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Enhanced Thermal Stability and UV-shielding Properties of Poly(vinyl alcohol) Based on Esculetin Yang Wang, Chennong Xiang, Ting Li, Piming Ma, Huiyu Bai, Yi Xie, Mingqing Chen, and Weifu Dong J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b11453 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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The Journal of Physical Chemistry

Enhanced Thermal Stability and UV-shielding Properties of Poly(vinyl alcohol) Based on Esculetin Yang Wang, Chennong Xiang, Ting Li, Piming Ma, Huiyu Bai, Yi Xie, Mingqing Chen, and Weifu Dong* * Corresponding author. E-mail: [email protected]. The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, 1800 Lihu Road, Wuxi 214122, China

ABSTRACT: In this paper, PVA composites with outstanding thermal stability, UV-shielding and highly transparent were fabricated based on traditional Chinese medicinal (esculetin). Characterization date have suggested that the resulting PVA/esculetin (ESC) composites display excellent thermal stability compared to pure PVA and most of PVA nanocomposites. The pyrolysis mechanism of PVA before and after modification with esculetin varies from chain unzipping degradation followed by chain random scission to chain random scission. DPPH scavenging activity and FTIR measurements have illustrated that esculetin can scavenge the reactive radicals, which leads to the improvement of thermal stability and the change in pyrolysis mechanism of PVA. More importantly, the resulting composites can almost completely block the whole UV region (200−400 nm) without any deterioration of the high transparency of the

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composites. Therefore, the composites can convert harmful UV light into blue light effectively, which is beneficial for their application as optical materials and devices.

INTRODUCTION Poly(vinyl alcohol) (PVA) (as shown in Scheme 1) is a commercially important polymer with many remarkable properties, such as good barrier properties, and can be widely used in packaging applications.1, 2 However, PVA has very strong inter- and intra-molecular hydrogen bond interactions, which substantially make its melting point (Tm ~230℃) even close to the thermal decomposition temperature (Td ~240℃).3, 4 Such a narrow temperature window makes the thermal melt processing of PVA almost impossible.5 In the past years, many plasticizers, such as glycerol, glycol, citric acid, etc. are facile to interact with hydroxyl groups of PVA to replace the original inter- or intra-molecular interactions combination and destroy the crystalline structure, hereby reducing the melting point of PVA.6, 7 Recently, improving the thermal stability of PVA becomes highly attractive for the thermalplastic processing. As known, the thermal decomposition of PVA consists of unzipping degradation and main chain random scission.8-10 At first degradation involves the elimination of water at weak points in PVA chains, thus the reactive hydroxyl radical are formed. Those hydroxyl radicals abstract hydrogen to generate the new free radicals. The reactive radicals are key factors on affecting the unzipping degradation. Therefore, the thermal stability of PVA can be improved mainly via suppress the information of those reactive radicals. Generally, the enhanced thermal stability of PVA could be reached via addition of fillers or chemical methods. For example, the thermal properties of PVA could be improved with addition of poly(glucosyloxyethyl methacrylate) (PGEMA) or alkyl boronic acids.11,


12

Although both

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methods can enhance the thermal stability of PVA, more loading of fillers are normally required. Recently, Song et al.13,

14

assumed that hydrogen bond crosslink may enhance the thermal

stability of PVA. They modified PVA by casting from solution with adding small mutiliame molecules (4-Aminopy rimidine or melamine). Ultraviolet radiation is widely applied in many field, including UV detection, photography, optical sensors, disinfection, medical diagnosis and therapy, and polymer process, due to its short wavelength and high energy.15-17 However, UV radiation deeply interact with human health, behaving not only as biological regulators but also as stressors that cause severe negative effects on cells and tissues.18 During the last few decades, much attention has been focused on developing photoprotectives materials, especially inorganic UV absorbers (e.g., TiO2, ZnO, SiO2, Al2O3, and CeO2) embedded in a polymeric matrix.19 However, those inorganic UV absorbers can induce photodegradation of the polymeric matrix under UV radiation. Moreover, their always only block the UV light below 320 nm due to their inherently wide band gaps. It should be noted that the UV region of the solar UV light includes wavelengths that fall in UVC (220−280 nm), UVB (280−320 nm), and UVA (320−400 nm). While most of the solar UV light reaching the Earth’s surface locates at UVA region. Therefore, it is necessary to develop a novel transparent UV-shielding material with excellent UV-shielding performance over the whole UVA region. Traditional Chinese medicinal herb are rich natural resource and have been extensively used to prevent and cure many disease that inflicted humans for thousands of years in China. There are several kinds of Chinese medicinal herb preparations, such as Yufengningxin pill and Xiaoke pill (composed of Cortex Fraxin and Fructus sophorae). Esculetin (6,7-Dihydroxycoumarin, as shown in Scheme 1) is the main bioactive components in Cortex Fraxin.20 It is a naturally



