Time-Salt Type Superposition and Salt Processing of Poly(methacrylamide) Hydrogel based on Hofmeister Series

Breakthrough to the extreme properties of polymer networks relies on new insights into their molecular dynamics. Time-salt concentration superposition has been discovered in polyelectrolyte coacervate systems, proving instrumental in tuning the mechanical performance of polyelectrolyte hydrogels. However, time-salt type superposition has never been mentioned in the existing literature. Herein, we reported that the mechanical properties of poly(methacrylamide) (PMAm) hydrogel can be systematically regulated in a vast range by treating with different salts, for example, Young’s modulus can be tuned from 10^-2 to 10³ MPa. The unusual behavior of salt-stiffening arises from the salt-enhanced phase separation of the polymer network and a subsequent glassy transition of the polymer-rich phase. Rheological results demonstrate that the dynamic behavior of the hydrogels can be superposed onto “time-salt type” master curves with the salt types aligning along the Hofmeister series. The time-salt type shift factor exhibits correlation with the mobility of water molecules as revealed by low-field nuclear magnetic resonance spectroscopy. A polymer-water-salt ternary interaction mechanism was proposed to elucidate the time-salt type equivalent behavior. Guided by the “time-salt type” superposition principle, a salt processing strategy was brought up to expand the property limits of the PMAm hydrogel. With simply switching the type of salt employed, the hydrogel could either be a stiff and wear-resistant material akin to glassy plastics or a soft and flowable gel utilizable for recycling.