Most hydrogels become frozen at subzero temperatures, leading to degraded properties and limited applications. Cryoprotectants are massively employed to improve anti-freezing property of hydrogels; however, there are accompanied disadvantages, such as varied networks, reduced mechanical properties, and the risk of cryoprotectant leakage in aqueous conditions. Reported here is the glassy hydrogel having intrinsic anti-freezing capacity, excellent optical and mechanical properties at ultra-low temperatures. Supramolecular hydrogel of poly(acrylamide-co-methacrylic acid) with moderate water content (~50 wt%) and dense hydrogen-bond associations is in a glassy state at room temperature. Since hydrogen bonds become strengthened as the temperature decreases, this gel become stronger and stiffer, yet still ductile, with Young’s modulus of 900 MPa, tensile strength of 30 MPa, and breaking strain of 35% at -45 ^oC. This gel retains high transparency even in liquid nitrogen (Figure 1), and exhibits unique phosphorescence that is further enhanced at subzero temperatures due to presence of confined carbonyl clusters. Further investigations elucidate that the intrinsic anti-freezing property is related to a fact that most water molecules are tightly bound and confined in the glassy matrix and become non-freezable. This correlation, as validated in several systems, provides a roadmap to develop intrinsic anti-freezing hydrogels for widespread applications at extreme conditions.

Figure 1. (a) Network of P(AAm-<i>co</i>-MAAc) hydrogel with dense entanglements and associative interactions. (b) Photos of P(AAm-<i>co</i>-MAAc) gel at 25 (room temperature), −45, and −196 °C (liquid nitrogen).

Figure 1. (a) Network of P(AAm-co-MAAc) hydrogel with dense entanglements and associative interactions. (b) Photos of P(AAm-co-MAAc) gel at 25 (room temperature), −45, and −196 °C (liquid nitrogen).

Zi Liang Wu

Zhejiang University

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