Supramolecular Adhesive Materials from Natural Acids and Sugars with Tough and Organic Solvent-Resistant Adhesion

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jun 2021Supramolecular Adhesive Materials from Natural Acids and Sugars with Tough and Organic Solvent-Resistant Adhesion Shuanggen Wu†, Changyong Cai†, Fenfang Li, Zhijian Tan and Shengyi Dong Shuanggen Wu† College of Chemistry and Chemical Engineering, Hunan University, Hunan 410082 , Changyong Cai† College of Chemistry and Chemical Engineering, Hunan University, Hunan 410082 Institute of Bast Fiber Crops, Chinese Academy of Agricultural Sciences, Hunan 410205 College of Chemistry and Chemical Engineering, Central South University, Hunan 410083 , Fenfang Li College of Chemistry and Chemical Engineering, Central South University, Hunan 410083 , Zhijian Tan *Corresponding authors: E-mail Address: email protected E-mail Address: email protected Institute of Bast Fiber Crops, Chinese Academy of Agricultural Sciences, Hunan 410205 and Shengyi Dong *Corresponding authors: E-mail Address: email protected E-mail Address: email protected College of Chemistry and Chemical Engineering, Hunan University, Hunan 410082 https://doi.org/10.31635/ccschem.020.202000318 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Natural adhesives have been widely replaced by industrial adhesives made from petroleum-based products. Compared with that of traditional natural adhesives, modern industrial adhesives show improved adhesion performance. However, the drawbacks of modern adhesives, including toxicity and nonbiodegradability, drive the need for new and high-performance adhesive materials from renewable and biocompatible natural feedstock. In this study, a new family of acid–sugar adhesive materials exhibiting excellent and long-term adhesion effects was developed inspired by the concept of deep eutectic solvents ( DESs). The supramolecular polymerization between natural sugars and acids gave rise to both strong cohesion and adhesion properties. Moreover, high resistance to organic solvents is an advantage of acid–sugar supramolecular adhesive materials. This study not only dramatically expands the applications of DESs but also sheds light on the development of supramolecular adhesive materials as promising alternatives to polymeric adhesives. Download figure Download PowerPoint Introduction In ancient times, sugars and starches were frequently used as ordinary adhesives to adhere papers, textiles, leathers, and other materials. Experimental and mechanistic studies clearly demonstrated that the hydrogen bond formation between water and sugar was crucially related to the adhesion behavior.1–3 Water molecules that are buried within sugars tighten sugar molecules by hydrogen bonds, to form supramolecular polymeric networks.4,5 Supramolecular polymer structures and hydrogen bond formation give rise to the cohesive and adhesive properties.6–9 However, water can be easily removed from the sugar–water supramolecular polymeric systems during the practical adhesion process, thus leading to a significant attenuation of adhesion strength or adhesion effect.10,11 With the rapid development of artificial polymeric adhesives in the past century, natural adhesives were no longer the research focus of modern adhesives.12–16 In addition, the relatively weak adhesion capacities and the presence of water become obvious disadvantages, hindering the functionalization of sugar adhesives.17–20 Sugar adhesives were therefore unable to keep pace with the expanding practical applications of modern polymeric adhesives,21–23 even though many modern polymeric adhesives are toxic or nonbiodegradable.24,25 To overcome the drawbacks of sugar adhesives, it is necessary to develop new adhesive components other than water. In natural systems, sugars are always linked with proteins to form glycoconjugates,26,27 such as glycoproteins and proteoglycans, via the bond formation between hydroxyl groups from sugars and carboxylic groups from amino acids. Inspired by the conjugation between sugars and amino acids, natural acids have great potential to develop new adhesive materials with sugars, because the hydrogen bond formation between sugars and acids are abundant, and natural acids are nonvolatile in comparison with water. Sugars and poly(acids) were then used to prepare adhesive materials by reacting at high temperature (178 °C).28 Such high reaction temperature not only yielded the adhesion property (the formation of high molecular weight polymers by the esterification between hydroxyl groups in sugars and carboxylic groups in acids) but also led to the caramelization of sugars and/or acids. The caramelization phenomena of adhesive materials dramatically affected the adhesion effect and became an obvious drawback of practical applications. In recent years, a large number of deep eutectic solvents ( DESs) based on natural sugars and acids have been successfully designed and synthesized.29–37 DESs are bulk eutectic mixtures of two or more monomers, which act as hydrogen bond acceptors and/or donors.29–38 From the view of supramolecular chemistry, DESs are typical noncovalently cross-linked polymeric supramolecular systems.39–42 In DESs, the formation of three-dimensional hydrogen bond networks leads to the occurrence of many interesting and useful features, including high viscosity.32–38 In fact, the high viscosity of sugar–acid supramolecular complexes was frequently ignored or even avoided in the wide applications of DESs.43–46 In our previous work, we developed cyclodextrin-acid supramolecular adhesives.9 However, the combination of DESs, adhesion, and supramolecular polymerization represents a new but still rarely acknowledged method to prepare supramolecular adhesives.9,38,45 Here, we report a series of acid–sugar adhesive materials that exhibit tough and long-term adhesion effects on various surfaces. Kilogram-scale and eco-friendly adhesives were prepared in a facile route. Water is replaced by natural acids in the compositions of sugar adhesive materials. The hydrogen bond formation between sugars and acids leads to the fabrication of supramolecular polymers, which dramatically improves the adhesion capacity and sustainability.9,39,40,41 Strong, recyclable, and long-term adhesion performance on a wide range of substrates originate from the reversible hydrogen bonding and dynamical supramolecular polymerization.42 Importantly, acid–sugar adhesive materials are highly resistant to organic solvents, and tough adhesion behavior is easily achieved in various organic solvents. Experimental Methods All materials and reagents were commercially obtained and used without further purification. Infrared (IR) spectra were collected on a Thermo Scientific Nicolet iS10 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, Massachusetts, USA). Thermogravimetric (Tg) analysis (TGA) was carried out using a NETZSCH STA 449 F3 Jupiter (NETZSCH-Gerätebau GmbH, Selb, bavaria, Germany), from 20 to 400 °C at a rate of 10 °C/min in nitrogen atmosphere. Differential scanning calorimeter (DSC) measurements were carried out using a TA Q200 (TA Instruments, New Castle, Delaware, USA), from −40 to 80 °C at a rate of 10 °C/min in nitrogen atmosphere. X-ray diffraction (XRD) data were collected on a Bruker D8 ADVANCE (Bruker, Karlsruhe, Baden-Württemberg, Germany). Rheology measurements were performed on an Anton Paar MCR 92 (Anton Paar, Graz, Steiermark, Austria). The laminator model PP15 was chosen with a diameter of 15 mm with the gap at 1 mm. Scanning electron microscopy (SEM) images were collected on a HITACHI UHR FE-SEM SU8010 (HITACHI, Tokyo, Japan). The adhesion strength measurements were performed on a HY-0580 Electronic tensile testing machine (Shanghai Heng Wing Precision Instrument, Shanghai, China). Proton nuclear magnetic resonance (1H NMR) spectra and concentration-dependent 1H NMR spectra were collected on a Varian Unity INOVA-400 (Varian, Palo Alto, California, USA) or Bruker-AV400 (Bruker, Karlsruhe, Baden-Württemberg, Germany). Two-dimensional correlation spectroscopy (COSY) and nuclear Overhauser effect spectroscopy (NOESY) experiments were performed on a Bruker-AV400 MHz spectrometer (Bruker, Karlsruhe, Baden-Württemberg, Germany). Two-dimensional diffusion-ordered spectroscopy (DOSY) spectra were recorded on a Bruker-AV600 MHz spectrometer (Bruker, Karlsruhe, Baden-Württemberg, Germany). Concentration-dependent viscosity experiments were done on a Brookfield DV2TLVTJ0 Viscometer (Brookfield, Middleboro, Massachusetts, USA). Microstructure of supramolecular material's fiber was observed under an Inverted light microscope LWD200-37T1 (Shanghai Cewei Photoelectricity Technology, Shanghai, China). Isothermal scanning calorimetric measurements were carried out on a Nano ITC (TA Instruments, New Castle, Delaware, USA). Preparation of acid–sugar supramolecular adhesive materials Twelve acid-sugar adhesive