Strong and Tough Cellulose Nanofibrils Composite Films: Mechanism

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Letter

Strong and Tough Cellulose Nanofibrils Composite Films: Mechanism of Synergetic Effect of Hydrogen Bonds and Ionic Interactions Kai Li, Lydia Skolrood, Tolga Aytug, Halil Tekinalp, and Soydan Ozcan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b03442 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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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.

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ACS Sustainable Chemistry & Engineering

Strong and Tough Cellulose Nanofibrils Composite Films: Mechanism of Synergetic Effect of Hydrogen Bonds and Ionic Interactions Kai Li,a Lydia N. Skolrood,a Tolga Aytug,a Halil Tekinalp,b Soydan Ozcanb,c,* a

Chemical Sciences Division, Oak Ridge National Laboratory,

1 Bethel Valley Road, Oak Ridge, Tennessee, 37831, United States b

Manufacturing Demonstration Facility, Energy and Environmental Sciences Directorate, Oak Ridge National Laboratory, 2350 Cherahala Blvd, Knoxville, TN 37932

c

Department of Mechanical, Aerospace, and Biomedical, University of Tennessee, Estabrook Rd, Knoxville, TN 37916 * Corresponding author: [email protected]

Abstract: Cellulose nanofibrils (CNFs) have been exploited for different applications, such as nanocomposites, gas separation, flexible electronics, and fuel cells, due to their unique properties. To fulfill different demands regarding the utilization of CNFs, one critical step is to develop strong and tough CNF composites. In this study, facile synthesis of strong and tough nanocellulose films was demonstrated using a strategy that employs the synergetic effects of hydrogen bonds and ionic interactions in the films. With the addition of chitosan (CS) and copper ion (Cu2+), the tensile strength and Young’s modulus of the newly developed film (CNF-CS-Cu) increased by 104% and 75%, respectively; more impressively, the toughness of CNF-CS-Cu improved by 560% compared to pure CNF. The hydrogen bonds and ionic interactions in the films were confirmed by attenuated total reflection infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. Results confirmed that the combination of hydrogen bonds and ionic interactions could yield much better performance in nanocellulose films. The development of facile, low-cost, scalable, and ecofriendly processing methods and formulations for the fabrication of strong nanocellulose-based films are essential and would significantly impact wide-spread utilization of such materials for a variety of applications requiring high performance. Keywords: cellulose nanofibrils, synergetic effect, hydrogen bond, ionic interaction, strong and tough.

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Cellulose nanofibrils (CNFs) have attracted significant research interest due to their excellent mechanical properties, large specific surface area, low weight, and efficient biodegradability.1-3 Numerous applications, such as nanocomposites,4 gas separation,5 flexible electronics,6 and fuel cells,7 have been developed based on CNFs. Many CNF-based applications use composites or films, for which individual CNFs are stacked together to form lightweight and strong materials. To fulfill different demands on effective utilization of CNFs, one key step is to develop mechanically strong and tough CNF films. Thus far, different processing strategies have been adopted to make CNF films (e.g., chemical modification,8, 9 physical reinforcement,10,

11

and

mechanical treatment12) for various applications, however, these methods generally involve elaborate, energy intensive processing schemes. Therefore, development of unique, cost-effective, and relatively simple manufacturing strategies for strong and tough CNF films is necessary for their wide-spread utilization in applications requiring high-performance materials. It is well known that weak molecular interactions (e.g., hydrogen bonds and ionic interactions) play an important role in interfacial engineering of materials which can lead to the emergence of excellent mechanical properties.13-15 For instance, Song et al. developed strong and tough poly(vinyl alcohol) (PVA) films by optimizing the hydrogen bonds between PVA and aminecontaining compounds.14 Cheng et al. realized graphene nanocomposite with integrated high tensile strength and toughness through poly(dopamine)-nickel (Ni2+) ionic interaction.16 Recently, Isogai et al. developed a dual counterion system of TEMPO-oxidized CNF film with tunable mechanical properties.17 Inspired by these studies, a strategy of utilizing the synergetic effect of hydrogen bonds and ionic interactions to make strong and tough CNF films was introduced. Mechanically fibrillated CNF, chitosan (CS), and copper ion (Cu2+) were chosen to facilitate the hydrogen bonds and ionic interactions in CNF films, where CS is incorporated as a soft phase in CNF film to induce additional hydrogen bond networks and Cu2+ is for the ionic interaction and also serves as “crosslink points” in the film. Mechanically fibrillated CNFs have negligible surface charge, making it ideal for investigations of hydrogen bonds. CS could form composites with metal ions such as Cu2+ through ionic interactions between the amino group (-NH2) and metal ions.18 In addition, CS has a similar structure to cellulose, which also possesses abundant OH groups, and thus could form strong hydrogen bonds with CNFs. Due to the synergetic effects of hydrogen bonds and ionic interactions, the presently obtained CNF-CS-Cu films demonstrated increased tensile strength, 2 ACS Paragon Plus Environment

