Optically transparent tough nanocomposites with a hierarchical structure of cellulose nanofiber networks prepared by the Pickering emulsion method

Journal Pre-proofs Optically transparent tough nanocomposites with a hierarchical structure of cellulose nanofiber networks prepared by Pickering emulsion method Supachok Tanpichai, Subir K. Biswas, Suteera Witayakran, Hiroyuki Yano PII: DOI: Reference:

S1359-835X(20)30049-X https://doi.org/10.1016/j.compositesa.2020.105811 JCOMA 105811

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Composites: Part A

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2 September 2019 29 January 2020 2 February 2020

Please cite this article as: Tanpichai, S., Biswas, S.K., Witayakran, S., Yano, H., Optically transparent tough nanocomposites with a hierarchical structure of cellulose nanofiber networks prepared by Pickering emulsion method, Composites: Part A (2020), doi: https://doi.org/10.1016/j.compositesa.2020.105811

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Optically transparent tough nanocomposites with a hierarchical structure of cellulose nanofiber networks prepared by Pickering emulsion method

Supachok Tanpichai1, 2, 3,⊥, *, Subir K. Biswas1,⊥, Suteera Witayakran4 and Hiroyuki Yano1 1Research

Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji,

Kyoto 611-0011, Japan. 2Learning

Institute, King Mongkut’s University of Technology Thonburi, Bangkok,

10140, Thailand. 3NanotecKMUTT

Center of Excellence on Hybrid Nanomaterials for Alternative

Energy, King Mongkut’s University of Technology Thonburi, Bangkok, 10140, Thailand. 4Kasetsart

Agricultural and Agro-Industrial Product Improvement Institute, Kasetsart

University, Bangkok, 10900, Thailand. *Corresponding author: [email protected] ⊥S.T.

and S.K.B contributed equally to this work.

Abstract Optically transparent tough cellulose nanofiber (CNF) reinforced composites were successfully prepared through a simple Pickering emulsion method. The emulsions of CNFs, acrylic resin monomer and water were vacuum filtered to form mats, and the mats were hot-pressed, and UV-cured. Hierarchical CNF networks where the

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homogenously-dispersed CNFs formed a network surrounding resin droplets in the composites were observed. As the CNF content increased, thicker CNF networks were formed, which could bear greater loads. The strength, modulus, strain and toughness of the composites with 25 wt% CNFs increased to be 18.6, 56.7, 1.5, and 36.3 times as great as those of the neat acrylic films with the great reduction in the thermal expansion to 7.3 ppm K-1 (1/26th of the neat polymer) and only 3% degradation in optical transmittance. The composites prepared by Pickering emulsion approach showed higher performances than those from the impregnation method, which could be used in optoelectronic applications.

Keywords: A. Nanocomposites; A. Cellulose; B. Mechanical properties; B. Optical properties

1. Introduction Cellulose nanofibers (CNFs) have a range of attractive properties, such as superior mechanical properties, low densities, low coefficients of thermal expansion and high surface areas [1-5]. Owing to these features and their width being less than the wavelength of visible light, CNFs have been widely studied to enhance properties of transparent resins without sacrificing the transparency of the material for optoelectronic applications [6-12]. Most studies have impregnated cellulose nanopapers with a resin matrix, and then cured by UV light or heat [6-10, 13-16]. Okahisa et al. [10] observed that introduction of 35–40 wt% CNFs increased the tensile strength and modulus, which was associated with the considerable decrease in thermal expansion of the composites to 12.1 ppm K−1 and a slight reduction in transparency. Such material

