Study of the aberrant retention behavior of a semi-crystalline polyamide in reversed-phase liquid chromatography

The elution behavior of semi-crystalline polyamides (PAs) in reversed-phase liquid chromatography (RPLC) was examined. RPLC of the aliphatic polyamide 4,6 (PA46) yielded a single broad peak. However, the partly aromatic polyamide X (PAX) displayed a deviating elution profile, encompassing early- and late-eluting portions, which changed in relative abundance when varying injection and gradient conditions. These bands were suspected to be due to formation of amorphous and crystalline phases, respectively. RPLC fractions of the PAs were subjected to the same RPLC system and to size-exclusion chromatography. The presumed amorphous PAX portion showed two bands in RPLC, suggesting that (largely) amorphous and crystalline phases are formed upon sample injection. Differential scanning calorimetry (DSC) demonstrated that reducing the crystallinity of PAX decreased the relative abundance of the late-eluting fraction, approaching the behavior of aliphatic polyamides. X-ray diffraction and static-light-scattering analyses confirmed the semi-crystallinity of the two solid PA samples. Although both small particles and larger aggregates were observed in solution, these findings could not be correlated to the differences in elution profile of the PAs. Cloud-point measurements indicated that the solubility of both PA46 and PAX was almost independent of temperature. Strikingly, at low column temperatures (i.e. below the depressed melting point), PAX eluted as a single broad PAX peak. To conclude, the semi-crystallinity of PAX influences its RPLC-elution behavior, and by ensuring complete dissolution of the crystalline phase useful chemical information can be extracted from the obtained chromatograms.