Field-flow fractionation of polymers: one-phase chromatography

Abstract

Field—flow fractionation (FFF) is introduced as a one—phase chromatographic system utilizing an external field to differentially retain high molecular weight polymeric and particulate species. The principles and theory of FFF are described. FFF and exclusion chroma— tography are then compared on the basis of their underlying separative mechanisms, and theway that these mechanisms influence and limit experi— mental capabilities. This comparison is continued in a more quantitative way by examining fundamental column selectivity requirement for polymer fractionation. Several examples of polymer fractionation by FFF are then shown. Finally, some of the extreme limits of FFF performance are discussed, including resolution, separation speed and high and low molecular weight limits. INTRODUCTION One of the outstanding challenges in the broad discipline of chemical separations is the general achievement of high—resolution polymer fractionation. The magnitude of the technical hurdle is not unlike that confronted in separating isotopes over three decades ago. One of the common problems is that in each case the fractional difference in transport or equilibrium properties between close—lying species is extremely small and separations based on these differences are accordingly hard to achieve. Polymer fractionation has its own unique problems, however, in that most chromatographic systems are at best only marginally, if at all, applicable to polymers (1). Exclusion methods of chromatography (gel filtration and gel permeation chromatography) have been developed, but these have inherent limitations and drawbacks that will be pointed out later, Non— chromatographic methods, including solubility fractionation and ultracentrifugation, have failed to provide high resolution. It is apparent, therefore, that effective approaches to polymer fractionation are extremely limited in scope. This fact makes the appearance of any truly new approach to polymer separations an uncommon event deserving close scrutiny to see if new capabilities have thereby been introduced. Field-f low fractionation (FFF) is a broad methodology capable of separating many complex, high molecular weight species, including nonpolar, synthetic polymers. It can be applied to biological particles such as viruses, nonbiological particles such as latex beads and chromatographic support particles, biological macromolecules such as proteins, and non— biological macromolecules. such as polystyrene and polyacrylic acid. Its scope appears to be very broad throughout this high mass range. However, this report will be limited to the potential applicability of FFF to nonbiological polymers alone. The concept of FFF was first proposed by the author in 1966 (2), and experimental work has been underway since that time. Progress at first was slow due to the difficulties of initiating a new type of experimental system. Recent years have witnessed increasing versatility, resolution and speed in the separation of both polymers and particles by FFF. Separations have now been carried out over the effective molecular weight range lOl&-, a trillion—fold mass range. In terms of particle size, this corresponds to diameters ranging from about 0.001 pm to over 10 pm. In another direction, separation times for polymers have been reduced to approximately one minute (3). FFF resembles chromatography in both the experimental and dynamical aspects of its operation, and it can therefore be thought of as a chromatographic method. However, there is no stationary phase: separation occurs in an open channel containing a single moving fluid. For this reason FFF has been described as one—phase chromatography (4,5). Because FFF functions like a chromatographic system, the theoretical analysis of intrinsic resolving power and separation speed is subject to the same principles and definitions used in exclusion chromatography (EC) and other chromatographic methods. However, in