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properties would be beneficial for fabricating organic light-emitting diodes (OLEDs) on optically transparent CNF reinforced composites. Although the impregnation approach provides composites with superior mechanical properties, high fiber contents and low thermal expansion associated with the slight degradation in optical transmittance, it is time-consuming. Attempts have also been made to prepare composites via emulsion polymerization [17-19]. Recently, we demonstrated a simple Pickering emulsion process as an alternative efficient process to prepare optically transparent composite materials with good mechanical performance and low thermal expansion [11, 12, 20]. In these studies, nanofiber networks formed at the oil and water interface, preventing flocculation and coalescence of the resin droplets [11, 12]. For composites with a CNF content of 16 wt%, tensile strength and modulus were respectively 20 and 50 times as great as those features of the neat resin; although the thermal expansion was 1/15th of a neat resin film, the composites maintained a similar transparency [11]. Importantly, the strain at break and toughness of the composites respectively increased to be ~2 and 53 times as great as those features of the neat resin. This considerable improvement is attributed to the formation of a unique reverse nacre-like microstructure of resin droplets encapsulated by CNF networks, which delayed or prevented crack propagation and improved stress-transfer processes. Furthermore, the composites formed by Pickering emulsification could be moldable before resin curing, allowing modification of the surface patterns with micro/nanoscale features with high dimensional stability up to high temperature, which is not possible by the impregnation method in the case of using cellulose nanopapers as reinforcement [11, 12]. We also developed high thermal-resilience transparent electrode materials using these composites as substrates [21]. The simplicity of the Pickering emulsification

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process has been previously used to prepare composites with other polymer matrices [3, 22]. Fujisawa et al. [3] reported an improvement in tensile strength and modulus of CNF reinforced polystyrene composites based on Pickering emulsification; however, strain at break was not improved in their work. Zhang et al. [22] reported the reduced tensile strength and modulus for CNF reinforced poly(lactic acid) composites formed from a Pickering emulsion when the CNF content was greater than 10 wt%. Hence, the relationships between hierarchical structures of composites prepared by the Pickering emulsion method and mechanical performances of the composites are closely related. Although we have previously published the performances of the composites with CNFs prepared by the Pickering emulsion method [11, 12, 20], we fixed the fiber content, and mainly focused on the preparation conditions of the Pickering emulsification method and the applications of the as-prepared composites. Here, we prepared CNF reinforced acrylic resin composites by the Pickering emulsion method with various CNF contents, and studied effects of CNF contents on changes in the hierarchical structure of the composites and material performances in terms of their optical transparency, mechanical properties and thermal stability. Moreover, performances of the composites prepared by the Pickering emulsification and impregnation approach were compared. The Pickering emulsion technique used in this study offers a potential way of enlarging transparent composite production for electronic applications such as flexible touchscreen displays or solar cells.

2. Materials and methods 2.1. Materials

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The sugarcane bagasse pulp used as a raw material for CNF preparation was kindly provided by Eastern Sugar & Cane Co., Ltd., Thailand. The UV-curable acrylic resin monomer (2.2 bis[4-(acryloxypolyethoxy)phenyl]propane (ABPE-10) (refractive index of 1.516) used as a matrix was supplied by Shin-Nakamura Chemical Co., Ltd., Japan, and photoinitiator 2-hydroxy-2-methylpropiophenone (0.25 wt%), supplied by Wako Pure Chemical Industries, Japan, was added to the monomer before use. Potassium hydroxide (KOH), acetic acid, and sodium chlorite (NaClO2) were purchased from Wako Pure Chemical Industries, Japan. 2.2. Preparation of Cellulose nanofibers The bagasse was initially pre-treated with 30% (% by weight of bagasse) potassium hydroxide (KOH) at a bagasse to liquor ratio of 1 to 10 for 2 h at 165 °C in a closed pulping unit, washed with distilled water, and completely dried. A bleaching process was applied to the pre-treated bagasse, based on a procedure reported by Abe et al. [23]. Briefly, the bagasse pulp was treated with acidified sodium chlorite (NaClO2) at 80 °C for 5 h, and then 4 wt% KOH at 90 °C for 2 h. The bagasse was additionally treated with acidified NaClO2 at 80 °C for 3 h to eliminate residual hemicellulose and lignin. The treated fibers were filtered, and washed well with distilled water until neutralization. The resulting slurry with a concentration of 0.8 wt% chemically treated bagasse fibers was initially fibrillated with a Vitamix blender (Osaka Chemical Co., Ltd., Japan) for 1 min at 5,000 rpm, and then passed through a grinder (MKCA6-2, stone type: MKGC6-80, Masuko Sangyo Co., Ltd., Japan) twice at 1,500 rpm. 2.3. Preparation of cellulose nanofiber reinforced nanocomposites The CNF reinforced nanocomposites were prepared by a similar procedure reported by Biswas et al. [11, 12, 21] with minor modifications. The resin monomer was

