Unique Stress Whitening and High-toughness Double-crosslinked

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Unique Stress Whitening and Hightoughness Double-crosslinked Cellulose Films Pingdong Wei, Junchao Huang, Ying Lu, Yi Zhong, Yongfeng Men, Lina Zhang, and Jie Cai ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05485 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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ACS Sustainable Chemistry & Engineering

Unique Stress Whitening and High-toughness Double-crosslinked Cellulose Films Pingdong Wei†, Junchao Huang†, Ying Lu§, Yi Zhong†, Yongfeng Men§, Lina Zhang†, Jie Cai†, ‡* †College §State

of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P. R. China

Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Renmin Street 5625, 130022 Changchun, China ‡Research

Institute of Shenzhen, Wuhan University, Shenzhen 518057, P. R. China

E-mail: [email protected] KEYWORDS: Cellulose, Film, Stress whitening, USAXS, Mechanical properties.

ACS Paragon Plus Environment

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ABSTRACT. Polysaccharide-based materials, which have the advantages of abundant reserves, excellent biocompatibility and biodegradability, have attracted growing interest due to public awareness of sustainable development. Herein, we demonstrate the formation of high-strength and thoughness double-crosslinked (DC) cellulose films. For the first time, stress whitening of DC cellulose films is reported, which has never been observed in cellulose-based films or other polysaccharide-based materials. The epichlorohydrin-to-anhydroglucose unit of cellulose (ECHto-AGU) molar ratio, ethanol concentration and relative humidity are critical parameters that influence the microstructure and stress whitening of DC cellulose films. Moreover, the incorporation of chemically and physically crosslinked heterogeneous structures, strong hydrogen bonding and irreversible chemical covalent interactions among cellulose chains endows DC cellulose films with excellent mechanical properties and superior toughness. The drawing orientation can produce extremely high-strength and high-toughness DC cellulose films with a tensile strength, Young’s modulus and work of fracture of 234 MPa, 9.3 GPa, and 28.2 MJ/m3, respectively. The developed DC cellulose films also exhibited excellent thermomechanical properties, moderate thermal stability and extremely low oxygen permeability and should contribute to potential applications in food and drug packaging, battery separators, and biodegradable flexible electronics.

INTRODUCTION Functional polymer materials possess high flexibility, good mechanical properties, optical transparency, thermal stability and gas-barrier properties and can be used in various applications, especially for sensors,1 flexible electronics,2 field-effect transistors,3 semiconductors4 and drug differentiation.5 However, most of these materials are fabricated from petroleum-based feedstocks,


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and their production and wide use are threatened by the shortage of raw petroleum and severe environmental problems, such as climate change and air pollution. In addition, nonbiodegradable materials are found throughout the oceans because of ocean gyres, and their presence has resulted in environmental consequences worldwide.6 Additionally, many materials cannot be entirely biodegraded, which means they only degrade into small parts, so-called “microplastics”.7,

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Microplastics further worsen the situation because they are ingested by marine fauna, such as seabirds and plankton as well as marine mammals, and accumulate in marine organisms due to their intrinsic inert characteristics.9, 10 In addition, microplastics, which are found in shallow and hadal zones of oceans,11 are always accompanied by persistent organic pollutants (POPs) that are extremely fatal because of their endocrine-disrupting properties.12 Thus, bio-based materials fabricated from renewable biomass have attracted attention because of they have the advantages of being green, biocompatible, and entirely biodegradable and conserve resources. Recently, bioplastics have been made from different sources, such as poly(lactic acid) (PLA), polyester (PET), polyamine (PA), polycarbonate (PC) and poly(butylene succinate) (PBS).13 Among various kinds of biofeedstocks, cellulose is the most abundant, renewable resource worldwide and has been widely utilized in many fields due to its biocompatibility, biodegradability, and thermal stability.14 Numerous advanced materials made from cellulose have been proposed for different applications,15-17 such as conductors,18 conductive aerogels,19,

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loudspeakers,21 sodium-ion batteries,22, 23 and electrochemical capacitors.24-26 Cellulose films with excellent mechanical strength and other functionalities have been widely applied in flexible electronics,27 photonics,28 fluidic devices29 and solar gain regulators.30 However, most of these films were obtained from bacterial cellulose and cellulose nanofibers, which suffer due to complex


