다중 검출법을 이용한 가지형 고분자의 이차원 액체 크로마토그래피 분석
- 다중 검출법을 이용한 가지형 고분자의 이차원 액체 크로마토그래피 분석
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- For the branching analysis of polymers, size exclusion chromatography (SEC) together with various detection methods (e.g. light scattering and viscometric detection) is the most popular method. The number of branching can be obtained according to the Zimm & Stockmayer theory from the molecular weight (MW) and the size of the polymers in the SEC fraction. This type of analysis is based on the assumption that the fractions of the SEC effluents are homogeneous in MW and branching number. However, owing to the separation mechanism, SEC cannot distinguish the polymers of the same hydrodynamic size but with the different MW or the branch number. In this dissertation study, branched polymers were characterized by two dimensional liquid chromatography (2D-LC) coupled with multiple detection methods such as light scattering detector and viscometry. Combining two different separation mechanisms is more effective for the precise analysis of branched polymers having multivariate distributions.
In chapter 1, basic principles of high performance of liquid chromatography (HPLC) and 2D-LC for polymers are briefly reviewed respectively. HPLC separation of polymers can be classified into three different separation mechanisms: size exclusion chromatography (SEC), liquid chromatography at the critical condition (LCCC) and interaction chromatography (IC). Combining two different separation mechanisms is the concept of 2D-LC which has been widely used for the analysis of polymers having multivariate distributions. In this chapter, some practical 2D-LC methodologies are also described.
In chapter 2, basic working principles of branching analysis are briefly reviewed. Qualitative branching analysis is based on the scaling relationship between chain sizes and MW and quantitative analysis is usually carried out on the basis of Zimm-Stockmayer theory. Several practical examples and issues are also included.
In chapter 3, a rigorous molecular characterization of comb-shaped polystyrene (PS) was carried out taking advantage of its molecular structure
a normal hydrogenated-PS (h-PS) backbone and deuterated-PS (d-PS) branches. Normal phase (NP) LC can separate the comb PS species according to their molecular weight well. Nonetheless, it cannot distinguish the backbone from the side-chains and the differently structured polymers having a similar molecular weight, e.g, a single backbone comb and a coupled backbone comb with fewer side-chains. In contrast to NPLC, the hydrogenous polymer is retained longer than the deuterated counterpart in reversed phase (RP) LC. Taking advantage of the isotope sensitivity of RPLC, the comb PS are cross fractionated by NPLC and RPLC, and a two dimensional mapping with respect to the backbone chain length and the number of branches is fully established.
In chapter 4, SEC with triple detection method (light scattering detector, viscometry detector and differential refractive index detector) was used to study comb-shaped PS as well as the side product effect arising in the synthetic routes. SEC with multiple detection method has been widely used to characterize branched polymers on the basis of Zimm-Stockmayer theory (contraction factor g based on radius of gyration, and the contraction factor g′ based on intrinsic viscosity). Several studies have investigated the relationship between contraction factors (g and g′) of comb-shaped polymers, but it is still under argument. Utilizing specially-designed structures (i.e. d-PS branches and h-PS backbone), single backbone comb PS, fully purified by isotope sensitivity in RPLC separation, was used and the comparative studies of as-synthesized mother comb and the fractionated comb were reported. The relationship between contraction factors (g and g′) by a constant exponent ε value was clearly confirmed after removing the side product effect.
In chapter 5, a problem conventional branching analysis using size exclusion chromatography (SEC) is demonstrated and a way of improvement is proposed by using a model polymer system, a mixture of linear and star-shaped polystyrene (PS). In the branching analysis by SEC/triple detection (concentration, light scattering and viscosity detectors) method, the branching number is calculated from the extent of chain size contraction due to chain branching relative to the linear polymer of the same molecular weight (MW). A problem arises from the fact that polymer chain size depends on both MW and chain branching. Since SEC separates polymers according to the chain size, an SEC fraction should contain polymer species polydisperse in both MW and chain architecture in general. As a solution of the problem, we propose a separation of polymers by interaction chromatography according to MW first then measure the chain size distribution of polymers in the homogeneous MW fraction by SEC/triple detection. The analysis scheme is successfully established by on-line two dimensional liquid chromatography (2D-LC) combining temperature gradient interaction chromatography and SEC/triple detection.
In chapter 6, SEC coupled with triple detection was used to characterize solution properties of star PS samples under a good solvent (tetrahydrofuran, THF) and a theta solvent (cyclohexane with 1% THF). For characterization of branched polymers, SEC with the multiple detection method has been widely used based on related theories such as the Zimm and Stockmayer theory and the scaling relation of chain sizes. Theoretical work assumes the ideal chain state (~theta solvent condition), but SEC separation is usually carried out in the good solvent condition. Using the SEC separation and detection system, chain sizes (e.g. intrinsic viscosity and radius) were measured as a function of MW and solution properties of star polymers were scrutinized. To further understand the scaling relationship of star polymers, the results obtained under the two solvent conditions were compared with the theory and the results for other polymers.
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