Cellulose

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Change in the Crystallite Orientation of Poly(ethylene oxide)/ Cellulose Nanofiber Composite Films Miki Noda Fukuya,*,†,‡,§ Kazunobu Senoo,† Masaru Kotera,∥ Mamoru Yoshimoto,⊥ and Osami Sakata‡,§ †

Corporate R&D Center, Sumitomo Bakelite Co., Ltd., 1-1-5 Murotani, Nishi-ku, kobe, Hyogo 651-2241, Japan Department of Innovative and Engineered Materials, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259-J3-16 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502, Japan § Symchrotron X-ray Station at SPring-8, National Institute for Materials Science (NIMS), 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan ∥ MORESCO Corporation, 5-5-3 minatojimaminamimachi, Chuo-ku, Kobe, Hyogo 650-0047, Japan ⊥ Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259-J3-16 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502, Japan ‡

S Supporting Information *

ABSTRACT: The crystallite orientation and crystallographic domain structure of poly(ethylene oxide) (PEO) in cellulose nanofiber-incorporated (CNF-incorporated) PEO films developed for packaging materials were observed using wide-angle X-ray diffraction for different CNF filling ratios. When a CNF filling ratio of 50 wt %, the PEO molecular chains were oriented in a direction parallel to the surface of the film. The fiber axis of the CNFs became parallel to the surface of the PEO/CNF composite film when the filling ratio was >25 wt %. The change in the orientation of the PEO crystals occurred because increasing the amount of CNF in the composite films decreased the space in which the PEO could be crystallized. Furthermore, the hydrogen bonds between the PEO and the CNF may behave as crystallization nuclei for the PEO. Our results thus pave the way toward the development of packaging materials that are more impermeable to gases than the current materials.

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INTRODUCTION Cellulose nanofibers (CNFs) derived from wood have interesting characteristics, such as a very small diameter (ca. 5 nm), high aspect ratio, and high modulus.1 A number of studies have added CNFs to polymers so as to improve their mechanical properties.2−4 One study found that the oxygen permeability of a polylactic acid film was reduced by a factor of roughly 700 when a coating of CNF was applied to its surface.5 Because poly(ethylene oxide) (PEO) is a hydrophilic polymer and forms hydrogen bonds with cellulose, PEO/CNF composite materials have been studied for use as biomaterials6,7 and for polymer electrolytes.8 Additionally, multilayered films composed of PEO have been shown to act as good oxygen barriers because the PEO crystals in those multilayered films were found to be highly oriented.9,10 The PEO crystallite orientation was observed in Laponite clay PEO composite films. The PEO crystallite orientation changed with the clay content.11 However, the crystallite orientation of PEO has not been studied in reports investigating PEO/CNF composite materials. We have previously focused attention on the fact that CNF and crystallite orientation are related to the oxygen barrier properties5,9,10 and have also previously developed packaging materials.12,13 In this report, before evaluating their physical properties (such as their barrier properties), we investigated the crystallite orientation and crystallographic domain structure of PEO/CNF composite films with CNF filling ratios of up to 75 wt %. © XXXX American Chemical Society

RESULTS AND DISCUSSION

Thermal Behavior. Figure 1 shows the first run differential scanning calorimetry (DSC) curve of each PEO/CNF composite film. The DSC curve of CNF (i.e., PEO/CNF = 0/100) exhibited no change in the heat flow. This indicates that any endothermic peaks found for the PEO/CNF composite films were due to the melting of the PEO crystallite regions.

Figure 1. DSC thermograms of the PEO/CNF composite films for 0, 10, 25, 50, and 75 wt % of CNF. Received: October 5, 2017 Revised: November 8, 2017

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DOI: 10.1021/acs.biomac.7b01434 Biomacromolecules XXXX, XXX, XXX−XXX

