Abstract
The mechanical properties of polyurethane ionogels prepared by UV light-curing are usually inferior to those of conventional polyurethanes. Highly entangled polymer chain networks with chemical crosslinking can potentially address this problem. Here, we prepare ionogels (PU-HRs) using UV curing technology with esterified rutin as a cross-linking agent. After optimization of the preparation process by response surface methodology, we obtain PU-HRs with a tensile strength of 34.96 MPa and toughness as high as 88.11 MJ m−3 (1.26-fold higher than that of silk from the silkworm, Bombyx mori (70 MJ m−3)). The high strength and toughness of PU-HR are mainly attributed to the three-dimensional cross-linked network structure formed by the “rigid-flexible” esterified rutin, the micro-phase separation structure between the soft-chain fragments, and the hard-chain fragments that form stable interfacial regions. These ionogels have great prospects in sunscreen coating applications, such as for sunscreen umbrellas and automotive or architectural sunscreen glass.
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Introduction
Polyurethane ionogels formed by ultraviolet (UV) light-curing would be ideal UV-resistant coatings in many applications. Compared with other curing methods, UV light-curing has the advantages of fast curing rate, low curing temperature, low emission of volatile organic compounds (VOCs), low energy consumption, and high production efficiency1,2. During UV-curing, decomposition of the initiator generates free radicals, which in turn triggers the polymerization of a chain of monomers or liquid oligomers to form a solid product. One downside of light-cured polyurethanes compared with conventionally cured polyurethanes is that they have lower molecular weights, which means that their mechanical properties are generally inferior to those of conventional polyurethanes3,4.
Several strategies, including optimization of molecular chain ratios and construction of polymer chain networks, have been used to enhance mechanical properties. By adjusting the ratios of methyl acrylate and butyl acrylate, Kim et al.5 produced poly(urethane-acrylate) elastomers with a tensile strength of 17 MPa and an elongation at break as high as 600%. Deng et al.6 designed and prepared three UV-cured polyurethane acrylate films with different hard-chain segment structures, different proportions of hard-chain fragments, and different NCO/OH molar ratios by modifying acrylates with poly(tetramethylene ether glycol), diisocyanate and polycaprolactone. The UV-cured film with the best performance, which had a 48.6% hard-chain segment and an NCO/OH molar ratio of 1.06, had a tensile strength of up to 50.5 MPa and an elongation at break as high as 238%. However, the study only adjusted the tensile strength of the polymers by changing the proportion of hard chain fragments and did not add cross-linking agents to enhance the tensile strength. Huang et al.4 constructed a polymer chain network with highly entangled polymer chains to improve the mechanical properties of the polymers. The resulting material exhibited a tensile strength of 25.9 MPa and an elongation at a break of 1605%.
Increasing the cross-linking density between molecular chains also improves the mechanical properties of UV-cured polyurethane ionogels. The linear molecules are crosslinked by covalent bonds to form bulk molecules with a more stable network structure and improved mechanical properties7,8. The addition of too much cross-linking agent, however, increases the proportion of hard-chain fragments within the polyurethane molecules, which drastically reduces the elongation at break. It is thus crucial to select a suitable cross-linking agent that can enhance strength without reducing elongation at break9,10,11. Dong et al.12 synthesized a trifunctional itaconic acid-based cross-linking agent (IHA), and regulated the mechanical properties of light-cured polyurethanes by adjusting the ratio of IHA. When the IHA dosage was 7.5 wt.%, the tensile strength was 21.9 MPa and the fracture elongation was 263.1%. It should be noted that we have been inspired by the idea of using itaconic acid as a green cross-linking agent or of crosslinking with non-covalent bonds (e.g., physical entanglement, hydrogen bonding, etc.) to increase the cross-linking density between molecular chains and thus improve mechanical properties.
Mechanical properties can also be improved by adjusting the morphology and phase distribution of the microphase-separated structures13. Hasa et al.14 adjusted the extent of photopolymerization, leading to a variety of different phase-separated morphologies and phase distributions, by adjusting the position of OH groups and the molecular weight of the prepolymers; when the molecular weight of the prepolymers was 30,400 g mol−1, the tensile strength was 8 MPa, and the elongation at break was 45%.
The addition of nanofillers can also be used to increase the cross-linking density between molecular chains and improve the mechanical properties through physical entanglement of the chains. Wang et al.15 prepared polyurethane acrylate nanocomposite films by promoting physical entanglement using MXene (Ti3C2TX) as a nano-filler; the nanocomposites containing 0.077 wt% of MXene showed a 39.9% increase in tensile strength and a 38.5% increase in elongation at break. Fei et al.16 enhanced the mechanical properties of polyurethane resins by using SiO2 nanoparticles modified with 3-(trimethoxymethylsilyl)methacrylic acid. The modified SiO2 nanoparticles increased the cross-linking density between the molecular chains, which led to a preferential fracture of sacrificial bonds and chain entanglements. This extended the local damage at the tip of the crack to the entire elastomer network, resulting in prepared elastomers with a tear strength of up to 23.4 N mm−1.
Compared with other UV-curable polyurethane resins, we believed that a fruitful line of research would be to select a plant-based natural cross-linker with which to modulate the mechanical properties of light-cured polyurethane ionogels by optimizing the ratio of molecular chain to cross-linker. Rutin is a natural polyhydroxy aromatic biopolymer, which is formed by the reaction of the hydroxyl group at the C3 position of quercetin with rutinose to form a bis-glycoside. The quercetin structure functions as a rigid subunit that enhances strength, and the glycoside structure functions as a flexible subunit that ensures elongation at break is not reduced17,18,19,20. This “rigid-flexible” structure is very suited to high toughness in polymers. It is also suitable for building a three-dimensional network structure within polyurethane ionogels as a crosslinking agent. Rutin also contains a large number of conjugated double bonds and phenolic groups, resulting in a p-π conjugated system, which confers strong UV shielding properties21,22,23,24.
We have now prepared polyurethane ionogels by construction of a highly entangled polymer chain network using a chemical cross-linking strategy. The prepolymer molecular chains contain both soft and hard fragments. A stable interfacial region is formed between these fragments, creating a micro-phase separation structure. The long chains of prepolymer molecules become entangled and form entanglement points. The esterified rutin’s multiple cross-linking sites enable the formation of ionogels with a high crosslinking density and a dense three-dimensional mesh structure. Specifically, we used an ionic liquid dispersion medium to cross-link esterified rutin containing a carbon-carbon double bond with a carbon-carbon double-bond terminated urethane acrylate pre-polymer, using diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide as the initiator. The resulting ionogels (PU-HRs) have a high tensile strength of 34.96 MPa and a toughness of 88.11 MJ m−3, which is 1.26 times higher than that of natural silk (70 MJ m−3). The new ionogels also have high visible light transmittance ( > 80%) and excellent UV shielding properties (0% transmittance in the UVB region). The PU-HRs possess excellent adhesive properties and are suitable for automotive glass coatings and sunshade coatings. This study shows that cross-linking agents can enhance the mechanical properties of PU-HRs.
