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Transparent and Hazy All-cellulose Composite Films with Superior Mechanical Properties Wen Hu, Gang Chen, Yu Liu, Yingyao Liu, Bo Li, and Zhiqiang Fang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00814 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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ACS Sustainable Chemistry & Engineering

Transparent and Hazy All-cellulose Composite Films with Superior Mechanical Properties Wen Hu†, Gang Chen†, Yu Liu†, Yingyao Liu†, Bo Li†, Zhiqiang Fang*,†,‡ †

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology,

Wushan Campus 381 Wushan Road, Tianhe District, Guangzhou 510641, China. ‡

South China Institute of Collaborative Innovation, South China University of Technology,

Songshan Lake High-tech Industrial Development Zone, Dongguan 523808, China E-mail addresses of the corresponding authors: [email protected] (Z. Fang)

ACS Paragon Plus Environment

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ABSTRACT: A general and scalable strategy to achieve transparent and hazy cellulose films with excellent mechanical properties, especially folding endurance, remains an obstacle. Herein, we incorporate transparent carboxymethyl cellulose (CMC) into opaque but hazy paper of uniformly distributed lignocellulosic fiber network by a facile and scalable impregnation to fabricate transparent and hazy all-cellulose composite films with strong mechanical properties. The CMC serves as a matrix to enhance the transparency and smoothness, while the isotropic intertwined network of long and slender lignocellulosic fibers simultaneously acts as a reinforcing phase to improve the mechanical properties and a light scatting source to enhance the optical haze. Consequently, the resulting composite film combines a high transparency (up to 90%) and optical haze (up to 82%) at 550 nm with superior mechanical properties (140 MPa tensile strength, 8.5 MJ m−3 toughness, and 3342 folding times). Moreover, this composite film demonstrated ultra-smoothness (Rq: 0.9 nm) and improved flame retardant performance. Mechanically robust, transparent, and hazy composite films with ultra-smoothness and enhanced fire retardancy may extend their opportunities in flexible optoelectronics. KEYWORDS: All-cellulose composite film, Mechanical properties, Optical properties, Ultrasmoothness.


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INTRODUCTION With ever-increasing environmental concerns and rapid depletion of the fossil fuel resources, tremendous interests have been drawn to incorporate biodegradable and renewable materials into electronic devices, aiming to realize the sustainable development of human society.1, 2 Substrates are a crucial component of optoelectronics that not only mechanically support the devices but also determine their fabrication methods and performance such as flexibility, biodegradability, and electrical properties to a large extent.3-6 Transparent all-cellulose films with built-in high transmission haze have recently been emerged as novel sustainable substrates for flexible optoelectronics such as solar cells7-9 and organic light emitting diodes,10-12 which worked simultaneously as a mechanical support to uphold the devices and a light management layer to enhance device performance in a broadband wavelength and wide incident angle by rationally coupling the light into or out of optoelectronics through it. A variety of manufacturing approaches were adopted to achieve transparent and hazy allcellulose films by using multiscale cellulose fibers.7,

12-15

For instance, microscale cellulose

fibers were utilized to fabricate a film with both high transparency (up to 90%) and high transmission haze (up to 90%) by vacuum filtration,7 or partial dissolution of fiber surface,15 and so forth. A top-down method involving delignification and mechanical pressing was used to prepare anisotropic nanopaper with aligned nanocellulose, exhibiting high transparency as high as 90% and high haze up to 90%.16, 17 Despite excellent optical properties were achieved for allcellulose films by aforementioned well-established manufacturing methods, so far there was still lack of a general and scalable strategy to attain transparent and hazy films with superior mechanical properties, especially folding endurance. For example, large-scale manufacturing techniques have been proposed to prepare transparent and hazy films consisting of micro-sized


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cellulose fibers,7, 12 but their tensile strength is lower than 100 MPa. For anisotropic transparent and hazy nanopaper, they exhibited superior mechanical strength up to 350 MPa, whereas upscaling production of the anisotropic nanopaper with large dimension is still challenging. Moreover, seldom attention has been paid on the bendability and fire retardancy of transparent and hazy all-cellulose films despite they are of significance for flexible optoelectronic applications. Herein we designed a transparent and hazy all-cellulose composite film by a facile and scalable impregnation, which exhibited not only superior mechanical properties (such as strong tensile strength, superb folding endurance, high toughness), but also ultra-smoothness and enhanced fire retardancy. The molecular (carboxymethyl cellulose) CMC serves as a matrix to enhance the transparency and surface smoothness, while randomly intertwined network of slender lignocellulosic fibers acts as a reinforcing phase to improve the mechanical properties and as a light scatting source to enhance the optical haze. Moreover, our composite film showed better flame retardancy than common paper and pure CMC film. Mechanically robust, transparent, and hazy paper with ultra-smoothness and enhanced flame retardancy may bring a step close to their commercial application in optoelectronic devices. EXPERIMENTAL SECTION Fabrication of common paper, CMC film, composite film, TEMPO-oxidized paper Northern wood pulp was kindly provided by Lee & Man Paper Manufacturing Ltd (Dongguan, China). Common paper with a grammage of 21 and 70 g/m2, respectively, was prepared by a RK3AKWT sheet former (PTI, Austria). Common paper with a basis weight of 21 g/m2 was dried at room temperature, but for 70 g/m2 common paper, it was dried at atmosphere condition under mechanical pressing. The resulting sample has a thickness of 160 µm.


