Nanocomposite Hydrogels Enhanced by Cellulose Nanocrystals Stabilized Pickering Emulsions with Self-Healing Performance in Subzero Environment

Nowadays, hydrogels as �exible materials have attracted considerable attention in frontier �elds such as wearable electronic devices, soft actuators and robotics. However, most hydrogels use water as matrix will inevitably freeze at subzero and damage in severe environment, which greatly reducing their service life and practical value. Herein, nanocomposite hydrogels with self-healing performance at subzero temperatures were proposed by introducing binary water-glycerol continuous phase and dual self-healing interactions. The e�cacy of binary solvents was emphasized in preventing formation of ice crystals, enhancing �exible and self-healing abilities of hydrogels in subzero environment. Particularly, linseed oil (LO) as healing agent was effectively loaded in Pickering droplets by cellulose nanocrystals (CNCs). Due to external healing agent and non-covalent bonding, hydrogels showed good self-healing performance at subzero temperature (the healing e�ciency could be up to 80.1% for 12 h at -20 ℃ ). Thus, the designed hydrogels demonstrated multifunctional properties to overcome adverse conditions, which greatly elevated their durability and practicality.


Introduction
As an ideal candidate for soft materials, hydrogels have caught great interest due to unique electrical, exible and tensile properties (Chen et al. 2020c; Shi et al. 2020).Varied hydrogels have developed rapidly and been universally applied in many advanced areas, such as arti cial skin (Lei and Wu 2018), wearable electronic equipment (Buenger et al. 2012), drug carrier (Song et al. 2021) and tissue engineering (Zhao et al. 2020).The most representative characteristic of hydrogels is the ability to absorb and retain large amounts of water without dissolving under three-dimensional structure (Li et al. 2016).However, conventional hydrogels rely on water as matrix, inevitably freeze at subzero temperatures, resulting in irreversibly damaged structure of polymer network (Wang et al. 2019b; Wei et al. 2020).Specially, owing to rigid aqueous phase, hydrogels lose the exible ability and could be easily destroyed, which threatens their service life and practical value.Thus, it is necessary to endow hydrogels with the performance of self-healing in subzero environment to resist external interference, which will broaden their underlying applications in a wider temperature range.
The cold resistance mechanisms in natural organisms provide wealth strategies for improving the frost resistance.Inspired by the phenomenon of increasing intracellular solute concentration to impede ice formation in natural plants, binary solvent system attracted a strong interest (Xu et al. 2020).Generally, these solvent systems involve two components with entirely opposite physiochemical properties, like water and organism (Zhang et al. 2018).Glycerol, with the merits of non-toxic and solving with water in any ratio, is commonly served as cryoprotective agent (Chen et al. 2020a;Yu et al. 2021).The strong hydrogen bonds formed between glycerol and water compete with the hydrogen bonds formed in water, thus reducing the hydrogen bond density between water molecules, which destroy ice crystal lattices and reduce freezing point (Liao et al. 2019).For instance, Yang et al. (Yang et al. 2019) designed anti-freezing hydrogels by introducing polyelectrolytes and glycerol, which could maintain excellent conductivity and self-healing properties at -20°C for 24 h.Ge et al. (Ge et al. 2020) proposed muscle-inspired hydrogel bioelectronic devices that possessed high stretchability, desirable sensitivity, durable adhesion feature and self-healing performance at low temperature, ascribing to glycerol/water binary solvent system.Thus, the anti-freezing property could be easily obtained by constructing facile and e cient waterglycerol system, contributing to preserving original performance in subzero environment.Furthermore, polymer chains within hydrogels lack uidity at low temperatures, which leading weak network to resist damages by deformation, further cripple mechanical properties and realistic applications (Lu and Chen 2020).Inspired by autonomously healing phenomenon of natural skin, selfhealing mechanisms in hydrogels have attracted numerous attentions (Lin et al. 2019;Son et al. 2018).Self-healing hydrogels exhibit excellent durability and mechanical properties that can autonomously heal damages or nonautonomously with external stimuli (Li et al. 2017b; Taylor and In Het Panhuis 2016).Self-healing mechanisms could be classi ed into extrinsic and intrinsic types according to the healing process.Extrinsic healing is realized with the participation of external healing agents, whereas intrinsic healing is based on dynamic reversible physicochemical action, like non-covalent and covalent interactions (Liu et al. 2019).Combining these two approaches can build double-healing construction, as well as obviously increase healing e ciency at low temperatures.Notable, Pickering emulsions (Wu and Ma 2016) stabilized by cellulose nanocrystals (CNCs) (Wang et al. 2020) has caught much attention, providing an effective strategy to store and release healing agent (Li et al. 2018).CNCs are considered suitable candidates as biomass solid emulsi ers, re ecting the advantages of high aspect ratio, small size, intermediate wettability, non-toxicity, biocompatibility and sustainable (Zhang et al. 2019).
