News Resource: School of Materials Science and Engineering

Editor: News Agency of BIT

Translator: Xu Yunlei, News Agency of BIT

The development of high-strength hydrogels has greatly broadened the application field of gel materials. However, most hydrogels have a limited temperature range for their application due to the large amount of water they contain. At low temperatures, the physicochemical properties of hydrogels change drastically, and the formation of ice crystals leads to toughness-brittleness transition, reduced transparency, and functional deterioration. The development of hydrogels with frost resistance and excellent low-temperature mechanical properties is of great scientific value.

The water in the hydrogel can be divided into freezing water and non-freezing water (also known as bound water). The presence of freezing water causes the hydrogel to freeze under certain supercooling conditions (approx. -10 °C). In order to improve the antifreeze performance of hydrogels, antifreeze agents such as inorganic salts or organic solvents are usually introduced. The disadvantage is that the antifreeze risks leaking into the surrounding water environment, thereby losing its antifreeze properties. Another method is to increase the content of bound water in the hydrogel, inhibit the transformation of the water molecular network to ice crystals at low temperatures, and make the water in the gel in an unfreezing state. The challenge lies in how to effectively increase the content of bound water in the gel to prepare intrinsic antifreeze hydrogels.

Professor He Zhiyuan's research group from the School of Materials Science and Engineering of BIT and the research group of Wu Ziliang, a researcher at Zhejiang University, found that glassy hydrogels containing dense hydrogen bond association structure have excellent intrinsic antifreeze properties and low temperature mechanical properties. In poly(acrylamide-co-methacrylic acid)[P(AAm-co-MAAc)] hydrogels, highly tangled and dense hydrogen bond association effectively reduces segment motility, leaving it in a glassy state at room temperature. At low temperatures, hydrogen bond association is further enhanced, resulting in an increase in modulus and strength. The gel still exhibits some ductility at -45 °C and remains transparent in liquid nitrogen. DSC, low-field NMR, low-temperature XRD and other results showed that most of the water in the glass gel was in an unfreezing state. The unique correlation between network dynamics and water molecular state provides a new idea for the design and preparation of intrinsic antifreeze hydrogels. The authors also explored other low-temperature properties and potential applications of the gel.

The gel has a water content of about 50wt%, and has excellent mechanical properties at room temperature. Due to the dynamic characteristics of hydrogen bonds, the mechanical and viscoelastic behavior show significant temperature dependence. The variable temperature stretching results showed that the gel still showed yield phenomenon at minus 45 °C and had good toughness. The gel remains highly transparent in liquid nitrogen, indicating excellent frost resistance. In contrast, polyacrylamide gels (PAAm) with the same water content quickly freeze and whitish at low temperatures, and their mechanical properties become brittle.

XRD and DSC results showed that glassy P (AAm-co-MAAc) hydrogels only freeze a small amount of water at low temperatures, which was much lower than that of conventional PAAm hydrogels. Further, the low-field NMR results showed that the water molecules inside the glassy gel were in a highly restricted state at room temperature, resulting in inhibiting the formation of ice cores at low temperatures. This is because , (1) There are a large number of hydrogen bonding sites in the gel network, which reduces the motility of water molecules; (2) The hydrogen bond is further enhanced at low temperature, and the confinement effect of the glass network hinders the nucleation and freezing of water molecules, so that it is in an unfreezing state. Therefore, the network glass state has an important contribution to the intrinsic antifreeze performance of hydrogels. Furthermore, a small amount of free water can’t provide the free volume required for segment movement, so the hydrogel remains in a glass state at room temperature. The correlation between network glass state and antifreeze performance provides a new basis for the preparation of intrinsic antifreeze hydrogels. Other glassy hydrogels also exhibit intrinsic frost resistance and excellent low-temperature mechanical properties, indicating the universality of the method.

In addition, P(AAm-co-MAAc) hydrogels have unique low-temperature luminescence behavior due to cluster luminescence caused by carbonyl groups in hydrogen bond associations. When the temperature is reduced from 50°C to -80°C, the phosphorescent intensity and lifetime of the gel increase significantly. This is because, as the temperature decreases, the hydrogen bond gradually strengthens and the motility of the chain segment decreases, thereby suppressing non-radiative transitions and improving luminous efficiency.

The above study was published in Advanced Materials under the title "Intrinsic Anti-Freezing and Unique Phosphorescence of Glassy Hydrogels with Ultrahigh Stiffness and Toughness at Low Temperatures". Professor He Zhiyuan, BIT and Professor Wu Ziliang of Zhejiang University are the co-corresponding authors of the paper. The research work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, and the Institute of Advanced Materials and Chemical Engineering of Shanxi-Zhejiang University.