1. Introduction
Leather is one of the most widely used and traded commodities across the world attributed to its incredible versatile nature and endless applications, thus it has received progressively more attention due to its outstanding performance. The leather and leather products industry play a prominent role in the world’s economy; however, as a natural polymer material, the warmth retention of leather needs to be further improved in extremely cold environment, and microorganisms such as bacteria, mold, and virus, can easily breed on it, which affects its hygiene performance [
1], [
2], [
3]. In order to improve the warmth retention of leather products such as leather jackets and leather boots, usually soft and breathable cotton lining is added. Although it can improve the warmth retention to a certain extent, it makes the leather products bloated and affects the wearing comfort and aesthetics [
4]. In view of the problem of easy breeding of bacteria and mold on leather, organic antibacterial agents such as pentachlorophenol are often added as leather preservatives in the leather tanning section, which are not only toxic and endanger the health of consumers, but also resourced from petrochemicals [
5], [
6], [
7]. Overall, there is almost no ideal solution to solve the problems associated with both the warmth retention and antibacterial properties of leather products simultaneously.
As a material that forms a film on the leather surface, coating materials for leather can cover injuries, beautify the leather surface, and impart different functions to leather, thus it is a key polymer material used in leather production process [
8], [
9]. Notably, light and bacteria first contact the leather surface; therefore, the development of coating materials for leather with photothermal conversion and antibacterial properties is an effective approach to achieving warmth retention and antibacterial functions of leather products, respectively. However, styrene (St)-butyl acrylate (BA) emulsion P(St-BA), a commonly used coating material for leather, exhibits neither photothermal conversion properties, nor antibacterial properties, thus it becomes difficult for leather products to achieve these two functions [
10], [
11]. Moreover, St, as the main raw material for the production of P(St-BA), has been recognized by the World Health Organization as an air pollutant and a carcinogen [
12], [
13]. Consequently, many countries and organizations have imposed strict St emission standards, which further limits the use of P(St-BA) in the leather production, construction, and other industries. Once a large amount of St is used, it can bring a heavy burden to the environment.
Recently, as a new type of two-dimensional (2D) layered material, MXene has attracted extensive research attention due to its excellent photothermal conversion and antibacterial properties. The general formula of MXene is M
n +1X
n T
x, where M represents an early transition metal, X represents carbon and/or nitrogen, T indicates surface termination (-O, -OH, and -F),
n = 1, 2, 3, and
x denote the number of functional termination groups [
14], [
15]. Typically, MXenes are fabricated by selective etching of Al layer of the corresponding layer-structured MAX phase [
16] and present unique and appealing photothermal conversion properties due to the localized surface plasmon resonance (LSPR) effect [
17]. Moreover, it can absorb sunlight to generate heat, which offers potential applications in the fields of desalination [
18], [
19], [
20], personal thermal management [
21], [
22], [
23], and tumor treatment [
24]. For example, Zha et al. [
18] prepared a flexible MXene/cellulose anti-biofouling photothermal film for efficient solar-driven water purification, which showed a light absorption efficiency of up to 94% in the wide solar spectral range. Yan et al. [
21] used van der Waals forces and hydrogen bond interaction for
in situ deposition of MXene nanosheets on silk fibers and obtained MXene-decorative silk fabric (MXene@silk), which exhibited excellent photothermal conversion properties, rapid thermal response, and long-term functional stability. Lu et al. [
24] deposited doxorubicin (DOX) as a model drug on ionic liquid (IL)-Ti
3C
2T
x -MXene nanosheets and obtained a new anticancer drug IL-Ti
3C
2T
x -MXene@DOX, which could effectively kill cancer cells in
in vitro and
in vivo experiments with synergistic photothermal/chemotherapy effects. Furthermore, MXene also offers significant application prospects in antibacterial field [
25], [
26], [
27], [
28]. MXene is an atomically thin inorganic nanosheet material with excellent photothermal properties and biocompatibility, which can kill microorganisms such as bacteria and molds through the physical cleavage of bacterial cells by sharp edges and via thermal denaturation of bacterial proteins by photothermal conversion. For instance, Rasool et al. [
26] reported that the antibacterial activity of Ti
3C
2T
x -MXene nanosheets against
Escherichia coli (
E. coli) and
Bacillus subtilis in colloidal suspension was higher than that of graphene oxide (GO) nanosheets, which has been widely reported as an antibacterial agent. Mansoorianfar et al. [
27] loaded MXene on bacteriophages through electrostatic bonding and obtained a novel antibacterial agent, which exhibited good antibacterial effect on bacteria and reduced the artificial contamination in water samples by 99.99%. Although the application prospects of MXene have been studied extensively, the research on MXene as a coating material for leather, with warmth retention and antibacterial properties, has never been investigated to date.