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mixed with CNFs and distilled water using a Vitamix blender (Osaka Chemical Co., Ltd., Japan) at 5,000 rpm for 2 min, and then at 37,000 rpm for 5 min twice. The original CNF contents in the Pickering emulsions were 3, 5, 10 and 15 wt% to the resin. The milky Pickering emulsion was then vacuum-filtered using a glass filter funnel with a 0.1 µm pore size polytetrafluoroethylene membrane filter to obtain a CNF/resin mat, the mat was hot-pressed at 150 °C and 1 – 2 MPa (depending on the CNF contents) for 5 min. During the hot compression, a small amount of the resin was lost. The pressed mat was subsequently UV-cured with a F300S UV lamp/LC6 conveyer system (20 J cm-2, Fusion UV Systems, USA) to fabricate a transparent composite film. The CNF weight percentages of the final composites determined using a thermogravimetric analyzer Q50 (TA Instruments, Japan) were 4, 9, 18, and 25. The composites were coded as PE X, where X refers to the CNF content in the composite. Fig. 1 illustrates the Pickering emulsification process used to prepare CNF reinforced composites. To obtain impregnated composites as a control sample, the 0.1 wt% CNF suspension was subjected to vacuum filtration, and the wet sheet was then hot-pressed at 110 °C for 30 min with the pressure of 0.1 MPa. The prepared CNF sheets were impregnated into the acrylic resin for 12 h, and the resin-impregnated CNF sheet composites placed between two glass slides were mechanically pressed under different pressures. The samples were subsequently UV-cured to obtain the composites with CNF contents of 15, 20 and 26%, respectively. The impregnated composites were coded as IM X. The neat acrylic films were also prepared using the same procedure as described above. 2.4. Field emission scanning electron microscopy

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The fracture surfaces of the CNF reinforced composites and one drop of the 0.05 wt% CNF suspension on a glass slide were sputter-coated with platinum by an ion sputter coater (JEC-3000FC, JEOL Ltd., Japan) to avoid charging, and the samples were then observed using a field emission scanning electron microscope (JSM-7800F, JEOL Ltd., Japan) with an acceleration voltage of 1.5 kV and a working distance of 10 mm. 2.5. UV-Vis spectroscopy The light transmittances of samples were measured using a UV-Vis spectrophotometer (U-4100, Hitachi High-Tech Corp., Japan) at wavelengths from 300 to 800 nm with an integrating sphere 60 mm in diameter. The linear transmittance was measured by placing a sample 25 cm from the entrance port of the integrating sphere while a sample was placed close to the entrance port of the integrating sphere to measure the total transmittance. The thickness of the composite samples varied between 80 and 180 µm. Therefore, the thickness of each sample was normalized to be 100 µm to eliminate the effect of thickness variations. 2.6. Mechanical testing Tensile properties of the composites were measured by an Instron 3365 universal testing machine (Instron Corp., USA) equipped with a 5 kN load cell. The 5 mm-wide and 35 mm-long samples were tested at a crosshead speed of 1 mm min−1 and a gauge length of 20 mm. Prior to testing, the specimens were maintained in a controlled room at 23 ± 2 °C and a relative humidity of 50 ± 2 % for 24 h. At least five samples were tested to obtain averages and standard deviations of the mechanical property results for each material. 2.7. Thermal expansion analysis

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Coefficient of thermal expansion (CTE) was analyzed by a thermomechanical analyzer (TMA/SS6100, Seiko Instruments, Japan) in tensile mode with temperatures in the range of 20–150 °C at a heating rate of 5 °C min−1 and preload force of 29.4 mN under a nitrogen atmosphere. The samples were cut to a width of 3 mm and a length of 30 mm. The CTE value was calculated between 20 and 150 °C.

