Photonic Properties and Applications of Cellulose Nanocrystal Films

Subscriber access provided by Kaohsiung Medical University

Photonic Properties and Applications of Cellulose Nanocrystal Films with Planar Anchoring Partha Saha, and Virginia A. Davis ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00233 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on May 1, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Nano Materials

Photonic Properties and Applications of Cellulose Nanocrystal Films with Planar Anchoring Partha Saha and Virginia A. Davis* Department of Chemical Engineering, Auburn University, Auburn University, AL 36849, United States KEYWORDS: cellulose nanocrystals; film; selective reflection; chiral nematic; helix defect

ABSTRACT: Above a critical concentration, aqueous dispersions of sulfonated cellulose nanocrystals (CNC) form chiral nematic liquid crystalline phases. Retention of microstructural order and planar anchoring of the helix during drying should result in films that exhibit selective reflection of specific wavelengths of light. Such films are of interest for use a variety of photonic applications including display components, narrow band optical filters, low threshold mirrorless lasing, sensors, and architectural, decorative and security coatings. However, non-uniformities in the initial CNC dispersions and microstructural changes during drying typically result in uniform selective reflection only being achieved over length scales on the order of ten microns. In this research, uniform photonic properties were achieved over orders of magnitude greater length scales by understanding the effects of initial concentration, orbital shear, surface anchoring, and drying conditions on the microstructure and photonic properties. In addition, biomimetic films

ACS Paragon Plus Environment

1

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

were produced which exhibited a double-peak spectra similar to that exhibited by the chiral nematic photonic structure in Lomaptera beetles.

1. INTRODUCTION Cellulose nanocrystals (CNC) extracted from biomass using sulfuric acid hydrolysis readily form chiral nematic liquid crystalline phases in water; and there is considerable interest in using this phase behavior to produce films with controlled photonic properties such as selective reflection. Depending on the source, the width and length of CNC vary from 3 - 20 nm and 50 - 1000 nm respectively.1-2 Although CNC are not truly cylindrical, they generally follow the lyotropic phase behavior associated with rigid rods; above a critical concentration they form chiral nematic (cholesteric) liquid crystals. In the chiral nematic phase, the local orientation of the CNC rods is defined by the director n, which rotates in a left-handed helical fashion resulting in slightly skewed continuous nematic microstructures.3-4 The helix direction is perpendicular to the director n. The distance that n travels to complete a full (360°) rotation is known as the helical pitch P which decreases with increasing chiral interaction during drying. Depending on the direction of helix alignment, chiral nematic ordering can be classified into three distinct categories (Figure 1): planar (vertical helix perpendicular to film surface), homeotropic (helix parallel to film surface revealing fingerprints under cross-polarized light), and focal conic (tilted helix or helix in both directions).5 In the case of planar alignment, circularly polarized light with the same handedness as the chiral nematic CNC is reflected as a single color; the wavelength of maximum reflection λmax is related to P by Braggs’ formula6-7 =


(1)

2

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where nav is the average refractive index of the crystalline material (1.56 for CNC8), and θ is the angle of incidence of circularly polarized light with respect to the film surface. Obtaining uniform selective reflection requires achieving planar alignment over macroscopic areas.

Figure 1. Schematic showing different orientation directions of the chiral nematic helix, (A) planar: vertical helix in the z-direction, 360° rotation of n over the pitch P, (B) homeotropic: helix parallel x-direction along the substrate, and (C) focal conic: helices tilted in both directions.

Retention of cholesteric microstructure during drying results in photonic films whose optical properties depend on the helix orientation and pitch;9-10 several researchers have shown that dried CNC films can display remarkable photonic properties including Bragg reflection and circular dichroism.11-12 There is considerable interest in using CNC films as photonic materials for optical sensing, security papers, selective reflectors, templates for plasmonic nanomaterials, and decorative films.12-14 In addition, cholesteric microstructure based photonic reflectors and humidity sensors have also been reported for composite CNC films.15-16 This indicates the importance of uniform planar microstructures for a range of sensing applications. However, achieving uniform planar or vertical helix ordering and cholesteric pitch over large areas has remained a challenge. Non-uniformities between isotropic and anisotropic phases in biphasic dispersions, forces associated with film casting and shrinkage, and gelation during drying all


3


Page 4 of 30

impair the ability to achieve the uniform helix orientation required for selective reflection of a single color from large areas.17-18 Typically photonic properties are limited by the size of the planar domains, which typically vary from a few microns to several tens of microns.19 In addition, poor control over the helix orientation during drying gives rise to defect-rich, mosaiclike domains that are typically observed in air-dried CNC films. Hence, increasing understanding of how to control planar domain size, pitch (photonic color), and spatial helix defects is critical for producing CNC films with uniform optical properties. Park et al. reported that orbital shear flow during drying of liquid crystalline CNC dispersions facilitated planar anchoring and uniform photonic properties on length scales on the order of 50 µm.20 In another work, Dumanli et al.19 reported more distinct colors in different domains with sizes on the order of 10 µm. Wilts et al. obtained relatively uniform film color by slowly drying CNC dispersions in a custom built humidity controlled chamber;21 other researchers have explored the effects of magnetic alignment and confined geometry.22-23 However, the combined effects of dispersion concentration and processing parameters on the uniformity of chiral nematic helix orientation and associated optical properties across a film are not yet fully understood. In fact, these individual studies suggest a need to better understand the combined effects of surface anchoring, concentration dependent gelation onset,24 orbital shear and slow drying. In addition, while there have been many recent advances in understanding the role of defects in photonic materials25 such as Lomaptera beetles26 and pollia condensata fruits,27 there is relatively little understanding of helical defects in cholesteric CNC films. Line defects in CNC have been attributed to varying pitch in adjacent domains,19 but stop band twist defects arising from cholesteric half-pitch variations and resulting in double-peak reflection have not previously been addressed.


