Research Article www.acsami.org
Modifying Mechanical, Optical Properties and Thermal Processability of Iridescent Cellulose Nanocrystal Films Using Ionic Liquid Ping Liu, Xin Guo, Fuchun Nan, Yongxin Duan, and Jianming Zhang* Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-Plastics, Qingdao University of Science & Technology, Qingdao 266042, China S Supporting Information *
ABSTRACT: Iridescent films formed from the self-assembly of cellulose nanocrystals (CNCs) are brittle and difficult to handle or integrate within an industrial process. Here we present a simple approach to prepare iridescent CNC films with tunable pliability and coloration through the addition of ionic liquids (ILs) of 1allyl-3-methylimidazolium chloride (AmimCl) as plasticizers. By using the undried CNC film as a filter membrane and ILs solution as a leaching liquid, it was found that the filtration process made ILs uniformly interpenetrate into CNC film due to the strong ionic interaction between CNC and AmimCl. Unexpectedly, the filtration process also gave rise to partial desulfurization of CNC film, which is conducive to the improvement of thermal stability. Benefiting from the improved thermal stability and the dissolving capacity of AmimCl for cellulose at high temperature, the incorporated ILs enable the cholesteric CNC film to be further toughened via a hot-pressing treatment. This study demonstrates that ionic liquids have great potential to modify the mechanical, optical properties as well as the thermal stability of iridescent CNC films. KEYWORDS: cellulose nanocrystal, ionic liquid, plasticization, structural color, hot-pressing
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concerned before putting the film into commercial applications. For one, solid films formed from pure CNC suspensions are brittle and difficult to handle or integrate within an industrial process.29 Brittleness of the solid film results from the intrinsic rigidity of CNC particles and the lack of soft energy dissipating phase as matrix. Till now, only a couple of studies have tried to address this issue. Water-soluble polymers such as poly(vinyl alcohol) (PVA), polyols, and polyethylene glycol (PEG) have been used for alleviating the brittleness of CNC films while maintaining its unique color.29−31 The adopted polymers are usually with low molecular weight and function as plasticizers or lubricants between the CNC particles. Styrene−butadiene (SB) latex was also used for making the film strong and flexible, in which the latex was believed to function as “glue” between CNC rods.30 However, the participation of SBR latex would in most cases undermine the iridescence of CNC film because of its large particle size that would interfere with the alignment of CNC rods. For another, adjusting the coloration of CNC film by modifying the chiral nematic organization is essential for developing its applications as anticounterfeit materials. Generally, to obtain iridescent CNC film with both flexibility and tunable coloration, extra sonication or addition of ionic
INTRODUCTION Among the various forms of nanocellulose, cellulose nanocrystals (CNCs) have attracted particular attention due to an attractive combination of properties including high stiffness, low coefficient of thermal expansion, optical transparency, the ability of self-organization, and modifiability.1−5 CNCs are usually extracted from cellulose fibers through a controlled acid hydrolysis process,6,7 with sulfuric acid being the most commonly used acid.8 Rodlike CNCs (typical diameter of 5− 10 nm and length of 50−300 nm) with negatively charged sulfate half-ester groups on surfaces would be obtained if vegetal cellulose (e.g., wood or pulp) was utilized as feed stock.9 The anisotropic rodlike CNCs show the intriguing ability to self-organize into chiral nematic liquid crystal phase in concentrated solution,10−13 which would lead to the generation of cholesteric CNC films after evaporation of solvents.14 Recent research works about liquid crystalline CNCs-based materials have been mainly evolving around the self-assembly characters, 15−18 as a template for inorganic/organic porous materials,19−21 the coassembly behavior with functional nanoparticles through guest−host interactions,22−25 and bioinspired cholesteric nanocomposites.26−28 Since the optical properties (iridescent color) of cholesteric CNC film cannot be reproduced by printing or photocopying, CNCs iridescent films are of latent applications in optical encryption technology. However, some issues need to be © XXXX American Chemical Society
