Synthesis and characterization of SPI-lignin and BN nanosheets. Fig. S1 shows the synthesis of SPI-lignin and an illustration of the three-dimensional (3D) chain microstructure of SPI-lignin. As shown in Fig. 2a, in SPI-lignin, the peaks at 1663 cm-1, 1445 cm-1, and 1251 cm-1 were assigned to amide I (C = O stretching), amide II (N-H bending) and amide III (C-N, N-H stretching) vibrational modes, respectively23,29. In addition, intensity of the characteristic peak of C = N significantly increased due to the formation of strong imine bonds from the reaction of amine and aldehyde groups30. The band at 3296 cm-1 in Fig. 2a corresponding to the stretching vibration of O-H/N-H groups of SPI-lignin sharpened compared to that of unmodified lignin, presumably due to SPI breaking the inter-/intra-molecular interactions of lignin chains, forming a homogeneous structure within the matrix31. Additionally, two sharp peaks at 2943 cm-1 and 2859 cm-1 appeared and were assigned to the -CH2 and -CH3 stretching of the long aliphatic carbon chain. As illustrated in Fig. S1, the crosslinked polymer networks were constructed via strong imine bonds. All of these observations revealed that SPI was successfully grafted onto the lignin skeleton.
Sonication-assisted liquid-phase exfoliation was used to prepare few-layer BN nanosheets in this study. Commercially available hexagonal boron nitride (h-BN) was used as the source of BN nanosheets and deionized water was used as solvent for both dispersion and exfoliation. After centrifuging and collection, BN nanosheets were found capable of forming a stable aqueous suspension that can last for over months. Fourier transform infrared (FTIR) spectra and thermogravimetric analysis (TGA) of BN and BN-OH convinced the successful graft of OH group onto the BN surface (Fig. S2a, b, Supporting Information). The morphology of the asobtained BN nanosheets was measured by scanning electron microscope (SEM, Fig. S2c, Supporting Information) and transmission electron microscope (TEM). As shown in Fig. 2b, BN nanosheets with lateral size above 500 nm were slightly transparent to the electron beam, suggesting their ultrathin feature. The electron diffraction pattern of the exfoliated BN nanosheets displayed typical sixfold symmetry of h-BN (inset of Fig. 2b), indicating that the exfoliated BN nanosheets retained the structural integrity of h-BN. Atomic force microscopy (AFM) revealed that the thickness of the exfoliated BN nanosheets (Fig. 2c) was around 2.79 nm (based on the corresponding height marks, Fig. 2d). The statistical distribution of thickness and length through counting over 100 pieces of BN nanosheets indicated that the exfoliated BN nanosheets were mainly 1–3 nm in thickness (Fig. 2e) and 100–700 nm in length (Fig. 2f).
Multiple hydrogen bonds and covalent bonds crosslinking networks of PVA-BN-QDs-SPI-lignin. A series of analogous PVA-based samples were prepared for reference and FTIR spectroscopy was used to determine the hydrogen bond interactions between PVA, BN, QDs, and SPI-lignin. As shown in Fig. 3a, incorporation of SPI-lignin into PVA, BN, and QDs yielded a significant blue-shift of the O-H group, in which the frequency of the O-H band increased from 3328 cm-1 (PVA-BN-QDs) to 3350 cm-1 (PVA-BN-QDs-SPI-lignin)32. Similarly, formation of hydrogen bonds among PVA, BN, QDs, and SPI-lignin33,34 was also implied by the change in UV absorption spectroscopy, evidented by the peak at 239 nm (PVA-BN-QDs, Fig. 3b) red shifting to 244 nm (PVA-BN-QDs-SPI-lignin). Combination of the blue-shift in FTIR spectra and red-shift in UV spectra validated the hydrogen bonding interaction among PVA, BN, QDs, and SPI-lignin (Fig. 3a, b). In addition, the C-OH group of PVA-BN-QDs-SPI-lignin showed a blue-shift similar to the O-H group (Fig. 3a)6,35,36. The stretching vibrations of C = O (1715cm-1), asymmetric and symmetric stretching vibrations of C-O-C (1305 cm-1 and 1235 cm-1) in the carboxylate groups were also detected27. These hydrophilic groups not only provided an insight into the luminescence mechanisms, but also promoted the environmental friendliness of the products, enabling their potential applications in biochemical investigations27.