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occurring coumarin derivative that can be isolated from many plants such as Artemisiacapillaries (Compositae),

Citruslimonia

and

Euphorbia

lathyris.

Esculetin

have

such

marked

pharmacological functions as antiinflammation,21 free radical and reactive oxygen species scavenging activity,22 liver-protective antioxidation,23 as well as cancer chemopreventive and anti-tumor activities. Although esculetin has been used so far externally (topically) as an active ingredient of ointments and gels, its internal administration, aside from reinforcing its actual known properties, may bring new application possibilities, since it may provide outstanding potential in augment the properties of polymers. Given that esculetin can be extracted from many natural sources makes it even more attractive for this purpose. In the present work, we have reported the preparation and the thermal, optical properties of novel UV-shielding composites based on esculetin with PVA matrices. It is shown that a significant improvement in thermal stability and UV-shielding performance of PVA films is achieved by loading a small amount of esculetin, and meanwhile, the composite films still keep high transparency. The work opens door toward fabricating an excellent thermal stability and UV-shielding material for optical materials and devices.

Scheme 1 Chemical structures of PVA and esculetin. EXPERIMENTAL SECTION Materials.


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Poly(vinyl alcohol) (PVA) (GB12010-89, polymerization degree: 1700; degree of hydrolysis: 99 mol%) was obtained from Sinopec Ningxia Energy Chemical Industry Works. Esculetin was obtained from Aladdin. Free radical scavenging by DPPH Method DPPH radical (2,2-diphenyl-1-picrylhydrazyl radical) scavenging capacity of esculetin was determined by monitoring the decrease in the absorbance at 517 nm due to depletion of DPPH radical in the mixture after reaction.24 Briefly, esculetin solution (0.1 mL) in ethanol was added to solution [3.9 mL, 0.004% (w/v)] of DPPH in ethanol. The results were expressed as the radical scavenging activity (RSA) in percentages and calculated as: RSC (%) = [(A0－At)]/A0×100 where A0 is the absorbance of DPPH alone in ethanol and At is the absorbance of the test samples. All tests were performed in triplicate. Preparation of PVA/ESC composites PVA/ESC composites were prepared by green casting method.25 Esculetin was added into aqueous PVA solution and stirred for 3 h. Then composite films were prepared by solution casting, and dried under vacuum oven at 80℃.

UV-shielding performance of PVA and composite films The degradation behavior of RhB solution in the presence of photocatalyst (TiO2) under high-pressure mercury lamp (150 W) was applied to evaluate the UV-shielding performance of films. 25 mg TiO2 was added to 50 mL of RhB solution. Prior to irradiation, the suspension was stirred in the dark for 30 min to reach adsorption-desorption equilibrium. The suspension was charged into a beaker covered by tinfoil to exclude light. Before UV irradiation, PVA or



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composite films was used to cover the mouth of the beaker. The distance between of the lamp and the film was about 10 cm. The photocatalytic degradation of RhB solution was carried out under constant stirring. At given time (t), 4 mL of the suspension were collected and centrifuged to remove the photocatalyst. The absorbance of RhB at 552 nm was measured by a TU-1901 UV−vis spectrophotometer. The UV-shielding performance was calculated as I =At/A0×100%, where A0 is the initial absorbance of RhB solution without UV radiation, At is the absorbance of the remaining RhB solution protected with film under UV radiation. Characterizations The thermal properties of PVA and PVA/ESC composites were evaluated by thermogravimetric analysis (TGA/SDTA851e Mettler Toledo). Samples (about 5-10 mg) were heated from 50 to 600 ℃ under a nitrogen atmosphere at different heating rates: 5, 10, 20, and 30 ℃ min-1 with a flow rate about 50 mL min-1. ATR-FTIR examinations were conducted on a Nicolet 6700 FTIR spectrometer. The surface topography images of PVA and composite films were observed by atomic force microscopy (AFM) (Bruker MuLtimode 8) with Nanoscope 7.20 image processing software. UV–vis spectra were observed with a TU1901 UV–vis spectrophotometer. The emission spectra were characterized using a FS5 fluorescence spectrophotometer. Results and discussion Free-radical-scavenging activity and UV-vis spectrum of esculetin solution. The radical scavenging properties of esculetin is measured by employing DPPH assay. As shown in Figure 1a, free radical scavenging activity of esculetin up to 97% at


