Observation of compositional heterogeneity in poly(styrene sulfonate) using frontal analysis continuous capillary electrophoresis

Sulfonated polystyrene is frequently studied as a model polyelectrolyte bec ause of its narrow molecular weight distribution, corresponding to that of the precursor, anionically polymerized polystyrene. Frontal analysis contin uous capillary electrophoresis (FACCE) was carried out for polystyrenesulfo nate (PSS) samples from various commercial sources. Whereas some of the ele ctropherograms appeared as abrupt and discontinuous, indicating a single un iform mobility, others were sigmoidal, corresponding to a distribution of m obilities. This distribution must reflect sample heterogeneity, possibly du e in part to the molecular weight distribution (MWD) for the low-MW samples , but only attributable to chemical heterogeneity for the higher-MW samples . Size-exclusion chromatography (SEC) was used to establish the possibility of some MWD contribution to the distribution of mobilities for the low-MW PSSs, but not for the high-MW ones. Because a high-MW sample with a broad M WD prepared by polymerization of styrenesulfonate did not show a mobility d istribution, the only plausible explanation for those distributions is comp ositional heterogeneity, i.e., chain-to-chain variations in the degree of s ulfonation.