Results and Discussion
Conceptual design of PU-HRs
Rutin is a polyhydroxylated aromatic biopolymer that is well suited to building crosslinked three-dimensional network structures in polymer ionogels. Rutin was adapted for use in UV light-curing systems by reaction with acryloyl chloride to give esterified rutin containing carbon-carbon double bonds. The “C = C” capped polyurethane prepolymers (PU) were added to the ionic liquid, 1-ethyl-3-methylimidazole ethyl sulfate ([EMIM][ESO4]), and diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO) was used as the photoinitiator. The ionogels (PU-HRs) were prepared by free radical polymerization and cross-linking of carbon-carbon double bonds, as shown in Fig. 1. In particular, the hard chain fragments of PU-HR, which consist of reacted IPDI and chain extender PDM, contain highly polar groups, such as aromatic and carbamate groups, which result in high cohesion energy and intermolecular hydrogen bonding, leading to aggregation and the formation of microphase regions of the hard chain fragments (for structural characterization of structures shown on a blue background in Fig. 1, see Supplementary Fig. 13). Polyethers, polyesters and other oligomeric polyols make up the soft chain fragments of the PU-HR molecular chain (structures shown on a pink background in Fig. 1), and due to thermodynamic incompatibility, the stable interfacial zone formed between the soft chain fragments and the hard chain fragments produces a micro-phase separation structure. The hard chain fragments are dispersed independently in the soft phase as physical cross-linking points and have a similar effect to reinforcing fillers dispersed in the soft phase, affecting the mechanical, thermodynamic, and functional properties of the ionogels by regulating the degree of microphase separation25.
By incorporating PCL into IPDI, high molecular weight linear polymer chains were prepared using the PDM chain expansion method, and HEMA was then added to produce the ‘C = C’ capped polyurethane prepolymers (PUs). The presence of soft chain fragments (shown on a pink background) and hard chain fragments (shown on a blue background) in the PUs forms a micro-phase-separated structure. PU-HRs with a three-dimensional mesh structure were rapidly prepared by cross-linking “C = C” double bonds of esterified rutin and PU by photoinitiated free radical polymerization using TPO as photoinitiator. The illustrations show, from left to right, digital photographs of a model umbrella half-coated with PU-HR taken under natural light and 365 nm UV light, and a schematic diagram showing the UV shielding effect of PU-HR irradiated with 365 nm UV light. PCL530, polycaprolactone diol (Mw ~530); PCL2000, polycaprolactone diol (Mw ~2000); IPDI, isophorone diisocyanate; DMG, dimethyl ethylene dioxide; PDM, 2,6-pyridinedimethanol; HEMA, hydroxyethyl methacrylate; TPO, diphenyl(2,4,6-trimethyl benzoyl)phosphine oxide; [EMIM][ESO4], 1-ethyl-3-methylimidazole ethyl sulfate.
Optimization of preparation
To determine which factors influence the properties of photoinitiation-prepared PU-HRs, the preparation process was optimized using Box-Behnken response surface methodology26,27,28. Specifically, tensile strength, which is closely associated with the degree of cross-linking, was used as the response index to investigate the effects of mass fraction of polyurethane ionogels prepolymer (PU) prepared by photoinitiation (X1), mass fraction of cross-linking agent esterified rutin (X2) and mass fraction of photoinitiator (X3). The results of the experiments were analyzed using Design Expert 11.0 (Table 1). The quadratic multiple regression model was obtained after regression analysis, as shown in the equation below:
Y = 19.84 + 10.27X1 + 3.79X2-3.81X3 + 0.4250X1X2-5.94X1X3 + 0.3075X2X3 + 1.77X12-3X22-3.16X32
Analysis of variance (ANOVA) was used to test the necessity and adequacy of the regression model. The regression model was found to have an F-value of variance of 19.99 ( > 1), which is a significant correlation (Table 2). The likelihood of such a large F-value being due to noise is only 0.03%. The P-value is also <0.05, again indicating that the model obtained from the regression analysis is statistically significant.
The adequacy of the Box-Behnken design model diagnostic plots and the 3D contour response surface are shown in Fig. 2. The normal probability and internal study residuals are extremely close to the same straight line, proving that the model is stochastic (Fig. 2a); the predicted values and actual values are close to the same straight line, indicating that the two correlations are high and that the fit is better and accurate (Fig. 2b). The internal study residuals for each experimental site are <4, further indicating that the regression model is reliable (Fig. 2c).
a Normal probability with internal study residuals; b Comparison of actual and predicted values; c Internal study residuals for each experimental point; d Response surface graphs of tensile strength showing effects of variables X1-X2 (X1 = mass fraction of PU; X2 = mass fraction of cross-linker esterified rutin); e Response surface graphs of tensile strength showing effects of variables X1-X3 (X3 = mass fraction of photoinitiator); f Response surface graphs of tensile strength showing effects of variables X2-X3.
According to the F-values of the regression model, the order of influence of each factor on the response index was determined to be PU mass fraction (X1) > photoinitiator mass fraction (X3) > cross-linker esterified rutin mass fraction (X2). The significance analysis of the P-values showed that X1, X2, X3, X1X3 and X32 were the important model terms (P < 0.05).
The steepness of the slope of the response surface of each factor reflects the degree of influence of that factor on the tensile strength of the ionogels; the steeper the response surface, the more significant the interaction. The X1, X2 and X3 curves are steeper (Fig. 2d–f), indicating that these factors have a greater influence on the tensile strength. In addition, after fixing a single factor, the tensile strength varies with the other two factors; the curve of X1 is steeper than that of X2 or X3, indicating a greater influence on the response index, tensile strength, which is consistent with the ANOVA results of the regression model.
The regression model was validated using the optimized process formulation. Experiments were carried out with X1 = 90 wt%, X2 = 0.3 wt%, X3 = 4.065 wt% and [EMIM][ESO4] = 5.635 wt%. The tensile strength of the resulting PU-HR was found to be 34.96 MPa, which is very close to the predicted value of 32.481 MPa, demonstrating that this methodology has an important reference value and guiding significance. The ionogels prepared using this method will be termed PU-HRs in the subsequent discussion, and the ionogels prepared without the addition of the cross-linking agent, esterified rutin, will be used as a control and termed PU-Hs.