4

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This research investigated the individual and combined effects of CNC concentration, orbital shear, surface anchoring, and drying conditions. The evolution of helix orientation and pitch during drying and the microstructure and properties of the solid films were investigated using a combination of polarized optical microscopy, reflectance spectroscopy, and scanning electron microscopy (SEM). Slow drying of CNC dispersions anchored between two glass surfaces on an orbital mixer in a water vapor-saturated environment (relative humidity 98%, at 22 °C) resulted in films with large planar domains on the order of a few hundred microns in length and width. Some of these domains exhibited the biomimetic property of double-peak reflectance spectra such as that seen in Lomaptera beetles.26 This unique stop band optical property was the result of variations in chiral nematic half-pitch within the films’ cross-section and can be modulated by changing the aspect ratio and chiral strength. To the best of the authors’ knowledge, such stop band properties28 have not previously been reported in cholesteric CNC films.

2. EXPERIMENTAL DETAILS 2.1. Materials. A sulfonated cellulose nanocrystals aqueous suspension (11.8 wt. %/7.7 vol. %) containing 1.2 wt. % sulfur (according to product specification) with Na+ as the counter ion was produced by the U.S. Forest Products Laboratory (FPL) and supplied by University of Maine Process Development Center (batch number 2015-FPL-077). Based on polarized optical microscopy, the onset of biphasic and liquid crystalline regimes were observed around 4.8 and 7.5 wt. % (3.0 and 4.8 vol. %), respectively. While CNC generally considered to follow “rodlike” phase behavior they do not have a cylindrical cross-section; CNC are actually three dimensional parallelepipeds. The three dimensional geometry and effective dimensions resulting from the sulfonated CNCs’ double layer have a significant influence on their lyotropic liquid


5


Page 6 of 30

crystalline phase behavior. Based on atomic force microscopy (AFM) measurements of dried samples the average height, width, and length of the CNC used in this investigation were 5.5 nm (standard deviation 1.3), 46.7 nm (standard deviation 8.5), and 160.8 nm (standard deviation 47.3), respectively (Figure S1). The width measurements included blind tip estimation (using Gwyddion’s) to deconvolute the shape of the particles from the geometry of the tip, but some artifacts may still persist. Many researchers only report two dimensions but the length and height measurements are consistent with previous research using FPL CNC.29-30 Among researchers reporting width, the values vary considerably but are all much greater than the height. Interestingly, in recent using a different FPL batch, the measurements based on AFM were similar, but the width and height obtained by analyzing small angle neutron scattering data (in D2O) were half of the AFM values.30 2.2. Dispersion preparation. The as received cellulose nanocrystal suspension, was dispersed to the desired concentration using ultrapure water (purified using Thermo Scientific Auto Dispenser, Millipore water, pH 6.5, conductivity 0.95 µS/cm at 23.3 °C). The dispersions were then vortex mixed for 10 minutes followed by overnight bottle rolling. The bottle rolled dispersions were allowed to rest for at least 1 hour before drop casting to make sure no bubbles were present. Densities of 0.997 g/cm3 for water and 1.60 g/cm3 for CNC were used for the conversion between mass and volume concentrations. 2.3. Drying. A 75 mm x 25 mm glass microscope glass slide (substrate) and 22 mm x 22 mm coverslip (anchoring surface) were cleaned with acetone and DI water and air dried. This was followed by further cleaning with air plasma (300 mTorr, 100 watts, 1 min) for ultra-cleaning. The microscope slide was placed inside a polystyrene Petri dish; 200-300 µL of the dispersion was then dropped onto the slide with a spatula. Water drops were added to the surrounding area


6

Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


using a micropipette and the lid was closed. Then, the closed Petri dish was placed on an orbital mixer (VWR Standard Analog Shaker, set speed 60 rpm) for 15 minutes to pre-shear the dispersion. After that, the lid was opened, and the coverslip was placed on the cast dispersion. The lid was put back again and left for 24 hours drying (under a relative humidity of 98% at 22°C). 2.4. Optical microscopy. A Nikon (Melville, NY) Eclipse 80i microscope with imaging workstation and Nikon Elements was used to capture all of the cross-polarized reflected light micrographs using 10x/0.30 and 20x/0.45 LU Plan Fluor objectives. The camera used was a Nikon DS-Ri2. 2.5. Scanning electron microscopy. A JEOL (Peabody, MA) JSM-7000F Field Emission Scanning Electron Microscope was used for SEM analysis using both 10kV and 15 kV electron beam voltage. Prior to SEM, gold sputtering was applied to the CNC films using a PELCO SC-6 SPU system run by argon (0.08 mbar pressure was used for plasma sputter). 2.6. Atomic force microscopy. A Pacific Nanotechnology Nano-R SPM atomic force microscope (AFM) was used in non-contact mode, to determine the size distribution of the CNC. Approximately 10 µL of 0.05 vol. % CNC dispersion was drop cast onto a freshly cleaved mica surface. The dispersion was allowed to sit for 5 min followed by air drying, leaving behind adsorbed CNC on the surface. Each 3 µm x 3 µm area scan was acquired at a scan speed of 0.2 Hz with a 1024 × 1024 pixel resolution. An image of the tip was obtained using a porous aluminum standard surface and Gwyddion’s blind tip estimation algorithm; this image was used to deconvolute the shape of the particles from the geometry of the tip. 2.7. Spectrophotometry. A CRAIC 2020 UV-Vis/ Fluorescence Microspectrophotometer was used to get the selective reflection (using 75 W Xenon reflectance power supply) spectra of the


7


Page 8 of 30

chiral nematic planar domains. A UV-vis-NIR polarizer with quarter wave plate retardation was used to obtain the circularly polarized light.