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Carboxymethyl cellulose (CMC), M.W. 700000 (DS=0.9), 2500~4500 mPa.s, was purchased from Aladdin biochemical technology Co., Ltd (Shanghai, China). CMC solution (1.5 wt. %) was prepared by adding 12 g of CMC into 788 mL deionized water, and kept at 72 °C with a starring speed of 1200 rpm for 1 h. The CMC film with a grammage of 70 g/m2 was directly fabricated by casting aqueous CMC solution on the surface of a square glass mold. The thickness of obtained CMC film is 65 µm. Aqueous CMC solution infiltrated into the intertwined network of common paper with a grammage of 21 g/m2, followed by an air drying process for 4 h in a constant-temperature humidity chamber at 50% relative humidity (RH%) and 42 °C. Finally, a CMC/lignocellulosic fiber composite film with a grammage of 70 g/m2 was obtained (thickness is 70 µm). Northern wood fibers were treated with TEMPO-oxidized system,31 then the treated wood fibers were used to prepare TEMPO-oxidized paper by a filtration method using a 20 cm filter membrane (0.45 µm, PVDF). The obtained paper has a grammage of 70 g/m2 and a thickness of 75 µm. Characterizations. The thickness was determined using a Thickness Tester (L&W, Sweden). Optical image was measured on a BX51 Optic Microscope (Olympus, Japan). Cross-section of the films was obtained under Ar+ ion beam using an ion beam milling systems (Leica EM TIC 3X). Scanning electron microscope measurements were carried out on an EVO 18 SEM (Carl Zeiss, Germany) with a voltage of 10 kV. Surface roughness was measured on Multimode 8 atomic force microscope (Bruker, Germany) in ScanAsyst mode in air. Optical transmittance and haze were measured on a LAMBDA 950 UV/ Vis/ NIR Spectrophotometer (PerkinElmer, America) with an integrated sphere. Three dimensional microtopography of composite film was obtained on a laser scanning 3D surface measurement system (China). Strain–stress curves were


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obtained on a 5565 universal material experiment machine (Instron, Boston, America) while the folding endurance was conducted on an MIT/U21B (America) with a folding angle of 135 degrees at 175 folds per minute. Thermogravimetric analysis was determined by using a Q500 TGA (TA, America). LOI was performed by means of FTT oxygen index tester (America). Vertical flame testing were carried out on an UL94 horizontal and vertical burning tester (FTT, America). RESULTS AND DISCUSSION Common paper presents an opaque appearance (Figure 1a and Figure S1a) and low mechanical properties (only 9 MPa tensile strength and 4 folding times). When a beam of light impinges on the intertwined virgin softwood fiber network of common paper (Figure 1a and Figure S1b), the incident light scatters largely because of the difference in refractive index between cellulose (1.5) and air (1.0), thereby leading to a transparency of < 40% (Figure S1c). The virgin softwood fibers have an average fiber length and diameter of 3.2 mm and 50 µm (Figure S2), respectively. The diameter of wood fibers is much larger than visible light wavelength, which can invoke strong forward light scattering that manifests as a transmission haze close to 100% over the visible wavelength (Figure S1d).18, 19 For CMC film, incident beam can directly pass through it without inducing light scattering, thus resulting in high transparency and low transmission haze (Figure 1b and Figure S3a). Moreover, it exhibits flat surface (Figure S3b) and good mechanical properties (54 MPa tensile strength and 1056 folding times). Atomic force microscope (AFM) height image suggests a root mean square roughness (Rq) of 0.7 nm over a 2 × 2 µm2 scanning area (Figure S3b). The optical properties of CMC film are illustrated in Figure S3c and d, showing a high total transmittance (up to 92%) and minimal transmittance haze (below 2%) in the visible spectrum.


