Enhanced Toughness and Thermal Stability of Cellulose Nanocrystal

Aug 20, 2017 - Iridescent films constructed by self-assembly of rod-like cellulose nanocrystals (CNCs) are mechanically brittle and of poor thermal st...
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Research Article pubs.acs.org/journal/ascecg

Enhanced Toughness and Thermal Stability of Cellulose Nanocrystal Iridescent Films by Alkali Treatment Fuchun Nan,† Selvaraj Nagarajan,† Yuwei Chen,† Ping Liu,† Yongxin Duan,† Yongfeng Men,‡ and Jianming Zhang*,† †

Key Laboratory of Rubber−Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber−Plastics, Qingdao University of Science & Technology, Qingdao City 266042, People’s Republic of China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China S Supporting Information *

ABSTRACT: Iridescent films constructed by self-assembly of rodlike cellulose nanocrystals (CNCs) are mechanically brittle and of poor thermal stability. Herein, we demonstrate that the toughness and thermal stability of vacuum filtered CNC iridescent films could be significantly improved by a simple alkali treatment. The results reveal that the NaOH treatment can simultaneously alter the condensed physical structure and surface chemical structure of CNCs in their liquid crystal state while the self-assembled CNC chiral nematic structure is still well retained. To be specific, the CNC was transformed from a higher crystallinity of form I to a lower crystallinity of form II, and the sulfate groups of CNCs were erased by alkali treatment, resulting in the remarkable enhancement in mechanical and thermal properties of CNC iridescent films. Of note, the unprecedented improvements in both tensile strength and toughness of CNC iridescent film have been achieved by alkali treatment. A sandwiched model with interdigitated molecular chains in amorphous layer as energy dissipation was proposed to explain the remarkable improvement in mechanical properties. This study shows that tailoring the condensed structure of CNC nanorods itself is a promising strategy to improve the performance of CNC iridescent films without blending with the second component. KEYWORDS: Cellulose nanocrystals, Structural color, Crystal transformation, Toughness, Thermal stability

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INTRODUCTION CNCs have attracted a great attention due to biodegradability, excellent mechanical properties, and natural abundance. Generally, CNCs were isolated from cellulose by a controlled hydrolysis using sulfuric acid. The acid hydrolysis results in negatively charged sulfate groups attached on the surface of the rod-like CNCs, and these sulfate groups maintained the stability of the suspension in aqueous media. CNCs obtained from sulfuric acid hydrolysis possess the intriguing ability to self-assemble into a chiral nematic structure in concentrated solution, which can be preserved in the dry film as well.1−3 The chiral nematic structure of CNCs consists of submicrometer pitches, and these pitches have undergone the Bragg reflection in visible light, which results in striking iridescent color on the films.4 This special optical property endows CNCs with potential applications in security papers,5 optical encryption,6 and as chiral templates.7−9 Evaporation-induced self-assembly (EISA)10,11 is the most widely used technology to prepare CNC iridescent films. However, EISA technology is time-consuming, and thus the obtained iridescent films usually show a polydomain structure due to the heterogeneity of evaporative speed.11 Recently, we © 2017 American Chemical Society

demonstrated that the vacuum-assisted self-assembly (VASA) technique could be used to prepare CNC iridescent films.12 In comparing with the CNC films via the EISA method, these films show a more uniform appearance in color because no 3D “coffee stain” rings formed during the VASA process. More importantly, VASA technology needs shorter time to obtain the iridescent films. Nevertheless, iridescent films prepared with both methods show poor toughness due to the intrinsic brittleness of CNCs13 and the lack of energy-dissipating binder phases among CNC particles.14 To improve the toughness of CNC iridescent films, the coassembly strategy of CNCs with plasticizers, such as watersoluble polymers,15−18 monomer of polymer,19 ionic liquid,20 and latex,15,21 has been used as toughening agents. However, these introduced toughening agents would easily disturb the liquid crystal self-assembly of the CNCs. Besides, the addition of plasticizers generally results in the detriment of tensile strength of CNC iridescent film. Thus, it remains a significant Received: June 2, 2017 Revised: August 17, 2017 Published: August 20, 2017 8951

