English

• Over the past decade, supramolecular adhesives have triggered a flurry of attentions in material science and supramolecular chemistry [1-5]. Non-covalent bonding in structures endows adhesive materials with various dynamic and reversible features [6-10]. Nowadays, supramolecular adhesives have become an important member of the high-performance adhesive family [11-18].

  In the application of supramolecular adhesives, low temperatures play a significant role in the adhesion effect and performance [19,20]. It is well-known that high temperature can destroy the non-covalent interactions and supramolecular structures, thus greatly weakening the adhesion strength [21-24]. However, the negative effect of low temperatures on adhesion is frequently ignored, as low temperatures can enhance the binding capacity of supramolecular pairs [25,26]. As a matter of fact, low temperatures dramatically affect the integrity of adhesion layers, and cause the debonding between surfaces and adhesive materials [27-30]. Different anti-freeze molecules, such as salts and alcohols, have been used in the preparation of adhesive materials to yield low temperature-resistance [31,32]. However, low-temperature-resistant supramolecular adhesives without any anti-freeze components are still limited [33-36].

  Herein, we report a serendipitously found supramolecular adhesive that exhibits good adhesion effects to various surfaces at low temperatures. Strong and tough adhesion performances of supramolecular adhesive on metal, glass, polymethyl methacrylate (PMMA), and polytetrafluoroethylene (PTFE) were observed, when temperatures were below freezing point. At extremely low temperatures (< −40 ℃), stable adhesion was still realized.

  During the synthesis of bis(B21C7)-substituted ZE [37], a wrong molar ratio of B21C7-NH2 and acid (1:1) was applied (Scheme 1). Firstly, a labile intermediate compound (O-acylisourea ester) was formed via the reaction of EDC with the carboxyl group of TA. Secondly, B21C7-NH2 partially substitutes the EDC group of O-acylisourea ester via a nucleophilic attack to produce amide. Finally, the residual EDC groups of O-acylisourea ester may undergo cyclic electronic displacement (N→O displacement) to generate N-acylurea (Fig. S1 in Supporting information) [38]. Therefore, OE instead of ZE was monitored and isolated. Compared with ZE and TE, OE is a glue-type material with high viscosity and poor fluidity (Fig. 1a), as confirmed by various rheology measurements. As shown in Fig. 1e and Figs. S25-S27 (Supporting information), OE displays excellent temperature-dependent rheological behavior, which can be attributed to the bulk state and thermal stable feature of OE. In addition, the composite viscosity of OE was increased by 10⁵ times as the testing temperature decreased from 80 ℃ to 10 ℃, and this rheological behavior was reversible (Fig. 1e). This observation can be ascribed to the existence of dynamical non-covalent interactions in the system, such as hydrogen bonding, van der Waals forces, and F–F interactions. In contrast, neither ZE nor TE exhibits any viscosity in a wide temperature range (from −80 ℃ to 100 ℃). In our previous studies, it has been demonstrated that benzo-21-crown-7 is essential to realize strong adhesion [39-41]. Thus, TE did not show any adhesion property due to the absence of the crown ether ring [41]. Meanwhile, as a 1,4-disubstituted compound, ZE favors aggregating together (strong cohesion), which dramatically attenuated the adhesion capacity to surfaces (weak adhesion) [41].

  Scheme 1

  

  Scheme 1.  Synthetic routes of OE, ZE, and TE.

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  Figure 1

  

  Figure 1.  Characterization of OE. (a, b) Macroscopic and SEM image of OE (scale bar is 10 µm). (c) PXRD pattern of OE. (d) DSC spectra of OE. (e) Temperature-dependent reversible composite viscosity of OE.

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  Based on above information, more investigations were carried out to study the physicochemical and mechanical properties of OE (Figs. S2-S24 in Supporting information). Only a wide peak was observed in powder X-ray diffraction (PXRD) pattern, demonstrating the amorphous nature of OE (Fig. 1c). Scanning electron microscopy (SEM) tests showed a dense, non-porous morphology of OE, without the existence of three-dimensional fibers (Fig. 1b and Fig. S23). A low glass transition temperature (T_g of 0.23 ℃) close to the freezing point was recorded in differential scanning calorimetry (DSC) measurements, indicating the potential low-temperature adhesion application of OE. Meanwhile, during the heating-cooling cycles, a similar T_g (−0.39 ℃) was found in the second heat-cooling cycle (Fig. 1d).