3. Results and discussion 3.1. Morphology Fig. 2(a) shows CNFs with widths in the range of 10–30 nm and lengths of more than 2 µm isolated from sugarcane bagasse, and Fig. 2(b) and (c) presents Pickering emulsions with various CNF contents after high-speed blending and storing in a dark container at room temperature for 4 weeks. After blending the milky emulsions were observed for all ratios of the resin and CNFs. Yet, the Pickering emulsions with 3 and 5 wt% CNFs started to fall to the bottom after one day. After 1 week, sedimentation was observed for the emulsions with 3 and 5 wt% CNFs whereas no sedimentation was seen for the emulsions with 10 and 15 wt% CNFs, even after 4 weeks. At a low content of CNFs in an emulsion, CNF assembly at the oil and water interface might not be sufficient to encapsulate all resin droplets dispersed in water. With increasing CNF contents in the Pickering emulsions, the CNF network formed at interfaces of the oil and water could cover oil droplet surfaces. A phase separation has been spontaneously found with no CNFs in the Pickering emulsion [3]. The formation of the CNF network at the interfaces of the oil and water played an important role to stabilize the Pickering emulsion against coalescence and Ostwald ripening [24-28]. Fujisawa et al. [29] has recently reported that the formation of an ~8 nm-thick layer of the CNF network at the

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interface of monomer and water in Pickering emulsions, resulting in individual CNFcovered monomer droplets dispersed in water, while the rest of the CNFs was probably dispersed in the water phase. Moreover, it has been reported that due to the high energy adsorption of particles at oil/water interfaces, higher stabilization of Pickering emulsions is generally found in comparison with emulsions stabilized by surfactants [24, 29]. The better emulsification stabilization of the Pickering emulsions has also been reported by Sanchez-Salvador et al. [26]. With higher concentrations of cellulose microfibers, the stabilization index of the Pickering emulsions increased due to higher entangled networks. The higher stability of the Pickering emulsions stabilized by bacteria cellulose nanofibers has been observed for 4 weeks [30]. The higher stability against sedimentation at high contents of CNFs might also be due to an increase of the viscosity of the emulsions, resulted from a three-dimensional CNF network covering resin droplets [31]. Kalashnikova et al. [32] studied effects of aspect ratios of cellulose nanorods in Pickering emulsions. Shorter nanofibers could cover the whole droplet surfaces, but multilayered networks on neighboring droplets were found for longer nanofibers. With increasing contents of longer fibers, interconnections of the droplets were formed. This entangled networking arrangement obtained from longer fibers could be more stable than single drops covered by shorter cellulose nanorods. Notably, the precipitation of the Pickering emulsions with 3 and 5 wt% CNFs did not occur during composite preparation owing to the short processing time. It is worth mentioned that if a large emulsion volume is prepared to make a composite, the sedimentation issue should be taken into consideration to reduce the effect of the phase separation. During the Pickering emulsion preparation, the resin droplets were covered by CNF, and CNF networks were formed at the interfaces between the resin droplets and

