Characterizing Polyamides Using Reversed-Phase Liquid Chromatography

Researchers from Vrije Universiteit Amsterdam, led by external PhD candidate Jordy Kruijswojk, studied how semi-crystalline polyamides (PAs) react when undergoing reversed-phase liquid chromatography (RPLC). Their findings were published in the Journal of Chromatography A (1).

Amsterdam, The Netherlands, 17.11.2023, The Vrije Universiteit Amsterdam, a public research university located at the Zuidas district of Amsterdam | Image Credit: © Milos - stock.adobe.com

Polyamides (PAs), commonly known as nylons, are characterized by having amide groups, or (CO–NH), in their main polymer chains (2). These compounds are used in many aspects of everyday life, including industries like textiles and engineering materials. To relate the chemical structure of these substances to their physical properties, it is important to have appropriate polymer-characterization techniques available. As novel polymeric materials are constantly developed, demand has grown for improved understandings of analytical methods. That said, analyzing synthetic polymers is challenging due to distributions in their molecular characteristics.

Synthetic polymers are intrinsically heterogeneous in chain length, meaning they exhibit molecular-weight distributions (MWDs). Other distributions, such as block-length or functionality-type, can further add to heterogeneity. Understanding polymer distributions is important, since they relate to physical characteristics. The number of solvents that can dissolve synthetic polymers is typically quite limited.

In this study, the researchers investigated the atypical RPLC-retention behavior of a semi-crystalline aromatic PA. The RPLC of an aliphatic polyamide 4,6 (PA46) yielded a single broad peak. 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 the creation of amorphous and crystalline phases, respectively. RPLC fractions of the PAs were subjected to the same RPLC system as well as size-exclusion chromatography (SEC).

The presumed amorphous PAX portion showed two RPLC bands, suggesting that phases that are largely amorphous and crystalline are formed upon sample injection. Using differential scanning calorimetry (DSC), revealed that reducing PAX crystallinity 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-crystalline nature of the two solid PA samples. However, while 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-points for PAX and PA46 were deemed to be relatively independent of temperature. This was compared to the HFIP percentage at elution from the RPLC experiments for the early- and late-eluting PAX portions. The difference between the eluted compositions were found to diminish below 35 °C. The broad band observed for PAX at 5 °C is eluted at a higher HFIP percentage than the late-eluting portion eluting at shigher temperatures. This resulted in the depressed melting point having a lower temperature than expected from the cloud point.

Within the studied temperature range, the depressed melting point is typically believed to be encountered earlier in a gradient than the conditions required for eluting PA46. This prevents the formation and dissolution of a mostly crystalline phase from affecting RPLC performance. In contrast, for PAX, the late-eluting portion is believed to correspond to a mostly crystalline phase.

Overall, the findings in this study helped to create a better understanding of the mechanisms involved in PA retention behavior. These can ultimately allow for more robust and accurate characterizations of semi-crystalline polymer samples.

References

(1) Kruijswijk, J. D.; Wijker, S.; Philipsen, H. J. A.; Schoenmakers, P. J.; Somsen, G. W. Study of the Aberrant Retention Behavior of a Semi-Crystalline Polyamide in Reversed-Phase Liquid Chromatography. J. Chromatogr. A 2025, 1750, 465887. DOI: 10.1016/j.chroma.2025.465887

(2) Polyamide. ScienceDirect 2013. https://www.sciencedirect.com/topics/materials-science/polyamide (accessed 2025-5-2)