Estimation of the Intrinsic Birefringence of Cellulose Using Bacterial

Letter Cite This: ACS Macro Lett. 2019, 8, 250−254

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Estimation of the Intrinsic Birefringence of Cellulose Using Bacterial Cellulose Nanofiber Films Kojiro Uetani,* Hirotaka Koga, and Masaya Nogi

ACS Macro Lett. Downloaded from pubs.acs.org by WASHINGTON UNIV on 02/24/19. For personal use only.

The Institute of Scientific and Industrial Research (ISIR), Osaka University, Mihogaoka 8-1, Ibaraki-shi, Osaka 567-0047, Japan ABSTRACT: The intrinsic birefringence of cellulose is one of the most fundamental optical parameters for analyzing and developing various cellulosic materials. However, the previously reported values greatly vary depending on the problems occurred due to the measured cellulose sample or method, and it is still a challenge to evaluate the intrinsic birefringence of cellulose using suitable cellulose samples and methodologies by taking account into the recent knowledge and techniques. Here, we estimated the intrinsic birefringence of cellulose to be 0.09 by a procedure with three valid factors: (1) bacterial cellulose nanofibers consisting of extended chain crystals of cellulose are used, (2) films with relatively small orientation degrees are fabricated, and (3) the in-plane retardation maps are measured. The resultant eigenvalue is greater than those reported for cellulose and many petroleum-based polymers. Therefore, cellulose could be used to develop highperformance light compensation films with large optical anisotropies for use in future optoelectronic devices. The birefringence of “ideally aligned cellulose I fibers” has been estimated to be 0.08 by analyzing chemically purified ramie fibers with very high X-ray orientation factors of ∼0.7− 0.9.14 Cellulose I crystals are believed to have the benefit of allowing the molecular conformation to be almost straight owing to their extended chain crystals, and the order parameters of their agglomerates have been precisely determined by two-dimensional X-ray diffraction (2D-XRD). However, a recent study raised doubts about the validity of a previous study14 because the order parameters of above 0.5− 0.6 for cellulose nanocrystal films derived by 2D-XRD differ from those derived from the linear dichroic ratio of the optical birefringence owing to the amorphous chains.15 Namely, XRD only gives orientation information about the crystals, whereas the optical birefringence gives information about the whole molecules. Therefore, it is still a challenge to evaluate the intrinsic birefringence of cellulose using suitable cellulose samples and methodologies. In this study, we experimentally estimated the intrinsic birefringence of cellulose using bacterial cellulose (BC) nanofiber films with various order parameters below 0.5 by 2D-XRD. BC is believed to be suitable as the cellulose I sample and the CNF orientation can be easily controlled by simple stretching treatment before drying.16 We used the general extrapolating method,17,18 which measures the birefringence Δn of BC samples with different orientational order parameters to linearly predict Δn for the ideal sample with the maximum orientation degree as the intrinsic birefringence Δn0. The

ensely packed films made of cellulose nanofibers (CNFs) are promising materials for optoelectronics, such as flexible displays and electronic paper, owing to their flexibility and high dimensional stability against the temperature change of each single CNF.1,2 For these optoelectronics, light compensation films, such as polarizers, phase difference films, and backlight films, with large optical anisotropy are essential to control the retardation (i.e., birefringence) owing to polymer chain alignment,3,4 and they are also indispensable to improve the contrast or reduce the rainbow unevenness and light leakage.5 CNF films are known to show some birefringence when the constitutive CNFs anisotropically align,6 and CNFs consisting of an extended chain crystal of cellulose molecules are also considered to have large birefringence.7,8 However, the intrinsic birefringence of cellulose, which is the origin to the birefringence of CNF films, greatly varies between cellulose samples,9 and its estimation represents a bottleneck for producing light compensation films with CNFs to develop high-performance next-generation optoelectronics. The cellulose molecule is considered to have large intrinsic birefringence because of the large anisotropy of the polarizability derived from its anisotropic molecular structure and high polarity.10,11 A theoretical study predicted the intrinsic birefringence to be ∼0.078 using the polarizability and density of cellulose.12 However, the intrinsic birefringence of cellulose has been estimated to be 0.013−0.034 or ∼0.055 using regenerated cellulose, with the large variation due to imperfect molecular alignment.9 Another study using regenerated cellulose reported different values, 13 so the intrinsic birefringence of cellulose is difficult to estimate using regenerated cellulose.