Intermolecular interactions between PVA, BN, QDs, and SPI-lignin were further evaluated by the rheological tests. The storage modulus (Fig. 3c) and viscosity curves (Fig. 3d) of PVA-BN-QDs-SPI-lignin were significantly higher than those of PVA-BN-QDs and PVA-BN-SPI-lignin. As shown in Fig. S3, the as-prepared QDs were well dispersed in narrow distributions of 2–4 nm diameter. Therefore, QDs nanoparticles can form hydrogen bonding interaction among PVA, BN, and SPI-lignin through the pendant hydroxyl end-groups of the QDs27. All of these observations revealed that PVA, BN, QDs, and SPI-lignin formed a physical cross-linked network19 via strong hydrogen bonding interactions (Fig. 3e).
Mechanical properties and enhancement mechanism. Attributing to the synergistic self-association hydrogen bonding effect37, PVA-BN-QDs-SPI-lignin showed significantly improved mechanical performances, exhibiting a tensile strength of 53.3 ± 3.0 MPa and toughness of 182.8 MJ·m-3, which was 2.1 and 8.6 times that of its anology PVA-BN-SPI-lignin, respectively (Fig. 4a-c; Fig. S4, Table S1, Supporting Information). The best mechanical performance obtained in PVA-BN-QDs-SPI-lignin coincided with the most intensive hydrogen bonding based physical cross-linking network, as disclosed by the rheological tests19. The short-chain network of QDs and the rigid aromatic and 3D molecular structure of SPI-lignin significantly strengthened and toughened thefinal product. BN nanosheets, again, as an alternative to β-sheets (in spider silk), greatly improved the deformability and energy dissipation capacity of the interconnection network17, and uniformized the stress of the multi-scale network38. Multiple hydrogen bond interactions and hard segment crosslinking, along with the physical entanglement bewteen the SPI moitey and PVA39 remarkably assisted to improve the mechanical strength40.
In order to further clarify the fluidity and viscoelastic behaviors of PVA-BN-QDs-SPI-lignin, the temperature dependence of storage modulus (G′) was tested (Fig. 4d) by dynamic mechanical analysis (DMA). Presence of more intensive multi-strength hydrogen bonding crosslinking40,54 endowed PVA-BN-QDs-SPI-lignin the highest G′ among these tested nanocomposites. Above about 50°C, G′ appeared to decrease more rapidly, indicating that the soft segment motion began40. When the temperature was higher than 70°C, G′ of all nanocomposites dropped sharply, implying that the strong hydrogen bonds in these samples began to dissociate. The nanocomposites softened and their shapes could not be maintained due to the dissociation of most of the intermolecular hydrogen bonds55 at around 160°C. Comparison of the mechanical properties between PVA-BN-QDs-SPI-lignin and commonly used plastic materials and other bio-based materials (Fig. 4e, f; Table S2, 3, Supporting Information) showed that PVA-BN-QDs-SPI-lignin had a superior stretchability and its mechanical properties were sufficient for the requirments of protective materials19. Meanwhile, as shown in Fig. 4g, PVA-BN-QDs-SPI-lignin presented very good flexibility and can withstand large deformations, such as twisting, bending, knotting and stretching, furthermore, it can smoothly bear at least a weight of 8 kg without any fracture. Moreover, our assembly method is effective and scalable, easily achiving massive production of all-natural bioinspired structural material (Fig. S5a, Supporting Information). Therefore, the high mechanical stability as well as desirable processing will allow to prepare various external packing products for daily use using PVA-BN-QDs-SPI-lignin (Fig. S5b, Supporting Information).
XPS was employed to examine the molecular bonding properties of the PVA-BN-QDs-SPI-lignin nanocomposites in order to study the enhancement mechanism of mechanical properties. As shown in Fig. 5a-c, the C 1s spectrum of PVA-BN-QDs-SPI-lignin can be fitted with three peaks at 284.8, 286.2, and 287.8 eV, corresponding to the C-C bonds, C = N/C-O-C bonds, and B-O-C bonds components in the polymer chains, respectively56. These data fully prove the etherification and crosslinking reaction between BN-OH and PVA. No C = O bond was found in PVA-BN-QDs-SPI-lignin, indicating that a large number of hydrogen bonds formed through self-association of oxygen-containing functional groups57, which agreed with the results in FTIR (Fig. 3a). Increase of intensity of the C = N bond also suggested that a strong imine bond was formed in the system30.
