Improving the Transparency of Ultra-Drawn Melt-Crystallized

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Improving the Transparency of Ultra-Drawn Melt-Crystallized Polyethylenes: Toward High-Modulus/High-Strength Window Application Lihua Shen, Koen Nickmans, John Severn, and Cees W. M. Bastiaansen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04704 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 25, 2016

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Improving the Transparency of Ultra-Drawn Melt-

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Crystallized Polyethylenes: Toward High-

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Modulus/High-Strength Window Application

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Lihua Shen1, Koen Nickmans1, John Severn1,2, Cees W.M. Bastiaansen1,3*

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1

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P.O. Box 513, 5600 MB (The Netherlands)

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E-mail: [email protected]

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2

DSM Ahead B.V., NL-6160 MD Geleen, The Netherlands

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3

School of Engineering and Materials Science, Queen Mary, University of London, London E1

Laboratory of Functional Organic Materials and Devices, Eindhoven University of Technology,

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4NS, UK.

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E-mail: [email protected]

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KEYWORDS: High-density polyethylenes, solid state drawing, transparency, modulus, strength

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ABSTRACT: Highly transparent, ultra-drawn high-density polyethylene (HDPE) films were

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successfully prepared using compression molding and solid-state drawing techniques. The low

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optical transmittance (< 50%) of the pure drawn HDPE films can be drastically improved (>

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90 %) by incorporating a small amount (> 1% wt/wt) of specific additives to HDPE materials

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prior to drawing. It is shown that additives with relatively high refractive index result in an

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increased optical transmittance in the visible light wavelength which illustrates that the

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improvement in optical characteristics probably originates from refractive index matching

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between the crystalline and non-crystalline regions in the drawn films. Moreover, the optically

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transparent drawn HDPE films containing additives maintain their physical and mechanical

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properties, especially their high modulus and high strength, which make these films potentially

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useful in a variety of applications such as high-impact windows.

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INTRODUCTION

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In the past decades, solid-state drawing has been extensively used to achieve high degree of

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alignment and chain extension in semi-crystalline polymers.1-3 For instance, ultra-drawing in the

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solid state of both melt- and solution-crystallized polyethylenes results in a very high degree of

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molecular orientation and chain-extension and consequently the ultra-drawn polyethylenes

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exhibit an unusually high modulus and high strength.4,5 However, the optical transparency of the

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drawn polyethylenes, especially for high density polyethylenes (HDPE), is usually rather poor

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after solid-state drawing, which limits their usefulness in certain applications. This poor

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transmittance in the wavelength range of 400 - 700 nm normally originates from extensive light

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scattering as a result of the bulk or surface structures in the drawn materials. For instance, it has

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been proposed that the light scattering is related to internal voids both longitudinal and

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perpendicular to the drawing direction.6

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Several studies have been reported on the improvement of optical transparency of linear low-

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density polyethylenes (LLDPE)7-9 and it was shown that high transparency in the blown LLDPE

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films can be achieved. However, ultra-drawing in the solid state was not performed and, as a

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consequence, highly oriented, chain-extended polyethylene films with a high modulus and

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strength were not obtained. According to our knowledge, only one study describes the

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preparation of optically transparent, solid-state drawn HDPE.10 In this particular case, the

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transparent ultra-drawn HDPE was obtained by ultra-drawing samples at a temperature of 100-

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105 oC. In addition, the study was limited to HDPE with a broad molecular weight distribution

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(Mw ~ 208,000, Mw/Mn = 18.9). In the case of HDPE with a narrower molecular weight

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distribution (Mw ~ 93,700, Mw/Mn = 4.3), the drawn films were always opaque at all drawing

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temperatures between 40~105 oC.10

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In general, additives such as light stabilizers and antioxidants are required in polyethylenes in

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order to preserve their properties during processing or to improve their long-term stability.11-13

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Usually, these additives are added in small quantities (less 0.1 % wt/wt) to reduce cost. We

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report here that the optical properties such as transparency in the visible light wavelength range

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(400-700 nm) of the ultra-drawn HDPE films can be dramatically improved by incorporating a

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relatively high content of specific additives while the corresponding mechanical properties of

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these ultra-drawn HDPE films are successfully preserved. This study predominantly focuses on

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2-(2H-benzotriazol-2-yl)-4,6-ditertpentylphenol (BZT) as an additive which is one of the most

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widely used ultraviolet absorbers (UVA) for polymers, coatings, adhesives and sealants.