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© XXXX American Chemical Society

Received: January 11, 2019 Accepted: February 20, 2019

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DOI: 10.1021/acsmacrolett.9b00024 ACS Macro Lett. 2019, 8, 250−254

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ACS Macro Letters microscope-type birefringence measurement system was used to determine the in-plane retardation maps of the BC nanofiber films. The key point of our method is that we use the retardation mapping images taken by the birefringence measurement system as multipoint sensing media. The single image includes 384 × 288 pixels with retardation data for each pixel. We collected the retardation data of more than 110000 points mapped within the single images of ∼1 × ∼0.8 mm as individual measuring areas using a 5× objective lens. Multiple images were taken for each sample to detect the retardation distributions of the BC nanofiber films with high accuracy. The retardation color maps shift to red with increasing draw ratio, as shown in Figure 1. The retardation of the nanofiber film

Figure 2. Retardation histograms for BC nanofiber films with draw ratios of (a) 0%, (b) 5%, (c) 10%, (d) 15%, (e) 20%, and (f) 25%. Each graph contains five histograms corresponding to the different observing areas within the single films, except for (f), which contains three histograms.

to align the CNF makes the BC films have more uniform internal structures by reducing the nata-de-coco-derived variation. We calculated the arithmetic averages of retardation from each histogram, and the median values in the same specimens were determined to be the representative average retardation R values. The R values are 214.7, 142.3, 750.1, 1335.7, 1671.5, and 1190.2 nm for the films with draw ratios of 0%, 5%, 10%, 15%, 20%, and 25%, respectively. The average in-plane birefringence Δn was calculated by Δn = R/d, where d is the film thickness (40−60 μm). The relationship between Δn and the orientational order parameter S (see Experimental Section) for all of the samples is shown in Figure 3. Δn linearly increases with increasing S and there is a good linear relationship. The intrinsic birefringence of cellulose Δn0 corresponds to Δn for the ideal nanofiber film with S = 1. Therefore, linear fitting was

Figure 1. Retardation mapping images for aligned BC nanofiber films with draw ratios of (a) 0%, (b) 5%, (c) 10%, (d) 15%, (e) 20%, and (f) 25%. The aligning direction was set vertical to each image. The inset in (f) shows the color scale of retardation.

increases as the orientation degree of the CNFs increases. The retardation value of the film with a draw ratio of 25% is small because of the small thickness of the film. The quantitative retardation data contained in each image were extracted to construct histograms to evaluate the variation (Figure 2). Within each film, the five histograms (three histograms for the film with a draw ratio of 25%) at different view angles agree well, and all of the samples have low regional difference of retardation. The histograms for draw ratios of 0% (Figure 2a) and 5% (Figure 2b) are bimodal. As shown in Figure 1a,b, some parts have slightly greater retardation (i.e., light blue) than the other parts (i.e., cobalt blue). This is considered to be because BC hydrogels contain vertical crosslinking CNFs between the mille-feuille-like layered CNF networks19 and slight variation of the local amounts of nanofibers occurs when they are compressed. The retardation reflects the whole molecules on the light path, so the local distributions of nanofibers are sensitively detected in the mapping images. As the orientation degree increases, the retardation histograms shift to greater values and the bimodal distribution changes to monomodal. The stretching treatment

Figure 3. Relationship between the orientational order parameter S derived by 2D-XRD and the optical birefringence Δn. 251