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water, as shown in Fig. 3(a) and (b). The diameters of the emulsion droplets covered by CNFs were 1.28 ± 0.53 µm for the emulsion with 3 wt% CNFs. As the CNF content increased, the droplet diameter increased. The average diameters of droplets dispersed in the Pickering emulsions with 5, 10 and 15 wt% CNFs increased to 1.90 ± 1.15, 2.00 ± 1.46 and 1.95 ± 2.76 µm, respectively. The droplet sizes became more polydisperse at higher CNF concentrations, which suggested that a thicker network of CNFs was developed. At high CNF contents, fiber aggregates might form during strong agitation, resulting in coalescence of droplets [33]. Fig. S1 (a) shows surface morphology of the resin droplets covered by the CNF network with a higher magnification. Moreover, the mixing speed has been reported to inversely control sizes of the oil droplets [34]. The droplets became smaller with increasing the rotational speed. As the mixing speed used in this study remained at 37,000 rpm, this might also affect the increase of the droplet size due to higher viscosity of the Pickering emulsions with increasing CNF concentrations [34]. The emulsion was subsequently vacuum-filtered to form a composite mat. The loss of monomer could be negligible during this state after deposition of the first few layers of the CNF network on a membrane filter, the resin penetration through the filter could be blocked by this CNF network. The fracture surfaces of the unpressed composite mats prepared from the Pickering emulsions with 3 and 10 wt% CNFs are shown in Fig. 3(c) and (d). Each individual round acrylic resin droplet was encapsulated by a network of CNFs, and the CNF networks reinforced the mat; neither penetration nor dispersion of CNFs in the resin could be observed (Fig. S1(b)). This indicated CNFs were resided within interfaces of oil and water or dispersed in a water phase not an oil phase, acting as an emulsifier agent in the system. The round droplet shape in the

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emulsion became an oval owing to the suction pressure during vacuum-filtration. Notably, resin penetration into the CNF networks was not discovered at this state. When a compressive force was applied to the mat with heat, the structural morphology of the material changed, forming oval-shaped droplets covered with CNF networks to platelets (resin was not cross-linked in this state). A homogenous distribution of multilayers of the CNF networks with the formation of the continuous matrix throughout the composites was observed, which explains the good dispersion of the droplets covered by CNF networks during the preparation of the Pickering emulsions. It should be pointed out that during this compression the resin came out of the mat, making the higher CNF contents in the final composites in comparison with those CNF contents in the emulsions and uncompressed mats. Conversely, in the impregnated composite, a CNF sheet was laminated between two layers of the resin with only interfacial interactions between the surface layer of the CNF sheet and resin (Fig. S2). 3.2. Transparency The effects of CNF contents on the optical transmittance of the composites are shown in Fig. 4(a) and (b), and the composite appearances before and after hot-pressing are compared in Fig. 4(c) and (d), respectively. The PE 4 mats before hot-pressing showed low total light transmittance of 48.9% at 600 nm. The total transmittance decreased markedly to 6.4% for the PE 25 mat. We attribute this poor transmittance at high CNF contents to the thicker CNF networks formed in the mat with unfilled pores, as shown in Fig. 3(d), which scattered more light. The optical transmittance of the films changed when the samples were hot-pressed, as shown in Fig. 4(d). At 4 wt% CNFs, a total light transmittance value of 91.5% was obtained from the composites, which is

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close to that of neat acrylic film (92.4%). The refractive index difference between the CNFs and matrix slightly degraded the transparency [35]. By increasing the CNF content to 25 wt%, the transmittance decreased by only 3% compared with that of the neat films. Hence, the change in transmittance induced by hot pressing indicates that the resin penetrated into the pores of the thick CNF network and formed a continuous matrix phase throughout the material to construct a compact hierarchical structure. In the impregnation method, the greater degradation of the transmittance was observed after curing at a high CNF loading. A total transmittance of 87.9% was observed from IM 26. The differences between the composites prepared by the Pickering emulsification and impregnation methods in this work were obviously observed from the linear transmittance results. The PE 25 showed a linear transmittance of 84.9% at 600 nm whereas 80.4% of light passed through the IM 26 samples. We attribute the lower transmittance of the impregnated composites to the resin penetrating through the pores only for the surface layer of the dense CNF sheet, leaving the core intact with large amounts of unfilled pores [36]. Moreover, compared with a previous work on bacterial cellulose embedded composites with a cellulose content of 5 wt% [9], the PE 25 sample showed higher transmittance despite having a high fiber content. These results in the present study indicate the advantages of the Pickering emulsion method in terms of optical transmittance of the CNF composites over cellulose nanopaper laminated composites. 3.3. Mechanical properties Stress-strain curves of the composites with CNFs are shown in Fig. 5, and the tensile properties of the CNF reinforced composites are summarized in Table 1. The increase in tensile strength and modulus was accompanied by a decrease in tensile strain

