Local polydispersity detection in size exclusion chromatography: Method assessment

Abstract

Local polydispersity is the term describing the variety of molecules present at the same retention volume in size exclusion chromatography (SEC) analysis. In the analysis of a linear homopolymer, local polydispersity is generally attributed to the effect of axial dispersion: it can cause molecular size variety (i.e., imperfect resolution) at each retention volume and thus local polydispersity in the molecular weight. In the analysis of polymer blends (copolymers and branched polymers), it is possible to have local polydispersity, even when the resolution is perfect, because molecules of different compositions (or degrees of branching) can have the same molecular size in solution. Conventional SEC interpretation assumes no local polydispersity if the axial dispersion effects are negligible. Three methods are currently available for detecting local polydispersity by using a combination of differential refractive index, light scattering, and viscometer detectors: the chromatogram comparison method, the conventional calibration curve comparison method, and the universal calibration comparison method. Here we experimentally assess these three methods using polymer blends and emphasize the chromatogram comparison method. All three are shown to be useful for assessing triple detector systems, but they are capable of detecting local polydispersity due to molecular heterogeneity only for very large differences in specific refractive index increments in the blend components.

Carbodiimide Induced Cross-Linking, Ligand Addition, and Degradation in Gelatin

“…Size exclusion chromatography has been used to identify and characterize macromolecules such as polymers, 20 proteins, 21 and gelatin. 13,17 One potential drawback of using this method with polydisperse random coil macromolecules like gelatin is the difficulty of separating entangled molecules, which results in unresolved areas between peaks of different species.…”

Theory of Multiple‐Detection Size‐Exclusion Chromatography of Complex Branched Polymers

“…The objectives of the present paper are to extend the work of Hamielec and coworkers as follows: To define distribution functions which provide the basis for interpretation of SEC and similar data for complex branched polymers; these distribution functions are a generalization of the familiar number and mass distributions for the MW of linear polymers. To show how SEC data for both linear and branched polymers using a range of different types of online detectors (refractometer, viscometry, light scattering (LS), etc.) can be expressed in a single formalism which incorporates current and potential future modes of detection. To show how this formalism enables band broadening to be taken into account for distributions from any mode of detection, thus enabling new knowledge to be obtained from the detailed shape of these distributions17–21 (rather than just averages such as $\overline M _{\rm w}$ and $\overline M _{\rm n}$ , which are usually not strongly affected by band broadening in contemporary SEC devices22). To present details of how true average molecular weights, $\overline M _{\rm n}$ and $\overline M _{\rm w}$ , can be obtained.…”

“…To show how this formalism enables band broadening to be taken into account for distributions from any mode of detection, thus enabling new knowledge to be obtained from the detailed shape of these distributions17–21 (rather than just averages such as $\overline M _{\rm w}$ and $\overline M _{\rm n}$ , which are usually not strongly affected by band broadening in contemporary SEC devices22). …”

“…The reason that it may be necessary to include the possibility of band broadening is the following. While band broadening is often considered as minimal in many modern SEC set‐ups,22 it has been found to significantly broaden MWD in several cases 38,39. It has been shown40 that this broadening is sufficient to affect the fine details of the shape of the MWD (or hydrodynamic volume distribution), and this detailed shape can be used to provide mechanistic information 17–21.…”

“…It is important to be aware that, except where specifically stated, the inclusion of G′ ( t el , V h ) enables signals for all detectors to be corrected explicitly for band broadening. Again it is emphasized that while band‐broadening corrections for modern SEC devices have little effect on averages such as $\overline M _{\rm n}$ ,22 they are essential to take into account if one wants detailed mechanistic information from the actual shape of the various distribution functions. If one knows the broadening function G′ ( t el , V h ), then a number of procedures have been given46 for numerically inverting the convolution equation, Equation (38).…”