DOI: 10.1021/acsmacrolett.9b00024 ACS Macro Lett. 2019, 8, 250−254

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ACS Macro Letters performed with direct weighting errors (coefficient of determination 0.997). The fitted line was then extrapolated to approximate Δn0. Weighting errors are beneficial when the measurement errors vary between the points. Δn0 was calculated to be 90.9 × 10−3. Although BC nanofibers have a high degree of crystallinity20 and an observable crystal lattice structure,21 they are still not a perfect crystal and they are believed to contain some disordered parts.22 Therefore, Δn of a single CNF is not strictly the same as Δn0. However, the method used in this study allows the slight effect of the disordered parts to be ignored because CNF films with relatively low orientation degrees (S < 0.5) give almost the same orientation factors by 2D-XRD and from the optical birefringence.15 We believe that because the estimation using the S range gives consistent S from 2D-XRD and the birefringence, Δn0 is close to the true value. The obtained Δn0 value is higher than previously reported values for cellulose (Table 1). Regarding the large Δn0

Figure 4. Relationship between Δn and the in-plane thermal conductivity (the data were obtained from ref 16) for the aligned BC nanofiber films.

The obtained Δn0 value of 0.09 for cellulose is relatively high among those of general petroleum-based polymers.23 The Δn0 values of cellulose and several petroleum-based polymers are shown in Figure 5. The Δn0 values of polyethylene,

Table 1. Comparison of the Reported Δn0 Values of Cellulose this work ref 9 ref 9 ref 9 ref 9 ref 12 ref 13 ref 13 ref 13 ref 14 ref 26 ref 27 ref 28

cellulose material

Δn0

bacterial CNFs ideally oriented cellulose I cotton ideally oriented cellulose II viscose rayon staple calculation regenerated cellulose regenerated cellulose regenerated cellulose purified ramie fiber cellulose II fiber ideally oriented regenerated cellulose fiber solvent spun cellulose fiber

0.090 0.047 0.045−0.062 0.055, 0.054 0.013−0.034 0.078 0.0624 0.086 0.055 0.080 0.081 0.056 0.061

Figure 5. Comparison of Δn0 of cellulose with various polymers, including poly(ethylene terephthalate) (PET), polycarbonate (PC), polypropylene (PP), polyethylene (PE), poly(vinyl chloride) (PVC), poly(methyl methacrylate) (PMMA), and polystyrene (PS).

difference when using cellulose I and II samples, we inferred that there are two reasons due to the imperfect alignment of cellulose molecules and the determination difficulty of the orientation degrees (order parameters) for cellulose II samples. First, the molecules in cellulose II crystals contain chain folding due to the antiparallel chain packing, and their comformation allow making the apparent anisotropy of polarizability smaller even if the crystallites fully aligned. Second, the orientational parameters for cellulose II with relatively low degrees of crystallinity have little choice to be indirectly estimated by the draw ratios or the optical circular dichroic analysis. These approximations might derive the large variations of maximum birefringence for cellulose II. The property-to-property relationships within a material described as Ashby plots are of importance for the application requiring multiple criteria. Interestingly, the optical birefringence of the BC nanofiber films is correlated with the thermal conductivity. Both Δn and the in-plane thermal conductivity proportionally increase with increasing S (Figure 4). The CNF alignment structure has been found to directly affect the movement of both photons and phonons within CNF films. In addition, the thermal expansion coefficient is also dependent on the CNF alignment.16 Our BC nanofiber films are peculiar materials because they simultaneously tune the optical birefringence, thermal expansion, and thermal conductivity.

