Anisotropic optical film embedded with cellulose nanowhisker

Carbohydrate Polymers 130 (2015) 448–454

Contents lists available at ScienceDirect

Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

Anisotropic optical film embedded with cellulose nanowhisker Dah Hee Kim, Young Seok Song ∗ Department of Fiber System Engineering, Dankook University, Jukjeon-dong, Yongin, Gyeonggi-do, Korea

a r t i c l e

i n f o

Article history: Received 18 March 2015 Received in revised form 6 May 2015 Accepted 8 May 2015 Available online 21 May 2015 Keywords: Cellulose Nanowhisker Orientation

a b s t r a c t We investigated anisotropic optical behaviors of composite films embedded with CNWs. To control the orientation of CNWs, elongation was applied to the composite film. Morphological and mechanical analyses of the specimens were carried out to examine the influence of the applied extension. The CNWs were found to be aligned in the elongated direction, yielding remarkable anisotropic microstructure and optical properties. As the applied elongation and CNW loading increased, the resulting degree of polarization and birefringence increased due to increased interactions between the embedded particles. This study suggests a way to prepare an anisotropic optical component with nanoparticles of which the microstructures, such as orientation and filler content, can be controlled. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Cellulose is a fascinating renewable and abundant natural material offering manifold advantages in production cost, physical and chemical properties, and production security (Brahmakumar, Pavithran, & Pillai, 2005; Dumanli et al., 2014b; Iwatake, Nogi, & Yano, 2008; Jacob & Thomas, 2008; Jang, Jeong, Oh, Youn, & Song, 2012; Siro & Plackett, 2010). Recently, cellulose-based nanomaterials, including cellulose nanowhisker (CNW) and microfibrillated cellulose (MFC), have attracted growing interest for engineering advanced functional materials in many applications, such as structural, optical, biomedical, and energy areas (Cranston & Gray, 2006; Li, Wu, Song, Qing, & Wu, 2015b; Lin & Dufresne, 2014). In particular, CNWs possess compelling features, such as high specific modulus, low thermal expansion coefficient, unique morphology, high aspect ratio, optical transparency, negative anisotropic diamagnetic susceptibility, biocompatibility, and inherent sustainability (Dumanli et al., 2014a; Lagerwall et al., 2014; Wicklein et al., 2015). Furthermore, this natural nanoparticle can allow the manifestation of structural colors based on photonic crystal structures of biological matter and open a bio-inspired route for developing new multifunctional material systems. They can be incorporated into a wide range of polymers due to their relatively flexible surface chemistry (Goffin, Habibi, Raquez, & Dubois, 2012; Pan, Hamad, & Straus, 2010; Ten, Bahr, Li, Jiang, & Wolcott, 2012). CNWs are generally prepared through sulfuric acid hydrolysis from various plant products (e.g., wood, hemp, sisal, cotton, and ramie),

∗ Corresponding author. Tel.: +82 31 8005 3567; fax: +82 31 8005 3561. E-mail address: [email protected] (Y.S. Song). http://dx.doi.org/10.1016/j.carbpol.2015.05.033 0144-8617/© 2015 Elsevier Ltd. All rights reserved.

