Preparation of Photochromic Wood Films Comprising Spiropyran-Based Wood Cellulose Scaffold Realized through Grafting and Densification

Abstract

The development of smart color-changing wood could facilitate its functional application. Herein, photochromic wood films (PWFs) were synthesized by grafting 1-(2-hydroxyethyl)-3,3-dimethylindolino-6’-nitrobenzopyrylospiran onto the cellulosic skeleton structure of delignified wood using hexamethylene diisocyanate as a bridging linker, followed by the densification process. The microstructural morphology, chemical composition, photochromic properties, and mechanical properties of the prepared PWFs were characterized by scanning electron microscope, Fourier transform infrared spectroscopy, colorimeter, and universal mechanical testing machine. The photochromic capability of the wood film was positively correlated with the grafting amount of spiropyran; the color change is evident when the grafting amount is high. Furthermore, the photochromic and recovery capability of the wood film weakened after sequentially irradiating 40 times using a UV lamp and daylight lamp, which could be partially recovered after a period of rest. However, the wood film strength was negatively correlated with the grafting amount of spiropyran. The grafted spiropyran affected the hydrogen bonding between cellulose nanofibers of the wood film during densification, which caused a reduction in the film strength. However, the strength was still >120 MPa. Meanwhile, temperature and humidity affected the photochromic capability of the wood film. Thus, the developed high-strength PWF has potential applications in various fields, such as intelligent sensing, personalized design, indoor and outdoor architecture, and optoelectronics.

1. Introduction

With the aggravation of problems such as energy consumption and resource shortages [1,2], tapping renewable resources to reduce energy consumption and carbon dioxide emissions from materials used in various fields has become increasingly important [3,4]. Forests are nature’s gift to mankind [5]. Wood, a forest product, has been accompanying the development of human civilization [6]. As one of the most abundant renewable green natural mesoporous materials on earth [7,8], wood has attracted the attention of scholars from various fields [9,10]. Owing to its natural properties [11] (such as hierarchical multilevel structures, excellent mechanical properties, and abundant functional groups), wood has also been developed into unique wood-based materials for numerous applications [12,13]. The chemical components of wood include cellulose, hemicelluloses, lignin, extractives, and ash [14]. As a major component of wood [15], cellulose has been the focus of wood processing and utilization. The molecular chain of cellulose contains numerous hydrogen bonds [16], which provide feasibility for cellulose modification, making it potentially useful as an advanced functional material [17]. Recent advancements in optoelectronics have extended the applications of cellulose functional materials with photochromic capability to various fields [18].

Photochromism is achieved under specific light intensity, frequency and temperature changes, and certain external conditions (e.g., ultraviolet (UV) light) [19]. Under these conditions, a photochromic material undergoes a color change; the material can be restored to its original color when these conditions disappear. The practical application of photochromic materials has high value: based on the property of generating different colors under light stimulation, photochromic materials can easily meet the personalized needs of various products [20]. These materials can be used in the field of electronic information by leveraging their reversible cyclicity [21], realizing the traceless input and deletion of information. Current works on photochromic materials mainly focus on the research, development, and evaluation of new photochromic systems; process and cost control of their preparation methods; and research on their actual application in technologies. Methods to bestow materials with a photochromic function can be mainly classified as chemical and physical, and the specific choice of the preparation method depends on whether a photochromic unit can produce a strong and stable chemical bond with a polymer. The chemical methods involve the process of firmly bonding photochromic substances with the carrier material at the molecular structure level. However, this process is usually complicated and requires extensive experimental efforts. The physical methods are generally more applicable and straightforward, but the bonding effect is often not as strong as that of chemical methods, which limits the applicability of such preparation methods of photochromic materials [22,23].

