Cellulose Nanocrystal Iridescence: A New Model ... - ACS Publications

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Cellulose Nanocrystal Iridescence: A New Model G. Picard,*,† D. Simon,† Y. Kadiri,† J. D. LeBreux,‡ and F. Ghozayel‡ †

Department of Physics, Collège Ahuntsic, 9155 rue Saint-Hubert, Montréal, Québec, Canada H2M 1Y8 Quebec Institute of Graphic Communications, 999 avenue Émile-Journault est, Montréal, Québec, Canada H2M 2E2

‡

ABSTRACT: A homogeneous aqueous dispersion of cellulose nanocrystals (CNs) that is left to evaporate in a Petri dish selforganizes into smectic liquid crystals that are actually liquid multilamellar structures. As evaporation proceeds, the liquid multilamellar structures solidify to become a solid multilamellar film. Each solid lamella is in the submicrometer range, and its iridescence is easily explained by classical light interference. A careful inspection of each solid lamella revealed long, oriented arrays of colloids. Interestingly, the array orientation is generally the same for each superposed layer. This is exceptional because the stratification appears first in the liquid, and the solid colloids are formed in each stratum at the very end of the process. Our findings are supported by optical, atomic force, and electron microscope observations and by laser diffraction observations. The multilamellar solid film model is easier to engineer than the helical model currently used to explain the iridescence and optical activities of CN solid films. This new understanding should promote the industrial production of colorful CN coatings and inks as a green alternative for decades to come.

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observation.10−12 In fact, the whole subject of mesoscopic helical structures is said to be shrouded in mystery.9 Although the structure’s elusiveness might seem surprising, the helical structure concept is based on assumptions that were reasonable decades ago but are now questionable. Rather than being identical rods, CNs are now known to be very irregular nanofibers in regard to their length, diameter, and twist, which is not surprising given that they are extracted from wood using corrosive acidic solutions.13 With this key assumption in the helical model not supported by observation and given the elusiveness of the helical structure in CN to observation, we came to ask the following question: Could it be that the helical model, a concept initially elaborated from optical observations, is no longer tenable? By way of answer, we posit a new color-generating structure in this report. On the basis of our own microscopy observations, the CN solid film is essentially made of superposed lamellae and each lamella is itself made of more or less parallel chains of colloids, with each colloid being a bundle of organized nanofibers. It is this simple structure, a solid multilamellar film, that is responsible for the CN iridescence and optical activity. We also propose that this solid multilamellar structure is the product of a simple self-organization process. The CN aqueous dispersion essentially self-organizes into smectic liquid crystals,14 which the reader may visualize as being stable liquid multilamellar structures in water. Clearly, each liquid plane is

INTRODUCTION In response to concerns about the environmental impact of traditional inks, the printing industry is searching for ecological ink alternatives. Cellulose nanocrystals (CNs) are one such prospect. Extracted from resinous trees, CNs have proven their potential in a variety of applications1−3 and are notable for the solid iridescent films that result after water evaporates from CN dispersions.4−7 If CNs are to be fully exploited as a prospect for the coating and printing industry, understanding the fundamental mechanism that is responsible for their iridescence and optical activity is essential. Until now, the solid film iridescence and optical activity of CNs have been attributed to helices in the CN dispersion that solidify after water evaporation. This is an application of the helical model concept that was developed several decades ago in an effort to explain the observed optical activity of liquid crystals in general.8 This model hinges on the notion that some molecules in the bulk liquid can self-arrange and ultimately form a mesoscopic helix. The helical model has been similarly applied to other large rod-shaped molecular systems.9 It assumes that each helix is composed of superposed nanorod monolayers and that each layer’s nanorods are similarly oriented and at a small angular shift relative to the layers above and below it. It is also implied that a helical structure should self-organize in the liquid dispersion and freeze in place in the solid film after complete evaporation. This concept offered a credible explanation of the optical activity and iridescence of CN solid films. Amazingly, although modern microscopy is more than powerful enough to photograph such a fine helical structure, the structure’s existence has yet to be recorded by direct © 2012 American Chemical Society