sea animals, and bacteria (Kim, Kang, & Song, 2013; Li et al., 2010; Nakagaito & Yano, 2008; Shafiei-Sabet, Hamad, & Hatzikiriakos, 2013). A CNW suspension with lyotropic liquid crystals can be used to prepare a film with optical chirality, due to the D-glucose building blocks of CNWs. Rod-like CNWs spontaneously produce left-handed chiral nematic structure (or cholesteric structure) at a critical concentration according to Onsager (Eichhorn, 2011). The cholesteric structure encompasses stacked planes of nematic CNWs aligned along a director with the orientation twisted from one plane to the next in an anticlockwise fashion (Kelly, Giese, Shopsowitz, Hamad, & MacLachlan, 2014). Also, the chiral nematic structure, the pitch of which is of the order of the wavelength of visible light, is capable of reflecting left-handed circularly polarized light, in general, across the near-IR and visible spectrum (Kelly, Shopsowitz, Ahn, Hamad, & MacLachlan, 2012). These attractive optical features enable applications such as security papers and chiral templates because they cannot be reproduced by photocopying (Pan, Hamad, & Straus, 2010). On the other hand, circular dichroism (CD), referring to the difference in absorption of left and right circularly polarized lights, and polarized microscopy measurements are used to examine chiral nematic structures embodied in the material (Shopsowitz, Qi, Hamad, & Maclachlan, 2010). Controlling the orientation of CNWs is essential for designing and manufacturing novel nanoparticle-embedded composites with desirable optical and mechanical characteristics. For example, unlike isotropic spherical nanoparticles, the helical self-assembly characteristics of CNWs allows a material to have more complex internal structure with chirality and more enhanced mechanical properties. When a particle is subjected to an external field, it can be orientated relying on the applied field direction. For

D.H. Kim, Y.S. Song / Carbohydrate Polymers 130 (2015) 448–454

this, shear force (Ebeling et al., 1999; Li et al., 2015a), electric (Bordel, Putaux, & Heux, 2006), and magnetic fields (Kvien & Oksman, 2007; Sugiyama, Chanzy, & Maret, 1992) have been used. CNWs tend to align along an applied shear force and electric field direction but orient in a direction perpendicular to a magnetic field direction due to their negative diamagnetic anisotropy. Compared with the physical behavior of helically oriented CNWs, that of unidirectionally aligned CNWs under external fields still remains poorly understood and needs to be investigated in a more systematic manner. In this study, elongation was used to unidirectionally align CNWs in a water soluble polymer, polyvinyl alcohol (PVA). We investigated the effects of applied elongation and concentration of CNWs on the mechanical and optical properties of CNW composite films. To our knowledge, this is the first report applying elongation to orient CNWs unidirectionally in a polymer and exploring the resulting degree of polarization and birefringence to look into the possibility of using the material as an optical element. As well as the influence of the draw ratio, we exploited the effect of the CNW concentration on the physical properties of the films. 2. Experimental 2.1. Materials and preparation of cellulose nanowhisker Microcrystalline cellulose (MCC) power was obtained from Acros Organics; the average particle diameter was 50 ␮m. Sulfuric acid and filter paper with a pore size of 400 nm were purchased from Ducksan Chemical (Korea) and Whatman (USA), respectively. To isolate cellulose nanowhisker (CNW), acid hydrolysis of MCC was carried out with a sulfuric acid of 64 wt% at 45 ◦ C for 2 h. The CNW suspension was sonicated in an ice bath and centrifuged. The supernatant was removed, followed by the addition of water. The suspension was diluted several times until an appropriate pH of the suspension was reached. When the suspension was stabilized, the supernatant was filtered and freeze-dried. The PVA used in this study had a molecular weight of 85,000–124,000 and a melting temperature of 200 ◦ C. CNW/PVA suspension was cast and dried on a petri dish. To align CNWs in the polymer matrix, the CNWs/PVA films were extended. Different draw ratios were considered. Here, the draw ratio means the ratio of the length of the undrawn film to that of the drawn film. 2.2. Morphological characterization The size of CNW was characterized using a Nano DS particle size analyzer (Cilas, France), which uses dynamic light scattering and static light scattering in a single optical system. Red and blue light wavelengths were employed, and the size distribution of CNW was determined based on the Mie scattering theory. Transmission electron microscope (TEM) measurement was carried out using JEM-200CX (JEOL). For the observation of CNWs, a droplet of the CNW suspension was deposited on a 200-mesh TEM grid, and carbon-coated. The applied voltage was 200 kV. The ImageJ software was used to measure the dimensions of the CNW. The cross-section of specimens was observed morphologically using scanning electron microscopy (JEOL, JSM-5410LV). Before the observation, all the specimens were coated with platinum using an ion sputter coater (JEOL, JFC-1100E). 2.3. Mechanical and optical characterization Mechanical properties such as tensile strength, Young’s modulus, and elongation at break were evaluated using a universal testing machine (UTM, Instron 3365) according to ASTM D638.

