Nanodiamond reinforced self-healing and transparent poly(urethane–urea) protective coating for scratch resistance

ABSTRACT With increasing demand for scratch-resistant flexible electronics, the development of transparent coatings with good scratch resistance and self-healing properties has emerged as a key research topic. In this study, a high-strength self-healing poly(urethane – urea) (PUU)-based nanocomposite coating was prepared by introducing functionalized nanodiamond (ND) into a PUU matrix via solution blending. The PUU matrix had hard-segment repeating units and was constructed using isophorone diamine and isophorone isocyanate. The ND particles were modified using a silane coupling agent, 3-aminopropyltriethoxysilane, to obtain well-dispersed KH-ND nanoparticles. KH-ND promoted microphase separation in the PU matrix, inducing the formation of dense and homogeneous hard domains that dissipated stress, prevented further crack development, and improved the mechanical properties and scratch resistance of the coating. In addition, the coating exhibited excellent self-healing properties. Fourier-transform infrared spectroscopy, scanning electron microscopy, and atomic force microscopy were used to characterize the self-healing and hardening mechanisms of the coating. The environmentally friendly PUU/KH-ND coating is easy to prepare and has broad application prospects in transparent and anti-scratch coatings for flexible electronics, automobiles, and home appliances. Graphical Abstract


Introduction
Flexible electronics, as a technology to assemble electronic circuits on flexible plastic substrates, have been developed rapidly with the advent of 5 G communication.Transparent organic polymer materials (i.e.transparent plastics), including polymethyl methacrylate (PMMA), polystyrene, and polycarbonate, have become indispensable owing to their optical transparency, light weight, and mass producibility [1,2].Transparent plastics are integral in manufacturing various high-tech consumer products, including display screens, windscreens, lenses, and optical components.However, their practical applications are hindered by their low surface hardness and susceptibility to scratching [3,4] Various methods have been employed to increase the surface hardness without compromising the intrinsic properties of these plastics.A particularly effective approach is the application of protective coatings [5,6].Such coatings play a crucial role in guarding against scratches, thereby extending the performance of the underlying material.Consequently, the coating must have high scratch resistance to ensure its durability.Previous research has led to the development of flexible and hard protective coatings that exhibit impressive characteristics.For example, Liu and coworkers [7][8][9][10] developed a series of coatings through the graft modification of polyhedral oligomeric silsesquioxane and photopolymerization.The coatings had good bendability (bending radius: <2 mm), remarkable hardness (nanoindentation hardness: 1.4 GPa, pencil hardness: >9 H), high light transmittance (>95%), and good oil/water repellency, providing a guideline for the production of coatings with improved omniphobicity and wear resistance.
In addition, when accidentally scratched by sharp objects, the coating with self-healing ability can heal the scratches and prolong the service life.Among various self-healing materials, transparent coatings capable of healing scratches is still in its infancy, despite their importance as protective materials in optical and display devices [11].The desirable self-healing/healable transparency coating requires optical transparency, scratch resistance, and the ability to repair severe damage, such as cuts and scratches in depth and width.Polyurethane (PU) synthesized through the addition polymerization of a polyol, polyfunctional isocyanate, and chain extender, shows promise in this domain owing to its microstructural inhomogeneity, toughness, and wear resistance [12,13].Xu et al. [14] demonstrated a PU coating with room-temperature self-healing capabilities based on multiphase hydrogen-bond dissociation and rapid restructuring.Notably, the coating was able to repair scratching damage multiple times.In addition, Liu et al. [15] recently proposed a PU-based material with biomimetic protective features and self-healing ability.Nevertheless, these coatings fall short in terms of hardness, making them susceptible to scratching and other forms of physical damage.
Efforts have been made to create intrinsically healable materials with enhanced mechanical properties by introducing hard components into the material [16,17].Incorporating nanofillers into polymer materials has emerged as an effective strategy [18,19].For example, Sun et al. [20] introduced CaCO 3 nanoparticles into polyelectrolyte multilayer films assembled from polyacrylic acid and polyethyleneimine and demonstrated that the nanoparticles made the healable films stronger and more scratchresistant to scratches.Similarly, Liang et al. [21] reported a self-healing and antifogging coating based on a polyzwitterion copolymer, wherein the addition of silica nanoparticles improved the wear resistance of the coating.Nevertheless, although the films in the above studies had high hardness and self-healing ability, the repair process required an aqueous environment, which is inconvenient for practical applications.Moreover, adding high concentrations of nanoparticles to increase the hardness can compromise the transparency of the coating and its dynamic healing process based on molecular chain movement [22,23].Therefore, fabricating highly transparent coatings with excellent scratch resistance and self-healing properties remains challenging.
Nanodiamond (ND) has emerged as a valuable nanomaterial owing to its high hardness, superior abrasion resistance, excellent Young's modulus, chemical stability, spherical nanomorphology, and moderate cost [24] Introducing a small amount of ND into a PU matrix could notably enhance the mechanical properties and scratch resistance of the composite coating.ND could prompt the separation of PU microphases, creating more hard domains to dissipate stress and preventing further expansion of scratches.On one hand, the highly dispersed ND with large specific surface area attracted a large number of polymeric molecules to its surface., increasing the physical cross-linking points between PU molecules and forming organic/inorganic composite hard domains.On the other hand, ND treated with a silane coupling agent could introduce a large number of amine groups to its surface.These groups could then generate hydrogen bonds with the carbonyl oxygens and amide groups in PU, further enhancing the microphase separation of PU.
In this study, we modified ND with a silane coupling agent, 3-aminopropyltriethoxysilane (KH-550), to obtain well-dispersed KH-ND nanoparticles.We then modified the PU structure by substituting the conventional hydroxy-capped chain extenders with aminocapped isophorone diamine (IPDA).This created a high-strength self-healing poly (urethane -urea) (PUU) matrix with repeating aliphatic ring units.Different concentrations of KH-ND nanoparticles were incorporated into the PUU matrix via solution blending, resulting in a series of scratch-resistant and self-healing PUU/KH-ND nanocomposite coatings.We evaluated the mechanical and self-healing properties of the coatings through tensile, hardness, scratch, and adhesion tests.The self-healing ability of the coatings was primarily attributed to the dynamic dissociation and reassociation of hydrogen bonds.The introduction of KH-ND promoted microphase separation in the PUU matrix and facilitated the formation of dense hard domains via organic/inorganic recombination and hydrogen bonding.This significantly enhanced the mechanical properties and scratch resistance.Importantly, owing to the considerable increase in hardness at low nanoparticle loadings (0.1-1.0 wt%), the transparency and self-healing ability of the coating were not significantly deteriorated.Consequently, this approach overcomes the tradeoff between hardness, transparency, and self-healing ability.The PUU/KH-ND coating demonstrated high hardness, scratch resistance, strong adhesion, self-healing ability, and optical transparency, demonstrating its potential for broad applications in flexible electronics, automobiles, and home appliances.