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fabrication processes and high costs. Hence, a facile process to fabricate biodegradable, highperformance cellulose materials via a “green process” is crucial. Recently, we established the formation of high-strength and high-toughness double-crosslinked (DC) cellulose and chitin hydrogels by a sequential chemical and physical crosslinking strategy via a high-efficiency, energy-saving, and “green” route.31, 32 A mechanism of “sacrificial bondsload carriers-scaffold” synergistic enhancement was proposed to elucidate the toughening mechanism of the double crosslinking structure of the DC cellulose hydrogels.32 To further utilize this strategy, we demonstrate the formation of high-strength and toughness DC cellulose films via two different routes. One route demonstrated abnormal stress whitening phenomena, and the other resulted in a material with excellent mechanical properties. We evaluated the influence of the epichlorohydrin-to-anhydroglucose (ECH-to-AGU) molar ratio, concentration of aqueous ethanol and relative humidity on the microstructure and stress whitening of the DC cellulose films. To the best of our knowledge, the stress whitening of DC cellulose films has never been reported for cellulose-based films or other polysaccharide-based materials. In addition, we controlled the draw orientation of the DC cellulose films to improve the mechanical properties of the resulting DC cellulose films, and the obtained DC cellulose films had extremely high strength and high toughness with a tensile strength, Young’s modulus and work of fracture of 234 MPa, 9.3 GPa and 28.2 MJ/m3, respectively. The developed DC cellulose films also exhibited excellent thermomechanical properties, moderate thermal stability and extremely low oxygen permeability and should contribute to potential applications such as battery separators, biodegradable flexible electronics and food packaging. EXPERIMENTAL SECTION


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Materials. Cellulose (cotton linter pulp) with a viscosity-average molecular weight (Mη) of 10.1 × 104 was provided by Hubei chemical Fiber Co. Ltd. (Xiangfan, China) and dried under vacuum at 60 °C for 24 h before use. LiOH·H2O, urea, epichlorohydrin (ECH) and ethanol were all purchased from Shanghai Chemical Reagent Co., Ltd. (China). All chemical reagents were used as received without any further purification. Fabrication of DC Cellulose Films. Cellulose was dissolved in an aqueous 4.6 wt% LiOH/15 wt% urea solution that was precooled to -12 °C to form a 6 wt% transparent cellulose solution within 5 min according to our previously reported method.33 Subsequently, ECH was injected into the cellulose solution as chemical cross-linker under stirring according to our recent works.31, 32 And then, the air bubbles were removed by centrifugation under 5 °C. After that, the obtained clear and viscous cellulose solution containing ECH was spread onto a glass plate to give a 0.5 mmthick film-like cellulose solution via solution casting method, then sealed and maintained at 5 °C for 24 h for the chemical crosslinking reaction between the hydroxyl groups on the cellulose chains and ECH. The film-like cellulose gels were then immersed into an aqueous ethanol solution as neutralization reagent at 5 °C for 4h to terminate the chemical crosslinking reaction and meanwhile, induce physical crosslinking. All the cellulose gels were washed thoroughly with deionized water to give double-crosslinked (DC) cellulose hydrogels with different molar ratio of ECH-to-AGU of cellulose (i.e., 0.34, 0.68, 0.86, 1.04, 1.72, named as DC1-4, DC2-4, DC3-4, DC4-4 and DC5-4, respectively) and different concentration of aqueous ethanol solution (i.e., 0 wt%, 25 wt%, 50 wt%, 75 wt%, 100 wt%, named as DC3-1, DC3-2, DC3-3, DC3-4 and DC3-5, respectively). After that, the DC cellulose hydrogels were handled in two routes, one route was posted them on a clean and smooth plastic plate and dried at ambient temperature to obtain undeformed DC cellulose films. For comparison, the undeformed physical crosslinked cellulose


