Development of a multifunctional nanocomposite film with record-high ultralow temperature toughness and unprecedented fatigue-resistance

doi:10.21203/rs.3.rs-841906/v1

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Development of a multifunctional nanocomposite film with record-high ultralow temperature toughness and unprecedented fatigue-resistance

https://doi.org/10.21203/rs.3.rs-841906/v1

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In the quest of materials that can tolerate extreme environments (i.e., aerospace, polar regions of earth), facile design of self-healing, high fatigue-resistant and multifunctional nanocomposite materials with excellent ultralow temperature toughness, especially by utilizing inexpensive and sustainable bioresources is still currently challengeable. In current study, we present a material that displays remarkable ultralow temperature toughness, shows excellent toughness (107.3 MJ·m^-3) at − 196°C and maintains high mechanical strength in highly humid environments. This material is a spider silk-inspired, poly(vinyl alcohol) (PVA)-based, autonomous room temperature self-healable nanocomposite by complexation of boron nitride (BN), quantum dots (QDs) and soybean protein isolate grafted lignin (SPI-lignin). The fabricated material, namely PVA-BN-QDs-SPI-lignin, simultaneously exhibits outstanding tensile strength (53.3 MPa), toughness (182.8 MJ·m^-3), fatigue-resistance as well as antiultraviolet and fluorescent properties and sets an impressive new record of folding-failure (900 000 times) and toughness, which are 10.6 to 45.7 times higher than other graphene-based nanocomposites. It can be impressively self-healed within only 2 minutes. Of particular interest is its facile, green, mild and inexpensive preparation method that can be easily scale up. It is believed that this work, beginning with abundant biodegradable resources, opens the door to develop biobased multifunctional materials in practical applications, such as flexible wearable materials.

Nanoscience

Atomic and Molecular Physics

PVA-BN-QDs-SPI-lignin film

nanocomposite materials

ultralow temperature toughness

fatigue-resistant

self-healing

Burgeoning progress of space technologies and booming of human’s activities in extreme environments (aerospace, South Pole, etc) stimulated the development of materials that can maintain their functionalities meanwhile tolerate ultralow temperatures^1,2. In particular, design of multifunctional materials that display a combination of degradability, flexibility, self-healability, sustainability and toughness at ultralow temperature is even more difficult and critical.

Poly(vinyl alcohol) (PVA), a widely used degradable and nontoxic material^3,4 that can be fully degraded into CO₂and H₂O by microorganisms in natural environments⁵, possesses a large number of hydroxyl units capable of forming robust intramolecular and intermolecular hydrogen bonds (H-bonds)^6,7. Despite various advances in toughening PVA-based materials by forming network⁸, adding nanofillers^9,10, and mechanical training¹¹ in the past decade, their mechanical performances are still less than satisfactory. On the other hand, there are only limited studies reporting PVA-based materials tested under sub-zero temperatures¹². With the gradual freezing of conventional PVA-based materials below ice point, these materials tend to lose their flexibility and stretchability towards low temperatures (such as -30 °C) and even fewer reliable approaches have been proposed to solve the poor stretchability of PVA-based materials under these circumstances^13,14. However, extreme and complex environments in practice like labeling and transfering foods and biosamples at cryogenic temperature pose great challenges to current materials, especially for the supercold tolerance¹⁵.Therefore, it is of great importance but still chanllengable to explore robust and toughness materials that can tolerate supercold conditions.

Natural creations, such as nacre, bone, lobster cuticle, muscles, and spider silk^16,17 present outstanding mechanical properties with a high-level combination of stiffness and toughness¹⁸.Most of them are composites with orderly hierarchical architectures that arrest crack propagation to prevent catastrophic failure¹⁹. These natural materials provide endless inspiration for the fabrication of novel bioinspired structural materials²⁰. One typical example is spider silk²¹, which exhibits a combination of high mechanical strength and high toughness at ultralow temperature. Unlike the mineralized natural materials (biological bone), both the structural unit (β-sheets nanocrystals) and “soft part” (semiamorphous matrix) of spider silk consist of protein, highlighting a key role of orderly hierarchical architecture (β-sheets nanocrystals and H-bonds)¹⁶. Recently, many other fascinating properties of spider silks were gradually disclosed, such as vibration propagation, rubber-like elasticity, shape memory, and water collection²². However, the ultralow temperature performance of spider silks was barely concerned probably. Low temperature is also a great threatening of the practical applications of commercial materials, under which most of them are unable to maintain excellent mechanical properties from the cryogenic shock. These unique properties of spider silks may provide a great opportunity to develop new materials with supercold tolerance.