Vanillin is a type of bio-based aromatic raw material and green antibacterial agent extracted from vanilla bean and lignin, which constitutes the only renewable monomer of lignin derivatives that can be industrialized [
29], [
30]. The molecular structure of vanillin contains rigid benzene ring, aldehyde group, and phenolic hydroxyl group. The aldehyde group in vanillin interacts strongly with the sulfhydryl group of cysteine in microbial cells, breaking the cell membrane structure and thus killing bacteria [
31]. Moreover, vanillin has been used as a renewable monomer in the preparation of various high-performance bio-based polymers due to its reactive active groups and rigid structure [
32], [
33], [
34]. Notably, the phenolic hydroxyl group in the molecular structure of vanillin can be chemically modified into vinyl group, epoxy group, and so forth [
35], [
36], [
37], [
38]. Methacrylated vanillin (MV) consists of a vinyl group and a rigid aromatic structure similar to St, thus it can be used as a bio-based green substitute to petroleum-based carcinogen and hazardous air pollutant St used for producing P(St-BA) as coating material for leather.
In this study, first, bio-based MV was synthesized by Steglich esterification using vanillin and methacrylic anhydride that was also obtained from biomass resources [
39]. Subsequently, MV not only as a bio-based green antibacterial agent endowed leather coatings with desired antibacterial properties, but also as a green substitute for petroleum-based carcinogen St copolymerized with BA (potential renewable monomer [
40], [
41]) by miniemulsion polymerization to provide novel bio-based P(MV-BA) miniemulsion as a coating material for leather. This green material was used as a green alternative to traditional coating material for leather; namely, P(St-BA), as shown in
Fig. 1. Finally, 2D nanosheets MXene with excellent photothermal conversion performance and antibacterial properties was introduced into the P(MV-BA) miniemulsion by ultrasonic dispersion, to fabricate P(MV-BA)/MXene nanocomposite coating. The coating exhibited excellent photothermal conversion efficiency and antibacterial efficacy, and realized the efficient warmth retention and antibacterial properties of the leather products. This study brings forward a novel idea for the preparation of new green bio-based nanocomposite coating materials for leather with desired warmth retention and antibacterial properties. It can not only realize zero-carbon heating based on sunlight in winter, thus reducing the use of fossil fuels and greenhouse gas emissions, but also improve people’s ability to resist microbial invasion.
2. Experimental
2.1. Materials
Lithium fluoride (LiF), titanium carbide MAX phase (Ti3AlC2-MAX), vanillin (98%), methacrylic anhydride (94%), 4-dimethylaminopyridine (DMAP), and BA (98%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (China). Hydrochloric acid (HCl), sodium dodecyl sulfate (SDS), potassium persulfate (KPS), and n-butanol were obtained from Macklin Biochemical Technology Co., Ltd. (China). Luria-Bertani (LB) medium, phosphate-buffered saline (PBS), and agar powder were obtained from Beijing Aoboxing Biotechnology Co., Ltd. (China). E. coli and Staphylococcus aureus (S. aureus) were provided by Shanghai Conservation Biotechnology Center, and deionized water was used in all experiments.
2.2. Preparation of Ti3C2Tx-MXene nanosheets
MXene was prepared by selective etching of the Al layer of the MAX phase in LiF with HCl. Briefly, LiF (3.0 g) was added to HCl (9 mol·L−1, 40 mL) and stirred for 30 min, and then Ti3AlC2 (2.0 g) was slowly added to the mixed solution. The above-mentioned mixture was magnetically stirred at 35 °C for 48 h. Then, the pH of the dispersion was adjusted to 6 with deionized water, and HCl and LiF were removed by centrifugation at 3500 revolutions per minute (rpm) for 5 min. The precipitates were ultrasonicated under N2 atmosphere for 1 h, and the supernatants were collected and freeze-dried to obtain Ti3C2T x -MXene nanosheets.
2.3. Synthesis of P(MV-BA) miniemulsion
2.3.1. Synthesis of MV
MV was prepared by Steglich esterification. First, vanillin (10.00 g) and DMAP (0.16 g) were mixed uniformly and added into a three-necked flask. Then, nitrogen was purged for 2.0 h to remove air and moisture. Finally, methacrylic anhydride (10.33 g) was slowly added to the system. The reaction products were stirred vigorously at 45 °C for 24 h, diluted with methylene chloride, and repeatedly washed with saturated sodium bicarbonate solution until no bubbles emerged. The organic layer was washed with NaOH solution (1.0 mol·L−1), NaOH solution (0.5 mol·L−1), HCl solution (1.0 mol·L−1), and saturated NaCl solution in turn. The white waxy solid MV was obtained by drying under excess anhydrous MgSO4, filtration, rotary evaporation, and finally drying in a vacuum oven at 45 °C for 2.0 h.
2.3.2. Synthesis of P(MV-BA) miniemulsion
The P(MV-BA) miniemulsion was prepared by miniemulsion polymerization. Briefly, the aqueous phase was formed by dissolving SDS (0.28 g) and n-butanol (0.40 g) in deionized water (30.00 g). At the same time, MV (6.00 g) and BA (14.00 g) were mixed uniformly to form an oil phase. The oil phase and the water phase were mixed and sonicated for 15 min to form a pre-miniemulsion. Then, the first part initiator of KPS (0.15 g) in deionized water (13.00 g) was added to the flask, and the second part initiator of KPS (0.45 g) in deionized water (23.50 g) and the pre-miniemulsion were added simultaneously at 2.0 h. The reactants were mechanically stirred for 2.0 h at 75 °C. The product was allowed to cool down naturally and filtered to obtain P(MV-BA) miniemulsion.