Spiropyrans are one of the most widely used classes of organic photochromic compounds [24], with color changes recognizable to the naked eye. Further, low-molecular-weight spiropyrans have excellent photochromic and adhesion properties [25]. Spiropyrans often possess a two-part structure, incorporating benzopyran and indoline [26]. Under certain conditions, some bonds in the molecular structure of spiropyrans undergo heterolytic cleavage to produce colored anthocyanine compounds [27,28], which can be restored to a closed-loop colorless state under light at other wavelengths or heat stimulation. In addition, spiropyran analogs can form a strong chemical bond with the cellulose molecular chain through covalent bonding due to their excellent reactivity [29], such that the prepared material has a stable and durable color-changing function. Grafting spiropyrans onto cellulose to bestow them with photochromic capability provides a new approach to the research and utilization of wood-based photochromic materials [30]. A cellulose skeleton is obtained after the wood is delignified to remove most of the lignin and hemicelluloses present in the wood, which can be modified to obtain wood-based cellulosic materials with various excellent properties [31,32]. However, delignified wood scaffolds are fragile, and their mechanical properties, including strength, hardness, and toughness, are considerably weak [33]. In order for this new material to be widely used, it needs to be subsequently processed to give it better mechanical properties, thus expanding its scope of application.

Compression densification is one of the effective methods used to improve wood strength [34]. Further, this method bestows wood with excellent physical properties [35], making it a structural material with high hardness, density, and strength [36,37,38]. Thus, densified wood can replace hardwood as well as other precious wood species. After compression densification treatment, the wood undergoes significant changes in organization at the cellular level and in overall physical and mechanical properties [39,40,41,42]. Results have shown that the strength and toughness of compressed wood pretreated by delignification are considerably higher than those of natural wood [43], and the specific strength is even superior to that of some metals and alloys [44,45].

Zhu et al. fabricated anisotropic/isotropic transparent films directly from basswood by delignification and densification treatments, respectively. The longitudinal tensile strength of the anisotropic wood film was about 350 MPa, and that of the isotropic wood film was about 150 MPa [46,47]. Thus, the densification method helps realize the sustainable management of forest resources [48].

Spiropyran is an organic red dye molecule that is widely used in electronics, medicine, textiles, and clothing [21,49]. In recent years, spiropyran compounds have also been used in analytical chemistry [50]. In terms of photochromism, spiropyran compounds have excellent color-collecting properties, and they change their hue by means of a coupling reaction in visible light, which can be used as a driving force for the initiation mechanism [51,52].

Spiropyran, which belongs to the organic photochromic system, was selected in this work. This selection was made on the basis of a comprehensive consideration of the preparation process of wood materials and properties of photochromic materials, as well as the progress of wood compression technologies described in the previous section. Further, the compression treatment of photochromic wood was carried out. The spiropyran compound 1-(2-hydroxyethyl)-3,3-dimethylindoline-6’-nitrobenzospiropyran (SP-OH; hereafter referred to as spiropyran) was used. Hydroxyl groups are present in the chemical structure of spiropyran and also exist in the molecular chain of wood cellulose in large numbers; therefore, it is possible to effectively graft both of them using highly active intermediates (hexamethylene diisocyanate) to form strong chemical bonds, which is suitable for the preparation of photochromic wood materials. The main objective here is to develop a wood-based material with photochromic function and good mechanical properties, improve its structure, and increase the possibility of its intelligent application.

2. Materials and Methods

2.1. Materials

The basswood (Tilia Linn.) with a thickness of 0.38 mm was purchased from Taobao (https://www.taobao.com/, accessed on 25 December 2021). The density is approximately 0.43 g cm⁻³, and the moisture content is approximately 8.2%. N,N-Dimethylformamide (DMF; 99.9%), sodium chlorite (NaClO₂; 80%), and glacial acetic acid (CH₃COOH; 99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Hexamethylene diisocyanate (HDI; 99%) and SP-OH (93%) were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China).

2.2. Lignin Removal

The longitudinal section of basswood was used as the raw material and was sawn into a specimen with a size of 50 mm (length) × 50 mm (width) × 0.38 mm (thickness). The wood specimen was then submerged in the delignification solution (1 wt% NaClO₂ solution with CH₃COOH, adjusting the pH of the solution to 4.6). The delignification process was performed in a water bath at 80 °C for 12 h, replacing the delignification solution with a new solution every 6 h. The delignification process was repeated until the wood pieces became white. The delignified wood was washed with hot water several times, and then the solvent was exchanged with and stored in DMF.