Preparation of KH-550-modified nanodiamond (KH-ND)
ND was subjected to oxidation -reduction treatment prior to functionalization with KH-550 (Scheme 1).Oxidation converted most of the hydroxyl, carboxyl, carbonyl, and methylene surface groups into hydroxyl and carboxyl groups, and reduction converted the carboxyl groups to hydroxyl groups, thereby facilitating subsequent silanization using KH-550 [25,26].
For ND oxidation, first, 1.0 g of ND was dispersed in a hydrochloric acid -nitric acid mixture (3:1 v/v) and stirred at room temperature (RT) for three days.The product was diluted with 150 mL of deionized water and centrifuged at 7000 rpm for 10 min, then the ND precipitate was washed with deionized water.Subsequently, the ND precipitate was treated with 0.1 mol/mL sodium hydroxide Scheme 1. Synthesis of KH-ND.
for 2 h at 90°C with stirring, followed by diluting, centrifuging, and rinsing as above.The ND precipitate was further treated with 0.1 mol/mL hydrochloric acid at 90°C for 2 h with stirring and then washed with deionized water until the mixture became weakly acidic.Finally, the ND precipitate was dried under vacuum at 60°C for 24 h to obtain oxidized ND.
Next, for ND reduction, oxidized ND (0.5 g) was dispersed in THF by ultrasonication in an ice-water bath, then 0.8 g of LiAlH 4 was added to the system and refluxed for two days.THF was removed by vacuum drying, and the unreacted LiAlH 4 was hydrolyzed by adding deionized water and subsequently soaking in 0.1 mol/mL hydrochloric acid for 2 h.The mixture was diluted, centrifuged, and rinsed as above, then dried under vacuum at 60°C for 24 h to obtain reduced ND with enriched hydroxyl surface groups.
Finally, for KH-550 functionalization, KH-550, anhydrous ethanol, and deionized water were mixed in a 20:72:8 volume ratio to form a uniform solution.Then, 0.3 g of the reduced ND was dispersed into 30 mL of this solution by ultrasonication in an ice-water bath, followed by heating to 80°C and stirring for 2 h.Subsequently, the mixture was centrifuged and rinsed three times with anhydrous ethanol, and the product was dried under vacuum at 60°C to obtain KH-ND.