DOI: 10.1021/acssuschemeng.7b01749 ACS Sustainable Chem. Eng. 2017, 5, 8951−8958

Research Article

ACS Sustainable Chemistry & Engineering

Subsequently, as-prepared CNC iridescent films were immersed in 16% NaOH at 70 °C for various times. The treated films were rinsed thoroughly and naturally dried for characterization. Characterization. Scanning electron microscopy (SEM) images were collected with a JSM-7500F field emission SEM at an accelerating voltage of 3 kV on samples. For observing the cross-section, CNC iridescent films were fractured in liquid nitrogen and sputter-coated with gold. WAXD measurements for the iridescent composite film were performed by a RIGAKU Ultima IV X-ray diffractometer with Cu Kα X-ray beam (λ = 1.54 Å). The films are recorded in the range of 5−30° with a step interval of 0.05° and scanning rate of 0.02° min−1. Before the WAXD test, films were ground to powder. The crystallinity index (CI) was calculated by

challenge to obtain CNC iridescent film with simultaneous improvement in strength and toughness. On the other hand, the sulfate groups on the surfaces of CNCs deteriorate the thermal stability of the CNC films at elevated temperature.22,23 Attempts were made to improve the thermal stability of CNCs by removing sulfate groups including mild acid hydrolysis,24 alkaline treatment,23 and solvolysis.25 Recently, post-treatments including autocatalyzed acidic desulfation at high temperature26 and vacuum ovendrying27 have been demonstrated as splendid methods to remove sulfate groups. It is well-known that the transformation from the form I to form II of cellulose is convenient to achieve via so-called mercerization treatment,28 and the cellulose fibers with form II shows higher tensile strength and larger ultimate strain than form I. Therefore, the mercerization treatment has been used to improve the mechanical properties of cellulose materials.29,30 However, we noted that this strategy has never been used to modify the CNC iridescent films. It is probably due to the fact that the disintegration of CNC iridescent films induced by the redisperse or partial degradation of CNC nanoparticles would take place during the mercerization treatment process by NaOH solution. In the present study, we surprisingly found that vacuum-assisted CNC iridescent films could remain their shape well in 16% NaOH solution at 70 °C for some period. Moreover, the unprecedented enhancement in tensile strength, toughness, and thermal stability of vacuum-assisted CNC iridescent films have been simultaneously achieved by this facile alkali treatment.

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CI =

1 − ham hcr

(1)

where hcr is the peak height of (002) reflection at 22.7° of form I or (10I)̅ reflection at 20.0° for form II, and ham is the height of the amorphous reflection at 18° for form I or 16° for form II.33,34 UV−vis curves of the composite films were collected with a UV− visible spectrophotometer (Shimadzu UV-2550). Circular dichroism spectroscopy experiments of the films were performed by a (CD) J815 spectropolarimeter. The reflected wavelength varied from 300 to 800 nm of each sample. Thermal analysis of the samples was performed using a PerkinElmer TGA 6 (PerkinElmer Instruments, USA). The temperature ranged from 30 to 600 °C with a heating rate of 10 °C min−1 under nitrogen. Mechanical properties of CNC iridescent films were evaluated using dynamic mechanical analyses (DMA Q800 TA). All films were conditioned before testing at 55% relative humidity for a minimum of 24 h. Specimens were cut into 25 mm × 2 mm. A 18 N load cell was used with a normal strain rate of 1 mm min−1 with a gauge length of 10 mm. At least four specimens were measured from each sample. The pictures of CNC iridescent films were captured by Pentax K30. The diagrammatic drawings were prepared by 3D-Max software 2012.

EXPERIMENTAL SECTION

Materials. Cotton linter pulp (CLP) provided by Hubei Chemical Fiber Co. Ltd. (Xiangfan, China) was used as cellulose material. The degree of polymerization (DP) was 700, measured by the viscosity method. Sulfuric acid (H2SO4, 98 wt %, analytically pure, Yantai, China) was of reagent grade and used without further purification. NaOH (over 96% purity) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Deionized water was used in all experimental parts. Preparation of CNCs Suspension. Cellulose nanocrystals (CNCs) suspension was prepared from sulfuric acid hydrolysis of cotton linter pulp. Briefly, cotton pulp milled by a food processor was first soaked with 4% NaOH for 24 h and filtration washed to neutral, then vacuum-dried to remove the surface impurities of the cellulose, which is beneficial to the isolation of CNC.31,32 Then the pretreated pulp was hydrolyzed in sulfuric acid at a concentration of 64 wt % with vigorous stirring for 60 min at 45 °C. To accelerate the acid hydrolysis reaction and improve the yield of CNCs, CuSO4 (CLP/CuSO4 = 100:1) was added to a 64 wt % sulfuric acid solution.12 Subsequently, cellulose suspension was then diluted with deionized water (approximately 10 times the volume of the acid used) to cease the hydrolysis. The cloudy white suspension was repeatedly centrifuged and washed with deionized water four times. The thick white suspension (3 wt %) obtained after the last centrifugation step was kept for use. CNC Iridescent Films Treated by NaOH Solution. CNC iridescent films were obtained by vacuum assisted technique (VASA) from 1 wt % CNC suspension using poly(tetrafluoroethene) (PTFE) filter paper (50 mm diameter, 220 nm pore size). Before vacuum filtration, the diluted CNCs suspension (16.5 mL) was sonicated for 20 min (Sonics Vibra-cell (PS-D40 240 W 50 kHz) with full energy input). Sonication operation was performed in an ice bath to avoid overheating. Vacuum filtration was performed under 0.09 MPa vacuum. After vacuum filtration, CNC iridescent films were naturally dried under ambient conditions. The water content of thus-dried CNC iridescent films is ca. 5.3 wt %, calculated by the dry-weighing method.