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and parallel to the surface of the film (Figure 2b). In the 50 wt % PEO/CNF composite film, the 1 2 0 reflections appeared only in the meridian direction; as such, the molecular chains were found to be oriented only in the direction parallel to the surface of the film (Figure 2c). In the 75 wt % PEO/CNF composite film, the reflections from the PEO crystals were found to have disappeared, which agreed with the result obtained from the DSC curve (Figure 1). However, the 1 1 0 and 1 1̅ 0 reflections of cellulose were oriented in the meridian direction in the edge-view patterns when the filling ratio was >25 wt %. It was found that the fiber axis of the CNFs was mostly oriented in a direction parallel to the surface of the film. Furthermore, the CNFs were randomly located on the surface of the films because the reflections from the CNFs were ringshaped in the through-view pattern when the filling ratio was >50 wt %. Supporting Information (SI) Figure 1 shows an atomic force microscopy (AFM) image of the surface of the 75 wt % PEO/ CNF composite film. In SI Figure 1 it can be seen that the surface of the 75 wt % PEO/CNF composite film consisted of a random CNF network. This result was consistent with the WAXD result shown in Figure 2. Moreover, the AFM and WAXD results, which prove the surface of the material and the inside of the material, respectively, demonstrate that the CNFs were randomly oriented both on the surface and in the bulk of the film. Figure 3 shows the PEO crystallite size of (1 2 0) estimated using the Sherrer equation14 as a function of the CNF filling ratio. Details of the estimated method are described in the SI. We found that the crystallite sizes of the PEO became smaller and the crystallinity decreased as the CNF filling ratio increased. We therefore expected that the network structure of the CNFs would be dense and that the crystallized area of the PEO molecular chains would be restricted. It has been reported that crystallization was restricted when an ultrathin film condition was used for PEO.15 There were two peaks in the DSC curve of the 25 wt % PEO/CNF composite film, which indicated that there were two different PEO crystal orientations in this film. We therefore recorded the edge-view WAXD patterns of the 10 and 25 wt % PEO/CNF composite films at each temperature.

The degree of crystallinity of PEO (Xc) is shown in Table 1. The Xc decreased compared with Xc in the PEO film once the Table 1. Crystallinity of the PEO in the PEO/CNF Composite Films for Different CNF Filling Ratios CNF filling fraction (wt %)

Xc (%)

0 10 25 50 75

68 69 43 12 0

CNF filling ratio in the PEO/CNF film reached 25 wt %. Furthermore, the PEO became almost amorphous at a CNF filling ratio of 75 wt %. This implies that the crystallization of the PEO was inhibited by the addition of large amounts of CNF. It was also found that the melting point decreased as the CNF filling ratio increased and two peaks corresponding to melting points appeared for the 25 wt % composite film (Figure 1). We therefore determined that there were two types of crystal that formed and that they had different melting points. Crystallite Orientation of the PEO/CNF Composite Films. Figure 2 displays 2D diffraction patterns of the PEO and PEO/CNF composite films. In the PEO film (i.e., CNF filling ratio of 0 wt %), the inner circle and arc consisted of reflections from the (1 2 0) lattice planes, and the outer circle and arc were defined as the (0 3 2) lattice planes. In the PEO film, the circles can be seen in the through- and edge-view patterns. This means that most of the PEO crystals were randomly arranged. In the edge-view pattern of the 10 wt % PEO/CNF composite film, the 1 2 0 reflections appeared in the equatorial direction. The (1 2 0) lattice planes were parallel to the molecular chains of the PEO crystallite regions. Therefore, the molecular chains of the PEO in this composite were oriented in a direction perpendicular to the surface of the film (Figure 2a). In the edge-view pattern of the 25 wt % PEO/CNF composite film, the 1 2 0 reflections appeared in both the equatorial and meridian directions. We found that there were two kinds of PEO crystallite orientations in which the molecular chains of the PEO crystallite regions were oriented both perpendicular

Figure 2. 2D WAXD patterns of the PEO and PEO/CNF composite films for 0, 10, 25, 50, and 75 wt % of CNF. The inset images (a−c) indicate the PEO crystallite orientation in the film as viewed edge-on. B

DOI: 10.1021/acs.biomac.7b01434 Biomacromolecules XXXX, XXX, XXX−XXX

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Figure 3. Crystallite sizes of PEO as a function of the CNF filling ratio. Figure 5. Ratio of the spectral intensities at three different wavenumbers as a function of the CNF filling ratio.

SI Figure 2 shows that for the 10 and 25 wt % PEO/CNF composite films, the peak intensities at ϕ = 90° decreased at temperatures above 65 °C. However, in the 25 wt % PEO/ CNF composite film, the peak intensity at ϕ = 180° decreased at temperature above 55 °C. These results approximately agreed with the peak in the DSC data. We therefore determined that the melting point of the PEO in which the molecular chains were perpendicular to the surface of the film was higher than for the PEO in which the molecular chains were parallel. Observation of the Hydrogen Bonds between CNF and PEO. It has been reported that PEO and CNF form hydrogen bonds.16−18 SI Figure 3a shows the Fourier-transform infrared spectroscopy (FTIR) spectra of the PEO/CNF composite films. The absorption bands resulted from OH group in the CNFs appeared to shift to a higher wavenumber as the CNF filling ratio was decreased. The absorption bands at around 3340 and 3285 cm−1 were assigned to the intramolecular O3−H and O5 hydrogen bonds (Figure 4 (1)) and the intermolecular O3−H

