Cong Zhang,Yujun Ma,Tao Lin,Xuefeng Yin,Xiaoyao Wei,Zhongxiang Wang

National Demonstration Center for Experimental Light Chemistry Engineering Education,Shaanxi Provincial Key Laboratory of Papermaking Technology and Specialty Paper Development,Key Laboratory of Paper based Functional Materials of China National Light Industry,College of Bioresources Chemical and Materials Engineering,Shaanxi University of Science&Technology,Xi'an,Shaanxi Province,710021,China

Abstract:Transparent wood(TW)is a wood-based biomaterial with several advantages,such as high optical transmittance,low thermal conductivity,and tunable haze.TW is functionalized according to its transparency to broaden its applications in different fields.Several studies have examined wood functionalization in recent years;however,few studies have reported photochromic TW(PTW)for ultraviolet(UV)-shielding window applications.Herein,PTW was obtained by infiltrating the delignified wood template with photochromic silver bromide(AgBr)nanoparticles and a pre-polymerized methyl methacrylate(MMA)mixture solution.The obtained PTW can adjust the luminous flux on change in the color in the visible light region.The photochromic properties were examined,and the optical properties of the composites were characterized using UV-Vis spectrophotometry.Light transmittance of PTWwas up to 86.5%at 800 nm before UV irradiation,and it decreased to 70.1%at 800 nm after UV irradiation,as the wood color changed from colorless to dark purple under UV irradiation.Thus,this work not only achieves high-value utilization of wood,but also produces a new material that can be used in varied fields,such as UV-shielding,energy saving,and smart building.

Keywords:transparent wood;photochromic;UV-shielding;nanoparticles

1 lntroduction

With the pervasive shortage of resources and growing environmental pollution,there is an urgent need to develop and utilize bio-resources to replace petroleumbased materials[1-2].As a natural material,wood is more easily renewable as well as degradable than petroleumbased materials.It is advantageous because of its extensive sources,low price,and excellent physical and chemical properties[3-7].However,the applications of wood is limited because of its opacification and poor water resistance.In recent years,transparent wood(TW) has become a popular topic in wood modification.TW is a new material with low density,high strength, low thermal conductivity, and transparent optical properties[8].Inclusion of substances with different refractive indices is a direct factor that affects the transparency of wood.TW composite is a new type of composite material obtained by removing lignin and other chromogenic materials in wood and then infiltrating the vascular structures and pores inside the wood with a single or mixed resin with a refractive index similar to that of cellulose[9-11].

Based on transparency,wood can be endowed with functions other than transparency by doping nanoparticles with specific functions[12-13].Composite materials utilize the advantages of their individual components and achieve unique functionalities beyond their building blocks[14].This type of TW composite has many excellent properties and potential application value in intelligent environmental protection green optoelectronics[15-20],smart bio-based materials[21],building materials[22-25],information energy storage[26],and other fields.For example,Gan et al[10]used a mixture of poly(methyl methacrylate)(PMMA)and luminescentγ-Fe2O3@YVO4∶Eu3+nanoparticles to infiltrate delignified wood for obtaining luminescent transparent wood-base composites.Tang et al[27]coated silver nanowires on flexible transparent wood films to obtain wood films with excellent electrical conductivity.Lang et al[28]coated TW with[poly(3,4-ethylenedioxy-thiophene)∶poly(styrene sulfonate)](PEDOT∶PSS)as an electrochromic material,which has potential applications in tuning indoor brightness.Yu et al[29]infiltrated delignified wood with a dispersion containing PMMA and CsxWO3nanoparticles to prepare wood-base composites with excellent near-infrared shielding properties.However,so far,few studies have been conducted on the aspects of photochromic TW(PTW)and less on its UVshielding properties.

Herein, we conducted in-depth studies on photochromic functionalized wood to obtain PTW by infiltrating the delignified wood template with a solution comprising AgBr and pre-polymerized methyl methacrylate (MMA). AgBr nanoparticles show promising photochromic performance and have been used as color-changing glasses[30].The photochromic performance of the PTW was characterized using a system that simulates the real environment.The results show that PTWis a potential candidate for applications as an environmentally friendly and new bio-based material in the fields of energy saving,smart building,and UV-shielding.

2 Experimental

2.1 Materials

In this study,a fast-growing poplar wood was used;samples(dimensions,20 mm×20 mm×1 mm)were obtained by cutting longitudinally.NaOH,AgNO3(≥85%)powder,ethanol,and NaClO2were obtained from Tianjin Damao Chemical Reagent Factory(China).Acetate buffer solution was obtained from Tianjin Yaohua Chemical Reagent Co.,Ltd.(China).NaBr powder and 2,2'-azobis(2-methylpropionitrile)were obtained from Beijing Chemical Plant(China).Oleic acid was obtained from Kaiyuan Chemical Reagent No.1 Factory(China).Acetone was obtained from Sinopharm Group Chemical Reagent Co.,Ltd.(China).All reagents were of chemical grade and used without further purification.

