HPLC를 이용한 모델 분지형 고분자의 분리 및 분석
- HPLC를 이용한 모델 분지형 고분자의 분리 및 분석
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- Chain branching occurs in many commercial polymers and affects their rheological properties and processing characteristics. The relationship between the properties and branching characteristic can be understood by studying well-defined model branched polymers. They can be prepared by tailored anionic polymerization to yield precisely controlled molecular weight (MW) and chain architecture. However, imperfect stoichiometry and side reactions lead to inhomogeneity. The various side products in model polymers may have similar size but different structures, so conventional size exclusion chromatography (SEC) cannot characterize them since it separates polymers by size only. Furthermore, large band broadening during SEC separation seriously deteriorates its resolution. In this study, high performance liquid chromatography (HPLC) was used to characterize model branched polymers.
Chapter 1 reviews basic principles of HPLC for the characterization of synthetic polymers. HPLC separation of polymers can be classified into three mechanisms: size exclusion chromatography (SEC), liquid chromatography at the critical condition (LCCC) and interaction chromatography (IC). The principles of the three HPLC modes and two-dimensional liquid chromatography (2D-LC), which combines two different LC modes, are described.
Chapter 2 reviews basic principles of branching analysis using contraction factors. Branched polymers occupy a smaller volume than the linear polymers with the same MW. The ratio of the volume of a branched polymer to that of a linear polymer of the same MW can be expressed as contraction factors g and g′. The geometric contraction factor, g is defined as the ratio of the mean square radius of gyration of the branched polymer to that of the linear polymer. The viscometric contraction factor, g′ is defined as ratio of the intrinsic viscosity of the branched polymer to that of the linear polymer. Theoretical approaches and applications of the contraction factors are described.
Chapter 3 introduces TGIC characterization of H-shaped polybutadienes (PBs) and their precursors. Well-defined model branched polymers are frequently used in rheological study. An H-shaped polymer that has four arms and one crossbar is the simplest example of a branched polymer having two branch points. It can be prepared by tailored anionic polymerization to yield precisely controlled MW and chain architecture. However, imperfect stoichiometry and side reactions lead to inhomogeneity in the final product. The components in H-shaped PBs are separated using TGIC, and their absolute MWs are determined using light scattering detection. The components are identified using peak fitting of the TGIC chromatograms, and the structures of the polymers are inferred from their MW and the synthesis mechanism.
Chapter 4 demonstrates a branching analysis of star-shaped polymers using TGIC triple detection (TD) method at a theta solvent condition. SEC coupled with TD (SEC-TD) is widely used for branching analysis of polymers. However, SEC-TD has the limitation that SEC cannot separate polymers that have different chain architecture (and probably different MW) but similar hydrodynamic size. As a solution to the problem, TGIC-TD was proposed, to exploit TGIC’s ability to separate branched polymers according to their MW with much better resolution than that of SEC. The potential of the method is demonstrated using regular star-shaped PBs prepared by anionic polymerization. They were characterized by both SEC-TD and TGIC-TD. TGIC-TD enabled detailed analysis of branch distribution of narrow-MW fractions, whereas SEC-TD could only provide average branch number.
Chapter 5 demonstrates 2D-LC analysis of a graft copolymer. A graft copolymer has side chains composed of different monomers from the backbone chain. Because the backbone and graft chains are generally incompatible, most graft copolymers are multiphase materials that exhibit unique and interesting morphologies. Usually graft copolymers are characterized using SEC. However, SEC cannot characterize graft copolymers precisely because they often include various side products. 2D-LC combines two adequately selected LC separation methods
it has been used to effectively characterize bivariate distributions existing in synthetic polymers. In this study, a polystryene-graft-polyisoprene (PS-g-PI) sample was analyzed using 2D-LC which combined two LCCC steps. The PS-g-PI was prepared by tailored anionic polymerization to yield precisely-spaced graft chains. During the synthesis, imperfect stoichiometry and side reactions caused inhomogeneity in both backbone length and number of grafts. LCCC permits characterization of individual blocks of a copolymer by making one block chromatographically "invisible". PS-g-PI was separated with respect to its PI or PS block length by two LCCCs, one at the critical condition of PS and one at the critical condition of PI, respectively.
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