MgAl

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Article Cite This: ACS Omega 2019, 4, 10151−10159

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Effect of Divalent Metals on the UV-Shielding Properties of MII/MgAl Layered Double Hydroxides Yi Zhang,†,§ Jiaqi Yang,†,§ Faying Fan,*,†,‡ Binju Qing,†,‡ Chaoliang Zhu,†,‡ Yifei Shi,†,‡ Jie Fan,†,‡ and Xiaochuan Deng*,†,‡ Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources and ‡Qinghai Engineering and Technology Research Center of Comprehensive Utilization of Salt Lake Resources, Chinese Academy of Sciences, Xining 810008, China § University of Chinese Academy of Science, Beijing 100049, China Downloaded via 37.9.41.93 on August 13, 2019 at 10:33:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Both the particle size and compositions have a strong influence on the UV-shielding performance of layered double hydroxides (LDHs), and they will interact with each other. To investigate the effects of divalent metal ions on the UV-shielding properties of layered double hydroxides (LDHs), MII/MgAl−CO3 LDHs (M = Mg, Co, Ni, Cu, or Zn) with the same primary and secondary particle size have been prepared and their UV-shielding performance have been studied in this work. The UV−vis spectra show that the ZnMgAl-LDHs exhibit the highest absorbance under the ultraviolet B and ultraviolet A rays among these LDH samples, but in the ultraviolet C region, the CuMgAl-LDHs show the highest absorbance and this result is in good accordance with the UV-shielding performance, which was examined by protecting the photocatalytic degradation of rhodamine B aqueous solution. Moreover, under UV rays, PP/ZnMgAl-LDH films show excellent resistance to UV aging, which can be attributed to the strong inhibition of ZnMgAl-LDHs during the production of free radicals in polypropene, as has been confirmed by electron spin resonance results.

1. INTRODUCTION Ultraviolet (UV) irradiation is a part of sunlight. In virtue of its lower wavelength (UVA: 400−320 nm, UVB: 320−280 nm, and UVC: 280−200 nm) and higher energy (over 3.2 eV per photon) than the visible light, it is well known that long-term UV irradiation will cause many problems in practical applications.1,2 For examples, the polymers and coatings will decompose, resulting in the impairments of their appearance and mechanical properties; inks and colors can be faded; living organisms including human beings also can be damaged through excessive exposure to sunlight, which leads to sunburn and/or skin cancer.3−5 Therefore, materials with UV-shielding properties are very important for many applications, and it is very necessary to prepare UV absorbers with excellent UVshielding performance. Generally, there are two types of UV absorbers: organic and inorganic.6 Organic UV absorbers are molecules with conjugated π-electron systems; for example, various derivatives of stilbene are capable of absorbing UV light. However, organic molecules are prone to direct photodegradation, migration, and loss in the radical products, which result in limited protection time.7,8 Inorganic UV-shielding agents are not decomposed under UV rays and are stable at neutral pH, thus being suitable for use in UV protection. Therefore, it is very urgent to search for novel UV absorbers with good stability and excellent UV light-shielding performances.9−12 © 2019 American Chemical Society

Layered double hydroxides (LDHs) are a class of anionic clay compounds, consisting of positively charged two-dimensional brucite type host layers and a negatively charged exchangeable interlayer guest.13,14 The chemical formula of LDHs can be generally described as [M1−x2+Mx3+(OH)2]x+Ann−·mH2O, where M2+, M3+, and An− represent divalent and trivalent metal cations and interlayer anions, respectively.15,16 Owing to the flexibility of compositional tailoring in the metal elements and interlayer anions, as well as their unique physical and chemical properties, such as ion exchange capacity, surface adsorption capacity, and memory effect, LDHs have been widely used as catalysts,17−19 adsorbents,20−22 nanocomposite hydrogels,23,24 molecular containers,25 polymer additives,26−29 and optical and electrical functional materials.30−35 Recently, MgAl-LDHs have been used as UV-blocking materials, and they have been added to polypropene (PP),36,37 poly(ethylene terephthalate),38 poly(methyl methacrylate),39 polyurethane,40 bitumen,41 and fabric42,43 to improve the antiUV ability, owing to the virtues of relatively low photocatalytic performance, being nontoxic, and of low cost compared with the currently used inorganic UV inhibitors, such as TiO2 and Received: March 19, 2019 Accepted: May 16, 2019 Published: June 11, 2019 10151