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films (PC, ECH-to-AGU molar ratio of 0) and chemical crosslinked cellulose films (CC, ECH-toAGU molar ratio of 3.44) were prepared using the same procedure. In the other route, the DC cellulose hydrogels were fixed in a unidirectional tensile clamp to desired draw ratios from 1.00 to 1.60, and then air dried at ambient temperature to obtain oriented DC cellulose films. The ECHto-AGU molar ratios and neutralization conditions for all of the cellulose hydrogels are listed in Table S1 and Table S2. Characterization. The attenuated total reflectance Fourier transform infrared spectrometer (ATR-FTIR) spectra were recorded in the wavenumber range from 4000 cm−1 to 600 cm−1 using a FTIR spectrometer (Perkin Elmer Spectrum) at ATR mode. The topographic images of the surface of the PC, DC, and CC cellulose films before and after tensile fracture were evaluated using atomic force microscopy (AFM, Cypher ES, Asylum Research), the images were recorded in AC mode. Silicon probe with spring constant of 2 N/m and resonance frequency of 70 KHz (OLTESPA-R3, Bruker) was employed. The average tip radius is 8 nm. All images were analyzed using the AFM accessory software and the images presented were treated only by flattening when necessary. The morphology of the fracture surface perpendicular to the deformation direction and the cross section along the deformation direction of the cellulose films after stretching was observed by scanning electron microscope (SEM, Zeiss Merlin Compact, German) with an operating voltage of 5 kV, and all the samples were coated with gold before observation. The wide-angle X-ray diffraction (WAXD) patterns of the cellulose films were recorded in reflection mode on a Rigaku Smartlab 9000 diffractometer equipped with a CuKα radiation source (λ = 0.154 nm) operated at 40 kV and 200 mA; the samples were scanned at 5° min–1 and at a step


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size of 0.01° in 2θ ranged from 5° to 40°. All of the cellulose films were ground into fine particles to eliminate the effects of crystalline orientation. The apparent crystal size of the cellulose films was calculated according to Sherrer equation:34 ACS=kλ/(βcosθ) (1) where k stands for Sherrer constant (0.89), λ represents the wavelength of CuKα radiation source, θ stands for the azimuth angle of the lattice plane, and β represents the width of half peak of the related lattice plane. In situ ultra-small angle X-ray Scattering(USAXS) experiments were conducted with a modified Xeuss system of Xenocs equipped with a CuKα radiation source (λ = 0.154 nm) (GeniX3D Cu ULD, Xenocs SA, France) and scatterless collimating slits at a sample-to-detector distance of 6508 mm providing effective scattering vector q (q = (4π sin θ)/λ, where 2θ is the scattering angle and λ is the wavelength of the X-rays) range from 0.022 to 0.24 nm-1. The size of the primary X-ray beam at the sample position was 0.8 × 0.8 mm2. USAXS images were recorded with a Pilatus 100K detector of Dectris, Swiss (487 pixels × 197 pixels, pixel size = 172 μm). We used a stepwise tension at a constant cross-head speed of 33 μm s-1 at ambient temperature. The collection times for USAXS patterns were set as 300 s after each step. The two-dimensional wide-angle X-ray diffraction (2D-WAXD) patterns of the cellulose films with different draw ratios were conducted in a Bruker Smart APEX DUO single crystal four-circle diffractometer operated at 40 kV and 0.65 mA with a Cu anode using Debye-Scherrer method. The angles of φ, ψ, ω and 2θ were set as 0°. The distance between CCD 4K detector and sample was 60 mm, and the exposure time was 30 s. The X-ray beam was spotted on the horizontal middle of the sample when exposure was conducted. The sample was cut into small pieces which were placed


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in a glass capillary of 1 mm diameter. The crystallite orientation parameter, f, for drawn films was estimated using empirical formula as followed:35 f = (180°-Ψ°)/180° (2) where Ψ° is the half-width of the azimuthal intensity distribution. Light transmittance (Tr) of the PC, DC and CC cellulose films before and after stretching was measured with a UV–vis spectroscope (UV-6100PCS, Meipuda instrument, China). The thickness of the cellulose films was ranged from 0.12 to 0.15 mm. Dynamic mechanical analysis (DMA) was performed on a DMA Q800 (TA instrument, U.S.A.) in tensile mode at a frequency of 1 Hz. The test temperature was ranged from -100 to 250 °C at a heating rate of 5 °C /min. Tensile measurements were performed on the cellulose films using a universal tensilecompressive tester (CMT 6503, MTS/SANS, China) using GB/T 1040.3-2006 Standard. The rectangular film with 0.12-0.15 mm in thickness and 10 mm in width was tested at a crosshead speed of 2 mm min–1. The Young’s modulus was calculated from the initial linear region of the stress–strain curves. Before each measurement, the cellulose films were conditioned in closed vessels containing saturated solution of MgCl2, NaBr·H2O, (NH4)2SO4, CuSO4·5H2O to give relative humidity of 30 %, 58%, 72%, 81% and 98%, respectively. The oxygen permeability of the cellulose films was determined by a Mocon Ox-Tran model 2/21MH (Modern Controls, U.S.) at 23 °C and 0 % relative humidity under standard conditions (ASTM 3985). All of the results were obtained until the O2 permeability rate reached a stable value.