  With the above information in mind, the driven force of supramolecular adhesion was studied (Figs. S9−S20). As shown in Fig. 2, concentration-dependent ¹H NMR spectra, concentration-dependent ¹⁹F NMR spectra, and concentration-dependent two-dimensional diffusion ordered NMR (DOSY) spectra are obtained to study the interaction between OE. No new covalent interaction was found in either diluted or concentrated OE solutions [39-41]. Meanwhile, as shown in Fig. 2a, the signals of NH protons H on OE moved down-field, from 6.27 ppm to 6.97 ppm, when the concentration of OE was increased from 5 mg/mL to 200 mg/mL, suggesting that the existence of hydrogen bonding among the amide groups (the amide groups act as H-bonding acceptors and donors simultaneously). In addition, the chemical shift of fluorine moved to the high field when the concentration of OE was increased from 5 mg/mL to 200 mg/mL (Fig. 2b), indicating that the existence of F–F interaction [38].

  Figure 2

  

  Figure 2.  (a) Partial concentration-dependent ¹H NMR spectra of OE (CDCl₃, 400 MHz, room temperature). (b) Partial concentration-dependent ¹⁹F NMR spectra of OE (CDCl₃, room temperature). (c) Concentration-dependent DOSY NMR spectra of OE (CDCl₃, 600 MHz, room temperature).

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  Moreover, by analyzing the DOSY NMR spectra of OE, two diffusion coefficients (D) of 6.31 × 10⁻¹⁰ m²/s (5.0 mg/mL, 1.28 nm) and 2.75 × 10⁻¹⁰ m²/s (200 mg/mL, 2.95 nm) were observed, respectively, suggesting the concentration-dependent aggregation of OE. Combining with results from macroscopic tests and rheological experiments, it is reasonable that OE forms high molecular weight species at concentrated solutions and in bulk state, based on the reversible non-covalent interactions.

  As a glue-like material, OE showed a good processability (Fig. 1a). Uniform, thin, and intact adhesion layer can be easily cast on various surfaces, including steel, glass, PMMA and PTFE. As shown in Fig. 3a, two steel plates are firmly cohered together by OE, without any tedious operations or additives. In the following weight-loading tests, no failure of the adhesion or destruction of the adhesive layer was observed (Fig. 3b). Low testing temperatures (from 25 ℃ to −18 ℃) did not affect the macroscopic adhesion effect of OE. As a matter of fact, OE exhibited better adhesion performances at low temperatures (such as 4 ℃ and −18 ℃) than that at 25 ℃. The information clearly demonstrate that OE has great potential as a powerful low-temperature-resistant adhesive material [30,42,43].

  Figure 3

  

  Figure 3.  Adhesion performance of OE at low temperatures. (a) Adhesion process of OE. (b) OE-steel system hung with a weight of 500 g. (c) Adhesion strengths of OE at −18 ℃ for different time. (d) Adhesion strengths of OE at various temperatures (−80 ℃, −50 ℃, −18 ℃, 4 ℃, and 25 ℃) for 12 h. (e) Adhesion strengths of OE at 4 ℃ under various relative humidities (10%, 60%, and 99%) for 12 h. (f) Cycle measurements of OE at −18 ℃.

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  Lap-shear measurements were later carried out to quantitatively evaluate the adhesion effect of OE (Figs. 3c-e). Compared with the adhesion strength of OE obtained at 25 ℃, stronger adhesion capacities of OE were realized at low temperatures. For example, moderate adhesion strengths (0.41−0.6 MPa) of OE to four tested surfaces were recorded at 25 ℃, while adhesion strength of OE up to 5.05 MPa (steel) was obtained at low testing temperature (4 ℃). As shown in Fig. 3d, 4 ℃ is the optimum temperature for the supramolecular adhesion of OE. This observation can be attributed to glass transition behavior of OE (T_g close to 0 ℃).