DOI: 10.1021/acsomega.9b00721 ACS Omega 2019, 4, 10151−10159

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ZnO.37,44 However, the MgAl-LDH filler tends to be excessively added in the matrix because it shows low absorbance to UV rays.45 Fortunately, recent studies have found that the UV-shielding properties of MgAl-LDHs can be further improved by introducing transition metal elements, such as Zn, Fe, and Ti.46−48 To optimize the UV-shielding performance and decrease the additive amount of LDHs, Wang et al.49 have synthesized a series of MgZnAl-LDHs with varying amounts of zinc ions in the host layers, and the result shows that the extent of UV absorption increases with the increasing amount of zinc in the host layers. Shi et al.46 have studied the UV-shielding mechanism of LDHs, and both the experimental and theoretical results indicate that the introduction of Zn element is an effective way to improve the UV-shielding properties of MgAl-LDHs by tuning the electron structure, band gap, and transition mode. Wang et al.48 have incorporated Fe3+ into MgAl-LDHs, which efficiently shift their UV absorption to cover the entire UV range (200− 400 nm), and the obtained coating exhibits excellent stability and long-term UV-shielding performance. However, there are rare reports that have systematically studied the influences of divalent metals for the UV-shielding performance of MgAlLDHs. In addition, it is reported that the chemical composition, physical morphology, and particle size have a strong influence on the UV-blocking properties of LDHs.46,48 Cao et al.38 have found that the lateral platelet size of LDHs has a significant effect on the UV-shielding performance of the nanocomposite film, and 460 nm-sized LDHs show a much enhanced UVshielding effect than 65 nm-sized LDHs. Wang et al.49 have prepared a series of LDHs with secondary particle size distributions from 136 to 218 nm, and the UV screening properties of ZnAl-LDHs increases with the enhancement of the particle size. Unfortunately, most of the reported literature have typically ignored the size-dependence effect when investigating the relationship between the UV-shielding property and chemical/structural parameters of LDH materials. In this work, to investigate the effect of metal elements on the UV-blocking performances of LDHs without the disturbance from the size-effect, MIIMgAl-LDH (M = Co, Ni, Cu, Zn) nanoparticles with the same primary and secondary particle size have been prepared by a green and simple method. The structures and UV-shielding performance of these LDHs were further studied in detail, and the anti-UV ability of PP/LDH composites has been evaluated. In addition, the UV-blocking mechanism of PP/LDH composites has been discussed.

Figure 2. Relationship between the doping ionic radius and a-axis cell parameters.

2. RESULTS AND DISCUSSION 2.1. Structural and Morphological Characterization of MII/MgAl-LDHs. Figure 1 shows the X-ray powder diffraction (XRD) patterns of the as-synthesized MII/MgAlLDH samples. The 2θ peaks located at 11.6, 23.4, and 34.5° are ascribed to the diffractions of basal planes of (003), (006), and (009) of LDHs. There are no peaks of by-products such as metal oxides or metal hydroxides observed, indicating the purity of the five as-synthesized LDHs. All of the positions of (003), (006), and (009) diffractions for five LDH samples are the same. This is because the interlayer ion for all LDH samples is the carbonate ion, and the c parameter was calculated to be 22.8 Å. The two peaks located at 60−63° are ascribed to (110) and (113) reflections for LDHs, and the

The metal element of MII/MgAl-LDHs was obtained by the inductively coupled plasma (ICP) test, and molar ratios of metal cations in the layers were calculated from the ICP testing results and are listed in Table 1. The MII/Mg2+ ratios for all LDH samples are close to 1.00, indicating that MII have replaced half of Mg in the host layer. The water molecule content was obtained by thermogravimetry−differential scanning calorimetry (TG−DSC) (the results presented in Figure S1), and the chemical composition of MII/MgAl-LDHs are listed in Table 1. Figure 3 shows the Fourier transform infrared (FT-IR) spectra of the obtained LDH samples. All of the samples show a strong and broad adsorption band between 3500 and 3100 cm−1, owing to the OH stretching mode of the hydroxyl