449

Dumbbell-shaped film specimens with a thickness of 0.2 ␮m were prepared and the elongation rate was 5 mm/min. A polarized optical microscope (Leitz-orthoplan, Leica) was used to obtain photomicrographs of CNW suspensions. The transmittance of the samples was measured at a wavelength in the range of 250–1000 nm using a UV–visible spectrometer (Lambda 1050, Perkin-Elmer). The degree of polarization (DP), a measure of the extent to which the light is polarized, was computed as follows: DP (%) =

Tmax − Tmin × 100 Tmax + Tmin

where, Tmax and Tmin are the maximum and minimum transmittances of light. X-ray diffraction experiments of the CNW/PVA films were carried out using D/MAX-2500 (Rigaku) at 40 kV and 45 mA with Cu K␣ radiation (wavelength = 0.154 nm) in the 2 range of 0–40◦ with a step interval of 0.02◦ .(Yang et al., 2013) The birefringence of PVA film was measured with a Leitz orthoplan polarizing microscope (Leica, Germany). Retardation of PVA films was determined with a B-type Berek compensator. The thickness of samples was 30–75 ␮m. The birefringence was calculated using the following equation:  = n × d where,  is the retardation, n is the birefringence, and d is the sample thickness. Circular dichroism (CD) experiments were conducted using a Jasco J-815 Spectropolarimer. Samples were scanned at 20 nm/min with a step resolution of 0.2 and 1 nm bandwidth. The light source was a xenon lamp covering the spectral range of 170–850 nm. 3. Results and discussion Morphological characteristics of the CNW suspension are demonstrated in Fig. 1. Fig. 1a presents a photograph of the CNW suspension. Birefringence is materialized in the CNW suspension when CNWs act as a liquid crystal material with a stable and homogeneous dispersion. The birefringence pattern between two crossed polarizers is presented in Fig. 1b. The anisotropy leads to a birefringent glassy-like state with iridescent domains. The polarized optical microscopy image of the suspension is displayed in Fig. 1c. The concentration of CNWs was 0.5 wt%. The CNW suspension exhibits liquid crystalline behavior above a critical concentration, consistent with Onsager’s theory for rigid rod-like particles (Onsager, 1949). As the concentration of CNWs increases, the suspension state changes from an isotropic phase with a random arrangement of CNWs, a chiral nematic phase, and a gel (Edgar & Gray, 2002). The CNW suspension encompasses anisotropic domains with the chiral nematic phase. Fig. 2a shows the TEM image of CNWs. The size distribution of CNWs is presented in Fig. 2b. The isolated CNWs have an average length of 440 nm and diameter of 45 nm, giving an average aspect ratio of around 10. Fig. 3 demonstrates the polarized optical microscopic images of CNW/PVA films. Similar to the birefringence images of the CNW suspension, interference colors were observed in the films. When the plane polarized light passes through CNWs with different refractive indices in their longitudinal and transverse directions in the polarized light microscopy, it is split into slow and fast lights in the CNW film, thereby generating a phase difference in the lights. When the lights pass through the rear polarizer, they interfere destructively or constructively, according to the phase difference induced by the birefringent CNW (Ross et al., 1997). Fig. 4a–c exhibit the mechanical results of the CNW films: tensile strength, Young’s modulus, and elongation at break as a function of the CNW content. Unlike conventional filler-reinforced

450

D.H. Kim, Y.S. Song / Carbohydrate Polymers 130 (2015) 448–454

Fig. 1. Morphological observation of CNW suspension: (a) photograph, (b) polarized light image, and (c) polarized optical microscopy image. The scale bar indicates 40 ␮m.