Received: July 26, 2012 Revised: September 17, 2012 Published: September 18, 2012 14799

dx.doi.org/10.1021/la302982s | Langmuir 2012, 28, 14799−14807

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nematic according to Onsager theory.15 When evaporation is finished, the liquid multilamellar structure has simply become a solid multilamellar structure. Basic interferential physics tells us that this simple structure alone is capable of iridescence. Indeed, the multilayer structure is responsible for the beautiful colors often encountered in nature, for instance, in butterflies, beetles, fish, and plants.16−20 Finally, we believe that the last evaporation phase generates periodic instabilities in each smectic liquid plane, leading to the formation of parallel liquid lines that finally divide into rows of solid bundles. It is likely that within each bundle there is a preferential nanofiber orientation or twist that creates the observed optical activities and that lateral capillary forces are what drive the nanofibers into organized twisted solid bundles. This is an end result of the initial nematic organization inside each liquid plane of the smectic liquid crystal. This interesting solid multilamellar structure made with CNs is well documented and closer to reality than the helical model. The proposed self-organization process offers real possibilities for the industrial production of coatings and inks that can be controlled with respect to both color and intensity. This new model should improve the research and industrialization of CNs for decades. Finally, this report questions the longstanding belief that nanorods self-organize as a helical structure in liquid dispersions.

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Figure 1. (A) Experimental setup for observing the laser diffraction pattern from a CN dispersion in a Petri dish. A laser beam traveling through the dispersion is reflected onto a white screen. Near the screen is an identical CN dispersion in a Petri dish on a weigh scale. The diffraction patterns and the scale’s digital display are photographed. (B) A typical laser diffraction pattern is presented. The central spot is seen with three diffraction orders. This indicates that a smectic liquid crystal is in the path of the laser beam. At the bottom of the picture is the luminous digital display. Also visible (middle right) is the Petri dish (87 mm i.d.) containing the CN dispersion that glows red in the dark.

MATERIALS AND EXPERIMENTS

CN Dispersion. The CNs were extracted with sulfuric acid, leading to the attachment of about 2% sulfonyl groups.21 To obtain color reproducibility, all CN dispersions were filtered through a 1.2 μm pore size membrane (GFC, glass microfiber, Whatman) to remove microcontaminants and aggregates. The filtrate was then passed through a resin (mixed bed, MTO Dowex, Marathon MR-3, Supelco Analytical) to remove ions, leaving only CNs in pure water and possible resin particles. The deionized dispersion was then passed through another 1.2 μm filter to remove any resin particles. The final result was our working dispersion. A portion of the working dispersion was then used to make solid films. Further treatments were also performed on the remaining working dispersion. In one case, tiny amounts of NaCl were added to partially screen out the CN electric charges. In another case, the working dispersion was further sonicated because the sonication of dispersed CNs in water is known to change the final color of the solid film. How these different preparations impact the CN solid film’s structure and color could provide a better understanding of how CN nanofibers self-organize in water. Light Diffraction. To follow the self-organization steps in the CN aqueous dispersion, in situ laser diffraction was used (Figure 1A). The CN dispersions were poured into a Petri dish (87 mm inside diameter). A 1 mW He−Ne laser beam (Metrologic Neon Laser, model ML 820, 632.8 nm) entered the Petri dish from the bottom and upon its emergence was reflected onto a white screen. The light patterns projected onto the screen were photographed with a Canon EOS 5D Mark II camera (Figure 1B). High-resolution raw 5616 × 3744 pixel array pictures were taken manually every 15 s for several minutes and then automatically every 15 min for 10 h. To follow the evaporation process, an identical CN preparation was placed on the plateau of a digital scale that had all of its doors open. The camera’s field of view also captured the luminous display of the scale. Each image taken thus corresponds to a specific CN concentration. The room was in darkness at all times. Optical Microscopy. A microscope (Nikon, Optiphot-2, Japan) equipped for phase contrast and polarization imaging was used. It allowed the observation of the formation of smectic liquid crystals in a thin liquid film or in capillary tubes. It was also used to see in situ the last evaporation phase of some CN dispersions. Finally, it was