Preparation of PUU matrix for scratch-resistant self-healing coatings
The PUU matrix for the scratch-resistant self-healing coatings was prepared as follows Scheme 2. First, PTMEG (20.0 g) was heated in a three-necked flask under vacuum at 120°C for 1 h to remove residual water.Subsequently, IPDI (5.2 g) and Scheme 2. Synthesis of self-healing PUU.
DBTDL (0.01 g) were dissolved in 20 mL of DMF and added to the flask.The mixture was stirred for 2 h at 80°C under a N 2 atmosphere to obtain the prepolymer, then the flask was cooled naturally to RT.Subsequently, the flask was placed in an icewater bath to prevent a violent reaction, and 50 mL of DMF and 2.13 g of IPDA were slowly added to the flask.Finally, the mixture was heated at 65°C for 12 h to obtain a high-strength PUU solution.
The asymmetric aliphatic ring structures of IPDI and IPDA form chain structures with loose hard segments, which are beneficial for preventing crystallization and ensuring the transparency of the coating.Meanwhile, IPDI and IPDA form chains with large numbers of repeating units and a continuous array of hydrogen bonds that provide stable supramolecular interactions.In addition, the soft segments provided by PTMEG, which have strong order, not only increase the binding energy between molecular chains but also provide carriers for the formation of hydrogen bonds.Benefiting from the combination of these organic structures, the prepared PUU has excellent mechanical strength and good self-healing efficiency and is therefore appropriate for the preparation of scratch-resistant and transparent selfhealing coatings.

Characterization
Bulk samples were characterized by Fourier-transform infrared (FTIR) spectroscopy in attenuated total reflectance (ATR) mode on a Bruker ALPHA II spectrometer (Germany).Variable-temperature FTIR was performed using a Nicolet 6700 spectrometer equipped with a thermo-controller.Scanning electron microscopy (SEM) was conducted using a high-resolution microscope (ZEISS G300).The morphologies of the ND and KH-ND nanoparticles were observed using transmission electron microscopy (TEM; FEI Talos-S).Transmittance spectra were recorded on an ultraviolet -visible (UV -vis) spectrophotometer with the substrate as a reference.The phase separation structure of the coatings was evaluated by atomic force microscopy (AFM; Bruker NT-MDT Prima).

Mechanical and self-healing tests
Tensile tests were performed using a universal electronic tensile testing machine (Instron 5944, U.S.A.) at a strain rate of 50 mm/min at RT. Dog-bone-shaped specimens with dimensions of 20 × 3 × 0.3 mm (L × W × H) were obtained using a sheet punching machine.The self-healing test involved cutting the samples with a scalpel or scissors, tightly joining the pieces, and self-healing them in air at 80°C for 12 h.Toughness is one of the most important mechanical properties for coatings; therefore, the self-healing efficiency (η) was defined as the recovery in toughness (calculated by integrating the stressstrain curve), and η* represents the self-healing efficiency of the sample after three cutrepair cycles.

Hardness tests
Nanoindentation tests were performed on a nanoindenter (CSM NHT-2) with a diamond Berkovich tip.The load was increased linearly from 0 to 10 mN at a constant rate of 20 mN/ min, held for 5 s at the highest load, and then reduced to 0 mN at the same rate.The pencil hardness of the coatings on glass sheets was determined according to GB/T6739-2006.

Micro-scratch tests
Micro-scratch tests were performed on a microindenter (G200, Anton Paar MST, Switzerland) with a diamond Rockwell tip (100 μm) according to ASTM D7027-2013.The scratch load was increased linearly from 30 mN to 15 N at a constant rate of 1 mm/ min.At least three tests were performed for each sample.