3. RESULTS AND DISCUSSION Drying drop cast CNC dispersion typically results in films with significant non-uniformities and significant regions of homeotropic anchoring which do not result in selective reflection. However, planar domains on the order of a few microns may also be observed under reflected light microscopy (Figure 2). In this work, a combination of orbital shear, two surface anchoring, and slow drying in a water-saturated environment was used to produce CNC films with uniform planar anchoring on length scales up to a millimeter. As shown in Figure 3A, a drop of CNC dispersion was placed on a microscope slide and covered with a (22 mm x 22 mm) glass coverslip and placed in a water vapor-saturated (relative humidity 98% at 22 °C) environment created within a closed Petri dish containing water droplets (Figure 3B). Based Park et al’s20 work on understanding the effect of orbital shear on planar alignment, the assembly was allowed to dry for 24 hours on an orbital mixer moving between 50 and 80 rpm. With increasing mixer speed, the domains generally became smaller, and at too high a speed, the applied shear field unwound the cholesteric microstructures, eventually resulting in nematic-like alignment. The drying scheme created a combination of drying induced capillary flow toward the edges of the sample and orbital flow created by the applied motion of the orbital mixer (Figure 3C). The authors believe that the weak shear field provided by the mixer coupled with the humid environment served to reduce capillary flow, and the CNCs’ tendency to align parallel to the two surfaces, all combined to enhance ordering. Since films made at 60 rpm exhibited less color variation than those made at 50 rpm, 60 rpm was chosen for the remainder of the experiments.


8

Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Key findings regarding the effects of concentration, surface anchoring, and drying conditions are described in the sections below; images from each possible combination of conditions are included in the supporting information (Figures S2 - S7).

Figure 2. Cross-polarized reflected light microscopy image of a typical CNC film made by drop casting a biphasic (6.5 wt. %) dispersion on a glass slide and drying under ambient conditions. Scale bar is 50 microns.

Figure 3. Photograph, schematic, and optical micrograph showing (A) CNC dispersion on a microscope slide with a glass coverslip on the top, (B) drying set up, (C) directions of capillary flow and orbital shear. 3.1. Concentration Effects. To understand the effects of the initial dispersion concentration, cross-polarized optical micrographs were compared for films prepared from a 2.0 wt. % (1.3 vol. %) isotropic, 6.5 wt. % (4.2 vol. %) biphasic, and a 8.0 wt. % (5.1 vol. %) liquid crystalline dispersions; Figure S8 of the supporting information contains images of the initial dispersions.


9


Page 10 of 30

As described in the seminal work by Park et al.,18 drying of isotropic or biphasic dispersions results in the formation of new chiral nematic domains (tactoids) at random locations within the continuous isotropic phase. Since each of these new domains can potentially adopt any helix orientation, Park et al. hypothesized that drying fully liquid crystalline dispersions would eliminate the possibility of forming new liquid crystalline domains with random helix orientations.18 They further showed that orbital shear facilitated planar anchoring (vertical helix orientation) by aligning the cholesteric helix to the short axis of the ellipsoidal or elongated tactoids (making n tangential to the interface between the tactoids). Figure 4A is a representative image of CNC films obtained from isotropic dispersions (2.0 wt. %, 1.3 vol. %) using the drying scheme shown in Figure 3. Islands of elongated planar domains with distinct reflectance colors were surrounded by dot-shaped smaller domains. Some of the dot-shaped domains had striped textures which are attributed to fusion defects between liquid crystalline domains or tactoids in a phase coexistence regime. As the isotropic dispersions dried, the growing tactoids coalesced or fused with each other followed by bending, folding, and elongation of the cholesteric microstructures.31 During ambient drying, two tactoids may not fully merge into a single uniform tactoid, possibly due to high viscosity and slow relaxation of the CNC microstructure. Figure 4B shows a CNC film obtained from a biphasic dispersion (6.5 wt. %, 4.2 vol. %). Despite some defects, this film had large bands of planar domains with uniform reflectance colors that spanned the 1.4 mm wide field of view. Films from the liquid crystalline dispersion (8.0 wt. %, 5.1 vol. %) showed the smallest domains (Figure 4C). We attribute this to two factors. First, the CNC in the liquid crystal dispersions had lower mobility; the low shear (0.01 s-1) viscosity of the liquid crystalline dispersion (8.0 wt. %) was over one order of magnitude higher than that of the biphasic dispersion (6.5 wt. %). Second, the lower


10

Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


concentration of the biphasic dispersions implies a longer time until the drying induced concentration increase results in a rheological gel and kinematic arrest. In contrast to the current work, Park et al.18 observed that the liquid crystalline dispersions resulted in the largest domains. We attribute to this discrepancy to a combination of both differences in drying conditions and differences in the initial dispersions. First, the ambient drying in Park et al.18 resulted in faster drying times, and less time for biphasic domains to rearrange and fuse as the concentration was increased. Second, Park et al.18 used CNC extracted from cotton which retained the H+ counterion. This research used dispersions from the US Forest Products Lab (USFPL), which are prepared from CNC extracted from woody biomass and the counterion exchanged to Na+. Although similar in size, the dispersions of these two types of CNC have markedly different phase boundaries and rheological properties. The effects of Na+ ion exchange on the electric double layer results in an increase in both the liquid crystal phase transition and viscosity at a given concentration; it can also effect the glass transition behavior during drying.32 As a result, the cholesteric domains’ mobility, relaxation, and ordering are strongly affected by the CNC counterion. In Park et al.,18 the liquid crystalline dispersion had a concentration of 4.8 wt. % (3.0 vol. %), compared to 8 wt. % (5.0 vol. %) in the present work. Park et al.18 did not report the viscosity data for their biphasic and liquid crystalline dispersions, but they were likely an order of magnitude lower than those in this work. This estimate is based on comparison of the viscosities of the USFPL dispersions used in the current research to the data we reported in Ureña-Benavides et al.33 for CNC prepared using a very similar method to that of Park et al.18