extensively used to explore CN iridescent solid films, either on the surfaces or at the rupture lines when the solid films were broken apart. AFM. A Nanosurf Easy Scan 2 AFM equipped with Image Metrology A/S SPIP 5.0.1 software was used. The microscope was operating in dynamic force mode. The tips acquired from Nano World were NCLR-20 with Al-coating. To obtain images of isolated CN nanofibers, the dispersion was diluted, sonicated, put onto mica, and dried. Solid films made from different CN preparations were also examined. The procedure was quite simple. The film surface was first scanned. Next, the CN film was broken and the rupture line was scanned. Using the AFM’s high degree of vertical precision, the step heights were measured and are presented in Table 1. To further our analysis, the solid films were also cut with a sharp razor blade, and the front face of the open edges was scanned.

Table 1. Attenuation and Reflection Peaks of CN Solid Films film

salta

son.b

AFMc

Δzd

attenuatione

reflectione

color violet blue blue-green green red transparent

Y/N more less no no no no

s no no no 15 600 1200

nm 113 150 136 204 344 460

nm 19 20 16 48 53 95

nm 358 371 454 539 795 1100

nm 350 N/A 450 520 700 off scale

a Whether a small amount of salt was added. bThe time of sonication in seconds. cThe lamella thickness, in nanometers. dThe standard deviation of the thickness. ePeak wavelengths, in nanometers.

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SEM. A Hitachi S-4300SE/N (VP-SEM) was used. The top and bottom surfaces of CN solid films were first explored. Next, the CN solid films were cut with a sharp razor blade and the front faces of the open edges were scanned. Precautions were taken to avoid sample deterioration due to the electron beam. Attenuation Spectroscopy. To quantify the optical properties of the solid film for further analysis, a UV−visible spectrophotometer was used (Ultraspec 2100 pro, model 80-2112-21). The solid films were installed perpendicular to the analyzing beam, resulting in the beam passing through the solid film before reaching the instrument detector. The reading was in absorbance units, although in the visible part of the spectrum this reading indicates only attenuation by reflection, interference, or diffusion. Reflection Spectroscopy. Iridescence arises from the reflection of light off of the CN solid film. To quantify the iridescence better, a spectrofluorometer (USB 4000, Ocean Optics Inc.) specially equipped for reflection spectroscopy was used. Flashes from a PS-2 xenon lamp emitting UV to IR flashes at a rate of 10 pulses/s were funneled into a UV fiber and conducted to the sample. Both sides of the solid film were systematically investigated. A special holder kept the fiber a few millimeters above the targeted surface and perpendicular to it. Surrounding the light-emitting fiber, a cluster of fibers conducted the reflected light to a detector array monochromator. This reflection spectroscopy setup permitted an entire film to be quickly and easily scanned before taking the most representative spectrum for further analysis.

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RESULTS AND DISCUSSION During Evaporation. At the beginning of evaporation, the laser beam made a bright, narrow spot on the screen surrounded by a uniformly diffuse speckled field. Seconds afterwards, a large diffuse ring appeared from the screen edges, shrank, and merged at the center about 5 min later. For the next 2 min, the central spot became somewhat larger and evolved toward a composition of large speckled spots. This is indicative of an evolution from Rayleigh to Mie scattering (i.e., from a random to an organized dispersion).22 About 10 min later, a halo emerged from the center, indicating the birth of smectic liquid crystallites. As the halo grew larger, it became segmented and evolved toward the wellknown diffraction fringe pattern of large smectics (Figure 1B). Because we know the distance to the screen, the laser wavelength, the halo diameter, the water content for each picture during the evaporation process, and the dispersion refractive index, it is easy to plot the plane-to-plane distance against the water content for every picture. The initial plane-toplane distance is 15 μm. After several hours, the diffraction pattern gradually widened and vanished at about 2 to 4 μm. Microscopic observation also revealed smectic liquid crystals in the aqueous dispersion that tend to fuse together (Figure 2A). The smoothness of the liquid planes tells us that the smectic liquid crystals are composed of dispersed CN nanofibers, confirming the conclusion of our analysis of the laser diffraction pattern. The CNs in water are essentially making a phase transition from being well dispersed to settling into liquid planes in smectic liquid crystals. The birefringence of the smectic liquid crystals indicates that the nanofibers are oriented. To test the smectic liquid crystal model further, a 4% CN dispersion was also inserted into a capillary tube. The same smectic liquid-crystal patterns as in Figure 2A were observed with the same microscopy setup. The tube was carefully rotated under the microscope objective while observing the space in between the liquid planes. When the main axis of the capillary