polypropylene, Nylon-6, and poly(methyl methacrylate) are 0.026,17 0.066,17 0.078,18 and −0.004,24 respectively, although these values can vary depending on the measurement method.25 In conclusion, we have experimentally estimated the intrinsic birefringence of cellulose to be 0.09. Our approach contains three key factors: (1) We used CNFs consisting of extended chain crystals of cellulose I as the most suitable sample to determine the birefringence of fully extended cellulose molecules as an eigenvalue. (2) We used aligned CNF films with S < 0.5 to match the order parameters derived by 2DXRD and the birefringence method.15 (3) We used the inplane retardation mapping technique as the large-volume sensing media to evaluate the mean birefringence behavior. The in-plane retardation mapping measurements allow twodimensional analysis, which is believed to be more suitable to develop retardation films than traditional single-point determination methods.17,18 Our results and methodology could help facilitate the use of pristine cellulose/CNFs as light compensation materials with tunable large birefringence. 252

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EXPERIMENTAL SECTION

ACKNOWLEDGMENTS K.U. thanks Mr. J. Endo, of the Photonic Lattice Inc., for cooperation in measuring the retardation mapping images. We thank Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

Five specimens of aligned BC nanofiber films with draw ratios of 0%, 5%, 10%, 15%, and 20% used in a previous study16 were reused in this study. The BC pellicles produced by incubating Acetobacter xylinum (Fijicco Co., Ltd., Kobe, Japan) were purified through a 2 wt % NaOH aqueous solution at 90 °C and then mechanically compressed by a press machine without heating to remove the extra water. The products (∼4 cm × ∼15 cm) were then drawn with a tensile testing machine (Strograph VES5D, Toyo Seiki Seisaku-sho, Ltd., Tokyo, Japan) at a tensile rate of 10 mm min−1. The specimens were dried by hot pressing at 110 °C to obtain the drawn BC nanofiber films. We also newly prepared nine specimens of drawn BC films with a draw ratio of 25% in the same manner. The 2D-XRD measurements were outsourced to EAG Inc. (California, U.S.A.). They were performed in the same manner as a previous study.16 A Siemens/Bruker GADDS system with a Hi-Star detector and a Huber goniometer was used at 50 kV and 40 mA with Cu Kα radiation (λ = 1.54059 Å) to acquire two-dimensional diffraction frames five times for 200 s per frame, which were summed and averaged for better statistics without detector saturation issues. The orientational order parameter S was calculated from the fitted intensity of the Debye−Scherrer ring I at 2θ = 22.8° with respect to the azimuthal angle ϕ using the following equations:16 S=

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2

cos2 ϕc , z = 1 − 2 cos2 ϕ200, z q

∑ p I(ϕ)sin ϕ cos2 ϕ q

∑ p I(ϕ)sin ϕ

The in-plane retardation mapping images at a wavelength of 543 nm were taken with a microscope-type birefringence measurement system (WPA-micro, Photonic Lattice Inc., Miyagi, Japan) using a 5× objective lens and WPA-View software (version 2.4.6.2, Photonic Lattice Inc.). The retardation images of the translucent BC nanofiber films often contained noise owing to surface scattering. We then slightly defocused the image to obtain the optimal concentration of transmitted light and avoid noise in the images. The mean retardation value R was determined by averaging the 384 × 288 retardation data contained in each image. The birefringence of the BC nanofiber film was calculated by Δn = R/d, where d is the film thickness. Linear fitting analysis with direct weighting errors was performed with OriginPro version 2019 (OriginLab Corp., Northampton, MA, U.S.A.).

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REFERENCES

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3 cos2 ϕc , z − 1

cos2 ϕ200, z =

Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kojiro Uetani: 0000-0003-3245-6929 Hirotaka Koga: 0000-0001-6295-1731 Funding

This research was partly supported by JSPS KAKENHI (Grant No. 17K18169 to K.U.) of the Japan Society for the Promotion of Science (JSPS), the JST-Mirai R&D Program (Grant No. JPMJMI17ED to M.N.) of the Japan Science and Technology Agency (JST), and the Cooperative Research Program “CORE Lab.” (Grant No. 20186002 to H.K.) of the Network Joint Research Center for Materials and Devices: Dynamic Alliance for Open Innovation Bridging Human, Environment, and Materials. Notes

The authors declare no competing financial interest. 253

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