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RESULTS AND DISCUSSION Influence of Alkali Treatment on the Optical Properties and Physical Structure of Vacuum-Assisted CNC Iridescent Dilms. Because of the existence of sulfate groups and the hydrophilic property of CNC nanorods, CNC films become negatively charged on hydration and could be able to be disintegrated in water for the electrostatic repulsive force among CNC nanoparticles. It has been reported that the dried CNC powder with residual water content (>4%) was easily redispersible in water via ultrasonication.35,36 As mentioned in the Experimental Section, the water content of as-prepared CNC iridescent film in this research is around 5.3%. The disintegration of CNC film in pure water indeed took place with the assistance of ultrasonication. However, we found that such CNC iridescent film could keep their shape intact in concentrated alkaline solution (16% NaOH, 70 °C) for a long time (12 h), even with ultrasonic treatment (Supporting Information, Figure S1c). Compared with the collapsibility of CNC iridescent film in water, the integrity and shape of this film could be well retained during picking up by forceps after alkali treatment. We believe that there are two main reasons for the improved stability of CNC films in concentrated alkaline solution. First, the elimination of sulfate groups from the CNC surface by alkali treatment23 resists the dispersity of CNC in water, which will be discussed later. Second, the diffuse rate of solvent into the films under the static alkaline treatment is slower than that under stirred conditions, which slows down the decomposition rate of CNC nanoparticles. 8952

DOI: 10.1021/acssuschemeng.7b01749 ACS Sustainable Chem. Eng. 2017, 5, 8951−8958

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Figure 1. (a) Illustration of the preparation of CNC iridescent films with tunable color and enhanced flexibility. (b) The DRCD and the UV−vis spectra of CNC iridescent films with various treated time in NaOH solution. (c) WAXD data of the iridescent films with various treated time by 16% NaOH solution (I means form I; II means form II).

λmax = nP

The sampling process is illustrated in Figure 1a. CNCs were produced through sulfuric acid hydrolysis of cotton pulp, which was described in detail in the Experimental Section. The prepared CNCs demonstrate a typical needle-like morphology with the diameter of 5−15 nm and the length of 100−200 nm (Supporting Information, Figure S2). Turbid CNCs suspension was sonicated to obtain uniform clear solution. Then the CNC iridescent films were prepared by vacuum filtration. The fabricated CNC iridescent films were immersed in alkali solution (16% NaOH) with different aging times at 70 °C. After being dried in air, it is found that this process endowed these films flexibility without any damage of iridescent color of CNC films; as demonstrated in Figure 1a, the film processed by 9 h can be rolled up easily. As shown in Figure 1a, the alkaline treatment directly influences the color of iridescent film. Orange, yellow, green, and blue show up one after the other as treated by 3, 6, 9, and 12 h, respectively. UV−vis and diffusion reflectance circular dichroism (DRCD) spectra were used to detect the color change of CNC iridescent films (Figure 1b). In general, a positive signal suggests the chiral structure with left-handed arrangement when the CD spectra were measured with transmission mode.7 However, under diffusion reflectance mode, the negative signal in DRCD spectra means that the system has left-handed chiral nematic structure.34 Therefore, the negative CD signal in Figure 1b illustrate that the alkali treatment does not change the handedness of the helical structure of iridescent films. These CD signals correspond to the reflection wavelengths measured by UV−vis spectroscopy, which demonstrates that the chiral nematic structure is the cause of iridescent color. For a chiral nematic structure, light at a normal incidence is selectively reflected according to

(2)

where λmax, n, and P are the maximum reflected wavelength, the average refractive index of the film, and the helical pitch length, respectively.37 Moreover, the periodic band in SEM crosssection of the iridescent CNC film is generally believed to be related with a 180° rotation of the chiral nematic director (i.e., a half helical pitch), that is, to P/2. Thus, the λmax can be calculated according to the SEM data. Table 1 shows that the Table 1. Maximum Reflected Wavelength of CNC Iridescent Films with Different Processing Time time (h) λmax calculated by SEM (nm) λmax observed by UV−vis spectra (nm)

0

3

6

9

12

560 550

644 651

548 548

477 480

450 450

values of λmax calculated by the helical pitches from SEM images in Supporting Information, Figure S3, are roughly consistent with those observed from UV−vis spectra. Previous studies30,32,38 have revealed that cellulose treated by concentrated NaOH solution could convert form I into form II, and the degree of phase conversion of cellulose depends on the concentration of NaOH, time, and temperature for the treatment. XRD data of the iridescent films with different processing time in concentrated NaOH solution were therefore collected and presented in Figure 1c. The results of XRD suggest that form I-to-form II crystal transition was finished in the first 3 h. Table 2 shows the change of crystallinity of CNC iridescent films with different processing time. The data in Table 2 reveals that the crystal conversion induced by NaOH treatment accompanied by the decrease in the degree of crystallinity. This should be related to the crystal transition, that 8953