Received: October 11, 2016 Accepted: December 27, 2016 Published: December 27, 2016 A
DOI: 10.1021/acsami.6b12953 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Information) of the prepared film samples obtained via a Stereo Microscope were used for measuring the thicknesses. Gravimetric analysis was employed for calculating the actual content of AmimCl in the films. First, mass measurement of CNC film without infiltration of AmimCl was made using a Mettler-Toledo analytical balance. Then the mass of plasticized films was obtained in the same way. Third, the difference between the two masses was divided by the mass of plasticized film to get the AmimCl content by weight percentage. Characterization and Measurement. Mechanical properties of the samples were evaluated by static uniaxial in-plane tensile tests using dynamic mechanical analyses (DMA Q800 TA). The samples were cut with a razor into rectangular strips with 5 mm width and 30 mm length for mechanical testing. All tensile tests were conducted in controlled strain rate mode with a preload of 0.01 N and a strain rate of 0.5 mm/min under ambient conditions, using an 18 N load cell. At least three specimens were measured from each sample. UV−vis extinction spectra of the film samples were obtained through a UV−vis spectrophotometer (Shimadzu UV-2550). Polarized optical micrographs (POM) were obtained through a BX51 Olympus polarized optical microscope (Olympus, Japan) equipped with an Olympus DP72 CCD camera. Scanning electron microscopy (SEM) images of the samples were obtained on a JEOL 6700 SEM at an accelerating voltage of 3 kV on samples sputter-coated with gold. Thermal analysis of the samples was performed using a PerkinElmer TGA 6 (PerkinElmer Instruments, USA). The temperature was set from 30 to 700 °C with a heating rate of 10 °C min−1 under nitrogen. IR spectra were obtained via a Bruker VERTEX 70 spectrometer equipped with a MCT detector using the normal transmission mode. To study the thermal dehydration process of the sample, the free-standing Film-(15) was set in a Linkam hot stage (FTIR-600, Linkam Scientific Instrument Ltd., Surrey, UK). Then the film was heated from room temperature (26 °C) to 150 °C at a rate of 2 °C min−1 in atmosphere. During the heating process, the FTIR spectra of the specimen were recorded at 1 min intervals, from 26 to 150 °C. The spectra were obtained by coadding 32 scans at a 4 cm−1 resolution. The intensities of the IR bands were calculated automatically using a numerical data-processor program, Spina Version 3 (developed by Yukiteru Katsumoto in the Ozaki Group of Kwansei-Gakuin University). Prior to calculating the intensities, all the spectra were baseline corrected. Atomic force microscope (AFM) image was acquired by using a Multimode V (VEECO) under contact mode. CNCs were coated on freshly cleaved mica plate for AFM measurements via spin-coating at 2000 rpm. Dynamic Light Scattering (DLS) was performed on a Malvern Nano ZS90 light scattering instrument. Wide angle X-ray diffraction (WAXD) patterns were collected at room temperature on a Rigaku UltimaIV diffractometer with Cu Kα radiation (λ = 0.154 nm), the diffraction signals were recorded in the range of 2θ = 5−40° with a step interval of 0.02° and a scanning rate of 5° min−1.
chemicals is indispensible while introducing the above polymers as plasticizers or glue.32 Nevertheless, oversonication or the addition of an excess of ionic can irreversibly inhibit the selforganization, resulting in a loss of coloration.29 Therefore, new approaches for fabricating iridescent CNC films with both pliability and tunable coloration are still anticipated. Here, we report a facile and effective method for preparing iridescent CNC films with both pliability and tunable coloration by infiltrating ionic liquids (ILs) of 1-allyl-3methylimidazolium chloride (AmimCl) into the voids between CNC particles. Our major interest is directed to the variations following the plasticization of the CNC iridescent films, particularly in their mechanical performances and optical properties. Attention is also dedicated to the changes in the structure and thermal stability of CNC film after the infiltration process. Besides, inspired by the fabrication of all-cellulose nanocomposites from cellulose microfibers using ionic liquidbased nanowelding,33 we approached the direct production of all-cellulose nanocomposite from the AmimCl plasticized CNC film via a hot-pressing process.