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Furthermore, a limited set of other solid- and liquid- additives are selected to explore the possible

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mechanism behind the observed improvement in optical characteristics of the ultra-drawn HDPE

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films.

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EXPERIMENTAL SECTION

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Materials. The high density polyethylene used was Borealis VS4580 (Burghausen, Germany)

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with a number- and weight-average molecular weight of approximately 3.7 × 104 and 1.3 × 105

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g/mol. 2-(2H-benzotriazol-2-yl)-4,6-ditertpentylphenol (BZT) was purchased from BASF

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(Germany). Cinnamon oil and Polystyrene oil (average Mw: 800 g/mol) were obtained from

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Sigma-Aldrich Co. (Germany). Paraffin oil was purchased from Thermo Fisher Scientific Inc.

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(Netherlands). 3M™ Dynamar™ Polymer Processing Additive FX 5911 was purchased from

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3M (Germany). All reagents were used directly as received without further purification.

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Preparation of specimens. HDPE samples containing 0.5%, 1.0%, 2.0% and 5.0% wt/wt of the

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BZT were mixed with HDPE pellets in a co-rotating twin screw extruder at 160 oC. As an

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example, the blends containing 0.5% BZT will be called PE0.5. Similar nomenclatures will be

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used for the other samples. The extrudates were cooled in a water bath at room temperature, air

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dried, then palletized into granules. Subsequently, isotropic sheets of approximately 1.5 mm

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thickness were produced by compression molding at 160 oC for 5 minutes, followed by

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quenching in water. Dumbbell-like samples with gauge dimensions 1.2 × 0.2 cm2 were then cut

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from the compression-molded sheets. These dumbbell-like samples were subsequently drawn to

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various draw ratios at 80 oC in air using a Zwick Z100 tensile tester at a crosshead speed of 100

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mm/min. The draw ratio was determined from the displacement of ink marks initially spaced at

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intervals of 1 mm or 2 mm on the sample surfaces. The thickness of the drawn samples was

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calculated by weighing assuming a density equal of 1 g/cm3,14 which can reduce errors related to

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variations of the cross-sectional area of the drawn samples along the plastically deformed zone.

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Characterization. The absorbance and transmittance spectra were measured in the range of 250-

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700 nm on a Shimadzu (Japan) UV-3102 PC spectrophotometer with a 1-nm interval, equipped

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with a MPC-3100 multi-purpose large sample compartment. In order to reduce surface scattering,

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a few drops of paraffin oil were coated on the surface of the drawn PE films and the films were

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sandwiched between two glass slides. The Young’s modulus and tensile strength of the drawn

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samples were measured at room temperature on a Zwick Z100 tensile tester at a crosshead speed

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of 100 mm/min. In all cases, at least three strips were measured and the mean values of Young`s

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modulus together with tensile strength and the corresponding standard deviation were calculated.

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The rheological properties of the polymers were characterized via a stress-controlled TA

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instruments AR-G2 rheometer (United States) using a 25 mm parallel plate-plate geometry and

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disk-shape specimens (25 mm in diameter; 1 mm in thickness). Frequency sweeps in the range of

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500 - 0.1 rad/s were performed at a temperature of 200 oC in a N2 atmosphere at a constant strain

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of 0.5%. Optical microscopy was performed on a Leica CTR 6000 microscope. Small-angle light

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scattering (SALS) patterns were recorded using a 15 mW He-Ne gas laser as a light source. VV

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patterns were obtained with the direction of polarizer and analyzer both vertical. The drawing

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direction of the films was always perpendicular to the polarizer. XRD measurements were

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performed on a Ganesha lab instrument equipped with a Genix-Cu ultra-low divergence source

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producing X-ray photons with a wavelength of 1.54 Å and a flux of 1×108 photons/second.

