was chosen as alternative. However, working at conditions for LCCC also did not lead to the desired PC separation according to branching. Thus, a novel chromatographic approach was developed where the mobile phase composition is varied around the critical point by a solvent gradient (solvent gradient at near-critical conditions, SG-NCC). To elucidate the underlying separation mechanism, a branched sample was subjected to SG-NCCC and the eluting fractions were collected. These were analyzed by MALDI-TOF-MS and TD-SEC, and the THPE contents in the individual fractions were quantified. Thus, it could be confirmed that the method indeed separated according to molar masses, hydroxyl end-groups as well as branching. In order to improve the separation performance the developed method was hyphenated with a separation according to molar mass to an on-line 2D-LC (SG-NCC × SEC). In the corresponding 2D contour plots individual linear and branched structures could be identified and their content could be determined by a quantitative detector. Based on theory of liquid adsorption chromatography an earlier elution of polymer chains with lower molar masses was expected, as the number of repeating units defines the interaction strength. However, a reversal of that elution order was observed in the SG-NCC chromatogram when moving from linear to branched chains. This phenomenon could be explained by end-group effects described in the literature, which may affect the strength of the interactions with the stationary phase depending on their arrangement along the chain. In order to further characterize the branched and linear structures that could be separated by the developed 2D-LC analysis the method was augmented by TD and additional insights into the polymer chain structure could be obtained. Moreover, for the first time, the degree of branching was investigated in a 2D-LC experiment, the corresponding 2D contour plot could be established and the individual structures were quantified. Furthermore, the absolute molar mass distribution of the specific spots was extracted. Therefore, 2D-LC with TD offered an experimental opportunity to determine the detailed polymer chain composition in a branched PC sample for the first time. This method could be exploited to investigate the predictive power of statistical Monte-Carlo (MC) simulation models. MC models have been developed aiming at a statistical determination of the chain structure of branched PC in order to predict the resulting material processing properties. However, MC simulations are based on empirical data evaluations and involve a number of model assumptions. The chain structures of branched PC predicted by MC simulations were compared with experimental data that are accessible by the newly developed 2D-LC with TD. For that purpose, the MMDs as well as proportions of linear and branched structures in a PC sample could be quantified by LC and the outcomes were compared with MC simulations. The results revealed fair agreement and deviations could be explained by assumptions in the MC models. In order to improve the predictive power of the MC simulations, additional branched PC samples differing in degree of branching and molar masses might be investigated in further studies. The determined proportions of branched and linear structures may be compared with the outcomes of MC models and the MC simulation should be further optimized, if required. Furthermore, the information on the degree of branching that could be determined in the 2D-LC with TD might be correlated with chain structures derived from MC simulations. In that way, the comprehension on the structure composition of branched PC might be deepened, the predictive power of rheological simulations could be improved and tailor-made synthesis of branched PC materials may be developed.