Fortunately, linseed oil (LO) with attractive virtues such as non-toxic, easily manage and the ability to ll cracks by polymerization with oxygen, showing promising applications as external healing agent encapsulated in Pickering droplets (Jadhav et  Herein, the composite hydrogels with enhanced self-healing performance at low temperatures were designed through binary solvent system and dual self-healing interactions.Facile one-pot thermal method was utilized for the construction of polyacrylic acid (PAA) skeleton.LO as external healing agent was encapsulated in Pickering droplets with CNCs as solid emulsi ers.In view of CNCs and LO were all biomass materials, this strategy provided a simple and green way to load the healing agent.The introduction of glycerol ensured the characters of anti-freezing, water retention and exibility of hydrogels.Double self-healing interactions were composed of two aspects.On the one hand, LO could be released from Pickering droplets and polymerization with oxygen.The other was realized with reversible dynamic non-covalent bonding, including hydrogen bonds and the chelation of Fe 3+ .Overall, the obtained hydrogels were comprehensively characterized and showed great potentials as exible materials.Experimental Materials Acrylic acid (AA, ≥ 98.0%, M n = 72.06g mol − 1 ), iron ( ) chloride hexahydrate (FeCl 3 •6 H 2 O, ≥ 99.0%, Mw = 270.29 g mol − 1 ) and ethanedioic acid dihydrate (≥ 99.5%, Mw = 126.07g mol − 1 ) were obtained from Sinopharm.Choline chloride (≥ 98.0%, Mw = 139.62g mol − 1 ) was obtained from Macklin.Potassium peroxydisulfate (KPS, ≥ 99.5%, Mw = 270.32g mol − 1 ) was obtained from Damao Reagent.Glycerol (≥ 99.0%, Mw = 92.09g mol − 1 ) was obtained from Fuyu Chemical.LO (≥ 99.0%, 0.93 g mL − 1 ) was obtained from Greeno.FeCl 3 •6 H 2 O was dissolved in distilled water to obtain Iron ( ) solution (12.0 mg mL − 1 ).
Potassium peroxydisulfate was dissolved in distilled water to obtain KPS solution (6.6 mg mL − 1 ).The other reagents were directly used without further transforming.

Extraction of CNCs
CNCs were extracted by acid hydrolysis procedure according to our previous research (Fan et al. 2020).0.25 g of cellulose, 15.3 g of ethanedioic acid dihydrate and 9.7 g of choline chloride were added in a ask, stirring at 350 r/min at 100 ℃ for 5 h.The mixture was centrifuged by adding distilled water for about 3 times, then dialyzed and freeze-dried.
Preparation of Pickering emulsions 0.02 g of CNCs and 6.0 mL of distilled water was added in a vial and ultrasonic for 10 minutes to obtain milky white CNCs suspension.Then, 150.0 µL of LO was added and stirring at 8000 r/min for 3 minutes to obtain Pickering emulsions.
Synthesis of hydrogels 4.0 mL of AA, 5.0 mL of KPS solution, 1.0 mL of Iron ( ) solution, different doses of Pickering emulsions and glycerol were added in a vial, then ultrasonic for 10 minutes to obtain uniformly dispersed mixture.The mixture was polymerized at 65 ℃ with stirring at 350 r/min for 15 minutes.Hydrogels were formed in a mold at 40 ℃ for a period of time.The hydrogels were marked as HG-P x G y , where x represented the content of Pickering emulsions and y represented the content of glycerol.

Evaluation of water retention and swelling abilities
The hydrogels were molded in circle shape (Φ = 2.5 cm), then cut in average sections for measurements.
To explore water retention capacity, the samples were kept at 25, 45 and 65 ℃ for 7 days.The reduction of water content in hydrogels was re ected according to the following formula: W t /W 0 (%), where W t was the weight at t time and W 0 was the original weight.In addition, the same size hydrogels were immersed in distilled water (pH = 6.5) at 25 ℃, then the samples were taken out and dried by lter paper to weight every 20 minutes.The increment of water content in hydrogels was calculated by the following equation: W/W 0 (%), where W was the change of weight and W 0 was the original weight.

