Elastic, tough and switchable swelling hydrogels with high entanglements and low crosslinks for water remediation

Abstract

Developing hydrogel adsorbents with both high mechanical property and high adsorption capacity is a big challenge. At the same time, the swelling-adsorption is in conflict. Swelling expands the polymer network and facilitates the exposure of adsorption sites and the diffusion of pollutants. On the other hand, swelling causes the hydrogel to increase in volume and soften, which increases the difficulty of operation and regeneration. Hence, we promote a method of preparation advanced hydrogel adsorbent by high entanglements and low crosslinks. The entanglements work as additional crosslinks, reducing swelling and making the hydrogel tough. Unlike crosslinks, entanglements are like springs, which are flexible and can efficiently transmit and store stress. Therefore, highly entangled hydrogel exhibits high elasticity, low hysteresis, and good fatigue resistance. Abundant carboxyl functional groups are introduced by copolymerizing acrylic acid. The carboxyl groups are not only sites for adsorbing cationic pollutants, but also a smart switch for regulation of swelling. When adsorbing pollutants, the carboxyl groups are deprotonated, and the flexible entanglements allow large swelling of the hydrogel while maintaining good rigidity, which efficiently facilitates adsorption. Compared with highly crosslinked hydrogel with similar polymer content and carboxyl abundance, the adsorption capacity and rate of highly entangled hydrogel increased by more than ten times. While regeneration, the highly entangled hydrogel de-swells efficiently under the tension of entanglements, significantly improving the operability of regeneration. This study provides a novel strategy for developing smart hydrogel adsorbent.