Research Article
Preparation of dynamic polyurethane networks with UV-triggered photothermal self-healing properties based on hydrogen and ion bonds for antibacterial applications

https://doi.org/10.1016/j.jmst.2022.06.019Get rights and content

Highlights

  • The building of Zn-dimethylglyoxime-polyurethane coordination complex provides a covalently cross-linked elastomer with triple dynamic bonds.

  • The self-healing behavior and mechanism of 3D micro-crack is investigated by molecular dynamics simulations.

  • The doping of copper on double-nut shaped zinc oxide generates more defects, preventing the recombination of photo-induced electron-hole pairs and generating more heat through lattice vibration.

  • The enhanced self-healing effect of UV-light-responsive polyurethane was due to the hybrid of Cu-doped ZnO, which provides coordination bonds and the photothermal effect.

Abstract

Photo-induced self-healing composites have attracted more and more attention as a kind of materials that can be controlled remotely and accurately in real time. Here, we report a strategy of a photo-responsive system based on hydrogen and ion bonds capable of performing self-healing process by ultraviolet wavelengths, which is covalently cross-linked zinc-dimethylglyoxime-polyurethane coordination network with triple dynamic bonds. The recombination of hydrogen bond and metal coordination bond produces effective healing performance. The self-healing behavior and temperature dependence of 3D micro-crack is investigated by molecular dynamics simulations to reveal the mechanism of self-healing at molecular level. Moreover, the hybrid of copper-doped zinc oxide not only provides metal coordination bonds to enhance the self-healing rate, but also enhances the photothermal effect and anti-bacterial properties of polyurethane. Importantly, doping of copper generates more defects and forms a space charge layer on the surface of zinc oxide. The defects could trap surface electrons and holes, preventing the recombination of photo-induced electron-hole pairs, generating more heat through lattice vibration. Therefore, under ultraviolet light irradiation, the polyurethane can reach 62.7 °C for 60 s, and the scratches of the polyurethane can be healed within 30 min and fully healed within 1 h.

Introduction

There has been an increasing interest in polymers that can spontaneously heal surface damage and recover protective properties in last decades [1,2]. Generally speaking, the self-healing function is mainly achieved from two aspects: One is extrinsic approaches that microcapsules or microvascular networks containing healing agents are embedded into polymer matrix [3,4]. The other is intrinsic approaches that polymer matrix are repaired by reversible chemical bonds or physical molecular arrangement [5,6]. Compared to the former method, the second method allows repeated healing cycles without the help of a special catalyst [7,8]. A common strategy for design materials that healing process at room temperature is to incorporate dynamic noncovalent interactions such as hydrogen bonds, metal-ligand coordination, host-guest interactions, ionic interactions, and van der Waals forces [9], [10], [11], [12]. Hydrogen bond is the most common structural force of intrinsic self-healing elastomers, which has a wide range of sources and good reversibility. Ionic bond is a strong electrostatic interaction between groups with opposite charges. The anions and cations on the surface of fracture can connect the fracture through ion interaction to achieve self-healing effect. The polymers based on ionic and hydrogen bonds can achieve excellent mechanical strength while maintaining high self-healing efficiency [13,14].

Self-healing process is usually initiated or enhanced in response to external stimuli such as heat, light, pH, magnetic fields, or electric fields. In particular, the photothermal self-healing materials have the advantages of space control and remote activation, which provide favorable conditions for the self-healing process of damaged materials [15]. On the one hand, the diffusion and heat of photothermal materials can promote chain entanglement again, in the above its glass transition temperature or when the melting temperature to repair thermoplastic polymer, and on the other hand, the light can also heal locally, which can be accurately illuminate damaged area for repair, and the "health" area was not damaged [16,17]. It is well known that many photothermal conversion agents have been applied to self-healing polymer materials, such as carbon-based materials and metal-based materials [18,19]. However, most of the nanocomposites have lower photothermal conversion efficiency and require surface modification to promote dispersion. Therefore, it is still a challenge to find a photothermal conversion agent that has good photothermal efficiency and compatibility with polymers [20,21]. Zinc oxide (ZnO) is an n-type semiconductor with excellent optical activity which has a band gap of 3.37 eV. In addition, ZnO exhibits good stability and high electron mobility, and has a good inhibitory effect on bacteria [22], [23], [24]. Most importantly, ZnO can provide Zn2+ coordination with polymers to form self-healing metal coordination bonds.

