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Thermally Superstable Cellulosic-NanorodReinforced Transparent Substrates Featuring Microscale Surface Patterns Subir K. Biswas,*,† Supachok Tanpichai,†,§ Suteera Witayakran,∥ Xianpeng Yang,† Md. Iftekhar Shams,‡,⊥ and Hiroyuki Yano*,† ACS Nano Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/31/19. For personal use only.
†
Laboratory of Active Bio-Based Materials and ‡Laboratory of Sustainable Materials, Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Kyoto 611-0011, Japan § Learning Institute, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand ∥ Kasetsart Agricultural and Agro-Industrial Product Improvement Institute, Kasetsart University, Bangkok 10900, Thailand ⊥ Forestry and Wood Technology Discipline, Khulna University, Khulna 9208, Bangladesh S Supporting Information *
ABSTRACT: The recent rapid expansion of thin-film, bendable, and wearable consumer (opto)electronics demands flexible and transparent substrates other than glass. Plastics are the traditional choice, but they require amelioration because of their thermal instability. Here, we report the successful conversion of a soft and thermally vulnerable polymer into a highly thermally stable transparent nanocomposite material. This is achieved by the meticulous choice of a polymer with a glass-transition temperature below 0 °C that gives stable mechanics above room temperature, reinforcing the polymer with a load-bearing hierarchical network of the incredibly strong and stable natural material: cellulose nanorods. Owing to the Pickering emulsification process, the nanocomposites inherit the self-assembled structural hierarchy from the cellulose nanorod-encapsulated resin droplets. The ameliorated nanocomposites have highly desirable high-temperature endurance (∼150−180 °C) in terms of the thermomechanical, thermodimensional, and thermo-optical performance. Any photonic nano- or microstructures can be directly molded on the surface of the nanocomposites in high precision for better light management in photonic and optoelectronic applications. The highlight of this work is the demonstration of a highly thermally stable microlens array on the ameliorated transparent nanocomposite. KEYWORDS: nanocellulose, Pickering emulsion, flexible electronics, polymer nanocomposites, thermal stability, microlens array molecule filtration,12 and cell biology.13 Plastics enable scalable, multitier, and R2R patterning by a simple molding and embossing technique (e.g., imprint lithography) with high resolution.14,15 Although plastics have high flexibility, transparency, and facile patternability, most plastics exhibit poor thermomechanical, thermodimensional, and thermo-optical stability. Most plastics lose stiffness drastically, sometimes an order of magnitude, when heated from room temperature to ∼100−150 °C. Another drawback of plastics is that they have very low thermal dimensional stability, and their coefficient of thermal expansion (CTE) often exceeds 200 ppm K−1.16 This high CTE causes problems in devices at high temperature during
T
ransparent substrates are used in optoelectronic devices, such as photovoltaics, displays, smart windows, touch screens, and so forth. Glass and plastics are the conventional choices for the substrate material. However, plastics are being ubiquitously investigated because thin-film, lightweight, flexible, and printable electronics can be prepared by the roll-to-roll (R2R) production process.1 Fabricating plastics with photonic surface patterns (e.g., microlens and nanopillar arrays) is a rapidly developing lightmanagement technique for optoelectronic devices, such as solar cells, light-emitting diodes, and optical sensors.2−6 Nanoand microscale surface features support strong optical resonance and reduce plasmonic loss, which can enhance and effectively control light absorption and scattering processes or light outcoupling to improve the device performance. Other application areas include three-dimensional (3D) and widefield-of-view imaging and microscopy,7,8 optical filters and stealth surfaces,9 data and energy storage,10 fuel cells,11 © XXXX American Chemical Society
Received: November 6, 2018 Accepted: January 28, 2019
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DOI: 10.1021/acsnano.8b08477 ACS Nano XXXX, XXX, XXX−XXX
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Figure 1. Fabrication of the transparent nanocomposites starting from (a) the preparation of CNs with different lengths and crystallinities and (b) their Pickering-stabilized resin-in-water emulsions. The FE-SEM images in panel a show that the chemically extracted microscale cellulose fibers are composed of aggregates of nanoscale cellulose fibers. Therefore, microscale cellulose fibers are termed as “nanostructured fibers”. The individual CNs with different lengths and crystallinities are extracted from the nanostructured fibers subsequently by mechanical treatment (grinding) and acid hydrolysis. The FE-SEM images in panel b show that the resin droplets are encapsulated by the CN network. The long CN4K produced a loose mesh-like network, whereas CN1K (combination of long and short CNs) and CN400/300 (short CNs) produced a dense network.