2.4. Preparation of P(MV-BA)/MXene nanocomposite film and coated leather
P(MV-BA)/MXene nanocomposite film was prepared by the ultrasonic dispersion method. First, the pH of P(MV-BA) miniemulsion was adjusted to 7.5 with ammonia water. Then, MXene dispersions with different MXene contents were added to P(MV-BA) miniemulsion (15.0 g) and sonicated for 30 min to obtain the nanocomposite miniemulsion. The nanocomposite miniemulsion was poured into a polytetrafluoroethylene (PTFE) mold with a size of 5 cm × 5 cm and dried naturally until the water was removed completely. The resulting nanocomposite films were kept in a desiccator for 24 h to balance the moisture content. The contents of MXene in the P(MV-BA)/MXene nanocomposites were 25.2 mg (0.8 wt%), 31.5 mg (1.0 wt%), 37.8 mg (1.2 wt%), 44.1 mg (1.4 wt%), and 50.4 mg (1.6 wt%), respectively. The corresponding samples were named as P(MV-BA)/MXene nanocomposite film (PMF)-0.8, PMF-1.0, PMF-1.2, PMF-1.4, and PMF-1.6, respectively. The coated leather was obtained by a spray method. Briefly, the nanocomposite miniemulsion (10.0 g) was sprayed on the surface of the goat leather with a size of 10 cm × 10 cm. Next, the sprayed leather was dried in an oven at 50 °C to obtain the coated leather. Notably, by controlling the number of sprays (i.e., 1, 2, and 3 sprays), leathers with different MXene loadings were obtained. The contents of MXene in the coated leather samples were 7, 14, and 21 mg·cm−2, respectively, and the corresponding leather samples were denoted as P(MV-BA)/MXene nanocomposite miniemulsion-coated leather (PML)-1, PML-2, and PML-3, respectively.
2.5. Characterizations
The structure of MV was characterized by
1H nuclear magnetic resonance (NMR) and
13C NMR spectroscopy using Bruker 400 MHz NMR spectrometer (Germany) with dimethylsulfoxide (DMSO) as the solvent. Rigaku D/max-2200pc X-ray diffraction (XRD; Japan) was used for analyzing the phase structure of the samples, with Cu Kα radiation (wavelength
λ = 0.154 nm) in the range of 5°-65°. Morphologies of MXene were characterized by scanning electron microscopy (SEM; Hitachi S4800, Japan). The molecular weight of the miniemulsion was characterized by gel permeation chromatography (GPC; PL-GPC50, Agilent, USA). Fourier transform infrared (FTIR) spectroscopy system (VERTEX 80, Bruker) equipped with an attenuated total reflectance (ATR) system was used to evaluate the chemical structure of films. Raman spectra were characterized with a Raman microscopy (InVia, Renishaw, UK). The absorbance of PMF was recorded by ultraviolet-visible-near infrared (UV-Vis-NIR; Cary5000, Agilent, USA) spectroscopy. The solid content, monomer conversion rate, gel rate, micromorphology, particle size, stability of P(MV-BA) miniemulsion, air permeability, water vapor permeability, dry friction firm level, wet friction firm level, softness, and antibacterial property of PML were determined according to the reported methods [
10]. Further, the photothermal conversion efficiency of PMF was calculated by commonly used method [
42]. The PMF and PML were cut into dumbbell-shaped films with a size of 30 mm × 5 mm using a dumbbell-shaped cutter for determining the mechanical properties. Further, the stress-strain curves were obtained using a servo material multifunctional high- and low-temperature control tester (AI-7000-NGD, Gotech Testing Machines Co., Ltd., China). The molecular dynamics (MD) simulations were conducted by using Materials Studio (MS) v8.0.
2.5.1. Photothermal performance evaluation
The photothermal performances of PMF and PML were tested under the infrared (IR) light with an optical power density of 65 mW·cm−2 using an IR lamp with a working power of 275 W. The xenon lamp (CEL-HXF300-T3, Beijing Zhongjiao Jinyuan Technology Co., Ltd., China) was used as the source for simulated sunlight. The light intensity was adjusted by changing the distance between the sample and the light source and the current of the xenon lamp. The light intensity of the simulated sunlight was determined with an automatic optical power meter (CEL-NP2000-2A, Beijing Zhongjiao Jinyuan Technology Co., Ltd.). The surface temperature of PMF and PML in real time was recorded using a thermocouple multi-channel temperature tester (ST1008, Sigma Technology, China), and an IR thermal imager was used to capture the images showing temperature variations during the heating process of the samples. Finally, the photothermal conversion performance of the samples was evaluated under outdoor sunlight.