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concentration of 0.1 mg mL-1. It could be explained that the excellent radical scavenging activity of esculetin is attributed to their hydrogens donated by phenolic hydroxyl groups which can convert the radical into stable molecules. Figure 1b reveals the UV-vis absorption spectrum for esculetin in water. The inset shows that esculetin solution is almost transparent. Absorption band at 382 and 200 nm can be observed, which is in conformity to its wellknown optical characteristic of liver-protective antioxidation.26, 27

2.0

a100

b

80

1.5

Absorbance

RSA (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0.05 mg/ml 0.08 mg/ml 0.10 mg/ml

0 0

10

20

30

40

50

1.0

0.5

0.0 200

300

400

500

600

Wavelength (nm)

Time (min)

Figure 1. (a) DPPH radical scavenging activities of esculetin of different concentrations (0.05, 0.08, and 0.10 mg mL-1). (b) UV-vis absorption spectra of esculetin (5×10-1 mol L-1). The inset shows a photograph of esculetin dispersed in water. FT-IR FT-IR spectra can provide powerful evidences to determine interactions between PVA and fillers. Figure 2a, 2b shows the FTIR spectra of PVA and PVA/ESC composites. Pure PVA exhibits characteristic peaks at 850 cm-1 (νC-C), 920 cm-1 (γC-O), 1340 cm-1 (δH-C-OH), 1425 cm-1 (δO-H + νCH2), 2910 and 2942 (νCH2), and 3000~3600 cm-1 (νO-H). The absorption peak at around 1652 cm-1 is ascribed to C−O and C=O stretching from the residual acetyl content (1%) present



in the PVA.28, 29 As shown in Figure 2b, the νO-H value of pure PVA is 3263 cm-1, arising from inter- and intramolecular hydrogen bond interactions. However, the νO-H value of PVA shifts upward to different extent with adding esculetin and about 11 cm-1 of blue shift is shown for PVA/ESC-2wt%. This reveals that esculetin molecules have stronger capabilities to form hydrogen bond, thus resulting blue shift of νO-H. Pure PVA film has very strong inter- and intramolecular hydrogen bond interactions. After dissolved into water, water molecules can destroy those hydrogen bonding, and esculetin molecules are uniformly dispersed in PVA solution. On the other hand, esculetin can be facile to form stable H-bonding interactions with PVA during the evaporation process, thus forming charge transfer complexes (CTCs) between esculetin and PVA (as shown in Figure 2c).

1425

a

1652

2942

1340

920

b

3263

PVA ESC-0.2 wt%

3265

ESC-0.5 wt%

3268

ESC-1.0 wt%

3268

Absorbance (a.u.)

ESC-2.0 wt%

ESC-1.0 wt%

ESC-0.5 wt%

ESC-2.0 wt%

3272

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ESC-0.2 wt%

PVA

4000

3500

3000

2500

ESC interacts with PVA via multiple H-bonds

2000

1500

1000

3600

-1

Wavenumbers (cm )


3400

3200

3000 -1

Wavenumbers (cm )