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EXPERIMENTAL SECTION
Materials. Bleached wood pulp with cellulose polymerization degree (DP) of 700 was supplied by Hubei Chemical Fiber Co. Ltd. (Xiangfan, China). Ionic liquid of AmimCl was kindly provided by Prof. Jun Zhang (Institute of Chemistry, Chinese Academy of Sciences) and used without any further treatment. H2SO4 (98 wt %) and NaOH (96%) with analytical grade were obtained from Zhengye Reagent Company (Qingdao, China). Double-distilled water was used for all experiments. Preparation of Cellulose Nanocrystals (CNCs). CNCs were extracted from bleached wood pulp via controlled sulfuric acid hydrolysis as described in the literature with minor modifications. The pulp milled with a food processor was first soaked by 4 wt % aqueous NaOH solution (40 mL for 1 g pulp) for 24 h at room temperature. The obtained slurry was thoroughly washed with distilled water to neutral pH and then completely dried at 60 °C in a vacuum oven. Thus, preprocessed pulp was then hydrolyzed in a 64 wt % sulfuric acid solution (17.5 mL for 1 g pulp) with vigorous stirring (600 rpm) at 45 °C for 60 min. The reaction was stopped by diluting with copious water at the end of hydrolysis. Let the mixture stand for 24 h and then discard the upper clear liquid. The lower turbid sediment was centrifuged and washed repeatedly until the pH was close to 3. Finally, an ivory-white CNC suspension with a solid content around 3 wt % was obtained and stored for use without dialysis. From the AFM image as shown in Figure S1a, the prepared CNCs deposited on freshly cleaved mica plate present rodlike morphology. Dynamic light scattering (DLS) profile (size distribution by intensity) as presented in Figure S1b was used to inspect the size distribution of the prepared CNC. A typical TEM image, which was presented as Figure S1c, reveals that the CNC rod is about 140 nm long and with a diameter of 5−10 nm. Preparation of Iridescent CNC Films Plasticized by AmimCl. In a typical procedure for preparing iridescent CNC film plasticized by AmimCl, 5 mL of the above prepared CNC suspension was diluted to a total volume of 15 mL and sonicated for 3 min. Then the suspension was pushed through a PTFE filter membrane (47 mm in diameter, 0.22 μm pore size) via vacuum suction to remove water. Through this step, solid CNC film with cholesteric structure took shape on the filter membrane. Then 1 mL of AmimCl solution with a concentration of 5−30 wt % was added onto the surface of the CNC film and vacuum filtered to pass through the film. After that, the obtained film supported by filter membrane was taken down and dried in a 70−80 °C oven for 1 h. The CNC films infiltrated by AmimCl with thicknesses of about 80−150 μm could be peeled off from the filter membrane. Fracture images (see Figure S2 in the Supporting
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RESULTS AND DISCUSSION Preparation of Iridescent CNC Films Plasticized by AmimCl. In our previous work, we have demonstrated that the vacuum filtration process can be used to fabricate highly oriented CNC iridescent films with hidden layered helical structures.34 It is known that there are pores and gaps in cellulosic nanomaterials due to the hierarchical nanostructures inside, which provides permeable paths for fluids such as water and different solvents.33,35 If some kind of fluid plasticizer is infiltrated uniformly into the cholesteric CNC film, it is possible to endow the iridescent film with pliability. AmimCl, as a powerful nonderivative solvent of cellulose, has been used as an effective plasticizer for cellulosic materials because of its nonvolatile and aggressive ability to break H-bonds of cellulose.36,37 In the present work, iridescent CNC films with pliability were prepared by vacuum filtration of aqueous CNC dispersion followed by pushing AmimCl aqueous solution B
DOI: 10.1021/acsami.6b12953 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. (A) Schematic illustration of the preparation process for iridescent CNC films plasticized by ILs of AmimCl. (B) Structure of cholesteric CNC film infiltrated with ILs and the proposed ionic interactions between cellulose and AmimCl.
