|Comprehensive Two-Dimensional Liquid Chromatography Coupled to Triple-Detection for Characterization of Branched Polymers
|Comprehensive Chromatography - The State of the Art
|Mr Nico Apel
|Dr Elena Uliyanchenko
Dr Stephan Moyses
Dr Christian Wold
Dr Tibor Macko
Dr Robert Brull
Abstract Information :
Polymer processing properties and their performance in final applications strongly depend on their composition. Branching, in particular, affects the rheological properties of the polymer melt and, thus, influences the processing conditions, enabling new forming processes and, consequently, new applications.
However, in addition to branching, branched polymers are distributed with regard to molecu-lar weight, end-group moiety and sometimes chemical composition. Because of this high structural complexity, their characterization is fundamentally challenging. There is no univer-sal analytical method or technique that is able to provide detailed information on branching distribution alongside with other molecular characteristics. Size-exclusion chromatography (SEC) in combination with triple-detection (TD) is the most common technique for characteri-zation of branched macromolecules. In SEC strong eluent is applied and the separation is based on hydrodynamic volume and driven by entropy effects. Therefore, some coelution of linear and branched structures with the same hydrodynamic volume can occur throughout the chromatogram. This leads to inaccurate branching information determined by TD, since only average degrees of branching are measured at each elution slice.
Depending on the experimental conditions (stationary phase, eluent strength), polymers may be separated by other chromatography modes according to different molecular metrics. Ap-plying weak eluents separation occurs in adsorption mode (liquid adsorption chromatog-raphy, LAC) and is mainly determined by the chemical composition of the polymer. At the transition between LAC and SEC the macromolecules elute independently of their molar mass for a given repeating unit (so-called liquid chromatography at critical conditions, LCCC). LCCC can be used to separate macromolecules according to end-groups.
In the present case, a new chromatographic method, namely solvent gradient at near-critical conditions, was developed applying a mobile phase gradient in a very narrow range around the critical point and allowing for a separation according to end-group moiety of the analyzed polymer. As branched polymers often vary with respect to their number of end-groups a sep-aration of linear and branched polymer structures could be attained (upper chart in Fig. 1). The separation was further augmented by hyphenating the developed method with SEC in a two-dimensional (2D) setup allowing detailed characterization of branched chains simultane-ously with molecular weight. Finally, the 2D-LC method was coupled with TD, comprising a concentration, a light scattering and a viscosity detector. That allowed for analyzing the degree of branching of individual branched structures as a function of their absolute molar masses without the coelution problem encountered in one-dimensional analysis. For the first time a 2D contour plot of the degree of branching could be presented highlighting the high potential of 2D-LC hyphenated with TD.