Synthesis of PU-HR
Structural characterization of esterified rutin
To ensure rapid acylation and prevent the formation of chlorinated alkanes, the nucleophilic substitution reaction of rutin hydroxyl groups with acryloyl chloride to produce esterified rutin and hydrochloric acid was carried out in the presence of triethylamine as an acid scavenger (Fig. 3a). The FT-IR spectrum of esterified rutin (Fig. 3b) showed the stretching vibrations of C=O (1676 cm−1) and C=C (1742 cm−1) groups and out-of-plane bending vibrations (902 cm−1). Additionally, the broad O-H stretching vibration of rutin (3400 cm−1) had essentially disappeared. The 1H-NMR spectrum of esterified rutin (for NMR spectra see Supplementary Fig. 1) showed an absorption peak for hydrogens attached to a “C=C” bond (5.92 ppm), indicating that the C=C bond of acryloyl chloride is connected to rutin. The peaks in the spectrum of rutin at 12.6 ppm, 10.8 ppm, 9.7 ppm, and 9.2 ppm, attributed to hydrogens attached to the carbon atoms adjacent to the phenol groups in the quercetin moiety of rutin, also disappeared from the 1H-NMR spectrum, again demonstrating the successful preparation of esterified rutin.
a Preparation of esterified rutin; b FT-IR spectra of rutin and esterified rutin over the range 500–4000 cm−1; c Molecular configuration of esterified rutin obtained by molecular dynamics simulation (red dashed line shows nucleophilic substituents and hydroxyl groups that did not react due to spatial barrier effect).
Molecular dynamics simulations carried out for the preparation of esterified rutin (Fig. 3c) indicated that the hydroxyl groups of rutin were not completely substituted because of the spatial site resistance effect (red dashed part of Fig. 3c) and that the esterified rutin contained both C=C bonds and hydroxyl groups. The root mean square deviation (RMSD) plot (for RMSD plot see Supplementary Fig. 2) shows an initial upward-sloping trend, indicating that the reaction of the hydroxyl groups of rutin with acryloyl chloride starts quickly; the later curve vibrates smoothly, indicating that the reaction reaches equilibrium and that esterified rutin has been successfully synthesized. The free energy shape change and small amplitude radius of the gyration curves from the molecular dynamics simulation results (for molecular dynamics simulation results see Supplementary Fig. 3, Supplementary Fig. 4 and Supplementary Fig. 5) prove that the esterified rutin structure has stability and compactness.
Structural characterization of PU-HRs
Firstly, the PU molecular chain was prepared by nucleophilic addition reaction of polycaprolactone diol and isophorone diisocyanate with dimethylglyoxime. Following the design principle, dynamic covalent oxime-carbamate bonds will be formed on the molecular chain, which should confer the characteristics of room-temperature cross-linking and high-temperature dissociation. To test this hypothesis, the shear energy storage modulus (G’) of PU-HR and PU-H was determined using a rotational rheometer (for rotational rheometer results see Supplementary Fig. 6). The slow decrease of G’ at 140 °C was attributed to dissociation of the dynamic oxime-carbamate bonds. The high-temperature dissociation of the dynamic bonds leads to disconnection of the molecular network and freer movement of the molecular chains, which spontaneously regulates the elastomer crosslinking network topology, confers reversibility on the network structure, and enhances the stability of the network structure after final equilibration29.
Secondly, the esterified rutin was polymerized and crosslinked with PU, forming the crosslinked network structure of PU-HR. The FT-IR spectra of PU-HR and PU-H were very similar (Fig. 4a), indicating that the basic skeletal structures of the two were identical. Closer inspection of the 3000–3900 cm−1 region of the spectra (Fig. 4b) showed that the unsaturated hydrocarbon C-H stretching vibrations of the benzene ring side chains seen at 3065 cm−1 in the spectrum of PU-H had disappeared in the spectrum of PU-HR, indicating that the esterified rutin had been successfully crosslinked. Additionally, when compared with the 1H-NMR spectrum of PU-H, the 1H-NMR spectrum of PU-HR appeared to have hydrogen absorption peaks associated with the -CH3 linked to C=C (3.82 ppm) and enhanced hydrogen absorption peaks (7.77 ppm, 7.68 ppm) on the group attached to the benzene ring (for NMR spectra see Supplementary Fig. 7), further indicating the cross-linking reaction of esterified rutin.
a FT-IR spectra of PU-H and PU-HR over the range 500–4000 cm−1; b FT-IR spectra of PU-H and PU-HR over the range 3000–3900 cm−1; c Storage modulus curves of PU-H and PU-HR over the temperature range −50 °C to 100 °C; d Surface SEM image of PU-H, inset: 2D SAXS map of PU-H; e Surface SEM image of PU-HR, inset: 2D SAXS map of PU-HR; f 1D scattering intensity profiles of PU-H and PU-HR.
It is generally believed that the elastic energy storage modulus (E’) and the elastic loss modulus (E”) are closely associated with the degree of elastomer network crosslinking30; E’ reflects the ability of ionogels to resist elastic deformation under cyclic stress and E” reflects the entanglement of molecular chains and intermolecular interactions within the ionogels. We measured the E’ and E” of the ionogels over the temperature range −50 °C to 100 °C using dynamic mechanical analysis (DMA) in order to obtain further information about the structure of the crosslinked network. As shown in Fig. 4c and Supplementary Fig. 8, E’ of both PU-HR and PU-H decreased with increasing temperature. This was attributed to the increased flexibility of the molecular chains and decreased rigidity with increasing temperature, leading to the gradual transformation of the materials from glassy to highly elastic states. The values of E’ and E” for PU-HR were higher than those for PU-H at −50 °C. During shear deformation of the ionogels, E’ and E” are mainly affected by the entanglement force of the elastomer molecular chains and the intermolecular interaction force. For PU-HR, E’ and E” are also affected by the formation of cross-links by esterified rutin, leading to restricted movement of the polyurethane molecular chains. This means that E’ and E” are larger for PU-HR than for PU-H under the same deformation. The glass transition temperature (Tg) of PU-HR also shifted towards higher temperatures (for glass transition temperature see Supplementary Fig. 9), again because the formation of crosslinked networks restricts the movement of the molecular chains. Thermogravimetric (TGA/DTG) analysis also showed that the crosslinked network structure of PU-HR resulted in a higher thermal decomposition temperature (for TGA see Supplementary Fig. 10) and a faster thermal decomposition rate over the range 380–445 °C (for DTG see Supplementary Fig. 11).