carbon nanotubes, and clays.19−24 However, high loading (>50 wt %) of these inorganic fillers is usually required to achieve satisfactory thermal stability, which often renders the plastic relatively brittle and/or optically inferior. In this context, strong cellulose nanofibers (CNFs), which are ∼0.1−2 μm long and ∼5−60 nm diameter semicrystalline fibers extracted from plants,25 have attracted great attention as a nanofiller to improve the mechanical and thermal performance of plastics without decreasing the transparency.16,26,27 The “crystalline” moieties (∼50−90%)28 in semicrystalline CNFs have an incredibly high Young’s modulus (E) of ∼130 GPa,29 a very low CTE of ∼0.1 ppm K−1,30 and high thermal and chemical durability. Recently, using “semicrystalline” CNFs, we fabricated transparent hierarchical composites by a facile Pickering emulsification method that have high strength, toughness, and 3D moldability compared with their counter-
both the fabrication processes (often requiring temperatures of >100 °C) and operation. Not only can this lead to detrimental mechanical stress between the functional materials (e.g., silicon, CTE ≈ 3 ppm K−1) and the substrate owing to the very large CTE mismatch,17 but also, the nano- and microscale surface features can easily become deformed or damaged, which can cause attenuation of the device performance. For example, decreased light extraction from an organic light-emitting diode (OLED) has been observed owing to fabrication-induced damage in the nanopatterned substrate.4 The optical clarity of plastics also decreases with heating.18 Transparent plastic substrates devoid of such issues are highly desirable, but they have rarely been reported. The thermal properties of plastics can be improved by incorporating micro- and nanofillers, such as silica, alumina, boron nitride, diamond, zinc sulfide, zirconium tungstate, B
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Figure 2. Evolution of the optical transparency and structural hierarchy of the high-strength and flexible nanocomposites. (a) Self-standing CN/resin mats containing liquid resin droplets. (b) Highly transparent nanocomposites obtained after hot compressing the mats. (c) Total optical transmittances of the nanocomposites (∼125−200 μm thick). The transmittances were normalized to 100 μm thickness to eliminate the effect of thickness variation. (d) Schematic of evolution of the transparency and self-assembled hierarchical microstructure. (e) FE-SEM image of the fracture surface of a CN/resin mat showing the oval-shaped resin droplets (highlighted by the yellow dashed line). (f) FE-SEM image of the fracture surface of a transparent nanocomposite showing the platelets of the resin (highlighted by the yellow dashed line). (g) Tensile stress−strain curves of the nanocomposites at 23 °C and 50% relative humidity.
parts prepared by a different method.31 The nanocomposites have a CTE of 13 ppm K−1 (1/15th that of the polymer used and close to that of glass) with only 16 wt % CNF content. Herein, we report the preparation of highly thermally stable nanocomposites using the stronger and more stable “crystalline” parts of the CNFs (i.e., nanocrystals or nanorods, CNs) in a transparent polymer matrix. A resin-in-water Pickering emulsion was first prepared using CNs as a stabilizer followed by dehydration, hot compression, and curing to obtain transparent nanocomposites (Figure 1). The Pickering emulsification approach was used because of its simplicity and the ability to directly and uniformly mix immiscible components (e.g., hydrophilic CNs and hydrophobic resin) in water without any chemical intervention. 2,2-Bis[4(acryloxypolyethoxy)phenyl]propane (ABPE-10) acrylic resin was used because of its soft nature (for high composite flexibility), low glass-transition temperature (Tg) below 0 °C (for stable mechanics above room temperature), and refractive index close to cellulose (for high optical clarity). The resulting nanocomposites have a hierarchical microstructure and very high thermomechanical, thermodimensional, and thermooptical stability for temperatures of ∼150−180 °C with only ∼10 wt % CN content. The highlight of this work is the demonstration of a highly thermally stable microlens array
(μLA) that was directly molded on the thermally ameliorated transparent nanocomposite.