2.3. Spiropyran Grafting

Figure 1 shows the grafting process of spiropyran onto wood cellulose. Reactions between the hydroxyl groups on wood cellulose and spiropyran were realized using the isocyanate group of HDI. These reactions resulted in the formation of stable amide bonds that grafted spiropyran onto the cellulose molecular chain. In particular, 0 (blank group), 2, 4, and 8 g of HDI were added into 100 mL DMF to prepare DMF solutions with different HDI concentrations. Thereafter, the delignified wood slices were placed in the solutions with different HDI concentrations and slightly stirred at 60 °C for 8 h for the reaction to initiate. Afterward, the DMF solution of spiropyran (2 mg/mL) was prepared, and the treated wood slices were placed in this solution at 60 °C for 12 h. After the reaction, the delignified wood slices were washed with deionized water several times and immersed in deionized water to obtain delignified wood slices grafted with spiropyran.

2.4. Densification Treatment

After the above-mentioned treatment, the wood slices underwent densification. The top and bottom surfaces of the delignified wood slices grafted with spiropyran were covered with two PES filter membranes and filter paper, respectively. Subsequently, these samples were pressed for 3 h using a press machine (HPC-600E, HENCH, Tianjin, China) at room temperature with a pressure of 10 MPa. Natural wood, the delignified wood film, and photochromic wood films (PWFs) with different addition amounts of HDI (0, 2, 4, and 8 g) were referred to as NW, DWF, PWF0, PWF2, PWF4, and PWF8, respectively.

2.5. Characterization

2.5.1. Microstructure Characterization

A field emission scanning electron microscope (FE-SEM, SU8010, Hitachi, Tokyo, Japan) was used to observe the microstructure of the PWFs. The PWF samples were adhered to the microscope observation carrier stage using conductive tape, sprayed with gold, and then observed. The accelerating voltage used in the scanning process was 15.0 kV.

2.5.2. Chemical Composition Characterization

The chemical compositions of the wood slices, delignified wood slices, and PWF samples were characterized by Fourier transform infrared (FTIR, Nicolet iS50, Waltham, MA, USA) spectroscopy using the ATR mode, with a spectral acquisition range of 4000–500 cm⁻¹, a scanning count of 32, and a resolution of 4 cm⁻¹.

2.5.3. Photochromic Performance Test

The samples were irradiated with a UV lamp (wavelength = 365 nm) and daylight lamp for discoloration and recovery, respectively, and the color difference was detected using a colorimeter. The samples were irradiated for several cycles to test their cyclic color change capability. Different patterns of laminating films were created to cover the surface of the PWF samples. Further, the UV lamp was used for irradiation, and the completeness of the pattern display, as well as the prominence of the laminating and discolored areas, were observed.

The following color difference value calculation formula was used:

$$\Delta E=\sqrt{{(\Delta L)}^2+{(\Delta a)}^2{+(\Delta b)}^2}$$

(1)

where ΔL, Δa, and Δb represent the difference between two color data points on the L, a, and b axes, respectively. In the CIELAB color space (used for color measurement in this study), the L (luminance) axis indicates black and white colors (0 represents black and 100 represents white), a (red–green) axis indicates red and green colors (a positive value is red, a negative value is green, and 0 is a neutral color), and b (yellow–blue) axis indicates yellow and blue colors (a positive value is yellow, a negative value is blue, and 0 is a neutral color).

2.5.4. Mechanical Property Test

The tensile strength of the samples was measured using a universal mechanical testing machine (SHIMADZU AGS-X, Kyoto, Japan). The size of the sample was 50 mm (length) × 5 mm (width), and the longitudinal tensile rate was 5 mm/min.

2.5.5. Environmental Resistance Test

The PWFs were placed in an oven at 80 °C and then in a refrigerator at −30 °C for 12 h. Subsequently, the films were tested to examine the color difference before and after discoloration. Further, the PWFs were placed in a high-humidity environment at room temperature for 24 h and again tested to examine the color difference before and after discoloration.

3. Result Analysis

3.1. Microstructure Analysis

Figure 2 shows the microstructural topography of the longitudinal section of the NW, DWF, PWF0, and PWF8 samples. Figure 2a presents the entire structure of the longitudinal section of NW, and the warping of some fibers on the surface due to rotary cutting resulted in an uneven surface. Figure 2a₁ shows the longitudinal section of NW with magnification, which reveals the original structural integrity of the wood and smooth inner cell walls. Figure 2b shows a denser and flatter surface of the longitudinal section of the DWF compared to NW. Figure 2b₁ shows a slight deformation of the cell wall structure of the DWF, indicating that the delignification process resulted in wood component degradation and structural damage. However, the wood fiber structure of the DWF remained intact. As shown in Figure 2c,c₁,d,d₁, the surface flatness of PWF8 was worse than that of PWF0, with a low densification degree. This was attributed to the weakening of hydrogen bonding between the molecular chains after the hydroxyl grafting of spiropyran on the molecular chain of cellulose. This resulted in deteriorated compression densification. As seen in Figure 2d₁, PWF8 exhibited a uniform and finely rough structure on the cell wall surface due to successful spiropyran grafting.

