Time-Salt Type Superposition and Salt Processing of Poly(methacrylamide) Hydrogel based on Hofmeister Series
Yijie Jin, Shan Lu, Xinran Chen, Qianyao Fang, Xin Guan, Liguo Qin, Chongyi Chen, Chuanzhuang Zhao
Abstract
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 10 3 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.