RESULTS AND DISCUSSION Sugar-cane bagasse pulp was used as the raw material for CN preparation. The bagasse was chemically purified by a series of acidified sodium chlorite (NaClO2) and potassium hydroxide (KOH) treatments to remove the matrix lignin and hemicelluloses. The purified nanostructured pulp fibers (crystallinity of 81%) were then mechanically disintegrated into individual long semicrystalline CNFs (length of 4542 ± 690 nm, crystallinity of 70%, CN4K) by passing them twice through a grinder in a water medium (Figure 1a). The decrease in the crystallinity is probably because of damage of the cellulose crystals during the mechanical action.32 The amorphous parts of semicrystalline CN4K were chemically cleaved by hydrochloric acid (HCl, 2 N) hydrolysis to produce nanocrystals or nanorods (Figure 1a). A total of three types of hydrolyzed CNs with decreasing length and increasing crystallinity were produced by modifying the hydrolysis time and temperature (Figure S1): 1278 ± 1029 nm long and 73% crystallinity (CN1K), 366 ± 142 nm long and 76% crystallinity (CN400), and 341 ± 94 nm long and 79% crystallinity (CN300). Thus, short but highly crystalline CNs were C
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Figure 3. Material properties of the transparent nanocomposites. (a) Storage modulus E′ as a function of temperature. (b) Thermal expansion in the x or y direction. (c) CTE of the CN300 transparent nanocomposite compared with a variety of other transparent optoelectronic substrates. aCN300 @ 20−100 °C; bCN300 @ 20−150 °C; borosilicate glass;38 CNF nanopaper;31 PI, polyimide;40 chitin nanopaper;40 PEN, polyethylene naphthalate;40 PET, polyethylene terephthalate;31 PES, polyether sulfone;40 PC, polycarbonate (this work); epoxy;26 and PS, polystyrene.42 See also Table S4 for more details. (d) Optical transmittance at 600 nm wavelength as a function of the heating time at 180 °C.
flattened to platelets, and their surrounding CN network (CN shell) is infiltrated by the resin (composite shell) to form a continuous matrix. Therefore, refractive index variation and scattering at the interfaces are minimized, which leads to the nanocomposites having high optical clarity. There is no significant effect of different CNs on the transmittance of the nanocomposites (Figure 2c). For the structural hierarchy, the submicrometer-thick CN network around the submicrometerto micrometer-thick resin platelets is the first order of the hierarchy (Figure 2f; also see Figures 1b and 2d,e). The second order of the structural hierarchy is the micrometer- to millimeter-scale CN network formed by interconnection and stacking of the numerous CN-encapsulated resin platelets in the bulk composite structure (Figure 2d−f). It should be noted that a small amount of resin is squeezed out of the CN4K mat during hot compression. This might be because a loose mesh-like network structure forms due to the long high-aspect-ratio CN4K on the droplet surface (Figure 1b). In contrast, CN1K, which contains both long and short CNs (Figure S1b,e), and short CN400/300 produce more dense network structures, thus prevented the resin from being squeezed out. Therefore, the amount of CN4K in the emulsion was carefully chosen to obtain transparent nanocomposites with ∼10 wt % CN content. The tensile stress−strain curves are shown in Figure 2g, and the tensile stress−strain data are given in Table S1. The results show incorporation of the hierarchical CN network results in a huge improvement in the mechanical properties of the soft resin, as previously reported,31 and the different types of CN lead to different mechanical behavior even for the same CN content (∼10 wt %). The strength, E, and toughness increase from 2.29 ± 0.54 MPa, 0.03 ± 0.00 GPa, and 0.09 ± 0.04 MJ
successfully obtained. However, the crystallinities are little lower compared with the crystallinities (77% − 86%) reported by Oliveira et al.33 In their work, the CNs were produced directly from the sugar cane bagasse fibers by treating with 65 wt % sulfuric acid (H2SO4). The width of CN4K is 20 ± 6 nm and the width of all the hydrolyzed CNs is 18 nm (CN1K: 18 ± 4 nm; CN400:18 ± 4 nm; and CN300:18 ± 3 nm). The CN/water slurry was then mixed with liquid acrylic resin monomer and vigorously agitated to obtain a resin-inwater emulsion stabilized by CNs (Figure 1b). The average diameter of the resin droplets is ∼2 μm. Field-emission scanning electron microscopy (FE-SEM) images reveal that the droplets are encapsulated by the CN network, thereby preventing their coalescence (Figure 1b). By taking advantage of the nanocellulose network, Svagan et al.34 encapsulated optically functional molecules contained in hexadecane oil to prepare a photon energy up-converting material and Li et al.35 encapsulated paraffin to prepare a thermal regulation nanocomposite. In this study, the emulsions were easily dehydrated by vacuum filtration and drying (40 °C) without any apparent resin leakage towing to the protecting CN network. The obtained translucent materials, which are called CN/resin mats, were then hot compressed (2 MPa, 150 °C) to obtain nanocomposites with a high optical transmittance of ∼90% in the wavelength range 400−800 nm (Figures 1b and 2a−c). Evolution of the transparency and microstructural hierarchy of the nanocomposites is shown in Figure 2d, which is supported by the FE-SEM images of the fracture surface of the mat and the transparent nanocomposite (panels e and f of Figure 2, respectively). The spherical resin droplets in the emulsion become oval-shaped in the mat owing to the vacuum suction pressure. During hot compression, the oval droplets are D