Figure 2. Mechanical performance of Film-(x): (a) Tensile stress−strain curves for Film-(x) (25% RH). Inset provides the curve of Film-(0) for comparison. (b) Overview of strain at break (εmax), Young’s modulus (E), and tensile strength (σUTS) as a function of ILs content in the adopted mixed solvent. (c) Digital photographs of curly Film-(15) showing the good deformability due to plasticization.
through the wet CNC cake via a second suction filtration, as summarized in Figure 1A. By pushing the CNC suspensions through a filter membrane to remove water via the first vacuum suction, CNC rods assembled into solid CNC film with a cholesteric structure inside. Then, a room temperature ILs of AmimCl was mixed with water and brought to pass through the prefabricated CNC film via a second vacuum filtration. As AmimCl is miscible with water at any ratio, it can be carried onto the skins of CNC particles together with water and filled those pores and gaps. To confirm the interactions between negatively changed CNC and AmimCl, FT-IR spectra of AmimCl, CNC, and CNC films infiltrated with AmimCl were collected and presented as Figure S3 in the Supporting Information. The spectra demonstrate that after infiltration with AmimCl, a peak belonging to the stretching vibration of CN in the imidazole cation of AmimCl appears, while a symmetrical C−O−S vibration associated with C−O−SO3 significantly diminished. Also, the broad band located in the range of 3000−3600 cm−1,
which is ascribed to O−H vibrations of cellulose, became narrower than that of pure CNC due to the interaction between chlorine anions (Cl−) and cellulose hydroxyl proton. Based on these results, we infer that alkylimidazolium cations (Amim+) of ILs adhered on CNC skins through interaction with negatively charged sulfate half-ester groups of CNC, while chlorine anions (Cl−) associated with the cellulose hydroxyl proton. The proposed sketch for depicting ionic interactions between cellulose and AmimCl was shown in Figure 1B. After removing excess water from the film by evaporation in an oven at 80 °C, nonvolatile AmimCl was left in the film. By varying the content of AmimCl in the mixed solvent, a series of samples were prepared for performance evaluation. For convenience, the produced samples were designated as Film(x), where x denotes a weight percentage of ILs in the adopted mixed solvent. Crack-free Film-(x) prepared at x = 5, 10, 15, 20, 25, 30 wt % were of different coloration. Iridescent CNC film without penetration of ILs, denoted as Film-(0), was included for comparison. During the infiltration process, ionic C
DOI: 10.1021/acsami.6b12953 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. Optical properties of Film-(x): (a) and (b) Actual photos of a series of Film-(x) viewed from orthogonal and oblique to the sample surfaces with a white background. The insets are sketches for indicating the viewing direction (c) UV−vis spectra of Film-(x). (d) and (e) Polarized optical microscopy (POM) images of Film-(15) and Film-(0). Insets are the real photos of the films to demonstrate their optical transparency.
