Depolymerization of cellulose to intermediate active cellulose by selective partial pyrolysis and its structural characterization and analysisSelective partial pyrolysis and depolymerization of cellulose can be achieved by gradually raising the temperature from 25 °C to 200 °C under controlled conditions. Significant changes in the macroscopic morphology and microscopic structure of cellulose before and after heat treatment and depolymerization (Fig. 2a–d) were observed. In the Fourier Transform infrared spectroscopy (FT-IR) spectrum (Fig. 2e), the stretching vibration peaks at 896 and 1165 cm−1 attributed to the glycosidic cellulose β-(1-4) bond significantly decreased in intensity after selective partial pyrolysis of MCC (MCC-SPP), indicating that a considerable number of these bonds were broken in the pyrolysis process. The peak at 1058 cm−1 belonged to the bending vibration peak of C-O at the C3 position of the glycosidic bond connection. This peak did not significantly change after pyrolysis, indicating that this bond was still intact and cellulose only underwent partial pyrolysis. The peak at 3420 cm−1 was the stretching vibration peak of the O–H bond in the hydroxyl group, which was significantly reduced in MCC-SPP due to the decomposition of a large number of hydrophilic hydroxyl groups, which improved the water resistance of this material. In addition, in the Ultraviolet and Visible spectrum(UV) spectrum (Fig. 2f), the wavelengths of 210 and 289 nm were assigned to hydroxyl groups (–OH) and β-(1-4) glycosidic bonds. The UV absorption transmittance of the latter significantly decreased after 3 h of pyrolysis, which corroborated the FT-IR results above.Fig. 2: Characteristics of cellulose before and after selective partial pyrolysis and depolymerization.a Macromorphology of cellulose before selective partial pyrolysis and depolymerization; b Macromorphology of cellulose after selective partial pyrolysis and depolymerization; c Microscopic morphology of cellulose before selective partial pyrolysis and depolymerization; d Microscopic morphology of cellulose after selective partial pyrolysis and depolymerization; e FT-IR spectra, f UV spectra; g 1H NMR spectra, h Water contact angle (The error bars in the figure represents the standard error), and i GPC-based molecular weight distribution before and after selective partial pyrolysis and depolymerization; j EDS spectra, k Reaction equations for pyrolysis and depolymerization.In the 1H nuclear magnetic resonance (1H NMR) spectrum (Fig. 2g), the peak at a chemical shift of 3.3 ppm was a typical O–H peak of the primary hydroxyl group at the C6 position. After 3 h of selective partial pyrolysis, the peak significantly decreased because a part of the hydroxyl groups was decomposed after pyrolysis, which reduced the hydrophilicity of cellulose. The absorption peak at a chemical shift of 4.5 ppm decreased considerably, caused by the breaking and disappearance of glycosidic bonds linking large cellulose molecules, and some remaining peak intensity indicated the partial pyrolysis of cellulose to form intermediate cellulose. Furthermore, it is important to note that the water contact angle of cellulose (Fig. 2h) significantly increased after selective partial pyrolysis and depolymerization. The great increase from 33.69° to 50.15° indicated that the water wettability of the treated cellulose material was poorer, and the hydrophobicity of the material significantly increased. At the same time, in the molecular weight distribution diagram (Fig. 2i), the molecular weight of the material decreased considerably from 89,400 to 74,244 after pyrolysis and depolymerization. The lower molecular weight indicated that the hydrogen bond network and cellulose glycosidic bonds between rod-shaped large cellulose molecules were broken, and the large rod-shaped cellulose decomposed into a secondary intermediate structure, making the cellulose molecules more active and malleable, indicating the smooth progress of selective partial pyrolysis and depolymerization. X-ray diffraction(XRD) analysis (see Supplementary Fig. 1) revealed that the crystallinity of partially pyrolyzed cellulose was 42.28%, which was slightly lower than the 44.00% of unpyrolyzed MCC. The decrease in cellulose crystallinity promoted the production of intermediate cellulose. The diffraction peak at 2θ of 22.85° originated from the cellulose crystal plane (002) and remained almost unchanged, indicating that the basic structure of cellulose did not undergo significant changes. This also proved the formation of intermediate active cellulose after the partial pyrolysis and depolymerization treatment of cellulose. This process eliminated