Evaluation of self-healing performance at subzero temperature
The subzero temperature of -20 ℃ was provided by a low constant temperature bath (DC-2006, SHP, -20 ~ 95 ℃).The self-healing ability was performed on Stress(σ)-Strain(ε) curves supported by tensile machine (LDW, Songdun).The regular dumbbell-shaped samples were cut in half and healed at -20 ℃ without external stimuli, then immediately used for testing at room temperature.With different healed time and different doses of Pickering emulsions, the healed samples were stretched at a uniform speed of 50 mm/min.Additionally, the self-healing e ciency was calculated by σ/σ 0 , where σ was the fracture stress of healed hydrogels and σ 0 was the fracture stress of initial hydrogels.

Characterization and methods
The morphology of CNCs was obtained from transmission electron microscope (TEM, Thermo Fisher Scienti c Talos F200X G2) and scanning electron microscope (SEM, Hitichi SU-8010).The crosssectional of hydrogels were performed on SEM.Surface charge of CNCs was tested via zeta potential measurement (Zetasizer Nano ZS90, Britain Malvern).The structural information was conducted on Fourier transform infrared (FT-IR, Nicolet500) and x-ray photoelectron spectroscopy (XPS, Escalab Xi+).
The UV-vis spectra were recorded in the 190-500 nm wavelength range using SolidSpec-3700 at room temperature.The healing process of hydrogels and morphology of emulsions were recorded by optical microscope (DMM-300C).Rheological curves were recorded by rheometer (TA DFR-2).The freezing points of hydrogels were investigated by differential scanning calorimeter (DSC, Netzsch 204 F1).

Results And Discussion
General preparation process of Pickering emulsions was to disperse solid particles in water phase, and then added oil phase to form emulsions under high-speed mixing.The schematic of CNCs-stabilized Pickering emulsions was illustrated in Scheme 1a.At present, recognized stabilization mechanism of Pickering emulsions mainly concentrated on solid particles adsorbing at the oil-water interface and forming monolayer or multilayer lm.The existence of interfacial layers could effectively prevent droplets aggregation and condensation (Low et al. 2020).
The emulsifying e ciency was affected by many factors, which mainly depended on the properties of solid emulsi er.With numerous active groups and high aspect ratio, CNCs can form the particle layer and re ect a nity for both phases, which enable droplets preserved during polymerization process ( From TEM and SEM images (Fig. 1a, Fig. S1), CNCs were rod-like particles with the length of 187.0 ± 37.4 nm and diameter of 17.6 ± 4.0 nm.From FT-IR spectra (Fig. S2), typical characteristic peaks of cellulose at 3340 cm − 1 , 2900 cm − 1 , 1320 cm observed, which attributing to carboxyl group (Ling et al. 2018).This peak indicated that weak esteri cation occurred between carboxyl groups in cellulose chains and oxalic acid (Li et al. 2020).Also, this was con rmed by XPS method.The wide scan spectra of cellulose and CNCs were shown in Fig. S3a.The carbon signal appeared at 286.5 eV, and the oxygen signal appeared at 532.8 eV.In Fig. S3b, the peaks at 286.5, 284.8, 289.Then, CNCs suspension was used to prepare Pickering emulsions (Fig. 1b).After adding LO and under high-speed homogenization, the creamy O/W emulsions were synthesized (Fig. S5).Even after leaving at room temperature for 1 day, no obvious grease was found in the stable system.On the other hand, to further study the effect of CNCs content and oil-water ratio on emulsions stability, different types of Pickering emulsions were observed via optical microscope images (Fig. S6).In a certain content of CNCs (0.33 wt%), the size of Pickering droplets has increased with oil-water ratio raised.The same phenomenon occurred when the content of CNCs reduced from 0.50 to 0.17 wt% at the oil-water ratio of 1:40.The results showed that the size of droplets was related to increasing oil-water ratio or reducing CNCs contents, ascribing to small amounts of particles covering the surface of LO to form dense interface layers, which leaving the droplets in an unstable state (Wang et al. 2019a).