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ACS Nano m−3 for the neat resin to 29.39 ± 1.15 MPa, 1.20 ± 0.06 GPa, and 2.38 ± 0.18 MJ m−3, respectively. Notably, the highest strength and toughness are obtained with CN1K reinforcement, whereas the E value of the CN300 nanocomposite is the highest. We investigated the fracture surfaces of the nanocomposites after tensile tests by FE-SEM (Figure S2). It is notable that the load-bearing CN networks around the resin platelets in the CN1K and CN300 nanocomposites are thicker and more densely structured than that of CN4K (Figures S2 and 1b). In addition, because CN1K is composed of both long and short CNs, the load transfer from the polymer matrix is effectively governed by both the breaking of long CNs (evident by protruded broken nanofibers, as in CN4K) and pulling out of short CNs (as in CN300).36,37 Furthermore, in a recent study, we demonstrated that polymer nanocomposites having a hierarchical CNF network with a brick-and-mortar architecture show a synergistic stress-bearing, stress-dissipating, and crackminimizing mechanism.31 In this study, a similar synergistic hierarchical structure was successfully obtained (Figures 2d−f and S2). Consequently, compared to the soft polymer, the transparent nanocomposites are highly flexible (fracture strain of up to ∼15% versus ∼7% for the neat resin) and strong (strength of up to ∼30 MPa), leading to much higher toughness at only ∼10 wt % CN content. In comparison, the strength of the thermal regulation paraffin/CNF nanocomposite prepared by a similar process is 30 MPa at a CNF content of 28 wt %.35 The strength and E of the nanocomposites, particularly those of the CN1K and CN300 nanocomposites, are comparable with those of typical engineering plastics, such as polyethylene terephthalate (PET) and epoxy.38 The toughness, strength, and E of the nanocomposites are higher than or comparable with those of delignified-wood/epoxy transparent composites.39 The thermal stability of the nanocomposites with respect to potential applications in the flexible (opto)electronics field is shown in Figure 3. The thermomechanical stability results in Figure 3a and Table S2 show that the storage modulus (E′) of the neat polymer decreases by more than 1 order of magnitude (4.04 to 0.08 GPa) from −50 to 150 °C owing to the glass transition. Notably, the storage modulus of the polymer is relatively stable in the temperature range from ∼20 to 150 °C, although it is extremely low (0.06 ± 0.01 GPa). We carefully chose a polymer with Tg < 0 °C that shows stable mechanics above room temperature. This is because when the polymer is reinforced with a hierarchical CN network, not only does the storage modulus greatly improve, but, most importantly, it also remains highly stable over a wide temperature range. The CN4K, CN1K, CN400, and CN300 nanocomposites retain high elastic moduli of 1.12, 1.60, 1.77, and 1.80 GPa, respectively, even at 150 °C. The high E′ values indicate good dispersion of the load-bearing hierarchical CN network with extensive hydrogen bonding and a synergistic interaction between the thermally stable CNs and the CN-networkreinforced polymer platelets, which inhibits thermal relaxation motion of the polymer phase.40 The values also indicate that a thick and dense CN network (Figure S2) and high crystallinity are beneficial. For comparison, a commercial thermally stable biaxially oriented PET film (Lumirror, Toray, Japan) used in the optoelectronic industry was also tested (Figure 3a). After being relatively stable from −50 to 85 °C, the E′ value of the film drastically decreases to 0.66 GPa at 150 °C. The E′ of polyimide (PI), another commercially used polymer film, has
similar high mechanical stability against elevated temperature to the nanocomposites, although it suffers from having an unwanted brownish tint.40 Therefore, the high and stable E′ at elevated temperatures is a promising characteristic of the transparent nanocomposites for fabrication of organic and inorganic optoelectronic devices. The thermal dimensional properties of the transparent nanocomposites are shown in Figure 3b and Table S3. The neat polymer expands by 2.42% in the planar direction (x or y direction) from 20 to 150 °C. However, for the CN4K, CN1K, CN400, and CN300 nanocomposites, thermal expansion is only 0.16, 0.11, 0.08, and 0.08% in the x or y direction, respectively. The CTE of the neat polymer from 20 to 150 °C in the x or y direction is very high (190.53 ppm K−1). However, owing to strong hierarchical reinforcement from the low-CTE CNs to the polymer platelets, the CTE drastically decreases to 12.10, 8.28, 6.29, and 5.49 ppm K−1 for CN4K, CN1K, CN400, and CN300, respectively. Clearly, the highly crystalline short CNs with a thick and dense network layer produce highly thermally dimensionally stable nanocomposites. The CTE values are incredibly low at only ∼10% CN content. In the previous studies, a high nanocellulose content of >60 wt % was used to obtain transparent nanocomposites with CTE values below 10 ppm K−1.26,41 To understand the underlying reason, we investigated the thermal dimensional stability of the nanocomposites in the thickness direction (z direction) (Figure S3 and Table S3). We found that thermal expansion of the neat polymer and nanocomposites is anisotropic. Thermal expansion and the CTE of the neat polymer in the z direction from 20 to 150 °C are 7.94% and 624.46 ppm K−1, respectively. In the z direction, thermal