11


Page 12 of 30

Figure 4. Cross-polarized reflected light micrographs showing planar chiral nematic domains of CNC in dried films using (A) isotropic (2.0 wt. %/1.3 vol. %), (B) biphasic (6.5 wt. %/4.2 vol. %), and (C) liquid crystalline (8.0 wt. %/5.1 vol. %) dispersion concentrations. All scale bars are 100 µm. Each image is a 1.4 x 1.1 mm field of view. 3.2. Surface Anchoring Effects. The microstructures of dispersions dried with and without a coverslip on the apparatus shown in Figure 3 were compared. The isotropic dispersion (2.0 wt. %) was chosen for study because films from isotropic dispersions have the most nonuniformities.18 During drying, tactoids (droplets of anisotropic phase dispersed in isotropic phase) form, evolve, and merge resulting in small planar domains with random mosaic defects.18 Figure 5 shows the difference in the optical texture with and without top surface anchoring to a coverslip. Without the coverslip (Figure 5A), there are many mosaic defects and fingerprint textures resulting from parallel helix ordering. There are also randomly oriented domains exhibiting different reflectance colors. The predominantly blue color could be the result of the oblique incidence of light on tilted helices resulting in a blue-shift. However, with the coverslip (Figure 5B), the film had dot-shaped planar domains with subtle mosaic defects. The similarity of the regular dot-shaped planar domains’ appearance (also shown in Figure S11B and S15I) to cholesteric blue phase defects is an interesting subject for future investigation. The marked differences between 5A and 5B show that two surface anchoring (slide and coverslip) has a


12

Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


significant effect on CNC ordering during drying. This effect may be partially due to interactions with the dispersion and glass surface. CNC will tend to align parallel to the slide and coverslip due to their anisotropic rod-like shape; therefore, the helices will tend to align vertical to the surfaces favoring a planar texture. However, is likely to primarily be the result of preventing evaporation from, and skin formation on, the top surface which is known to limit microstructural uniformity by locking everything into a glassy state.17-18

Figure 5. Cross-polarized reflected micrographs showing the effect of surface anchoring on planar orientation, CNC film dried in water vapor saturated environment assisted by orbital shear with a coverslip (A) off and (B) on during drying. Scale bars are 100 µm. Larger images may be found in the supporting information (Figure S11). 3.3. Humidity Effects. Films produced using the scheme shown in Figure 3 did not exhibit the typical coffee-ring effect observed for open surface drying of colloid dispersions, including CNC dispersions.34 For a CNC drop dried on a surface with contact line pinning, the higher evaporation rate at the edge results in capillary flow of the CNC to the edge of the drop and a thick iridescent ring in the dried film.35 This affects not only the alignment of the nanocrystals at the edge, but also the helix alignment in the rest of the drop cast dispersion.19


13


Page 14 of 30

Drying in a water vapor-saturated environment effectively slowed down the capillary flow, allowing more time to the helix ordering before the kinetic arrest of the microstructure.36 Thus, this drying scheme allowed more time for the liquid crystalline domains to achieve planar ordering. We hypothesized that the interplay of weakened capillary flow and orbital shear may have enabled vertical helix orientation while the coverslip surface pinned the nematic directors of the adjacent liquid crystal domains at the dispersion coverslip interface. To confirm this hypothesis, the evolution of the vertical helix during drying was investigated using crosspolarized reflected light microscopy. A biphasic CNC dispersion sandwiched between the slide and coverslip was taken out from the water vapor saturated environment after two hours of slow drying. Then, the dispersion was left out to dry under ambient conditions (relative humidity 10 % at 22°C). Because of the slow drying followed by fast drying, some of the chiral nematic domains were trapped in between horizontal and vertical helix orientations displaying a tilted helix transition. As shown in Figure 6, fingerprints, tilted helix, and planar alignment were all observed in close proximity (circled areas). This kind of helix transition was previously observed by Zola et al. for thermotropic liquid crystals in the presence of a rubbed anchor surface.37

Figure 6. Cross-polarized reflected micrograph showing the evolution of the helix orientation in a CNC film obtained using 6.5 wt. % (4.2 vol. %) dispersion. 1) Fingerprint texture resulting


14

Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


from homeotropic anchoring indicated by closely spaced parallel stripes, 2) tilted helix texture signified by irregularly spaced stripes, 3) planar anchoring indicted by domains of uniform selective reflection. Arrows indicate the relative directions of shear and capillary flow. Scale bar is 50 µm. 3.4. Combined Effects. Since the biphasic (6.5 wt. %) dispersion resulted in the largest domains (Figure 4), it was studied in more detail than the other dispersions. Figure 7A shows a mosaic strip of the film stitched from forty images; the entire film is shown in Figure S12. Based on the size, shape, and uniformity of reflectance colors, this film strip can be divided into three regions of broadband reflectance colors, ranging from violet to deep red. The colorful regions from the near center to the edge, the film thickness varied from 2 µm to 10 µm (Figure S13) However, the dark spot in the center had a submicron thickness. The thickness variations in the CNC film are attributed to the motion of the orbital mixer not being able to completely overcome capillary flow toward the edge as well as non-uniformities in the mixer’s rectangular orbital path.18, 38 Region 3L (Figure 7B) near the edge of the coverslip, displayed elongated planar domains; the subtle shift from green to blue was due to variations in pitch within the film cross section; this is further discussed later in the manuscript. The uniform yellow to red domains in Region 2 (Figure 7C) are consistent with having greater and more uniform thickness than Region 3L. In Region 1 (Figure 7D), the domains were less elongated; and uniform over larger regions. This may be the result of less capillary flow, and lower velocity, near the center of the drying drop, as well as the inverse relationship between domain size and shear rate.5 The large macroscopic domains in this region resemble the “Grandjean” textures reported previously for planar aligned thermotropic liquid crystals.5, 39 In this region, the significant change in thickness over a small distance near the center caused a red shift in reflectance colors compared to the deep