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Thermogravimetric analysis (TGA) of PC, DC, and CC cellulose films was carried out on TA Q500 (TA instrument, U.S.A.). The sample was put into platinum pan and remained at 80 °C for 20 min to remove residual water, and heated from 70 to 700 °C at a heating rate of 10 °C /min under air atmosphere. RESULTS AND DISCUSSION Figure 1a shows the structural changes that occur as the DC cellulose hydrogels are transformed into DC cellulose films by subjecting the hydrogels to two different sequential drying-drawing (Route 1) and drawing-drying (Route 2) procedures. The incorporation of chemical covalent crosslinking by ECH, cellulose II crystallite hydrates, and hydrogen bonding interactions and chain entanglements between self-assembled cellulose nanofibrils afforded transparent DC cellulose hydrogels with high toughness and good extensional deformation (Figure 1b).32 In Route 1, the postdrawn DC cellulose films were obtained by fixing and drying DC cellulose hydrogels at ambient temperature and then uniaxial drawing the films under a certain humidity. After drying, the nonporous structure of the DC cellulose hydrogels entirely collapsed, resulting in a dense structure of transparent, undeformed DC cellulose films with an approximately 10% decrease in thickness and a light transmittance of 89% at 600 nm (Figure 1c; Figure 2a). Upon uniaxial drawing under mechanical force at 72% relative humidity, the postdrawn DC cellulose films demonstrated a stress whitening phenomenon with a light transmittance of 50% at 600 nm during the drawing process (Figure 1d and 1e; Figure 2a and Video S1), which suggested the formation of heterogeneities with a length on the scale of the wavelength of visible light. The PC and CC cellulose films show clear differences in their light transmittance, and both are transparent during the whole drawing process (Figure 2a and Videos S2 and S3). Notably, the ECH-to-AGU molar ratio, ethanol concentration, and relative humidity were the main parameters influencing the stress


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whitening behavior of the postdrawn DC cellulose films. During the uniaxial-drawing deformation, stress whitening occurred in the postdrawn DC cellulose films macroscopically changing the light transmittance, especially for moderate ECH-to-AGU molar ratios from 0.68 to 1.04 and a relative humidity higher than 72%. However, the stress whitening was independent of the concentration of the aqueous ethanol solution (Figure 2b-d). In Route 2, the DC hydrogels were uniaxially drawn under mechanical force to a certain draw ratio and then dried at ambient temperature. The cellulose nanofibrils and cellulose II crystallite hydrates within the hydrogels were aligned along the direction of the mechanical force in the presence of water molecules, which acted as a plasticizer. Finally, after the drying procedure, the hydrogen bonding interactions between the cellulose chains were reconstructed, the cellulose II crystallites recrystallized, and the cellulose nanofibrils aggregated and adhered when the water molecules evaporated from the cellulose hydrogels, which resulted in a dense alignment and oriented structure in both the physically and chemically crosslinked domains and a tough predrawn DC cellulose film. Remarkably, these DC cellulose films exhibited high toughness, foldability, and dyeability (Figure 1g-1i).