materials were synthesized according to reported methods.30,42,43 Taking preparation of citric acid/d-(+)-fructose supramolecular adhesive material as an example, citric acid (19.2 g, 0.1 mol) and d-(+)-fructose (18.0 g, 0.1 mol) were added into a 100 mL flask, and water (20 mL) was added into the flask. The mixture was stirred at 25 °C for 10 min, then the mixture was dried at 60 °C for 12 h without striring (the flask is open). A transition from a mixture of solids to a viscous liquid was observed during the heating process. The viscous liquid was treated under vacuum for 1 h. Without further purification or isolation processes, the viscous liquid was directly used for the adhesion and other tests. The details of the preparation of acid–sugar adhesive on a large scale were described in the Supporting Information ( Supporting Information Figure S1). The adhesive coating on different surfaces (with the coating on the glass as an example) Acid–sugar adhesive materials were coated on one glass slice which was covered by another glass slice. The adhered pair of glass slices was heated at 60 °C for 30 min and was stored under different conditions before the pull-off adhesion tests. The amount of the adhesive was kept at ca. 10 mg/cm2. Results and Discussion Two natural acids, citric acid ( C) and l-(−)-malic acid ( M), were chosen to replace water. d-(+)-trehalose ( T), d-(+)-fructose ( F), d-(+)-glucose ( G), and sucrose ( S) were used to form hydrogen bond networks with natural acids (Figure 1 and Table 1). Acid–sugar adhesive materials were prepared by heating a mixture of different natural sugars and acids at 60 °C for 30 min, as a typical method to prepare DESs.43,45,46 Temperature-induced transitions from mixtures of nonsticky solids to highly viscous soft materials were observed (Figure 1b and Supporting Information Videos S1 and S2), indicating the occurrence of supramolecular polymerization between the sugars and acids.9 In constrast, individual acids or sugars were nonviscous and did not show any adhesion effect. Compared with the 1H NMR spectra of the simply mixed sugars and acids, no changes were observed in the 1H NMR spectra of deep eutectic supramolecular adhesive materials (Figure 1b and Supporting Information Figures S2–S13), which demonstrated that no esterification or etherification took place at 60 °C.47,48 The 1H NMR results also confirmed that deep eutectic supramolecular adhesives were hydrogen-bonded supramolecular complexes, which can decomplex to individual acids and sugars in D2O.39,40 As shown in concentration-dependent 1H NMR spectra ( Supporting Information Figures S14–S25), it was demonstrated that when the concentrations of DESs were increased from 4 to 1200 mg/mL, no supramolecular polymerization behavior was observed. This information indicated that at the solution state, due to the strong solvation effect of water molecules (the interaction between water molecules and natural sugar/acid molecules), natural sugars and acids existed in a monomer-type state. Two-dimensional DOSY, NOESY, and titration experiments further confirmed these observation ( Supporting Information Figures S26–S65). Concentration-dependent viscosity tests clearly showed that at the solution state, the increase of the concentration only exerted slight influence on the viscosity ( Supporting Information Figures S66 and S67). However, when in a mixture of DESs and water (water content is below 25–15%), a rapidly increased visocosty was observed, indicating the formation of supramolecular polymeric structures ( Supporting Information Figure S68).42 These observations fit the features of our formerly reported supramolecular polymers with deep eutectic properties.9 The attenuated total reflectance IR (ATR-IR) spectra of the acid–sugar adhesive materials further confirmed the hydrogen-bonding formation ( Supporting Information Figures S69 and S70).9,49,50 Acid–sugar supramolecular adhesive materials were amorphous and thermostable, according to their powder XRD (PXRD) patterns and TGA, respectively ( Supporting Information Figures S71–S74). Figure 1 | Acid–sugar supramolecular adhesive materials. (a) Chemical structures of natural acids and sugars. (b) 1H NMR spectra (400 MHz, D2O, 25 °C) of the simple mixture of citric acid and d-(+)-trehalose (molar ratio = 1∶1) without heating, and supramolecular adhesive materials CT-1. (c) Composite viscosity (|η*|) value of MS-2 at reversible temperature-dependent rheological