Rational design of multiple hydrogen bonds to improve the mechanical property of rigid PANI
Introduction
Excellent properties are important for flexible materials, such as stretchability, outstanding self-healing ability, and so on [1], [2]. Materials possessing stretchable property and self-healing characteristic pave the way for its application to reduce device damage by repeat stress [3], [4], [5]. Gel is desired for the development of intrinsically flexible materials which has displayed a large variety of application [6]. Nevertheless, conventional gels generally fall short of the self-healing performance, which may hinder its practical application. Moreover, some self-healing gels are hard to surmount some inherent defects, such as long self-healing time, low healing efficiency, and stimulus dependence [7], [8]. Therefore, the designing and fabricating gel with outstanding performance is an interesting topic [4], [9], [10].
Polyaniline (PANI) is a conductive, flame-retardant andcorrosion-resistant polymer. However, rigid PANI generallypresents less desirable mechanical properties which limits its practical application. To overcome this disadvantage, PANI was cross-linked with soft molecular chains by molecular interaction [11], [12]. The multicomponent gel can combine mechanical performance of soft polymer chain with the intrinsically conducting paths of PANI [11], [12]. The hydrogen bond as an important molecular interaction is widely used in the preparation of bicomponent or multicomponent gels [13], [14], [15], [16]. The mechanical properties of bicomponent or multicomponent gel can be enhanced by hydrogen bonding interaction [13], [17]. The bicomponent or multicomponent gel can combine mechanical performance of soft polymer chain to improve the elongation at break of the PANI. Therefore, the rigid PANI can be improved by constructing hydrogen bonding interaction. It is generally accepted that polyvinyl alcohol (PVA) and polyacrylic acid (PAA) are nontoxic polymer and have soft molecular chains [18], [19]. The hydrogen bonding interaction can be formed because of the hydroxyl group of PVA and the carbonyl group of PAA. Although reversible bonds (hydrogen bond) provide an approach to achieve self-healing [20], pure PVA or PAA is difficult to heal itself [21], [22] or the self-healing efficiency is less desirable [7]. Generally, the gel contains a large number of solvent molecules, such as water. PAA and PVA can easily form hydrogen bonds with water. The effective hydrogen bonds are not readily to form between molecular chains when there is only water. Designing the effective hydrogen bond interactions between molecular chains is an important prerequisite for the preparation of self-repairing gel. In the previous reports, PVA gel with phytic acid exhibited good stretchability and self-healing properties, suggesting phytic acid contributes to form hydrogen bonds [7]. Phytic acid with groups is favorable to form the reversible hydrogen bonds with PANI [23]. Therefore, a PANI/PVA gel can be prepared by phytic acid, PANI and PVA. However, its self-healing time is long (12 h) and self-healing efficiency is low [24]. The rapid spontaneous self-healing behavior and high self-healing efficiency are urgently needed [7], [8]. PAA with carboxyl facilitates to form reversible hydrogen bonds, which is beneficial to improve the self-healing efficiency [25]. The carbonyl group may provide abundant and stable hydrogen bonding. Meanwhile, phytic acid can form reversible hydrogen bond with the different molecular chains, similar to the bridge among the molecular chains. Therefore, a self-healing composite gel is promising to be obtained based on the above molecules.
In this work, we report a facile approach for the synthesis of a composite (polyvinyl alcohol/polyacrylic acid/polyaniline, PVA/PAA/PANI) gel with superior mechanical property. PVA and PAA were employed as soft chains in composite gel. Phytic acid containing groups is introduced into the gel, which could form multiple hydrogen-bonding network with the OH of PVA and the COOH of PAA. Meanwhile, the NH of PANI can interact with groups. Thus, the mechanical property of composite gel is improved greatly. The composite gel demonstrates excellent stretchability and self-healing efficiency (about 100%, within two minutes). Extraordinary performance is beneficial for its application. Thereafter, a stretchable strain sensor can be assembled directly via the composite gel. All in all, we believe that this work may provide a practical and meaningful insight into the design of multifunctional gel.
Section snippets
Characterization of composite gel
Schematic illustration of the synthesis has been revealed in Fig. 1. The brief experimental procedure as follows: Initially, PANI was prepared by chemical oxidation polymerization and was doped with hydrochloric acid. PVA and PAA were evenly mixed with deionized water. Then, PANI and phytic acid were added and stirred. Dark green composite gel can be formed quickly at room temperature. Let stand for 24 h at ambient temperature, composite gel was obtained. The gel is under acidic conditions due
Conclusions
In conclusion, a stretchable and self-healing composite gel was prepared by a facile method. The mechanical property of rigid PANI is improved by the physical crosslink of multiple hydrogen bonds with soft chains in composite gel. The elongation at break of composite gel shows high stretchability and the self-healing efficiency is enhanced to around 100%. DFT was employed to further explore and assess hydrogen bonding interaction. To understand its application, the stretchable strain sensor
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 study was supported by the Natural Science Foundation of Tianjin, People’s Republic of China (Grant No. 19JCYBJC17400) and Doctoral Foundation of Dezhou University, People’s Republic of China (2019xjrc303).
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