DOI: 10.1021/acssuschemeng.7b01749 ACS Sustainable Chem. Eng. 2017, 5, 8951−8958

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ACS Sustainable Chemistry & Engineering

Mechanical Properties of CNC Iridescent Films Treated by Alkali. As mentioned in the Introduction, the CNC iridescent films generally demonstrate stiff and brittle characters due to intrinsic rigidity of CNC particles and the lack of soft energy dissipating phase as matrix.14 As shown in Figure 3a, the stress−strain curve of untreated CNC film just includes a linear part corresponding to elastic deformation and there was no plastic deformation happening before the fracture of original CNC iridescent films. However, stress−strain curves corresponding to CNC iridescent films treated by alkali are the combination of a linear part and a platform part, which indicates that films had undergone inelastic deformation before failure. To one’s surprise, the simultaneous improvements in strength and toughness have been achieved for all the CNC iridescent films with different processing time in 16% NaOH solution. To evaluate the effect of alkali treatment time on the mechanical performance parameters in detail, the tensile strength (σUTS), strain at break (εmax), and toughness of the CNC iridescent films under various alkali treatment time are depicted in Figure 3b−d. After alkali treatment, it is clear to see that the films show both improvement on σUTS and εmax, which could increase to 91.4 MPa and 8.42% at the optimum processing time (9 h). Such large σUTS and εmax result in the toughness of treated CNC iridescent film increasing to 6.30 MJ/m3, around 92 times higher than that of untreated CNC iridescent film. When processing time is in excess of 9 h, the εmax of specimens reduced, which might be caused by the over decomposition of CNCs as shown in Figure 2. To the best of our knowledge, CNC iridescent films with such extraordinary improvement in both strength and toughness have never been achieved in previous studies.14,17,18,42 The comparison of mechanical properties for CNC iridescent films obtained by the alkali treatment to the previous reports about CNC/polymer iridescent films is shown in Figure 4. In general, the enhancement of tensile strength and toughness are in conflict in polymer composites. The addition of a second component like PEG, PVA,14 Zwitterionic surfactants (DMAPS),42 and latex nanoparticles21,43 could

Table 2. Crystallinity and Degradation Temperature of CNC Iridescent Films with Different Alkali Processing Time time (h) crystallinity (%) degradation onset temperature (°C) maximum degradation temperature (°C)

0

3

6

9

12

92.5 142

78. 7 255

75.5 263

72.4 249

71.7 242

177

309

336

349

317

is, the disruption of the cellulosic crystalline structure caused by the drastic chemical reaction of alkali treatment.31 The chemical reaction induced by the alkali treatment could be confirmed by the weight loss of CNC film as depicted in Figure 2. The results show that the film lost 28% of its weight

Figure 2. Effect of treatment time on the weight change of CNC iridescent film as a function in 16% NaOH solution.

compared to the untreated film after 3 h of alkali treatment. Previous studies39−41 reported that the cellulose will degrade into low molecular weight products with carboxylic acid groups and hydroxy groups after alkali treatment. In summary, the alkali treatment results in the form I-to-form II phase transition, the decrease of crystallinity, and the weight loss of CNC film. All these changes will affect the helical pitch, which is related to the color of CNC iridescent film as demonstrated in Figure 1a.

Figure 3. (a) Stress−strain curves of the CNC iridescent films with different processing time in 16% NaOH solution. (b−d) Tensile strength, strain at break, and toughness of iridescent films varying the alkali processing time, respectively. 8954