(E), and tensile strength (σUTS) of the prepared samples were plotted versus ILs percentage as shown in Figure 2b. With the increasing ILs percentage in the adopted mixed solvent, both the strength and modulus of the Film(x) reduced consistently, while the strain at break increased continuously. Compared with pure CNC film, εmax of the films infiltrated with ILs increased significantly. This may be attributed to the plasticization effect of the incorporated ILs, which softened the rigid CNC film by virtue of the aggressive ability to break H-bonds between CNC particles, and the frictional sliding of rigid CNC particles was permitted. Unlike those biocomposites using polymer as soft phase targeting at enhanced toughness and flexibility, ionic liquid functioned as plasticizer gifting CNC films with both pliability and coloration. Compared with those biocomposites, CNC films plasticized by AmimCl present pliability with strength and modulus as sacrifices. Among the prepared samples, Film-(15) with bright iridescence is easier to be bended or fashioned without crack during the actual processing operation, and the deformation did not need any additional force to maintain, as manifested in Figure 2c. Therefore, we choose Film-(15) as a typical sample for further analysis. Optical Properties of CNCs Films Infiltrated with ILs. Iridescence refers to the property of some surfaces to vary color with viewing angle and includes a color travel phenomenon or a structural color phenomenon. In general, the angular-dependent color (iridescence) is an outward manifestation of CNCs film with chiral nematic order.38 The iridescence originates from the orderly alignment of CNC crystals that can selectively reflect the incident light.39,40 Figure 3a is the actual photo of the prepared Film-(x) viewed from orthogonal to the sample surfaces. It can be seen that with the increasing ILs percentage, the color changed from light brown to light blue sequentially. When viewed from an oblique direction as indicated by the insets in Figure 3b, the films presented shiny colors that were different from Figure 3a. This kind of coloration, which varies with observing angle, is acknowledged as structural color. The prepared series of films present different coloration when viewed from orthogonal to their surfaces with a white background. This indicates that light with certain wavelength was extinct when passed through the films. Given that cellulose itself has no chromophore to absorb UV lights, we employed
liquid goes through the membrane and is partly lost during vacuum filtration. The actual contents of 1-allyl-3-methylimidazolium in the films were calculated through gravimetric analysis, and the results were presented as Table S1 in the Supporting Information. With the increasing percentage of AmimCl in the mixed solution employed, the content of AmimCl in the film sample increased accordingly. The superposed energy-dispersive X-ray (EDX) spectra of key elements including N, S, and Cl were presented as Figure S4a and S4b in the Supporting Information. The superposed spectra demonstrate that the content of sulfur in the film decreased significantly while that of nitrogen and chlorine increased after the infiltration process. The allocations of these elements in the same fractural section of films were obtained via energy-dispersive X-ray (EDX) mapping, and the data (by area fraction) were also presented in Table S1 in the Supporting Information. Evaluate the Mechanical Performances of CNCs Films Plasticized by ILs. The obtained crack-free films allowed us to perform the static uniaxial in-plane tensile tests to evaluate their mechanical performances. Room temperature tensile stress− strain curves for specimens conditioned at 25% RH (RH = relative humidity) are displayed in Figure 2a. Since both cellulose and ionic liquid are hygroscopic, dried CNC film infiltrated with ionic liquid would quickly get damp when it is exposed in ambient environment with higher relative humidity. To minimize the plasticization effect contributed by absorbed moisture, we performed the tensile test in the dry spring when the relative humidity in the atmosphere is only about 25%. The obvious softening and significant increase in ductility signify the plasticization effect of ILs. From the inset of Figure 2a, Film-(0) demonstrates a stiff and brittle mechanical response because the stress−strain curve includes only a linear part that corresponds to elastic deformation. There was no plastic deformation before Film-(0) fractured upon the tensile stress. However, all the latter parts of the curves correspond to Film-(x) (x = 5, 10, 15, 20, 15, and 30) evidently deviated from the first linear parts, indicating the specimens had undergone inelastic deformation before failure. To demonstrate the strong effect of incorporated ILs on the mechanical performance of rigid CNC film, performance parameters including strain at break (εmax), Young’s modulus D
DOI: 10.1021/acsami.6b12953 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. (a)−(b) Scanning electron microscopy (SEM) photograph of fracture surface morphology of (a) Film-(15) and (b) Film-(0). Insets are a whole view of the fracture surfaces with lower magnification. (c) The full energy-dispersive X-ray (EDX) spectra with the corresponding elements of Film-(15) and Film-(0). (d)−(f) EDX mapping of the different components: (d) chlorine of Film-(15), (e) sulfur of Film-(0), and (f) sulfur of Film(15).