3.2. Chemical Composition Analysis

Figure 3 shows the FTIR spectra of NW, DWF, PWF0, PWF2, PWF4, and PWF8 samples. The characteristic peaks of hemicelluloses at 1734 and 1238 cm⁻¹ and the absorption signal peaks of lignin at 1592, 1504, and 1460 cm⁻¹ are observed for NW [31,53]. For the DWF, the intensity of the peaks at 1734 and 1238 cm⁻¹ is lower, and the absorption peaks located at 1592, 1504, and 1460 cm⁻¹ almost disappear, indicating that the lignin and hemicellulose components of the DWF were partially removed after delignification. Unlike the case of NW, DWF, and PWF0, peaks were observed at 1635 and 1541 cm⁻¹ for the PWF2, PWF4, and PWF8 samples. The absorption peak located at 1635 cm⁻¹ was the characteristic peak of the amide bond, and the peak at 1541 cm⁻¹ was attributed to the bending vibration of the N–H of the amide II bond [54,55]. The peak strengths increased with an increasing HDI concentration, which indicated the formation of amide bonds during spiropyran grafting using HDI as an intermediate, thereby confirming the successful grafting of spiropyran.

3.3. Photochromic Property

Figure 4 shows the color change in the PWF samples under daylight and UV light, wherein the inset pictures from left to right correspond to PWF0, PWF2, PWF4, and PWF8. The figure reveals an evident discoloration effect and a substantial color difference for high HDI concentrations in the sample. This can be attributed to HDI as an intermediate connecting cellulose and spiropyran. Increased HDI addition indicates increased spiropyran grafting onto the wood scaffold, affording a superior photochromic effect. In addition, the poor photochromic effect of the PWF0 samples indicated that the hydrogen bonding between spiropyran and wood cellulose hydroxyl groups was weak without the bridging effect of HDI, and spiropyran units fell off during the subsequent cleaning process, resulting in poor photochromic performance. The strength of the covalent bonding formed by spiropyran grafting onto wood cellulose using HDI intermediates was high. The color difference analysis data showed that the difference between the color values before and after discoloration increased with the number of grafted spiropyrans. This phenomenon was specifically manifested in the decrease in brightness, increase in a (red–green) value, and decrease in b (yellow–blue) value.

Figure 5 shows the color-changing patterns of PWF2, PWF4, and PWF8 under UV irradiation and photomask coverage. The PWF samples with their corresponding patterns can be generated under photomask coverage. Under UV irradiation, the patterned photomask was covered on the PWF surface, and the photomask portion was unaffected by the UV light and maintained its original color. By contrast, the spiropyran grafted on the unmasked wood film was excited to turn pink, resulting in a certain pattern. The pattern can be preserved for a period of time and then completely restored to its original color. Therefore, PWF can be used as a functional film material with information storage.

Daylight and UV light were utilized to irradiate the PWF samples repeatedly and characterize their fatigue resistance. Therefore, the samples were cyclically transformed between the original and pink colors, and the color difference was detected using a color difference analyzer. The PWF8 sample underwent cyclic irradiation at room temperature. Figure 6 shows the color difference values before and after discoloration under different numbers of light cycles. Figure 6d reveals that the color difference value ΔE gradually decreases with the increase in cycle times. This gradual reduction indicates the weakening of the short-term discoloration capability of PWFs with the increase in the number of discolorations. As shown in Figure 6a–c, the values of a (red–green) and b (yellow–blue) gradually decreased and increased with the increase in the number of cycles, respectively. The difference between the color of the photochromic film after discoloration and the original color is small with additional cycles. The color difference between before and after discoloration increased slightly compared with that in the last cycle (ΔE = 17.8) after the samples were rested for 48 h. The discoloration capability recovered but was still lower than the initial level. Compared with the initial ΔE (28.9), the color difference value decreased by approximately 38.4%. Therefore, the photochromic capability of the PWF after several cycles of discoloration will result in a certain degree of irreversible loss of the color change capability.