2.2 Methods

2.2.1Wood sample preparation and delignification

The original wood(OW)chips were pretreated with 5 wt%NaOH at 100℃for 80 min in a microwave,followed by rinsing in deionized(DI)water.The purpose of this step was to open the microchannels of the wood to facilitate rapid and even delignification at later stages.Thereafter,the treated wood slices were extracted and kept in a solution of 5 wt%NaClO2and acetate buffer solution(pH value=3)and at 85℃.The degree of delignification was achieved by controlling the processing time.Experiments showed that the optimum delignification effect was achieved when the reaction time was 2 h.The extracted samples were carefully washed using DI water;thereafter,they were dehydrated,sequentially,using 40%,60%,80%,90%,and 100%ethanol,a 1:1(volume ratio)mixture of ethanol and acetone,and pure acetone;each step was maintained for 1 h.The dehydrated wood samples were stored in anhydrous ethanol prior to use.

2.2.2Preparation of PMMA

PMMA was prepared by mixing purified MMA and 2,2′-azobis(2-methylpropionitrile)in a weight ratio of 1000 to 3,which was prepolymerized at 75℃for 15 min.Thereafter,the prepolymerized polymer was cooled to room temperature in an ice water bath to cease the polymerization.

2.2.3Preparation of AgBr nanoparticles

The AgBr sol was synthesized by dissolving NaBr powder(0.2 mmol,99%)in DI water(33 mL)to titrate 0.3 mmol of AgNO3powder(99.8%)dissolved in DI water(50 mL),with constant stirring.The AgBr nanoparticles were prepared under the following conditions:oleic acid∶AgBr=150∶1 in a 30℃water bath for 15 min under a shading environment,followed by static stratification for 12 h in a 30℃water bath;AgBr nanoparticles were then extracted while the solution was hot.

2.2.4Fabrication of PTW

PTW was prepared by infiltrating the delignified wood template with the as-obtained AgBr/PMMA mixed solution at a volume ratio of 1:20 and under vacuum for 4 h to complete infiltration.The infiltrated wood was loaded into a mold of certain size and heated in an oven at 70℃for 4 h for further polymerization,leading to PTW(Fig.1).

Fig.1 Schematic illustration of the fabrication strategy of PTWas UV-shielding material

2.2.5General characterizations

The sample was fixed on a metal sample stage using conductive glue,and its surface was completely covered with gold spray(thickness,approximately 25 nm);thereafter,surface morphology of the sample was observed using scanning electron microscopy(SEM,voltage,25 kV;JEOL JSM-6700F,Japan Electronics Co., Ltd.). Differential scanning calorimetry(DSC)analyses were carried out using a NETZSCH DSC200F3 (Netzsch, Germany)calorimeter at 20℃/min from-40℃to 40℃under a nitrogen atmosphere.Thermo gravimetric analysis(TGA)was performed using a TGA Q500(TA instrument,USA)at a heating rate of 10℃/min from 25 to 800℃under a nitrogen atmosphere(50 mL/min).The wood chips were fixed on an UV-752 spectrophotometer ultraviolet-visible instrument(Agilent,USA),and the transmittance test interval was set to 200-800 nm;low slit and double beam modes were selected for measurement.

3 Results and discussion

3.1 Photochromic mechanism of AgBr solution

Fig.2 presents pictures of photochromism of the AgBr solution.The solution darkened on exposure to light.When the light source is removed,the light color is restored,which continues to deepen,indicating the excellent and reversible photochromic effect of the AgBr solution.

Fig.2 Photochromism of AgBr solution

For an incomplete AgBr crystal,the loss of an ion at the defect and dislocation site of the lattice means that the six ions around it are negatively charged and can attract positively charged particles to form a photosensitive center.The effect of light on the AgBr particles is that it opens the gap from the photosensitive center, thus altering the whole particle. The photosensitive center has the ability to absorb electrons because of its high activity and mobility.Under the irradiation of short-wavelength light,such as UV or solar light,the produced electrons are absorbed in the photosensitive center. The negatively charged photosensitive center absorbs a moving positive charge.The silver ions produced by the photodecomposition of AgBr combine with this negative charge to form silver atoms.Silver colloid was formed and the color deepened when the silver atoms were concentrated to a certain extent.Because light-decomposed halogen cannot escape from the surrounding matrix,when light irradiation is stopped,the silver atom and halogen are recombined under the influence of heat or longwavelength light,and then,the system returns to the original silver halide state.

Equation(1)shows the principle of photochromic reaction of AgBr solution:the reaction goes to the left whenhv1＞hv2,which is the coloring process.Whenhv1＜hv2,the reaction proceeds to the right and becomes the decoration process.In the dark and at room temperature,molecular thermal motion also slowly fades.The color of the system slowly fades in the dark and at room temperature owing to molecular thermal motion.