expansion and the CTEs of the nanocomposites decrease to 6.46−7.00% and 506.49−542.50 ppm K−1, respectively. This again shows that the nanocomposites reinforced with a thick and dense network of highly crystalline short CNs show high thermal dimensional stability. Compared with the x or y direction, thermal expansion and the CTE values of the nanocomposites in the z direction are extremely high. These phenomena have also been observed for a nanocomposite with a similar layered structure.16 This is probably because the CNs are randomly oriented in-plane in the nanocomposites. In addition, owing to the use of a soft polymer (E = 0.03 GPa), the induced thermal stresses are very small.16 Therefore, the high thermal expansion of the polymer in the x or y direction is dramatically suppressed by the hierarchical in-plane rigid network of short and highly crystalline CNs, and it is also accommodated by the huge expansion in the z direction. Note that the absolute value of z-direction expansion is insignificant owing to the thinness of the nanocomposites. Thermal expansion of the transparent nanocomposites is much lower than those of the transparent polymeric films reported as optoelectronic substrates (Figure 3c and Table S4).26,31,40,42 In particular, the CTE of the CN300 nanocomposite is 3.4 and 5.5 ppm K−1 in the temperature ranges of 20−100 and 20−150 °C, respectively. These values are comparable with those of highly stable borosilicate glass (∼4 ppm K−1)38 and semiconducting silicon crystals (∼3 ppm K−1).16 In addition to the thermomechanical and thermodimensional stability, the thermo-optical stability of a transparent substrate is also an important requirement. The substrate must maintain high optical clarity after the high-temperature E
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Figure 4. Thermal stability of the micropattern on the surface of the transparent nanocomposite. (a) Fabrication of a μLA on the surface of the CN300 nanocomposite. (b) Thermal stability of the μLA on the nanocomposite compared with that on the neat resin film. (i, iv, vii, and x) Thermal images, (ii, v, viii, and xi) laser beam diffraction patterns (μLAs are highlighted by the yellow dashed line), and (iii, vi, ix, and xii) FE-SEM images of the μLA before and after heating.
(∼100−150 °C) processing steps, such as fabrication of the transparent conductive electrode (TCE) and deposition of functional materials.18 Amazingly, the nanocomposites still transmit ∼83% (at 600 nm) of specular light (light parallel to the incident light), even after 120 min heating at 180 °C, which is ∼86% before heating (Figure 3d). The diffusive transmittance (all parallel and forward-scattered light) is ∼90%, and it decreases to ∼88% after 120 min heating (Figure 3d). This high-level thermo-optical stability is because of the highly stable polymer. The slight decreases in the transparencies of the nanocomposites with increasing time is probably because of initiation of thermal decomposition of the cellulose at about 180 °C.32 However, thermogravimetric analysis (TGA) reveals that the nanocomposites are fairly stable against thermal decomposition (Figure S4). The CN4K
and CN1K nanocomposites lose 1% of their weight up to ∼290 °C, and the CN400 and CN300 nanocomposites lose 1% of their weight up to ∼280 °C. A PET film was heated at 180 °C. The film starts to turn hazy after 30 min heating. After only 60 min of heating, the specular transmittance decreases from 86% to 55%, and the diffusive transmittance decreases from 88% to 83% (Figure 3d). Similar degradation in the optical quality of the PET film at 150 °C has been reported, which is because of the increase in the surface roughness induced by migration of cyclic oligomers to the film surface.18 The highly thermally stable nanocomposites developed here have excellent transparency, good load-bearing capacity, and amenable flexibility for R2R processing, indicating that they are extremely promising for a range of future smart optoelecF
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CONCLUSIONS We have fabricated highly transparent and highly thermomechanically, thermodimensionally, and thermo-optically stable nanocomposites. The high thermal stability originates from the hierarchical layered assembly of the rigid network of ultrastable crystalline CNs. This hierarchical assembly, where the resin platelets are trapped and encapsulated by the sub-microscale to macroscale nanorod network, is inherited from the nanorodstabilized resin-in-water Pickering emulsion. The hierarchical nanocomposites have also good mechanical strength, toughness, and flexibility. The thermal, optical, and mechanical performances of the nanocomposites compare favorably with many cellulose-based transparent materials and widely used transparent plastics. Notably, the nanocomposite reinforced with highly crystalline short nanorods possesses thermal dimensional stability similar to silicon crystals. Finally, we directly molded a highly thermally dimensionally stable flexible μLA on the surface of the CN300 nanocomposite. The highly thermally stable nanocomposites developed here have excellent transparency, favorable load-bearing ability, amenable flexibility for R2R processing, and easy patternability with thermally stable photonic surface structures that they are attractive for applications in a range of future photonics, electronics, and optoelectronics, as well as in other areas, such as microscopy, stealth surfaces, data and energy storage, molecule separation, and fuel cell technology.
