Facile preparation and characterization of tough poly(vinyl alcohol) organohydrogels with low friction and self-cleaning properties

https://doi.org/10.1016/j.jiec.2022.09.011Get rights and content

Abstract

Although many hydrogels have been applied to wearable sensors, it is still challenging to simultaneously realize hydrogels with optical transparency, superior mechanical properties, excellent sensing performance, and anti-freezing by using inexpensive raw materials and an easy preparation process. Herein, using ethylene glycol/H2O (EG/ H2O) as a solvent, poly(vinyl alcohol)/EG organohydrogel (PVA/EG OHG) was prepared by a simple heating and frozen-thawing method. Owing to the multifunctionality of EG (i.e., physical cross-linker, anti-freezer and co-solvent), PVA/EG OHG demonstrated excellent integrated properties, including high strength, high toughness, and anti-freezing performances. Besides, PVA/EG OHG also showed low friction, self-cleaning, and frost resistance properties. After the introduction of LiCl, ionically conductive PVA/EG @LiCl organohydrogel was served as a self-cleaning strain sensor, which could be long-term stable to detect the motions of human under room and low temperatures. The study provides to further understanding of the organohydrogel, which will help us design next-generation high-performance organohydrogels.

Introduction

Because of the favorable nontoxicity and good biocompatibility, Poly(vinyl alcohol) (PVA) hydrogels have been widely expanded, such as 3D-printed tissue engineering materials [1], [2], self-healing gels [3], [4], antimicrobial gel [5], wearable electronic skin [6], [7], and sensors [8], [9]. Although PVA hydrogel can be prepared through either physical crosslinking or chemical crosslinking processes, obtaining high-performance hydrogel materials by easy-to-operate methods has always been pursued by researchers.

The frozen-thawing method is one of the most commonly used methods to obtain PVA hydrogels [10]. The crystallization domain is formed through intermolecular or intramolecular hydrogen bonds and then physical cross-linking occurs [11]. The mechanical properties of PVA hydrogels can usually be improved by increasing the freezing/thawing cycles to form microcrystalline structures in the hydrogel as the physical crosslinking points. Therefore, hydrogen bonding interaction plays a key role in constructing the cross-linking network structure of PVA hydrogels and has a great influence on many characteristics of PVA hydrogels. PVA hydrogels have porous structures similar to cartilage and good hydrophilicity [12]. During the wear process, the water stored in the pores is squeezed by external loads, forming fluid lubrication and thus achieving a low coefficient of friction (COF) [13], which leads to the PVA hydrogels with potential application in engineering lubrication. However, the poor mechanical properties of pure PVA hydrogels limit their application in complex loading conditions. The performance of hydrogel can be improved by constructing a double network (DN) structure [14], [15] or adding nanofillers [16], [17]. However, the irreversible covalent bonds in DN gels often cause permanent damage and poor fatigue resistance of hydrogels [18], [19], [20]. While nanomaterials can improve the strength of PVA hydrogels, they are usually opaque and increase production costs [21], [22].

Solvent exchange or in situ cross-linking methods are also effective ways to improve the mechanical properties of PVA gels [23], [24]. Common organic solvents are dimethyl sulfoxide (DMSO), ethylene glycol (EG), glycerin (Gly) [25], [26], [27], etc. For example, Xu et al. reported a solvent exchange strategy for PVA hydrogels with high tensile strength of 3.13 MPa using DMSO as the good solvent and H2O as the poor one [28]. Jing et al. constructed an ionic organohydrogel based on PVA and graphene oxide (GO) in DMSO/H2O binary solvent [29], and the strength can reach 3.1 MPa. In the Gly/H2O or EG/H2O binary solution, Gly or EG can interact with H2O, which destroys the hydrogen bond network of water and prevents the formation of ice crystals. The presence of EG leads to a decrease in the saturated vapor pressure of the binary solution, resulting in a decrease in freezing points [30], [31]. In 2007, Wang’s group [27] and Liu’s group [4] separately found PVA organohydrogel could be significantly enhanced in the presence of a large amount of Gly and EG as cosolvent, and the hydrogen-bonding interactions between PVA and Gly or EG were also attributed to the physical cross-linking in the network structure. Our group also reported the tough, transparent, anti-freezing, and photochromic nanocomposite PVA/Gly organohydrogel by adding WO3 nanoparticles to the gels [32]. During our previous research, we found the Gly/H2O binary solvent could be exuded from the PVA/Gly organohydrogel as the gels set along with the time, which was similar to the natural seaweed. However, the freeze resistance and self-cleaning properties of these PVA organohydrogels weren’t investigated systematically.

In this study, the high-strength, tough, anti-freezing, low friction, and self-cleaning PVA/EG organohydrogel (PVA/EG OHG) were prepared by a facile heating and frozen-thawing process. The structure characterization, mechanical properties, self-healing ability, water retention, anti-freezing ability, and surface characteristics were investigated in detail. Finally, it was designed as a self-cleaning and flexible sensor to monitor human motions.

Section snippets

Materials

Polyvinyl alcohol 1750 ± 50 (PVA) was purchased from Sinopharm Chemical Reagent Co., ltd. (Shanghai, China). Ethylene glycol (EG, A.R.), glycerol (A.R.), D-mannitol (A.R.), sodium chloride (A.R.), and lithium chloride (A.R.) were obtained from Aladdin Biochemical Technology Co., ltd. (Shanghai, China). Isopropyl alcohol (A.R.) was accepted by Tianjin Fengchuan Chemical Reagent Technology (Tianjin, China).

Preparation of PVA/EG OHG and PVA hydrogel

Firstly, EG and deionized water were dissolved evenly according to the weight proportion of

Preparation and structural characterization of PVA/EG OHG

The preparation process of PVA/EG OHG was illustrated in Fig. 1. Briefly, PVA was firstly dissolved in the EG/H2O binary solution at high temperature, and then the PVA/EG OHG was formed by a facile frozen-thawing method. The network structure of PVA/EG OHG is shown in Fig. 1. During the cooling process, PVA chains were firstly cross-linked by the hydrogen bonding interactions between PVA and EG, which could be identified in Fig. 2 and rheological tests. Upon cyclic frozen-thawing, the

Conclusion

In summary, PVA/EG OHGs with high strength, high toughness, anti-freezing, low friction, self-cleaning and high transparency were prepared by a facile heating and frozen-thawing method. Owing to the strong hydrogen bonding interaction between PVA and EG as well as the crystallization of PVA, PVA/EG OHG exhibited a tensile strength of 1.24 MPa and fracture toughness of 3500 J/m2. Moreover, PVA/EG OHG could also display anti-drying and anti-freezing properties. More interestingly, PVA/EG OHG

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Excellent Youth Fund Project of Henan Natural Science Foundation (202300410166), Major Project of WIUCAS (WIUCASQD2021004 and WIUCASQD2021035), Natural Science Foundation of China (21504022), Chinese Postdoctoral Science Foundation (2018M642745 and 2020M672179), Science and Technology Project of Henan Province (212102210201 and 212102310015) and Project from Department of Education in Henan Province (21A430017 and 2020GGJS052).

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