DOI: 10.1021/acssuschemeng.7b01749 ACS Sustainable Chem. Eng. 2017, 5, 8951−8958

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remarkable and simultaneous enhancement in tensile strength and toughness of CNC iridescent films. To explore the origin of the remarkable improvement in mechanical properties, SEM of the fracture surface of CNC iridescent films with different alkali treatment time were measured. As shown in Figure 5a, it is found that alkali treatment decomposes the boundaries of CNCs whereas the ordered layer structure is well remained in the film. For the untreated film (Figure 5a-i), the individual CNC particles were pulled out from the film and the film underwent brittle fracture. After NaOH treatment for 3 h, CNC particles coalesced with each other (Figure 5a-ii). With increasing treatment time to 12 h, further fusion between ordered layers occurred so that the fracture surface looks smoother (Figure 5(a-iii)). SEM images of the surface of CNC iridescent film before and after the NaOH treatment (Supporting Information, Figure S4) also suggest that the surface of the CNC film becomes smoother after the NaOH treatment. Previous structural analysis based on WAXD data reveals that the transformation of form I to form II during the alkali treatment was accompanied by the increase of amorphous phase. The improvement of mechanical properties could be interpreted by the combination of WAXD data and the crosssectional SEM images. First, it is known that the lack of energy dissipation phase among CNCs is the key reason for the poor mechanical properties of CNC iridescent films. Here, the amorphous phase introduced by alkali treatment could be considered as the energy dissipation binder phase, making CNC iridescent films more flexible. Compared with adding polymer method to obtain flexible CNC iridescent film, the interface interaction between the energy dissipation binder phase provided by CNC amorphous region and the inner crystal part of CNC nanocrystals is covalently bonded, which is stronger than a noncovalent interaction between the additional polymer and cellulose matrix. Second, the amorphous phase could be interdigitated, which is beneficial for the improvement of mechanical properties. Therefore, we believe that the amorphous phase generated by alkali treatment in the CNC iridescent films should be the reason for the remarkable

Figure 4. Comparison of tensile strength vs strain at break of iridescent films based on CNCs. CNCs/PEG17 (PEG: polyethylene glycol). CNCs/PVA14 (PVA: poly(vinyl alcohol)). CNCs/DMAPS42 (DMAPS: (dimethylmyristylammonio)propanesulfonate). CNCs/ EGPUy18 (EGPUy: a methacrylate derivative with 2-ureido-4pyrimidinone groups). CNCs/PHEMA19 (PHEMA: poly(2-hydroxyethyl methacrylate)). CNCs/NPs43 (NPs: soft reactive latex nanoparticles)

improve the tensile strength of CNC iridescent films to some extent, whereas the improvement in strain at break is limited or even negative. In another case, Walther et al.18 designed the intermolecular hydrogen bonding to make CNC iridescent films tough, but the addition of this polymer sacrifices the tensile strength as shown in the right corner of Figure 4. In contrast, the CNC iridescent films modified by the alkali treatment at optimum condition show the unprecedented enhancement in both tensile strength and toughness. Origin of the Remarkable Enhancement in Tensile Strength and Toughness of CNC Iridescent Films. Ishikawa et al.44 had reported that form I and form II of ramie fibers give the tensile strength/strain break at 755 MPa/ 3.2% and 798 MPa/5.0%, respectively. The toughness of form II crystal is a bit higher than that of form I. However, the contribution of crystal transformation to the improvement of mechanical properties is limited and not enough to explain the

Figure 5. (a) SEM images on the fracture surface of CNC iridescent films treated by 0 h (i), 3 h (ii), and 12 h (iii) in 16% NaOH solution. (b) A cartoon to illustrate the change of cholesteric liquid-crystal structure and the helical pitch and (c) morphology change of single CNC nanorod with increasing the alkali treatment time. 8955

DOI: 10.1021/acssuschemeng.7b01749 ACS Sustainable Chem. Eng. 2017, 5, 8951−8958

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Figure 6. (a) Infrared spectra of the CNC iridescent films with different NaOH processing time. (b) Digital photos of CNC iridescent films with different NaOH processing time taken at various temperatures during the heating process. (c) TGA and DTG curves of CNC iridescent films with different NaOH processing time. The degraded specimens are denoted by the dotted red line.

presented in Supporting Information, Figure S5, suggests that the S content of CNC film reduced after alkali treatment. Meanwhile, from the results we realized that there is some Na remaining in the film even after being totally washed by water. Figure 6b vividly shows that the thermal stability of CNC iridescent films was improved largely by NaOH treatment due to the elimination of sulfate groups. The color of untreated CNC iridescent films started to change at a low temperature of 140 °C in air conditions. However, all CNC iridescent films treated by NaOH solution show remarkable improvement on thermal stability and their original colors never change until the temperature is as high as 260 °C. More detailed information about the thermal stability of CNC iridescent films can be understood from TGA analysis in Figure 6c. The CNC films without alkali treatment show the lowest onset weight loss at 142 °C, which is because the sulfate groups could decompose CNCs at high temperature. The highest onset degradation temperature is up to 263 °C when the processing time is 6 h, which is consistent with the FITR result that sulfate groups were removed in the first 6 h (Figure 6a). The NaOH treatment for 9 h improves the maximum degradation temperature of the CNCs from 175 to 349 °C. When extending the NaOH treatment time to 12 h, the maximum degradation temperature of CNC film decreased to 317 °C, which was due to the increasing of amorphous region and over degradation of cellulose molecules.46

