Multiple Detection in Size-Exclusion Chromatography of Macromolecules

摘要

Chemical and physical detectors characterize the distributions of macromolecular parameters that have a critical effect on the end product. Synthesizing polymers is not as exact a science as we would like it to be. Natural syntheses and even well-controlled laboratory syntheses often yield macromolecules that vary in length, molar mass, branching, chemical composition, and other properties. Characterizing these properties and their distributions is important because of their critical effect on end-use structure-property relations and, hence, on the end product itself. The most commonly studied properties are the molar mass averages (M_(n), M_(w), M_(z), etc.) and the molar mass distribution (MMD). Various processing characteristics of macromolecules can be related to the individual averages, for example, flow properties and brittleness (related to M_(n)) and flex life and stiffness (related to M_(z)). Similarly, properties such as tensile strength and abrasion resistance tend to increase as MMD narrows, and properties such as elongation and yield strength tend to increase as MMD broadens. During the past four decades, size-exclusion chromatography (SEC) has been established as the premier method for characterizing M averages and the distribution of natural and synthetic polymers. Equally important, or more important, is the ability of SEC to characterize the distributions of many other macromolecular parameters (Table 1). This ability is imparted by the many analytical techniques that serve as detection methods and that are the primary subject of this article (1). Because of space limitations, this article will focus on SEC as the sole separation method prior to detection and ignore its growing role in 2-D LC. Detection methods will be divided into two classes. Chemical detectors, such as UV-vis (when not used as a concentration-sensitive detector), IR, NMR, and MS, usually combine in additive fashion. Physical detectors, such as the viscometer (VISC) and light-scattering (LS) photometers, on the other hand, can combine in synergistic fashion. Some detectors appear to bridge the gap between classes; for example, IR and NMR can function as either chemical or physical detectors. The type of information provided by each class will be explored. Because of the large number of detectors covered here, it is impossible to include an explanation of the fundamentals of each. Many excellent reference sources exist on SEC, as well as on each of the individual topics (2, 3); a few relevant references are given where appropriate.