  Tough, long-term adhesion of OE at low temperatures (such as −18 ℃) was successfully realized. As shown in Fig. 3c, good adhesion effects of OE to glass, PMMA, and PTFE, were observed, indicating the potential application of OE as a long-last low temperature-resistant adhesive. After adhering to steel for 46 days, only a moderate decrease in adhesion strength was found, from 2.45 MPa to 1.81 MPa (Fig. 3c). The adhesion effects of OE between −4 ℃ and −80 ℃ demonstrate the strong adhesion capacity of OE at low temperatures. Compared with crown ether-based supramolecular adhesives, OE exhibited stronger adhesion effect at long-time adhesion tests [37,39-41].

  Humidity plays an important role in the adhesion performance of OE. OE showed the strongest adhesion strengths to steel (5.05 MPa) and glass (2.83 MPa) at RH 60% and 4 ℃ (Fig. 3e). Either elevating or decreasing RH% obviously weakened the adhesion effects of OE (4 ℃). A possible explanation is that water molecules can participate in the supramolecular polymerization and adhesion, via the hydrogen bonding formation with OE (crown ether unit) and/or adhered surface [31,39,40,44]. Therefore, high or low content of water may interfere with the assembly pattern of OE.

  After multiple cycle measurements at low temperatures (Fig. 3f), no obvious decay or fatigue of adhesion capacity of OE on testing substrates was observed, indicating that OE can be used as a recyclable adhesive material. For example, after seven cycles, the adhesion strengths of OE to steel and glass were about 84.7% (0.94 MPa) and 72.1% (0.31 MPa) of that of the first time, respectively (Fig. 3f). A possible explanation is that the non-covalent interactions, such as hydrogen bonds, could be weakened upon heating and reconstructed at low temperatures [45-50]. Moreover, compared to the glass without the treatment of OE, no obvious changes of contact angel and FTIR were observed from the glass with the treatment of OE (Figs. S30 and S31 in Supporting information). Meanwhile, the ¹H NMR spectrum of OE coated on a glass surface for 48 h is the same as that of freshly obtained OE (Fig. S32 in Supporting information). These results suggested that OE did not participate in the covalent bonding with the surface of the materials.

  Interestingly, OE exhibited strong affinity to steel at extremely low temperatures. As shown in Fig. 4a, two steel plates are firmly adhered together in liquid nitrogen by OE. Meanwhile, good adhesion of OE was also observed when testing temperature was quickly increased from −196 ℃ to 25 ℃. Lap-shear tests clearly proved the good adhesion effect of OE to steel in liquid nitrogen, with the adhesion strength at 0.87 MPa (Fig. S33 in Supporting information). No obvious decay of interaction energy between OE and steel was observed as the temperature was decreased from 298 K to 77 K. Similar result was found from cohesive energy density (CED, cohesion effect) of OE. The simulation model clearly showed the non-covalent interactions in adhesion and cohesion steps of the bulk OE cluster to iron surface. These simulation results are fully consistent with the macroscopic adhesion behavior of OE (Figs. 4b and c, Figs. S35−S43 and Tables S1−S3 in Supporting information).

  Figure 4

  

  Figure 4.  Supramolecular adhesion behavior of OE at low temperatures. (a) Adhesion behavior of OE in liquid nitrogen. (b) The independent gradient model (IGM) isosurfaces for the interaction of OE and OE in a molar ratio of 1:1. (c) Molecular dynamic simulation of configurations of the molecular model of OE and steel (with Fe atom as model) at 298 K and 77 K, respectively. The scale bar represents 2 nm.