2.5.2. Evaluation of antibacterial properties
Antibacterial properties of samples were evaluated by inhibition circle method and plate colony counting method. E. coli and S. aureus were cultured in a water bath at 37 °C with constant shaking for 18 h, and the bacterial suspension was diluted to 1.0 × 105 colony-forming unit per milliliter (CFU·mL−1) with PBS. The obtained bacterial solution was poured into a 15 mL beaker, PML was immersed in the bacterial solution, and irradiated under simulated sunlight for 30 min (bacterial solution was magnetically stirred during the irradiation process). Uncoated leather served as the control group for comparative analysis. After irradiation, the bacterial suspension (1 mL) was taken from each group and diluted to suitable concentration with PBS. Then, diluted bacterial solution (100 μL) was smeared on agar medium and cultured at 37 °C for 24 h. Finally, the results were recorded with a camera.
3. Results and discussion
3.1. Structural characterization of MV
Fig. 2 (a) illustrates that MV was prepared by Steglich esterification using bio-based vanillin as raw material, and it was used as a green antibacterial agent and a substitute for petroleum-based carcinogenic St. The structure of MV was confirmed by 1H NMR, 13C NMR, and FTIR spectroscopy. The NMR spectra of vanillin and MV were recorded, as presented in Figs. 2(b)-(e). The chemical shifts of protons and carbon in different chemical environments are shown in Appendix A.
Comparative analysis of the
1H NMR spectra of vanillin and MV (
Figs. 2(b) and (d)) indicates that the peak of phenolic hydroxyl protons (Ar-OH) at
δ = 9.81 ppm of MV completely disappeared, and new peaks appeared at
δ = 1.98 ppm,
δ = 6.11 ppm, and
δ = 6.43 ppm. These peaks are assigned to the methyl protons (-CH
3) and vinyl protons (-C=CH
2) of the methacrylate groups, respectively. Comparison of the
13C NMR spectra of vanillin and MV (
Figs. 2(c) and (e)) reveals that MV shows a characteristic peak of carbonyl carbon (-C=O) at
δ = 162.34 ppm, the peaks of carbon in carbon-carbon double bond (-C=C) appeared at
δ = 137.63 ppm and
δ = 138.21 ppm, and peak of methyl carbon (-CH
3) at
δ = 18.73 ppm. Moreover, in FTIR spectrum, the absorption peaks of MV at 1731, 2748, 1731, and 1130 cm
−1 were attributed to the stretching vibration of carbonyl -C=C, aldehyde C-H, methyl propylene -C=C, and ester group C-O-C, respectively (Fig. S1 in Appendix A). The abovementioned results indicate that the phenolic hydroxyl group on vanillin and the acid anhydride group on methacrylic anhydride successfully underwent Steglich esterification and synthesis of MV.
3.2. Structure and micromorphology of Ti3C2Tx-MXene
Ti
3AlC
2 was etched in a mixture of HCl and LiF to remove the Al layer, which was followed by centrifugation and sonication to obtain few-layer Ti
3C
2T
x -MXene (
Fig. 3(a)).
Fig. 3(b) shows the XRD patterns of Ti
3C
2T
x and Ti
3AlC
2. The (104) peak at 39.1° disappeared from the XRD patterns of Ti
3C
2T
x, indicating the successful removal of the Al layer of Ti
3AlC
2. Moreover, the (002) peak shifted from 9.6° to 7.3°, confirming the increase in the layer space.
Fig. 3(c) shows the SEM image of the few-layer Ti
3C
2T
x nanosheets, with the size of about 100 nm. The Raman spectra of Ti
3C
2T
x are shown in
Fig. 3(d). The characteristic peaks of Ti
3C
2T
x at 154, 395, and 621 cm
−1 represent A
1g (Ti, C, and T
x ), E
g (T
x = O), and A
1g, respectively.
Fig. 3(e) shows that the contact angle of Ti
3C
2T
x nanosheets to water was about 88°, which was near to the hydrophilic and hydrophobic boundary (90°), indicating that the Ti
3C
2T
x was amphiphilic and could reduce the oil-water interfacial tension. These abovementioned results indicate that the amphiphilic few-layered Ti
3C
2T
x nanosheets were prepared successfully.
