With the development of supramolecular chemistry and the increasingly obvious limitations of traditional polymer adhesives, supramolecular adhesives, which are constructed by combining non-covalent interactions with covalent bonds, have attracted extensive attention from academia and industry. Due to the dynamic network characteristics of non-covalent interactions, supramolecular adhesives exhibit many excellent properties, such as reusability, responsiveness, and circularity, making them show great potential for practical applications in the automotive, biomedical and electronics fields. However, supramolecular adhesives usually require additional conditions such as heating or ultraviolet light in practical applications, and often have poor performance under harsh conditions such as low temperature and high humidity, so there is still a lot of room for improvement in the optimization of adhesive performance.
Recently, the team of Prof. Cao Pengfei and Associate Professor Chen Jiayao from Beijing University of Chemical Technology published the latest paper "Constructing Crown Ether-Based Supramolecular Adhesives with Ambient-Temperature Applicable and Durable Adhesion" in the journal Advanced Functional Materials. The authors of the paper propose a series of room-temperature crown ether-based supramolecular adhesives that do not require harsh curing conditions by introducing a variety of physical interactions into the four-arm polymer supramolecular system through ingenious molecular and network structure design. This type of adhesive exhibits excellent adhesion performance in the bonding of a variety of matrix materials, among which TCE-PEG-C1/PEI after complexation with hydrochloric polyethylenimine PEI-HCl shows the best adhesion performance, and the adhesive strength can reach 7.21 MPa. These adhesives also exhibit excellent adhesion properties in extreme environments such as underwater and low temperature (-170 °C), and their overall performance is better than that of most reported crown ether-based adhesives, providing a unique new strategy for the design of multifunctional adhesives.
1. Molecular design of crown ether-based supramolecular adhesives
In this paper, we propose a simple and efficient strategy for constructing multi-interaction network structures, as shown in Figure 1, using four-arm polyethylene glycol (4-arm-PEG) as the matrix backbone and hexamethylene diisocyanate (HDI) as the end capping to obtain isocyanate prepolymers. Then, a range of crown ether-based adhesives with different adhesive properties can be obtained by adjusting the type and size of the crown ether ring. In addition, a supramolecular complexing adhesive (TCE-PEG/PEI) was obtained by using PEI-HCl with high secondary amine content as a physical crosslinker to coordinate with TCE-PEG, and a supramolecular network with multiple interactions was successfully constructed, including hydrogen bonding between the crown ether ring and hydroxyl groups, metal ions, water molecules, ionic interaction, π-π stacking and van der Waals force physical interaction, which can improve the macroscopic adhesion performance of the material by enhancing the substrate adhesion and adhesive cohesion.
Fig. 1 (a) Design and preparation of crown ether-based supramolecular adhesive and (b) multi-interaction adhesion mechanism
2. Structural characterization of crown ether-based supramolecular adhesives
The authors used 1H NMR, FTIR, and GPC to characterize the chemical structure, composition, and molecular weight of the synthetic adhesive. From the NMR data (e, f, and g peaks in Figure 2a), the -CH2-O-CH2- characteristic peak of the crown ether ring was observed to appear within 3.7-4.2 ppm, and the -CH2-proton signal adjacent to the isocyanate group disappeared (peak d in Figure 2b), demonstrating the successful preparation of the crown ether-based adhesive. As shown in the FT-IR spectrum of Figure 2c, the characteristic peak of the isocyanate group appears at 2249 cm-1, which verifies the successful termination of the isocyanate group, and the proportion of the characteristic peaks in the NMR image further proves that no significant cross-linking is formed. In addition, the results of gel permeation chromatography (GPC) showed that the molecular weight of isocyanate was slightly shifted after termination (from 1900 g/mol in the four-arm PEG-OH to 2600 g/mol in the four-arm PEG-NCO), and the molecular weight was consistent with the 1H NMR results.
Figure 2: Basic synthetic characterization of crown ether-based supramolecular adhesives
3. Adhesion properties of crown ether-based supramolecular adhesives
The synthetic TCE-PEG adhesive exhibited excellent adhesion properties on wood, stainless steel, aluminum, glass, paper and other substrates (Fig. 3), among which TCE-PEG-C1 had the highest adhesion strength at room temperature (25°C, 50 RH%), which was 4.21, 3.10 and 3.00 MPa on wood, stainless steel and aluminum, respectively. Due to the excellent adhesive properties, some substrate materials (glass, paper, cardboard) break before the bond layer breaks. The difference in the bond strength of TCE-PEG-C1 on different substrates mainly depends on the chemical groups on its surface, among which wood, glass and paper are mainly bonded by forming hydrogen bonds, while metals are mainly bonded by forming metal complexes. In addition, the adhesion performance of TCE-PEG adhesive on different substrates was improved after 7 days of curing compared to the adhesive cured for 1 day. Among them, TCE-PEG-C1 had the most significant enhancement performance, and the adhesive strength of TCE-PEG-C1@Wood (5.50 MPa), TCE-PEG-C1@Steel (4.23 MPa) and TCE-PEG-C1@Aluminum (3.71 MPa) increased by 30%, 36% and 24%, respectively, after 7 days of curing This is due to the direct contact of the sample with water molecules in the air, the crown ether ring forms hydrogen bonds with the water molecules, and a stronger supramolecular network structure is formed inside the TCE-PEG adhesive.