Rheology tests were further applied to investigate the stability of emulsions (Farias et al. 2020; Maestro et al. 2020).With frequency sweep data increased from 0.1 to 200 rad/s, both of the values of modulus improved, which manifested non-Newtonian uidlike behavior (Fig. 1c) (Alam et al. 2008).With strain sweep data from 1 to 1000%, two curves showed a downward trend and G′ was in a greater range (Fig. 1d).Before the intersection, G′ was greater than G′′, suggesting the emulsions were in a stable geltype structure.When strain continued to increase, gel structure was destroyed and the emulsions were in an unstable liquid state.The point where two curves intersected was at 60%, which represented the maximum strain value that the emulsions could bear (Debeli et al. 2020).Besides, the values of G′ and G′′ showed monotonicity over time (Fig. 1e).G′ (0.12 Pa) still dominated over G′′ (0.03 Pa) from 0 to 200 s, manifesting the emulsions could remain stable for a long time (Liu et al. 2017).
Next, hydrogels were synthesized through facile and simple one-pot method (Scheme.1b).A certain percentage of Pickering emulsions, Iron ( ) solution, KPS solution, glycerol and AA were added in the vial and sonicated to obtain well-dispersed mixture.Then, the polymerization process was completed during 65 ℃ with stirring for 15 minutes.Poly (acrylic acid) (PAA) networks were constructed by conventional free radical polymerization of acrylic acid in aqueous system (Fernandes et al. 2015), which was composed of water as matrix, AA as monomer and KPS as initiator.The polymerization produced strong chemical cross-linked backbone, while Fe 3+ further developed the network through physical crosslinking (Wei et al. 2013).As shown in Fig. S7, the transformation process from solution to gel was well visualized.At the beginning of the reaction, the system was in a uid state.After the reaction, the hydrogels with stable and uniform state were obtained.
In order to analyze the internal structure and surface morphology, hydrogels were freeze-dried and observed by SEM.As shown in Fig. 2a and Fig. S8, compared with HG-P 0 G 0 , HG-P 1.0 G 0 demonstrated porous three-dimensional networks with uniform apertures of 21.3 ± 5.4 µm.This was owing to noncovalent bonding interaction between Pickering droplets and PAA chains, which increased entanglement density within hydrogels.Obviously, HG-P 1.0 G 1.0 exhibited a more rm and smooth structure with the smaller apertures of 8.9 ± 2.1 µm (Fig. 2b).The introduction of glycerol provided abundant hydroxyl groups, contributed to improve the hydrogen bonds and interaction between polymer chains.The uniform pores with smaller sizes signi ed that hydrogels possessed dense networks, which leading to higher mechanical properties (Lin et al. 2021).
The porous three-dimensional network endowed hydrogels with the abilities to absorb and retain a certain amount of water.Figure 2c showed the swelling kinetics curves of hydrogels with different content of Pickering emulsions immersed in distilled water at room temperature.The swelling rate increased rapidly in the rst 120 minutes, on account of substantial water molecules were quickly ll internal porous channels.After that, the swelling rate slowed down until it reached dynamic equilibrium.At the same immersion time, water absorption was related to emulsions content.As Pickering emulsions increased from 0.5 to 1.0 and 1.5 mL, the water absorption changed from 269.4-234.1% and 192.1%, respectively.These results demonstrated that the denser and stable structures within hydrogels were enhanced by Pickering droplets, which was consistent with the SEM and mechanical results.The tenacious internal network prevented the structures from collapsing due to excessive water absorption.Besides, water retention kinetics curves of hydrogels at 25, 45 and 65 ℃ were shown in Fig. 2d.At 45°C and 65°C, the water content dropped rapidly within the rst two days.The water retention was related to temperatures that high temperature accelerated water evaporation.After 7 days, the weight retention ratio at 25, 45 and 65 ℃ was 86.2%, 70.7% and 65.1%, respectively.By comparison, HG-P 1.0 G 0 could only maintain 57.2% of their original weight.The water content determined uidity of polymer chains further exibility of hydrogels, and glycerol was underscored as a crucial factor that retarded evaporation process.
To verify the low-temperature tolerance performance, the freezing points of hydrogels were obtained from DSC thermograms in Fig. 2e.Interestingly, HG-P 1.0 G 0 showed a sharp exothermic peak at -17 ℃, which was far below the freezing point of pure water.Because PAA chains provided a wide platform to form hydrogen bonds with water molecules, inhibiting the formation of ice crystals (Deller et al. 2014).After introducing glycerol, freezing point of HG-P 1.0 G 1.0 was depressed at -40 ℃ as well as the exothermic peak became smaller and broader, which intuitively proved that binary water-glycerol system bestowed hydrogels with freezing tolerance attribute (Lin et al. 2021).Additionally, the anti-freezing performance were characterized by optical photographs in Fig. 2f.All hydrogels presented exibility that could be easily bent at 25 ℃.After being placed at -20 ℃ for 24 hours, HG-P 1.0 G 0 were in a frozen solid-like state that could not be bent.Inversely, HG-P 1.0 G 1.0 showed good anti-freezing ability that no ice area was observed and still could be bent.