tronics. In addition to these promising characteristics, the nanocomposites also show no bi-refringence because of the random in-plane orientation of the CNs (Figure S5), which is advantageous for display applications. In conjunction with enhancement of the transparent substrate properties, amelioration of substrates using nanoand microscale surface structures for better light management in optoelectronic devices is a rapidly developing field. Nanoand microscale surface features improve the efficiency of solar cells, OLEDs, and optical sensors by supporting strong photonic resonance, reducing plasmonic loss, and enhancing photon absorption, scattering, and out-coupling.2−6 The surface of the CN/resin nanocomposites can be patterned in situ during hot compressing by simply sandwiching the CN/ resin mat between a glass slide and an oppositely patterned template (one-sided) or between two templates (two-sided), as shown in Figure 4a. Formation of the pattern is facilitated because the strong CN network holds the resin in the liquid state in the mat. Hence, nano- and microscale polymeric structures can be easily and directly obtained with great precision on the surface of the CN-reinforced transparent nanocomposites. This molding strategy can be regarded as a simple form of the micro- and nanoimprint lithography techniques. Based on the results in Figure 3, in situ fabricated patterns on the nanocomposites are expected to be highly thermally stable. As a demonstration, a concave μLA that shows rainbow colors owing to diffraction of light was fabricated (Figure 4a). A custom experiment was designed to demonstrate the thermal stability of the μLA in real time (Figure S6). The μLAs on the highly stable CN300 nanocomposite and a neat polymer film were separately clipped on glass slides, and a green laser beam was pointed at them. As shown in Figure 4bi,ii, the laser beam is diffracted by the periodic microlenses on the polymer film and produces a well-defined pattern on the back screen at room temperature (∼17 °C). When the μLA on the polymer film was heated to ∼140 °C for only 2 min with a heat gun, the film expanded and produced a deformed diffraction pattern, as shown in Figure 4biv,v. FE-SEM images before and after heating (Figure 4biii,vi) show that the diameter of the microlenses increases from ∼2.15 to ∼2.19 μm owing to very large planar expansion of the neat polymer film. The increase of the diameter is about 1.86%, which is less than the aforementioned planar expansion of the neat polymer film from 20 to 150 °C (2.42%). This is probably because of shrinkage of the polymer film between the experiment and FESEM imaging. In contrast, the μLA on the transparent CN300 nanocomposite is highly stable, and there are no significant differences in the laser diffraction patterns and microlens diameters before and after heating (Figure 4bvii−xii). The strong hierarchical CN network restricts planar thermal expansion of the polymer, so the microlenses on the nanocomposite film are thermally stable. This is advantageous for optoelectronic device processing because any deformation or damage in the nano- and microscale features might affect the performance of the device.4 The high thermal stability of the nano- and micropatterned films could be desirable in other application areas, including 3D imaging and microscopy, optical filters and stealth surfaces, data and energy storage, fuel cells, and molecule filtration.