Adhesion tests
The adhesion of each coating was measured using a pull-out adhesion tester (PosiTest, Model AT-A) according to ASTM D4541-09.The test area was circular with a diameter of 20 mm, and the pull rate was constant at 0.2 MPa/s.Each sample was tested at nine different locations and the average value was reported.The test samples comprised coatings on PMMA, inorganic glass, and 50# steel plate substrates.

Synthesis and characterization of functionalized ND
In this study, the as-received ND particles underwent three steps of modification (Scheme 1).First, ND was oxidized to convert various surface groups to carboxyl groups.Subsequently, the carboxylic groups on the ND surface were reduced to hydroxyl groups that could readily bind to the silane coupling agent for further functionalization.Finally, KH-550 was reacted with the hydroxyl groups on the ND surface to generate KH-ND.The successful functionalization of the ND particles was confirmed using various analytical methods.The FTIR spectrum of the bare ND particles (Figure 1(a)) contained a broad peak at 3435 cm −1 , which was attributed to −OH groups, as well as peaks at 2929 and 2857 cm −1 , which were assigned to C -H bonds [27].The FTIR spectrum of KH-ND also contained these characteristic Dispersions of ND and KH-ND in DMF (0.5 mg/mL) are shown in Figure 1(b).Both ND and KH-ND were well-dispersed in DMF after 1 h of ultrasonication and no significant separation occurred after standing for 10 min.However, after standing for 3 h at RT, the ND particles had completely settled, leaving a transparent DMF layer on top of a sediment layer, whereas the KH-ND dispersion did not show noticeable settlement.These observations suggest that the KH-550-functionalized ND particles exhibited better stability and less aggregation, which is consistent with previous reports [28].The morphologies of the ND and KH-ND particles were observed by TEM, which further confirmed the dispersion states before and after functionalization [29].As shown in Figure 2(a), the ND particles prepared by the explosion method exhibited a strong tendency to aggregate.This aggregation tendency hinders the practical application of ND.In contrast, the KH-ND particles existed as isolated nanoparticles with diameters of 10-20 nm, as shown in Figure 2(b), indicating the success of functionalization [30].Notably, smaller nanoparticles have larger specific surface areas, which increases the potential contact sites with the polymer matrix and thus induces stronger interactions [31].

