Self-healing supramolecular waterborne polyurethane based on host–guest interactions and multiple hydrogen bonds

Abstract

Polymers which contain 2-Amino-4-hydroxy-6-methylpyrimidine (UPy) usually show good self-healing capabilities under certain conditions. In this study, the UPy was grafted into the side chain of waterborne polyurethane (WBPU), which was synthesized from 4,4-diisocyanate dicyclohexylmethane (H₁₂MDI) and polycarbonate diol (PCDL), for giving the function of self-healing to the polymer. Meanwhile, the methylated β-cyclodextrin (M-β-CD), which was able to enhance the mechanical properties of WBPU and realize the synergistic effect between M-β-CD and UPy to raise the performance of self-healing, was introduced onto the main chain of WBPU. The T_g (glass transition temperature) of the polymer can be observed from dynamic mechanical property analysis (DMA) below room temperature, which facilitated the movement of the chain in self-healing process. From the tensile test, it showed that the cut polymer spline displayed a self-healing efficiency of 92.29% at 100 °C within 36 h. Furthermore, the disappearance of surface scratch of polymer film was observed by heating under a polarized microscope (POM) with a heating table. The results manifested that SWBPU exhibited great application as an intelligent response-type material.

Introduction

The damage of polymer materials, as the one of the most important affecting factors on polymer lifespan, attracted more and more attention nowadays. [1] In particular, self-healing materials were usually used to solve the problems of the polymer damages that could be unavoidable. At present, the self-repair materials should be fall into two classes of non-intrinsic and intrinsic. The non-intrinsic material was self-healed by adding the active agent, which consisted of microcapsules and liquid core fibers. For example, to achieve self-healing performance, White and his coworkers firstly reported a glass fiber, where the active healing agent was embedded into the epoxy resin matrix. [2] The intrinsic material constructed a healing material from reversible covalent bonds or non-covalent bonds under certain external conditions. Such as, the conversion of reversible covalent bond could be achieved in light or heat by the introduction of Diels-Alder chemistry. [3,4] The transformation between disulfide and thiol groups [5,6] also made the polymer self-healable. Moreover, the ester exchange reaction, the reversible boric acid ester bond [7] and the reversible oxime bond were able to be introduced into self-healing materials either. On the other aspect, the non-covalent bonding system like macromolecular rearrangement, host-guest recognition, [[8], [9], [10]] van der Waals forces, [11] hydrogen bonds, [[12], [13], [14]] metal-ligand coordination bonds [15,16] and π-π stacking interactions which had a weak bonding energy were usually used in some flexible and soft self-healing polymer materials.

A lot of efforts had been done to endow multifunctional materials with self-repairing capability. The host-guest recognition interaction was the one of the most effective ways to prepare the self-repairing materials. Moreover, cyclodextrins (CDs), as a series of semi-natural oligosaccharides, were commonly connected with 6–8 glucose units by α-1,4 glycosides. Due to the hydrophilic surface and hydrophobic cavity, CD could selectively identify different guest molecules. [17] Benefited from the non-toxic, CDs were used in various fields, including pharmaceutical, [[18], [19], [20]] food, cosmetics, catalyst, [21,22] adsorption, [[23], [24], [25], [26], [27]] and so forth. Interestingly, CDs were able to conveniently graft into the polymer chains to construct self-healing materials through host-guest complexation. For example, Chen et al. prepared a supramolecular self-healable polymeric hydrogel based on host–guest recognition between the β-CD polymer and the α-bromonaphthalene polymer, which could self-heal in macroscopic scale within only one minute and in just air without any additive. But it had a poor tensile strength that the pristine polymer was only 2.5Mpa. [28] Zhu et al. reported a self-healable supramolecular hydrogel, which was induced by inclusion complexation between cholic acid and β-CD. Interestingly, due to the natural origin of cholic acid and β-CD, these supramolecular hydrogels may become attractive candidates for pharmaceutical and biomedical applications. [29] Li et al. made a hydrogel crosslinked polymer built on dynamic host–guest interactions which exhibited both of outstanding biocompatibility and self-healing properties. The supramolecular hydrogels exhibited good self-healing properties, which not only could be seen visually, but also confirmed by step-strain rheological measurements. [30] Wang's group synthesized a novel graphene/β-CD supramolecular nanocontainer composite based on host-guest chemistry, where the benzotriazole (BTA) loaded nanocontainers were used to endow materials with great passive and active anticorrosion, and self-healing performance respectively. With the graphene added, the mechanical property of polymer was improved distinctly. [31] By taking the advantage of the dynamic binding process of the host-guest complexes connects, polymer was equipped with excellent self-healing ability.

On the other hand, the air quality problem seriously affected people's healthy life in recent years. Thus, the researches on environmental protection materials became more and more popular. Waterborne polyurethane (WBPU) was emerging as an environmentally friendly material with reducing the release of volatile organic compound (VOC). Due to its non-toxic and pollution-free, it was widely used in versatile fields such as coatings, [32] adhesives, smart textiles, plasticizers and so on. In recent years, many researches about the self-healing of WBPU were reported. For example, Wang et al. synthesized a series of self-healing waterborne polyurethane/graphene oxide nanocomposites with extraordinary self-healing performance. Graphene oxide greatly improves the mechanical properties of waterborne polyurethane. The tensile strengths of the nanocomposites were 22–47% higher than that of neat waterborne polyurethane. [33] Wan et al. synthesized self-healing environmentally friendly waterborne polyurethane, which had disulfide bonds in the main chain. The tensile strength of the scratched WBPU film was recovered to 90.5% at a modest healing temperature of 65 °C in 10 min. However, this tensile strength of original WBPU film was 0.5Mpa merely. [34] Interestingly, Ding et al. reported waterborne polyurethane and β-CD were used to enhance anti-corrosive performance of polyurethane CaCO₃ composite film recently. A novel polyurethane-CaCO₃ film was developed via in situ mineralizing in seawater. Moreover, β-CD with rich hydroxyl and hydrophobic cavity promotes mineralization. However, this work didn't deeply explain the effect of β-CD in self-healing process. [35] It was found that the self-repairing polymers usually showed poor mechanical properties no matter in host-guest interaction hydrogel system or reversible covalent bond system. Hence, the way to improve the mechanical properties of self-healing material was much significant.

According to previous studies, [35,36] the polymers were able to increase their mechanical properties by introducing rigid CDs. Meanwhile, the host-guest complexation abilities owned by CDs provided much more intelligences onto the self-healing polyurethane. However, so far as we know, the research was rarely reported in the field of self-repair on the introduction of CD into polyurethane. At the same time, the study of self-repair mechanism was able to provide the theoretical foundation for the comprehending about the host-guest interaction between β-CD and guest in water-based polymer. Herein, in this manuscript, the waterborne polyurethane was prepared from the environmental protection perspectives. In the first step, PCDL, H₁₂MDI and DMPA reacted to form prepolymer. The reactions of chain growth of prepolymer were carried out by U₂-diol and β-CD molecules with the DBTDL to catalyze the reaction. The UPy and β-CD molecule were incorporated to self-healing function of waterborne supramolecular polyurethane to achieve hydrogen bond synergy host-guest interaction. The synthesized polyurethanes were characterized by different analytical techniques. Various properties including physical, mechanical, chemical, thermal along with self-healing capability were evaluated. The obtained results and the analysis of self-healing mechanism were showed that the supramolecular waterborne polyurethane (SWBPU) was expected to be applied in electronic flexible devices and waterborne environmentally friendly coatings future.