Fig. 2. (a) transmission electron microscopy (TEM) image and (b) size distribution of CNWs of CNWs. The scale bar indicates 0.2 ␮m.

Fig. 3. Polarized optical microscopic images of CNWs/PVA films: (a) 1 wt% CNW, (b) 2 wt% CNW, (c) 5 wt% CNW, and (d) 7 wt% CNW content.

D.H. Kim, Y.S. Song / Carbohydrate Polymers 130 (2015) 448–454

451

Fig. 4. Mechanical properties of CNWs/PVA films: (a) tensile strength, (b) Young’s modulus, and (c) elongation at break as a function of the CNW content. SEM images of CNWs/PVA films: (d) pure PVA, (e) 1 wt% CNW, (f) 2 wt% CNW, (g) 5 wt% CNW, and (h) 7 wt% CNW content.

composites showing generally a decrease in elongation at break with respect to the filler content, the three mechanical properties were all enhanced significantly with increasing the filler concentration, up to the addition of 5 wt% CNWs. In particular, the composite film with a CNW content of 5 wt% showed outstanding mechanical properties compared with the pure PVA film. However, the incorporation of 7 wt% CNWs resulted in a decrease in the tensile strength, Young’s modulus, and elongation at break. This may be due to aggregation of CNW particles at a high filler concentration. The SEM images of the fractured CNW films are shown in Fig. 4d–h. There existed agglomerated particles in Fig. 4h, which may help explain the mechanical results for the 7 wt% CNWs embedded composite film. The UV-visible transmittances of the specimens are presented in Fig. 5a–b. While the transmittance of the composite films diminished with increasing the CNW content, the film even with 7 wt% loading of CNWs showed considerable transmittance. As the draw ratio increased, the corresponding transmittance increased. The inset shows photographs of the prepared composite

films placed on a background paper to demonstrate their transparency. Fig. 6a–b present the degree of polarization as a function of wavelength, which shows the opposite results to the transmittance results. That is, the degree of polarization of the composite films increased with increasing the CNW loading but decreased with increasing the draw ratio. There was a maximum value for the degree of polarization, at around 550 nm, which may be related to the dimension of CNW. Fig. 7 presents the wide-angle X-ray diffraction spectrum (WAXS) patterns of pure PVA film and the nanocomposite films with different CNW contents. The PVA film showed a diffraction peak at 19.4◦ corresponding to the crystalline phase of PVA (Salavagione, Martinez, & Gomez, 2009). The spectra of the composite films showed two characteristic peaks of CNWs, at 14.8◦ and 22.6◦ (Yang et al., 2013). Considering the increase in the mechanical properties, some interactions between PVA and CNWs due to the hydrogen bonding exists. The WAXS photographs are shown in Fig. 7b–f. They revealed that the higher loading of CNWs resulted

Fig. 5. UV–visible transmittance: (a) effect of the CNW content and (b) effect of the draw ratio. A draw ratio of 3 and a CNW content of 7 wt% were applied for the results, respectively. The inset indicates photographs the PVA/CNW nanocomposite films placed on a background paper for demonstrating their transparency.

452

D.H. Kim, Y.S. Song / Carbohydrate Polymers 130 (2015) 448–454 6

7

a

b 6

4

3 pure PVA CNW 1wt% CNW 2wt% CNW 5wt% CNW 7wt%

2

1

Degree of polarization (%)

Degree of polarization (%)

5

5

4

3 Draw ratio 1 Draw ratio 2 Draw ratio 3

2

1 400

500

600

λ (nm)

700

400

500

600

700

λ (nm)

Fig. 6. Degree of polarization: (a) effect of the CNW content and (b) effect of the draw ratio. For the results, a draw ratio of 3 and a CNW content of 7 wt% were applied, respectively.

Fig. 7. Wide angle X-ray diffraction spectra of PVA/CNW films with respect to (a) the CNW content and wide Angle X-ray diffraction photographs of (a) pure PVA, (b) PVA/CNW 1%, (c) PVA/CNW 2%, (d) PVA/CNW 5%, and (e) PVA/CNW 7%.