nav and P. Following the facile vacuum filtration of the CNC suspension, the self-assembled cholesteric structure is preserved within the solid film, whose fracture surface presents typical layered structures. Compared with the CNC films via the traditional solvent evaporation method, these films show more uniform color and microscopic structure for not having suffered from the 3D “coffee stain” rings. Structure Characterization of CNC Films Infiltrated with ILs. From SEM images at low magnification (inset in Figure 4a, 4b), both Film-(15) and Film-(0) demonstrate the parallel aligned layered structures which were similar to the SEM observations reported in the literature.34,42 The helical pitch heights can be quantified through SEM images at high magnification. As indicated by the white solid line in Figure 4a and 4b, the half of the pitch heights increased from about 186 nm for Film-(0) to approximately 258 nm for Film-(15). The increased helical pitch in Film-(15) contributes to the red shift of λr as revealed by UV−vis spectra. In addition, from Figure 4a and Figure S7, one can see that CNC rods stick to each other and the boundaries of CNC particles become obtuse and fuzzy compared to those in Film-(0). These variations could be ascribed to the uniform infiltration of ILs between CNC particles after the second vacuum filtration, which would be further verified via energy-dispersive X-ray (EDX) analysis in the following section. To confirm the uniform interpenetration of ILs into CNC films, the full EDX spectra with the corresponding elements in the fractural surfaces of Film-(15) and Film-(0) were collected and presented in Figure 4c. Comparing with Film-(0), the content of carbon, oxygen in Film-(15) takes on a pronounced increase, while the additional new peak corresponds to nitrogen and chlorine appears in the spectrum of Film-(15), which is an indication of ILs existing in CNC film. The content of sulfur is strongly diminished in Film-(15). This means that some of the sulfate half-ester groups in CNC film were removed when infiltrating ILs into CNC films via a second vacuum filtration. From the molecular structure of cellulose and the employed ILs as illustrated in Figure 1B, cellulose is free from chlorine, while ILs contain negatively charged chloride ions. The homoge-
the UV−vis reflection spectroscopy to qualitatively descript the variations. As shown in Figure 3c, the spectra show a gradual red shift in the reflected wavelength with the increasing ILs percentage. In addition, when viewed from orthogonal to their surfaces with a black background, the coloration of the films originated from the reflection of incident light does appear with a tendency of getting red with the increasing of ILs percentage (see Figure S5 in the Supporting Information). These results indicate that the proposed method is effective in controlling the optical properties of CNC films. The chiral nematic structure from the assembly of anisotropic nanocrystals in CNC film can be established through monitoring the birefringence via polarized optical microscopy (POM). Taking Film-(15) as an example and Film-(0) as the control sample, the marblelike texture in Figure 3d is quite similar to that of Film-(0) (see Figure 3e) except for some slight differences in color. When Film-(15) was placed on some capital letters as a cover layer (see inset in Figure 3d), the letters were as legible as those under Film-(0) (see inset in Figure 3e). In addition, the increasing ILs content seemingly did not influence the transparency of films samples (see Figure S6 in the Supporting Information), which indicates that the degrees of light scattering in the samples were similar. These results suggest that the incorporation of ILs using the proposed method preserves the assembled cholesteric structure of CNC film. The angular-dependent color of chiral nematic CNCs solid films, in terms of the reflected peak wavelength (λr), can be correlated to their microstructure (in terms of helical pitch P, the distance over which is observed a 360° rotation of the chiral nematic director) according to the equation proposed by H. de Vries41 λr = nav P sin θ
(1)
where nav is the average refractive index of the film, and θ is the glancing angle between the incident light and the sample surface. As demonstrated in the previous section, the incorporation of ILs into CNCs film led to the red shift of λr. According to eq 1, when the incident light is perpendicular to the surface, the red shift of λr is related to some changes of E
DOI: 10.1021/acsami.6b12953 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 5. (a) Collective TGA measurements profiles of Film-(15) and Film-(0). The curves of cellulose and AmimCl are included for comparison. (b) Temperature-dependent IR spectra of Film-(15) in the region of 1780−1500 cm−1. The spectra are displayed from 26 to 150 °C with 2 °C intervals. (c) Peak intensities of the δ (−OH) band as a function of temperature. (d) Tensile stress−strain (25% RH) curves for Film-(15) before and after the hot-pressing treatment. Inset provides the bar chart to display toughness of Film-(15) before and after the hot-pressing treatment for comparison. (e) A photograph to illustrate the optical transparency of Film-(15) before (on the right) and after (on the left) the hot-pressing treatment. (f) Cross-sectional schematic model for demonstrating the formation mechanism of a welding layer between assembled CNC particles. Green rods, orange layer, and green random coil denote CNC particles, AmimCl, and cellulosic chain, respectively.
