Light scattering and the absolute characterization of macromolecules

Light scattering (LS) has returned to the arsenal of analytical chemists with an impact scarcely imaginable a decade ago. Several important developments have restored this absolute measurement technique to its present status. The advent of HPSEC (high-performance size exclusion chromatography) and a variety of other techniques for macromolecular separation were of particular significance in stimulating the application of low angle laser light scattering (LALLS) for the determination of polymer molecular weights, their averages, and their distributions. However, it has been the more recent introduction of on-line scattering measurements over a broad range of scattering angles (differential LS) that has permitted many important deductions of light scattering theory to be extracted and applied for the first time. These include determination of the mean square radius and its various averages, molecular conformation and structure, branching ratios, better characterizations of co-polymers, as well as those polymers soluble only at high temperatures. In addition, some instrumental broadening effects now may be measured directly. Tremendous advances in personal computers, together with flexible and versatile data collection and processing software, have played significant roles as well. This paper summarizes the theoretical basis of the measurements and presents some examples of the application of this theory for the deduction of molecular parameters from unfractionated polymer solutions. In passing, the operating principles of differential refractive index detectors (the most important and commonly used detection system for HPSEC) are described, as well as their importance to differential LS measurements. The general operating characteristics of modern LS detectors/photometers with their associated software and, most importantly, the combination of on-line LS detection and HPSEC separation is discussed with numerous examples of its analytical capabilities.