Synthesis and mechanical properties of PUU/KH-ND coatings
KH-ND was uniformly dispersed in the PUU matrix by solution blending, which improved the dispersion of the nanoparticles in the bulk polymer [32].First, the KH-ND particles were sonicated and dispersed in DMF for 30 min in an ice-water bath to prepare a welldispersed 0.1 mg/mL nanoparticle suspension.Next, the suspension was added dropwise to the PUU solution under vigorous stirring.The composite coating was prepared by a solution-casting method.The polymer solution was applied to a substrate (PMMA panel) with a dropper, followed by drying at 80°C for 3 h to obtain a coating.The thickness of the dry coating was approximately 200 μm.According to the KH-ND content in the polymer solution (0, 0.1, 0.5, and 1.0 wt%), the coatings were denoted as PUU, KH-ND × 0.1%, KH-ND × 0.5%, and KH-ND × 1.0%, respectively.
The influence of the nanoparticle content on the mechanical properties of the composite coatings was evaluated by tensile testing using the indices of ultimate tensile strength (σ), elongation at break (ε), self-healing efficiency (η), and self-healing efficiency after three cut -repair cycles (η*) [33].As shown in Figure 3(a) and Table 1, the composite coatings exhibited high σ and ε values.With increasing KH-ND content, σ increased from 32.8 MPa for PUU to 44.3 MPa for KH-ND × 1.0%, whereas ε decreased from 874% for PUU to 598% for KH-ND × 1.0%.The improvement in σ is attributed to the good dispersion of inorganic nanoparticles with a high specific surface area.Large numbers of polymer chains wrap around the inorganic nanoparticles and generate strong interactions [34,35].Furthermore, the abundant amine groups on the surface of KH-ND lead to the formation of hard -hard segment hydrogen bonds with the carbonyl oxygens and amide groups of PUU, further improving the microphase separation.These hard domains effectively dissipate stresses and prevent crack expansion, as discussed later.However, the increase in cross-linking upon increasing the KH-ND content reduces the mobility and stretchability of the molecular chains, resulting in reduced elasticity.Consequently, KH-ND × 1.0% has high strength (σ > 44 MPa) but low elasticity (ε < 600%) (Table 1).
Coatings are designed to protect the substrate from corrosion and physical damage, and self-healing coatings can improve the protection effectiveness and extend the service life.Owing to the dynamic reversibility of its hydrogen bonds, PUU is expected to have high self-healing ability.As shown in Figure 3(b), a PUU sample could be stretched by over 800% after cutting and self-healing for 12 h at 80°C. Figure 3(a) and Table 1 show the  stress -strain curves and σ and η values, respectively, of the original and cut -healed specimens.The pure PUU sample had a high self-healing efficiency and achieved almost complete self-repair.In contrast, the nanoparticle-containing samples had lower η values, indicating that the addition of KH-ND decreased the motility of the molecular chains and the contact between hydrogen bond pairs.However, the KH-ND × 0.5% coating still showed considerable self-healing capacity (η = 88.5%).In addition, it had a promising multi-repair capability, as η* remained greater than 80% after three cut -repair cycles.
The main driving force for the self-healing process of PUU comes from the successive arrays of loose hard-segment hydrogen bonds.The mobility of the molecular chains increases significantly upon heating, enabling the molecular chains in contact at the cut surfaces to re-form numerous hydrogen bonds over time.Thus, the self-healing process is guided by the reassociation of hydrogen bonds to connect the separated parts [36].Variable-temperature FTIR was performed on the synthesized elastomers to verify the effect of temperature on the hydrogen bonding behavior.Figure 4(a) shows the changes in the FTIR spectra as the temperature was increased from 40 to 90°C at 10°C intervals, and Figures 4(b)-(d) show enlargements of different parts of the curves.A pronounced redshift in the characteristic peaks was observed with increasing temperature.The peaks intensity of ν (C=O) amide I (1725 cm −1 ), ν (C-N) + δ (N -H) amide II (1502 cm −1 ), and ν (C-N) + δ(N -H) amide III (1220 cm −1 ) gradually increased in intensity, while the corresponding hydrogen-bonding peaks (at 1700, 1560, and 1235 cm −1 , respectively) gradually decreased in intensity [37].These intensity changes suggest that the association -dissociation of hydrogen bonds occurred within the testing temperature.As the temperature increased, the electron movement became dynamic, which resulted in a potential difference at the hydrogen bonding sites.Consequently, the C=O stretching vibrations were no longer restricted by hydrogen bonds, thereby increasing the vibration strength [38].