The hydrogels possessed excellent self-healing performance depended on double healing interactions (Scheme 1c).PAA chains with rich active carboxyl groups on building the three-dimensional network structure and providing connection sites.When hydrogels were cracked, the healing process was carried out by reversible noncovalent interactions, including vigorous hydrogen bonding and metal ionscoordination.Hydrogen-bond interactions were widely existed between oxygen-containing groups of Pickering droplets, PAA, water and glycerol.Self-healing mechanisms based on hydrogen bonds have been widely reported (Chen et  Fe 3+ was con rmed by UV-vis.As shown in Fig. S9a, AA solution showed a characteristic peak at 195 nm. After introducing Fe 3+ , the characteristic peak was shifted to 203 nm, which indicated the chelation interactions existed between Fe 3+ and hydroxyl groups (Rao et al. 2016).
Bene ting from LO broken from Pickering droplets, the self-healing e ciency was further enhanced.LO as healing agent was encapsulated in microcapsules has been widely used in self-healing coating (Lang and Zhou 2017).This was because it contained unsaturated fatty acids, which provided lots of C = C bonds.In the presence of oxygen, LO could rapidly oxidized to form a solid lm and ll the cracks (Suryanarayana et al. 2008).The ability of LO as external healing regent was con rmed by FT-IR spectra (Fig. S9b).The fresh LO had an absorption peak at the wavelength of 1652 cm − 1 , attributing to the transitions of C = C bonds.For the LO/PAA hydrogels, this characteristic peak disappeared, suggesting that LO polymerized with oxygen in hydrogel matrix (Fan et al. 2019).The release of LO from Pickering droplets were recorded by UV-vis spectra.Firstly, absorbance versus concentration calibration curve of LO dissolved in petroleum ether was obtained (Fig. S9c).In Fig. S9d, LO showed characteristic peaks in ultraviolet region.The concentrations of LO were calculated as 2.64×10 − 6 , 1.65×10 − 5 and 1.98×10 − 5 mol/L at the time of 4, 8 and 12 h, respectively, which exhibited a slowly release process within hydrogels.
These results indicated that LO could be used as healing regent as well as effectively encapsulated in Pickering droplets.
The self-healing properties of hydrogels in subzero environment were demonstrated via typical stressstrain curves.It was well known that the shape and size of the samples in uenced the tensile data, hydrogels were made strictly into the standard templates (Fig. 3a).The effects of Pickering emulsions and healing time on mechanical abilities and self-healing e ciency were proved in Fig. 3b-d.With increasing Pickering emulsions contents of 0, 0.5, 1.0 and 1.5 mL, the corresponding healing e ciency was 57.1, 71.0, 80.1 and 76.9% for 12 h.Similarly, the value of stress also showed a trend of increasing then decreasing.With addition of certain contents of Pickering droplets, polymer chains within hydrogels entangled tightly and caused denser structure, leading to higher mechanical properties and healing e ciency.However, excess Pickering droplets prevented the construction of hydrogen bonds between polymer chains, and the presence of polymer lms at cracks may reduce the effect of reversible noncovalent bonding, resulting in a loose network structure.Besides, as the healing time were 4, 8 and 12 h, the corresponding healing e ciency were 21.3, 44.9 and 80.1%.The e ciency was noticeably improved with healing time increased, which implying more non-covalent bonds were rearranged.The strain of original HG-P 1.0 G 1.0 was 1900% and stress was 0.24 MPa at -20 ℃, which also re ected reliable stretchable and anti-freezing characters.For comparison, the healing performance was assessed at room temperature that hydrogels were cut and self-healing for 12 h.The self-healing e ciency of HG-P 0 G 1.0 and HG-P 1.0 G 1.0 were 64.7% and 90.0%, respectively (Fig. S10).These results were higher than those at -20 ℃, because the uidity of polymer chains and active groups at high temperatures facilitated the healing process.
Furthermore, the self-healing ability and energy dissipation process were evaluated by loading-unloading stress-strain curves under 100% strain for ten successive cycles without waiting (Fig. 3e-f).The hysteresis loops revealed larger areas during the 1st to 3rd process, which indicated that energy consumption was mainly in the previous cycles.Because of the broken bonds within healed hydrogels were partially restored, the stress value was lower than original hydrogels (Ma et al. 2020).Even after ten successive cycles, the value of stress remained at 69.5% as compared with the rst cycle.These results showed that the hydrogels possessed anti-fatigue properties, which rose from the double self-healing interactions.