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In our design, opaque but hazy paper of random distributed softwood fiber network is simultaneously used as a reinforcing phase to improve the mechanical properties and a light scattering source to enhance optical haze of composite film, while transparent and clear molecular CMC functions as a matrix phase to enhance its mechanical properties, light transmittance, and surface smoothness. To execute our design, a facile and scalable impregnation widely used for paper conversion was applied to marry the strong light scattering and reinforcing effect of randomly distributed fiber network with the high transparency and super smoothness of CMC film. The infiltration of CMC in the porous structure of common paper not only successfully transformed the strong interior backward light scattering into forward light scattering, but also increased the numbers of hydrogen bonds. As a consequence, the resulting composite film successfully combines a high transparency and optical haze with superior mechanical properties and ultra-smoothness (Figure 1c) that are of great interest for flexible optoelectronic applications.

Figure 1. Schematic showing the incorporation of molecular CMC into common paper to prepare mechanical robust, transparent, and hazy all-cellulose composite film with excellent mechanical properties. (a) Opaque but hazy common paper with low mechanical properties. (b) Transparent and clear CMC film with medium mechanical properties (c) Transparent and hazy all-cellulose composite film with excellent tensile strength (T) and folding times (F).


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A detailed study in the surface roughness and optical properties of our composite film is of significance for device applications. The CMC infiltrated into the intertwined fiber network to form fiber-reinforced all cellulose composite film with dense structure. Figure 2a demonstrates a piece of large size transparent and hazy composite paper with excellent mechanical properties. As we can see from this photo that the two sides of the composite paper are quite distinct in terms of surface smoothness. A smooth surface is observed from the top-view scanning electron microscopy (SEM) image in Figure 2b, indicating the superior surface smoothness of glass is successfully transferred to the composite film. The smooth side presents an average surface roughness of 0.9 nm over a scanning area of 2*2 µm2 (inset in Figure 2b), which is close to that of glass (0.3 nm) in Figure S4 and better than that of nanopaper (~1.8 nm)20 and polyimide (~2 nm).21 The nanoscale surface roughness lays the foundation for the direct fabrication of electronics on it. The SEM image in Figure 2c shows the surface morphology for the rough side of the composite film and its 3D morphology is revealed in the inset of Figure 2c. The influence of surface roughness on the optical transparency and transmission haze of the composite film across the visible wavelength was also investigated and their corresponding results are shown in Figure 2d, e. Overlapping curves for both total light transmittance and transmission haze over the visible spectrum are observed, illustrating the negligible effect of surface roughness on the optical properties of the composite film that manifests remarkable light scattering (inset in Figure 2e).


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Figure 2. (a) Digital image of large size transparent and hazy composite film (25*35 cm2). (b) Top-view SEM image for the smooth surface of the composite paper, inset is the 3D AFM images showing a surface roughness of 0.9 nm for the smooth side of the composite paper. (c) Top-view SEM for the rough surface of the composite paper, inset is a 3D microtopography image showing a surface roughness of 3.7 µm for the rough surface of the composite paper. (d) Total light transmittance and (e) transmission haze of the composite film in the visible region, indicating the surface roughness has a negligible effect on optical properties. Inset shows the light scattering behavior of the composite film.

In addition to the excellent optical properties and superior surface roughness, our composite film could offer intriguing mechanical properties such as strong tensile strength, high toughness, and superior folding endurance. Figure 3a illustrates the stress–strain curves of common paper, CMC film, and our wood fiber-reinforced all-cellulose composite film. Our composite film presents highest tensile strength and toughness of 140 MPa and 8.51 MJ m−3, respectively. To explore the underlying mechanism for the excellent stress and toughness, the structure and tensile-broken surfaces of common paper and our composite film after tensile testing were


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investigated by SEM analysis. Figure S5 a, b show the cross-sectional SEM images of common paper and composite film. A loosely packed structure is seen in common paper, and inset shows the hollow structure of the virgin softwood fibers. The porous structure and hollow fibers rendered not only heavy light scattering but also poor mechanical properties of the common paper. Whereas composite film showed a densely packed structure due to the uniform infiltration of CMC into fiber network (Figure S5b, S6), where CMC served as a matrix to fill up the porous structure while the randomly distributed fiber network functioned as a reinforcing phase to enhance the mechanical properties of the composite film. The tensile-broken surfaces of the samples are also analyzed to visualize the effect of the virgin softwood fibers and CMC on the fracture behavior. As shown in Figure S5c, amounts of pull-out fibers are observed in the fracture area of common paper due to the slippage of individual wood fiber in the fiber network. However, for our composite film, we can observe the fracture of the wood fibers in the tensile-broken surface (Figure S5d). The strong hydrogen bonding between the fibers and CMC enables the complete utilization of the ultra-strength of wood fibers (0.3-1.4 GPa).16 Therefore, the enhanced tensile strength and toughness are ascribed to the high levels of hydrogen bonding between wood fibers and CMC, uniformly distributed wood fiber network, and the intrinsic strength and toughness of individual wood fiber. Our composite film also exhibits superb folding endurance. A MIT folding testing machine was used to quantitatively evaluate the folding endurance of the composite film and the results are displayed in Figure 3b. The sample strip with a size of 15×150 mm2 was pulled with a constant force of 9.8 N and folded to 135° repeatedly until it broke down (Figure 3c and Figure S7). Our composite film presents highest folding times of over 3000, which is three times improvement