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Diffraction patterns were collected on a Pilatus 300K silicon pixel detector with 487 × 619 pixels

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of 172 µm2 placed at a sample to detector distance of 91 mm (WAXS). The detector consists out

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of three plates with a 17 pixels spacing in between, resulting in two dark bands on the image.

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Azimuthal integration of the obtained diffraction patterns was performed to obtain the intensity

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versus the scattering vector (q). The percentage crystallinity (Xc) was measured by WAXS using

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the following equation:  =

 + 1.46  ∙ 100%  + 0.75 + 1.46 

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Where  ,   , and  are the integral areas of the (110), (200) and the amorphous peak of

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polyethylene, respectively.15 The extent of the crystal orientation of the drawn samples was

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quantitatively described by Herman’s orientation function ( ),16,17

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3 <   > −1  = 2 1

where  is the angle between chain axis and the drawing direction and <   > is defined as, $

<   >=

%    !"# $

%  !"# 2

where () is the scattering intensity along the angle . Polarized absorption spectra of samples

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in the range of 250-500 nm were measured on a Perkin Elmer, Lambda 750 spectrophotometer

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with a 1-nm interval. The films were sandwiched between two fused quartz glasses and coated

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with paraffin oil.

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RESULTS AND DISCUSSION

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High-density polyethylene films containing up to 5% wt/wt of additives (2-(2H-benzotriazol-2-

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yl)-4,6-ditertpentylphenol), BZT) were first prepared by melt blending in a co-rotating twin

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screw extruder followed by compression-molding. The rheological properties (Figure S1) and

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solid-state drawing behaviors (Figure S2) of the mixtures were explored and the Young’s

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modulus and strength (Figure S3) after uniaxial drawing were measured. It was found that all

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these properties were hardly changed upon addition of BZT additive, even at a comparatively

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high content. In accordance with previous studies on the ultra-drawn HDPE films, it is observed

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that high draw ratios (25-30) can be obtained when the compression-molded films were drawn at

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80 oC and that melt-crystallized, uniaxially drawn films with a high modulus (> 30 GPa) and

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strength (> 0.6 GPa) can be produced.

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In Figure 1A, the absorption spectra of the drawn polyethylene films in the wavelength range

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(250-500 nm) are shown. As expected, the maximum absorbance in the ultraviolet wavelength

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range increases with increasing additive content, indicating that the BZT was successfully

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incorporated into the polyethylene matrix with little weight loss. The most striking feature of

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these drawn films is the decreasing scattering in the visible light wavelength range (400-700 nm)

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as a function of the increasing BZT content. Therefore, the corresponding transmittance of the

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drawn films was further characterized, as shown in Figure 1B. The results suggest that the

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transmittance of the drawn HDPE films without additive is relatively low (~ 49% at 550 nm) and

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it is drastically enhanced by incorporating a comparatively large amount of BZT additive (1-5 %

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wt/wt) into the films. For instance, the drawn HDPE films containing 2 % wt/wt additive (PE2)

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transmit more than 90% of the visible light. As shown in Figure 2, the drawn PE films

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containing 2 % wt/wt BZT additive (PE2) show a high degree of optical transparency and exhibit

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a glass-like appearance, even at a high draw ratio of 20. By contrast, the drawn HDPE films at an

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identical draw ratio without additives (PE) become white upon drawing, regardless of draw ratio.

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Of course, the film thickness decreases with increasing draw ratio and this might have an effect

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on transparency. On the other hand, at an identical draw ratio and film thickness, an obvious

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improvement in transparency of the drawn films is observed after addition of additives, which

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clearly illustrates that another mechanism is also operational.

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A

B

2.0 Pure PE PE0.5 PE1 PE2 PE5

1.5 1.0

Transmittance (%)

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Absorbance (a.u.)

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0.5 0.0 250

300

350 400 450 Wavelength (nm)

500

100 80 60 Pure PE PE0.5 PE1 PE2 PE5

40 20 0 400

450

500 550 600 650 Wavelength (nm)

700

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Figure 1. (A) Absorbance and (B) transmittance of the drawn HDPE films (DR=10) as a

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function of additive content (BZT). The thickness of the drawn films is around 160 µm and the

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numbers in the legends indicate the BZT content in the ultra-drawn films.