3.2 Light transmittance of AgBr sol

The stirring speed considerably influences the light transmittance of the AgBr nanoparticles.The light transmittance of the AgBr sol obtained at different stirring speeds was determined,and the measurement results are shown in Fig.3.When the stirring speed was increased from 200 to 2000 r/min,the light transmittance of the AgBr sol first increased and then decreased.When the stirring speed reached 400 r/min,the light transmittance of the AgBr sol reached its maximum value.With continuous increase in the stirring speed,the light transmittance of the AgBr sol gradually decreased.

Fig.3 Relationship between stirring speed and light transmittance of AgBr sol

Fig.4 shows that when the stirring speed was too low,the dispersion in the colloid(AgBr sol)was uneven and it appeared milky white(Fig 4(a1)).When the stirring speed was increased to 400 r/min,the colloid showed a clear light blue color(Fig 4(b1)),the light path of the Dindal effect was clear(Fig 4(b2)),and the light transmittance reached 92.1%.When the stirring speed was further increased to＞1600 r/min,the transmittance of the colloid decreased and obvious large particles appeared,which indicates that the collision probability of AgBr particles in the aqueous solution increased when the stirring speed was too high,resulting in serious agglomeration(Fig 4(e1)and Fig 4(f1)).

Fig.4 (a1)-(f1)AgBr sols prepared at stirring speeds of 200-2000 r/min,respectively;(a2)-(f2)the corresponding test diagrams of Dindal effect

3.3 Anti-glare ability of PTW

Fig.5(a)and Fig.5(b)show that on laser irradiation of the glass and PTW,the glass could not disperse the light as the light still converged at a point after passing through the glass.While the light was dispersed when the light passed through PTW,which had anti-glare ability compared with traditional building materials.As building material,glass often produces shadow effects and uncomfortable glare effects.Glare refers to the appearance of excessive brightness in a certain part of the field of vision or the occurrence of excessive and sudden brightness changes,that is,extreme brightness contrast in space and time so that human eyes cannot adapt to the change sufficiently quickly.In addition to visual fatigue by glare,frequent exposure to glare may cause nausea,discomfort,and even blindness in severe cases.Therefore,PTW can be used for indoor lighting as it does not cause partial overbrightness or instant brightness changes.

The text on the paper was clearly visible when the colorless PTW was close to the object;in contrast,the text become blurred when the dark purple PTW template was close to the object(Fig.5(c)and Fig.5(d)).PTW is characterized by high transmittance and haze,which affects indoor lighting as well as ensures privacy indoors when used as building lighting materials.

Fig.5 Laser irradiation of glass(a)and PTW(b);the colorless PTW(c)and dark purple PTW(d)were laid on a text

3.4 Characterization

3.4.1SEM analysis

Fig.6(a)-Fig.6(c)show the longitudinal sections of the SEM images of OW,TW,and PTW.Several pits were observed on the cell wall of the longitudinal section of OW(Fig.6(a)).The cell cavities of TW and its pits on the longitudinal section surface were filled with PMMA during the filling process for delignified wood(Fig.6(b)).Nanoparticles dispersed in PMMA were visible in the longitudinal section of the composite material obtained by filling the delignified wood with a solution comprising PMMA and AgBr nanoparticles(Fig.6(c)).

Fig.6(d)-Fig.6(f)show cross-sectional SEM images of OW,TW,and PTW.Clear pits and cell cavities of OW were observed(Fig.6(d)),the cell cavities of TW were filled with PMMA(Fig.6(e)),and the cell cavities of PTW were filled with the PMMA solution containing AgBr nanoparticles(Fig.6(f)).The AgBr nanoparticles were uniformly distributed in PMMA so that wood can achieve high light transmittance and photochromic function.

Fig.6 The longitudinal section SEM images(a-c)and cross-sectional SEM images(d-f)of OW,TW,and PTW

3.4.2UV-vis spectrophotometry

Fig.7 shows the light transmittance curves of PTW before and after UV irradiation.When the wavelength was within the visible range,the light transmittance of the PTW was up to 86.5%,higher than that in the visible region before UV irradiation.The light transmittance of PTW was detected using 365 nm UV irradiation for 5-10 min until the color of PTW no longer changed;thus,it was determined that the PTW light transmittance of the visible light region decreased to 70.1%.Therefore,the PTW has the function of adjusting light transmittance by changing color under UV light,relieving the discomfort caused by the sudden increase in indoor light.Moreover,the light transmittance was higher,and the purple PTW changed back to colorless PTW when the sunlight was weak.Therefore,PTWcan avoid light flicker and stabilize the indoor light when it is used as indoor lighting material.Fig.8 shows the light transmittance curves of PMMA and PTW before and after UV irradiation in each section.The UV light has three sections:UVC(200-290 nm),UVB(290-320 nm),and UVA(UVA II:320-340 nm and UVA I:340-400 nm),among which,the human body is most susceptible to UVB rays.The damage caused by UVA to the human body is only onethousandth that caused by UVB,but it can strongly penetrate glass,clothing,and water,and has an irreversible cumulative effect on human skin.The main harm of UVA to the human body is making it more susceptible to damage by UVB,so UVA harm to the human body is indirect.Thus,the main band of UVshielding should be UVB,followed by UVA.