Furthermore, ZnO can excite photogenerated electrons under ultraviolet light, and the hot carrier will cause lattice vibration, thus raising the temperature [22,25]. As a photothermal agent, ZnO can immediately respond to light stimulation and quickly repair the damaged area [26]. In order to further improve the photothermal properties of ZnO, different ZnO doping strategies were proposed. Among them, the substitutional doping of different types of metal dopants, such as anion, cation and rare earth dopants, in ZnO semiconductors can enhance the charge separation between electrons and holes, thus solving the recombination problem [27]. Copper ions (Cu2+) is considered to be the best choice because it has the same ionic radius as zinc ions (Zn2+) (0.73 Å, and 0.74 Å). Cu-doping into the ZnO matrix can not only improve the photothermal properties of ZnO, but also promote the bactericidal effect of ZnO [28,29]. Due to Cu-doping on ZnO, deep level defects in ZnO are caused, and vacancy defects become non-radiative recombination centers of electron holes. The interaction between energy band and interface structure promotes photothermal effect [30].

Herein, we synthesized a series of Cu-doped ZnO-based shape memory polyurethane composites based on hydrogen and ion bonds. The structure and photothermal properties of Cu-doped ZnO were determined by a series of characterization methods. Moreover, the photothermal and self-healing effects and antibacterial properties of Cu-doped ZnO-based shape memory polyurethane composites under 405 nm UV-light were further executed.

Section snippets

Materials

Polytetramethylene ether glycol (PTMEG, Mn = ∼1000 g/mol), isophorone diisocyanate (IPDI, 99%), toluene diisocyanate (TDI) and dibutyltin dilaurate (DBTDL, 95%) were purchased from Aladdin. Dimethylglyoxime (DMG, 98%), glycerol (99%), Zinc acetate and copper nitrate (Cu(NO3)2, 99%) were purchased from Sinopharm Chemical Reagent. Propyl isocyanate (98%) and phenethyl isocyanate (98%) were purchased J&K Chemical. Acetone (99.8%) were purchased from Yonghuachem. All reagents were used as received

Synthesis principle of polyurethane composites

As shown in Fig. 1, the self-healing polyurethane hybrid network based on hydrogen and ion bonds was firstly synthesized by one-pot polycondensation method using polytetramethylene ether glycol (PTMEG), isophorone diisocyanate (IPDI), toluene diisocyanate (TDI), glycerol and dimethylglyoxime (DMG) as raw materials. We selected PTMEG as the soft segment because the flexible chain segment can promote the chain movement in the self-healing process. IPDI and TDI are selected as hard segments. The

Conclusions

This work shows a strategy for constructing a photo-responsive self-healing system through constructing a zinc-dimethylglyoxime-polyurethane coordination elastomer with triple dynamic bond. The hybrid of Cu-doped ZnO enables the system to perform UV-induced self-healing property. Cu-doping causes more defect centers on the surface of ZnO and increases the transfer channel of photogenerated carriers. By increasing the light absorption range of ZnO and inhibiting the recombination of

Acknowledgments

This work was financially supported by the National Natural Science Foundation Joint Fund (No. U1806223), the National Natural Science Foundation of China (Nos. 51572249, 42076039), the Foundation of Key Laboratory of National Defense Science and Technology (No. JS220406), the Natural Science Foundation of Shandong Province (No. ZR2020ME016).

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