Finally, the stable interfacial region formed between the soft-chain fragment, polycaprolactone diol, and the hard-chain fragments, isophorone diisocyanate, and 2,6-pyridinedimethanol, in PU-HR results in microphase-separated structures. Analysis by scanning electron microscopy (SEM) showed an obvious interface between the soft and hard phases (Fig. 4d, e), with uniform distribution of the microphase separation structure. The agglomerated particles were uniformly dispersed in the continuous phase with a “sea-island” structure (where the sea is the soft phase and the island is the hard phase). This structure helps the PU-HR to withstand and transfer stresses, thereby increasing its strength and toughness. The SEM image of PU-HR also showed more pronounced circular scattering rings, smaller scattering vectors (qmax), lengthier long periods (L) (for characteristic SAXS data see Supplementary Table 2) and a higher degree of micro-phase separation (insets of Fig. 4d–f), which contribute to its improved strength and toughness.
In a word, in PU-HR, the polyurethane molecular chains contain dynamic covalent oxime-carbamate bonds and crosslink with esterified rutin to form a crosslinked network structure, while the soft-chain fragments between the molecular chains form a distinct microphase-separated structure from the hard-chain fragments.
Mechanical properties of PU-HR
Ionogels have broad prospects in applications such as wearable electronic products, sensors, electroluminescent devices and interfacial bonding, in which good mechanical properties are a prerequisite. We therefore tested the mechanical properties, including tensile strength, toughness, cyclic tensile resistance, puncture resistance and stress relaxation of PU-HR.
Firstly, stress-strain curves and toughness were measured (Fig. 5a, b). The tensile strength of PU-HR was 34.96 MPa, which is 2.6-fold higher than that of PU-H (13.39 MPa), the elongation at break was 538%, and the toughness was as high as 88.11 MJ m−3, which is 1.95-fold higher than that of PU-H (45.08 MJ m−3) and also higher than that of the nylon fiber (80 MJ m−3) and natural domestic silkworm silk (70 MJ m−3)31. The sharp increase in stress of PU-HR in the second half of the stress-strain curve indicates that the esterified rutin cross-linker increases the cross-linking density of the whole system, which in turn enhances the tensile strength and toughness of PU-HR. Compared with the tensile strength and elongation at break of previously described elastomeric materials with UV shielding properties (Fig. 5c), PU-HR has both high tensile strength and high elongation at break, thus overcoming the challenge of obtaining UV shielding elastomeric materials with both high tensile strength and elongation at break31,32,33,34,35,36,37,38,39,40.
a Stress–strain curves of PU-H and PU-HR; b Toughness of PU-H and PU-HR; c Comparison of tensile strength and elongation at break of other ionogels with UV shielding properties32,33,34,35,36,37,38,39,40,46; d Stress–relaxation curves of PU-H and PU-HR (inset shows stress-relaxation curves for 1–30 s); e Tensile force variation curves over 50 cycles of loading and unloading at 50% fixed strain; f Tensile rate of 100 mm min−1 (inset shows digital photograph of puncture process of PU-HR).
We next analyzed the stress relaxation curves of the PU-HR ionogels, which can be divided into three stages (Fig. 5d). Stage I (0–14 s) is the pre-stress relaxation stage (Fig. 5d inset), which is due to elastic deformation in the crosslinked network: the greater the elasticity, the faster the relaxation. The rate of relaxation of PU-HR and PU-H is the same in Stage I because they have the same molecular chain structure. Stage II (14–85 s), the mid-point of the middle stage of stress relaxation, is due to the presence of covalent cross-linking points in PU-HR. The cross-linked structure is dense, which manifests as a slowing down of the rate of stress reduction of PU-HR— both structural stability and the dynamics of the polymer network are present. Stage III ( ≥ 85 s) is the late stage of stress relaxation (for the derivative curves of the stress relaxation curves see Supplementary Fig. 12) and is mainly associated with the physical entanglement and viscous motion of the molecular chains of the ionogels, as well as with chain-section movement. In Stage III, PU-HR and PU-H exhibited the same slow relaxation rate. Over time, the stresses of both PU-HR and PU-H relax to a value > 0, showing that both are cross-linked polymers. Based on the Maxwell model for viscoelastic fluids, the relaxation time (τ*) is the time at which the stress relaxation reaches G/G0 = 1/e41. The longer relaxation time of PU-HR allows for a slower release of internal stress than is the case for PU-H, suggesting that the denser crosslinked structure in PU-HR hampers the relaxation of the chain segments, thus providing better elastic properties.
To investigate the ability of the PU-HR ionogels to recover quickly, we carried out 50 stretch-unload cycles at 50% strain (Fig. 5e). The tensile stress remained at 84.7% after many cycles, showing that the ionogels have a high degree of resilience. Structural changes in the ionogels in the stretched versus relaxed state are shown in Supplementary Fig. 13. The tensile stress of the ionogels was highest at the initial stage of cycling and remained constant after several cycles. This was attributed to hydrogen bonds between the ionic liquid and the polyurethane prepolymer molecular chains being broken to dissipate energy during the stretching process and then reforming after the stress was released whilst the covalent bonds of the crosslinked network remained intact, i.e., the breaking and remodeling of hydrogen bonds provide the PU-HR ionogels with good and rapid recovery and the covalent bonds of the crosslinked networks remain intact. This means that the breaking and reforming of hydrogen bonds gives PU-HR ionogels a good ability to recover quickly, and the intact covalent bonding of the crosslinked network results in a constant tensile stress after many cycles.
Puncture resistance enables ionogels to withstand large loads without being damaged. This ensures that they will not be punctured during storage, transport and use, which is very important for their safe application. The puncture force-displacement curves (Fig. 5f) show that, due to the presence of covalent cross-linking points in the PU-HR, the cross-linking density of the whole molecular chain increases. This improves macroscopic puncture resistance; the maximum displacement of PU-HR is 15.47 mm at a maximum puncture force of 44.44 N, which is greater than that of PU-H and shows great potential for application in flexible devices.
In summary, the high toughness and puncture resistance of PU-HR comes from the micro-phase separation of the soft chain fragments from the hard chain fragments, the cross-linking mesh structure formed between the molecular chains, and the formation of hydrogen bonding energy dissipation between ionic liquids and linear polymer molecular chains (sacrificial bonding toughening principle).