enhancement in the tensile strength and toughness of CNC iridescent films. The cartoons in Figure 5 were used to illustrate the crosssection morphology change of CNC iridescent films. The green CNC nanorods represent the untreated CNC iridescent films with high crystallinity. At the early stage of NaOH treatment, the increase of amorphous region in CNC films is dominated so that the helical pitch increase. However, at the later stage, due to the decomposition and the reduced size of nanocrystals, the helical pitch will decrease. This trend in the change of helical pitch is in agreement with the previous results as listed in Table 1. During this process, CNCs transform from high crystalline form I to low crystalline form II, as supported by the data in Table 2. It means that the increasing amount of amorphous layer with the interdigitated cellulose molecular chains will occur. Improvement on the Thermal Stability of CNC Iridescent Films. As mentioned in the Introduction, the negatively charged sulfate groups are helpful for the stable dispersion of CNCs in aqueous solution. However, the existence of sulfate groups deteriorates the thermal stability of the CNC films as well. It had been reported that mild acid hydrolysis or alkaline treatment can remove sulfate groups. We therefore measured our alkali treated samples with IR spectroscopy. As shown in Figure 6a, the characteristic band at 807 cm−1 (C−O−S vibration) of sulfate groups45 does disappear with increasing the NaOH treated time. It suggested that the sulfate group could be partially eliminated from the surface of CNCs by such NaOH treatment. The EDX data 8956

DOI: 10.1021/acssuschemeng.7b01749 ACS Sustainable Chem. Eng. 2017, 5, 8951−8958

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ACS Sustainable Chemistry & Engineering

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(2) Revol, J. F.; Marchessault, R. H. In vitro chiral nematic ordering of chitin crystallites. INT. Int. J. Biol. Macromol. 1993, 15 (6), 329−35. (3) Habibi, Y.; Lucia, L. A.; Rojas, O. J. Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications. Chem. Rev. 2010, 110 (6), 3479−3500. (4) Lagerwall, J. P. F.; Schütz, C.; Salajkova, M.; Noh, J. H.; Park, J. H.; Scalia, G.; Bergström, L. Cellulose nanocrystal-based materials: from liquid crystal self-assembly and glass formation to multifunctional thin films. NPG Asia Mater. 2014, 6 (1), e80. (5) Fleming, K.; Gray, D. G.; Matthews, S. Cellulose crystallites. Chem. - Eur. J. 2001, 7 (9), 1831−1835. (6) Zhang, Y. P.; Chodavarapu, V. P.; Kirk, A. G.; Andrews, M. P. Nanocrystalline cellulose for covert optical encryption. Proc. SPIE 2012, 8258, 825808. (7) Shopsowitz, K. E.; Qi, H.; Hamad, W. Y.; Maclachlan, M. J. Freestanding mesoporous silica films with tunable chiral nematic structures. Nature 2010, 468 (7322), 422−425. (8) Shopsowitz, K. E.; Hamad, W. Y.; Maclachlan, M. J. Flexible and Iridescent Chiral Nematic Mesoporous Organosilica Films. J. Am. Chem. Soc. 2012, 134 (2), 867−870. (9) Khan, M. K.; Bsoul, A.; Walus, K.; Hamad, W. Y.; Maclachlan, M. J. Photonic patterns printed in chiral nematic mesoporous resins. Angew. Chem., Int. Ed. 2015, 54 (14), 4304−4308. (10) Dumanli, A. G.; van der Kooij, H. M.; Kamita, G.; Reisner, E.; Baumberg, J. J.; Steiner, U.; Vignolini, S. DigitalColor in Cellulose Nanocrystal Films. ACS Appl. Mater. Interfaces 2014, 6 (15), 12302− 12306. (11) Khan, M. K.; Giese, M.; Yu, M.; Kelly, J. A.; Hamad, W. Y.; Maclachlan, M. J. Flexible mesoporous photonic resins with tunable chiral nematic structures. Angew. Chem., Int. Ed. 2013, 52 (34), 8921− 8924. (12) Chen, Q.; Liu, P.; Nan, F.; Zhou, L.; Zhang, J. Tuning the iridescence of chiral nematic cellulose nanocrystal films with a vacuumassisted self-assembly technique. Biomacromolecules 2014, 15 (11), 4343−4350. (13) Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J. Cellulose nanomaterials review: structure, properties and nanocomposites. Chem. Soc. Rev. 2011, 40 (7), 3941−3994. (14) Wang, B.; Walther, A. Self-Assembled, Iridescent, CrustaceanMimetic Nanocomposites with Tailored Periodicity and Layered Cuticular Structure. ACS Nano 2015, 9 (11), 10637−10646. (15) Zou, X.; Tan, X.; Berry, R.; Godbout, J. D. L. Flexible, iridescent nanocrystalline cellulose film, and method for preparation.WO2010124378A1, 2010. (16) Kelly, J. A.; Yu, M.; Hamad, W. Y.; Maclachlan, M. J. Large, Crack-Free Freestanding Films with Chiral Nematic Structures. Adv. Opt. Mater. 2013, 1 (4), 295−299. (17) Bardet, R.; Belgacem, N.; Bras, J. Flexibility and color monitoring of cellulose nanocrystal iridescent solid films using anionic or neutral polymers. ACS Appl. Mater. Interfaces 2015, 7 (7), 4010− 4018. (18) Zhu, B.; Merindol, R.; Benitez, A. J.; Wang, B.; Walther, A. Supramolecular Engineering of Hierarchically Self-Assembled, Bioinspired, Cholesteric Nanocomposites Formed by Cellulose Nanocrystals and Polymers. ACS Appl. Mater. Interfaces 2016, 8 (17), 11031−11040. (19) Tatsumi, M.; Teramoto, Y.; Nishio, Y. Polymer composites reinforced by locking-in a liquid-crystalline assembly of cellulose nanocrystallites. Biomacromolecules 2012, 13 (5), 1584−1591. (20) Liu, P.; Guo, X.; Nan, F.; Duan, Y.; Zhang, J. Modifying Mechanical, Optical Properties and Thermal Processability of Iridescent Cellulose Nanocrystal Films Using Ionic Liquid. ACS Appl. Mater. Interfaces 2017, 9 (3), 3085−3092. (21) Thérien-Aubin, H.; Lukach, A.; Pitch, N.; Kumacheva, E. Structure and properties of composite films formed by cellulose nanocrystals and charged latex nanoparticles. Nanoscale 2015, 7 (15), 6612−6618.