It is highly desirable to use easily available, renewable and abundant biomass, as the H-bond cross-linked polymer materials to imitate the advanced orderly hierarchical architecture design¹⁷. Soy protein isolate (SPI) is an inexpensive and biodegradable material with good film-forming property²³. Lignin, one of the main recalcitrant substances in pulping black liquor, is the most abundant aromatic polymers in nature²⁴. SPI grafted lignin (SPI-lignin) has a 3D complex molecular structure with abundant hydroxyl, amino, and sulfonic acid groups, which can form high density of H-bonds with PVA thus is envisioned as a promising external H-bond crosslinker²⁵. Glucose-based fluorescent carbon nanoparticles contain plenty of oxygen-containing groups^26,27, which can form various H-bond interactions with PVA. The multiple sacrificial H-bonds can dissipate energy on molecular scale through dynamic rupture and reform upon stretching, accompanied with the deformation of granules, thus endowing PVA-based materials with super strong, tough, and multi-functional properties²⁸. Among numerous two-dimensional (2D) structural materials, application of boron nitride (BN) nanosheets in material design are surprisingly underestimated. The unique surface chemistry of BN can potentially provide new soft and hard boundaries, which can improve the mechanical strength of resulting composite material. The new soft boundary refers to hydrogen bond between BN and PVA and the hard boundary refer to the BN nanosheets. In addition, as an alternative of β-sheet nanocrystals, BN can form interlocked regions and transfer the load between partially extended and oriented macromolecular chains under loading¹⁷.

In this work, inspired by natural spider silk, super-tough PVA nanocomposite films with exceptional mechanical strength and toughness, fatigue-resistance, thermal stability, excellent fluorescence, self-healing, mand excellent ultralow temperature toughness properties were prepared via a facile strategy using SPI-lignin and quantum dots (QDs, external H-bond cross-linkers). QDs contain numerous oxygen-containing groups that are readily available to form H-bond interactions with PVA. BN nanosheet was used as a substitute for β-sheet to improve mechanical properties. Simultaneously, self-association of multiple H-bonds cross-linking and external H-bonding cross-linkers were used to design multiple H-bonds cross-linked polymers (Fig. 1). From the perspective of material fabrication and industrial applications, this work offers a facile method by using renewable and abundant biopolymers, and simply using water as the medium to prepare biodegradable and strong polymeric materials for a series of promising applications. To the best of our knowledge, this degradable material presents excellent ultralow temperature toughness and high folding fatigue-resistance, surpassing the performances of most of the reported degradable materials.

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, respectively^23,29. 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-lignin^33,34 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 crosslinking^40,54 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 (d_tr), and the length of the crystalline layer plus transition layer (l_c)³¹. The core-crystalline layer length (d₀ = l_c-d_tr) and the amorphous layer length (l_a = L-l_c) were then calculated accordingly. Increase of d_tr, l_c, 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 properties^6,33,56. 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 materials^62,63, 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 properties^80–82, 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 reports^22,93−95, 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.

A facile biomimetic strategy was implemented to prepare a nanocomposite material consisting of poly(vinyl alcohol), boron nitride (BN), qunatum dots (QDs) and lignin grafted soy protein isolate (namely PVA-BN-QDs-SPI-lignin). The fabricated material presented high strength, toughness, fold endurance, self-healing, excellent fluorescence, thermal properties, and UV-blocking performance. Inspired by the architecture of natural spider silk, BN nanosheets were used as a substitute for the β-sheet to strengthen this nanocomposites material. Through a nano-microphase separation and intermolecular sacrificial hydrogen-bond interactions, PVA and BN were highly orientated in the nanocomposite matrix, affording nanocomposites similar to natural spider silk, with high strength, high toughness and folding resistance. In addition, PVA, BN, lignin, SPI and QDs are all green materials, and the resulting nanocomposites exhibited promising applications in biomarkers, biological imaging and biosensors due to their low cost, green, and easy labeling, etc.

The detailed experimental section can be found in the Supporting Information.

Acknowledgements

The research was supported by the National Natural Science Foundation of China (Grant No. 31901253), Natural Science Research Project Foundation of Colleges and Universities of Jiangsu Province of China (No. 19KJB220009), Natural Science Foundation of Jiangsu Province (No. BK20200795), and Postgraduate Research &Practice Innovation Program of Jiangsu Province. S. J. and W. Q. contributed equally to this work. J. L. and Z. F. conceived the idea and designed the experiments. J. L. supervised the project. S. J. and W. Q. carried out the synthetic experiment and analysis. S. J., W. Q., S. S., Z. F., and J. L. wrote the paper, and all authors discussed the results and commented on the manuscript.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary information The online version contains supplementary material.

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Development of a multifunctional nanocomposite film with record-high ultralow temperature toughness and unprecedented fatigue-resistance

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