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as has been widely reported in polymer composites with cellulose particles owing to poorer compatibility or debonding between the cellulose and polymer matrix. Moreover, cellulose aggregation is the main cause of the decreased mechanical properties at a high concentration of the cellulose particles [37-43]. However, the hierarchical structured composite in this work showed good mechanical properties. A great improvement in the mechanical properties of the CNF composites was found compared with acrylic resin films, where the Young’s modulus and tensile strength of the neat acrylic films were 0.03 GPa, and 2.2 MPa, respectively. At 4 wt% CNFs, the tensile modulus and strength were 0.66 GPa and 21.3 MPa, respectively, which were 19.5 and 9.7 times as great as those features of the neat films, and remarkably the PE 4 composites withstood tensile deformation better than did neat resin. The strain at break and toughness of the PE 4 composites were 12.7 % and 1.8 MJ m-3, respectively which were 2 and 21.8 times as great as those features of the neat films (7.0 % and 0.08 MJ m-3 for strain and toughness). We attribute the considerable improvement in mechanical properties and toughness to the compact two-tier hierarchical nanofiber network architecture in the composites which allowed stress to efficiently transfer from the soft resin to the CNF network. The two-tier hierarchical structure of CNF networks in composites prepared by the Pickering emulsification could limit/delay crack growth initiation by crack deflection, delamination, and bridging by microcrack formation accompanied with energy dissipation from resin platelet sliding during the tensile deformation [11, 12, 44]. The two-tier hierarchical structure found in these composites resembles the brick-andmortar structure, which is found in nacre. Nacre consists of ~95 vol% of 400 nm stacked and aligned CaCO3 glued together with 5 vol% of proteins and polysaccharides. Thanks to the hierarchical architecture, the toughness of nacre has been reported to be up to

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three orders of magnitude orders greater than that of pure CaCO3 [44, 45]. The similar increase in tensile stress and toughness has been recently reported for cellulose acetate with graphene oxide [45]. This improvement was due to the nacre-like structure of graphene oxide sheets covered by cellulose chains. The effect of this hierarchical structure on the mechanical performance of the composites was maintained as the CNF content increased, as shown in Fig. 5. Great increases of strength and modulus by 18.3 and 48.7 times to 41.0 MPa and 1.7 GPa were measured from PE 25, whereas the tensile strain of the PE 25 was 10.6 %, which was 1.5 times as great as that of the neat resin films and toughness of the PE 25 composites was 2.9 MJ m−3, which was 38 times as great as that of the neat resin films. The thicker CNF networks played a vital role to bear more load. It has been reported that better stress transfer between cellulose nanofibers within the nanopaper was found for the nanopaper with a higher number of fibers interacted in a network plain [46, 47]. Furthermore, it should be noted that the degree of cross-linking of all composite materials should be similar as the initiator was pre-mixed with the acrylic resin before blending with CNFs and water. The understanding of the failure mechanisms of the two-tier hierarchical architecture of the composites prepared by the Pickering emulsion which provided higher toughness with increasing CNF contents would be our further investigation. We compare the performances of composites prepared by the Pickering emulsion and impregnation method. At a similar content of CNFs, higher tensile strength, strain, and toughness were observed from Pickering emulsion composites whereas a higher modulus was obtained from the impregnated composites. The hierarchical structure of CNF networks (multilayers of the networks) in composites