15


Page 16 of 30

blue closer to the center. The pitch P can be related to the film thickness d according to this following relation40 = 2 ⁄ With increasing thickness, the number of chiral half-pitches (N, periodic spacing indicating half of the pitch in the dried film) typically decreases (lateral direction), resulting in planar domains with a shift of reflectance colors. However, the number of chiral half-pitches may also vary even if the thickness remains constant. The purple Region 3R (Figure 7E) showed mainly focal conic and homeotropic ordering (see larger images in Figure S14). Similar parabolic focal conic domains were reported by Roman et al. for self-assembled CNC in films.41 However, crosspolarized reflected colors of these domains from region 1, 2, and 3L did not change upon rotation of the film; this means there was no strong birefringence induced by the optical axis of the homeotropic helix (Figure S15).

Figure 7. Cross-polarized reflected light micrographs showing a stitched image of (A) distinct planar regions across the CNC film between the coverslip and glass substrate. It also illustrates


16

Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


variable shapes and sizes of planar domains shown from left to right in (B) Region 3L, (C) Region 2, (D) Region 1 (E) Region 3R. All scale bars are 100 µm. Within the film, many areas had dominant uniform reflectance colors throughout each 1.4 x 1.1 mm image (Figure 8). For blue clusters, bleached green was the second reflective color indicating subtle pitch transitions (Figure 8A). As the sample thickness remains constant, a domain with one or two extra half-pitches must have a shorter pitch than the neighboring domain, exhibiting a blue-shifted color. The same observation was found for green (with the orange transition) and deep red (with green transition) clusters as shown in Figures 8B and 8C, respectively. However, though the overall colors were very uniform, there were clear disclinations walls between adjacent domains. These line defects are attributed to non-integer changes in the number of chiral half-pitches, N between adjacent domains; these non-integer changes are incomplete half-turns of the director n.5, 39, 42 More research is needed to fully understand, and remove, these disclination features and achieve uniformity over the large areas required for some applications. However, for some applications domain sizes on the order of hundreds of microns are sufficient. Also, as was the case in the polymer industry, and is the case for other nanomaterials, reducing polydispersity and improving quality control of the raw material will facilitate more uniform assembly and properties.


17


Page 18 of 30

Figure 8. Cross-polarized micrographs showing uniform reflectance colors of (A) blue, (B) green, and (C) red, in planar domains with slight transition colors of green, orange, and green, respectively. Scale bars are 50 µm. The origins of the colors and disclinations were investigated using elliptically polarized reflected microspectroscopy and scanning electron microscopy. Figures 9A and B show the selective reflectance spectra that were observed using both left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) reflected light on selected domains with blue, green, and orange colors. Figure 9C is a micrograph under LCP light showing reflection by the planar domains (using 0º incident angle of illumination); the same domains transmitted RCP light due to opposite handedness of chiral nematic ordering (Figure 9D). Figures 9E and 9F show selected areas of domains for spectra analysis. As shown in Figures 9A, some areas (e.g. 1, 2, and 3) had single-peak reflectance spectra. As expected, cross-polarized reflectance, micrographs of those spots (1, 2, and 3) were uniform in color. On the other hand, areas such as 4, 5, and 6 showed a subtle change in the reflectance colors within a domain showing doublepeak reflection. The double-peak LCP reflection spectra (Figure 9B) with a narrow trough were observed for these types of domains (i.e. green with yellowish green, spot 5). Interestingly, similar double-peak reflectance spectra have recently been reported in Lomaptera beetles’ cuticle by Carter et al.26 Two possible variations in a planar chiral nematic ordering can give rise to double-peak spectra. First, cholesteric microstructures with variable half-pitch (throughout the film’s cross section) cause an abrupt change to the periodic modulation of the refractive indices. A sudden phase jump at the interface of cholesteric helices with different half-pitches takes place, causing twist defects in films.25, 28 The shape and width of the double-peak reflection bands depend on the birefringence and maximum and minimum values of the half-pitch. Second,


18

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


even with constant half-pitch, any discontinuity between cholesteric helices may also cause twist defect modes resulting in double-peaks.28 The different shapes of the double-peak bands in Figure 9B are attributed to different phase jumps caused by twist defects. To the best of the authors’ knowledge, this is the first report of such a narrow photonic stop band defect in CNC photonic films, resolved by LCP. Interestingly, Ličen et al. performed using Fast Fourier Transformation (FFT) on SEMs of CNC film cross sections and found that films dried using orbital shear had multiple peaks associated with periodic variation and thickness dependent changes in cholesteric half-pitch.43 Photonic CNC films with such twist band defects can have potential use in advanced applications including narrow band optical filters, displays, and lowthreshold mirrorless lasing.28, 44-45

Figure 9. Selective reflectance spectra using circularly polarized light showing (A) single-peak, and (B) double-peak reflection from planar domains of CNC film using both left handed