owing to the strong intramolecular hydrogen bond in the crystalline region.20 The ratio of the absorption at 3285 cm−1 (Figure 5 mark ▲) increased, and the band at 3390 cm−1 (Figure 5 mark *) decreased as the filling ratio increased. This indicates that the weak intermolecular hydrogen bond in CNF was converted to a hydrogen bond between PEO and CNF. Here the absorption ratio at 3390 cm−1 (Figure 5 mark *) for the CNF film was not 0 despite the fact that PEO was not included. This is because the Gaussian function at 3390 cm−1 contained not only the hydrogen bonds between PEO and CNF but also other components.19 The mobility of the PEO molecular chain was reduced by the hydrogen bonding between CNF and PEO, which reduced the Xc of PEO (Table 1).17,18 The absorption ratio at 3390 cm−1 suddenly decreased when the CNF filling ratio reached 50 wt %. This indicated that the ratio of the hydrogen bonds between PEO and CNF rapidly decreased at this filling ratio. Here, as in SI Figure 1, the spaces between the CNFs became smaller as the CNF filling ratio increased. PEO molecules exist in the spaces between the CNFs, which may explain the sudden decrease in the hydrogen bond ratio at 50 wt %: The spaces between the CNFs may have become so small that they no longer allowed PEO molecules to enter them. Potential Mechanism Underpinning the Change in the Crystallite Orientation of PEO. The crystallite orientations in the PEO/CNF composite films are summarized in Figure 6. The change in the orientation of the PEO crystals has previously been observed in Laponite clay PEO composite films.11 It has been reported that the orientation of the PEO crystallite changed as the amount of clay was increased because the distances between the clay platelets became narrower and the space in which PEO could crystallize became more restricted.11 In this report, we used nanofibers instead of clay. Because both nanoclays and nanofibers are high aspect ratio materials, the same orientational changes that occurred in the clay composites should also be expected to occur in the CNF composites. In the film with a ratio of 10 wt %, the diffraction peak due to cellulose did not appear from the WAXD results. Although this was likely because the amount of CNFs added was very low, it is considered that the CNFs were arranged so as to be parallel to the surface of the film due to the high aspect ratio of the CNFs. The space for PEO crystallization was open in the direction parallel to the surface of the film but was restricted in direction of the film thickness for the composite films that had CNF filling ratios of 10 wt % or less. As a result, the molecular chains of the PEO were oriented perpendicular

Figure 4. Chemical structure of cellulose.

and O6 hydrogen bonds (Figure 4 (2)), respectively.19 The absorption band of the OH group is known to shift toward higher wavenumbers when PEO and CNF form hydrogen bonds.16 We therefore concluded that the absorption band at ∼3390 cm−1 was due to the hydrogen bond that formed between CNF and PEO. Figure 5 shows the ratios of the areas of under the curves the three Gaussian peaks representing the above hydrogen bonds (SI Figure 3b) as a function of the CNF filling ratio. The ratio of the absorption at 3340 cm−1 (Figure 5 mark □) was almost constant independent of the CNF filling ratio. This occurred C

DOI: 10.1021/acs.biomac.7b01434 Biomacromolecules XXXX, XXX, XXX−XXX

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Figure 6. Summary of the preferred PEO crystalline structures from the edge-view of the film.

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to the surface of the film.9,12,21 When the filling ratio was >50 wt %, the space between each CNF was believed to be small in the plane of the film due to the decrease in the ratio of the hydrogen bonds between PEO and CNF (Figure 5). In addition, if we assume that the hydrogen bonds between PEO and CNF act as a crystallization nucleus for PEO, then it can be interpreted that the crystallite size became small because the density of the crystal nuclei increased. It has been reported that the molecular chains of PEO became oriented parallel to the surface of a film as the crystal nucleus density increased.22 It therefore seems that the molecular chains of PEO oriented parallel to the surface of a film when the crystal nuclei increased on the added CNFs. The change in the PEO crystallite orientation is thought to occur due to the size of the spaces between each CNF and the hydrogen bonds between CNF and PEO.