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prepared by the Pickering emulsion was a key, while for the impregnated composites, we attribute the stiffer behavior to the high-density CNF sheet acting to control the mechanical behavior of the impregnated composites. The only interfacial bonding limited the stress transfer from the matrix to the CNFs, leading to lower strain at break and toughness. The bio-based epoxy composites reinforced with a bacterial cellulose (BC) network have exhibited similar mechanical properties with increasing fiber contents [16]. With 30 vol% BC network, the modulus and strength of the composites massively increased to 8.8 GPa and 84 MPa from 1.2 GPa and 60 MPa of those of the neat resin. But, the degradation in the tensile strain to around 2 % was found when 5 vol% BC network was introduced in the composites. Notably, the lower modulus of the Pickering emulsion composites than that of the impregnated composites became less noticeable when a higher content of CNFs were introduced, owing to the thicker network formation in the Pickering emulsion composites. The Young’s modulus of the CNF networks might be dominated by the fiber interaction [46, 47]. With increasing CNF contents, more interaction between CNF fibers in the network could be formed, improving Young’s modulus of the CNF network in the composite [46, 47]. The advantages of Pickering emulsions could be an alternative technique to prepare flexible transparent CNF composite films with superior mechanical properties instead of the time-consuming impregnation method of the nanopaper reinforced composites. 3.4. Coefficient of thermal expansion and thermal stability Low thermal expansion is an attractive property for CNF reinforced composites as well as optical transparency and flexibility to enable use in electronic devices. Fig. 6(a) shows thermal expansion curves of the composites as a function of the CNF contents. The CTE of the neat acrylic resin was 192.3 ppm K−1, and this value decreased

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to 15.6 ppm K−1 on addition of a CNF content as low as 4 wt%. Taking into account the similarity of the structures observed in resin-impregnated compressed bacterial cellulose pericles [7] and Pickering emulsion composites, we attribute the considerable reduction of the CTE to the appearance of multiple layers of homogenous randomly oriented inplain networks of rigid and low thermal expansion CNFs, which suppressed the thermal expansion of the polymer in the x and y directions [9, 12]. At CNF contents greater than 4 wt%, the CTE value of the composites decreased. Composites with a CNF content of 25 wt% exhibited a value of 7.3 ppm K−1, which is close that of glass substrates (7–10 ppm K−1) [11]. Hence, thick and dense network layers and strong bonding formed in the hierarchical structure, restraining the movement of resin. Similarly, the CTE of the impregnated composites decreased as the content of CNFs increased, as shown in Fig. 6(b). It has been reported that there is a negative relationship between thermal expansion and Young’s modulus [48]. Both Pickering emulsion and impregnated composites presented a good linear correlation between the CTE and inverse Young’s modulus (Fig. 7). However, the less-stiff Pickering emulsion composites had a lower CTE than that of the impregnated composite samples. To obtain similar thermal dimensional stability, the Pickering emulsion composites required a CNF content as low as 9 wt% whereas 20 wt% CNFs was required for the impregnated composites. This result might be attributed to the compact hierarchical structure, which effectively hindered the mobility of the polymer chains [48] whereas the dense CNF sheets, having weak interfacial bonding with the resin, controlled the properties of the impregnated composite. Thus, both the impregnated and Pickering emulsion composites were reinforced with networks of CNFs; however, the two-tier hierarchical nanofiber network

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(multiple layers) in the Pickering emulsion composites more effectively restrained deformation of the resin matrix, compared with the impregnated composites where only single dense network was laminated between the resin with weak connections. The high thermal stability of the transparent substrate is another advantage for electronic applications. The addition of CNFs decreased the thermal stability of the composites, but no thermal degradation of the composites was observed until 210 °C (Fig. S3). This limitation should not hinder the high efficiency of the composites prepared in this work in terms of high transparency, strength and toughness for use from room temperature up to 150 °C.

4. Conclusions Tough and transparent CNF reinforced composites with a hierarchical structure were successfully fabricated by Pickering emulsification process without the use of emulsifiers or coupling agents. CNF networks formed during the emulsion preparation at the interfaces between the resin and water to encapsulate resin droplets was the main mechanism to stabilize the emulsion from flocculation and coalescence. Because of this encapsulation, the soft resin penetrated through pores within the CNF networks and formed a hierarchical architecture of the two-interconnected networks between the encapsulated resin by the networks and the CNF networks filled with resin when the pressure was applied to the composite mats at high temperature. The feature contributed to a considerable enhancement of the mechanical properties and toughness associated with a reduction in thermal expansion close to glass substrates and no major loss of the optical transparency. The superior performance became more pronounced as the CNF content increased owing to multilayers of the thicker network of CNFs, which required

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more energy to deform and had better stress-transfer behavior. Finally, the composite preparation based on the Pickering emulsion process features a simple method than that used for the impregnation method. The tough transparent composites formed by our new approach might have applications as an alternative to glass substrates in electronic applications such as flexible displays and solar cells.