O–H moieties in some hydroxyl groups, reduced hydrophilicity, and improved water resistance while retaining the basic structure of intermediate cellulose that can continue to undergo secondary crosslinking reactions.Further analysis of the elemental composition before and after pyrolysis was conducted using energy dispersive X-ray spectroscopy (EDS) (Fig. 2j), and typical signal peaks of C and O elements belonging to cellulose were observed. The most significant decrease in content in MCC-SPP was noted for the O element at 520 eV. Not only did the occurrence of the dehydration reaction in cellulose lead to a decrease in water, but the disappearance of some C–O bonds in glycosidic bonds also led to a decrease in O element and C element at 270 eV. Differential scanning calorimetry (DSC) was used to determine the enthalpy values and peak intensities of heat absorption and release before and after selective partial pyrolysis and depolymerization (see Supplementary Fig. 2). Based on this analysis, the glass transition behavior and degree of crystallization remained almost unchanged, and since basic composition and structure did not undergo fundamental changes, some of the excellent basic properties of the material were preserved during treatment. To summarize, the intermediate active cellulose obtained by selective partial pyrolysis of cellulose had a significantly reduced content of hydrophilic hydroxyl groups (–OH) in its molecular structure, which enhances its hydrophobicity and water resistance when used as an adhesive.Characteristics of the prepared all-cellulose colloidal adhesiveCellulose reacted with sulfuric acid in a dilute sulfuric acid environment to form broom-like filamentous fibers after acid hydrolysis. As shown in Fig. 3c, these fibers were characterized by ultra-high polymerization and self-assembly properties and were able to self-assemble under certain conditions to form a crosslinked network with a stable structure. In this work, the self-assembly performance refers to the spontaneous formation of larger structures between filamentous fiber molecules through non-covalent interactions. This self-assembly and polymerization behavior is enabled by its unique molecular structure and physicochemical properties, which are related to its unique broom-like filamentous fibers, high aspect ratio, and the presence of hydroxyl groups on the surface. The molecular chains of filamentous cellulose are rich in hydroxyl groups (–OH), which can form hydrogen bonds between them. Under certain conditions, the hydroxyl groups on the surface of cellulose interact through hydrogen bonds, promoting the spontaneous combination of nanocellulose molecules to form an ordered structure. This process is driven by various forces, including van der Waals forces, hydrogen bonding, electrostatic interactions, and hydrophobic interactions. These forces cause the filamentous fiber particles to arrange in a specific direction, forming a hierarchical structure. The presence of hydroxyl groups enables strong hydrogen-bonding interactions between particles, resulting in the formation of a three-dimensional cross-linked network. In the self-assembly and solidification process, cellulose molecules intertwined and crosslinked to finally form a stable and very strong crosslinked three-dimensional network, which was responsible for the excellent physical and chemical properties of the material. The broom-like filamentous fiber morphology was clearly observed after treatment in the microstructure diagram in Fig. 3c and Supplementary Fig. 29, which also confirmed our theory. In the FT-IR spectrum (Fig. 3d), the peak at 2915 cm−1 was the stretching vibration peak of O–H and C–H bonds, which became weaker with the progress of the reaction and the degree of substitution. The peaks at 1170 cm−1, 880 cm−1, and 1740 cm−1 were attributed to the stretching vibrations of S=O, C–O–S, and C=O in the ester group, respectively. The spectra indicated the presence of the –SO4 group, which is strong evidence that sulfuric acid molecules replaced or underwent hydration reactions with the primary hydroxyl group at C6 and the secondary hydroxyl groups at C2 and C3, respectively, to form more active sites. Furthermore, the peak exhibited by the adhesive at 3420 cm−1 was the stretching vibration peak of the O–H bond in the hydroxyl group. The significant reduction in the number of hydrophilic hydroxyl groups also led to a decrease in the hydrophilicity and a significant improvement in the water resistance of the adhesive. The chemical shift at 4.50 ppm in the 1H NMR spectrum (Fig. 3f) also indicated the presence of substituent groups. Here, –HSO4 serves as a protective group to