Application of PU-HR in UV shielding coatings
The conjugated aromatic rings of rutin confer good UV shielding properties to PU-HR ionogels (Fig. 6a). PU-HR ionogels can partially shield the long-wave dark spot effect UV region (UVA region, 320–400 nm), which can reach the dermis of the skin and destroy elastin and collagen fibers, resulting in skin lesions; the UV absorption of PU-HR is about 2-fold higher than that of PU-H. PU-HR can also completely absorb mid-wavelength erythema-causing UV (UVB region, 275–320 nm, Fig. 6a). UVB radiation is particularly strong in the summer and in the afternoon and can cause sunburn in as little as 25 min. PU-HR can be used as the UVB screen in UV protection materials. Under 365 nm UV radiation, the anti-counterfeiting mark on an uncovered 10 yuan RMB is very obvious (Fig. 6j1), as is the anti-counterfeiting mark covered with PU-H (Fig. 6j2), whereas the anti-counterfeiting mark covered with PU-HR essentially disappears (Fig. 6j3). The PU-HR thus has the prospect of being used as a general UV protection material in applications such as sunshades, UV-protected car windows and food packaging boxes for foods susceptible to photo-oxidation.
a UV transmittance curves of PU-H and PU-HR; b XRD spectra of PU-H and PU-HR, with inset photographs comparing light transmission properties of PU-H and PU-HR thin ionogels adhered to transparent glass; c Adhesion strength–displacement curves of PU-H and PU-HR on glass pane; d Adhesion strength-displacement curves of PU-H and PU-HR on PVC panel; e Microstructure of bonding interface of PU-HR and glass pane; f Microstructure of bonding interface of PU-HR and PVC panel; g SEM-EDX mapping showing distribution of C, O, N and S at bonding interface of PU-HR and PVC panel; h 2D morphology of friction damage to PU-H coating and PU-HR coating; i Friction coefficients of PU-H coating and PU-HR coating; j Macroscopic images showing UV shielding properties of PU-H and PU-HR. 1 shows RMB 10 note with no covering under 365 nm UV lamp irradiation, 2 shows RMB 10 note covered with PU-H under 365 nm UV lamp irradiation, 3 shows RMB 10 note covered with PU-HR under 365 nm UV lamp irradiation, 4 shows digital photograph of model umbrella under natural light, 5 shows digital photograph of uncoated half of model umbrella under 365 nm UV light, 6 shows digital photograph of PU-HR coated half of model umbrella under 365 nm UV light.
Interestingly, PU-HR has excellent fast-curing properties and can be fully cured within 5 s (for PU-HR fast curing movie see Supplementary Movie 1). The X-ray diffraction (XRD) patterns of the fast-curing PU-HR and PU-H (Fig. 6b) show significant broad dispersion peaks around 20°, indicating that PU-HR has an amorphous structure and that its unique asymmetric molecular structure does not disrupt the order of the polycaprolactone diol chain segments but rather inhibits crystallization of the polycaprolactone diol, which results in both PU-HR and PU-H being transparent (PU-HR is light yellow and transparent, and PU-H is colorless and transparent, Fig. 6b insets).
The ionogel PU-HR also has excellent adhesive properties. The adhesive strength-displacement curves of PU-HR and PU-H on glass and polyvinyl chloride (PVC) sheets (Fig. 6c, d) show that the adhesive strength of PU-HR on glass reaches 1.21 MPa and the adhesive strength of PU-HR on PVC sheet is as high as 2.11 MPa. This high adhesion, particularly to PVC, is the result of the coupling of two aspects. On the one hand, the elastomer itself has a strong toughness that can effectively disperse the external stress and dissipate energy. The covalent cross-linking sites of PU-HR increase the cross-linking density and enhance the strength of the elastomer itself. The long molecular chains of the polyurethane prepolymers increase the possibility of physical entanglement between the chains. The entanglement sites act as slip rings that transfer external stresses to the intermolecular chain segments so that external stresses are dispersed before the molecular chain breaks. When a covalent break occurs in one of the molecular chains, the polymer transfers elastic potential energy to each molecular chain segment through the entanglement sites, thus dissipating external energy42,43. On the other hand, a strong interfacial interaction is formed between the elastomer and the bonded material. Analysis of the morphology and chemical composition of the cross sections by scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDX) is shown in Figs. 6e, 6f. The elastomer was tightly connected to the PVC (Fig. 6f), and the adhesive was highly permeable before light-curing. After light-curing, strong interfacial interactions were formed between the elastomer and the PVC. Hydrogen atoms on the PVC sheet were able to form strong interfacial interactions with the free urethane and ether groups in the elastomer bonds and the nitrogen and oxygen atoms in the ionic liquid to form hydrogen bonds, resulting in a strong interfacial interaction (Fig. 6g).
Figure 6h shows the 2D morphology of the wear of glass sheets coated with PU-HR and PU-H. It can be clearly seen that PU-HR is less abrasive (for three-dimensional morphology of friction damage see Supplementary Fig. 14, Supplementary Fig. 15). The friction coefficient of the PU-HR coating in the stabilization stage is about 0.21, which is significantly lower than that of PU-H (Fig. 6i), and the wear-reducing performance is better. This can be attributed to the extrusion and deformation of the PU-HR during the friction process, which produces adhesion at the interface. The cohesion of PU-HR is, however, greater, and the presence of acylated rutin increases the density of cross-linking points in the light-curing system; the formation of the three-dimensional network structure also has a reinforcing effect, which reduces interfacial adhesion. The degree of abrasion is much smaller, thus improving the wear resistance of the ionogels.
As one example of its UV shielding properties, we observed the effect of coating the anti-counterfeiting mark on a 10 yuan RMB with PU-HR. Under 365 nm UV radiation, the anti-counterfeiting mark on an uncovered 10 yuan RMB is very obvious (Fig. 6j1), as is the anti-counterfeiting mark covered with PU-H (Fig. 6j2), whereas the anti-counterfeiting mark covered with PU-HR essentially disappears (Fig. 6j3). To further demonstrate its high UV shielding, rapid curing, strong adhesion and excellent abrasion-resistance properties, we half-coated a sunscreen umbrella made of PVC with PU-HR. One-half of the model PVC umbrella was coated with PU-HR, and the PU-HR coating was then tightly bonded to the umbrella (Fig. 6j4). The coating did not separate when the umbrella was opened and closed repeatedly 100 times (for umbrella opening and closing movie see Supplementary Movie 2). When the uncoated side of the umbrella was irradiated with 365 nm UV light, the UV light penetrated all the way through (Fig. 6j5); when the coated side was irradiated, the UV light was absorbed (Fig. 6j6). These experiments demonstrate that the ionogel PU-HR has the prospect of being used as a general UV protection material in applications such as sunshades, UV-protected car windows, architectural sunscreen glass and food packaging boxes for foods susceptible to photo-oxidation.