3.3. Preparation of P(MV-BA) miniemulsion and P(MV-BA)/MXene nanocomposite films
Fig. 4 (a) shows the schematic illustration of preparation process of PMF. First, P(MV-BA) miniemulsion was prepared by miniemulsion polymerization, and its basic properties were tested, as shown in Fig. S2 in Appendix A. The number-average molecular mass (
M n) of P(MV-BA) miniemulsion was found to be 15 243 g·mol
−1 and the molar-mass dispersity (
Ð M) was 2.13 (Fig. S2(f)). Subsequently, the MXene nanosheets were dispersed in P(MV-BA) miniemulsion by ultrasonication. In the preparation process of P(MV-BA) miniemulsion, first, MV and BA were mixed as the oil phase, and the surfactant was dissolved in aqueous solution as the water phase (
Fig. 4(a-i)). Second, the oil phase and water phase were mixed and subjected to ultrasonication to obtain an oil-in-water pre-miniemulsion in the presence of surfactants (
Fig. 4(a-ii)). Finally, the initiator was introduced into the reaction system (
Fig. 4(a-iii)) to trigger free radical polymerization, and a uniform milky P(MV-BA) miniemulsion was obtained (
Fig. 4(a-iv)). For preparing PMF, first, MXene nanosheets and P(MV-BA) miniemulsion were uniformly mixed by ultrasonic dispersion (
Fig. 4(a-v)). Then, the nanocomposite miniemulsion was poured into the PTFE film forming plate and dried at room temperature (
Fig. 4(a-vi)). Amphiphilic properties of MXene nanosheets, being similar to those of GO, were attributed to oxygen-containing terminal groups at the sheet edges and the large framework of conjugated carbon groups with hydrophobic properties [
43], [
44]. During the drying process of nanocomposite miniemulsion, amphiphilic MXene nanosheets migrated from the oil-water interface to the air-water interface with the volatilization of water, which caused MXene nanosheets to migrate to the surface of nanocomposite films. Moreover, the cavitation effect generated by ultrasound led to the adhering of the MXene nanosheets to the surface of rising microbubbles [
45], which further promoted the migration of MXene nanosheets to the surface of nanocomposite films (
Fig. 4(a-vii)). Figs. 4(b)-(g) illustrate that MXene nanosheets and Ti elements aggregated in the upper section of PMF (yellow circle marking), indicating that MXene nanosheets migrated to the surface of nanocomposite films during the drying process. The migrated MXene nanosheets films could fully contact with the sunlight and bacteria, thus producing the maximum warmth retention and antibacterial effects. Moreover, MXene nanosheets were wrapped with P(MV-BA) polymer chain, which isolated MXene nanosheets from air and moisture, thereby delaying their oxidation. Analysis of the XRD patterns indicates that PMF showed the characteristic peak of MXene at 6.9° compared with pure P(MV-BA) films, indicating the successful combination of MXene nanosheets and P(MV-BA) miniemulsion, as shown in
Fig. 4(h). The FTIR spectra exhibit the typical peaks at 3680, 1628, and 1062 cm
−1, corresponding to the terminal oxygen-containing groups (i.e., -OH, Ti-O, and C-O) of Ti
2C
3T
x -MXene. Other than the characteristic peaks of MXene, the absorption peaks of P(MV-BA) films at 2966, 1728, and 1167 cm
−1 corresponding to -C-H, -C=O, and C-O-C stretching vibration were also observed in the FTIR spectra of PMF. It indicates the presence of MXene nanosheets in P(MV-BA) films (
Fig. 4(i)). The abovementioned results reveal the successful preparation of PMF.
3.4. Properties of P(MV-BA)/MXene nanocomposite films
MXene nanosheets as inorganic nanofillers can affect the properties of PMF. Fig. S3 in Appendix A illustrates that with the increase of the amount of MXene from 0.8 wt% to 1.6 wt%, the mechanical properties of PMF gradually increased and then decreased. Further, when the amount of MXene was below 1.4 wt%, the mechanical properties of PMF were improved compared with pure P(MV-BA) films, and its Young’s modulus changed slightly, which could meet the application requirement of coated-leather. For systematic evaluation of the compatibility, free volume, and molecular chain mobility of P(MV-BA) polymer with MXene nanosheets, MD simulation of P(MV-BA)/MXene nanocomposite system was carried out, and the simulation unit diagram is shown in
Fig. 5. Owing to the presence of Ti atoms in P(MV-BA)/MXene composite system (
Fig. 5(a)), two different force fields were used to simulate the intermolecular interactions of pure P(MV-BA) and P(MV-BA)/MXene composite system. The COMPSAAII force field was used to simulate pure P(MV-BA) system, and the universal force field for P(MV-BA)/MXene composite system [
46]. The P(MV-BA) model contains five polymer chains, each consisting of five repeating units (
Fig. 5(c)). For the parameters of molecular simulation operation of complex system, the reported methods were referred to Ref. [
47].
Noteworthy, binding energy (
E bind) can be used to evaluate the compatibility between polymer matrix and filler. The greater the binding energy, the better the compatibility of the system, and vice versa [
48]. Table S1 in Appendix A presents that the
E bind between P(MV-BA) polymer chains and MXene nanosheet was 32.898 kcal·mol
−1, indicating that MXene nanosheets exhibited good compatibility with P(MV-BA) polymer matrix, which was conducive to the construction of P(MV-BA)/MXene nanocomposite coating for leather, with both warmth retention and antibacterial properties.
The ratio of free volume to total volume is defined as the free volume fraction (
f) of the nanocomposite system [
49]. In this study, the free volume fraction (
f) of P(MV-BA)/MXene nanocomposite system was found to be 25.41% (
Fig. 5(d) and Table S2 in Appendix A), which is significantly higher than that of pure P(MV-BA) polymer system (13.11%), as presented in
Fig. 5(e) and Table S2. This result indicates that with the addition of MXene nanosheets, the free volume of P(MV-BA) polymer system increased, and more air and water vapor channels could be provided, which was conducive to improving the air permeability and water vapor permeability of the leather coating.