replace the active hydrogen atom of O–H on the primary or secondary hydroxyl groups in cellulose molecules, thereby forming an active group structure that can generate covalent bonds during hot-pressing curing reactions. The peak at 1.22 ppm also originated from the H–S–O bond of the substituent, which replaced the hydroxyl groups at different C positions in the cellulose molecule. The UV spectrum of the prepared adhesive (see Supplementary Fig. 5) also showed significant changes in the functional groups and the effect of protective substituents. In the XRD spectrum of the all-cellulose colloidal adhesive (Fig. 3e), a broad peak was observed at 2θ = 15.12°–16.80°, indicating its amorphous structure and strong fluidity. Furthermore, the crystallinity decreased from 58.22% to 52.89%, which also resulted in a better liquid effect.Fig. 3: Various characteristics and performance of all-cellulose colloidal adhesive.a A gel system prepared directly with sulfuric acid without partial pyrolysis and depolymerization treatment; b all-cellulose colloidal adhesive macroscopic performance; c microstructure of the all-cellulose colloidal adhesive system; d FT-IR spectra before and after all-cellulose colloidal adhesive preparation; e XRD pattern of all-cellulose colloidal adhesive; f 1H NMR spectra of all-cellulose colloidal adhesive; g DSC spectra before and after all-cellulose colloidal adhesive preparation; h Particle size distribution map of all-cellulose colloidal adhesive; i Bar chart of viscosity (Vis) and solid content (Sol) of all-cellulose colloidal adhesive; j Equations of chemical reactions that may occur in a sulfuric acid environment.The thermal curing performance is one of the most important properties of adhesives. Therefore, the thermal properties and reaction process of the sample were determined by DSC (Fig. 3g), revealing significant changes in the enthalpy and peak intensity of the absorbed and released heat of all-cellulose colloidal adhesive. During the thermal reaction process of all-cellulose colloidal adhesive, the peaks observed at 131.91 °C and 163.86 °C were assigned to the most important curing processes to generate the high-performance and highly water-resistant adhesive. The phase transition and reaction process represented by the peak region at 131.91 °C were the volatilization of solvents and the continued transition of active intermediate cellulose to a more active molten state. The endothermic peak at 163.86 °C represented the secondary crosslinking reaction and condensation of the reactants of the previous stage, resulting in the transformation of the material morphology and final solidification and bonding. The thermal properties and reaction processes displayed in the graph confirmed that the used hot-pressing curing temperature of 150 °C was indeed the optimal temperature for this process. The thermal reaction peak region at 346.65 °C in MCC indicated the most significant weight loss stage, confirming the classic pyrolysis theory of cellulose. In addition, according to the thermogravimetric analysis (TGA) of the dynamic thermodynamic analysis of all-cellulose colloidal adhesive(see Supplementary Fig. 6), the first stage of weight loss during heating occurred between 35.24 and 140.15 °C, during which water and solvent were removed. In the second stage of weight loss from 140.15 to 200.23 °C, all-cellulose colloidal adhesive continued to undergo crosslinking and curing reactions during heating, forming a dense and high-strength crosslinked network to provide the product with excellent mechanical properties (for DTG spectra before and after all-cellulose colloidal adhesive preparation see Supplementary Fig. 7). This is also consistent with the earlier DSC analysis, providing strong evidence for our proposed heating and curing behavior and the adopted hot-pressing curing temperature. The third stage of weight loss was observed between 200.23 and 249.43 °C and was the main stage of thermal decomposition and performance loss of this adhesive. The molecular network structure underwent extensive cracking, decomposing into small molecules, accompanied by partial carbonization and the precipitation of volatile matter.In addition, the particle size of active molecules in adhesives has a direct and important impact on reaction activity, permeability, and adhesive performance after crosslinking and curing at the composite interface. Dynamic light scattering was used to study and analyze the particle size distribution range of all-cellulose colloidal adhesive molecules (Fig. 3h), showing that their particle size was concentrated between 250 and 550 nm. This uniformly dispersed micro- and nano-sized molecular system