2800

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Figure 2. (a) (b)FTIR spectra of PVA and PVA/ESC blends. (c) Effect of esculetin on the molecular architecture of PVA/ESC composites. Thermogravimetric analysis. TGA measurement is used to determine the thermal properties of PVA and PVA/ESC composites. For pure PVA, the first degradation peak takes place between 230 and 320 ℃ is mainly due to the dehydration of PVA accompanied by the formation of volatile products, while the followed degradation (320~420 ℃ ) is dominated by chain-scission. By incorporation of esculetin, the thermal degradation temperature shifts to higher temperature, as shown in Figure 3a. The initial decomposition temperature (Ti), the decomposition temperature at loss mass 50% (T50%), are important parameters of polymer.13 It can be seen that Ti, and T50% of pure PVA are 254, 349 ℃, respectively. With the addition of 0.5 wt % esculetin, Ti, and T50% increase to 302, and 364 ℃ respectively. On increasing the amount of



esculetin, the thermal stability of PVA/ESC composite is gradually enhanced. When the amount of esculetin is 2 wt %, Ti, and T50% reach up to 334 and 372 ℃ respectively, Ti of which is approximately 80 ℃ higher than that of PVA. Figure 3b presents DTG curves of PVA and PVA/ESC composites. It have been extensively that PVA thermally degrades in two steps: (1) unzipping degradation and (2) main chain random scission. The unzipping degradation takes place within 230−320 ℃ due to the elimination reactions, while the followed degradation is dominated to main chain random scission. However, there is a significant difference between PVA and PVA/ESC composites. PVA/ESC composite shows only single

a

100

a

Tia T50% 5 wt%

PVA ESC 0.2 wt% ESC 0.5 wt% ESC 1.0 wt% ESC 2.0 wt%

80

254 268 302 314 334

349 361 364 360 372

60 50 wt%

40

20

0 200

250

300

350

400

450

500

b

0.000

Derivative weight loss (%/℃ )

stage decomposition (350~450 ℃) due to the main chain random scission.

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-0.004 -0.008 -0.012 -0.016 PVA ESC 0.2 wt% ESC 0.5 wt% ESC 1.0 wt% ESC 2.0 wt%

-0.020 -0.024 -0.028 200

250

Temperature (℃ )

300

350

400

450

500

Temperature (℃)

Figure 3. TGA curves (a) and DTG curves (b) of PVA and PVA/ESC composites in N2 condition at heating rate of 10 ℃/min-1. Ti and T50% refer to the temperature where 5 wt% and 50 wt% mass loss rate take place. The thermal behavior of composite can be markedly influenced by interaction of matrix and fillers. The experimental TGA and the theoretical TGA of the composites are shown in


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Figure S1. The value of interaction can be calculated by ratio ∆α=(αexp-αtheo)/αtheo, where αexp and αtheo are experimental and calculated degree of conversion.30 As shown in Figure 4, the value ∆α＜0 indicates better stability than predicted between 200 and 400 ℃, which clearly in turn demonstrate that esculetin play the key role in the improved thermal stability. 0.4 0.0 -0.4

∆α

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.8 -1.2 ESC 0.2 wt% ESC 0.5 wt% ESC 1.0 wt% ESC 2.0 wt%

-1.6 -2.0 200

250

300

350

400

450

500

Temperature (℃)

Figure 4. Dependence of the interaction ∆α on the degradation time for PVC/ESC composites. Isothermal TG To futher studying the effects of esculetin on thermal stability, isothermal TGA are conducted. The temperature of 250 and 280 ℃ are chosen, because between 250 and 280 ℃ unzipping degradation takes place at suitable and measureable rate. The isothermal TGA shows PVA/ESC composites express same stability phenomenon as the nonisothermal TGA (as shown in Figure 5). During 30 min of heating PVA loses 20% and 32% of the initial



mass, respectively. Compared with PVA, PVA/ESC (2 wt %) loses barely 1% and 3%, confirming its superior thermal stability to pure PVA.

a

100

100

b

95

T=250℃

90

90

Weight (%)

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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85 PVA

80

0

PVA

70

ESC (0.5 wt%)

70

80

ESC (0.2 wt%)

ESC (0.2 wt%)

75

T=280℃

ESC (0.5 wt%)

ESC (1.0 wt%)

ESC (1.0 wt%)

ESC (2.0 wt%)

ESC (2.0 wt%)