In summary, its high UV shielding, rapid curing, high adhesion and excellent abrasion resistance endow PU-HR with broad application prospects as a UV shielding coating.
Conclusions
In this study, ionogels (PU-HRs) were prepared by construction of a highly entangled polymer chain network. A chemical cross-linking strategy involving cross-linking esterified rutin-containing carbon-carbon double bonds with carbon-carbon double bond-capped polyurethane acrylate prepolymers in an ionic liquid dispersion medium was optimized by response surface methodology. The main conclusions are as follows: the “rigid-flexible” structural characteristics of rutin are suitable for crosslinking polymer molecular chains to form a three-dimensional crosslinked network, the rigid quercetin structure of rutin enhances strength, and the glycoside structure acts as a flexible unit that maintains elongation at break. The tensile strength of the ionogel (PU-HR), based on esterified rutin cross-linking, was 34.96 MPa and the toughness was as high as 88.11 MJ m−3, which is 1.26-fold higher than that of natural domestic silkworm silk (70 MJ m−3). The three-dimensional cross-linking network structure formed by the cross-linking of esterified rutin, the micro-phase separation structure between the soft and hard chain fragments in the formation of stable interfacial regions, and the hydrogen bonding energy dissipation between the ionic liquid and the polymer chain act together to endow the ionogels (PU-HR) with high tensile strength and high toughness. After light-curing of esterified rutin by radical polymerization, the structure retains a complete conjugated system, which confers excellent UV protection, with a transmittance of >80% in the visible region and a transmittance of 0% in the UVB region. Due to the “rigid-flexible” structure of esterified rutin and the accompanying conjugated structure, the preparation of multifunctional ionogels by photoinitiation provides a way of thinking for the rapid preparation of high-performance polymer UV coatings, which have great potential for application in automotive glass films.
Methods
Materials
Polycaprolactone diol (PCL2000, Mw~2000), polycaprolactone diol (PCL530, Mw~530), methacryloyl chloride (95%), triethylamine (TEA, AR, 99%), isophorone diisocyanate (IPDI, 99%), dimethylglyoxime (DMG, AR, 98%), 2,6-pyridinedimethanol (PDM, 97%), rutin (R, 95%), hydroxyethyl methacrylate (HEMA, 99%), diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO, 97%), ethyl 1-ethyl−3-methylimidazole sulfate ([EMIM][ESO4], 99%) and dibutyltin dilaurate (DBTDL, 95%) were purchased from Aladdin (Shanghai, China). Other solvents were purchased from Sinopharm Chemical Reagent Company (Shanghai, China). All reagents, except PCL2000 and PCL530, were used as received without further purification. PCL2000 and PCL530 were dried under vacuum at 115 °C for 1.5 h before use and cooled to 40–60 °C.
Preparation of esterified rutin
Under a nitrogen atmosphere and with external cooling (ice water bath), rutin (0.6105 g, 1 mmol), methacryloyl chloride (1.568 g, 15 mmol) and TEA (1.518 g, 15 mmol) were added to N, N-dimethylacetamide (10 ml). After stirring at 25 °C for 72 h, the reaction mixture was slowly added dropwise to a large volume of deionized water. The resulting precipitate was filtered, washed thoroughly with deionized water and freeze-dried to give esterified rutin.
Preparation of polyurethane acrylate prepolymer (PU-H)
PCL2000 (9.6 g, 4.8 mmol), PCL530 (3.816 g, 7.2 mmol) and DMG (0.227 g, 2.4 mmol) were stirred at 50 °C in acetone (8 mL) until completely dissolved. IPDI (5.340 ml, 25.573 mmol) and DBTDL (0.5 wt.%) were added, and the reaction was stirred at 50 °C under nitrogen for 30 min. PDM (0.3686 g, 2.649 mmol) was added, and the reaction mixture was stirred at 50 °C for 1 h. The intermediate linear N=C=O-terminated polyurethane prepolymer was treated with HEMA (1.140 ml, 9.377 mmol), and the reaction mixture was stirred at 50 °C for 2 h to prepare C=C-terminated polyurethane acrylate prepolymer PU.
Preparation of ionic liquid gels (PU-HRs)
TPO was added to a mixture of PU, esterified rutin and ([EMIM][ESO4] and curing was carried out in a 365 nm UV light box to prepare the photocured ionogel PU-HR. Box-Behnken response surface methodology was used to explore the effects of the mass fractions of prepolymer (X1), crosslinker (X2) and initiator (X3) on the tensile strength of the ionogels. Process optimization was carried out using three design factors and three levels (for factors and levers table see Supplementary Table 1), and the mass fraction of [EMIM][ESO4] is
Preparation of model sunscreen umbrellas
The PU-HR prepolymer solution, prepared as described above, was coated onto model PVC umbrellas, which were then subjected to rapid curing in a 365 nm UV light box to prepare model sunscreen umbrellas.
1H nuclear magnetic resonance spectroscopy
1H-NMR spectra were recorded using a 400 MHz Bruker AVANCE III spectrometer (Bruker, Billerica, MA, USA), with DMSO-d6 as the solvent.
Infrared spectroscopy
FT-IR spectra were recorded at room temperature using a Nicolet iN10 FT-IR spectrometer (Thermo Fisher Scientific, Carlsbad, CA, USA). The resolution was 32 scans and the scan range was 4000–400 cm−1.
Thermal analysis
PU-HR and PU-H were analyzed using a Discovery TGA 550 thermo-gravimetric analyzer (TA Instruments, New Castle, DE, USA.). An alumina crucible was used and the sample was heated from 30 °C to 800 °C at a rate of 20 °C min−1 under a nitrogen atmosphere.
Ultraviolet spectroscopy
Transparency tests were carried out using a TU-1950 UV–vis spectrophotometer (Beijing, China), with a scanning range from 800 to 200 nm, using air as the reference.
X -ray diffraction
X-ray diffraction (XRD) maps were recorded using a Panalytical X’Pert Pro X-ray diffractometer (Malvern Panalytical, Malvern, UK). Data were acquired over the range 10–80°, with a scanning speed of 10°.min−1.
Analysis of microstructure and surface morphology
An Apreo S HiVac scanning electron microscope (Thermo Fisher Scientific) was used to examine the surface structure of the samples.
Dynamic mechanical analysis
The glass transition temperatures, storage modulus curves, loss modulus curves and loss factor curves of the samples were investigated using a Q800 dynamic mechanical analyzer (TA Instruments). The measurements were carried out under a nitrogen atmosphere over the temperature range −50 °C to 100 °C, with a heating rate of 3 °C min−1 and a frequency of 1 Hz.