Figure 1. XRD analysis of different MII/MgAl-LDHs washed with acetone: (a) Mg2Al-LDHs, (b) CoMgAl-LDHs, (c) NiMgAl-LDHs, (d) CuMgAl-LDHs, and (e) ZnMgAl-LDHs.

unequal position of samples is owing to the various metal ionic sizes of M2+ in the host layer. The unit cell parameter a of LDHs can be calculated by the formula a = 2d110, and the a parameters for Mg2Al-LDHs, NiMgAl-LDHs, ZnMgAl-LDHs, CoMgAl-LDHs, and CuMgAl-LDHs were calculated to be 3.0448, 3.0346, 3.0582, 3.0592, and 3.0506 Å, respectively. Figure 2 shows that the a parameters and metal ionic radius of M2+ on the host laminate have a good linear relationship.50

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Table 1. Chemical Composition of MII/MgAl-LDHs samples

MII/Mg2+ ratioa

Mg2Al-LDHs

M2+/M3+ ratioa 2.00

CoMgAl-LDHs

1.04

2.08

NiMgAl-LDHs

0.96

1.96

CuMgAl-LDHs

1.00

2.08

ZnMgAl-LDHs

0.93

1.89

chemical compositionb [Mg0.667Al0.333(OH)2] (CO3)0.167·0.70H2O [Co0.345Mg0.331Al0.324(OH)2] (CO3)0.162·0.74H2O [Ni0.325Mg0.340Al0.338(OH)2] (CO3)0.169·0.73H2O [Cu0.338Mg0.338Al0.324(OH)2] (CO3)0.162·0.66H2O [Zn0.315Mg0.339Al0.346(OH)2] (CO3)0.173·0.73H2O

a Molar ratio obtained from ICP data. obtained from TG−DSC data.

b

Water molecule content

Figure 3. FT-IR spectra of the LDH samples: (a) Mg2Al-LDHs, (b) CoMgAl-LDHs, (c) NiMgAl-LDHs, (d) CuMgAl-LDHs, and (e) ZnMgAl-LDHs.

groups and the interlayer water molecules, and the band slightly shift for different samples due to the replacement of Mg2+ by M2+. The sharp peaks at 1384 and 1500 cm−1 are ascribed to the interlayer carbonate species.44 The bands observed in the low-frequency region (1000−500 cm−1) are assigned to the vibration mode of the Al−OH vibration and M−O vibration.51 Scanning electron microscope (SEM) was carried out to observe the micromorphology characteristics and platelet diameters of MII/MgAl-LDHs (M = Mg, Co, Ni, Cu, and Zn). As shown in Figure 4a−e, MII/MgAl-LDHs are plate-like, and all samples are well dispersed, and no severe agglomeration was observed. The diameters of platelets are measured from the SEM images by counting about 100 nanoparticles, and the diameters of Mg2Al-LDHs, CoMgAl-LDHs, NiMgAl-LDHs, CuMgAl-LDHs, and ZnMgAl-LDHs are 63.2 ± 11.5, 61.9 ± 12.0, 60.7 ± 15.7, 53.8 ± 11.1, and 70.9 ± 12.3 nm, respectively, which demonstrates that all of the LDH samples exhibit the same primary particle size. In addition, the particle size distributions of the samples are narrow. Figure 5 shows the secondary particle size distribution of LDHs, measured by a nanoparticle size analyzer. All of the LDH samples show a narrow size distribution with d0.5 ≈ 150 nm. Combining this with the results from SEM, it indicates that all of the assynthesized LDH samples exhibit the same particle size. As reported in the literature, the chemical composition and

Figure 4. SEM analysis of different MII/MgAl-LDHs washed with acetone: (a) Mg2Al-LDHs, (b) CoMgAl-LDHs, (c) NiMgAl-LDHs, (d) CuMgAl-LDHs, and (e) ZnMgAl-LDHs.