19


Page 20 of 30

circularly polarized (LCP) and right handed circularly polarized (RCP) light, (C) selective reflection micrograph under LCP illumination, (D) no reflection under RCP light illumination over the same planar domains, (E), (F) cross-polarized reflected micrographs showing selected planar domains giving reflectance spectra of both single-peak (1, 2, and 3) and double-peak (4, 5, and 6). To better understand the microstructural origin of the optical properties, perpendicular and oblique cross-sections of the film were investigated by SEM. Figure 10A shows the individual chiral nematic half-pitch segments (N) in a perpendicular cross-section view of the fractured film. The thickness of each layer is half of the pitch; dividing the total number of chiral half-pitch (N) by film thickness would give the average half pitch in that planar domain. Figure 10B shows the oblique cross-section (slanted to the helical axis); the parallel arc-like appearance indicates uniform planar chiral nematic ordering. These continuous stacks of nested arcs resemble the “twisted plywood” or Bouligand structure reported by Livonent et al. for several biological cholesteric systems.46 Figure 10C shows an example of a planar domain with uniform or constant pitch that resulted in single peak spectra. On the other hand, Figure 10D shows a domain with a varying pitch in the film direction (perpendicular to the film surface) which resulted in subtle color variations and double-peak spectra (Figure 9B). Figure 10E shows an example of the number of chiral nematic half-pitch segments N changing from 10 to 11, which resulted in domains with different reflectance colors. Figure 10F is from one of the films showing a tilted helix optical texture (Figure 6, spot 2) and shows a distinct tilt in the planar ordering (helix not being perpendicular to the substrate).


20

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. Scanning electron microscope (SEM) images of cross-sections of CNC films showing (A) chiral nematic half-pitch segments viewed at 90° cross-section, (B) uniform parallel arc-like morphology of planar ordering viewed at oblique cross-section, (C) constant pitch and (D) spatially varying pitch within a planar domain, (E) line defect due to change of the number of chiral nematic half-pitch, and (F) tilted domains next to planar ones. The dome-shaped topography on the surface of the film is attributed to electron beam damage. Scale bars are 1 µm. To confirm the relationship between the microstructure and the optical properties, the pitch calculated based on the reflectance peaks (λpeak) was compared to that measured using SEM (Table 1). For the blue, green, and orange planar domains the spectra-based pitch values were 284, 328, and 384 nm, respectively (shown in Table 1). SEM measurements on the same planar domains (Figure S16) were in generally good agreement with pitch values of 288 ± 40, 342 ± 58, and 416 ± 38 nm, respectively. The stop band reflection spectra (Figure 9A) indicates a possible range of pitch values which would be in agreement with the SEM pitch.


21


Page 22 of 30

Table 1. Measured pitch using optical spectra and SEM Domain color

λpeak (nm)

dSEM (nm)

NSEM

Pspectra (nm)

PSEM (nm)

Blue

444

1150

8 ± 1.0

284

288 ± 40

Green

512

1200

7 ± 1.0

328

342 ± 58

Orange

601

1250

6 ± 0.5

384

416 ± 38

4. CONCLUSIONS In this work, a combination of surface anchoring, shear, and drying in a humid environment were used to produce photonic CNC films. The effects of initial dispersion concentration and slow drying were investigated with and without the aid of orbital shear and two surface anchoring. In all cases, biphasic dispersions resulted in films with more uniform microstructures than isotropic or liquid crystalline dispersions. Similarly, slower drying in a humid environment reduced the effects of capillary forces and resulted in more uniform microstructures. The addition of a coverslip to provide a second anchoring surface further facilitated vertical helix formation. The combination of using a biphasic dispersion, orbital shear and drying between two surfaces in a humid environment resulted in the largest areas of uniform reflectance color. This macroscopic planar ordering of the cholesteric helices in dried films exhibited broadband selective reflection spectra of circularly polarized light. Compared to air-dried CNC films, the lateral dimensions of regions of selective reflection were enhanced from a few microns19 to millimeters in size. The interplay of slower capillary flow and orbital shear flow, in presence of surface anchoring, facilitated the vertical or planar helix orientation to the film surface. SEM analysis revealed uniform planar ordering throughout the film indicated by long range arc-like structural ordering. This research also unveiled a new insight into spatially varying pitch in planar CNC films,


22

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mimicking the double-peak defects observed in the natural photonic materials’ cholesteric microstructures. The results are promising for advancing CNC films toward their potential applications in display components, narrow band optical filters, sensors, and architectural, decorative and security coatings. In addition, uniform planar alignment of lyotropic CNCs can be used as a versatile template for long range planar ordering of inorganic nanowires, nanorods, plasmonic nanoparticles, and nanosheets for optical and sensing applications.47-50 ASSOCIATED CONTENT Supporting Information AFM based size distribution of CNC. Cross-polarized transmitted light microscopic images of the following: isotropic, biphasic, and liquid crystalline aqueous CNC dispersions, results from additional experiments on the effects of concentration, orbital shear, drying humidity, and anchoring; additional images

showing effect of surface anchoring on planar ordering;

fingerprints and focal conic textures; cross-polarized reflected microscopic images showing no linear birefringence of the planar domains stitched image of entire CNC film. SEM images showing variable thicknesses of planar CNC film and larger images of film cross sections showing the pitch of different domains. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions


23


Page 24 of 30

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by National Science Foundation (NSF) CBET, Award Number: 1437073. This project was also supported by Auburn University Research Initiative on Cancer Graduate Fellowship 2016-2017. The authors also express appreciation to Martin Pospisil and Professor Micah Green of Texas A&M University, USA, for useful discussions. REFERENCES (1)

Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J., Cellulose Nanomaterials Review: Structure, Properties and Nanocomposites. Chem. Soc. Rev. 2011, 40, 3941-3994.

(2)

Revol, J.-F.; Bradford, H.; Giasson, J.; Marchessault, R.; Gray, D., Helicoidal Self-Ordering of Cellulose Microfibrils in Aqueous Suspension. Int. J. Biol. Macromol. 1992, 14, 170-172.

(3)

Majoinen, J.; Kontturi, E.; Ikkala, O.; Gray, D. G., SEM Imaging of Chiral Nematic Films Cast from Cellulose Nanocrystal Suspensions. Cellulose 2012, 19, 1599-1605.