is more conducive to increasing the adhesive’s penetration and mechanical interlocking abilities at the bonding interface during bonding. At the same time, the higher reactivity of micro- and nano-sized particles promoted curing and resulted in the formation of a dense crosslinked network structure, thereby achieving excellent mechanical properties of high strength and high water resistance. In addition, as important properties of adhesives, their viscosity and flowability determine whether cost-saving and improved application methods such as spraying and roller gluing can be carried out with these adhesives. According to Fig. 3i, the viscosity distribution range of all-cellulose colloidal adhesive was 11–15 mPa s, which is almost the lowest viscosity of all compared types of adhesives and is close to the viscosity of water. Compared to the drawbacks of traditional adhesives such as high viscosity and high solid content, our invented adhesive achieved a breakthrough by exhibiting both ultra-low viscosity and high solid content. This undoubtedly has huge advantages in actual industrial production, and apart from plywood, there are also significant advantages in the application of the adhesive to particleboards and fibreboards since its use not only improves the utilization rate of raw materials through spraying and gluing but the permeability of the adhesive and bonding interface is also significantly enhanced, thereby improving bonding strength and performance. Electrostatic interactions are also present between the bonding interfaces of plywood, which is one of the main reasons why the prepared plywood has excellent properties such as high strength and ultra-high water resistance. In the Zeta potential diagram (see Supplementary Fig. 8), the gel system exhibited a high positive potential in the range of 15–75 mV, for which reason the gel system was relatively stable, which is beneficial for long-term preservation to achieve a long shelf life. More importantly, the large number of hydroxyl groups on the surface of wood carry negative charges and can interact with the positively charged adhesive system through electrostatic interactions, making the bonding interface more secure. This also provides evidence for our bonding theory(see Supplementary Fig. 10).Here, we have conducted a detailed characterization and data analysis on the rheological properties of the adhesive. In its flow curve test (see Supplementary Fig. 36), the viscosity changes of the adhesive at different shear rates can be observed. From the graph, it is evident that as the shear rate increases, the viscosity of the adhesive gradually decreases. When the shear rate increases from 0 s−1 to 1000 s−1, there is a significant decrease in viscosity, demonstrating that the adhesives exhibit pronounced shear-thinning properties. The excellent shear-thinning characteristics enable the adhesive to be more uniformly and smoothly coated during gluing or spraying processes, effectively optimizing process parameters and improving production efficiency and product quality. Additionally, these outstanding shear-thinning properties allow for further applications of adhesives in various fields such as 3D printing, coatings, and films, highlighting that all of these characteristics are crucial material properties. Furthermore, in thixotropy tests (see Supplementary Fig. 37), the viscosity of the adhesives decreases with time or under stress, thus indicating their thixotropic properties. Good thixotropy ensures easy application and good fluidity during coating processes. However, when coating stops or application ceases temporarily, the viscosity of adhesive fluids increases to prevent sagging.Bonding performance of the prepared all-cellulose colloidal adhesiveA single-sided gluing strategy was followed, where three layers of beech logs were placed orthogonally and overlapped along the vertical direction of the texture. Then, they were hot-pressed and cured at 150 °C and 1.3 MPa for 5 min to obtain plywood (Fig. 4b), which was subsequently cut into test specimens according to the standard test requirements (see Supplementary Fig. 14). Four different types of plywood were prepared under identical experimental conditions with only one changing variable: pure cellulose adhesive plywood (PCA), partially pyrolyzed and then sulphated; pure cellulose adhesive plywood (UPCA), unpyrolysis directly sulphated; pure cellulose adhesive plywood (HPCA), partially pyrolyzed and then acidified with hydrochloric acid (HCl); all-cellulose colloidal adhesive plywood (PCA (15 days)), stored for 15 days. These different plywood samples were used to explore the impact of important influencing factors on