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Figure 2. Photographs of the melt-crystallized, ultra-drawn HDPE films (draw ratio: 10, 15 and

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20) containing 0, 1 and 2 % wt/wt of additive (BZT). Surface scattering is reduced by coating

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with paraffin oil.

11 12 13 14 15

A

B 100

40

Transmittance (%)

10

Maximum draw ratio (-)

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30 20 PE PE1 PE2 PE5

10 0

60

80 100 120 o Drawing temperature ( C)

PE PE1 PE2 PE5

80 60 40 20 0

0

10

20 Draw ratio (-)

30

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Figure 3. (A) Maximum attainable draw ratio of the melt-crystallized HDPE films as a function

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of draw ratio, (B) Transmittance of the drawn films as a function of the draw ratio.

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The influence of the solid-state drawing conditions on optical transparency of the drawn HDPE

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films was explored more extensively. In Figure 3A, the maximum attainable draw ratio of the

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HDPE films is plotted as a function of the drawing temperature. It can be found that the optimum

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drawing temperature of the melt-crystallized HDPE films is around 80 oC and is hardly changed

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upon addition of additives, even at a high additive content of 5% wt/wt. More importantly, it is

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also found that the optical transmittance of the drawn films in the visible light wavelength

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dramatically increases with increasing additive content at an identical draw ratio and starts to

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decrease at draw ratios higher than 20. In other words, a high optical transparency of the drawn

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HDPE is only achieved at a moderate draw ratio (10-20) which corresponds to maximum

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Young’s modulus and strength of ~30 GPa and ~0.65 GPa, respectively.

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Figure 4. (A) Schematic representation of the small-angle light scattering SALS) (VV) and the

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corresponding VV patterns of (B) glass, the drawn PE films (DR=10) containing 2% wt/wt BZT

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without (C) and with (D) coating. The thickness of the drawn films is around 160 µm and the

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arrows inset indicates the drawing direction of the samples.

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It is well-known that the drawn polyethylene films possess highly fibrillar structures and these

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structures on the surface might cause surface light scattering, thus resulting in a loss of

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transparency.18 In order to avoid surface scattering, a few drops of paraffin oil were coated on the

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surface of the drawn HDPE films and the films were subsequently sandwiched between two

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glass slides. As shown in Figure S4, a slight further improvement in optical transmittance is

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observed upon coating with a low viscous fluid. The effect seems to be small in UV-VIS

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measurements and therefore the small-angle light scattering (SALS) experiments were performed

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to further study this. In Figure 4A, a schematic representation of the SALS VV measurements is

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presented and the corresponding SALS patterns of pure glass and the ultra-drawn HDPE films

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with and without coating are compared because VV patterns have been demonstrated to be highly

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sensitive to density fluctuations, i.e., refractive index mismatch in the drawn films.19,20 It is

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illustrated that highly transparent drawn HDPE films with a truly glass-like appearance are solely

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obtained by incorporating the additive in combination with suppressing the surface scattering

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with an appropriate coating.

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Figure 5. SALS Vv patterns of the drawn pure PE films (A, B) and the drawn PE films

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containing 2% wt/wt BZT (C, D). Left: draw ratio=10; Right: draw ratio=25. Surface scattering

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is reduced by coating with paraffin oil and the arrows inset indicates the drawing direction of the

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samples.

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The experimental results in Figure 3B also show that the transmittance of the ultra-drawn HDPE

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films decreases again at high draw ratios and this despite the use of additives. In Figure S5,

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optical micrographs of the ultra-drawn HDPE films are shown, which illustrate that micro-cracks

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are generated at high draw ratios (> 20) in the films (perpendicular to the drawing direction). The

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perpendicular cracking structures formed at high draw ratios are further verified by SALS

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patterns. As shown in Figure 5B and 5D, apparently scattering patterns along the drawing

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direction of the samples are observed, thus demonstrating the presence of numerous micro-crack

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structures in the highly drawn PE films perpendicular to the drawing direction and this in

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contrast to the scattering of the PE films at low draw ratios (Figure 5A and 5C). In fact, the

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appearance of these micro-cracks is probably related to the onset of sample failure during ultra-

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drawing. More importantly, these micro-cracks are probably present mainly in the bulk of the

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drawn films.1 SEM images in Figure S6 also indicate the absence of the micro-cracks on the

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surface of the drawn samples. Apparently, defect structures scatter light and have a detrimental

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effect on the optical properties of the ultra-drawn films at high draw ratios.