Fig.7 Light transmittance of PTWbefore and after UV irradiation

It can been seen from Fig.8 that the UV-shielding effect of PMMA in the UVB section was very weak,and there was only a slight UV-shielding effect in the UVA section.However,PTW(before and after UV irradiation)could completely shield the UV rays in the UVB and UVA II sections,and a few UV rays were transmitted within the UVA I section.In addition,the transmittance of PTW within the UVA region was lower after UV irradiation.Thus,PTW can completely shield UV rays in the UVB section,endowing it good UV-shielding property which would greatly reduce the harm of UV rays to human body.

Fig.8 Light transmittance of PMMA,PTWbefore and after UV irradiation in different sections

3.4.3DSC analysis

Fig.9 shows the thermal stability of PMMA,OW,and PTW.The TG curves(Fig.9(a))and DTG curves(Fig.9(b))show that the mass of PMMA,OW,and PTW decreased slowly with increasing temperature.The mass loss of the three samples changed when the temperature reached 400℃,and then plateaued.The TG curve shows PMMA went through three stages of mass loss.The first step started at approximately 160℃because of the fracture of the hydrogen-hydrogen(H—H)bond.The second step started at approximately 275℃as the vinyl broke at the unsaturated end.The third step started at approximately 340℃due to the random breakage of polymer chains.However,OW mainly went through two steps of mass loss.The first step started～80℃due to the evaporation of bound water in the wood.The mass loss of the second step started from ～260℃ due to the degradation of components such as cellulose,hemicellulose,and lignin in the wood.The degradation behavior of PTW was a combination of those of PMMA and OW.The mass loss of PTW in the range of 100-260℃was due to evaporation of bound water as well as fracture of H—H bonds.Degradation in the range of 260-420℃was caused by the degradation of wood components and PMMA.The TG curves indicate that PTW has more pyrolysis residues than PMMA,which can be explained by the residual carbon in the wood template.Less residue in PTWcompared to the TG curve of OW was attributed to the complete impregnation of PMMA in wood.The mass loss rate of PTW was the slowest,and the maximum loss rate was only generated when the temperature reached 400℃.The exothermic peaks of PTW moved forward compared to the DTG curves of PMMA and OW.Therefore,compared with PMMA and OW,PTW has a higher decomposition temperature and better thermal stability as well as a lower mass loss rate,making it suitable for building materials.

Fig.9 TG(a)and DTG(b)curves of PMMA,OW,and PTW

3.4.4Surface properties

Fig.10 and Table 1 show the water contact angle(WCA)detection of OW and PTW.TheWCAon the PTW surface was larger than that on the OW surface(Fig.10).TheWCAon PTW was obviously increased from 29.8°to 66.7°,and the swelling resistance rate and moisture resistance rate of PTW were as high as 36.85%and 96.90%,respectively.OW is unstable because the free hydroxyl groups in the amorphous region of cellulose adsorb water molecules from the air to form hydrogen bonds.Moreover,hemicellulose,lignin,and other substances in the wood can also adsorb water molecules,leading to the entry of water molecules,which increases the distance between the components of wood and causes volume expansion.PTW was obtained by filling the delignified wood template with PMMA.PMMA almost completely filled the pores of wood,blocking the contact between water and wood composition,which greatly improved the dimensional stability and reduced the moisture absorption of wood.The size stability of PTWis better,and it does not easily deform in a humid environment,which ensures its wide application.

Fig.10 WCA detection of OW(a)and PTW(b)

Table 1 Swelling resistance rate,moisture resistance rate,and WCA of OWand PTW

4 Conclusions

We demonstrated a functionalized photochromic transparent wood(PTW)composite for smart window applications,which has a high light transmittance of 86.5%at 800 nm before UV irradiation in the visible light region and excellent UV-shielding performance in the UV region.The composite can also be used for antiglare and privacy protection requirements.The real environment simulation test also showed that PTW has excellent photochromic performance.Thus,these results confirm that PTW is a potential candidate for applications in UV-shielding,energy saving,and smart building.

Acknowledgment

This work was supported by the Basic Research Project of Natural Science in Shaanxi Province(2019JM-485)and National College Students'Innovation and Entrepreneurship Training Program(S202010708007).