their performance. The macroscopic bonding interface (Fig. 4b) of plywood was smooth and the adhesive lines were tight and uniform, which directly reflected the excellent adhesive performance of all-cellulose colloidal adhesive. The microstructure of the bonding interface (Fig. 4c) revealed the uniform and deep penetration of the adhesive between the bonding interfaces as an indication of its excellent penetration effect, resulting in a strong mechanical interlocking ability between the bonding interfaces and significantly enhancing the bonding performance (mechanical properties) of the plywood prepared with all-cellulose colloidal adhesive. In addition, the bonding effect between the interfaces was also excellent, with continuous and tight bonding without any cracking, which is also strong proof of the excellent bonding performance of all-cellulose colloidal adhesive.Fig. 4: Adhesive and water resistance properties of the prepared all-cellulose colloidal adhesive.a Schematic diagram of three-layer plywood composite; b Display of the prepared plywood and morphology of the bonding interface; c Microstructure of the bonding interface; d Stress-strain curve of dry bonding shear performance and wet bonding shear performance after exposure to boiling water; e bar chart of dry bonding shear performance and wet bonding shear performance after exposure to boiling water (The error bars in the figure represents the standard error); f Tests of adhesive shear strength and tensile failure specimens in boiling water; g Comparison of the resistance to boiling water between the prepared PCA and various popular adhesives on the market.We conducted bonding strength tests on standard samples of three-layer plywood and studied the bonding performance by analyzing their stress conditions. In the stress-strain curve of the shear performance of adhesives (Fig. 4d) and the shear strength bar graphs of adhesives (Fig. 4e), PCA exhibited a significantly higher dry bonding strength of 1.97 MPa compared to cellulose adhesive that was not partially thermally decomposed and directly sulphated (UPCA) (1.28 MPa). This indicated that the broom-like filamentous active fibers, formed by partial pyrolysis and depolymerization followed by acid hydrolysis in a sulfuric acid environment, can continue to undergo reactive crosslinking and condensation reactions under self-assembly, covalent-bonding, hydrogen-bonding, and electrostatic interactions, producing strong bonding forces(see Supplementary Fig. 16). In its tensile failure specimen, the wood damage rate was 100%, the specimen was completely destroyed from the internal structure of the wood itself, but the bonding interface maintained a stable and good bonding effect without being damaged, proving that all-cellulose colloidal adhesive had excellent bonding performance. The dry bonding strength of HPCA using another acidic substance, hydrochloric acid, was only 1.09 MPa, far lower than that of PCA. After exposure to the acidic environment for 15 days, PCA (15 days) had a dry bonding strength of 1.66 MPa (see Supplementary Fig. 11), indicating that all-cellulose colloidal adhesive still demonstrated a good performance after being subjected to the corrosive environment for a considerable time. The observed resistance can be attributed to the high positive Zeta potential formed in the adhesive system, which makes the system more stable and enables longer-term storage for industrial production and application.The wet bonding strength was tested by heating a test sample in water at 63 °C for 3 h. The stress-strain curve and wet bonding strength are shown in Supplementary Fig. 15. As shown in Supplementary Fig. 15, the wet bonding strength of PCA still reached 1.96 MPa after heating at 63 °C for 3 h and hardly changed compared to its dry bonding strength of 1.97 MPa. Furthermore, the tensile failure specimen (see Supplementary Fig. 14) exhibited a 100% wood slope, and the tensile failure of the specimen was entirely caused by the structural damage of the wood itself. The adhesive bonding interface was not damaged and still maintained a good bonding effect. These results indicated that heating at 63 °C will not cause damage to the bonding interface of PCA, and the bonding performance of all-cellulose colloidal adhesive will not be affected. As an explanation, the combined forces of electrostatic interaction, hydrogen bonding, and covalent bonding between the interfaces provide ultra-high-strength bonding ability. After being placed for 15 days, PCA (15 days) still exhibited an excellent wet bonding strength of 1.30 MPa, but that of HPCA was only 0.31 MPa, almost lacking any water resistance (see Supplementary Figs. 11 