Scratch resistance of PUU/KH-ND coatings
Surface hardness is an important indicator for the scratch resistance of a coating [39].The hardness of the PUU coatings with different nanoparticle contents was investigated by nanoindentation (Figure 5) and pencil-hardness testing (Table 2).As shown in Figure 5(a), the nanoindenter penetrated deeper into the coating as the load increased, and the displacement decreased immediately after unloading, demonstrating the excellent elasticity of the coating.The maximum indentation depth of the pure PUU coating was approximately 10 μm.By comparison, the nanoparticle-containing coatings all had significantly lower penetration depths, confirming that the KH-ND addition enhanced the penetration resistance.Indeed, the maximum penetration depth of the KH-ND × 0.5% coating was approximately 8.6 μm.The hardness of the KH-ND × 1.0% coating was approximately 70% higher than that of the pure PUU coating (Figure 5(b)).The pencil hardness increased from H for the pure PUU coating to 3 H for the KH-ND × 0.5% coating, further demonstrating the positive effect of the nanoparticles on the coating hardness.
Micro-scratch tests are an effective and intuitive means of evaluating the scratch resistance of a coating.The scratch resistance can be estimated from the starting position  of damage from a scratch implemented with increasing load.As shown in Figure 6, the critical load for scratching of the pure PUU coating was 1.3 N, indicating its poor scratch resistance.By contrast, the starting point of damage was delayed as the KH-ND addition increased.For KH-ND × 0.5%, the critical load was 4.2 N (3.2 times that of the pure PUU coating), indicating that the composite coating had better scratch resistance.Figure 7 shows how the nanoparticles improve the mechanical properties and scratch resistance of the PUU coating.Numerous polymeric molecules are attracted to and wrap around the highly dispersed nanoparticles, which have large specific surface areas.This increases the physical cross-linking points between the polymer network and KH-ND particles.These organic/inorganic interactions lead to the development of dense hard domains, facilitating the microphase separation of PUU.Moreover, the abundance of amine groups on the KH-ND surface facilitates the formation of hard -hard segment hydrogen bonds with the carbonyl oxygens and amide groups of PUU, promoting the aggregation of hard segments and further enhancing the microphase separation of PUU.These dense hard domains effectively dissipate stress, which prevents further crack development and significantly improves the scratch resistance of the coating.
The distributions of hard domains at the surfaces and cross-sections of the coatings were observed by SEM.As shown in Figures 8(a), the surface of pure PUU was smooth   without obvious hard domains, and its cross-section had no obvious agglomeration or particles.Figure 8(b) demonstrates that the surface and cross-section of KH-ND × 0.1% were similar to those of pure PUU, with minimal ND aggregation and only a few dispersed hard domains (red dashed oval in Figure 8(b)) owing to the low KH-ND content.However, increasing the nanoparticle content resulted in the surface of the composite coating becoming relatively rough.For KH-ND × 0.5%, numerous fine white hard domains were distributed uniformly across the surface (Figure 8(c)).The same organic/inorganic composite hard domains were observed in the cross-sectional image (red dashed oval in Figure 8(c-1)).These hard domains had substantially higher hardness than the polymer matrix.Such obstructions help to dissipate stress and prevent further crack expansion.The uniform distribution of hard domains also increases the surface hardness of the coating, thereby providing effective scratch resistance.However, by increasing the nanoparticle content to 1.0 wt%, the hard domains on the surface tended to agglomerate (Figure 8(d)), and the domain size became significantly larger.The cross-section of the coating was rough with obvious aggregates (Figures 8(d-1)) owing to the high nanoparticle content.
AFM is widely used to study the microphase separation of PUU.In general, AFM images of PUU present two distinct regions, namely, a bright region related to the hard segments or crystalline phases, and a dark region related to the soft segments [40,41].The two-and three-dimensional surface topographies of the pure PUU and KH-ND × 0.5% coatings were characterized by AFM in tapping mode.As shown in Figure 9(a), a small volume of hard domains was observed in pure PUU, and there was no obvious microphase separation because of the highly asymmetric structure of IPDI and IPDA.With the introduction of KH-ND, the degree of microphase separation became more significant (Figure 9(b)).Compared to pure PUU, the KH-ND × 0.5% coating had a clearer distribution of soft and hard domains, and the hard domains were larger in size and more densely distributed.Therefore, the incorporation of KH-ND contributed to the microphase separation of the composite coating, consistent with the SEM results.
Coatings are used to protect a substrate from corrosion and physical damage, and a self-healing coating can repair the damage autonomously and achieve extended service life.We tested the self-healing ability of the KH-ND × 0.5% coating.As shown in Figure 10, two separate scratches with depths of approximately 100 μm and different widths were created on the surface of the coating using a scalpel or scissors to simulate the damage caused by sharp objects.The ability of the coating to repair these scratches was evaluated by observing the scratches over time via optical microscopy.After treating the surface with a heat gun at 100°C, the scratches became lighter and thinner over 300 s until they completely disappeared.These results demonstrate that the KH-ND × 0.5% coating has promising self-healing ability and is able to repair scratches of different widths.Compared to previous reports on transparent self-healing protective coatings [20,21], the PUU/KH-ND coating can repair scratches in a fast and simple way.Therefore, it has promising practicality.

Adhesion of PUU/KH-ND coatings
The adhesion of a coating is an important parameter for its applicability.Therefore, the adhesion of the KH-ND × 0.5% coating was tested on PMMA, glass, and steel substrates using a pull-out adhesion tester.As shown in Figure 11, the KH-ND × 0.5% coating had better adhesion to PMMA (2.05 ± 0.26 MPa) than to glass (0.56 ± 0.13 MPa) and steel (1.32 ± 0.25 MPa).This is likely because the DMF solvent in the coating solution minor corrodes the PMMA surface, thereby reviving the hydrogen-bonding sites on the surface.These regenerated hydrogen-bonding sites combine with the large number of hydrogenbonding sites in PUU, which increases the adhesive strength between the composite coating and PMMA.By comparison, the van der Waals and hydrogen-bonding forces between the coating and silica glass were weak, leading to poor adhesion.Meanwhile, the adhesion of the coating to different substrates decreased as the nanoparticle content increased owing to the stress concentrations generated by agglomerated nanoparticles.Nevertheless, the adhesion of the KH-ND × 0.5% coating was only slightly reduced.Considering its higher pencil hardness (3 H) and stronger penetration resistance compared to pure PUU, KH-ND × 0.5% could still offer good protection for the PMMA substrate.