Figure 2. (A) Tactoids made of dispersed CN nanofibers in water. The structure is very similar to smectic liquid crystals in which liquid planes are spaced by water. Here, the liquid planes are 3.5 μm thick and 10 μm apart. The nanofibers are believed to be perpendicular to the liquid plane surface. Similar structures were observed in capillary tubes. The liquid planes were positively identified as disks by rotating the tubes. The image is 200 μm wide. (B) Smectic liquid planes evolving toward a solid multilamellar structure create a moving polychromatic front. Optical activity emerges as the solid bundles are created. The image field is 0.9 mm wide.

tube was perpendicular to the planes, the liquid planes remained unchanged in size, position, color, and shape. If the CN had formed a helical structure, a barber pole effect would instead have been observed. The absence of this effect indicates that the liquid planes are really disk-shaped. This simple observation alone is enough to invalidate the helical model and confirm the smectic-like liquid-crystal model. Moreover, together with the observed birefringence of the liquid planes, this tends to demonstrate that the nanofibers are oriented with their long axis generally perpendicular to the liquid plane. In summary, the laser diffraction patterns and the optical microscopy observations indicate that the dispersed CNs are gathering in smectic liquid crystals. Throughout evaporation, the liquid planes are brought closer to one another until a solid multilamellar structure results. The weakening of the diffraction pattern’s intensity could be explained by the decreasing separation between the liquid planes and the planes becoming parallel to the Petri dish floor. Near the End of Evaporation. An interesting photograph was taken of the Petri dish while the CN dispersion was near the end of evaporation (Figure 2B). A polychromatic front was seen to be moving quickly from right to left when the picture was taken at low magnification. The colorless surface first became red, then green, then blue, and finally violet. The colors 14801


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Figure 3. (A) Mosaic made of dried out smectic-like liquid crystals. Each colored area is a smectic liquid crystal that became a solid multilamellar film. The image is 1.3 mm wide. (B) The multilamellar structure is observed with the optical microscope. Lamellae parallel to the Petri dish floor are beautifully stratified at the end of the process. The image is 700 μm. (C) A close-up view of a portion of image B is closely observed. Parallel arrays are clearly seen on the top lamella. As expected, fracture lines parallel to the arrays are also seen. The other fracture lines have a dented edge that is due to the ruptured arrays. The image is 120 μm. (D) A tiny piece of a lamella is closely examined, revealing parallel chains of bundles. The bundle size is close to the limit of the optical microscope’s range, about half a micrometer. The length of the chains is in the 100 μm range. Isolated bundles are also seen. In other pictures of shattered solid films, isolated arrays are scattered like solid needles. The image is 350 μm.