CONCLUSIONS In summary, a facile post-treatment by NaOH solution was developed to prepare color-tunable CNC iridescent films with extraordinary improvement for toughness and thermal stability. Compared to the untreated specimen, the tensile strength and toughness of CNC iridescent films could be improved up to 285.7% and 9250%, respectively. Simultaneously, the onset temperature of thermal degradation of CNC iridescent films has increased remarkably from 142 to 263 °C. It was found that the formation of the amorphous region as the energy dissipation binder phase among the rigid CNC nanorods is the key factor for the remarkable enhancement in the tensile strength and toughness of CNC iridescent film. The improvement of thermal stability results from the elimination of sulfate groups by alkali treatment, which is also the reason why CNC iridescent films could hold their integrity in solution. Moreover, due to the transition of aggregation structure and the decomposition of CNCs after NaOH solution treatment, the changes in helical pitch affect the color of iridescent films. This study demonstrates that the regulation on the condensed structure of CNCs itself can be used to design CNC iridescent films with high performance, which will lead a step forward to their practical application.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01749. Comparison of the stability of CNC iridescent films in NaOH solution and water, morphology of CNC, SEM images of cross sections of CNC iridescent films treated in NaOH solution for various times, SEM images of surfaces of CNC iridescent films before and after treatment, EDX images of CNC iridescent films with different time in NaOH solution (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 532 84022604. Fax: +86 532 84022791. E-mail: [email protected]. Address: no. 51-1, Wuyang Road, Qingdao 266045, China. ORCID

Yongfeng Men: 0000-0003-3277-2227 Jianming Zhang: 0000-0002-0252-4516 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from National Natural Science Foundation of China (51573082), Taishan Mountain Scholar Foundation (TS20081120 and tshw20110510), and the Open Research Fund of the State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (201606).

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REFERENCES

(1) Revol, J. F.; Bradford, H.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Helicoidal self-ordering of cellulose microfibrils in aqueous suspension. Int. J. Biol. Macromol. 1992, 14 (3), 170−172. 8957