The orientation of PVA and BN were further studied by XRD, in which the diffraction peaks at 19.7° and 22.7° were attributed to the (101) and (200) of the PVA crystal, respectively (Fig. 5d)19. Previous studies have shown that the peak intensity ratio of (002) and (100) could reflect the orientation of BN in polymer matrix35. The ratio of I002/I100 in PVA-BN-SPI-lignin was only 2.2, which rose sharply to 60.4 after adding QDs, suggesting that BN had a higher in-plane orientation under the action of multiple hydrogen bonds12. In addition, PVA-BN-QDs, PVA-BN-SPI-lignin, and PVA-BN-QDs-SPI-lignin all exhibited the same diffraction patterns, indicating that the incorporation of BN, QDs, and SPI-lignin in a certain amount did not obviously disrupt the crystal cell structure of PVA. However, the diffraction intensity (200) of the PVA crystals in PVA-BN-QDs-SPI-lignin was stronger than that of PVA-BN-QDs and PVA-BN-SPI-lignin, offering evidence that the molecule could confine the amorphous PVA chain by dynamically sacrificing hydrogen bonds (Fig. 5d)58, which could facilitate the orientation of PVA chain segments59, thus causing reinforced strength and hardness. As shown in Fig. 5f, the cross-section of PVA-BN-QDs-SPI-lignin was highly ordered and showed strong anisotropic orientation in the parallel and vertical directions.
Small-angle X-ray scattering (SAXS) analysis was implemented to further explore the deformation mechanism in the PVA-BN-QDs-SPI-lignin. As shown in Fig. 5e. the structure parameters could be obtained from the analysis of the 1D correlation function curves, including the long period length (L), the transition layer length (dtr), and the length of the crystalline layer plus transition layer (lc)31. The core-crystalline layer length (d0 = lc-dtr) and the amorphous layer length (la = L-lc) were then calculated accordingly. Increase of dtr, lc, d0 and long period length L for PVA-BN-QDs-SPI-lignin validated the formation of thicker crystals, which was consistent with the increase in the diffraction intensity of the PVA crystals, as revealed by XRD analysis in Fig. 5d. Arc-like scattering features on the 2D pattern of SAXS were presented in Fig. S6. With the increase of hydrogen bond interactions, the corresponding sharp scattering peak decreased gradually, implying that the QDs and SPI-Lignin molecules could constrain the amorphous PVA chains via dynamic sacrificial H-bonds, facilitating the chain extension and alignment upon stretching.
Fatigue-resistant and self-healing properties. In view of the outstanding ductility and toughness, a series of rigorous tests were conducted to further examine the folding endurance of PVA-BN-QDs-SPI-lignin. Firstly, the influence of cyclic folding-unfolding on its mechanical properties (Fig. 6a, b; Fig. S7a, Table S4, Supporting Information) showed that the tensile strength and toughness can still preserve after 1000 folds. The tensile strength ratio before and after folding (< 1000 times) was kept 0.92. Nanocomposites in multiple previous studies could only endure bending or limited folding cycles (100 times at most) with a gradual decrease of mechanical properties6,33,56. PVA-BN-QDs-SPI-lignin showed excellent folding failure resistance and can amazingly bear up to 900 000 folds. In addition, file bag, a commonly used polypropylene (PP) material in market, can tolerate 58 000 times of folding, in contrast, our material in this study can suffer as high as 390 000 times. These results exposed that the super tough multi-hydrogen bonding network based on PVA-BN-QDs-SPI-lignin nanocomposites had unprecedented bending resistance. To the best of our knowledge, it is the first time to explore an excellent folding fatigue-resistant film, and current work is believed to enormously expand the scope of its applications.
The fracture behavior of PVA-BN-QDs-SPI-lignin was investigated through SEM of propagated crack and fracture surface (Fig. 5g). Benefiting from its outstanding ductility and toughness, a stable crack was easily arrested. According to the fracture behavior, a multi-scale continuous network was proposed to delay the fatigue fracture, which linked its unique microstructure with macroscopic tensile properties. PVA-BN-QDs-SPI-lignin is tough and self-healing, consisting of reversible hydrogen bonds and imine bonds (1-nm scale, Fig. 3e), a cross-linked polymer network (10-nm scale, Fig. 3e) and bicontinuous hard/soft phase (PVA/BN) network (100-nm scale, Fig. 3e)60. During the stretching process, higher energy was required to break the highly ordered nanostructures than the amorphous polymer chains59. Due to the synergistic effect between different scales, PVA-BN-QDs-SPI-lignin exhibited high fatigue resistance61. This hierarchical structure-based anti-fatigue mechanism not only provides the understanding for the anti-fatigue behavior of biological tissues with complex hierarchical structures, but also offers a design strategy for toughness and fatigue-resistant materials that use non-covalent bonds as components to form a multi-scale network structure60.