AFM Microscopy. Figure 4A shows CN nanofibers deposited on a freshly cut mica sheet. The nanofibers are not straight rods, contrary to what is routinely reported.1−3,13 Instead, they are irregular in length and diameter. The formation of mesoscopic helices from such material, as described in most of the literature, is highly improbable. Figure 4B shows the rupture line. A nice, regular staircase structure is observed. The steps are superposed lamellae. Although there are minor variations in the thickness of the lamellae, their overall regularity suggests a fundamental thickness unit. Because the AFM microscope automatically adjusts colors to minimize their range on the screen for the comfort of the observer, what appears at first glance to be a Michelson echelon grating is actually an illusion. When this coloring function is disabled, the illusion disappears and the reality of the descending staircase is evident. The stair heights were measured without the coloring function and are reported in Table 1. Finally, if we look closely at each stair surface, a fibrous material seems visible. The microfibers are oriented in the same direction on all stairs. This is interesting and merits the closer examination of the next figure. In Figure 4C, the fibrous material seen in the previous image is further examined. Careful examination reveals differing widths of roughly parallel chains of disk-shaped objects. Two chains sometimes merge, creating a larger one. The chains are roughly parallel and vary in size. This indicates long- and shortrange forces that assembled the nanofibers within each smectic liquid plane into compact disk-shaped colloids. In Figure 4D, another image is shown to appreciate the whole process better. Nice rows of petal-like motifs are seen. The whole appearance is remarkably elegant and very different

were generated as water evaporated from the smectic liquid crystals. The broad range of colors is likely due to the optical activity that begins as the nanofibers start organizing into twisted bundles and progresses as evaporation proceeds. We discuss this further below. Solid Film Investigated. The solid multilamellar film was investigated by optical, AFM, and SEM microscopy. The basic principles governing their operation are completely different. By comparing the images, we can deduce a safe conclusion. Optical Microscopy. In Figure 3A, a CN solid film is observed. A mosaic of plates with different shapes and two dominating colors is seen. This indicates that almost all smectic layers dried up parallel to the Petri dish floor. Then the film was broken in two. At the rupture line, the next photograph (Figure 3B) shows that each smectic liquid crystal is a multilamellar film. This visual aspect is also regularly reported in the literature23 although our interpretation differs considerably. If a broader view of the image is taken, a finer structure is observed (Figure 3C). On the top lamella, parallel arrays are clearly seen. The distance from one array center to the next is about 3.6 μm. The general impression is that the bundles were formed after the arrays were created (which we discuss further below). When small pieces exposed at the rupture point are more closely examined (Figure 3D), it becomes evident that the lamellae are made of rigid needlelike structures that are loosely arranged alongside one another. The needlelike structures themselves resemble an array of colloids, but the resolution limit of the optical microscope is not high enough to confirm this and an examination by more powerful microscope was therefore necessary. 14802


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Figure 4. AFM pictures of CN. (A) Nanofibers are isolated after sonication. It can be seen that they are not uniform in shape and length. The nanofibers are typically highly heterogeneous. (B) A solid green film was split, and the edge was scanned. The multilamellar structure is seen clearly. Moreover, each lamellar surface reveals a fibrous texture. (C) A close-up view of the fibrous structure reveals nearly parallel arrays of bundles. (D) Another solid film was scanned in the same manner as in the previous picture. Rows of petal-like motifs are seen. The phase transition from liquid dispersion to solid objects can follow different paths. The basic result remains the same: the lamella thickness depends on the initial smectic liquid planes. (E) A film cross-section was observed. The multilamellar structure is clearly demonstrated.

multilamellar organization. At first glance, it looks very much like a typical colloidal stack, which gives rise to iridescence through light interference. Although the question of the colloidal aspect is interesting24 and is discussed further at the end of this section, what is most important is that the multilamellar structure that is the central point of this report is confirmed. Interestingly enough, the lamellae appear to be progressively thicker from top to bottom, although the overall variation is small. The iridescent CN solid

from the previous image. However, beyond this appearance, two fundamental properties are observed for each image: the rows are parallel, and the colloids have roughly the same size, shape, and orientation. This indicates that the dispersed CN nanofibers may organize into different bundle shapes from one smectic liquid crystal to another. Figure 4E presents another point of view. The edge of the solid film was scanned after the film was cut with a sharp razor blade. This image provides an additional indication of the 14803


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film may therefore be a chirped multilayer reflector (also discussed further below). SEM Microscopy. Figure 5 shows the solid film under experimental conditions that are very similar to those in Figure

The observed optical activity necessarily arises from the bundles themselves. We do not yet know the precise arrangement of the CN nanofibers inside each bundle. Our best assumption is that the CN nanofibers, which are themselves twisted, most likely thread together into a larger twisted structure.29 This question is not resolved in this report and could be an interesting subject for further exploration. Attenuation Spectroscopy. The attenuation spectra of the CN solid film were obtained and are reported in Figure 6.