5

10

15

20

25

30

60

0

5

10

15

20

25

30

Time/min

Time/min

Figure 5. TGA curves of PVA/ESC composites isothermal degradation in N2 at (a) 250 ℃ and (b) 280 ℃. After degradation at 280 ℃ for 30 min, an intense, broad infrared absorption band is detectable in range of 1600−1700 cm-1 for PVA, which can be assigned to unsaturated C=C double bonds in polymer backbone as a result of sever degradation. In contrast, PVA/ESC composite, which contains only about 0.5 wt% esculetin, shows almost no new absorption band. The ratio of absorption peak areas corresponding to C=C and C-H stretching of PVA/ESC composites are utilized to illustrate the change of C=C content. As shown in Figure 6a, AC=C and A-CH- are the absorbance peak areas of C=C groups at ~1660 cm-1 and C-H stretching at ~2939 cm-1, respectively.31 With increasing esculetin contents, a significantly reduce in the absorbance peak areas ratio (AC=C/A-CH-) is observed, which further confirms esculetin suppress the unzipping degradation of PVA (Figure 6b). As shown in Scheme 2, the renascent hydroxyl


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radicals can lead to a chain unzipping degradation of PVA. Esculetin can scavenging the radicals and convert those radicals into stable molecules, thus inhibiting the unzipping decomposition of PVA. Therefore, the thermal stability can be dramatically improved.

0.6

a

b After thermal degradation

0.5

Ac=c/A-CH-

ESC-2.0 wt%

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ESC-1.0 wt%

ESC-0.5 wt%

0.4

0.3

ESC-0.2 wt%

0.2

Before thermal degradation

PVA

4000

3500

3000

2500

2000

1500

1000

0.1

0.0

0.5

-1

Wavenumbers (cm )

1.0

1.5

2.0

ESC (wt%)

Figure 6. (a) FTIR spectra of PVA and PVA/ESC blends after thermal degradation in nitrogen at 280 ℃ for 30 min. (b) Relative absorbance intensity (AC=C/A-CH-) as a function of the contents of ESC for PVA/ESC composites before and after thermal degradation.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Scheme 2 The reactions of esculetin with radicals.

Nonisothermal degradation kinetics analysis The Kissinger-Akahira-Sunos model was determined to evaluate the activation energy (E) of samples .32, 33 The reaction kinetic parameters are obtained from TGA data in Figure S2. By integrating the equation. ோ஺ ா ଵ ߚ ݈݊ ቌ ൘ܶ ଶ ቍ = ݈݊ ቂாீ(ఈ)ቃ − ோ · ் ഀ ఈ

‫= )ߙ(ܩ‬

ோ் మ ாഀ

݁ ೃ೅

షಶഀ


(1)

(2)

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where dα/dt is reaction rate, α the conversion, β the heating rate, T the temperature, β the heating rate, Eα the activation energy, A is Arrhenius parameters, and R gas constant. E can be obtain from the plot of ln(β/T2) versus 1/T (Figure 7). E of PVA is about 90~130 kJ mol-1. While PVA/ESC (2 wt %) own higher value (about 150 kJ mol-1).With the addition of esculetin, PVA is more difficult to be degraded, and more activation energy is required for decomposition.

a

-9.0

b -9.5

-9.5 -10.0

-10.0 2

ln(β/T )

2

ln(β/T )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-10.5

10%

10%

-11.0

-10.5

-11.0

20%

30%

30%

40%

40%

-11.5

1.5

20%

1.6

1.7 1.8 -1 1000/T (K )

1.9

2.0

-11.5 1.44

1.52

1.60

1.68

1.76

-1

1000/T (K )

Figure 7. The relation between ln (β/T2) and 1000/T for (a) PVA and (b) PVA/ESC (2 wt %) composite. Table 1 Activation energies of PVA and PVA/ESC (2 wt %) composite PVA α (%)

PVA/ESC (2 wt %)

E(kJ mol-1 )

R2

E(kJ mol-1)

R2

10

94.4

0.97193

146.1

0.98840

20

92.7

0.98810

148.2

0.99062

30

109.3

0.98830

149.3

0.99193

40

131.8

0.99125

153.2

0.99283



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

AFM The thermal properties of PVA/ESC composite is further investigated by measuring the surface topography of the samples before and after heating in nitrogen. As shown in Figure 8, PVA and PVA/ESC (2 wt %) composite all show continuous and smooth surfaces. After heating the samples to 280 ℃ in nitrogen, clear bright dots and surface cracks are observed in pure PVA. Normally, the increase in surface roughness is caused by the migration of low molecular weight products in the gas phase due to degradation. Compared to PVA, PVA/ESC composite still retain smooth surface after heating in nitrogen indicating high thermal stability of composites.