Macroscopically, the self-healing ability and mechanical properties of hydrogels were characterized by optical photographs.The hydrogels were dyed by Prussian blue and Rhodamine B, respectively.In Fig. 3g, two different colors of hydrogels were molded in circle shapes (Φ = 2.5 cm), then, cut in half and healed at -20 ℃ for stretching.The samples could be stretched to a certain length.In a same way, the dyed hydrogels were cut into standard dumbbell shapes and broken down in the middle.Different color sections were stuck together and healed at -20 ℃.The healed samples could be stretched out by 20 cm and withstand up to a weight of 200 g (Fig. 3h-i), due to the tight healing layer formed at cracks.To further con rm this, linear cracks were scratched on the surface of hydrogels and then exposed to -20 ℃ environment, while the morphology of healing areas were recorded by microscopic images.It was found that all of the separate sections gradually merged closely, as well as the color of scratches became lighter and size became smaller.The traces were clearly observed on HG-P 0 G 1.0 at healing time of 0, 1 h, 4 h and 8 h (Fig. S11a-d).With the participation of Pickering droplets, HG-P 1.0 G 1.0 showed a markedly enhanced healing process that the cracks disappeared on the surface after 8 hours (Fig. S11e-h).All the results demonstrated brilliant self-healing and exibility performance of nanocomposite hydrogels in subzero environment.

Conclusions
In summary, the durable nanocomposite hydrogels were successfully synthesized by an ingenious strategy, which templating on Pickering emulsions and binary water-glycerol system.The obtained hydrogels showed attractive stretchable, anti-drying, anti-freezing and self-healing characters.Highly selfhealing e ciency pro ted from LO loaded in Pickering droplets and noncovalent bonding (metal ionscoordination and vigorous hydrogen bonds).Glycerol strongly locked water molecules and prevented the formation of ice crystals, played an active role in stretchable, moisturizing and anti-freezing.It was worth noting that the hydrogels could achieve 80.1% healing e ciency without external interventions at -20 ℃, meanwhile, the excellent mechanical properties (strain of 1900%, stress of 0.24 MPa) were preserved.Thus, we believe that these hydrogels possessed reliable durability and practicality to acclimatize for long service life, exhibiting great potentials in widespread applications as exible materials.

Declarations Figures
− 1 and 1050 cm − 1 were O-H stretching, C-H stretching, O-H bending and C-O-C pyranose ring stretching vibration, respectively (Hong et al. 2020b; Jordan et al. 2019).The spectra of cellulose and CNCs showed similar peaks, but a new slight peak located at 1730 cm − 1 was 5 and 288.1 eV corresponding to C-OH of alcohols, C-C/ C-H linkages, O-C = O of ester carbon and O-C-O, respectively (Oliveira et al. 2016).Additionally, from Fig.S4, CNCs exhibited a relatively negative zeta potential at -27.3 mV, because of the presence of the carboxyl groups on the surfaces.These results demonstrated CNCs with attractive morphology and negative charged surface were suitable to be used as stabilizer for Pickering emulsions.

Figure 4 (
Figure 4 al. 2011; Navarchian et al. 2019).Recently, studies on preparing self-healing materials based on LO as healing reagent in Pickering droplets have been reported.Li et al. (Li et al. 2019) designed Poly(urea − formaldehyde)/SiO 2 hybrid microcapsules to load LO by in (Chen et al. 2020b;Hong et al. 2020aobtained, DESs method was used in the extraction and modi cation of CNCs.In this way, CNCs were modi ed with hydrophilic carboxyl groups, resulting in satisfactory dispersion.Meanwhile, the existence of active carboxyl groups could improve the hydrophilicity of CNCs, contributing to reduce surface energy of the phases(Chen et al. 2020b;Hong et al. 2020a).
Jiang et  al. 2020).The quality of CNCs were rstly characterized and discussed.Deep eutectic solvents (DESs), composited of hydrogen bond donors and hydrogen bond acceptors in a certain ratio, has been used as hydrolytic medium to degrade the amorphous regions in cellulose(Douard etal.2021; Sirviö et al. 2015).
al. 2018; Dai et al. 2015; Phadke et al. 2012).For example, Li et.al prepared double-network hydrogels composed of PEG and PVA, while the self-healing ability resulted from hydrogen bonding between the hydroxyl side groups (Li et al. 2015).And the coordinate interactions of