tests. Download figure Download PowerPoint Table 1 | The Components of Supramolecular Adhesive Materials from Natural Acids and Sugars Numbers Acids Molar Ratios Sugars Abbreviations 1 Citric acid ( C) 1∶1 d-(+)-fructose ( F) CF 2 1∶1 Sucrose ( S) CS-1 3 1∶2 Sucrose ( S) CS-2 4 1∶1 d-(+)-glucose ( G) CG 5 1∶1 d-(+)-trehalose ( T) CT-1 6 1∶2 d-(+)-trehalose ( T) CT-2 7 l-(−)-malic acid ( M) 1∶1 d-(+)-fructose ( F) MF 8 1∶1 Sucrose ( S) MS-1 9 1∶2 Sucrose ( S) MS-2 10 1∶1 d-(+)-glucose ( G) MG 11 1∶1 d-(+)-trehalose ( T) MT-1 12 1∶2 d-(+)-trehalose ( T) MT-2 As shown in Supporting Information Figures S75 and S76, supramolecular adhesive materials possessed low glass transition temperature. Dense and smooth surface morphology of the acid–sugar adhesive materials were observed by SEM ( Supporting Information Figure S77). All acid–sugar adhesive materials showed high viscosities (1.8 × 104 to 5.9 × 105 Pa·s) detected by rheological characterization (Figure 1c and Supporting Information Figures S78–S89), which was consistent with their macroscopic performances. The zero-shear viscosities of the acid–sugar adhesive materials decreased with the increased temperature, exhibiting the thermosensitive nature of hydrogen-bonding networks and supramolecular polymerization processes.51 However acid–sugar adhesive materials still had relatively high viscosities at high temperature, which is the basis of their high-temperature adhesion. Temperature-dependent changes of the viscosities were totally reversible. Compared with individual sugars or acids, the newly formed adhesive materials were easier to process. Acid–sugar adhesive materials can be conveniently coated on different surfaces (glass, steel, polymethyl methacrylate PMMA, and polytetrafluoroethylene PTFE) to form uniform adhesion layers with various thicknesses (from tens of microns to several millimeters). Smooth and thin fibers with lengths of several meters were mechanically pulled out from the acid–sugar adhesive materials (Figure 2a), indicating the formation of high-molecular-weight supramolecular polymers.52,53 Figure 2 | Adhesion strengths of acid–sugar supramolecular adhesive materials. (a) The adhesion process for supramolecular adhesive materials and the macroscopic adhesion behavior. (The adhesion area is 3 × 3 cm2 and the substrate is glass in the right photograph, the weight of the person is 70 kg.) (b–e) Adhesion strengths of acid–sugar supramolecular adhesive materials on iron, glass, PMMA, and PTFE. The dashed lines demonstrate the highest values of adhesion strengths on different surfaces. Error bars show standard deviation of the measured adhesion strengths of acid–sugar supramolecular adhesive materials (n = 4). Download figure Download PowerPoint Adhesion tests were easily carried out because of the good processability of acid–sugar adhesive materials. Using the adhesion on glass as an example, acid–sugar adhesive materials were deposited onto the surface of a glass slice, which was then covered immediately. When weights of up to 70 kg were attached to the glass surface, no separation or displacement of the adhering area (3 × 3 cm2; Figure 2a and Supporting Information Video S3) was observed from shear forces. The adhesion effects of acid–sugar adhesives were not only strong but also long-lasting. Similarly, acid–sugar adhesive materials displayed strong, stable, and long-lasting adhesion effects on other surfaces, from hydrophilic steel to hydrophobic PMMA and PTFE. Recyclable adhesion performances were also realized by acid–sugar adhesive materials.7–9 Quantitative tests were performed to measure the adhesion strengths of acid–sugar adhesive materials on different surfaces and under different conditions (temperature), and to find the relationship between adhesive composition and adhesion effect (Figure 2). The shear strengths on glass surfaces are all higher than 1.10 MPa for all acid–sugar adhesive materials, with malic acid and fructose ( MF) having the highest value at 2.85 MPa. Ultrastrong adhesion effects were obtained between acid–sugar adhesives and steel surfaces. The lowest adhesion strength of acid–sugar adhesive materials on steel was achieved by MF with a value of 3.0 MPa, which is greater than the adhesion strengths of many previously reported supramolecular or polymeric adhesives.9,12 Malic acid and glucose ( MG) show the strongest adhesion effect on steel, with an adhesion strength at 5.42 MPa. Moreover, acid–sugar adhesive