Molecular composition size exclusion chromatography

It is basically a fractionation process that depends not only on molecular size, but also on chemical composition, stereo-configuration, branching, and crosslinking. For multicomponent systems, fractionation with different ion polymolecularity, chemical heterogeneity and sequence length distribution, solubility or elution fractionation is of primary importance. Therefore, gel permeation chromatography or size exclusion chromatography is used as an important tool for the characterization of PBAs. 

Advanced computational models are also developed to understand the formation of polymer microstructure and polymer morphology. Nonuniform compositional distribution in olefin copolymers can affect the chain solubility of highly crystalline polymers. When such compositional nonuniformity is present, hydrodynamic volume distribution measured by size exclusion chromatography does not match the exact copolymer molecular weight distribution. Therefore, it is necessary to calculate the hydrodynamic volume distribution from a copolymer kinetic model and to relate it to the copolymer molecular weight distribution. The finite molecular weight moment techniques that were developed for free radical homo- and co-polymerization processes can be used for such calculations [1,14,15]. 

HE FIELD OF SIZE EXCLUSION CHROMATOGRAPHY (SEC) remains a viable and lively area of polymer characterization. Over the past several years, there has been considerable research activity in the area of SEC detection and data analysis in order to obtain more comprehensive information concerning the composition and molecular architecture of complex polymer systems. 

Size-exclusion chromatography (SEC) has been also used as complementary technique for characterizing changes in the composition of synthetic resins due to aging. In particular, it enables the determination of molecular weight distributions and the glass transition temperatures of resins. 

A wide variety of polymers have been analyzed by gel-permeation, or size-exclusion, chromatography (sec) to determine molecular weight distribution of the polymer and additives (86—92). Some work has been completed on expanding this technique to determine branching in certain polymers (93). Combinations of sec with pyrolysis—gc systems have been used to show that the relative composition of polystyrene or acrylonitrile—polystyrene copolymer is independent of molecule size (94). Improvements in gpc include smaller cross-linked polystyrene beads having narrow particle size distributions, which allow higher column efficiency and new families of porous hydrophilic gels to be used for aqueous gpc (95). 

Gel Permeation Chromatography (GPC), also known as Size Exclusion Chromatography (SEC), is a technique used to determine the average molecular weight distribution of a polymer sample. Using the appropriate detectors and analysis procedure it is also possible to obtain qualitative information on long chain branching or determine the composition distribution of copolymers. 

Reemtsma,T.,These, A., Springer, A., and Linscheid, M. (2008). Differences in the molecular composition of fulvic acid size fractions detected by size-exclusion chromatography—On fine Fourier transform ion cyclotron resonance (FTICR-) mass spectrometry. Water Res. 42, 63-72. 

Size exclusion chromatography is the premier polymer characterization method for determining molar mass distributions. In SEC, the separation mechanism is based on molecular hydrodynamic volume. For homopolymers, condensation polymers and strictly alternating copolymers, there is a correspondence between elution volume and molar mass. Thus, chemically similar polymer standards of known molar mass can be used for calibration. However, for SEC of random and block copolymers and branched polymers, no simple correspondence exists between elution volume and molar mass because of the possible compositional heterogeneity of these materials. As a result, molar mass calibration with polymer standards can introduce a considerable amount of error. To address this problem, selective detection techniques have to be combined with SEC separation. 

This book covers some of the significant advances in hyphenated chromatographic separation methods for polymer characterization. Chromatographic separation techniques in this volume include size-exclusion chromatography, liquid chromatography, and field flow fractionation methods that are used in conjunction with information-rich detectors such as molecular size-sensitive or compositional-sensitive detectors or coupled in cross-fractionation modes. 

Size-exclusion chromatography (SEC) is the premier polymer characterization method for determining MWDs. As discussed in this volume and summarized in the following section, by hyphenating SEC with selective detectors, one can, in principle, completely characterize a polymer in terms of its molecular parameters and chemical composition in the time it takes to do a typical SEC analysis. 

It was recognized early in the development of ThFFF that different molecular weight components are separated because of the dependence of ordinary (Fickian) diffusion on hydrodynamic radius, whereas thermal diffusion is responsible for variations in retention with polymer composition (6). The ability to separate polymers based only on compositional differences was first demonstrated by Gunderson and Giddings (7) when poly(methyl methacrylate) and polystyrene polymers that were similar in size, and therefore coeluted in size-exclusion chromatography, were separated in a ThFFF channel. 

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