MATERIALS AND METHODS Materials. Sugar-cane bagasse was kindly supplied by Eastern Sugar & Cane Co., Ltd., Thailand. ABPE-10 monomer (reflective index = 1.516, Tg = −12 °C) was provided by Shin-Nakamura Chemical, Japan. The UV photoinitiator 2-hydroxy-2-methylpropiophenone (Wako), NaClO2 (Sigma-Aldrich), KOH (Wako), acetic acid (Wako), and HCl (Wako) were used as received. The photoinitiator (0.25% w/w) was mixed with the monomer before use. Preparation of the CNs. Bagasse (100 g) was initially treated with 4% KOH solution (1 L) at 165 °C for 2 h in a closed pulping unit. The washed material was then treated with acidified NaClO2 (80 °C, 5 h), KOH (90 °C, 2 h), and again with acidified NaClO2 (80 °C, 3 h) to completely remove lignin and most of the hemicelluloses. The purified pulp and water slurry (0.8% w/w) was then mechanically disintegrated into long semicrystalline CN4K by passing the slurry twice through a grinder (MKCA6-2, stone type: MKGC6-80, Masuko Sangyo, Japan). The grinding treatment was performed at 1,500 rpm with a clearance gauge of −2 (corresponding to a 0.2 mm shift) from the 0 position. Highly crystalline short CNs were obtained by hydrolyzing CN4K using 2 N HCl. The hydrolysis temperatures and times to prepare CN1K, CN400, and CN300 with different lengths and crystallinities were 50 °C and 2 h, 70 °C and 2 h, and 70 °C and 4 h, respectively. The hydrolyzed CNs were then washed by centrifugation until neutral and dialyzed in running water for 2 days. Preparation of the Nanocomposites. The CNs (0.2 g for CN4K and 0.4 g for CN1K, CN400, and CN300), monomer (3.8 g for CN4K and 3.6 g for CN1K, CN400, and CN300), and water (196 g) were vigorously blended (37 000 rpm, Vita-Mix Absolute 3, Osaka Chemicals, Japan) for 10 min with an interval of 5 min after 5 min. The obtained Pickering emulsions were subjected to vacuum filtration to obtain a CN/resin mat on a PTFE filter membrane (pore size 0.1 μm, Advantec) and then oven-dried overnight at 40 °C. The dried mats were hot compressed (150 °C, 2 MPa, 5 min) by placing them between glass slides to fabricate transparent nanocomposites. For the surface micropatterned nanocomposite, the mat was placed between a patterned sapphire substrate (PSS, SAMCO, Japan) and a glass slide followed by hot compression under the same conditions. The hotcompressed materials were finally UV polymerized by a F300S UV lamp/LC6 conveyer system (20 J cm−2, Fusion UV Systems). G
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ACS Nano Characterization. FE-SEM imaging was performed with a JSM7800F Prime scanning electron microscope (JEOL) after platinum coating. The resin droplet diameter was directly determined from the FE-SEM images of the UV-cured and oven-dried emulsions using ImageJ software. The lengths and widths of the CNs were also measured from the FE-SEM images using ImageJ software. X-ray diffraction (XRD) analysis to determine the crystallinity was performed for pellets of freeze-dried CN samples (pressed at 1.4 MPa) by irradiating with nickel-filtered CuKα (k = 0.154 nm) radiation generated by an UltraX 18HF X-ray diffractometer (Rigaku) operated at 40 kV and 200 mA in reflection mode. The crystallinities were determined from the normalized XRD profiles using the Segal equation.43 The optical transmittance was measured with an ultraviolet−visible spectrophotometer (U-4100, Hitachi) by placing the sample at and 25 cm from the entrance port of the integrating sphere. The tensile tests were performed with an Instron 3365 universal testing machine (Instron) with a span length of 20 mm at a cross-head speed of 1 mm min−1. The stress−strain data were obtained for at least 5 rectangular nanocomposite specimens (5 mm × 35 mm) conditioned at 23 °C and 50% relative humidity for 48 h. The thermomechanical properties were determined with a dynamic thermomechanical analyzer (DMS 6100, Seiko Instruments). The 5 mm wide samples were analyzed in a N2 atmosphere from −50 to 150 °C at a ramp of 2 °C min−1 in tension mode with a span of 20 mm, amplitude of 10 μm, frequency of 1 Hz, and a preload force of 0.1 N. The thermodimensional properties were determined for 3 mm wide nanocomposite samples by a thermomechanical analyzer (TMA/SS 6100, Seiko Instruments) in tensile mode with a 20 mm span and a ramp of 5 °C min−1 under a N2 atmosphere. The thermodimensional property (CTE) of PC was determined in the identical condition by the same instrument for 5 mm wide and 1 mm thick samples prepared from PC powder (Iupilon, Mitsubishi Engineering-Plastics Corporation) by injection molding (injection and die temperatures of 260 and 85 °C, respectively). To measure the thermo-optical properties, the samples were heated at 180 °C on a hot plate, and the optical transmittance was measured every 30 min with the U-4100 spectrophotometer over a period of 120 min. The TGA data were obtained with a Q50 thermogravimetric analyzer (TA Instruments) for ∼5 mg samples placed on a platinum pan, stabilized at 110 °C for 10 min to remove moisture, and then ramped to 600 °C at a rate of 10 °C min−1 under a N2 atmosphere. The bi-refringence was observed by placing the samples between two cross-polarizing films. A thermal imaging camera (FLIR E6, FLIR Systems) was used to monitor the real-time temperature of the μLAs during the thermal stability tests by laser diffraction.
