Tertiary ammonium counterions outperform quaternary ammonium counterions in ionic comb polymers: overcoming the trade-off between toughness and the elastic modulus

Abstract Ionic motifs incorporated into polymers, such as ionomers, polyelectrolytes, polyampholytes, and poly(ionic liquid)s (PILs), serve as physical crosslinks through ionic interactions, presenting an effective molecular design for improving both the toughness and elastic modulus of the resulting polymer. However, in glassy polymers, ionic motifs, particularly organic base counterions, seldom effectively contribute to toughening mechanisms, as observed in soft materials. Here, we investigate the influence of tertiary and quaternary ammonium counterions on the mechanical properties of ionic comb polymers with a focus on the counterion incorporation ratio. We discovered a specific ion content range where only tertiary ammonium counterions, such as triheptylamine, improved both the toughness and Young’s modulus of a precursor polymer containing carboxylic acid groups and oligo(ethylene glycol) side chains. Fourier transform infrared (FT-IR) analysis revealed the presence of neutral amines in the tertiary ammonium systems, as evidenced by slightly less intense carboxylate peaks compared with the quaternary ammonium system peak intensities. Furthermore, rheological analysis revealed that tertiary counterions induced plasticization and reduced relaxation times up to the rubbery region. This study demonstrated the distinct mechanical effects of organic bases on glassy polymers.