Fig. 8. Wide angle X-ray diffraction spectra of PVA/CNW films with respect to (a) draw ratio and wide angle X-ray diffraction photographs of (b) draw ratio 1, (c) draw ratio 2, and (d) draw ratio 3.

D.H. Kim, Y.S. Song / Carbohydrate Polymers 130 (2015) 448–454

the pitch, P, of chiral nematic structure, which is the distance necessary for one rotation cycle of the director vectors. The pitch is associated with the wavelength of maximum reflection, , as follows:  = nP, where, n is the average refractive index.(Pan, Hamad, & Straus, 2010) As shown in Fig. 10, the CNW-embedded films with higher concentrations showed higher CD. The interaction between embedded particles increases with an increase in the concentration of CNWs, thus enhancing the anisotropic optical properties. These results indicate that the composite films form left-handed chiral nematic structure, as expected. Considering the transmittance in Fig. 5a and the CD result that have the same maximum peak value (Kelly, Giese, Shopsowitz, Hamad, & MacLachlan, 2014), the CNW composite films prepared in this study may possess a pitch length smaller than 300 nm.

0.035 Draw ratio 2 Draw ratio 3

0.030

Birefringence

0.025

0.020

0.015

0.010

0.005

0.000 0

1

2

5

4. Conclusions

7

CNW (wt%) Fig. 9. Birefringence of PVA/CNW films with respect to the CNW content.

in the better orientation of CNWs. Fig. 8 explains the effect of draw ratio on the WAXS. Similar to the results of Fig. 7, the characteristic peaks of the CNW/PVA films were observed. As the draw ratio increased, the alignment of CNWs increased, due to the applied elongation. Fig. 9 presents the birefringence results of the specimens with respect to the CNW concentration. The birefringence increased as the CNW loading and draw ratio increased. This finding can be explained by the following. First, CNWs have anisotropic refractive indices: i.e., n1 = 1.681 in the longitudinal direction and n2 = 1.544 in the transverse direction (Wertz, Mercier, & Bédué, 2010). Second, there is a preferred orientation of the PVA molecules as a result of the applied extension. The birefringence results may allow the CNW-embedded composite film to be used as an optical element, such as a wave plate. Additionally, the higher draw ratio yields more orientation of the molecules. Fig. 10 shows the circular dichroism (CD) of the composite films as a function of wavelength. Circularly polarized light passing through an optically active material is differentially absorbed, leading to differences in the speeds, wavelength, and absorbance of right- and left-handed circularly polarized lights. The optically active material – i.e., the chiral nematic structure of the samples – can be analyzed using CD at a macroscopic level. The CD signal, the discrepancy between the absorptions of the left- and right-handed circularly polarized lights, provides information on 200

In the current study, we prepared thin composite films incorporated with unidirectionally orientated CNWs and investigated their optical properties. To align CNWs in one direction, elongation was applied to the composite film. The mechanical properties and morphology of the specimens were explored to figure out the influence of the elongation. It revealed that the CNWs were aligned along the elongational direction, thereby leading to outstanding anisotropy in microstructure and optical properties. In addition, the influence of the CNW loading was investigated experimentally. The degree of polarization and birefringence were found to increase with increasing the elongation and CNW loading due to the increased interactions between the embedded particles. This result may offer the possibility of using CNW-incorporated composites as an optical element. This study is expected to provide meaningful insights into composite films embedded with unidirectionally aligned CNWs as a result of elongation. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013R1A1A2059827). Also, this work was partially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2014R1A2A1A11053897). This work was partially supported by the Center for Advanced Meta-Materials (CAMM) funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CAMM-no. 2014M3A6B3063707). 40

a

20

pure PVA CNW 1wt% CNW 2wt% CNW 5wt% CNW 7wt%

150

100

b

0 -20

Ellipticity (mdeg)

Ellipticity (mdeg)

453

50

0

-40 -60 -80 pure PVA CNW 1wt% CNW 2wt% CNW 5wt% CNW 7wt%

-100 -120

-50

-140

-100

-160

300

400

Wavelength (nm)

500

600

300

400

Wavelength (nm)

Fig. 10. Circular dichroism as a function of wavelength: (a) transmittance and (b) reflection.