(4)

Dong, X. M.; Gray, D. G., Induced Circular Dichroism of Isotropic and MagneticallyOriented Chiral Nematic Suspensions of Cellulose Crystallites. Langmuir 1997, 13, 30293034.


24

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(5)


Dierking, I., Textures of Liquid Crystals. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, FRG, 2003; pp 51-90.

(6)

de Vries, H., Rotatory Power and Other Optical Properties of Certain Liquid Crystals. Acta Crystallogr. 1951, 4, 219-226.

(7)

John, W. S.; Fritz, W.; Lu, Z.; Yang, D.-K., Bragg Reflection from Cholesteric Liquid Crystals. Phys. Rev. E 1995, 51, 1191.

(8)

Klemm, D.; Philipp, B.; Heinze, T.; Heinze, U.; Wagenknecht, W., In Comprehensive Cellulose Chemistry: Fundamentals and Analytical Methods; Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, FRG, 2004; Chapter 2, pp 9-29.

(9)

Habibi, Y.; Lucia, L. A.; Rojas, O. J., Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications. Chem. Rev. 2010, 110, 3479-3500.

(10)

Pan, J.; Hamad, W.; Straus, S. K., Parameters Affecting the Chiral Nematic Phase of Nanocrystalline Cellulose Films. Macromolecules 2010, 43, 3851-3858.

(11)

Querejeta-Fernández, A.; Kopera, B.; Prado, K. S.; Klinkova, A.; Methot, M.; Chauve, G. g.; Bouchard, J.; Helmy, A. S.; Kumacheva, E., Circular Dichroism of Chiral Nematic Films of Cellulose Nanocrystals Loaded with Plasmonic Nanoparticles. ACS Nano 2015, 9, 1037710385.

(12)

Zhang, Y. P.; Chodavarapu, V. P.; Kirk, A. G.; Andrews, M. P., Structured Color Humidity Indicator from Reversible Pitch Tuning in Self-Assembled Nanocrystalline Cellulose Films. Sens. Actuators, B 2013, 176, 692-697.


25


(13)

Page 26 of 30

Beck, S.; Bouchard, J.; Chauve, G.; Berry, R., Controlled Production of Patterns in Iridescent Solid Films of Cellulose Nanocrystals. Cellulose 2013, 20, 1401-1411.

(14)

Shopsowitz, K. E.; Qi, H.; Hamad, W. Y.; MacLachlan, M. J., Free-Standing Mesoporous Silica Films with Tunable Chiral Nematic Structures. Nature 2010, 468, 422-425.

(15)

Lu, T., Pan, H., Ma, J., Li, Y., Bokhari, S. W., Jiang, X., Zhu, S., Zhang, D., Cellulose Nanocrystals/Polyacrylamide Composites of High Sensitivity and Cycling Performance to Gauge Humidity. ACS Appl. Mater. Interfaces 2017, 9, 18231-18237.

(16)

Espinha, A., Guidetti, G., Serrano, M. C., Frka-Petesic, B., Dumanli, A. G. m., Hamad, W. Y.; Blanco, Á., López, C., Vignolini, S., Shape Memory Cellulose-Based Photonic Reflectors. ACS Appl. Mater. Interfaces 2016, 8, 31935-31940.

(17)

Gray, D. G., Recent Advances in Chiral Nematic Structure and Iridescent Color of Cellulose Nanocrystal Films. Nanomater. 2016, 6, 213.

(18)

Lagerwall, J. P.; Schütz, C.; Salajkova, M.; Noh, J.; 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, e80.

(19)

Dumanli, A. G. M.; van der Kooij, H. M.; Kamita, G.; Reisner, E.; Baumberg, J. J.; Steiner, U.; Vignolini, S., Digital Color in Cellulose Nanocrystal Films. ACS Appl. Mater. Interfaces 2014, 6, 12302-12306.

(20)

Park, J. H.; Noh, J.; Schütz, C.; Salazar‐Alvarez, G.; Scalia, G.; Bergström, L.; Lagerwall, J. P., Macroscopic Control of Helix Orientation in Films Dried from Cholesteric Liquid‐ Crystalline Cellulose Nanocrystal Suspensions. Chem. Phys. Chem 2014, 15, 1477-1484.


26

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(21)


Wilts, B.; Dumanli, A.; Middleton, R.; Vukusic, P.; Vignolini, S., Invited Article: Chiral Optics of Helicoidal Cellulose Nanocrystal Films. APL Photonics 2017, 2, 040801.

(22)

Frka‐Petesic, B.; Guidetti, G.; Kamita, G.; Vignolini, S., Controlling the Photonic Properties of Cholesteric Cellulose Nanocrystal Films with Magnets. Adv. Mater. 2017, 29.

(23)

Parker, R. M.; Frka-Petesic, B.; Guidetti, G.; Kamita, G.; Consani, G.; Abell, C.; Vignolini, S., Hierarchical Self-Assembly of Cellulose Nanocrystals in a Confined Geometry. ACS Nano 2016, 10, 8443-8449.

(24)

Gray, D. G., Order and Gelation of Cellulose Nanocrystal Suspensions: An Overview of Some Issues. Phil. Trans. R. Soc. A 2018, 376, 20170038.

(25)

Kopp, V. I.; Genack, A. Z., Twist Defect in Chiral Photonic Structures. Phys. Rev. Lett. 2002, 89, 033901.

(26)

Carter, I.; Weir, K.; McCall, M.; Parker, A., Variation in the Circularly Polarized Light Reflection of Lomaptera (Scarabaeidae) Beetles. J. R. Soc. Interf. 2016, 13, 20160015.

