Separation of Polymers by Chromatography

Abstract

Abstract While mother nature can produce uniform macromolecules as found in many biological systems, man‐made polymers are hardly homogeneous. Synthetic polymers are usually complex mixtures of molecules differing in various molecular characteristics such as molecular weight (MW), chemical composition, functionality, microstructure, and chain architecture. For precise characterization of polymers, the multivariate distributions in all these molecular characteristics need to be measured, which is an extremely difficult task, if not impossible. In practice, however, it is often sufficient to analyze a limited number of molecular characteristics. For the purpose, liquid chromatography is the most widely used method that has made a remarkable progress during the last several decades in both instrumentation and understanding of the separation mechanism. Size exclusion chromatography (SEC) has been the most widely employed chromatographic separation technique for the characterization of synthetic polymers since its inception about a half‐century ago. Compared to traditional fractionation methods of polymers, SEC is an excellent method in precision, analysis speed, required amount of a sample, etc. Nonetheless, SEC separates the polymer molecules according to the chain size that depends on not only MW but also other molecular characteristics such as composition, chain architecture, and microstructure. Therefore, for instance, SEC cannot separate nonlinear homo‐polymers according to MW or copolymers according to the composition. In contrast to SEC in which the different pore‐accessibility of macromolecules is the main separation mechanism, interaction chromatography (IC) fractionates polymers according to the interaction strength between the polymer molecules and the sorbent. In results, IC is more sensitive to the chemical nature of the polymers than SEC. By judiciously selecting the IC separation condition, it is possible to separate polymers according to specific molecular properties with higher resolution than SEC. Furthermore, one can access to the chromatographic critical condition for a given polymer system by adjusting the solute interaction strength with the sorbent until the exclusion and interaction contributions to the solute retention are exactly compensated. At the critical condition, polymers of different MW coelute. Therefore, LC at the critical condition (LCCC) can be used as a unique tool for the characterization of polymers since one can suppress the peak dispersion by MW distribution and focus on other molecular characteristics. Recently, polymer separations using two‐dimensional LC by a rational combination of SEC, IC, or LCCC have become more and more popular for the characterization of polymers with multivariate distributions. In this article, chromatographic separation methods of synthetic polymers are reviewed with an emphasis on non‐SEC methods.