3.4. Mechanical Performance

To some extent, the thickness and density of WF samples reflect the effect of the spiropyran grafting amount on the hydrogen bonding formation between wood cellulose fibers. As shown in Figure 7a, the density of DWF was the largest at 1.05 g cm⁻³, and the density of PWF samples decreased with the increase in spiropyran grafting amount. This condition is due to the increased number of spiropyrans grafted on the wood cellulose and the increased addition of HDI intermediates. This phenomenon hindered the formation of hydrogen bonds between cellulose during the densification process. As shown in Figure 7b, the thickness of DWF was the thinnest at 0.153 mm, and the thickness of PWF increased with the amount of grafted spiropyran.

Figure 8 shows the results of the mechanical tensile properties of NW, DWF, PWF0, PWF2, PWF4, and PWF8. The longitudinal tensile strength of NW was approximately 80 MPa, and the strain was approximately 8.4%. Meanwhile, the tensile strength of DWF can reach approximately 240 MPa, and the strain is approximately 9.1%. Most of the substance retained after delignification was cellulose, and the hydrogen bonding formation of cellulose fibers during compression densification led to a significant increase in its tensile strength. The tensile strength of PWF0 was approximately 220 MPa, and that of PWF2, PWF4, and PWF8 gradually decreased. This phenomenon was attributed to the weak hydrogen bonding capability between celluloses due to the high amount of spiropyran grafting, which, in turn, reduced their tensile strengths.

3.5. Environmental Resistance

Spiropyran-like organic photochromic materials are characterized by fast color change speeds and sensitive light responses. After spiropyran was grafted onto the wood scaffold, the PWFs are expected to be used in numerous fields, such as UV sensing devices, anticounterfeit markings, indoor and outdoor decorations, and art designs. These applications cover a wide range of scenarios. Therefore, testing and analyzing the environmental resistance performance of these materials is necessary to formulate a reasonable application plan. The PWF8 samples were placed in a high-humidity environment, in an 80 °C environment, and in a −30 °C condition for 24 h. The color difference values before and after discoloration were compared with those of the original samples, and the results are shown in Figure 9. The high-humidity environment resulted in the deterioration of the discoloration effect of PWF8. Meanwhile, the high- and low-temperature environments also resulted in the loss of discoloration capability, with the low-temperature environment demonstrating the smallest effect on the photochromic capability. The smallest effect was observed, wherein ΔE was only reduced by 3.61584, and the high temperature was slightly inferior to the low temperature. By contrast, the high-humidity treatment had the largest effect on the PWF, wherein ΔE was reduced by approximately 28.36%. As shown in Figure 9b, the effect of environmental changes on photochromism was specifically observed in the changes in b (yellow–blue) values, while L (lightness and darkness) and a (red–green) values only slightly changed.

Humidity and temperature changes will have a certain impact on the photochromic performance of grafted spiropyran wood films. Therefore, further improving or increasing the protection measures in the later stage of the practical application of this new material is necessary. Such an improvement reduces the impact of the external environment on the photochromic effect according to the applicable field of cooperation, such as the application of a layer of transparent varnish.

4. Conclusions

A PWF was prepared using wood slices as raw materials. Delignified wood scaffolds were prepared via delignification treatment, spiropyran was grafted onto wood cellulose fibers using HDI as an intermediate, and the wood scaffolds were then densified. The grafting of photochromic components on a cellulose skeleton that retains its integrity is a practical way of preparing photochromic wood materials with strong chemical bonds. The PWF0 samples without the intermediate HDI lost their photochromic capability after washing, indicating that adsorption alone could not guarantee the stable photochromic capability of the wood. The mechanical properties of the delignified wood scaffolds significantly improved after compression and densification with a pressure of 10 MPa. However, the mechanical properties of the PWFs gradually worsened as the grafting amount of spiropyran increased. The photochromic effect of PWF was directly proportional to the amount of spiropyran grafting; the photochromic effect is evident when an increased amount of spiropyran is grafted on wood cellulose. The photochromic capability decreases after sequentially irradiating 40 times using a UV lamp and daylight lamp. However, it can be partially recovered after a period of rest. In addition, high humidity and low or high temperatures have a certain inhibitory effect on the photochromic capability.