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over CMC film and is two orders of magnitude higher than that of transparent and hazy paper made of TEMPO-oxidized wood fibers (TEMPO-oxidized paper). The remarkable improvement in folding endurance is due to the amounts of hydrogen bonds and utilization of the excellent mechanical properties of individual virgin wood fibers. The composite film can be arbitrarily folded without leaving a crease. An extreme folding testing was further applied to investigate the mechanical flexibility of our composite film and TEMPO-oxidized paper. Figure 3d shows the schematic for the extreme folding measurement. A flat sample strip was folded to 180° and the folding area was then pressed with a 1Kg weight for one minute. After being restored to plane, the folding area was observed for comparison. An apparent crease can be seen for TEMPO-oxidized paper, but there is no crack observed in composite film, indicating its excellent anti-cracking performance. Such superior mechanical flexibility and foldability enables the composite film to be potentially used in foldable electronics.


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Figure 3. Characterizations of the mechanical properties for wood fiber-reinforced all-cellulose composite film. (a) Stress–strain curves of common paper, pure CMC film, and our composite film. (b) Folding endurance of common paper, TEMPO-oxidized paper, CMC film, and our composite film. Not that all samples have the same base weight of 70g/m2. (c) Schematic showing the principle of MIT folding testing machine. (d) Folding measurement of our composite film and TEMPO-oxidized paper.

Table 1 summarized the fabrication time, optical and mechanical properties, and surface smoothness of various transparent and hazy cellulose films. In comparison to other transparent and haze cellulose films, our composite film exhibited competitive optical properties, medium tensile strength, much higher folding times, and smoother surface while requiring lesser fabrication time. Table 1. A comparison in various transparent cellulose films Materials

Transmittance (T) and haze (H)

Fabricate time / h

Tensile stress / MPa

Toughnes s / MJ m−3

Folding time (constant force)

Surface roughness

Refs

Oriented nanofibrous BC film

N/A

24.5

179

0.1

N/A

RMS: 40 nm

22

ESO-plasticized EC films

T: 97%; H: N/A

7

39

N/A

57 (4.9 N)

Nanoscale

23

Transparent paper

T: 92%; H: 41%

N/A

122

8.8

N/A

Rq: 42 nm

14

Nanopaper

T: 93%; H: 50%

N/A

300

N/A

N/A

RMS: 7.7 nm

10

Novel Nanostructured Paper

T: 96%; H: 60%

7~8

105

1.9

N/A

Microscale

7

Anisotropic, Transparent Film

T: 90%; H: 70%

4~8

350

7.4

N/A

Nanoscale

16

Plastic-paper

T: 85%; H: 90%

24

65

N/A

275 (14.7 N)

Rq: 3 nm

12

Hazy Transparent

T: 89~92%;

Nanopaper Hybrid cellulose nanopaper Super hazy paper Transparent

N/A

N/A

Nanoscale

24

7

N/A

RMS: 5.8~22.1 nm

13

T: 90%; H: 91%

9

N/A

RMS: 137 nm

15

T:91.5%; H: 70%

18

Nanoscale

19

H: 27~87% T: 79~86%; H: 10~62%


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hybrid paper Our composite film

T: 90%; H: 82%

5

140

8.5

3342 (9.8 N)