thermal stability between Film-(15), Film-(20), Film-(25), and Film-(30). Considering AmimCl is an efficient solvent of cellulose at high temperature,44−46 it is possible to toughen Film-(15) by a further hot-pressing treatment process, during which cellulose on CNC skin would be partially dissolved by AmimCl and compressed to make a welding layer between CNC particles. Room-temperature ionic liquids (RTILs) are hydroscopic, and the absorbed water from the atmosphere may influence their solvent properties.47 In order to find out the optimum temperature for removing the absorbed water during a hotpressing treatment process, temperature-dependent IR spectra of Film-(15) in the region of 1780−1500 cm−1 were collected as shown in Figure 5b. In this region, IR bands correspond to the bending mode (δ (−OH) of water (1595−1650 cm−1), and the stretching vibration of CN in the imidazole cation (ca. 1563 cm−1) are included. The upper temperature is set as 150 °C, which is below the onset temperature of thermal degradation of CNC. Peak intensity of the δ (−OH) band is used to monitor the moisture content in Film-(15). Quantitative results of the peak intensities as a function of temperature are plotted as shown in Figure 5c, from which one can see that the peak intensity of δ (−OH) becomes weak with heating until temperature reaches 130 °C. Based on this result, we performed the hot-pressing treatment process under 130 °C and 10 MPa for 10 min, with a 5 min preheating period to remove water. CNCs extracted from native cellulose such as cotton and pulp is cellulose I as indicated by the typical diffraction peaks at about 2θ = 14.8°, 16.3°, and 22.6°.48 The crystallinity indexes of Film-(15) before and after hot-pressing were calculated using wide-angle X-ray diffraction (WXRD) data (see Figure S9 in the Supporting Information) according to the following empirical equation6
neous distribution of chlorine throughout the cross-section of Film-(15) unveiled by EDX mapping (Figure 4d) confirms the uniform infiltration of ILs into CNC film. Comparing the EDX mapping of sulfur in Film-(0) (Figure 4e) with that of Film(15) (Figure 4f), the distribution of sulfur in Film-(15) is still uniform but much sparser, which also proves the homogeneous incorporation of ILs and the incomplete desulfurization of CNC film. Thermal Stability and Thermal Reprocessing Ability of CNCs Films Infiltrated with ILs. The introduction of negative sulfate groups onto the outer surface of cellulose during hydrolysis facilitates the easy dispersion of CNCs in water, and yet, the existence of acid sulfate groups would impair the thermal stability by the dehydration reaction.43 As revealed by thermal analysis (Figure 5a), the thermal degradation onset temperature of Film-(0) lowered about 140 °C compared to pure cellulose. The homogeneous interpenetration of AmimCl between CNC particles leads to an increase of the onset temperature of thermal degradation by 80 °C when comparing Film-(0) (Tdeg = 160 °C) with Film-(15) (Tdeg = 240 °C). It has been shown in the previous section that the acid sulfate groups in Film-(15) diminished due to the interpenetration of ILs. In general, the higher acid sulfate groups’ content results in a lower temperature of thermal degradation of cellulose. The better thermal stability of Film-(15) may be attributed to the sulfur removal during the second vacuum filtration process. In addition, the introduction of chlorine by ILs would also contribute to the improvement of thermal stability by means of preventing the thermal oxidation of cellulose. We also performed TGA test for films with higher content of AmimCl in addition to Film-(15). The TGA and DTGA profiles were collected and presented as Figure S8 in the Supporting Information. The results demonstrate that after the infiltration process with AmimCl solution, the thermal stability of CNC films improved significantly, and there is little difference in F
DOI: 10.1021/acsami.6b12953 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces Cr I =
I200 − Iam × 100 I200