Figure 5. SEM picture of a CN solid film taken with 15 keV electrons. The film was cut with a sharp knife and observed on its edge. The stratification is easily seen. Inside each lamella, bundles are also seen. This type of structure is the basis of the color generation by light interference. The image is 9 μm large.

4E. When the SEM was pushed for high magnification, the cellulosic material became severely damaged. We therefore had to limit the SEM to only relatively moderate magnification. The image was further amplified and contrasted via computer for better clarity. The image clearly shows a multilamellar structure. Moreover, we can see that each lamella is made of colloids. This corresponds to the CN nanofiber bundles that we reported seeing with the other microscopes we used. The lamella thickness is also consistent with our observations from the other microscopes. It is interesting that bundles are not always clearly seen at the edge of each lamella. This is consistent with the AFM observations that indicate variety in the end results of the evaporation process (Figure 4C,D). Collectively, the optical, AFM, and SEM microscopes all reveal that the CN solid film is a multilamellar structure. This is the principal finding of this report and should replace the longheld helical structure model for CN solid film. It is also demonstrated that each lamella is made of a colloidal material, which we refer to as bundles. The bundles can be in one piece or can be a highly structured assembly of smaller bundles. Overall, the aspects of the bundles vary throughout the solid film−as disks, spheres, coils, shells, etc.−but within a given solid film area, because of local conditions and phase-transition dynamics, their aspects are similar (Figure 4C,D). Another significant fact is that the bundles have a tendency to be in parallel rows. Instabilities in the planar solidification front most probably occurred in each smectic liquid plane during the final evaporation phase,25 leading to a liquid periodic structure. In other words, the CN liquid planes (2D suspension) become parallel CN liquid lines (1D suspension). Further along in the evaporation phase, the lines are broken into small CN liquid islands, and capillary forces finally gather the CN nanofibers into solid bundles.26−28 This sequence of events can explain the formation of rows of bundles (Figure 4C,D). As the capillary forces pull the nanofibers toward each other, short-range forces then shape the bundles into their final compact configurations.

Figure 6. Attenuation spectra of CN solid films (in absorbance units). Unsonicated (magenta line); sonicated for 15 s (yellow line); sonicated for 10 min (cyan line); sonicated for 20 min (transparent film−purple line); and unsonicated 0.016% w/v NaCl (blue line). The 10 min sonication spectrum has a shoulder at 700 nm.

Sonication of the CN dispersion is seen to red shift the attenuation peak, which is very typical,30,31 and the red shift reaches a limit after extended sonication. Sonication also clearly leads to thicker lamellae (Table 1). This is consistent with the fact that CN nanofibers form thicker smectic liquid planes in the dispersion after sonication. It has also been reported that sonication cleans the nanocrystals.32,33 Taken together, these characteristics indicate that sonication removes material that would otherwise screen the CN electric charges from each other and therefore increases the electrostatic repulsive forces in the dispersion. In contrast, the addition of NaCl creates a blue shift. Salt is known to screen the CN electric charges, diminishing the strength of the repulsive forces. The CNs are therefore closer to each other, resulting in thinner smectic liquid planes and ultimately thinner solid lamellae (Table 1). The attenuation spectra were simulated via computer. Because the spectrophotometer light source emitted incoherent light, basic physics tells us that the solid film essentially behaves as an interferential filter that operates at a depth of two layers at most, even if the film has over 100 layers. Such a poor filter shows a large bandwidth, and the results of the computer simulation are consistent with our observations of the CN film attenuation spectra because a similarly large bandwidth was found. This interesting aspect of our work merits further exploration. Reflection Spectroscopy. The reflection spectra were obtained and reported in Figure 7. The setup was efficient and easy to use and permitted a detailed inspection of the whole film on both sides. For a given film, the reflection spectra on both sides were quite comparable. However, some variations in the reflection spectra were seen across different areas, indicating 14804