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DR=1

DR=7

DR=10

DR=15

DR=20

13 14

PE

500 Intensity

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PE + 1% BZT

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0

PE + 2% BZT

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Figure 6. Wide-angle X-ray scattering (WAXS) patterns of the drawn HDPE as a function of

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draw ratio and BZT content. The drawing direction is in the horizontal axis.

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Table 1. Crystallinity (Xc), orientation function ( ), a-axis and b-axis of the orthorhombic unit

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cell of the drawn HDPE

Samples

Pure PE

PE + 1% BZT

PE + 2% BZT

Draw ratio (DR)

Xc (%)



a (Å)

b (Å)

1

65.3

0

7.32

4.88

7

62.1

0.911

7.33

4.90

10

65.1

0.928

7.33

4.90

15

68.9

0.939

7.37

4.89

20

71.4

0.944

7.37

4.89

1

65.5

0

7.31

4.88

7

61.1

0.898

7.31

4.89

10

62.3

0.91

7.32

4.90

15

68.8

0.933

7.34

4.89

20

70.4

0.938

7.37

4.88

1

62.9

0

7.35

4.90

7

61.9

0.884

7.34

4.89

10

62.3

0.899

7.35

4.90

15

66.4

0.921

7.38

4.88

20

67.5

0.932

7.37

4.89

3 4

Wide-angle X-ray scattering (WAXS) measurements with the incident X-ray beam perpendicular

5

to the film surfaces (Figure 6) were also recorded. In all cases, the (110) and (200) reflections

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were used to calculate the lattice parameters of the orthorhombic polyethylene unit cell.21 As

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shown in Table 1, there is no observable changes in lattice parameters prior to and after addition

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of BZT. Therefore, it is reasonable to conclude that the BZT is largely rejected by the crystalline

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regions and is mainly concentrated in the non-crystalline regions like amorphous regions,

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interstices between microfibrils, or both in the samples. Moreover, the crystallinity (Xc) and the

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orientation function ( ) of the samples were also extracted from the scattering data and the

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results in Table 1 illustrate the lack of detectable changes in Xc and  of the samples as a result

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of introduction of BZT.

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The absorption spectra in the range of 250-500 nm of the drawn HDPE containing 2% wt/wt

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BZT for parallel and perpendicular linearly polarized light are plotted in the Figure S7. The

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results show that the dichroic ratio of the drawn samples is low which indicates that the BZT in

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the drawn films is little oriented and non-birefringent. The drawn samples exhibit birefringence,

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mainly originating from the orientation of the HDPE.22

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Table 2. Characteristics of specific additives investigated in this study.

Density 3

(g/cm ) Melting Point o

( C) Refractive index (RI)

BZT

FX 5911

Paraffin oil

Cinnamon oil

Polystyrene oil

0.958

1.90-1.96

0.85

1.03

1.06

84

115

-

-

-

1.575

1.36

1.473

1.533

1.576

11 12 13 14 15 16 17 18 19

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Figure 7. (A) Appearance and (B) Transmittance of drawn HDPE films (DR=10) as a function

7

of various additives (2% wt/wt) without surface coating. The thickness of the films is around 160

8

µm.

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Light scattering of the undrawn polyethylenes is probably spherulitic in origin and the

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corresponding spherulites are deformed upon drawing at very low draw ratios (7) and consequently the

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spherulites are not present anymore in the drawn samples. Therefore, several additional

13

experiments were performed in order to explore the underlying mechanism behind the enhanced

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transparency of the drawn HDPE films upon addition of BZT at a moderate draw ratio. A limited

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series of solid- and liquid- additives were selected with different chemical and physical

16

characteristics which are listed in Table 2. Photographs and transmission measurements of the

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drawn HDPE films are shown in Figure 7A and 7B, respectively. A general trend can be

18

observed i.e. additives with high refractive index tend to improve transparency and this in

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contrast to additives with low refractive index. Moreover, other physical characteristics of these

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additives such as density, melting temperature or compatibility with polyethylene matrix hardly

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seem to matter.