500

600

454

D.H. Kim, Y.S. Song / Carbohydrate Polymers 130 (2015) 448–454

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbpol.2015.05. 033

References Bordel, D., Putaux, J. L., & Heux, L. (2006). Orientation of native cellulose in an electric field. Langmuir, 22(11), 4899–4901. Brahmakumar, M., Pavithran, C., & Pillai, R. M. (2005). Coconut fibre reinforced polyethylene composites: Effect of natural waxy surface layer of the fibre on fibre/matrix interfacial bonding and strength of composites. Composites Science and Technology, 65, 563–569. Cranston, E. D., & Gray, D. G. (2006). Morphological and optical characterization of polyelectrolyte multilayers incorporating nanocrystalline cellulose. Biomacromolecules, 7, 2522–2530. Dumanli, A. G., Kamita, G., Landman, J., Kooij, H., Glover, B. J., Baumberg, J. J., et al. (2014). Controlled, bio-inspired self-assembly of cellulose-based chiral reflectors. Advanced Optical Materials, 2, 646–650. Dumanli, A. G., van der Kooij, H. M., Kamita, G., Reisner, E., Baumberg, J. J., Steiner, U., et al. (2014). Digital color in cellulose nanocrystal films. ACS Applied Materials & Interfaces, 6(15), 12302–12306. Ebeling, T., Paillet, M., Borsali, R., Diat, O., Dufresne, A., Cavaille, J. Y., et al. (1999). Shear-induced orientation phenomena in suspensions of cellulose microcrystals, revealed by small angle X-ray scattering. Langmuir, 15, 6123–6126. Edgar, C. D., & Gray, D. G. (2002). Influence of dextran on the phase behavior of suspensions of cellulose nanocrystals. Macromolecules, 35, 7400–7406. Eichhorn, S. J. (2011). Cellulose nanowhiskers: Promising materials for advanced applications. Soft Matter, 7, 302–315. Goffin, A. L., Habibi, Y., Raquez, J. M., & Dubois, P. (2012). Polyester-grafted cellulose nanowhiskers: A new approach for tuning the microstructure of immiscible polyester blends. ACS Applied Materials & Interfaces, 4, 3364–3371. Iwatake, A., Nogi, M., & Yano, H. (2008). Cellulose nanofiber-reinforced polylactic acid. Composites Science & Technology, 68, 2103–2106. Jacob, M., & Thomas, S. (2008). Biofibers and biocomposites. Carbohydrate Polymers, 71, 343–364. Jang, J. Y., Jeong, T. K., Oh, H. J., Youn, J. R., & Song, Y. S. (2012). Thermal stability and flammability of coconut fiber reinforced poly(lactic acid) composites. Composites Part B, 43, 2434–2438. Kelly, J. A., Giese, M., Shopsowitz, K. E., Hamad, W. Y., & MacLachlan, M. J. (2014). The development of chiral nematic mesoporous materials. Accounts of Chemical Research, 47(4), 1088–1096. Kelly, J. A., Shopsowitz, K. E., Ahn, J. M., Hamad, W. Y., & MacLachlan, M. J. (2012). Chiral nematic stained glass: Controlling the optical properties of nanocrystalline cellulose-templated materials. Langmuir, 28(50), 17256–17262. Kim, D. H., Kang, H. J., & Song, Y. S. (2013). Rheological and thermal characteristics of three-phase eco-composites. Carbohydrate Polymers, 92, 1006–1011.