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Figure 1. (a) Schematic displaying the structural changes in the DC cellulose hydrogels and the postdrawn DC cellulose films (Route 1) and predrawn DC cellulose films (Route 2). (b-f) Photographs of the DC cellulose hydrogel without and with stretching (b), undeformed DC cellulose film (c), postdrawn DC cellulose films with stress whitening (d) and 5 mm above (e) the steel ruler, and predrawn DC cellulose film (f). (g-i) Photographs of the DC cellulose films demonstrating high toughness (g), foldability (h) and dyability (i). The existence of hydrogen bond interactions and chemical covalent crosslinking among the cellulose chains in the DC cellulose films were confirmed by the attenuated total reflectance Fourier transform infrared spectroscopy (ART-FTIR) spectrum (Figure S1). The intensity of the peak at 2920 cm-1 for C-H stretching in –CH2 increased from PC to CC cellulose films, indicates that the hydroxyl groups in cellulose chains reacted with ECH. The peak for the C(6)-OH and C(2)-OH hydrogen bond interactions shifted from 3327 to 3341 cm-1, which confirmed that the


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inter- and intra-molecular hydrogen bonding interactions was weakened (Supporting Information Figure S2).32 We further investigated the wide-angle X-ray diffraction (WAXD) patterns of the PC, DC and CC cellulose films before fracture to understand the effect of the ECH-to-AGU molar ratio and ethanol concentration on their aggregate state structures (Figure S2). The WAXD curve of the PC cellulose film showed three characteristic peaks at 12.4°, 20.0° and 21.4°, which were attributed to the (110), (110) and (200) reflections, respectively, of cellulose II crystals. Compared to those of the PC cellulose film, the positions of the (110) and (200) reflections of the DC cellulose films remained constant; however, the peak intensities significantly decreased and the peaks broadened at higher ECH-to-AGU molar ratios, which indicated the chemical crosslinking reaction disrupted the ordered state of the cellulose chains and resulted in less recrystallization. In addition, we observed that increasing the ECH-to-AGU molar ratio induced the (110) reflection to gradually shift by approximately 1.1° to lower diffraction angles, suggesting a reduction in the crystal size of the cellulose II crystallites. Application of multipeak fitting and the Scherrer equation indicated that the degree of crystallization and apparent crystal sizes of the (110) reflections decreased from 57% and 3.6 nm for the PC cellulose film to 37% and 1.6 nm, respectively, for the CC cellulose film (Table S1, samples PC, DC1-4 to DC5-4 and CC). On the other hand, the intensity of the (1 10) reflection of the DC3-4 cellulose films increased as the ethanol concentration increased from 0 wt% to 75 wt% and then decreased at a 100 wt% ethanol concentration. Additionally, the position of the (110) reflection of the DC3-4 cellulose films decreased from 11.7° to 11.4° as the ethanol concentration increased from 0 wt% to 75 wt% and then shifted to 11.7° at a 100 wt% ethanol concentration. The experimental results show that the cellulose chains were rapidly and completely neutralized in high ethanol concentrations and tended to more easily form cellulose II crystalline hydrates; however, the chain entanglements and hydrophobic interactions among cellulose chains


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were suppressed by the rapid diffusion of water molecules at low ethanol concentrations.32 These trends are more obvious than the trends of the DC cellulose and chitin hydrogels incorporated with physically and chemically crosslinked structures,31, 32 demonstrating that the ECH-to-AGU molar ratio and ethanol concentration play important roles in controlling self-assembly of cellulose chains into nanofibrils as well as the recrystallization of cellulose II crystallites during the formation of DC cellulose films.

Figure 2. (a) Optical light transmittance curves of PC, DC3-4 and CC cellulose films before and after fracture. Plot of light transmittance of PC, DC and CC cellulose films after tensile fracture with different ECH-to-AGU molar ratios (samples PC, DC1-4 to DC5-4, and CC) (b), ethanol concentrations (samples DC3-1 to DC3-5) (c) and relative humidities (DC3-4) (d) at wavelength of 600 nm. The insert are photographs of corresponding fractured PC, DC and CC cellulose films. To obtain a comprehensive understanding of the unusual stress whitening phenomenon in the DC cellulose films, we examined the microscopic morphologies of the surfaces of PC, DC3-4 and CC