(27)

Vignolini, S.; Rudall, P. J.; Rowland, A. V.; Reed, A.; Moyroud, E.; Faden, R. B.; Baumberg, J. J.; Glover, B. J.; Steiner, U., Pointillist Structural Color in Pollia Fruit. PNAS 2012, 109, 15712-15715.

(28)

Chen, J. Y.; Chen, L. W., Twist Defect in Chiral Photonic Structures with Spatially Varying Pitch. J. Phys. D: Appl. Phys. 2005, 38, 1118.

(29)

Reid, M. S.; Villalobos, M.; Cranston, E. D., Benchmarking Cellulose Nanocrystals: From the Laboratory to Industrial Production. Langmuir 2017, 33, 1583-1598.


27


(30)

Page 28 of 30

Haywood, A. D.; Weigandt, K. M.; Saha, P.; Noor, M.; Green, M. J.; Davis, V. A., New Insights into the Flow and Microstructural Relaxation Behavior of Biphasic Cellulose Nanocrystal Dispersions from RheoSANS. Soft Matter 2017, 13, 8451-8462.

(31)

Wang, P. X.; Hamad, W. Y.; MacLachlan, M. J., Structure and Transformation of Tactoids in Cellulose Nanocrystal Suspensions. Nat. Commun. 2016, 7, 11515.

(32)

Xu, Y., Atrens, A. D., Stokes, J. R., “Liquid, Gel and Soft Glass” Phase Transitions and Rheology of Nanocrystalline Cellulose Suspensions as a Function of Concentration and Salinity. Soft Matter 2018, 14, 1953-1963.

(33)

Urena-Benavides, E. E.; Ao, G.; Davis, V. A.; Kitchens, C. L., Rheology and Phase Behavior of Lyotropic Cellulose Nanocrystal Suspensions. Macromolecules 2011, 44, 8990-8998.

(34)

Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A., Capillary Flow as the Cause of Ring Stains from Dried Liquid Drops. Nature 1997, 389, 827-829.

(35)

Gençer, A.; Schütz, C.; Thielemans, W., Influence of the Particle Concentration and Marangoni Flow on the Formation of Cellulose Nanocrystal Films. Langmuir 2016, 33, 228234.

(36)

Honorato Rios, C.; Kuhnhold, A.; Bruckner, J.; Dannert, R.; Schilling, T.; Lagerwall, J. P. F., Equilibrium Liquid Crystal Phase Diagrams and Detection of Kinetic Arrest in Cellulose Nanocrystal Suspensions. Front. Mater. 2016, 3, 21.

(37)

Zola, R. S.; Evangelista, L.; Yang, Y. C.; Yang, D. K., Surface Induced Phase Separation and Pattern Formation at the Isotropic Interface in Chiral Nematic Liquid Crystals. Phys. Rev. Lett. 2013, 110, 057801.


28

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(38)


Mu, X.; Gray, D. G., Droplets of Cellulose Nanocrystal Suspensions on Drying Give Iridescent 3-D “Coffee-Stain” Rings. Cellulose 2015, 22, 1103-1107.

(39)

Dierking, I., Chiral Liquid Crystals: Structures, Phases, Effects. Symmetry 2014, 6, 444-472.

(40)

Zapotocky, M.; Ramos, L.; Poulin, P.; Lubensky, T.; Weitz, D., Particle-Stabilized Defect Gel in Cholesteric Liquid Crystals. Science 1999, 283, 209-212.

(41)

Roman, M.; Gray, D. G., Parabolic Focal Conics in Self-Assembled Solid Films of Cellulose Nanocrystals. Langmuir 2005, 21, 5555-5561.

(42)

Huang, C. Y.; Stott, J. J.; Petschek, R. G., Routes to Self-Assembling Stable Photonic BandGap Phases in Emulsions of Chiral Nematics with Isotropic Fluids. Phys. Rev. Lett. 1998, 80, 5603.

(43)

Ličen, M.; Majaron, B.; Noh, J.; Schütz, C.; Bergström, L.; Lagerwall, J.; Drevenšek-Olenik, I., Correlation between Structural Properties and Iridescent Colors of Cellulose Nanocrystalline Films. Cellulose 2016, 23, 3601-3609.

(44)

Yang, Y. C.; Kee, C. S.; Kim, J. E.; Park, H. Y.; Lee, J. C.; Jeon, Y. J., Photonic Defect Modes of Cholesteric Liquid Crystals. Phys. Rev. E 1999, 60, 6852.

(45)

Hodgkinson, I. J.; Wu, Q. H.; Thorn, K. E.; Lakhtakia, A.; McCall, M. W., Spacerless Circular-Polarization Spectral-Hole Filters Using Chiral Sculptured Thin Films: Theory and Experiment. Opt. Commun. 2000, 184, 57-66.

(46)

Livolant, F.; Giraud, M.; Bouligand, Y., Goniometric Effect Observed in Sections of Twisted Fibrous Materials. Biol. Cellul. 1978, 31, 159-168.


29


(47)

Page 30 of 30

Davis, V. A., Liquid Crystalline Assembly of Nanocylinders. J. Mater. Res. 2011, 26, 140153.

(48)

Lagerwall, J.; Scalia, G.; Haluska, M.; Dettlaff‐Weglikowska, U.; Roth, S.; Giesselmann, F., Nanotube Alignment Using Lyotropic Liquid Crystals. Adv. Mater. 2007, 19, 359-364.

(49)

Hamley, I. W., Nanotechnology with Soft Materials. Angew. Chem. Int. Ed. 2003, 42, 16921712.

(50)

Dierking, I.; Scalia, G.; Morales, P.; LeClere, D., Aligning and Reorienting Carbon Nanotubes with Nematic Liquid Crystals. Adv. Mater. 2004, 16, 865-869.

Table of Contents Image


30

Photonic Properties and Applications of Cellulose Nanocrystal Films

Recommend Documents