Orthogonal Chromatography: Chromatographic Cross-Fractionation of Polymers

Polymers are generally extremely multi-component materials. Performance of a product is often dictated by the individual concentrations of their wide variety of different molecules. We conventionally consider the importance of molecular weight distribution (i.e. concentration of each molecular weight present) and attempt to characterize it via one or two averages (a number (Mn) and/or a weight average (Mn)) for correlation with performance properties. However, most industrial polymers are copolymers or branched polymers or both. Even “simple” linear copolymers often contain molecules which differ from each other by at least three properties: molecular weight, composition and sequence length. [Sequence length here refers to the number of one type of repeating chemical unit in a row within a macromolecule before other types are encountered.] Three simultaneous property distributions are then present. Branched polymers contain the added complication of different branch lengths and/or “density of branching per molecule”. Despite the potential importance of all of these distributions, with the exception of molecular weight distribution, at most one average value can readily be measured to characterize each (e.g. average composition, average sequence length, average branch length). Also, as will be discussed later, molecular welght distribution measurements of such polymers are highly uncertain.