materials exhibited remarkable adhesion capabilities on hydrophobic PMMA. An adhesion strength of up to 1.18 MPa was detected between citric acid and glucose ( CG) and PMMA. Though acid–sugar adhesive materials did not show as strong an adhesion capacity on PTFE as that on glass or steel, acid–sugar adhesives still exhibit moderate adhesion strengths on PTFE (from 0.22 to 0.38 MPa; Figure 2e), in comparison with those of commercially available adhesives or previously reported supramolecular adhesives (usually below 0.1 MPa).9,12 Acid–sugar adhesive materials are typical supramolecular polymers containing two comonomers (acid and sugar), with hydrogen bonding as the main driving force.9 Density functional theory (DFT) simulations clearly revealed the three-dimensional networks of hydrogen bonding in acid–sugar complexes (Figure 3a and Supporting Information Figure S91) which drive the supramolecular polymerization between sugars and acids.8,9 Owing to the cross-linked polymeric structures and the high density of hydrogen-bonding networks, acid–sugar complexes can strongly bond to hydrophilic surfaces. With the adhesion behavior of MF on glass surface as an example, multiple hydrogen bonds were observed between MF and the glass surface (Figures 3b and 3c). Meanwhile the easy surface spreading and/or gap-filling properties originating from the high-molecular-weight polymeric structures further enhanced the adhesion effects. In addition, abundant carboxylic groups in either malic acid or citric acid were capable of hydrogen bonding with the fluorine groups of PTFE, which leads to effective adhesion behaviors on PTFE.12 Figure 3 | Illustration of adhesion mechanism. (a) Molecular dynamic simulations of configurations of molecular models of MF and glass with (HO)3SiOSi(OH)2OSi(OH)3 as model or steel (with Fe atom as model). (b) Carton representation of the adhesion modes with different surfaces. (The adhesion area in the inserted photograph is 2 × 3 cm2, and the substrates used are PTFE.) (c) DFT simulations of the hydrogen bond formations of d-(+)-fructose and d-(+)-fructose, l-(−)-malic acid and l-(−)-malic acid, MF, and MF@(OH)3SiOSi(OH)2OSi(OH)3. Download figure Download PowerPoint Though acid–sugar adhesives containing citric acid (citric acid–sugar adhesives) or malic acid (malic acid–sugar adhesives) all exhibited excellent adhesion capacities, the two types of adhesive materials showed several differences in adhesion performance (Figure 2). Citric acid–sugar adhesives had relatively stronger adhesion effects when adhering to hydrophobic surfaces (PMMA and PTFE), compared with malic acid–sugar adhesives. The average adhesion strengths of citric acid–sugar adhesives on PMMA and PTFE were 1.07 and 0.30 MPa, respectively. These were 34% and 13% higher than that of the corresponding malic acid–sugar adhesives (0.71 and 0.26 MPa). In contrast, malic acid–sugar adhesives were more suitable for adhesion on hydrophilic surfaces, with an average adhesion strength on steel of 4.54 MPa. This average value was higher than the best adhesion value of citric acid–sugar adhesives on steel ( CT-2, 4.37 MPa). When comparing the adhesion effects of all 12 acid–sugar adhesive materials, CT-2, MF, and MG showed relatively stronger adhesion effects than the rest of the adhesive materials. Unfortunately, MG was highly hygroscopic, and the adhesion effect was gradually attenuated. We then focused on CT-2 and MF to further investigate the adhesion capacities of acid–sugar adhesive materials (Figures 2b and 2c). Though both high and low temperatures were responsible for the decay of adhesion strengths of acid–sugar adhesive materials, CT-2 and MF had wide temperature ranges for tough adhesion applications up to 80 °C ( Supporting Information Figure S90). When the temperature was kept at 55 °C, the adhesion strength of CT-2 on steel was 4.09 MPa, which is about 93% of that at 25 °C (4.37 MPa). When the temperature was increased to 80 °C, an adhesion strength at 0.43 MPa was still measured, exhibiting the good adhesion capability of CT-2 at high temperature. Similar results were obtained from adhesion tests of CT-2 on other surfaces. MF also has strong adhesion effects at high temperature. In contrast, a total loss of adhesion was observed in our previously reported supramolecular adhesive system driven by hydrogen bonding, when the temperature was increased from 25 to 80 °C.9 These observations demonstrate that the adhesive components