Transparency of PUU/KH-ND coatings
Visible-light transmission is an important criterion for protective coatings applied to transparent PMMA.As illustrated in Figure 12(a), the PUU and nanocomposite coatings (~200 μm thick) were applied to a PMMA substrate and the transmittance was compared to that of an uncoated PMMA substrate to investigate the effect of nanoparticles.The quantitative data revealed that the pure PUU coating did not affect the optical transmittance of PMMA.By contrast, as the KH-ND content increased from 0.1 to 0.5 wt%, the transmittance at 650 nm decreased from 91.8% to 81.2%, although the transparency in the visible-light region remained above 70%.However, when the KH-ND concentration reached 1.0 wt%, the transparency rapidly decreased to 51.1% (Figure 12(b)).This was attributed to the formation and agglomeration of dense hard domains with larger areas owing to the high nanoparticle loading, which caused the coating to scatter visible light and reduced the transmittance and transparency.But even for the PUU/KH-ND coating with 1.0 wt% of KH-ND loading, the background logo could be clearly perceived in the optical photographs.Owing to the inherent differences between inorganic and organic materials, adding inorganic components to organic polymers usually reduces the transparency because of increased light scattering [6].The KH-ND × 0.5% coating had an optical transmittance of above 80% (λ = 650 nm), which is suitable for a transparent protective coating.

Conclusion
In this study, the silane coupling agent KH-550 was used as a surface modifier to functionalize ND to obtain well-dispersed KH-ND nanoparticles.IPDA and IPDI were used to form hard-segment repeating units in a high-strength PUU matrix, to which KH-ND nanoparticles were introduced via solution blending.The resulting PUU/KH-ND nanocomposite coating exhibited good scratch resistance and self-healing ability.Tensile tests showed that the KH-ND particles promoted interfacial interactions with polymer chains through chain entanglement, thereby improving the mechanical properties of the composite coating.Because of the restricted chain movement, the selfhealing ability diminished as the nanoparticle content increased; however, the KH-ND × 0.5% coating still achieved a self-healing rate of over 88% at elevated temperature (80°C).By adjusting the nanoparticle content, the surface hardness and scratch resistance of the composite coatings were significantly enhanced, with the first critical load for micro-scratching increasing from 1.3 to 5.1 N. SEM and AFM observations revealed that the uniformly distributed organic/inorganic composite hard domains on the surface of the nanocomposite coating were responsible for the improved hardness and scratch resistance.At the same time, the coating exhibited good substrate adhesion (adhesion strength: >2 MPa).The introduction of nanoparticles had a negative influence on the visible-light transmission; however, the KH-ND × 0.5% coating maintained a light transmission of more than 80% at λ = 650 nm.The PUU/KH-ND × 0.5% organic/inorganic composite coating in this study shows excellent performance and can be prepared by a simple and sustainable method.Thus, it is promising for application as a transparent and anti-scratch coating in flexible electronics, automobiles, and home appliances.Furthermore, this paper provides guidelines for the development of nanoparticle-enhanced PU materials with integrated strength, which we hope will open a new avenue for the application of autonomously healable materials.

Figure 1 .
Figure 1.(a) FTIR spectra of ND and KH-ND; (b) dispersibility of pristine ND and KH-ND in DMF at RT after different times.

Figure 3 .
Figure 3. (a) stress-strain curves of the cut and self-repaired samples.The solid lines correspond to the mechanical properties of the original samples and the dotted lines correspond to the mechanical properties of the self-repaired samples.(b) photographs of healed PUU sample, which could be stretched to 800% elongation.

Figure 5 .
Figure 5. (a) nanoindentation and (b) hardness of coatings with different KH-ND contents.

Figure 6 .
Figure 6.Optical micrographs of micro-scratches on coatings with different KH-ND contents.

Figure 10 .
Figure 10.Optical micrographs demonstrating the self-healing capability of the KH-ND*0.5% coating to repair scratches of different widths.

Figure 11 .
Figure 11.Adhesion of coatings with different KH-ND contents.

Figure 12 .
Figure 12.(a) photographs and (b) optical transmittances of composite coatings with different KH-ND concentrations.