Rq: 0.9 nm

Our composite film exhibits enhanced fire retardancy. Thermogravimetric analysis (TGA) was used to assess the thermal degradation behavior of our composite film, common paper, CMC film and TEMPO-oxidized paper in a nitrogen atmosphere, and the results are shown in Figure 4a. All samples are tested as the temperature increased from 25 °C to 600 °C. Among these cellulose-based films, common paper made of virgin softwood fibers exhibits highest thermal stability, illustrating an initial degradation temperature of approximately 321 °C, followed by our composite film with an onset decomposition temperature of 254 °C, which is comparable to that of the CMC film (253°C). TEMPO-oxidized paper shows lowest thermal stability (240 °C) due to the anhydroglucuronic acid units formed by TEMPO-mediated oxidation.25 In addition, limiting oxygen index (LOI) analysis and vertical flame test were employed to study the flammability of the composite film, CMC film, common paper, and TEMPO-oxidized paper. Our composite film has a highest LOI of 30% (Figure 4b), followed by a sequence of CMC film (24%), TEMPO-oxidized paper (22%), and common paper (16%). The digital photos in Figure 4c demonstrate the time-lapse flammability of the composite film (A), CMC film (B), common paper (C), and TEMPO-oxidized paper (D) during the vertical flammability testing. After exposed to the flame for 5 seconds, the consumption of sample A by flame is slower than that of sample B, resulting in its larger unburnt area. For sample C and D, they almost burnt out. When the fire application time increased to 10s, all testing strips were fully consumed by flame. without leaving behind any residues. However, a smoldering phenomenon was observed for both sample A and B after the fire removal. The afterglow time for sample A and B is 3s and 57s, respectively. Consequently, almost half of the sample A was maintained due to its enhanced fire


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retardant performance, but for sample B, half of weight was consumed by smoldering and only a small amount of residue was left. The enhanced fire retardant performance of the composite film is ascribed to the sodium ions in the CMC molecules, which can affect the kinetics of thermal decomposition and pyrolysis behavior of cellulose,

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as well as high DP and crystallinity of

virgin fibers.29, 30

Figure 4. Thermal stability and flammability of our composite film. (a) Thermogravimetric and LOI (b) analysis of composite film, CMC film, common paper and TEMPO-oxidized paper. (c) Vertical flame testing of composite film (A), CMC film (B), common paper (C) and TEMPO-oxidized paper (D) as a function of time. Note that the size of the testing strip is 100 mm*15 mm*0.1 mm, 100µm thick. Fire application time is 10s.

CONCLUSION In summary, we married intertwined network of lignocellulosic fibers acting as a reinforcing phase and a light scattering source with transparent and clear molecular CMC serving as a matrix to prepare mechanically robust, transparent, and hazy all-cellulose films with ultra-smoothness and enhanced fire-retardant performance by a facile and scalable impregnation. The resulting composite film combined a high transparency as high as 90% and optical haze up to 82% at


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550nm with strong tensile strength of 140 MPa, high toughness of 8.5 MJ m−3, superior folding times up to 3342. The strong tensile strength, high toughness, and ultra-high folding endurance are ascribed to the high levels of hydrogen bonding between wood fibers and CMC, uniformly distributed virgin softwood fiber network, and the intrinsic excellent mechanical properties of individual virgin wood fiber. Moreover, the obtained composite film demonstrated ultrasmoothness of 0.9 nm over a 2×2 µm2 scanning area and reduced flammability that was due to the synergistic effect of sodium ion in CMC, as well as high DP and crystallinity of virgin wood fibers. Such transparent and hazy composite film with excellent mechanical properties, ultrasmoothness as well as enhanced fire retardancy may extend its uses in flexible optoelectronic applications such as solar cells, OLED, and lighting system. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Digital photos, AFM height images, top-view SEM images, and optical properties of lignocellulosic fibers, common paper, CMC film, composite film, and commercial glass. EDS and elemental mapping analysis for composite film. Clamp in MIT folding testing machine to hold the testing sample strips. AUTHOR INFORMATION Corresponding Author *Zhiqiang Fang, Email: [email protected] Notes Any additional relevant notes should be placed here.


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ACKNOWLEDGMENT We appreciate the Young Scientists Fund of the National Natural Science Foundation of China (31700508), the Natural Science Foundation of Guangdong Province (2017A030310635), Pearl River S&T Nova Program of Guangzhou (201806010141), Science and Technology Program of Guangdong Province (2017B090903003), the Fundamental Research Funds for the Central Universities (2015ZM156), and the State Key Laboratory of Pulp and Papermaking Engineering (201709).


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films

of

cellulose

nanofibers

prepared

by

TEMPO-mediated

oxidation.

Biomacromolecules, 2008, 10, 162-165.


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For Table of Contents Use Only

TOC/Abstract graphic

SYNOPSIS We designed mechanically robust, transparent, and hazy all-cellulose composite films by combining intertwined network of lignocellulosic fibers acting as a reinforcing phase and a light scattering source with transparent and clear molecular CMC serving as a matrix via a facile and scalable impregnation.


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Transparent and Hazy All-cellulose Composite ... - ACS Publications

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