and 12). The wet bonding strength of UPCA was a relatively low 0.63 MPa, far from reaching the wet strength standard of 0.7 MPa in the GB/T 9846-2015 standard for plywood.The boiling water resistance (boiling-drying-boiling) performance is one of the most important properties of high-end, high-performance, and water-resistant adhesives and is crucial for the application scenarios, service life, and cost of adhesives. We conducted boiling-drying-boiling experiments and tested the performance of the final product to demonstrate the excellent water resistance of all-cellulose colloidal adhesive. Firstly, the prepared three-layer plywood sample was subjected to boiling water at 100 °C for 4 h. Then, the sample was cooled down in cold water, dried in a blower drying oven at 63 °C for 20 h, and finally placed again in boiling water at 100 °C for 4 h to obtain a boiled-dried-boiled test sample. According to the stress-strain curve (Fig. 4d) and bonding performance bar chart (Fig. 4e) of the treated sample, the bonding performance of all-cellulose colloidal adhesive still reached a maximum of 0.81 MPa, and its tensile failure sample displayed a wood fracture rate of about 95% (Fig. 4f), whereby almost all damage originated from the structure of the wood itself. The degree of damage to the bonding interface was very small, and the performance was excellent. At this point, the bonding performance of poplar wood with stronger universality was only 0.49 MPa because the weaker wood structure of this wood type was damaged after multiple rounds of boiling, drying, and boiling. In the early stage of tensile failure testing, the wood structure was destroyed and pulled apart, making it difficult to accurately detect the boiling water resistance of all-cellulose colloidal adhesive. Therefore, we chose beech wood with higher strength as the test wood to better demonstrate the performance of all-cellulose colloidal adhesive. In addition, no deformation or cracking occurred during the boiling-drying-boiling process (see Supplementary Fig. 33), which was also strong proof of the excellent performance of all-cellulose colloidal adhesive. However, when HPCA was heated in boiling water at 100 °C for 5 min, the interface already ruptured and separated; when UPCA was subjected to boiling water for 15 min, the bonding interface also ruptured and separated, indicating that both samples were not resistant to boiling water (Fig. 4e). In contrast, according to the comparison chart of the all-cellulose colloidal adhesive plywood samples before and after the harsh boiling-drying-boiling treatment (see Supplementary Figs. 33 and 34), the samples still showed a tightly bonded and good bonding interface, and the appearance and morphology did not indicate any deformation or cracking. This test was the most direct indicator of the ultra-high water resistance of all-cellulose colloidal adhesive. In addition, a demonstration with a large-size (50 cm × 50 cm) product was conducted for all-cellulose colloidal adhesive (see Supplementary Fig. 38), proving that this colloidal adhesive has the potential for further large-scale industrial production and application. Furthermore, the wide range of raw material sources and the simple preparation method of all-cellulose colloidal adhesive also enable it to quickly enter large-scale industrial production and application, achieving effective utilization of renewable, low-cost, and degradable biomass cellulose resources.Characteristics and morphology of the cured all-cellulose colloidal adhesive filmTo better demonstrate and explain the curing and bonding mechanism of the newly prepared pure natural biomass adhesive, we conducted separate curing and film-forming experiments on all-cellulose colloidal adhesive solution and studied its chemical composition and the state and changes in the material structure during the curing and film-forming process. First, the changes in the crystalline structure of the adhesive before and after the curing reaction were analyzed by XRD (Fig. 5e). After the film-forming and curing reactions of the adhesive, the degree of crystallinity decreased significantly, approaching zero, from 52.89% to 3.50%, indicating excellent curing performance of the adhesive and demonstrating the smooth progress of the curing reaction and the generation of a more stable structure after curing. In the FT-IR spectra (Fig. 5f), the peak at 3420 cm−1 originated from the original active hydroxyl groups, which were continuously consumed and reduced as they provided the active groups and active sites for the smooth progress of the curing reaction. At the same time, hydrogen bonding interactions occurred to stabilize the cured