particle size have an influence on the UV-blocking performance of LDHs.38,49 In consideration of the fact that all of the as-synthesized LDH samples in this work exhibit the same primary and secondary particle size, and narrow particle size distribution; the influence of particles on the UV-blocking performance can be ignored, which is an essential feature for the study of the influence of chemical components on the UVshielding performance of LDHs without considering the size effects. 2.2. UV-Shielding Performances and Mechanism of MII/MgAl-LDHs. The UV-shielding performance of particles is attributed by absorption and scattering.49,52 Therefore, the UV-shielding performance of the LDHs mainly decides on absorbed energy and the scattering effect on light, and they are 10153


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ZnMgAl-LDHs show high absorbance values than Mg2AlLDHs, CoMgAl-LDHs, CuMgAl-LDHs, and NiMgAl-LDHs; however, in the UVC region, the CuMgAl-LDHs show the highest absorbance value. To further evaluate the UV-shielding performance of samples, the photocatalytic performance of TiO2 to degrade rhodamine B (RhB) solution was carried out in the presence of 365 nm (UVA), 302 nm (UVB), 254 nm (UVC) light sources under the protection of ethylene-vinyl alcohol copolymer (EVOH)/LDH films. As shown in Figure 7, the pure EVOH film-protected or not protected RhB solution showed almost complete degradation after 60 min of irradiation, while the degradation is slow under the protection of EVOH/LDH films, indicating that the EVOH/LDH films have excellent UVshielding efficiency. In addition, the EVOH/ZnMgAl-LDH film exhibited the best UV-shielding performance under UVA and UVB rays and the EVOH/CuMgAl-LDHs showed the best UV-shielding performance under UVC rays, which are in good accordance with the absorbance intensity result from the UV− vis absorption spectra. Polypropylene (PP) is a type of polymer that has been widely used in the fields of food, electronic, pharmaceutical, and chemical industries owing to its high flexibility, good insulation, and low cost. However, it is unstable under longterm sunlight irradiation, and photostabilizers or UV-shielding agents must be added to the PP composites. Herein, the PP has been used as a candidate to test the UV-shielding performance of LDH samples with different divalent metals; 3 wt % of LDHs were added to PP by melt blending and pressed into a film with a thickness of about 50 μm, and the LDH samples were washed with acetone before being added into PP for the purpose of improving the dispersion of LDHs in PP.53,54 Figure 8 shows the FT-IR spectra of pure PP and PP/LDH composite films after different UV exposure times. The peak at 841 cm−1 is ascribed to the C−H out-of-plane deformation of −CH3, and the peak at 1724 cm−1 is a characteristic absorption associated with the carbonyl group of the decomposition products. After 10 h of UV irradiation, the absorption strength of PP at 1724 cm−1 gradually increased, but the intensity of 841 cm−1 was unchanged (as depicted in Figure 8A). The photo-oxidative degradation mechanism of PP has been widely studied, and the photodegradation process includes three steps: initiation, propagation, and termination reactions.55,56 Initiation involves the formation of an initial radical. A small amount of catalyst residues and groups, such as carbonyl and hydroperoxide, mixed with commercial PP during synthesis, processing, and storage. After exposure to UV radiation, the C−H bond of tertiary carbon was oxidized and undergoes cleavage, followed by the generation of initial radicals, such as carbonyl and hydroxy radicals, as well as the breaking of chains. In the propagation reaction, more and more carbonyl groups and free radicals were generated accompanied by serious chain breaking. The termination of photodegradation is achieved by

Figure 5. Particle size distributions of MII/MgAl-LDHs measured by a nanoparticle size analyzer.

correlated with the intrinsic optical properties and particle size or morphology of the samples. Since the morphology and the primary and secondary particle sizes of the five LDHs are similar, their scattering effect on light is the same, and the UVshielding difference is mainly caused by the intrinsic optical properties of LDHs. UV−vis absorption spectra in an aqueous dispersion (80 mg/L) of MII/MgAl-LDHs (M = Mg, Co, Ni, Cu, and Zn) were employed to characterize the optical properties of samples (shown in Figure 6). ZnMgAl-LDHs

Figure 6. UV−vis absorption spectra of Mg2Al-LDHs and MII/MgAlLDHs (M = Co, Ni, Cu, and Zn) in aqueous dispersion.