Acknowledgements S. Tanpichai is grateful to King Mongkut’s University of Technology Thonburi, Thailand for the KMUTT 55th Anniversary Commemorative Fund and Skill Development Grant and the Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program the Center of Excellence Network. Subir K. Biswas acknowledges the Monbukagakusho scholarship (no. 143492) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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Figure and figure captions

Fig. 1. Fabrication of the composites by the Pickering emulsification process.

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Fig. 2. (a) Morphology of cellulose nanofibers (CNFs) prepared from bagasse and photograph of Pickering emulsions with various contents of CNFs taken (b) immediately after high-speed blending and (c) after storage at room temperature for 4 weeks.

Fig. 3. SEM images of the dehydrated and UV-cured Pickering emulsion with (a) 3 and (b) 10 wt% CNFs and fracture surfaces of the mats after vacuum-filtration of the emulsions with (c) 3 and (d) 10 wt% CNFs and of composites after hot-pressing ((e) PE 4 and (f) PE 18).

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Fig. 4. (a) Total transmittance spectra of the CNF reinforced composites, (b) total and linear transmittance values obtained from composites prepared by Pickering emulsion and impregnation approaches with respect to CNF contents and their composite appearance with different CNF contents (c) before and (d) after hot-pressing.

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Fig. 5. Tensile tress-strain curves of the CNF reinforced composites prepared by (a) Pickering emulsification and (b) impregnation method.

Fig. 6. (a) Thermal expansion curves of the hierarchical CNF reinforced composites and (b) coefficient of thermal expansion of the composites formed by the Pickering emulsification and impregnation processes.

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Fig. 7. Relationship between thermal expansion and inverse of the Young’s modulus of the composites prepared by Pickering emulsion and impregnation method.

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Table Table 1. Values of Tensile strength, Young’s modulus, tensile strain and toughness for the composite films with CNF contents prepared by the Pickering emulsification and impregnation method. Tensile strength

Young’s

Tensile strain

Toughness

(MPa)

modulus (GPa)

(%)

(MJ m-3)

2.2 ± 0.5

0.03 ± 0.01

7.0 ± 1.6

0.08 ± 0.03

PE 4

21.3 ± 1.2

0.66 ± 0.09

12.7 ± 1.7

1.8 ± 0.3

PE 9

22.0 ± 1.0

0.68 ± 0.04

13.4 ± 1.7

1.9 ± 0.3

PE 18

29.2 ± 1.0

1.2 ± 0.1

11.0 ± 0.8

2.2 ± 0.2

PE 25

41.0 ± 2.6

1.7 ± 0.2

10.6 ± 0.9

2.9 ± 0.2

IM 15

17.9 ± 4.3

1.2 ± 0.2

5.1 ± 1.0

0.6 ± 0.2

IM 20

22.8 ± 2.2

1.5 ± 0.2

5.4 ± 0.9

0.9 ± 0.2

IM 26

32.3 ± 2.3

2.1 ± 0.2

6.4 ± 0.5

1.5 ± 0.1

CNF sheet

140.2 ± 10.8

8.6 ± 0.3

10.3 ± 2.0

11.2 ± 3.1

Materials Acrylic resin

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Credit author statement

Supachok Tanpichai and Subir K. Biswas contributed equally to the laboratory work. Suteera Witayakran conducted the bagasse treatment. Supachok Tanpichai, Subir K. Biswas and Hiroyuki Yano contributed to the design of methodology, to the analysis of the results and to the writing and editing of the manuscript.

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