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toughness, and Young’s modulus by 104%, 560%, and 75%, respectively, compared to unmodified CNF. In this study, we demonstrate a simple, industrially scalable, and environmentally benign approach for the fabrication of strong and tough CNF-based films. The characters of the CNF itself was firstly examined. CNF has a degradation temperature at 291.2 oC as determined by thermogravimetric analysis (TGA) (Figure 1a). Scanning electron microscope (SEM) images (Figure 1b, c) of the CNF shows CNF has a width of 50 nm and length of hundred micrometers. These fibers also have the tendency to bond together to form bundles. The preparation of CNF-CS-Cu films is illustrated in Figure 1d, and details are provided in the electronic supporting information (ESI) file. Aqueous CS (in 2% acetic acid) and CuCl2 solutions were prepared separately and then mixed to make CS-Cu complex, which later was combined with CNF dispersion to form the CNF-CS-Cu complex solution. The films were fabricated by vacuum filtering the CNF-CS-Cu complex solution through a membrane, followed by drying at 60 °C. The obtained CNF-CS-Cu is of a light blue-green color (Figure 1d, e, the diameter of these films are 4.7 cm and 19.5 cm, respectively). The CNF-CS, CNF, and CNF-Cu films were prepared in a similar fashion. Detailed compositions of the films are shown in Table S1. To optimize the concentration of CS in the CNF-CS-Cu films, CNF-CS films with different CS content (mass ratio) were prepared: CNF-CS-1 (CNF:CS = 98.0:2.0), CNF-CS-2 (CNF:CS = 96.0:4.0), CNF-CS-3 (CNF:CS = 93.5:6.5), and CNF-CS-4 (CNF:CS = 91.0:9.0). The tensile strength of the pure CNF film (CNF-CS-2 in Table S1) increased from 96.3 MPa to 124.6 MPa with the addition of 4.0 wt% CS, indicating that the addition of CS into CNF improves the mechanical properties significantly. This is due to the formation of hydrogen bonds between OH groups in CNF and OH/NH2 groups in CS. Similar results were also reported by Shih et al.19 on cellulose film, where a small amount of CS improves the tensile strength due to the formation of hydrogen bonds. Moreover, the soft chain of CS improves the interaction between CNF and CS and acts as a “glue” to bind relatively rigid CNF,20 which in turn enhances the mechanical properties of CNF. Further optimization of CS content (6.5 wt%) in CNF-CS film yield highest tensile stress (Table S1), demonstrating ~54.7% increase compared to pure CNF. Based on these results, the CNF-CS mass ratio was fixed to 93.5:6.5 in CNF-CS-Cu films. Four CNF-CS-Cu films with different Cu contents: CNF-CS-Cu1 (CNF-Cu = 99.0:1.0), CNF-CS-Cu-2 (CNF-Cu = 98.0:2.0), CNF-CS-Cu-3 (CNF-Cu = 97.5:2.5), and CNF-CS-Cu-4 (CNF-Cu = 97.0:3.0) were fabricated. For comparison purposes, films without CS, (i.e., CNF-Cu = 97.5:2.5) were also prepared. As shown in cross-sectional SEM images 3 ACS Paragon Plus Environment


(Figure f-m) CNF, CNF-Cu, CNF-CS-3, and CNF-CS-Cu-3 films exhibited similar fibrillar and layered structures. This layered microstructure, caused by the cellulose assembly during the filtering process, has also been reported in other studies of vacuum-filtered cellulose films.21 Further comparison of the morphology revealed that CNF film has bigger layered distance compared with other films. Meanwhile, layers in CNF-Cu, CNF-CS, and CNF-CS-Cu films are interconnected, especially for the CNF-CS and CNF-CS-Cu. This is because the CS is relative soft and helpful to bond the CNF together and thus help to improve the mechanical properties, which could explain the results why CS containing films have better mechanical properties compared with pure CNF (Table S1).

Figure 1. (a) TGA of the CNF fiber, (b, c) SEM images of the CNF, (d) Schematic of CNF-CSCu nanocomposites process flow, (e) CNF-CS-Cu sample with diameter of 19.5 cm, SEM images of films: (f, g) CNF, (h, i) CNF-Cu, (j, k) CNF-CS-3, and (i, m) CNF-CS-Cu-3.