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Figure 8. AFM high images of the PVA (a, a’) and PVA/ESC (2 wt %) composite (b, b’) before and after heating the sample to 280 ℃ in nitrogen.

Generally, the thermal stability determines the processing and applications of polymeric material. The advantages of PVA/ESC composites compare with other PVA based material are shown in Figure 9. It is noted that PVA/ESC composites exhibit much higher Ti and T50% values as compared to PVA composites based on other fillers. All the results show above consistently indicate that the thermal stability of PVA/ESC is much more superior to pure PVA and the other PVA composites.

420 PVA/VTMS-5 [42] PVA/GMGO [38]

400

360

PVA/ESC-2 [this work]

PVA/CS [37]

PVA/SiO2 [44]

PVA/GO-1 [36]

380

T50% (℃ )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PVA/4N-1 [14]

PVA/Fe3O4 [40]

PVA/CuO [39]

PVA/sPPTA-6.5 [35]PVA/MMT-5 [43]

PVA/ESC-1 [this work] PVA/Nanodiamond-5 [45]

340

PVA/TiO2-30 [34]

PVA/go-0.7 [15]

Pure PVA

320

PVA/MA-0.5 [13] PVA/CNS [41]

300 200

250

300

350

400

Ti (℃) Figure 9. Comparison of Ti and T50% of PVA/ESC and other PVA based composites.34-45 UV-Shielding Performance of PVA/ESC Films.



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During the last few decades, many approaches have been employed to develop photoprotective materials, especially UV absorbing embedded in a polymeric matrix. Thus, in this part an attempt is made to research the influence of esculetin on the optical properties of PVA system. Figure 10 shows UV-vis transmittance spectra of PVA and PVA/ESC composites in the wavelength range from 200 to 800 nm. There is no obvious absorbance of PVA in UVB region, and PVA is almost transparent to all of the UVA light and visible light. In contrast, PVA/ESC composite has relatively wider UV-absorption. When only 0.5 wt% of esculetin is used, most of the UVC light and a part of the UVA light are filtered out. When the ESC content increases to 1.0 wt %, all of the UVA light is efficiently blocked without any deterioration of the high transparency (approximately 80%) of the film. the efficient UVA-shielding properties of the composites makes the composites suitable for use as shielding materials for various optical applications, including optical filters and lightshielding glass.


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100

UVC UVB

UVA

80

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40 PVA ESC (0.2 wt%) ESC (0.5 wt%) ESC (1.0 wt%) ESC (2.0 wt%)

20

0 200

300

400

500

600

700

800

Wavelength (nm)

Figure 10. UV−vis light transmittance spectra of PVA and PVA/ESC composites. Background: digital photographs of PVA/ESC (2 wt %) films. The absorbance spectra recorded in the UV−vis region is depicted in Figure 11a. PVA exhibits almost no absorption in the region for wavelength greater than 220 nm. The absorption edge can be extended to approximately 450 nm upon addition of a small amount of esculetin. The appearance of new peak and there relative broadening support the possible interaction between the PVA matrix and esculetin dopant owing to the absorption edge of PVA and filler in the composite film. The increase in the filler content increases the number of light absorbing molecules, which in turn accounts for the increased absorbance. This agrees with the transmittance result. The optical energy band gap was calculated by Tauc’s expression ߙℎߥ = ߚ(ℎߥ − ‫ܧ‬௚ )ଶ


(3)


ߙ=

ଶ.ଷ଴ଷ ௗ

݈‫ ݃݋‬ூ = ( ூ

బ

)‫ܣ‬

ଶ.ଷ଴ଷ ௗ

(4)

where, A is the absorbance and d is the film thickness, h is Planck’s constant, ν is the frequency of the incident photons, β is a constant, Eg is optical energy band gap. The polts of (αhν)1/2 and as a function of photon energy (hν) yields a straight line. The extrapolation of linear portions of these curves gives the values of optical band gaps for the composites. Ascan be seen from Figure 11b, Eg for pure PVA film is around 5.11 eV, indicating that the material is an insulator. However, incorporation of esculetin results in a decrease in band gap energies from 5.11 to 4.46 eV, with an increase in esculetin loadings from 0 to 2 wt%. The observed variation in Eg with may be due to the formation of CTCs between esculetin and −OH groups of PVA, as supported by FTIR analysis. This decrease in Eg with increase in filling levels enhances the suitability of such materials for optoelectronic applications which demands band gap tunability.