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The solid-state drawing behavior of melt-crystallized linear polyethylenes has been investigated

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extensively in the past.23-25 In general, it is observed that chain folded lamellae tend to break up

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in the initial stages of drawing and tilt with their c-axis in the drawing direction. In the course of

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further drawing, the lamellae unfold and additional orientation as well as chain extension along

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the drawing direction is generated, which accounts for the drastic improvement in Young´s

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modulus and tensile strength. Despite this, defect structures originating from, for instance,

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impurities, oriented amorphous regions and longitudinal or perpendicular voids6 are still

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inevitably present in the ultra-drawn HDPE although often in very small quantities. Generally

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speaking, these defect structures are expected to possess relatively low refractive index

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compared to that of the crystalline elements, resulting in a refractive index mismatch between

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them and, as a consequence, the optical transparency of the drawn PE is low. Therefore, it seems

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reasonable to assume that additives are at least partially incorporated in these defect regions

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which decreases the refractive index mismatch and improves the optical transparency of the

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drawn HDPE films. This is already well demonstrated by our experiments that incorporation of

15

specific additives with high RI, e.g. BZT (RI=1.575), Polystyrene oil (RI=1.576) and Cinnamon

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oil (RI=1.533) can drastically improve the optical transmittance in the drawn HDPE at an

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additive content above 1% wt/wt and this in contrast to low refractive index additives like

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Paraffin oil (RI=1.473) or FX-5911 (RI=1.36). In addition, it has to be mentioned that in the case

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of ultra-drawn HDPE films close to the maximum draw ratio, extensive defects are formed due

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to longitudinal separation of fibrils and perpendicular cracks (see Figure S5) which also was

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reported in the past.6,26 The extensive light scattering from these structures causes the significant

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reduction of optical transmittance of the drawn films and the presence of high refractive index

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additives cannot efficiently improve the transmittance in the visible light wavelength, even at an

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additive content of 5% wt/wt.

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A few critical remarks are appropriate with respect to the experimental results presented above

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and their interpretation. Firstly, the proposed mechanism for generating a high transparency in

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melt-crystallized, drawn HDPE films with high refractive index additives might appear rather

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speculative. The bulk morphology, long-range periodicity as well as the molecular mobility of

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the drawn HDPE prior to and after addition of additives were therefore further investigated and

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the experimental results will be presented elsewhere. Secondly, the above-described study was

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performed on a single melt-crystallized linear high density polyethylene with a comparatively

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low molecular weight (Mw = 134,000, Mw/Mn = 4). Nowadays, linear high density polyethylenes

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with an enormous range in molecular weight, molecular weight distribution and co-monomer

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content are available, which opens a wide range of opportunities to further explore the

13

phenomena described here.

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CONCLUSION

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Ultra-drawn high-density polyethylene (HDPE) films with a high optical transmittance (> 90%)

16

in the visible wavelength range and a glass-like appearance are successfully prepared by

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incorporating small amounts of specific additives with high refractive index into the polymer

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prior to tensile drawing. It is argued that the observed phenomena originate from an improved

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refractive index matching between the crystalline and non-crystalline regions in the drawn HDPE

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films after addition of high refractive index additives. The rheological properties, drawing

21

behavior and the corresponding physical and mechanical properties are also preserved, especially

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their high modulus and high strength, which make these films potentially useful in a variety of

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applications such as high-impact windows.

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ASSOCIATED CONTENT

4

Supporting Information

5

Results showing the rheological properties, drawing behaviors, mechanical properties,

6

transmittance spectra and optical microscopy as well as polarized absorption spectra. This

7

material is available free of charge via the Internet at http://pubs.acs.org.

8

AUTHOR INFORMATION

9

Corresponding Author

10

*(C.W.M. B) E-mail: [email protected]

11

Notes

12

The authors declare no competing financial interest.

13

ACKNOWLEDGEMENTS

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The authors acknowledge the financial support by the China Scholarship Council (CSC).

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REFERENCES

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