A homopolymer is an assembly of chains with varying numbers of monomeric units (or degrees of polymerization). Apart from SEC, conventional LC methods, such as reversed-phase liquid chromatography (RPLC) and normal-phase liquid chromatography (NPLC), are applicable to polymers. For individual members of the series the common log-linear retention model is a useful approximation. For the entire ensemble it has been found that often both the intercept (ln k0) and the absolute slope (S) of such retention lines vary linearly with the degree of polymerization. This implies that ln k0 also varies linearly with S. The result is a bundle of lines as shown below. For all but he smallest members of a series (the lines tend to be very steep, causing retention to be either very long or very short (“on-off mechanism”).
It is possible that all lines intersect in a common (“critical”) point. At these so-called critical conditions retention is independent op molecular weight and polymers may potentially be separated based on small differences in chemical composition (e.g. end groups). In practice, isocratic critical chromatography is very sensitive to changes in mobile-phase composition, temperature, or even pressure (and thus flow rate).
This is illustrated by the vertical white arrow. At a composition that differs even slightly from the critical value, the different members of the series exhibit vastly different retention factors, resulting in very broad signals.
It is easier to study polymer composition with gradient-elution experiments, where the mobile-phase composition varies with time. This will make the members of the series elute in a typical narrow range of compositions, as illustrated by the horizontal white arrow. Low-molecular-weight oligomers may be separated, but higher members increasingly overlap. Using fast gradients, polymers can be separated essentially based on composition. “Such conditions are known as “pseudo-critical”.
For more information see Section 4.3.