To verify the chemical interaction among the active moieties, the films were characterized using various techniques. The element states in the CNF-CS-Cu films were confirmed with X-ray photoelectron spectroscopy (XPS). The deconvoluted C1S and N1S spectra are shown in Figure 2a–b. For C1s, the broad peak was fitted with five peaks centered at 287.7, 286.7, 286.2, 285.8, and 284.8 eV. These peaks are assigned to C = O, C-O-C, C-N, C-OH, and C-C, respectively. The existence of CS in the films is confirmed by the C = O and C-N signals, while the four peaks observed in the N1s XPS spectra, —amine, amide, protonated amine, and H-bonded N—indicate 4 ACS Paragon Plus Environment

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the formation of hydrogen bonds between CS and CNF. Comparison of the Cu2p XPS spectra (Figure 2c) of CNF-CS-Cu-3 and CNF-Cu films reveal a large symmetric peak at 934.6 eV that is assigned to a core feature,22 accompanied by the characteristic Cu (II) shakeup satellite peaks at ~940 and 943 eV. The existence of these satellite peaks signifies the formation of chelation in tetrahedral sites between CS and Cu in CNF-CS-Cu-3 and chelation of CNF and Cu in CNF-Cu films.23 To further confirm the interactions in the films, attenuated total reflection infrared spectroscopy (ATR-IR) was studied. Based on the results (Figure 2d), all CNF-based films contain characteristic peaks of CNF, such as typical OH stretching at ~3300 cm-1 and C-H stretching at 2800 cm-1. Other peaks, such as 1371 cm-1 and 897 cm-1, which belong to O-H bending vibration and C-H deformation vibration, respectively, arise from CNF and CS. Interestingly, the OH stretching region (3000–3600 cm-1) of the CNF-CS-3 and CNF-CS-Cu-3 became broader as compared to CNF, indicating formation of hydrogen bonds between CNF and CS. In contrast, the OH stretching of CNF-Cu became narrower as compared to CNF, suggesting a reduction in the hydrogen bonding network. The OH group could also chelate with Cu2+ to form complex,24 resulting in a reduced amount of OH groups available for hydrogen bonding, leading to sharper peaks with the incorporation of Cu. In the region of 1500–1750 cm-1 (Figure 2e), N-H bending at ~1580 cm-1 in CS shifted to a lower wavenumber (ca. 1560 cm-1) in both CNF-CS-3 and CNF-CSCu-3 films, which also indicates formation of hydrogen bonds after CS addition to CNF, while the peak in the region of 1500–1750 cm-1 in CNF-Cu is relatively remained the same. The Raman spectroscopy was performed to develop a deeper understanding of the synergetic interaction mechanisms of ionic interactions and hydrogen bonds in the films. As shown in Figure S2, typical nanocellulose bands are observed. The spectral region of 1010–1215 cm-1 of films was deconvoluted into five individual peaks (Figure 2f) centered at 1177, 1150, 1121, 1095, and 1080 cm-1 in CNF (Table S2), which are assigned to C-OH, C-C, C-O, and C-O-C stretching, respectively. For CNF-Cu, CNF-CS-3, and CNF-CS-Cu-3, while no shift was observed for the peaks at 1150, 1121, and 1095 cm-1 compared to CNF, a clear shift is evident for peaks centered at 1177 cm-1 (C-OH) and 1080 cm-1 (C-O-C). Note that when Cu was added into CNF, peaks at 1177 and 1080 cm-1 shifted to a lower wavenumber, whereas with CS addition, these peaks shifted toward a higher wavenumber. This implies (1) formation of an additional hydrogen bonding network between CNF and CS when CS was added to CNF; and (2) reduced strength of the 5 ACS Paragon Plus Environment


hydrogen bonds in CNF associated with the coordination of Cu and the OH group, in the case of the Cu addition. The Raman spectra is also consistent with the observed changes in the IR spectra. These results confirm our hypothesis that hydrogen bonds and ionic interactions could be realized by addition of CS and a Cu ion in the films.

Figure 2. (a) XPS spectrum of C1s in CNF-CS-Cu-3 film; (b) XPS spectrum of N1s in CNF-CSCu-3 film; (c) XPS spectrum of Cu2p in CNF-Cu and CNF-CS-Cu-3 films; (d) ATR-IR spectra of CNF, CS, CNF-CS-3, CNF-Cu, and CNF-CS-Cu-3 in the region of 4000–600 cm-1; (e) ATRIR spectra in the range of 1750–1500 cm-1; (f) Raman spectra of CNF-Cu, CNF, CNF-CS-3, and CNF-CS-Cu-3.

