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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ACS Applied Materials & Interfaces

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Improving the Transparency of Ultra-Drawn Melt-

2

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]

12 13

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

14 15

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

17

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

18

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

19

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

ACS Paragon Plus Environment

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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

15

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

17

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-

21

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

7

temperatures between 40~105 oC.10

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

9

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

10

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

11

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

12

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

13

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

16

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

18

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

19

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

2

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

3

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

4

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

5

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

6

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

7

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

8

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

9

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

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10

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

11

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

13

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

14

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

15

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

17

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-

19

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

20

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


4

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

2

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

3

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

4

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

5

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

6

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

8

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

9

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

10

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

11

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

12

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

13

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

14

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

15

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

17

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

18

the following equation: =

+ 1.46 ∙ 100% + 0.75 + 1.46

19

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

20

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

21

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

3

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

4

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

5

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-

8

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

9

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

10

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

11

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

12

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

13

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

15

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

16

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

18

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

19

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

20

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)

2

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

3

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

4

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

5

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

6

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

7

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

8

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

9

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

10

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

12

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

13

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

14

clearly illustrates that another mechanism is also operational.

16 17 18 19 20

A

B

2.0 Pure PE PE0.5 PE1 PE2 PE5

1.5 1.0

Transmittance (%)

15

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


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

22

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

23

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


7


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

8

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

9

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

16

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

17

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

18 19

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

20

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

21

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

22

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

23

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

24

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


8

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

2

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

3

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

4

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

5 6 7 8 9 10 11 12 13 14

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

15

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

16

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

17

arrows inset indicates the drawing direction of the samples.

18 19

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

20

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

21

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

22

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

23

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

2

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

3

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

4

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

5

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

6

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

7

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

8

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

9

with an appropriate coating.

10 11 12 13 14 15 16 17

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

18

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

19

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

20

samples.

21

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

22

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

23

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


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1

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

2

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

3

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

4

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

5

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

6

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

7

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

8

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

9

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

10

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

11

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

12

DR=1

DR=7

DR=10

DR=15

DR=20

13 14

PE

500 Intensity

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15 16

PE + 1% BZT

17 18

0

PE + 2% BZT

19 20

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

21

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

22


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

2

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

6

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

7

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

8

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

9

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


12

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1

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

2

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

3

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

4

of introduction of BZT.

5

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

6

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

7

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

8

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

9

mainly originating from the orientation of the HDPE.22

10

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


13


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Page 14 of 31

1 2 3 4 5 6

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.

9

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

10

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

12

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

14

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

15

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

17

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

19

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

20

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

21

seem to matter.


14

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1

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

2

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

3

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

4

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

5

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

6

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

7

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

8

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

9

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

10

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

11

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

12

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

13

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

14

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

16

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

17

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

18

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

19

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

20

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

21

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

22

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


15


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Page 16 of 31

1

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

2

additive content of 5% wt/wt.

3

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

4

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

5

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

6

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

7

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

8

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

9

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

10

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

11

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

12

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

13

phenomena described here.

14

CONCLUSION

15

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

17

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

18

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

19

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

20

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


16

Page 17 of 31

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


1

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

2

applications such as high-impact windows.

3

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

14

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

15

REFERENCES

16 17 18 19

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Improving the Transparency of Ultra-Drawn Melt-Crystallized

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