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  With the abovementioned adhesion behavior of OE in mind, underwater adhesion tests at low temperatures were designed and carried out. No separation or displacement of adhered steel by OE was found, when the adhesion was taken place in the presence of water (Fig. 5d). Similar observations were recorded in the underwater weight-loading tests. Changing the surfaces from steel to glass, PMMA or PTFE, did not exert obviously influences on the adhesion effects of OE (Fig. 5). When the water temperature was cooled to 0 ℃ (in an ice-water bath), tough adhesion behaviors of OE to four tested surfaces were still successfully achieved.

  Figure 5

  

  Figure 5.  Underwater adhesion performance of OE. (a) Schematic diagram of underwater adhesion of OE. (b-f) Macroscopic underwater adhesion behavior of OE to various substrates, including glass piece (b, c), steel piece (d), PMMA piece (e), and PTFE piece (f). Two weights are 500 g and 100 g, respectively.

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  As shown in Fig. S34 (Supporting information), the average adhesion strength of OE (in an ice-water bath) at 2.73 and 2.84 MPa on steel and glass are observed, respectively. Similarly, good adhesion ability of OE was obtained, when steel or glass was replaced by other testing substrates (PMMA or PTFE). Although moderate decay in adhesion strength was observed along with prolonged soaking time, tough adhesion of OE to glass and PMMA was still obtained under water.

  In conclusion, a low-molecular-weight supramolecular adhesive was serendipitously obtained. Tough and long-term adhesion of OE to diverse substrates at low temperatures (from −4 ℃ to −80 ℃) was successfully realized. Compared with traditional adhesive materials with good resistance to low temperatures, OE did not have any anti-freeze components. OE exhibited excellent resistance to low temperatures (including extremely low temperatures). Moreover, OE showed a potential as an underwater adhesive material. This work not only enriches the family of supramolecular adhesives, but also is helpful for designing low temperature-resistance adhesive materials.

  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

  The authors gratefully acknowledge the Outstanding Youth Scientist Foundation of Hunan Province (No. 2021JJ10010), the Huxiang Young Talent Program from Hunan Province (No. 2018RS3036), the Fundamental Research Funds for the Central Universities from Hunan University. Moreover, this work is supported by the Hunan Provincial Innovation Foundation for Postgraduate (No. CX20200446).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.107830.

  
  
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  Scheme 1  Synthetic routes of OE, ZE, and TE.

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  Figure 1  Characterization of OE. (a, b) Macroscopic and SEM image of OE (scale bar is 10 µm). (c) PXRD pattern of OE. (d) DSC spectra of OE. (e) Temperature-dependent reversible composite viscosity of OE.

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  Figure 2  (a) Partial concentration-dependent ¹H NMR spectra of OE (CDCl₃, 400 MHz, room temperature). (b) Partial concentration-dependent ¹⁹F NMR spectra of OE (CDCl₃, room temperature). (c) Concentration-dependent DOSY NMR spectra of OE (CDCl₃, 600 MHz, room temperature).

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  Figure 3  Adhesion performance of OE at low temperatures. (a) Adhesion process of OE. (b) OE-steel system hung with a weight of 500 g. (c) Adhesion strengths of OE at −18 ℃ for different time. (d) Adhesion strengths of OE at various temperatures (−80 ℃, −50 ℃, −18 ℃, 4 ℃, and 25 ℃) for 12 h. (e) Adhesion strengths of OE at 4 ℃ under various relative humidities (10%, 60%, and 99%) for 12 h. (f) Cycle measurements of OE at −18 ℃.

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  Figure 4  Supramolecular adhesion behavior of OE at low temperatures. (a) Adhesion behavior of OE in liquid nitrogen. (b) The independent gradient model (IGM) isosurfaces for the interaction of OE and OE in a molar ratio of 1:1. (c) Molecular dynamic simulation of configurations of the molecular model of OE and steel (with Fe atom as model) at 298 K and 77 K, respectively. The scale bar represents 2 nm.

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  Figure 5  Underwater adhesion performance of OE. (a) Schematic diagram of underwater adhesion of OE. (b-f) Macroscopic underwater adhesion behavior of OE to various substrates, including glass piece (b, c), steel piece (d), PMMA piece (e), and PTFE piece (f). Two weights are 500 g and 100 g, respectively.

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