Content area
Full text
1. Introduction
Over the past few decades block copolymers have attracted significant scientific and economic interest since they are able to self-assemble into long-range-ordered structures in bulk, while in selective solvents they form micellar aggregates with a core-shell structure in which the insoluble block forms the core and the soluble block forms the surrounding corona. Stability and an ability to vary the core-shell dimensions make block copolymers widely applicable in various fields, for example, for colloidal stabilization, the compatibilization of polymer blends, controlled drug delivery, water purification, gene therapy, phase-transfer catalysis, viscosity, and surface modifications [1]. Block copolymers can show a distribution in more than one property, that is, chemical composition, functionality, architecture, and molar mass, and as such they are considered as complex polymers. By using conventional methods of polymer characterization, such as IR or NMR, only the average structural features of copolymers without their distributions can be determined [2]. The chemical composition distribution (CCD) and functionality distribution of copolymers can be determined by liquid chromatography (LC) based on different mechanisms of separation, for example, liquid-adsorption chromatography (LAC), which is a gradient method and liquid chromatography under critical conditions (LCCC), which is an isocratic method [3]. LAC separation is directed by enthalpic interactions, while in LCCC the enthalpic interactions are compensated by entropy losses of the solute [4].
The molar-mass averages and distributions of polymers are usually determined using size-exclusion chromatography (SEC), which is governed by entropic effects and results in the elution of macromolecules according to their hydrodynamic volume. SEC in combination with different detectors (e.g., UV/VIS, IR) also gives information about the chemical composition as a function of the molar mass. However, only the average composition at a certain molecular size can be determined. For high-throughput screening and analysis, short and wide-bore columns were developed, making a high-speed separation possible [5].
By combining different LC techniques (LAC or LCCC with SEC) into a two-dimensional chromatographic system, it is possible to correlate a certain composition (or functionality) with the molar mass [6, 7]. The optimal analysis time in two-dimensional liquid chromatography (2D-LC) is achieved if the second dimension is as short as possible, while giving sufficient resolution, which is fulfilled by wide-bore high-speed SEC columns. The complete transfer of fractions between both...