structure. The stretching vibration peak of C=O in the ester group at 1740 cm−1 weakened, and together with the stretching vibration peak of O–H and C–H bonds at 2915 cm−1, the trend of the FT-IR peaks indicated the participation of this group in the curing reactions to generate a more stable covalent ether bond in the crosslinked network structure. In the 1H NMR spectrum (Fig. 5g), the chemical shift at 6.75 ppm also confirmed the formation of relatively stable ether bond structures at the primary and secondary hydroxyl groups at positions C6, C2, and C3 in the original structure of the cured cellulose molecules.Fig. 5: Characteristics and morphology of cured all-cellulose colloidal adhesive film.Macroscopic image (a) and microscopic image (b) of the adhesive before curing; Macroscopic image (c) and microscopic image (d) after curing into a film; XRD patterns (e), FT-IR spectra (f), and 1H NMR spectra (g) before and after curing; h Free energy morphology obtained by molecular dynamics simulation; i Spiral radius diagram of the molecular dynamics simulation; j Changes in the number of hydrogen bonds based on molecular dynamics simulations; k Schematic diagram of the curing reaction structure.The dynamic thermal performance analysis of the prepared adhesive after curing was conducted by DSC. In the DSC spectra (see Supplementary Fig. 19) of the adhesive, the peak width of the cured adhesive was significantly broader after curing. The wider peak width in DSC indicates a longer melting range, which is caused by the formation of a higher molecular weight supramolecular structure during the curing process, making it more stable. At the same time, a wider DSC peak width can also indicate a higher degree of material crystallization and a wider distribution of crystal chip thickness as further reasons for its excellent thermal stability. In addition, the differential thermogravimetry spectrum (see Supplementary Fig. 17) also revealed the excellent thermal stability of the adhesive after curing. Furthermore, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were employed to study the microstructure and morphology of the prepared adhesive before and after curing, and the results are shown in Fig. 5b, d. In the microstructure image of the gel state, cellulose presented a broom-like filamentous fiber morphology after treatment. This filamentous fiber had ultra-high polymerization and self-assembly properties and could undergo self-assembly under certain conditions to form a crosslinked network with a stable structure. In the further solidification process, a large number of broom-like filamentous fibers continuously formed a network-like structure by spiraling and crosslinking with each other, ultimately forming a stable and high-strength three-dimensional crosslinked network structure, providing the prepared materials with excellent physical and chemical properties. In addition, we conducted molecular dynamics calculations for this curing reaction system to elucidate the curing reaction process and the stability of the cured structure. The free energy morphology and radius of gyration of the simulation results are shown in Fig. 5h, i, respectively, and can provide evidence for the stability and dense compactness of the cured structure (For other molecular dynamics simulation results see Supplementary Figs. 20–28). The change in the number of hydrogen bonds during the simulated curing reaction process is shown in Fig. 5j. A large number of hydrogen bond interactions are generated in the curing structure formed in a dilute sulfuric acid environment, resulting in an increase in the total number of hydrogen bonds in the curing structure system. We also compared the all-cellulose colloidal adhesive with mainstream wood adhesives currently available on the market (Fig. 4g), including the most popular traditional chemical raw material adhesives (urea-formaldehyde resin adhesive, phenolic resin adhesive, melamine-formaldehyde resin adhesive) and lignin adhesive as an emerging biomass-based adhesive. The comparison revealed that the dry bonding strength and 63 °C hot water wet bonding strength of all-cellulose colloidal adhesive are comparable to mainstream wood adhesives and even exceed those of lignin adhesives. Its boiling water resistance after the boiling-drying-boiling cycle is superior to that of urea-formaldehyde resin adhesives and lignin adhesives and slightly lower than that of melamine-formaldehyde resin adhesives. However, all-cellulose colloidal adhesive can be produced by a much simpler preparation method and has a lower viscosity than other adhesive products, which renders this product more attractive and competitive in the market.