Tensile tests were carried out to investigate the synergetic effects of CS and Cu ion incorporation in the mechanical properties of the films. It can be clearly seen from the stress–strain curves (Figure 3a) that the incorporation of Cu increases the strain of the film (CNF-Cu) as compared to pure CNF, while the tensile stress and Young’s modulus (Figure 3b) values remain similar. Such results suggest that the ionic interaction of Cu2+ with the OH groups (in CNF)) is favorable to improve the tensile strain without decreasing tensile stress. That is, when the sample is stretched, the additional ionic interaction helps for absorption of energy and thus, leads to 6 ACS Paragon Plus Environment

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tougher films. With the addition of CS, the CNF-CS film showed substantial increase in tensile stress compared to pure CNF. Remarkably, the addition of both CS and Cu ions not only increased the tensile strength of CNF-CS-Cu films to ~200 MPa, more than two times that of CNF (96.3 MPa), but also significantly enhanced the strain (10.8%) and Young’s modulus (7.5 GPa). More importantly, a 560% increase in toughness [from 2.3 MJ/m3 (CNF) to 15.2 MJ/m3] for the CS and Cu incorporated film is obtained. Clearly, these results imply that the addition of both CS and Cu is highly effective in improving the mechanical properties of CNF films. Next, the effect of Cu concentration in the films was investigated. We have observed that increase in Cu content also increases the tensile stress (Figure 3c), reaching to highest value at 2.5 wt% Cu loading. However, further increase in the Cu content to 3.0 wt% demonstrated a decrease in performance. Note that, in addition to improvement in tensile stress, the toughness (Figure 3c) and Young’s modulus (Figure 3d) also showed relatively higher values when the Cu ion content is 2.5 wt%. As described earlier, Cu2+ interacts strongly with CS through the ionic interaction between the -NH2 group and Cu2+, and CS is coordinated to CNF with hydrogen bonds. Hence, it appears that the Cu2+ ion acts as a “cross-linking point” in the films and an optimum mechanical performance is achieved with ≤ 2.5 wt% Cu content. It is well established that cross-linking is favorable for the mechanical properties enhancement,25 while too many cross-linking points (or too much cross-linking density) results in material embrittlement. This result is also consistent with findings reported by Cheng et al. on graphene-based nanocomposites, where specific amount of Cu additions demonstrated improved mechanical properties .26



Figure 3. Mechanical properties of the films. (a) Stress–strain curve of CNF, CNF-CS-3, CNFCu, and CNF-CS-Cu-3; (b) Young’s modulus of CNF, CNF-CS-3, CNF-Cu, and CNF-CS-Cu-3; (c) Relationship of stress and toughness as a function of Cu content in CNF-CS-Cu films; (d) Young’s modulus of CNF-CS-Cu film for different Cu contents.

As elucidated above, for the case of CNF-CS-Cu films (Figure 4a), CS is chelated with Cu ions through ionic interactions and is bonded with CNF through hydrogen bonds as well. When an external force, such as stretching, is applied to the film, these interactions will absorb energy, rendering the film strong and tough. On the other hand, in the case of CNF-CS (Figure 4b) and CNF-Cu (Figure 4c), while the additional hydrogen bonds or ionic interactions help to improve the mechanical properties compared to pure counterparts (Figure 3a), the effect is relatively weaker compared to the synergistic influence of the hydrogen bonds and ionic interactions that simultaneously take place in CNF-CS-Cu samples.


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Figure 4. Schematic illustration of the interaction in the films: (a) CNF-CS-Cu, (b) CNF-CS, (c) CNF-Cu. The synergetic effect of hydrogen bonds and ionic interaction in the CNF-CS-Cu-3 film bring unique mechanical properties of the CNF films. As shown in Table 1, CNF-CS-Cu-3 film have higher tensile strength than other reported nanocellulose composites, such as CNF-CNC film, CNC-Latex film, LCNF film, CNF-HPC, and others. CNF-CS-Cu-3 film have the highest toughness compared with them. In general, the CNF films are brittle without treatment, as shown in Table 1. 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidized (TEMPO-CNF) is tougher compared with CNF, and certain treatment, such as super-critical CO2 drying, can also lead to tough film.27 In contrast, simply filtration of CNF-CS-Cu complex suspension can result strong and tough CNF film. What’s more, our fabrication strategy is also simple and capable for large size sample preparation (Figure 1 e). As reported before,28-30 vacuum filtration has also been used for large size composite preparation, such as polymer/carbon nanotube composites and graphene composites, and demonstrates the feasibility of this method for large-scale materials fabrication.