DOI: 10.1021/acssuschemeng.7b01749 ACS Sustainable Chem. Eng. 2017, 5, 8951−8958

Research Article

ACS Sustainable Chemistry & Engineering (22) Roman, M.; Winter, W. T.,Effect of Sulfate Groups from Sulfuric Acid Hydrolysis on the Thermal Degradation Behavior of Bacterial Cellulose. Biomacromolecules 2004, 5 (5), 1671−1677. (23) Wang, N.; Ding, E.; Cheng, R. Thermal degradation behavior of spherical cellulose nanocrystals with sulfate groups. Polymer 2007, 48 (12), 3486−3493. (24) Pomin, V. H.; Valente APPereira, M. S.; Mourao, P. A. Mild acid hydrolysis of sulfated fucans: a selective 2-desulfation reaction and an alternative approach for preparing tailored sulfated oligosaccharides. Glycobiology 2005, 15 (12), 1376−1385. (25) Jiang, F.; Esker, A. R.; Roman, M. Acid-catalyzed and solvolytic desulfation of H2SO4-hydrolyzed cellulose nanocrystals. Langmuir 2010, 26 (23), 17919−17925. (26) Beck, S.; Bouchard, J. Auto-catalyzed acidic desulfation of cellulose nanocrystals. Nord. Pulp Pap. Res. J. 2014, 29 (1), 6−14. (27) Bardet, R.; Roussel, F.; Coindeau, S.; Belgacem, N.; Bras, J. Engineered pigments based on iridescent cellulose nanocrystal films. Carbohydr. Polym. 2015, 122, 367−375. (28) O’Sullivan, A. C. Cellulose: the structure slowly unravels. Cellulose 1997, 4 (3), 173−207. (29) Zuber, M.; Zia, K. M.; Bhatti, I. A.; Ali, Z.; Arshad, M. U.; Saif, M. J. Modification of cellulosic fibers by UV-irradiation. Part II: after treatments effects. Int. J. Biol. Macromol. 2012, 51 (5), 743−748. (30) Abe, K.; Yano, H. Cellulose nanofiber-based hydrogels with high mechanical strength. Cellulose 2012, 19 (6), 1907−1912. (31) Yu, L.; Lin, J.; Tian, F.; Li, X.; Bian, F.; Wang, J. Cellulose nanofibrils generated from jute fibers with tunable polymorphs and crystallinity. J. Mater. Chem. A 2014, 2 (18), 6402−6411. (32) Han, J.; Zhou, C.; Wu, Y.; Liu, F.; Wu, Q. Self-assembling behavior of cellulose nanoparticles during freeze-drying: effect of suspension concentration, particle size, crystal structure, and surface charge. Biomacromolecules 2013, 14 (5), 1529−1540. (33) Bansal, P.; Hall, M.; Realff, M. J.; Lee, J. H.; Bommarius, A. S. Multivariate statistical analysis of X-ray data from cellulose: a new method to determine degree of crystallinity and predict hydrolysis rates. Bioresour. Technol. 2010, 101 (12), 4461−4471. (34) Oh, S. Y.; Yoo, D. I.; Shin, Y.; Kim, H. C.; Kim, H. Y.; Chung, Y. S.; Park, W. H.; Youk, J. H. Crystalline structure analysis of cellulose treated with sodium hydroxide and carbon dioxide by means of X-ray diffraction and FTIR spectroscopy. Carbohydr. Res. 2005, 340 (15), 2376−2391. (35) Beck, S.; Bouchard, J.; Berry, R. Dispersibility in water of dried nanocrystalline cellulose. Biomacromolecules 2012, 13 (5), 1486−1494. (36) Huo, H.; Wang, S.; Lin, S.; Li, Y.; Li, B.; Yang, Y. Chiral zirconia nanotubes prepared through a sol−gel transcription approach. J. Mater. Chem. A 2014, 2 (2), 333−338. (37) de Vries, H. Rotatory power and other optical properties of certain liquid crystals. Acta Crystallogr. 1951, 4 (3), 219−226. (38) Halonen, H.; Larsson, P. T.; Iversen, T. Mercerized cellulose biocomposites: a study of influence of;mercerization on cellulose supramolecular structure, water retention;value and tensile properties. Cellulose 2013, 20 (1), 57−65. (39) Knill, C. J.; Kennedy, J. F. Degradation of cellulose under alkaline conditions. Carbohydr. Polym. 2003, 51 (3), 281−300. (40) Johansson, M. H.; Samuelson, O. End-wise degradation of hydrocellulose during hot alkali treatment. J. Appl. Polym. Sci. 1975, 19 (11), 3007−3013. (41) Bourbon, X.; Toulhoat, P. Influence of organic degradation products on the solubilisation of radionuclides in intermediate and low level radioactive wastes. Radiochim. Acta 1996, 74 (s1), 315−320. (42) Guidetti, G.; Atifi, S.; Vignolini, S.; Hamad, W. Y. Flexible Photonic Cellulose Nanocrystal Films. Adv. Mater. 2016, 28, 10042− 10047. (43) Vollick, B.; Kuo, P. Y.; Thérienaubin, H.; Yan, N.; Kumacheva, E. Composite Cholesteric Nanocellulose Films with Enhanced Mechanical Properties. Chem. Mater. 2017, 29, 789−795. (44) Ishikawa, A.; Okano, T.; Sugiyama, J. Fine structure and tensile properties of ramie fibres in the crystalline form of cellulose I, II, III I and IV I. Polymer 1997, 38 (2), 463−468.

(45) Wang, M. J.; Xie, Y. L.; Zheng, Q. D.; Yao, S. J. A Novel, Potential Microflora-Activated Carrier for a Colon-Specific Drug Delivery System and Its Characteristics. Ind. Eng. Chem. Res. 2009, 48 (11), 5276−5284. (46) Poletto, M.; Zattera, A. J.; Forte, M. M. C.; Santana, R. M. C. Thermal decomposition of wood: Influence of wood components and cellulose crystallite size. Bioresour. Technol. 2012, 109 (1), 148−153.

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DOI: 10.1021/acssuschemeng.7b01749 ACS Sustainable Chem. Eng. 2017, 5, 8951−8958