Static SAXS measurement (SAXS)
Static SAXS measurements for PU-H and PU-HR were recorded using a Xeuss 3.0 instrument (Xenocs SAS, Grenoble, France). The conditions were as follows: copper target; light tube power, 30 W; focal spot diameter, 30 μm; maximum luminous flux at sample, 4.5 × 108 phs s−1; individual pixel size, 75 μm; acquired q range (theoretical value of the standard): 2θmin ≤ 0.013°, qmin ≤ 012 nm−1, 2θmax ≥ 75°, qmax ≥ 49 nm−1.
Molecular dynamics simulation
VESTA and VMD software packages were used to construct the initial rutin structure44, which was modeled using periodic boundary conditions to simulate macroscopic chain length. The GROMACS tool was used to insert multiple rutin and methacryloyl chloride into a N, N-dimethylacetamide environment, using a force field obtained from AMBER14SB44,45.
Stress-strain curve testing
Stress–strain curves were measured using a UTM2203 electronic universal testing machine (Shenzhen, China). The samples were cut into dumbbell shapes (20 mm × 4.0 mm × 1 mm) for testing. The tensile rate was set at 50 mm min−1.
Loading and unloading curves
Cyclic tensile profiles were measured using an INSTRON 5982 electronic universal testing machine (Instron, Norwood, MA, USA). The samples were cut into dumbbell shapes (20 mm × 4.0 mm × 1 mm) for testing and cyclic tensile deformation was determined by gradually increasing the tensile strain, with the strain rate fixed at 10 mm.min−1. In each step, once the sample reached the appropriate tensile strain, the coupler orientation was reversed and the sample strain was removed at the same rate as it had been applied until zero strain was reached. The coupler was then immediately reversed and the sample was stretched again at the same constant strain rate until the next target maximum strain was reached. The cyclic deformation test was continued until the target final strain was reached in the final cycle. In addition to the different strain cycles described above, the sample was stretched to 50% strain for 50 cycles.
Puncture resistance curve
The puncture resistance of the composites was measured according to ASTM F1342-9. A puncture needle with a diameter of 0.35 μm was compressed at a rate of 100 mm min−1 until a thin gel with a thickness of 0.5 mm was completely punctured.
Stress relaxation curves
Stress relaxation curves were measured using a UTM2203 electronic universal testing machine. The samples were cut into dumbbell shapes (20 mm × 4.0 mm × 1 mm) for testing and kept at room temperature, with a fixed strain of 10% and a relaxation time of 10 min. The dissipated energy, which is the area of the hysteresis curve of the ionic liquid gel during loading and unloading and during stress relaxation, was calculated using Origin software.
Friction and wear test
A CETR-UMT-2MT instrument (Bruker) was used to measure friction and wear. The friction coefficients and wear volume of PU-H and PU-HR were evaluated using a wear time of 20 min, a distance of 5 mm and a steel ball (GCr15) as the standard friction pair.
Determination of adhesive properties
The adhesion strength–displacement curves of the ionogels were obtained by gluing two slides together, with a slide size of 75 mm × 25 mm × 1 mm, using ionic liquid gel as the adhesive, and then performing an adhesive peel test using a UTM2203 universal electronic testing machine at a speed of 50 mm min−1.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
References
Won-Bin, L. et al. A novel UV-curable acryl-polyurethane for flexural 3D printing architectures. Addit. Manuf. 51, 102625 (2022).
Mendes-Felipe, C. et al. Lithium bis(trifluoromethanesulfonyl)imide blended in polyurethane acrylate photocurable solid polymer electrolytes for lithium-ion batteries. J. Energy Chem. 62, 485–496 (2021).
Zhang, Y. D. et al. UV-curing behaviors and general properties of acrylate-urethane resins prepared from castor oil-based polyols. Prog. Org. Coat. 188, 108204 (2024).
Huang, X. M. et al. 3D Printing of High Viscosity UV-Curable Resin for Highly Stretchable and Resilient Elastomer. Adv. Mater. 30, 2300211 (2023).
Kim, S., Lee, J. & Han, H. Synthesis of UV Curable, Highly Stretchable, Transparent Poly(urethane-acrylate) Elastomer and Applications Toward Next Generation Technology. Macromol. Res 28, 896–902 (2020).
Deng, Y. Z. et al. Preparation and relationship between structure and properties of PTMEG based polyurethane acrylate UV cured films. J. Macromol. Sci. 60, 63–70 (2022).
Dokyung, W. et al. Biomass-derived closed-loop recyclable chemically crosslinked polymer composites for green soft electronics. Chem. Eng. J. 488, 150818 (2024).
Wu, Y. Y. et al. In situ crosslinking-assisted perovskite grain growth for mechanically robust flexible perovskite solar cells with 23.4% efficiency. Joule. 7, 398–415 (2023).
Fei, J. et al. Progress in Photocurable 3D Printing of Photosensitive Polyurethane: A Review. Macromol. Rapid Commun. 44, 2300211 (2023).
Xiong, L. L. et al. Tailoring crosslinking networks to fabricate photocurable polyurethane acrylate (PUA) dielectric elastomer with balanced electromechanical performance. React. Funct. Polym. 183, 105498 (2023).
Zhang, H. et al. A photocurable polyurethane elastomer for digital light processing with comprehensive reusability based on dynamic hindered urea bonds. Polym. 292, 126657 (2024).
Dong, X. B. et al. Preparation and properties of green UV‐curable itaconic acid cross‐linked modified waterborne polyurethane coating. J. Appl. Polym. Sci. 139, 552042 (2022).
Zhou, S. & Wu, L. Phase Separation and Properties of UV-Curable Polyurethane/Zirconia Nanocomposite Coatings. Macromol. Chem. Phys. 209, 1170–1181 (2008).
Erion, H., Tai Yeon, L. & Allan, G, C. Controlling phase separated domains in UV-curable formulations with OH-functionalized prepolymers. Polym. Chem. 13, 3102–3445 (2022).
Wang, Y. et al. Preparation and Properties of UV-Curable Waterborne Polyurethane Acrylate/MXene Nanocomposite Films. Nanomater. 13, 3022 (2023).
Fei, J. H. et al. Vat photopolymerization printing of strong tear-resistance and stretchable polyurethane elastomer for highly sensitive strain sensors. Polym. Compos. 45, 2615–2628 (2024).
Baaran, E. et al. Rutin And Quercetin-Rutin Incorporated Hydroxypropyl β-Cyclodextrin Inclusion Complexes. Eur. J. Pharm. Sci. 172, 106153 (2022).
