Modeling of the molar mass distribution of six-arm star-branched poly-ε-caprolactam using size-exclusion chromatography

This paper presents a modeling-based approach to the prediction of the molar mass distribution of the various species in a star-branched polycondensation mixture. The interpretation of experimental SEC data of the mixture of linear, cyclic and star-branched molecules is not straightforward, because of the different sizes of those molecules (having the same molecular mass). Therefore we have opted to use SEC analysis with only a concentration detector and fit the experimental data to the theoretical mass distribution, corrected for the volume of the various molecules. This allows the relative fraction and the distribution of the various species in the mixture (linear, cyclic and star-branched) to be determined. To demonstrate this, the six-arm star-branched poly-ε-caprolactam based on the six-functional coupling molecule, hexa(6-caproic acid) melamine has been analyzed. Five polymer mixtures with different initial concentration of coupling molecule have been synthesized. As the initial concentration of coupling molecule increased, we found that the weight fraction of star-branched molecules increased, while the weight fraction of linear and cyclic molecules decreased. We also found that the weight-average molar mass and the arm length decrease as the initial fraction of the coupling molecule increases.