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cellulose films using atomic force microscopy (AFM) before and after fracture (postdrawn) (Figure 3a-3f). The topological images of the undeformed PC, DC3-4 and CC cellulose films exhibited homogeneous morphologies and dense percolation network structures composed of intertwined cellulose nanofibrils approximately 88.3 ± 21.4, 93.8 ± 18.3 and 141.8 ± 14.3 nm in diameter and some nanopores approximately 122.5 ± 32.0, 123.1 ± 31.2 and 95.2 ± 25.5 nm in size, respectively, before fracture (Figure 3a-3c; Figure S3), which contributed to the high light transmittance of the films. The larger diameter of the nanofibrils and smaller pore size of the nanopores in these cellulose films relative to those in the cellulose hydrogels are most likely due to aggregation and adhesion of cellulose nanofibrils by hydrogen bonding interactions among cellulose chains and the collapse and shrinkage of pore and network structures in the hydrogels due to capillary forces after drying.32, 36 Postdrawing leads to further adhesion of the cellulose nanofibrils and diameters of approximately 207.5 ± 36, 122.8 ± 12.8 and 145.3 ± 30.8 nm for the postdrawn PC, DC3-4 and CC cellulose films, respectively (Figure 3d-f). The pore sizes of the postdrawn PC and CC cellulose films were approximately 230.5 ± 39.0 and 124.5 ± 24.5 nm, respectively, which are much smaller than the wavelength of visible light, resulting in the transparency of the PC and CC cellulose films after tensile fracture (Figure 3d and 3f; Figure 2a). However, the AFM images of the postdrawn DC3-4 cellulose films show a large quantity of cavities with a width of 454 ± 83.1 nm and length of several micrometers homogenously dispersed on the surface. The size of these cavities is within the wavelength of visible light, resulting in light scattering and obvious stress whitening (Figure 3e). As reported previously,37 there are two independent processes of cavities during tensile deformation of semicrystalline polymers: the formation of cavities and the development of cavities. Under tensile deformation, the cellulose chains in amorphous regions disentangle and even break, which creates nucleation sites for the


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formation of cavities. However, only cavities that develop with a size on the scale of the wavelength of visible light could generate the stress whitening phenomenon. The strong hydrogen bonding interactions among the cellulose chains in the PC cellulose film and the dense chemical crosslinking points in the CC cellulose film significantly limited the development of the as-formed cavities and resulted in cavities with sizes far smaller than the wavelength of visible light. At a moderate molar ratio of ECH to AGU (e.g., from 0.68 to 1.04; Figure 2b and c) and a higher relative humidity (e.g., larger than 72%; Figure 2d), the crystalline structure was destroyed to some extent, leading to more amorphous regions for cavities to develop, and the size of these cavities could reach the wavelength of visible light, which was indicated by macroscopic strong stress whitening. To gain deeper insight into the inner structure of the cellulose films after fracture under stretching, we further conducted scanning electron microscopy (SEM) observations of the cross-section in the direction of and perpendicular to the deformation direction for the postdrawn PC, DC and CC cellulose films after fracture (Figure 3g-3l). In the cross-section images perpendicular to the tensile direction, all of the postdrawn PC, DC and CC cellulose films exhibited the characteristic features of ductile fracture (Figure 3g-3i).38-40 However, the cross-section of the postdrawn DC cellulose film was smoother than that of the PC and CC cellulose films, demonstrating better ductility. Meanwhile, in the cross-section images along the tensile direction, both the PC and CC cellulose films exhibited a dense structure without any voids (Figure 3j and 3l); however, cavities were present in the cross-section along the tensile direction of the DC3-4 film. The lengths of those cavities were near several hundred nanometers (Figure 3k), which are within the wavelengths of visible light, leading to strong stress whitening.


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Figure 3. (a-c) AFM images of the surface of undeformed PC (a), DC3-4 (b) and CC (c) cellulose films before fracture. (d-f) AFM images of the surface of the corresponding postdrawn PC (d), DC3-4 (e) and CC (c) cellulose films after fracture. (g-i) SEM images of the cross-section of the postdrawn PC (g), DC3-4 (h) and CC (i) cellulose films perpendicular to the deformation direction after fracture. (j-l) SEM images of the cross-section of the postdrawn PC (g), DC3-4 (h) and CC (i) cellulose films along with the deformation direction after fracture. The scale bars are 1 μm. The double-headed arrows represent the tensile direction.