Table 1. Comparison of mechanical properties of CNF-CS-Cu composites with other nanocellulose-based composite films.a Tensile Nanocellulose Toughness Preparation methods strength Strain (%) Ref b composite films (MJ/m3) (MPa) CNF-CNC

Vacuum filtration

130-150

~8

n/a

31

CNC-Latex

18-33

0.4-0.9

0.058-0.12

32

90

n/a

n/a

33

LCNF

Casting Vacuum filtration, hot-press Vacuum filtration

97-132

1.7-3.5

n/a

34

TEMPO-CNF

Solvent evaporation

170-280

3-5

n/a

35

CNF-HPC

Hand casting

16-30

n/a

n/a

12

TEMPO-CNF



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CNF

Solvent evaporation

20-88

n/a

n/a

36

CNC-Latex

Casting

14-23

0.03-2.88

0.1-0.55

37

Acetylated CNF

Vacuum filtration

73-118

0.82-2.55

n/a

38

CNC

Casting

20-35

1.5-2

n/a

39

TEMPO-CNF

Solvent evaporation Vacuum filtration, (Dried in liquid CO2 and supercritical CO2, or Freeze-Drying)

25-200

5-10

n/a

40

84-120

8.8-16

7.3-8.5

27

Vacuum filtration

197

15

15.2

TEMPO-CNF

CNF-CS-Cu-3

This work

Notes: a n/a: not reported. b CNF: Cellulose nanofiber; CNC: Cellulose nanocrystal; TEMPO-CNF: 2,2,6,6-tetramethylpiperidine-1oxyl (TEMPO) oxidized CNF; HEC: hydroxyethylcellulose; HPC: Hydroxypropyl cellulose; LCNF: lignocellulose nanofibrils; CS: Chitosan.

The thermal stability of the films was also investigated by thermogravimetric analysis in N2 (Figure S4). The results suggest that all the films have similar decomposition behavior and the addition of Cu and CS decreases the decomposition temperature slightly compared to that of pure CNF (Table S3). The residual weight (Rw) increased and CNF-CS-Cu sample showed the highest Rw, which is due to the remaining Cu and N species. We have also confirmed that CS and Cu were incorporated into the CNF films. The effect of the additional CS and Cu on the crystallinity of the film was studied by powder X-ray diffraction (Figure S5). The diffraction peaks, corresponding to (200) and (110) planes, were found to be similar for all the films. However, it appears that the crystallinity (Table S1) slightly decreased after adding CS and Cu, suggesting the disruption of the ordered packing of CNF in the films. In summary, this study introduces a strategy of utilizing the synergistic effects of hydrogen bonds and ionic interactions in production of strong and tough nanocellulose films. Specifically, CS and Cu2+ were introduced to form extra hydrogen bonding networks and ionic interactions with CNF. It was demonstrated that, while single additions of CS (CNF-CS) or Cu (CNF-Cu) species can improve the pure CNF’s mechanical properties, the combination of both CS and Cu can boost the performance of CNF films significantly. In particular, results showed that the tensile strength and Young’s modulus of the CNF-CS-Cu films can reach up to 200 MPa and 7.5 GPa, respectively; signifying 104% and 75% higher performance characteristics than those of pure CNF counterparts. More impressively, the toughness of CNF-CS-Cu showed an increase of 560%. Because the 10 ACS Paragon Plus Environment

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overall process is straightforward and scalable, the present approach offers potential for the design and fabrication of practical and cost-effective strong and tough nanocellulose-based high performance films for various applications.

Supporting information Experimental details; Table of detail composition, mechanical properties, and crystallinity index of the films; Characterizations including tensile test, scanning electron microscopes (SEM), attenuated total reflectance infrared spectra (ATR-IR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD); Young’s Modulus calculation; Raman spectra of films; Table of deconvolution result of Raman spectra; XPS survey of films; TGA of films; PXRD data of films.

Acknowledgments Research is sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office, under contract DE-AC05-00OR22725 with UT-Battelle, LLC. This manuscript has been authored by UT-Battelle, LLC, under contract DEAC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). Microscopy and spectroscopy studies were completed at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility. Authors would like to thank Dr. Harry Meyer for his help on XPS measurement and Rick R. Lowden for the access of mechanical test.

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Synopsis: Synergetic effect of hydrogen bonds and ionic interactions enable super-strong and tough cellulose nanofibril based bio-composites.


Strong and Tough Cellulose Nanofibrils Composite Films: Mechanism

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