Figure 3: Viscosity behavior of crown ether-based supramolecular adhesives
As the range of applications for adhesives expands, their adhesion performance at low temperatures is also critical. The study found that TCE-PEG-C1 adhesive has low temperature applicability on different matrix materials (Fig. 4). After curing at -15 °C, the adhesive strength of TCE-PEG-C1@Wood and TCE-PEG-C1@Steel was maintained at 2.0 MPa and 1.5 MPa, although the adhesive properties decreased slightly. Especially in the liquid nitrogen immersion experiment (-196 °C), TCE-PEG-C1 adhesive still showed good adhesion at extremely low temperatures, and maintained its adhesive properties after recovering from extremely low temperatures to room temperature.
Figure 4: Viscosity of crown ether-based supramolecular adhesives at low temperatures
4. Exploration of the complexation mechanism of crown ether-based supramolecular adhesives
In order to further study the complexation of the synthetic crown ether-based supramolecular adhesive, the authors selected TCE-PEG-C1 with the highest adhesive strength and the linear polymer PEI-HCl for compounding, and characterized its properties. The adhesive strength of TCE-PEG-C1/PEI on wood, stainless steel and aluminum substrates was 7.21 MPa, 3.88 MPa and 4.82 MPa, respectively, which was significantly higher than that of the original TCE-PEG-C1. The coordination interaction between the secondary amine and the crown ether ring in PEI/HCl significantly enhances the adhesive performance, and the crown ether ring with a large ring cavity can be used as a ligand for the complex of protonated ammonium ions, and NH··· O hydrogen bond interaction to achieve stable coordination. In addition, the addition of linear polymers also enhances the molecular chain entanglement in the network structure, thereby enhancing the cohesion of the system. Compared with the typical shear response behavior of the oligomeric system of TB-PEG in the control group, the storage modulus of TCE-PEG-C1 at high frequency is close to 1 GPa, and the surface TCE-PEG-C1 system has higher rigidity at low temperature, while the shear response behavior of TCE-PEG-C1/PEI shows the characteristics of strong cross-linked polymer network. In order to further reveal the mechanism of the interaction between TCE-PEG-C1 and PEI-HCl, the authors gradually simulated the complex from a simple small molecule system to a complex complexed system, and elucidated the influence of complexation interaction on supramolecular networks at the molecular structure scale. The DFT calculation and optimization of various supramolecular orientations, followed by frequency calculations, determined the variation of the Gibbs free energy (ΔG) of the system. For the small molecule system, 2MEAE and PA4 were selected as the guests to enhance the adhesion of TCE-PEG-C1/PEI, and C1 was selected as the host. The results show that there is considerable repulsion between the C1/2MEAEs, and the guest molecule fails to cross the center of the coronal ether ring, which is consistent with the previous hypothesis. In addition, the authors designed and calculated the optimized interaction between TCE-PEG-C1 and PA4, and the ΔG value of the optimized TCE-PEG-C1 and PA4 interaction system was the lowest (-6.48 kcal/mol) under different types of crown ether ring endcapping, which was consistent with the macroscopic experimental results.
Figure 5: Long-term curing performance of crown ether-based supramolecular adhesives and rheology/DFT mechanism exploration
Ju Hao, a master's student from Beijing University of Chemical Technology, is the first author of the paper, and Professor Cao Pengfei and Associate Professor Chen Jiayao of Beijing University of Chemical Technology and Professor Zoriana Demchuk of Oak Ridge National Laboratory are the co-corresponding authors of the paper.
About the Corresponding Author:
Jiayao Chen, trainee associate professor at the School of Materials Science and Engineering, Beijing University of Chemical Technology, will return to China in 2023 to join the team of Professor Cao Pengfei at Beijing University of Chemical Technology. His main research interests are the design and synthesis of elastic polymers, the degradation and upcycling of thermosetting polymers, and the 3D printing of functional polymer materials and composite materials. So far as the first or corresponding author in Advanced Materials, Angew. He has published more than 10 papers in internationally renowned journals of materials and engineering, such as Chem. Int. Ed., Advanced Science, Additive Manufacturing, and Chemistry of Materials, written 1 chapter of English monographs, and applied for 3 PCT international patents.
Source: Frontiers of Polymer Science