show high absorbance values than Mg2Al-LDHs, CoMgAlLDHs, and NiMgAl-LDHs in the UVB and UVA regions, but the CuMgAl-LDHs show the highest adsorption in the UVC region. Table 2 shows the absorbance values of LDH samples by integrating the absorbance in the wavelength range of 200− 400 nm. In the whole UV, UVA, and UVB regions, the

Table 2. Absorbance Values of LDH Samples at Different UV Regions

absorbance values

samples

Mg2Al

CoMgAl

NiMgAl

CuMgAl

ZnMgAl

UV (200−400 nm) UVC (200−280 nm) UVB (280−320 nm) UVA (320−400 nm)

33.45 20.06 5.57 7.82

53.56 35.73 7.62 10.21

30.05 17.51 5.13 7.41

59.42 44.70 6.52 8.20

60.30 38.39 9.67 12.24

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Figure 7. (a) Schematic illustration of UV-shielding performance testing of the EVOH/LDH films. (b−d) Photocatalytic degradation rate curve of EVOH/LDH film-protected RhB solutions under (b) UVA (λmax = 365 nm), (c) UVB (λmax = 302 nm), and (d) UVC (λmax = 254 nm).

cm−1 shows an unapparent change with irradiation time compared to the pure PP, indicating that the UV stability of the PP nanocomposite is improved by adding a small amount of LDHs. To further study the difference in antiaging properties of different PP/LDH composites, the internal standard method is used to compare the behavior of the pure PP and PP/LDH composites. As the intensity of the peak at 841 cm−1 (C−H out-of-plane deformation) is unchanged under UV irradiation, it is often used as a standard peak.57−59 The intensity of the peak at 1724 cm−1 (carbonyl stretching vibration) increases gradually under UV irradiation, and the absorption intensity ratio I1724/I841 is plotted as a function of aging time in Figure 9a. The results clearly show that I1724/I841 increases with irradiation time, but the change of PP/LDHs is small than pure PP, indicating that the addition of LDHs significantly improved the photostability of PP, and the aging of PP was inhibited. Figure 9b shows the I1724/I841 increasing intensity of the pure PP and PP/LDH composites after 10 h of UV irradiation. The increasing intensity of PP/ZnMgAl-LDH films (0.10) is less than those of PP/Mg2Al-LDH films (0.38), PP/ CoMgAl-LDH films (0.14), PP/NiMgAl-LDH films (0.27), and PP/CuMgAl-LDH films (0.37). It shows that ZnMgAlLDHs are the most advantageous anti-UV agents in inhibiting PP photoaging. To investigate the mechanism for photostability of PP/ LDHs, the spin-trapping electron spin resonance (ESR) was employed to monitor the presence of active radicals in PP/ LDH composites. As presented in Figure 10, all PP/LDH composites have a signal at about g = 2.000, and all of the signals increase after 365 nm UV light irradiation for 10 min, indicating that the fresh PP/LDH composites have unpaired electrons or radicals, and the amount of the unpaired electrons or radicals increases under the UV light irradiation. Interestingly, the increasing intensity of the signal for pure PP, PP/Mg 2Al-LDHs, PP/CoMgAl-LDHs, PP/NiMgAlLDHs, PP/CuMgAl-LDHs, and PP/ZnMgAl-LDHs is 124, 54, 46, 118, 51, and 5%, respectively. It demonstrates that the

Figure 8. FT-IR spectra of (A) pure PP and (B) PP/Mg2Al-LDH, (C) PP/CoMgAl-LDH, (D) PP/NiMgAl-LDH, (E) PP/CuMgAlLDH, and (F) PP/ZnMgAl-LDH nanocomposite (3 wt %) films after different UV exposure times (a: 0 h, b: 2 h, c: 4 h, d: 6 h, e: 8 h, and f: 10 h).

“mopping up” the free radicals and by the formation of inert products. With the increase of UV irradiation time, the intensity of the carbonyl group at 1724 cm−1 increases in Figure 8A,a−f, indicating that pure PP is rapidly decomposed. Fortunately, the peak of PP/LDH nanocomposites at 1724 10155


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Figure 9. (a) Ratios of I(carbonyl)/I(C−H) and (b) increase of I(carbonyl)/I(C−H) for pure PP and PP/Mg2Al-LDH, PP/CoMgAl-LDH, PP/NiMgAlLDH, PP/CuMgAl-LDH and PP/ZnMgAl-LDH nanocomposite films after different UV exposure times.