Author Contributions
S.K.B. and S.T. contributed equally to the laboratory work. S.K.B. also prepared the μLAs and tested their thermal stability. S.K.B. analyzed the data and wrote the manuscript with the help from the coauthors. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS S.K.B. thanks MEXT, Japan for a scholarship grant (no. 143492). We thank A. Miura for measuring the CTE of PC and SAMCO Inc., Japan for providing PSS. S.T. acknowledges a KMUTT 55th Anniversary Commemorative Fund and Skill Development Grant and the Center of Excellence Network program of the Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand. We thank the Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript. REFERENCES (1) Forrest, S. R. The Path to Ubiquitous and Low-Cost Organic Electronic Appliances on Plastic. Nature 2004, 428, 911−918. (2) Brongersma, M. L.; Cui, Y.; Fan, S. Light Management for Photovoltaics Using High-Index Nanostructures. Nat. Mater. 2014, 13, 451−460. (3) Wang, Z.; Peng, L.; Lin, Z.; Ni, J.; Yi, P.; Lai, X.; He, X.; Lei, Z. Flexible Semiconductor Technologies with Nanoholes-Provided High Areal Coverages and Their Application in Plasmonic-Enhanced Thin Film Photovoltaics. Sci. Rep. 2017, 7, 13155. (4) Hippola, C.; Kaudal, R.; Manna, E.; Xiao, T.; Peer, A.; Biswas, R.; Slafer, W. D.; Trovato, T.; Shinar, J.; Shinar, R. Enhanced Light Extraction from OLEDs Fabricated on Patterned Plastic Substrates. Adv. Opt. Mater. 2018, 6, 1701244. (5) Xiang, H.-Y.; Li, Y.-Q.; Zhou, L.; Xie, H.-J.; Li, C.; Ou, Q.-D.; Chen, L.-S.; Lee, C.-S.; Lee, S.-T.; Tang, J.-X. Outcoupling-Enhanced Flexible Organic Light-Emitting Diodes on Ameliorated Plastic Substrate with Built-in Indium-Tin-Oxide-Free Transparent Electrode. ACS Nano 2015, 9, 7553−7562. (6) Abramowitz, M., Davidson, M. W. Microlens Arrays. https:// www.olympus-lifescience.com/zh/microscope-resource/primer/ digitalimaging/concepts/microlensarray/ (accessed Jun 30, 2018). (7) Song, Y. M.; Xie, Y.; Malyarchuk, V.; Xiao, J.; Jung, I.; Choi, K.J.; Liu, Z.; Park, H.; Lu, C.; Kim, R.-H.; Li, R.; Crozier, K. B.; Huang, Y.; Rogers, J. A. Digital Cameras with Designs Inspired by the Arthropod Eye. Nature 2013, 497, 95−99. (8) Wu, D.; Wang, J.-N.; Niu, L.-G.; Zhang, X. L.; Wu, S. Z.; Chen, Q.-D.; Lee, L. P.; Sun, H. B. Bioinspired Fabrication of High-Quality 3D Artificial Compound Eyes by Voxel-Modulation Femtosecond Laser Writing for Distortion-Free Wide-Field-of-View Imaging. Adv. Opt. Mater. 2014, 2, 751−758. (9) Diao, Z.; Kraus, M.; Brunner, R.; Dirks, J.-H.; Spatz, J. P. Nanostructured Stealth Surfaces for Visible and Near-Infrared Light. Nano Lett. 2016, 16, 6610−6616. (10) Zhou, M.; Xu, Y.; Lei, Y. Heterogeneous Nanostructure Array for Electrochemical Energy Conversion and Storage. Nano Today 2018, 20, 33−57. (11) Zhou, Z.; Dominey, R. N.; Rolland, J. P.; Maynor, B. W.; Pandya, A. A.; DeSimone, J. M. Molded, High Surface Area Polymer Electrolyte Membranes from Cured Liquid Precursors. J. Am. Chem. Soc. 2006, 128, 12963−12972. (12) Yasui, T.; Rahong, S.; Motoyama, K.; Yanagida, T.; Wu, Q.; Kaji, N.; Kanai, M.; Doi, K.; Nagashima, K.; Tokeshi, M.; Taniguchi, M.; Kawano, S.; Kawai, T.; Baba, Y. DNA Manipulation and Separation in Sublithographic-Scale Nanowire Array. ACS Nano 2013, 7, 3029−3035. (13) Kim, M.-H.; Sawada, Y.; Taya, M.; Kino-oka, M. Influence of Surface Topography on the Human Epithelial Cell Response to
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b08477. Figures showing FE-SEM images, dimensions, and XRD results, thermal expansion in the z direction of the nanocomposites, TGA thermograms, a photograph of birefringence, and the experimental setup for the thermal stability test of μLA; tables showing summaries of the mechanical, thermomechanical, and thermodimensional data (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Subir K. Biswas: 0000-0002-9538-4726 Xianpeng Yang: 0000-0003-2287-7834 H
DOI: 10.1021/acsnano.8b08477 ACS Nano XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acsnano.8b08477 ACS Nano XXXX, XXX, XXX−XXX