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Returning to a more general discussion, we note that our liquid dispersions and solid CN films are optically very similar to those described in previously published studies.1−3 It is also significant that breaking a CN solid film introduces a staircase aspect with steps in the submicrometer range. This is consistent with the multilamellar model, where each lamella is made of chains of bundles. Isolated chains and isolated bundles were also observed. According to the helicoidal model, we should have observed lamellae with thicknesses in the tens of nanometers range. In addition, the lamellae surfaces should have been flat under the helical model with a roughness of approximately 10 nm. Instead, we always observed a roughness in the submicrometer range. When we directed a laser beam through the iridescent solid film onto a white screen, only the central spot was seen, with no secondary diffraction spot. This means that no trace of the grating-like objects, which are numerous at the beginning of the evaporation process in the Petri dish, could be detected. This proves that the iridescence is based on classical light interference from a multilamellar structure. One crucial question is whether our CN dispersions differed from others. This is unlikely because the CNs were extracted following the quasi-standard procedure of using sulfuric acid.21 Furthermore, our microscopic observations are essentially the same as those published: fingerprint motifs, tactoids, optical effects, absorption spectra, SEM images, and reactions to salt content.1−3 The only difference, a radical one, is the interpretation of the observations that are based on finer optical and AFM analyses and interferential physics. Interestingly enough, after drying out, the smectic liquid planes become solid multilayers of colloids in the submicrometer range. This is exceptional because stratification begins in the liquid and the solid colloids (i.e., bundles) are created in each stratum only at the very end of the process. Typically, the creation of structural colors is achieved as a colloidal dispersion evolves toward stratification.36−44 As interesting as this exception may be, because it has already been demonstrated that perfect 2D crystal ordering within a monolayer is not necessary to produce colors,41 the most important point to retain from our observations remains the superposition of the lamellae. We also established during our numerous experiments that the presence of smectic liquid crystals is critical to creating a colorful solid film. In a variety of experiments with electric fields,44 lateral flow,24,42,43 and other techniques, nanofibers have been driven to stack against each other in films that are tens of micrometers thick, and the final result was always a colorless film. However, by controlling the preparation and coating parameters, we can use the same CN nanofibers to produce a film that ranges from red to blue to transparent. Interesting optical observations have also been reported on oriented ultrathin films deposited either by the Langmuir− Blodgett method35 or by electrodeposition.45 The ultrathin film stratification produced light interference and colors as demonstrated by Langmuir.46 Finally, it might be asked whether the bundles are playing a role in iridescence by scattering light. This is in principle possible because the bundles are primarily in the submicrometer range.20 If so, because the color blue is more scattered than red, the reflected light should be bluer and the transmitted light should be redder. However, a comparison of the transmission and reflection spectra indicates that this effect is negligible

Figure 7. Reflection spectra of CN solid films. Unsonicated (red line); sonicated for 15 s (blue line); sonicated for 10 min (purple line); sonicated for 20 min (transparent film−yellow line); and unsonicated 0.016% w/v NaCl (green line). The 10 min sonication spectrum has a peak at 700 nm.