In addition, the self-healing properties of PVA-BN-QDs-SPI-lignin nanocomposites were also studied. Because of the dynamic nature of hydrogen bonds, PVA-BN-QDs-SPI-lignin nanocomposites can be conveniently welded with the assistance of water. Briefly, a 1 cm notch was drawn in the PVA-BN-QDs-SPI-lignin strip with a knife, and then moistened with deionized water (Fig. 6g). The film was self-healed within only 2 minutes and its mechanical properties did not significantly alter after self-healing. In contrast to the dynamically reversible non-covalent bonds or covalent bonds that were largely present in other self-healable materials62,63, PVA-BN-QDs-SPI-lignin possesses a large number of dynamic reversible hydrogen bonds (Fig. 6h), and the broken hydrogen bonds could also be theoretically self-healed indefinitely.
Compared to various types of other self-healable materials (Fig. 6d; Table S5, Supporting Information)54,64−77, PVA-BN-QDs-SPI-lignin displayed exceptional mechanical strengths (tensile strength and elongation at break) and a superior self-healing accomplishment in terms of overall healing performance and healing efficiency (Table S5, Supporting Information). The fluidity and mechanical properties of polymers were a pair of contradictory78, that was, self-healing properties and mechanical properties were usually contradictory79. The reported self-healing materials had excellent mechanical properties80–82, but relatively declined self-healing ability. Furthermore, the mechanical properties of self-healing materials initiated by water will be significantly reduced in highly humid environments79, however, for practical applications, PVA-BN-QDs-SPI-lignin is required to maintain high mechanical strength under these conditions. Pleasly, PVA-BN-QDs-SPI-lignin also retained an excellent flexibility even after storing at RH 80% for 30 days, with a high tensile strength of 43.7 MPa and tensile strain of 616.6% (Fig. 6a, b; Fig. S7b, Supporting Information). These values were higher than that of traditional PE plastics, which usually exhibited breaking strengths ranging from 15 to 30 MPa. In current study, all of these observations reveal that PVA-BN-QDs-SPI-lignin demonstrated an outstanding combination of high mechanical strength and self-healing properties, as well as excellent oil resistance (Fig. S8, Supporting Information).
The stress-to-failure and toughness of PVA-BN-QDs-SPI-lignin (fatigue-resistant and self-healing) are carefully compared with previous high-performance nanocomposites inspired by nature (Fig. 6e). Detailed compositions and tensile data of the reference natural bioinspired materials are listed in Table S6 (Supporting Information). It is clear that PVA-BN-QDs-SPI-lignin sets a new record of stress-to-failure and toughness, which were 10.6 to 45.7 times higher than other graphene-based nanocomposites. Moreover, toughness of the highly ductile PVA-BN-QDs-SPI-lignin stands out clearly from all platelet/polymer, platelet/nanofiber, and platelet/3D nanofiber network nanocomposites (5.3 to 140.6 times)18,46, 48–50,53, 83–91. In addition, after 1000 folds and self-healing, the tensile strength of PVA-BN-QDs-SPI-lignin was still much higher than that of typical plastics (Fig. 6f; Table S7, Supporting Information), illustrating its advanced application as durable advanced material. This natural bioinspired material is light weighted, strong, and tough, providing sufficient mechanical properties as a plastic alternative. Of particular interest is its facile, effective and scalable preparation method, which could realize massive production to replace plastics in practical applications, such as flexible wearable materials.
Ultralow temperature toughness and highly humid environments tolerance. Ultralow temperature threatens the practical applications of commercial materials, as they barely survive from the cryogenic shock. Previous studies have disclosed that the spider silk exhibits excellent ultralow temperature toughness, showing ductile failure even at the temperature of liquid nitrogen (-196°C)92. Likewise, PVA-BN-QDs-SPI-lignin retains the structural hierarchy, encouraging us to further evaluate its mechanical performance at ultralow temperature (-196°C). As indicated in Fig. 7f, the folded PVA-BN-QDs-SPI-lignin nanocomposite strip (10 mm × 80 mm) was immersed in liquid nitrogen and then stretched to uncoil the folding structures. To study the ultralow temperature toughness of the PVA-BN-QDs-SPI-lignin, the tensile stress-strain test was carried out at -196°C (maintained in liquid nitrogen, Fig. 7a-d; Fig. S9a-c, Supporting Information). The result implies that PVA-BN-QDs-SPI-lignin exhibits excellent ultralow temperature toughness (107.3 MJ·m-3, pure PVA is 22.0 MJ·m-3).