Furthermore, in the nanocomposite system, the diffusion coefficient can be used to evaluate the mobility of the molecular chain, the larger the value, the stronger the mobility, and vice versa [
50]. The diffusion coefficient of P(MV-BA)/MXene nanocomposite system was 0.000665 (
Fig. 5(f)), which is an order of magnitude lower than that of pure P(MV-BA) polymer system with value of 0.002700 (
Fig. 5(g)). It indicates that the introduced MXene nanosheets can limit the mobility of molecular chain to a certain extent. This makes it beneficial to improve the thermal stability of P(MV-BA)/MXene nanocomposite system and the heat-resistance of the coating. Fig. S4 in Appendix A shows that the residual carbon of PMF was 12.80%, which is 3.81% higher than that of pure P(MV-BA) film (Fig. S4(a)). The pure P(MV-BA) film showed obvious thermal degradation behavior at around 270 °C, while PMF did not undergo obvious thermal degradation (Fig. S4(b)), indicating that the thermal stability of PMF was better than that of pure P(MV-BA) film.
3.5. Photothermal performance of P(MV-BA)/MXene nanocomposite films
MXenes exhibit excellent light-to-heat conversion performance due to the LSPR effect under solar irradiation [
16], [
15], [
51], [
52]. This unique and appealing advantage allows MXene-based materials to absorb and utilize clean inexhaustible solar energy, converting it into thermal energy. The development and utilization of solar energy can alleviate the energy crisis caused by the excessive use of traditional fossil fuels and is conducive to the formation of a green and sustainable energy system.
Fig. 6 shows the photothermal performance of PMF, which was verified by recording the surface temperature under IR light, simulated sunlight, and sunlight.
Fig. 6(a) illustrates that the photothermal performance of PMF enhanced with the increase of MXene nanosheets content from 0.8 wt% to 1.6 wt% under IR light. When the ambient temperature was 25 °C, the surface temperature of pure P(MV-BA) film increased to only 55.6 °C after being illuminated under 65 mW·cm
−2 IR light for 5 min. In contrast, the PMF could effectively convert incident light into thermal energy, and the surface temperature of PMF-1.4 ascended to 77.8 °C, which was 22.2 °C higher than that of the pure P(MV-BA) film. Subsequently, the photothermal performance of PMF was tested under simulated sunlight with different illumination intensities. Figs. 6(b)-(f) exhibit that the surface temperature of PMF increased with the enhancement of light intensity. The surface temperature of PMF-1.4 increased from 49.2 °C under 50 mW·cm
−2 to 84.5 °C under 150 mW·cm
−2. Furthermore, under the same light intensity, the surface temperature of the PMF increased with increasing MXene content. When the light intensity was 50 mW·cm
−2, which was close to the average light intensity in winter in Xi’an, China, the surface temperature of PMF-1.4 was 49.2 °C, which was 4.0 °C higher than that of the PMF-0.8. Furthermore, the surface temperatures of PMF-1.4 and PMF-1.6 were almost the same, showing that the content of MXene in the PMF-1.4 reached the limit of photothermal performance. Based on the comprehensive consideration of the photothermal performance and mechanical properties (Fig. S3) of PMF, the optimal dosage of MXene nanosheets was determined to be 1.4 wt%.
Besides exploring the photothermal performance of the nanocomposite films under simulating sunlight, outdoor experiments were also performed under sunlight. The photothermal performance of PMF was tested under sunlight at different time periods on the same day (Xi’an, China, November 25, 2021, 10:00, 12:00, and 14:00). Figs. 6(g)-(i) demonstrate that PMF exhibited excellent photothermal performance in different time periods under sunlight. For example, at 12:00, the surface temperature of PMF-1.4 increased from 17.4 to 38.2 °C after being exposed to sunlight for 10 min, which was 12.8 °C higher than that of pure P(MV-BA) film (25.4 °C). Significantly, when the ambient temperature was around 15 °C, the surface temperatures of PMF-1.4 were 29.8, 38.2, and 34.2 °C at 10:00, 12:00, and 14:00, respectively. This can be attributed to the different incident angles of sunlight in different time periods, thus affecting the absorption rate of PMF under sunlight [
21], [
53].
Fig. 6(j) presents that the IR image clearly shows the surface temperature of the PMF exposed to sunlight for 10 min, intuitively indicating the excellent photothermal performance of PMF in sunlight.
Fig. 6(k) shows the absorptance of PMF. With the increase of MXene content, the absorptance of PMF also increased, which is consistent with the result of the photothermal performance. Significantly, the photothermal conversion efficiency of PMF-1.4 reached 60.08%, as shown in
Fig. 6(l). Furthermore, in order to investigate the photothermal cycling stability of PMF, the evolution of the surface temperature of PMF was investigated under constant solar illumination intensity (100 mW·cm
−2) by repeated light on/off tests with a time interval of 5 min, as shown in
Fig. 6(m). The results show that the saturated temperature and the heating rate of the PMF remained stable throughout the process, and its photothermal performance could remain stable without any significant deterioration after 5 light on/off cycles, indicating high sensitivity and stable photothermal performance of PMF.