5

b

PVA ESC (0.2 wt%) ESC (0.5 wt%) ESC (1.0 wt%) ESC (2.0 wt%)

800

(αhν) (cm eV)

4


2

a

600

-1

3

2

2

Absorbance

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1

200

0 200

400

300

400

500

600

700

800

0 2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Photon energy (eV)

Wavelength (nm)

Figure 11. (a) UV-vis absorption spectra of PVA and PVA/ESC composite films. (b) Tauc’s plots for PVA and PVA/ESC composite films.


Page 21 of 29

For quantify the UV shielding performance, photocatalytic degradation of RhB solution in the presence of TiO2 nanoparticles are carried out. As shown in Figure 12, PVA-protected RhB solution is almost completely degrade after irradiation for 60 min, while the RhB solution protected by PVA/ESC-0.5 wt % shows a decrease of 40%. When PVA/ESC (2 wt %) is used as the protecting film, only 3% of RhB degrade, indicating the excellent UVshielding efficiency of composite. Interestingly, by incorporating esculetin into PVA can efficiently shifts their UV-absorption to cover the entire UV range, allowing them to qualify as a candidate for blocking the whole UV light region.

100

80

At/A0 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60


20

0

0

10

20

30

40

50

60

Time (min)

Figure 12. Photodegradation curves of RhB solutions protected by PVA and composite films (The inserted photographs: the original RhB solution, and the solution protected with PVA and with composite).



Photoluminescent behavior of PVA/ESC composites For reconfirming the structural change and UV-shielding of PVA by addition of esculetin, the fluorescence spectrophotometer were carried out. As can be seen in Figure 13, PVA shows no absorbance, while PVA/ESC composites display typical dual fluorescence behaviors with a peak at 460 nm after UV excitation. With increasing concentration of esculetin, the emission intensity gradually increases and towards longer wavelength due to the formation of CTCs of esculetin-PVA. The results also clearly reveals the interaction between esculetin and PVA, inducing the changes of structure of PVA. The photograph of composite is shown in the inset if Figure 13. The composite film shows distinct blue luminescence under UV irradiation, which correspond to their emission spectra. On the other hand, the mechanism for excellent UV-shielding of PVA/ESC composite can be explained like this. The composite film can absorb and effectively convert UV light into blue light.

PVA ESC (0.2 wt%) ESC (0.5 wt%) ESC (1.0 wt%) ESC (2.0 wt%)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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400

450

500

550

600

Wavelength (nm)


650

700

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Figure 13. Emission spectra of PVA and PVA/ESC composite films excited with the excitation light of 380 nm (inset: photograph of the composite film under a 365 nm lamp). Conclusions In summary, we have successfully prepared PVA/esculetin composites film using simple solution processing technique like solution-casting and air-drying. Only adding 2 wt % of esculetin can make Ti of PVA increase by 80 ℃ . Esculetin can inhibit the unzipping decomposition of PVA due to its radical scavenging capabilities. The result will provide the fundamental knowledge for thermal processing of PVA. Most importantly, PVA composites can block the entire UVA range without side-effect, i.e., photocatalytic activity and remain its optical property. The films show distinct blue luminescence and can dissipate and effectively convert UV light into blue light. Their simple and large-scale thermal melt process, long-term stability, varying solid device structures, bright luminescence, and high full-UV-shielding efficiency render these materials highly promising and applicable for optical materials and devices. Supporting Information. Experimental and theoretical TGA curves of PVA/ESC composites nonisothermal degradation at 10 ℃ min-1 (Figure S1); TGA curves of PVA and PVA/ESC (2 wt %) composite with different heating rates (Figure S2). AUTHOR INFORMATION Corresponding Author



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

*Weifu Dong, E-mail: [email protected].

Tel.: +86-510-8532-6290; Fax: +86-510-

8591-7763. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (51373070), the Research Project of Chinese Ministry of Education (No.113034A) and the Fundamental Research Funds for the Central Universities (JUSRP51624A).

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