were important in the realization of strong adhesion behavior at high temperature. Tough adhesion at low temperature is an excellent advantage of adhesives for practical applications. CT-2 and MF have strong adhesion abilities at extremely low temperatures, though the low temperature partially weakens the adhesion effects ( Supporting Information Figure S90). After storage at −80 °C for 24 h, surfaces were still adhered by CT-2 or the testing at −80 °C from 24 h to 1 no obvious loss of adhesion strengths was observed. example, CT-2 has an adhesion strength of 4.37 MPa on a steel surface 25 °C, an adhesion strength of MPa was detected by pull-off adhesion tests at −80 no decay of adhesion abilities of CT-2 and MF was observed, when the adhesion surface was to PTFE MPa at 25 °C and MPa at −80 °C for CT-2, MPa at 25 °C and MPa at −80 °C for The adhesion strengths and adhesion effects of CT-2 and MF showed the potential for their as adhesives. The good adhesion capabilities of CT-2 and MF at low temperature can be to the of hydrogen-bonding networks, which the adhesives not as as traditional polymeric adhesives at low adhesion in organic solvents an important in material supramolecular adhesives and polymeric adhesives are either organic or their adhesion effects are attenuated in organic solvents because of These drawbacks not only the applications of adhesive materials but also the development of material The natural sugars and acids used in this study are in a of organic solvents, including ( ( ( ( and ( acid–sugar adhesive materials have behaviors to those of the individual sugars or acids. tests also confirmed the nature of the acid–sugar adhesives in various organic solvents. Meanwhile no phenomena were observed when acid–sugar adhesive materials were in different organic solvents. on these it is to investigate the organic adhesion behaviors of acid–sugar adhesive materials (Figure 4 and Supporting Information Video long-term adhesion tests demonstrate that storage in organic solvents ( and for more than 3 no separation or displacement was observed when a weight of 2 kg was attached to the adhered glass slices (Figure and Supporting Information Video As shown in Figure a glass can be easily by CT-2, which is highly resistant to However, is not suitable for these acid–sugar adhesive materials as it gradually the adhesion which can be to hydrogen bond formation between and acid–sugar polymeric due to the properties and hydrogen bond formation of acid–sugar adhesives, adhesion applications under water were not To adhesion behavior under it is to prepare supramolecular adhesives from Figure 4 | Adhesion performances of acid–sugar adhesive materials in organic solvents. (a) Adhesion strengths of CT-2 in different solvents for 24 h. (b) Adhesion strengths of MF in different solvents for 24 h. Adhesion strengths of CT-2 in different solvents for and of adhesion strengths of CT-2, MF, and commercially available in for 7 and adhesion behavior of CT-2 in In was added to the solution to a Error bars show standard of the measured adhesion strengths of acid–sugar supramolecular adhesive materials (n = 4). Download figure Download PowerPoint With the adhesion tests in as an example, storage for 24 h, the adhesion strength of CT-2 on glass is MPa (Figure which is with the adhesion values MPa). In contrast, commercially available and show adhesion strengths in and other organic solvents (Figure As shown in Figures tough adhesion effects are still in long-term adhesion tests in organic solvents. After storage in organic solvents for 2 ( CT-2 on about adhesion strengths were compared with that of tests. MF even organic adhesion MF on glass and on steel in organic solvents for 24 and adhesion strengths of adhesion, respectively (Figure comparing with the adhesion strengths obtained in and long-term (3 tests in organic solvents, it is that no obvious decay of adhesion strengths took place long-term to organic solvents. These observations demonstrate that acid–sugar adhesive materials are promising as organic adhesives. viscosity is an important for DESs, which from the three-dimensional hydrogen bond networks and the bulk state, the viscous property and of DESs are a the high viscosity of DESs is always ignored or even DESs with low viscosity and high are In this study, we used the high viscosity of acid–sugar DESs to and long-lasting adhesion performances on various surfaces. We reported a family of new adhesive materials of