Figure 10. Dimethylpyridine N-oxide (DMPO) spin-trapping ESR spectra recorded for films under dark conditions (red line) and after 365 nm UV light irradiation for 10 min (blue line): (a) pure PP, (b) PP/Mg2Al-LDHs, (c) PP/CoMgAl-LDHs, (d) PP/NiMgAl-LDHs, (e) PP/CuMgAlLDHs, and (f) PP/ZnMgAl-LDHs.

3. CONCLUSIONS To investigate the effect of metal elements on the UV-blocking performances of LDHs without the influence of the particle size, MIIMgAl-LDH (M = Co, Ni, Cu, and Zn) nanoparticles with the same primary and secondary particle size have been prepared by a method involving separate nucleation and aging steps (SNAS). XRD reveals the relationship between the unit cell parameters and the ionic radius of the constituent elements of host layers. SEM and particle size analyses showed that all LDH samples have the same primary and secondary particle size. The UV−vis spectra and the UV-shielding performance that was examined by the photocatalytic degradation of RhB

increasing intensity of the ESR signal for PP/LDHs is lower than that of pure PP, indicating that the presence of LDHs could inhibit the production of radicals in composites. In addition, the ZnMgAl-LDHs show the strongest inhibition for the production of radicals in composites, followed by CoMgAlLDHs, CuMgAl-LDHs, Mg2Al-LDHs, and NiMgAl-LDHs. The above results indicate that the LDHs inhibit the production of radicals in PP and hinder the chain degradation, increasing the antiaging performance of PP under UV irradiation. 10156


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of LDHs were determined by thermogravimetry and synchronous thermal analyzers (METTLER-TOLEDO, TGA/DSC 3+, Switzerland) in the temperature range of 20−700 °C with a heating rate of 5 °C/min in an N2 atmosphere. FT-IR spectra were carried out using FT-IR spectrometer (Thermo Nicolet, Nexus). The generation of active radicals in PP composite films before and after UV illumination was measured by an electron spin resonance (ESR) spectrometer (Bruker, ER200-SRC-10/12, Germany) with dimethyl pyridine N-oxide (DMPO) as a spin-trapping reagent in the central magnetic field of 3500.00 G with a microwave power of 3.99 mW. Subsequently, the films were illuminated with a UV lamp (λmax = 365 nm) for 10 min and then tested by the ESR spectrometer again. 4.4. UV-Shielding Performance Testing of EVOH Nanocomposite Films. The photodegradation behavior of the RhB solution was carried out in the presence of TiO2 (Degussa P25) and UV lamps with different wavelengths (254, 302, and 365 nm) to evaluate the UV-shielding performance of the EVOH nanocomposite films.63 Briefly, 40 mg of TiO2 were mixed and completely dispersed into 60 mL of RhB solution (0.01 g/L). Prior to irradiation, the suspension was stirred in the dark at ambient temperature for 60 min to achieve an adsorption/desorption equilibrium. The mouth of the beaker was covered with an EVOH or EVOH/LDH nanocomposite film prior to UV irradiation. At a given time interval (t), the absorbance at 555 nm for 3 mL of the suspension from which the photocatalysts were removed was measured by a TU1810 UV−vis spectrophotometer. The UV-shielding performance of EVOH/LDHs was calculated as I = At/A0 × 100%, where A0 is the initial absorbance value of the RhB solution without UV radiation, and At is the absorbance of the solution with UV radiation. 4.5. UV Aging Resistance Test of PP Nanocomposite Films. Samples of pure PP film and PP/LDH films were rapidly photoaged in a UV photoaging instrument (a UV highpressure mercury lamp as the UV light source, a power of 400 W, and λmax = 340 nm) with a temperature controlling system at 60 °C. Fourier transform infrared (FT-IR) spectra of the PP composite films were recorded after UV irradiation every 120 min.

aqueous solution in the presence of TiO2 indicate that the ZnMgAl-LDHs show the highest shielding performance in the UVB and UVA regions, but in the UVC region, the CuMgAlLDHs show the highest shielding performance. Furthermore, PP/ZnMgAl-LDHs exhibit good photoaging resistance for UV irradiation. This is because the strong inhibitory effect of ZnMgAl-LDHs on the production of free radicals in PP under UV irradiation was confirmed by the ESR tests. Therefore, it is expected that the ZnMgAl-LDHs show potential practical application as anti-UV agents for PP as well as for a variety of polymers that are also prone to UV aging.