3.6. Photothermal performance of P(MV-BA)/MXene nanocomposite miniemulsion-coated leather
In extremely cold weather, it is essential to improve the cold-proof and warmth-retention performance of leather products. The photothermal coating can undergo self-heating under sunlight, thereby reducing heat dissipation of the human body, which endows the leather products with lightweight and better wearing comfort. P(MV-BA)/MXene nanocomposite miniemulsion with photothermal performance was sprayed on leather surface using a spray gun to obtain the photothermal thermally insulated leather.
Fig. 7 shows the characterization results of the photothermal performances of the P(MV-BA)/MXene nanocomposite miniemulsio coated leather.
Fig. 7(a) shows the preparation process of the photothermal thermally insulated leather and leather clothing. For systematic investigation of the relationship between photothermal performance of the coated leather and dosage of P(MV-BA)/MXene nanocomposite miniemulsion, the dosage of P(MV-BA)/MXene nanocomposite miniemulsion was controlled by varying the spraying times. Next, the photothermal performance of the coated leather was tested under different lighting conditions.
Fig. 7(b) shows the time-dependent curve of the surface temperature of coated leather under IR light with intensity of 65 mW·cm
−2 for 5 min. Compared to the uncoated leather, the surface temperature of the coated leather increased significantly. The surface temperature of PML-1 ascended from 24.5 to 64.6 °C after irradiating with IR light. The surface temperatures of the leather samples after spraying twice (PML-2) and three times (PML-3) were 65.3 and 66.9 °C, respectively, which indicates that the number of spraying times showed little effect on the photothermal performance of leather. Moreover, the saturated value of the photothermal coating was reached by one spraying time. Figs. 7(c)-(g) show the relationship between the surface temperature and the illumination time of the coated leather after being illuminated for 5 min under simulated sunlight with different illumination intensities. The results showed that the coated leather exhibited excellent photothermal performance under simulated sunlight with different intensities. When the applied intensity of simulated sunlight was increased from 50 to 150 mW⋅cm
−2, the surface temperature of the coated leather (PML-1) increased from 44.9 to 76.3 °C, which indicates excellent photothermal controllability of the photothermal coating. Moreover, the saturation temperature of PML-1 corresponding to different irradiation intensities was linearly fitted, as shown in
Fig. 7(l). The light intensity (
I l) and saturation temperature (
T) exhibited almost linear relationship and the linear correlation coefficient was approximately 0.99628, which confirms the photothermal controllability of photothermal leather coatings. For testing the practical application performance of photothermal coating, the coated leather was tested outdoor under sunlight at different time periods on the same sunny day (Xi’an, China, December 25, 2022, 10:00, 12:00, and 14:00). The experimental device was placed under sunlight, and the photothermal data of the different coated leather samples were recorded at 10:00, 12:00, and 14:00 by IR thermography and using a thermocouple thermometer (Figs. 7(h)-(k)). Owing to the excellent photothermal conversion efficiency of MXene nanosheets, the surface temperature of PML-1 increased from 7.4 to 22.0 °C by about 15 °C at 12:00, which was nearly 5.0 °C higher than that of the uncoated leather. IR images clearly show that the coated leather also exhibits excellent photothermal and thermal insulation performance outdoors (
Fig. 7(k)). The heating cycling stability is an important property of photothermal coating. To evaluate the heating cycling stability of coated leather, the surface temperature evolution under constant irradiation (100 mW·cm
−2) was evaluated with repeated light-on/off cycles, as presented in
Fig. 7(m). The results show that the saturated temperature and the heating rate of PML-1 remained unchanged during the entire process, and its photothermal performance remained stable without significant deterioration after five cycles of switching the lamp on and off. It indicates that the nanocomposite coating shows favorable cyclic photothermal stability performance. Furthermore,
Fig. 7(n) and Table S3 in Appendix A present the comparative analysis of the temperature rise value (Δ
T) of PMF and other MXene-based photothermal films (such as soy protein isolate (SPI)/polyethyleneimine (PEI)/cellulose nanofibrils (CNF)@MXene, MXene/silver nanowire (AgNW)/polyurethane (PU), and carboxylated carbon nanotubes (CNT)@MXene/cellulose (CF) film [
42], [
54], [
55], [
56], [
57], [
58], [
59]. Under simulated sunlight with intensity of 100 mW·cm
−2, the temperature rise value of PMF was as high as 23 °C, which was significantly higher than those of other MXene-based photothermal films at low MXene content. The abovementioned results indicate that PML, as a photothermal thermal insulation coating, exhibits controllable and excellent photothermal conversion ability, and has the potential to be used in leather industry, wall insulation coating, reaction equipment insulation, and other fields.
3.7. Comparison of comprehensive performance of P(MV-BA)/MXene nanocomposite miniemulsion and commercial product P(St-BA) emulsion coated leather
Comparative analysis of the comprehensive performance of leather coated with P(MV-BA)/MXene nanocomposite miniemulsion and the commercial product P(St-BA), was evaluated via radar chart array analysis. The key performance evaluation parameters included photothermal performance, mechanical properties, hygienic properties, dry and wet friction firm level, and softness.
