"Soft Crosslinking" Trick Makes Brittle Plastics Both Harder and Tougher

Glassy polymers are plastics whose molecular chains become immobilized below a certain temperature, making them hard and stiff but also brittle, so they fracture when stretched. One strategy for overcoming this trade-off is to add ionic groups whose electrostatic attractions form reversible physical crosslinks, but this approach has been difficult to apply to conventional glassy polymers without also making them brittle in a new way.

Researchers from Tokyo University of Science (TUS) in Japan, working with the Japan Science and Technology Agency (JST), developed an ionic comb-shaped polymer using bulky 4-dimethylaminopyridine (DMAP) counterions to keep the ionic interactions evenly distributed. The study, led by then-Junior Associate Professor Daisuke Aoki along with Kotaro Uchiyama, Ryotaro Miyazawa and Professor Koji Arimitsu, was published in the journal Macromolecules on September 4, 2026.

The team synthesized comb polymers and neutralized them with either bulky DMAP counterions or conventional sodium ions to compare their effects. High concentrations of sodium ions made the polymers increasingly brittle, but DMAP improved mechanical performance across a broad range of concentrations. The best-performing DMAP-neutralized polymer achieved a toughness of about 137 MJ/m³ together with a Young's modulus of about 0.9 GPa, roughly four times tougher and twice as stiff as the non-ionic starting material.

X-ray scattering showed that DMAP forms a homogeneous nanostructure with no detectable phase separation, while infrared spectroscopy confirmed ionization within the polymer. Rheological tests found DMAP lowers the glass transition temperature from 96°C to 81°C, giving polymer chains greater mobility by acting as a plasticizer. "DMAP functions both as a plasticizer and a physical crosslinking point, creating a unique nanostructure that we call 'soft crosslinking'," said Aoki.

"This research overturns the conventional wisdom that glassy polymers become brittle when designed using ionic interactions," Aoki said. "Applying this finding to more widespread molecular designs will lead to the development of next-generation plastic materials that are less prone to breakage and have a longer lifespan. In the future, these longer-lasting materials could reduce plastic waste, and if used as structural materials for transportation equipment such as drones and automobiles, they could also contribute to lower carbon dioxide emissions and improved energy efficiency through lightweight designs."