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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