Fig. 8 demonstrates that the tensile strength, elongation at break, and softness of leather coated with P(MV-BA)/MXene nanocomposite miniemulsion were comparable to that of leather coated with the commercial product P(St-BA). Moreover, with the introduction of MXene nanosheets, the free volume of the leather coating increased (
Fig. 5(d)), which could provide more channels for air and water vapor to pass through, and the friction resistance of the leather coating also improved. Therefore, the air permeability, water vapor permeability, dry friction firm level, and wet friction firm level of leather coated with P(MV-BA)/MXene nanocomposite miniemulsion were slightly higher than those of leather coated with the commercial products P(St-BA). Noteworthy, the temperature rise value (Δ
T) of leather coated with P(MV-BA)/MXene nanocomposite miniemulsion was up to 15 °C, which was obviously higher than that of leather coated with the commercial product P(St-BA) with the value of 4.0 °C. This result indicates that leather coated with P(MV-BA)/MXene nanocomposite miniemulsion exhibited excellent photothermal performance. In general, the larger the closed-loop area of each property of a coated leather, the better its comprehensive performance. Therefore, the comprehensive performance of leather coated with the P(MV-BA)/MXene nanocomposite miniemulsion was significantly better than that of the leather coated with the commercial P(St-BA) emulsion. Overall, P(MV-BA)/MXene nanocomposite miniemulsion can be used as an ideal leather coating materials for producing leather products with photothermal and warmth retention properties. On the bright side, it is believed that the bio-based P(MV-BA)/MXene nanocomposite miniemulsion shows the broad prospects to substitute the commercial products styrene-based P(St-BA), thus promoting green sustainable production of leather products.
3.8. Antibacterial properties of P(MV-BA)/MXene nanocomposite miniemulsion-coated leather
In order to evaluate the antibacterial properties of PML, the bacterial growth inhibition rate was determined by inhibition zone method and colony counting method using
E. coli and
S. aureus (
Fig. 9). In the absence of simulated sunlight, the uncoated leather did not show antibacterial properties, and
E. coli and
S. aureus were clearly observed on the edges and surfaces of leather after 24 h of culture (Figs. 9(a-i) and (e-i)). Although bacterial growth was not observed on the surface of PML, many bacteria still grew on the edges, indicating its slight antibacterial properties (Figs. 9(b-i) and (f-i)). Figs. 9(c-i), (d-i), (g-i), and (h-i) show the digital images of
E. coli and
S. aureus on agar plates after being treated with the uncoated leather and PML, respectively. The results show that the growth inhibition rates of PML against
E. coli and
S. aureus were 90.7% and 95.2%, respectively. Under the simulated sunlight treatment, PML showed obvious inhibition zone (Figs. 9(b-ii) and (f-ii)) and almost 100% bacterial growth inhibition rate was observed (Figs. 9(d-ii) and (h-ii)) for
E. coli and
S. aureus, respectively, which indicates significant antibacterial activity toward
E. coli and
S. aureus. Furthermore, the antibacterial effect of leather coated with P(MV-BA)/MXene nanocomposite miniemulsion under the simulated sunlight treatment was obviously better than that without treatment, mainly because MXene nanosheets convert light energy into heat energy, and the high temperature inactivates the bacterial proteins, resulting in bacterial death [
60], [
61]. In addition, as a 2D layered nanomaterial, MXene nanosheets destroy the structure of bacterial cell membrane and cell wall through physical cutting, which was also the main reason for its antibacterial effect [
26]. According to the literature, the hydrophilic MXene nanosheets can get effectively attached to bacteria and destroy its membrane structure by direct contact, and the cell membrane of
S. aureus lacking the outer membrane is more easily damaged by direct contact with MXene nanosheets, compared to
E. coli with the outer membrane [
62], [
63], [
64], [
65]. This leads the better antibacterial effect of PML on
S. aureus than on
E. coli.
4. Conclusions
In this study, MV was synthesized using biomass-based aromatic monomer and green antibacterial agent vanillin, to prepare coatings for leather surface, with satisfactory antibacterial property. Next, MV was copolymerized with BA to prepare a novel green waterborne bio-based P(MV-BA) miniemulsion. Subsequently, P(MV-BA) miniemulsion and MXene nanosheets were combined by ultrasonic dispersion to prepare the photothermal thermally-insulated antibacterial nanocomposite miniemulsion P(MV-BA)/MXene. The surface temperature of PML increased by about 15 °C in an outdoor environment under sunlight in winter, and its antibacterial rate against E. coli and S. aureus was about 100% under the simulated sunlight treatment. Overall, the as-synthesized P(MV-BA)/MXene nanocomposite coating material is beneficial to improve the wearing comfort and hygiene performance of leather products, and can be used as a green alternative to the petroleum-based coating material P(St-BA). This study promotes the green and sustainable development of coating materials for leather, paper, architectural coatings, and other industries in the future.
Acknowledgments
This work was supported by the National Natural Science Foundation of China and (52073164 and 21838007). The authors also gratefully acknowledge Jianbin Qin in Northwestern Polytechnical University for providing Materials Studio 8.0 for MD simulations.
Compliance with ethics guidelines
Jianzhong Ma, Li Ma, Lei Zhang, Wenbo Zhang, Qianqian Fan, and Buxing Han declare that they have no conflict of interest or financial conflicts to disclose.
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