In addition, we compared the toughness at different low temperatures and found that QDs played an advanced role in ultralow temperature toughness than SPI-Lignin (Fig. 7c). According to previous reports22,93−95, we surmise that the dynamic properties of multiple H-bonds endowed this material with excellent ultralow temperature toughness. To test this hypothesis, we prepared two hyperbranched polyamide (HBPA) that can form multiple hydrogen bond interactions with the system and QDs-CHO polymers that can form covalent crosslinks. Hydroxyl-containing hyperbranched polysiloxane (PVA-BN-HBPA-1) as well as hydroxyl and primary amine-containing hyperbranched polysiloxane (PVA-BN-HBPA-2) exhibited excellent ultralow temperature toughness (Fig. 7e; Fig. S9d, e, Supporting Information), owing to multiple hydrogen bond interactions in the system. On the contrary, PVA-BN-QDs-CHO exhibits low ultralow temperature toughness (Fig. 7e) because of formation of only covalently crosslinked network in the system.
To further understand this fascinating supercold-tolerant phenomenon, we employed variable-temperature XRD and DSC to observe the crystallization behavior and phase transition of the dry films in wide low-temperature range. In the variable-temperature XRD, the samples all exhibited the same diffraction patterns (Fig. 7g), indicating that low temperature did not significantly damage the unit cell structure of PVA. However, when the temperature lowered below − 110°C, the diffraction intensity of PVA crystals in the material was slightly stronger, indicating that the degree of crystallization increased. In addition, a typical phase transition peak was found on the DSC curve (Fig. 7h), confirming that a significant phase transition occurred during the cooling process. Compared to PVA-BN-QDs, the higher amount of H-bonds network of PVA-BN-SPI-Lignin limited the self-recombination of PVA molecules96, weakened the interaction between molecules, and led to the decrease of crystallization temperature97, which further validated our assumption.
Multifunctional properties of PVA-BN-QDs-SPI-lignin. The thermal behavior of materials is critical to a variety of applications, especially for high or variable temperatures in service conditions. Due to the inferior thermal behaviors (e.g., poor thermal stability), application of plastics is often limited18. As show in Fig. 8a, the initial decomposition temperature (referred to as the temperature corresponding to 5% weight loss) of PVA-BN-QDs-SPI-lignin under nitrogen was around 251.5°C, which was were thermally stable at temperatures below ≈ 251.5°C (Fig. 8a, black and purple traces). Meanwhile, the PVA-BN-QDs-SPI-lignin exhibited a water content of ≈ 2.0 wt%, which was determined by the mass loss below ≈ 120°C in the TGA curve. And the residual amount of PVA-BN-QDs-SPI-lignin is obviously higher than that of PVA-BN-QDs and PVA-BN- SPI-lignin.
Pure PVA film had a higher light transmittance in the wavelength range of the ultraviolet spectrum, showing no UV absorption property. With the addition of BN, QDs, and SPI-lignin, the light transmittance decreased successively and the resulting film simultaneously exhibited excellent UV blocking performances, reaching a UV blocking rate up to 99.8% (Fig. 8b, green trace), benefiting from the phenylpropane structure, phenolic hydroxyl group and unsaturated C = O group in the SPI-lignin molecule which had strong UV absorption capacity19.
Materials with excellent fluorescent properties have broad application prospects in packaging anti-counterfeiting and light-emitting devices30. As shown in Fig. 8c, both PVA-BN-QDs and PVA-BN-QDs-SPI-lignin showed excellent fluorescence characteristics, mainly associated with the abundant surface traps and functional groups that endowed QDs with bright, stable luminescence and excellent water dispersion31, which further coincided the fact that PVA-BN-SPI-lignin (absence of QDs) showed no fluorescence characteristic. These two nanocomposites are promising in applications of biomarkers, biological imaging and biosensors due to their low cost, green, and easy labeling, etc30.
The degradability of the PVA-BN-QDs-SPI-lignin was examined by recording the mass changes as a function of time under soil. As shown in Fig. 8d, the weight of the PVA-BN-QDs-SPI-lignin continuously decreased as prolonged burying time and it completely degraded after ≈ 80 days. When buring under soil, PVA-BN-QDs-SPI-lignin gradually absorbed water from the soil, which partially broke the hydrogen bonds and coordination interactions within the plastic and led to its swelling. Furthermore, the microorganisms in the soil adhered to PVA-BN-QDs-SPI-lignin surfaces and secreted enzymes, such as dehydrogenase, oxidase, hydrolase, and aldolase also promoted material degradation.