(2)
The total peak height of the [200] crystal planes (I200) was located at 2θ around 22.5°, whereas the amorphous contribution (Iam) was observed by the intensity of the baseline at 2θ around 18.0°. The crystallinity index declined slightly after hot-pressing because of the partial dissolution of cellulose by AmimCl, which gave rise to an increase in the fraction of the formed amorphous phase. Tensile stress−strain curves for Film-(15) before and after the hot-pressing treatment were presented in Figure 5d. After hot-pressing, the strain at break (εmax) and tensile strength (σUTS) were apparently improved, and the toughness of Film-(15) had an increase of almost 3fold. Probably, cellulose on CNC surface was partially dissolved to form a matrix during hot-pressing, which led to a strong interface and good stress transfer capability.33,44 Besides, the optical transparency of Film-(15) is also improved after hotpressing as demonstrated in Figure 5e, which may be attributed to less light scattering in the film due to more effective adhesion between CNC particles and less voids in the film after hotpressing. A cross-sectional schematic model for demonstrating the formation mechanism of a welding layer between assembled CNC particles is proposed as shown in Figure 5f.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jianming Zhang: 0000-0002-0252-4516 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51573082), the “973” Project (NO. 2014CB460160). The authors gratefully acknowledge Prof. Jun Zhang (Institute of Chemistry, Chinese Academy of Sciences) for offering AmimCl.
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REFERENCES
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CONCLUSIONS In conclusion, a feasible vacuum infiltration method was reported in this work for preparing iridescent CNC films with both pliability and tunable coloration, in which ILs of AmimCl were incorporated as plasticizers. Tensile test results manifest that both the strength and modulus of the rigid fragile CNC film were undermined, while the strain at break was increased due to the infiltration of AmimCl. The plasticization effect of AmimCl makes the film soften and pliable. The gradual red shift in the reflected color with the increasing ILs percent content in the film, which was monitored by UV−vis spectra of the films, indicates that the proposed method is also effective in tuning the coloration of CNC films. SEM and EDX analysis show the uniform interpenetration of AmimCl and partial desulfurization in CNC film, which benefits the improvement of film thermal stability. The prepared AmimCl plasticized cholesteric CNC film can be further toughened via a hotpressing treatment, during which cellulose on skins of CNCs was partially dissolved by AmimCl and compressed to make a welding layer. The presented method will serve as a significant instance of promising routes to improve the mechanical performances of cellulosics with a mesomorphic ordered structure.
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orthogonal to the sample surfaces with a black background, collective TGA and DTGA profiles of film samples with higher AmimCl content, WAXD profiles of Film-(15) before and after a hot-pressing treatment for calculating their crystallinity indexes (PDF)
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12953. AFM image of the prepared CNC nanorods spinning coated on mica plate, the DLS profiles of CNCs suspension for evaluating the particle size, fracture images of the prepared film samples obtained via a Stereo Microscope for measuring the thicknesses, TEM image of the prepared CNC nanorods, collective FT-IR spectra of AmimCl, CNC and plasticized CNC film, EDX spectra focusing on corresponding elements in the film samples, photos of a series of samples viewed from G
DOI: 10.1021/acsami.6b12953 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
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DOI: 10.1021/acsami.6b12953 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