Alsaif, N. A. et al. Multi-spectroscopic investigation, molecular docking and molecular dynamic simulation of competitive interactions between flavonoids (quercetin and rutin) and sorafenib for binding to human serumalbumin. Int. J. Biol. Macromol. 165, 2451–2461 (2020).
Cui, X. et al. Regulating inhibitory activity of potato I-type proteinase inhibitor from buckwheat by rutin and quercetin. J. Food Biochem. 45, 13780 (2021).
Pan, R. Y. et al. Sodium rutin ameliorates Alzheimer’s disease-like pathology by enhancing microglial amyloid-β clearance. Sci. Adv. 5, 6328 (2019).
Xu, Y. L. et al. Synthesis of efficient broad-spectrum UV-absorbing carbon dots as UV absorbers using natural rutin as raw material and their multifunctional applications under acid-base amphoteric conditions. Surf. Interfaces 38, 102810 (2023).
Waterman, M. J. et al. Antarctic moss biflavonoids show high antioxidant and ultraviolet-screening activity. J. Nat. Prod. 80, 2224–2231 (2017).
Wang, H. Y. et al. Rutin-Loaded Stimuli-Responsive Hydrogel for Anti-Inflammation. Rutin-Loaded Stimuli-Responsive Hydrogel for Anti-Inflammation. ACS Appl. Mater. Interfaces. 14, 26327–26337 (2022).
Roy, S. & Rhim, J. W. Fabrication of bioactive binary composite film based on gelatin/chitosan incorporated with cinnamon essential oil and rutin. Colloids Surf., B 204, 111830 (2021).
Jiang, J. et al. Effect of fillers on the microphase separation in polyurethane composites: A review. Polym Eng Sci. 63, 3938–3962 (2023).
Yin, B. et al. Multi-scale synergistic modification and mechanical properties of cement-based composites based on in-situ polymerization. Cem. Concr. Compos. 137, 104945 (2023).
Rezgar, H. et al. A novel systematic multi-objective optimization to achieve high-efficiency and low-emission waste polymeric foam gasification using response surface methodology and TOPSIS method. Chem. Eng. J. 430, 132958 (2022).
Jiang, X. C. et al. Simultaneous adsorption of Ciprofloxacin and Ni(II) from wastewater using poly(vinyl alcohol)/poly(sodium-p-styrene-sulfonate) semi-interpenetrating polymer network@Ni foam: Insights into the synergistic and antagonistic mechanisms. Chem. Eng. J. 486, 150391 (2024).
Zheng, N. et al. Dynamic Covalent Polymer Networks: A Molecular Platform for Designing Functions beyond Chemical Recycling and Self-Healing. Chem. Rev. 121, 1716–1745 (2021).
Wang, S. et al. Facile mechanochemical cycloreversion of polymer cross-linkers enhances tear resistance. Science 380, 1248–1252 (2023).
Channa, I. A. et al. UV Blocking and Oxygen Barrier Coatings Based on Polyvinyl Alcohol and Zinc Oxide Nanoparticles for Packaging Applications. Coatings 12, 897 (2022).
Wang, Y. C. et al. Nanolignin filled conductive hydrogel with improved mechanical, anti-freezing, UV-shielding and transparent properties for strain sensing application. Int. J. Biol. Macromol. 205, 442–451 (2022).
Bai, Y. et al. UV-shielding alginate films crosslinked with Fe3+ containing EDTA. Carbohydr. Polym. 239, 115480 (2020).
Zhu, H. et al. Stretchable and recyclable gelatin Ionogel based ionic skin with extensive temperature tolerant, self-healing, UV-shielding, and sensing capabilities. Int. J. Biol. Macromol. 244, 125417 (2023).
Nan, L. et al. Rapid fabrication of xylan-based hydrogel by graft polymerization via a dynamic lignin-Fe3+ plant catechol system. Carbohydr. Polym. 269, 118306 (2021).
Zhang, X. et al. Robust Conductive Hydrogel with Antibacterial Activity and UV-Shielding Performance. Ind. Eng. Chem. Res. 59, 17867–17875 (2020).
Si, C. Q. et al. A Polyvinyl Alcohol–Tannic Acid Gel with Exceptional Mechanical Properties and Ultraviolet Resistance. Gels 8, 751 (2022).
Lv, H. et al. Room Temperature Ca2+-Initiated Free Radical Polymerization for the Preparation of Conductive, Adhesive, Anti-freezing and UV-Blocking Hydrogels for Monitoring Human Movement. ACS Omega 8, 9434–9444 (2023).
Huang, J. B. et al. Lignin nanorods reinforced nanocomposite hydrogels with UV-shielding, anti-freezing and anti-drying applications. Ind. Crops Prod. 187, 115324 (2022).
Yang, Y. T. et al. Rapid fabricated in-situ polymerized lignin hydrogel sensor with highly adjustable mechanical properties. Int. J. Biol. Macromol. 260, 129378 (2024).
Ohzono, T. et al. Internal constraints and arrested relaxation in main-chain nematic elastomers. Nat. Commun. 12, 787 (2021).
Kim, J. et al. Fracture, fatigue, and friction of polymers in which entanglements greatly outnumber cross-links. Science 374, 212–216 (2021).
Momma, K. & Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 44, 1272–1276 (2011).
Abraham, M. et al. GROMACS: High performance molecular simulations through multi-level parallelism from laptops to supercomputers. SoftwareX 1, 19–25 (2015).
James, A. et al. ff14SB: Improving the accuracy of protein side chain and backbone parameters from ff99SB. J. Chem. Theory Comput. 11, 3696–3713 (2015).
Cui, S. Y. et al. Tannic acid-coated cellulose nanocrystal-reinforced transparent multifunctional hydrogels with UV-filtering for wearable flexible sensors. Carbohydr. Polym. 323, 121385 (2023).
Acknowledgements
We gratefully acknowledge funding support provided by the Fundamental Research Funds for the Central Universities (Grant Number 41424056) and the National Undergraduate Training Programs for Innovations (Grant Number 202310225161). We also thank Shiyanjia Lab (www.shiyanjia.com) for the SAXS characterization.
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Z. Z., formal analysis, data curation and writing original draft; D. P. and X. S., investigation; X. Z., methodology and visualization; S. R., J. P. and S. L., supervision, writing, review and editing, supervision. All authors contributed to the scientific discussion.
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Zhang, Z., Peng, D., Shang, X. et al. Ultra-tough light-curing ionogels for UV shielding. Commun Mater 5, 259 (2024). https://doi.org/10.1038/s43246-024-00702-1
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DOI: https://doi.org/10.1038/s43246-024-00702-1