4. EXPERIMENTAL SECTION 4.1. Preparation of LDH Samples. The LDH samples in this work were synthesized using a method involving separate nucleation and aging steps (the SNAS method).60 Typically, equal volumes of the salt solution (including 0.4 M MIICl2, 0.4 M MgCl2, and 0.4 M AlCl3; M = Mg, Co, Ni, Cu, and Zn) and the alkali solution (including 2.4 M NaOH and 0.4 M Na2CO3) were simultaneously added to the reactor at the same rate of 10 mL/min and obtained the slurry. To obtain the LDH samples with the same particle size, the obtained slurry was crystallized at different temperatures for a suitable period of time under continuous stirring. Mg2Al-LDHs, ZnMgAlLDHs, and CoMgAl-LDHs were crystallized at 80 °C for 8 h, NiMgAl-LDHs were crystallized at 140 °C for 24 h, and CuMgAl-LDHs were crystallized at 80 °C for 24 h. The products were washed several times with deionized water until the pH of the supernatant was close to 7. The products were washed three times with acetone to have a good dispersion in the polymer and finally dried in an oven at 60 °C.61 4.2. Preparation of EVOH/LDH and PP/LDH Nanocomposite Films. EVOH/LDH nanocomposite films prepared by solution casting.62,63 Typically, MIIMgAl-LDH samples were respectively added into a solution of dimethyl sulfoxide containing 5 wt % of EVOH and stirred to obtain a homogeneous mixture. The nanocomposite films with an average thickness of ∼50 μm were then obtained by drying the homogeneous mixture in a vacuum at 60 °C for 24 h. PP/LDH nanocomposite films with the weight of LDHs is 3% were prepared by melt blending. For this, 0.9 g of MIIMgAl-LDHs were mixed with 30.0 g of PP in a mixer equipped with two counter-rotating rotors at 180 °C and mixed for about 10 min to produce the corresponding composite. The obtained composites were melded into a sheet with a thickness of 50 μm at 180 °C. The pure PP film was also prepared under the same conditions without the addition of LDHs. 4.3. Characterization. The crystal structure of the samples was measured using a Dutch PANalytical X’Pert Pro type X-ray diffractometer (Cu target, Kα ray, λ = 0.15406 nm, scanning speed 5°/min, and diffractions range from 5 to 80°). Elemental analyses were done by inductively coupled plasma (ICP) emission spectroscopy, and the LDHs were dissolved in dilute hydrochloric acid before testing. The microstructure of the sample was measured by a field-emission scanning electron microscope (Hitachi, SU8010, Japan) with an acceleration voltage of 20 kV. The particle size of the sample was measured by a nanoparticle size and ζ-potential analyzer (Malvern, ZS, U.K.). The UV−visible absorbance curves of the samples were measured using a UV−vis spectrophotometer (Purkinje, TU1810, China) by dispersing the LDH samples in water (with the concentration of 80 mg/L). The thermal properties

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00721. TG−DSC curves of Mg2Al-LDHs, CoMgAl-LDHs, NiMgAl-LDHs, CuMgAl-LDHs, and ZnMgAl-LDHs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.F.). *E-mail: [email protected]. Tel/Fax: +86-0971-6369067 (X.D.). ORCID

Faying Fan: 0000-0003-0964-0138 Notes

The authors declare no competing financial interest. 10157


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ACKNOWLEDGMENTS This work was financially supported by the Science & Technology Projects of Qinghai province (2016-GX-103 and 2016-ZJ-927Q) and the funds of the Chinese Academy of Sciences (Y610101037 and KFJ-HGZX-008).

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