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CONCLUSIONS

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ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81 78 992 3902. Fax: +81 78 992 3919. ORCID

Miki Noda Fukuya: 0000-0001-6872-0037 Osami Sakata: 0000-0003-2626-0161 Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Nishiyama, Y. J. Wood Sci. 2009, 55, 241−249. (2) Suzuki, K.; Sato, A.; Okumura, H.; Hashimoto, T.; Nakagaito, A. N.; Yano, H. Cellulose 2014, 21, 507−518. (3) Jonoobi, M.; Harun, J.; Mathew, A. P.; Oksman, K. Compos. Sci. Technol. 2010, 70, 1742−1747. (4) Frone, A. N.; Berlioz, S.; Chailan, J.-F.; Panaitescu, D. M. Carbohydr. Polym. 2013, 91, 377−384. (5) Fukuzumi, H.; Saito, T.; Iwata, T.; Kumamoto, Y.; Isogai, A. Biomacromolecules 2009, 10, 162−165. (6) Brown, E. E.; Laborie, M.-P. G. Biomacromolecules 2007, 8, 3074−3081. (7) Tercjak, A.; Gutierrez, J.; Barud, H. S.; Domeneguetti, R. R.; Ribeiro, S. J. L. ACS Appl. Mater. Interfaces 2015, 7, 4142−4150. (8) Samir, M. A. S. A.; Alloin, F.; Sanchez, J.-Y.; Dufresne, A. Polymer 2004, 45, 4149−4157. (9) Wang, H.; Keum, J. K.; Hiltner, A.; Baer, E.; Freeman, B.; Rozanski, A.; Galeski, A. Science 2009, 323, 757−760. (10) Wang, H.; Keum, J. K.; Hiltner, A.; Baer, E. Macromolecules 2009, 42, 7055−7066. (11) Chu, C. Y.; Chen, M. H.; Wu, M. L.; Chen, H. L.; Chiu, Y. T.; Chen, S. M.; Huang, C. H. Langmuir 2014, 30, 2886−2895. (12) Fukuya, M. N.; Senoo, K.; Kotera, M.; Yoshimoto, M.; Sakata, O. Polymer 2014, 55, 4401−4404. (13) Fukuya, M. N.; Senoo, K.; Kotera, M.; Yoshimoto, M.; Sakata, O. Polymer 2014, 55, 5843−5846. (14) Alexander, L. E. X-ray Diffraction Methods in Polymer Science, 1st ed.; Wiley-Interscience: New York, 1969. (15) Iwasa, M.; Emoto, K.; Wakairo, R.; Nishimura, S.; Yoshida, H. Netsu Bussei 2014, 26, 203−208. (16) Kondo, T.; Sawatari, C.; Manley, R. S. J.; Gray, D. G. Macromolecules 1994, 27, 210−215. (17) Nishio, Y.; Hirose, N.; Takahashi, T. Polym. J. 1989, 21, 347− 351. (18) Guo, Y. Q.; Liang, X. H. J. Macromol. Sci., Part B: Phys. 1999, 38, 439−447. (19) Hofstetter, K.; Hinterstoisser, B.; Salmen, L. Cellulose 2006, 13, 131−145. (20) Perez, S.; Mazeau, K. Polysaccharides: Structural Diversity and Functional Versatility, 2nd ed.; Marcel Dekker Inc.: New York, 2005. (21) Schonherr, H.; Frank, C. W. Macromolecules 2003, 36, 1199− 1208.

The orientation of the PEO molecular chains in the crystals changed from perpendicular to the surface of the film to parallel to the surface of the film as the added CNF increased. The intermolecular hydrogen bonds of the cellulose replaced the hydrogen bonds between PEO and CNF in the PEO/CNF composite films according to our FTIR measurements. We discussed why the crystallite orientation of the PEO changed. The fiber axis of the CNFs was arranged parallel to the surface of the film due to the high aspect ratio of the CNFs, and the crystal growth of the PEO was restricted in the direction of the film thickness. The change in the crystallite orientation of the PEO was caused by the available space in which the PEO could crystallize, decreasing when the CNF filling ratio increased. Therefore, the crystallite orientation of the PEO changed depending on the amount of CNF added to the composite film.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.biomac.7b01434. SI Figure 1: AFM images of the 75 wt % PEO/CNF composite film. SI Figure 2: Azimuthal angle dependence of the PEO 1 2 0 diffraction intensity. SI Figure 3: FTIR spectra of the PEO/CNF composite films and the fitting result of the 75 wt % PEO/CNF composite film for the FTIR spectra. (PDF) D

DOI: 10.1021/acs.biomac.7b01434 Biomacromolecules XXXX, XXX, XXX−XXX

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Biomacromolecules (22) Zhu, L.; Calhoun, B. H.; Ge, Q.; Quirk, R. P.; Cheng, S. Z. D.; Thomas, E. L.; Hsiao, B. S.; Yeh, F.; Liu, L.; Lotz, B. Macromolecules 2001, 34, 1244−1251.

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DOI: 10.1021/acs.biomac.7b01434 Biomacromolecules XXXX, XXX, XXX−XXX