Taihyun Chang, POSTECH PART 2/2

Synthetic polymers are not homogeneous molecules but have distributions in a number of molecular characteristics such as molecular weight (MW), chemical composition (in copolymers), chain architecture, and so on. Therefore an efficient separation according to a molecular characteristic of interest is necessary to measure the distribution. Among the various separation methods of polymers, size exclusion chromatography (SEC) has been used most widely since early 60’s by virtue of its high speed, facility, and precision compared to other classical fractionation techniques. However, SEC separates polymer molecules in terms of molecular size not MW, thus SEC has its limitation in resolving branched polymers or copolymers according to MW. In addition, SEC suffers from serious band broadening.
Interaction chromatography (IC) mainly utilizes the enthalpic interaction of polymeric solutes with the stationary phase. The enthalpic interaction depends on chemical composition as well as MW while less sensitive to chain architecture. Therefore, IC can complement the weakness of SEC effectively. In addition, band broadening is much lower in IC than SEC, thus IC exhibits a far better resolution than SEC. In early days of IC application to polymer separation, the solvent gradient elution method was widely used but we found that the temperature gradient elution is a powerful method for the molecular characterization of synthetic polymers with relatively narrow molecular weight distribution. Temperature gradient interaction chromatography (TGIC) exploits the thermodynamic interactions of polymer molecules with the stationary phase of chromatographic columns, in which the column temperature is controlled in a programmed manner during the elution to control the interaction strength of polymeric solutes with the stationary phase. TGIC exhibits much higher resolution than SEC in the separation according to MW and it allows many useful detectors for polymer characterization to be utilized, which is impossible in the solvent gradient elution.
TGIC has been applied to resolve a number of long-standing problems in polymer characterization, particularly for the polymers prepared by anionic polymerization which is expected to have precise molecular characteristics. For examples, TGIC resolved the issues such as rigorous determination of MW distribution of the anionic-polymerized polymers and separation according to chain architectures demonstrating clearly that the branched polymers or ring polymers prepared by the current state-of-art synthesis/purification process are still contaminated by a large amount of byproducts enough to influence their rheological property significantly.