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Branching is a molecular metric that strongly influences the application properties of polymers. Consequently, detailed information on the microstructure is required to gain a deeper understanding of structure-property relationships. In the present case, we employ high-performance liquid chromatography to characterize the branching in a poly(bisphenol A carbonate) (PC). To this end, a method was developed based on a mobile phase gradient in a very narrow range (±1.4 vol %) around the point of adsorption (98.9/1.1 vol % chloroform/methyl tert-butyl ether), which we refer to as solvent gradient at near-critical conditions. Application of such gentle gradient enabled separation of PC according to end-groups. The separation mechanism was confirmed by collecting fractions of a separated sample and subsequently analyzing these by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Hyphenating the developed gradient method with size-exclusion chromatography as the second dimension (2D-LC) enabled separation of linear and branched PC chains and determination of the molar mass distribution of the fractions. A reversed elution order was observed for branched species in 2D-LC, meaning that low molar mass chains exhibited higher elution volumes in the first dimension than higher molar masses. This finding was explained by influences of end-groups as well as the architecture of the branched polymer chains.

Detailed knowledge on structural information is required to derive the rheological properties of branched polymers. Size-exclusion chromatography with triple-detection (TD-SEC), comprising a concentration, a light scattering, and a viscosity detector, is a powerful tool to analyze the degree of branching of polymers as a function of their molar masses. However, TD-SEC alone is incapable of fully deconvoluting complex polymer systems. In this study we discuss a more sophisticated approach that includes coupling of TD to our recently described novel online two-dimensional liquid chromatography method (2D-LC), based on solvent gradient at near-critical conditions in the first dimension. Thus, a contour plot of the branching ratio is presented, and unique detailed information on the degree of branching can be derived for branched polycarbonate (PC) sample. Furthermore, the molar mass distributions of separated linear and branched PC chains as well as their fractions in the polymer are quantified. The corresponding data are correlated to Monte Carlo simulations of the polycondensation process of a branched PC, and both methods show a high level of agreement in the determined molar mass distributions of the linear and branched PC chains as well as their fractions. Finally, the influence of chemical structure on rheological properties is demonstrated.

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