Kvien, I., & Oksman, K. (2007). Orientation of cellulose nanowhiskers in polyvinyl alcohol. Applied Physics A, 87, 641–643. Lagerwall, J. P., Schutz, C., Salajkova, M., Noh, J., Park, J. H., Scalia, G., et al. (2014). Cellulose nanocrystal-based materials: From liquid crystal self-assembly and glass formation to multifunctional thin films. NPG Asia Materials, 6, e80. Li, D., Liu, Z., Al-Haik, M., Tehrani, M., Murray, F., Tannenbaum, R., et al. (2010). Magnetic alignment of cellulose nanowhiskers in an all-cellulose composite. Polymer Bulletin, 65, 642–653. Li, M. C., Wu, Q., Song, K., Lee, S., Qing, Y., & Wu, Y. (2015). Cellulose nanoparticles: Structure–morphology–rheology relationships. ACS Sustainable Chemistry & Engineering, 3, 821–832. Li, M. C., Wu, Q., Song, K., Qing, Y., & Wu, Y. (2015). Cellulose nanoparticles as modifiers for rheology and fluid loss in bentonite water-based fluids. ACS Applied Materials & Interfaces, 7, 5006–5016. Lin, N., & Dufresne, A. (2014). Nanocellulose in biomedicine: Current status and future prospect. European Polymer Journal, 59, 302–325. Nakagaito, A. N., & Yano, H. (2008). Toughness enhancement of cellulose nanocomposites by alkali treatment of the reinforcing cellulose nanofibers. Cellulose, 15, 323–331. Onsager, L. (1949). The effects of shape on the interaction of colloidal particles. Annals of the New York Academy of Sciences, 51, 627–659. Pan, J., Hamad, W., & Straus, S. K. (2010). Paramters affecting the chiral nematic phase of nanocrystalline cellulose films. Macromolecules, 43, 3851–3858. Ross, S., Newton, R., Zhou, Y. M., Haffegee, J., Ho, M. W., Bolton, J. P., et al. (1997). Quantitative image analysis of birefringent biological material. Journal of Microscopy, 187, 62–67. Salavagione, H. J., Martinez, G., & Gomez, M. A. (2009). Synthesis of poly(vinyl alcohol)/reduced graphite oxide nanocomposites with improved thermal and electrical properties. Journal of Materials Chemistry, 19, 5027–5032. Shafiei-Sabet, S., Hamad, W. Y., & Hatzikiriakos, S. G. (2013). Influence of degree of sulfation on the rheology of cellulose nanocrystal suspension. Rheologica Acta, 52, 741–751. Shopsowitz, K. E., Qi, H., Hamad, W. Y., & Maclachlan, M. J. (2010). Freestanding mesoporous silica films with tunable chiral nematic structures. Nature, 468(7322), 422–425. Siro, I., & Plackett, D. (2010). Microfibrillated cellulose and new nanocomposite materials: A review. Cellulose, 17, 459–494. Sugiyama, J., Chanzy, H., & Maret, G. (1992). Orientation of cellulose microcrytals by strong magnetic fields. Macromolecules, 25, 4232–4234. Ten, E., Bahr, D. F., Li, B., Jiang, L., & Wolcott, M. P. (2012). Effects of cellulose nanowhiskers on mechanical, dielectric, and rheological properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/cellulose nanowhisker composites. Industrial & Engineering Chemistry Research, 51, 2941–2951. Wertz, J. L., Mercier, J. P., & Bédué, O. (2010). Cellulose science and technology. Lausanne: EPFL Press. Wicklein, B., Kocjan, A., Salazar-Alvarez, G., Carosio, F., Camino, G., Antonietti, M., et al. (2014). Thermally insulating and fire-retardant lightweight anisotropic foams based on nanocellulose and graphene oxide. Nature Nanotechnology, 10, 277–283. Yang, Z. Y., Wang, W. J., Shao, Z. Q., Zhu, H. D., Li, Y. H., & Wang, F. J. (2013). The transparency and mechanical properties of cellulose acetate nanocomposites using nanowhiskers as fillers. Cellulose, 20, 159–168.