that whereas the structure of the film is mostly the same there are local differences. Optical activity values displayed similar behavior. These variations in optical properties for a single film are not surprising.34 This simply means that although the chemical conditions were initially the same, the 3 days of selforganization and the last stage of evaporation yielded local structural differences as might be expected. Table 1 reports the spectrum values that were most commonly found for each film. The progressive thickness variation of the lamellae from top to bottom also merits some attention in discussing the reflection spectroscopy. This is in principle a chirped multilayer reflector,20 a type of interferential filter that has a wide bandwidth just as our solid film does. However, because the reflection spectra on both sides of the film were found to be nearly identical, this means that the optical path lengths are the same for all layers, despite variations in the geometric lengths (i.e., the lamellae thicknesses). A simple explanation of this effect is that the quantity of matter is initially constant within each liquid lamella in the smectic liquid crystal. Interestingly, there is a simple relationship between the lamellae thicknesses measured with AFM and the peak values measured with attenuation and reflection spectroscopy. As noted above, given that the length of coherence of a single photon is around 500 nm, its range is limited to one to two lamellae only. Plotting the peak wavelengths against the lamellae thickness, we obtain a slope of 1.93 ± 0.15. Taking into account the hard reflection on the top surface and the soft reflection on the second interface, straightforward physics tells us that the average refractive index is n = 1.45 ± 0.11. Because the CN refractive index has been reported to be 1.45 and 1.47,35 voids comprise less than 10% of the solid CN film. Applying the physics of light interference, we thus found the spectroscopic and AFM measurements to be consistent with the conclusion that a CN solid film is essentially a multilamellar structure that generates color. The distances between the final solid lamellae depend on the initial electrostatic forces between the CN nanofibers dispersed in water. Both the sonication of the reference dispersion and the addition of tiny amounts of salt essentially modify the ionic environment of the CN nanofibers. 14805


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compared to the strong light interference that is a dominant characteristic of iridescence. The iridescence, therefore, depends essentially on the multilamellar structure.

REFERENCES

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CONCLUSIONS In this article, it was demonstrated that sulfated CN nanofibers dispersed in water first become smectic liquid crystals, with the nanofiber’s long axis perpendicular to the smectic planes. A solid multilamellar structure is obtained after evaporation, with each lamella made of roughly parallel rows of bundles. The final CN solid film is a multilamellar structure inducing light interference, not a helical cholesteric liquid crystal as previously thought. Absorption and reflection spectra, laser diffraction, and optical, SEM, and AFM microscope observations are all consistent with the multilamellar structure. This is our principal finding. Moreover, on the basis of this understanding, we propose that instabilities in the planar solidification front occur in each smectic liquid plane during the final evaporation phase, leading to a liquid periodic structure that solidifies in parallel rows. The capillary forces generated in the thin liquid film in the last moments of evaporation are in all likelihood driving the vertically standing CN to make the bundles. This very interesting topic should be investigated further. The shape, size, and orientation of each bundle are matters requiring a complex analysis that is well beyond the scope of this article. However, it can be safely concluded that the observed optical activity is effectively related to each bundle and that the latter are most probably CN nanofibers that are twisted together. Overall, the solidification of nematic CN in smectic liquid planes into solid bundles is very interesting from an academic point of view and should be investigated further. This report is an important breakthrough in our understanding of the generation and properties of the CN color structure, and it has significant consequences for the direction of future research on the self-organization of CNs and the engineering of CN industrial applications. It would be interesting to study the evaporation rate and the temperature effects on the bundle and multilamellar structures. Moreover, special effort should be devoted to obtaining a better understanding of the bundles’ final aspects and internal organization. Finally, this article poses a serious challenge to the long-standing belief that nanorods in general self-organize as helical structures in liquid dispersions. The existence of such structures should be revisited in light of this article.

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Article

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministère du Développement économique, de l’Innovation et de l’Exportation du Québec for financial support, G. Lachaine (Collège Ahuntsic, physics) for the laser setup, R. St-Amour (Collège Ahuntsic, chemistry) for valuable discussions, L. Brouillette (Collège Ahuntsic, biology) for AFM and optical microscopy, D. Schwinghamer (Collège Ahuntsic, English) for editing, M. Beaulieu (Quebec Institute of Graphic Communications) for photographing the halos, and FP Innovation for providing raw CN. 14806


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Cellulose Nanocrystal Iridescence: A New Model ... - ACS Publications

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