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Stress whitening phenomena are more common in semicrystalline synthetic polymers, such as polypropylene, polyethylene and polycarbonate, when they are subjected to cold drawing at atmospheric pressure and ambient temperature, and the stress whitening is attributed to the formation of cavities around the yield point.41, 42 To the best of our knowledge, stress whitening phenomena have never been observed in traditional cellophane, regenerated cellulose films, bacterial cellulose films and cellulose nanofibril films. To further understand the structural evolution of the PC, DC and CC cellulose films during uniaxial drawing deformation, we carefully conducted ultrasmall angel X-ray scattering (USAXS) measurements on cellulose films with different ECH-to-AGU molar ratios, ethanol concentrations and relative humidity (Figure 4; Figure S4). As depicted in Figure 4, the integrated scattering intensity of the PC cellulose film began to increase slightly as the strain increased to 26%, which suggested a slight reorientation parallel to the stretching direction because of the reorientation of cellulose nanofibrils within the PC cellulose film (Figure 4, sample PC; Video S2). A strong increase in the integrated scattering intensity parallel to the tensile direction appeared in the DC cellulose films with ECH-to-AGU molar ratios of 0.86 and 1.04 at certain strains where stress whitening was detected by USAXS measurements (Figure 4, samples DC3-4 and DC4-4; Video S1), which could be ascribed to the elongated cavities with dimensions much larger than those of the crystalline structure with low density.37 Moreover, low integrated scattering intensity occurred in the vertical direction of the DC cellulose films when the tensile strain reached 30%, which indicated that cavities with sizes within the wavelength of visible light formed along the tensile direction, and these results were consistent with the AFM and SEM measurements in Figure 3. Thus, these low-density cavities possessed different reflective indexes than the cellulose matrix, which led to macroscopic stress whitening. Afterward, the integrated scattering intensity gradually declined as the molar ratio further


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increased. When the ECH-to-AGU molar ratio was 3.44, no stress whitening was observed, and no scattering streaks appeared because of the relatively high homogeneity of the films (Figure 4, sample CC; Video S3). As reported in earlier studies, the formation and development of the cavities mainly occurred in the amorphous regions.37 When ECH was added, the dense physically crosslinked structure of the PC cellulose film was gradually destroyed, resulting in a heterogeneous structure within the DC cellulose films that allowed cavities to form and develop. When these cavities had sizes comparable to the wavelength of visible light, strong stress whitening was observed. However, as the ECH-to-AGU molar ratio increased, the amorphous region formed a denser chemically crosslinked structure, which hampered the formation and development of cavities, like the physically crosslinked structure did at low ECH to AGU molar ratios, and reduced the extent of stress whitening. We also examined the influence of the ethanol concentration and relative humidity of the atmosphere during the in situ USAXS measurements (Figure S4). The integrated scattering intensity minimally differed at high strains with different concentrations of ethanol (50 wt%, 75 wt% and 100 wt%). This result could be because the ethanol concentration mainly affected the structure of the crystalline region but had little direct effect on the structure of the amorphous region. However, all of those samples exhibited a strong stress whitening phenomenon (Figures S4a and Figure 2c). Regarding the relative humidity of the atmosphere, the integrated scattering intensity weakened remarkably as the relative humidity decreased from 75% to 30% (Figure S4b). A low relative humidity caused the water content of the cellulose films to sharply decrease from 16% to 11% (Table S2), strengthened the hydrogen bond interactions and chain entanglements among cellulose chains and reduced the flexibility and mobility of cellulose chains in the amorphous region, which strictly limited the formation and development of cavities and resulted


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in macroscopic transparency after tensile deformation until fracture (Figures 2d). These results along with the AFM, SEM and WAXD results confirmed that the ECH-to-AGU molar ratio, ethanol concentration and relative humidity are critical parameters that influence the microstructure and stress whitening of DC cellulose films, and the cavities in these DC cellulose films that form due to the heterogeneous structure of the chemically and physically crosslinked domains are responsible for the stress whitening phenomenon. Therefore, the formation mechanism is different with those of synthetic polymers.

Figure 4. Selected USAXS patterns of PC, DC and CC cellulose films taken at different strains as indicated on the graph under 75% relative humidity. The stretching direction was horizontal.


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The mechanical properties of the DC cellulose films were strongly affected by the ECH-to-AGU molar ratio and ethanol concentration (Figure 5a and 5b). For the DC cellulose films, when the ECH-to-AGU molar ratio and ethanol concentration increased from 0.34 to 1.72 and 0 to 100 wt%, the tensile strength (σb), Young’s moduli (E) and elongation at break (εb) of the DC cellulose films gradually increased with maximum values of 97.1 MPa, 3.9 GPa and 33%, respectively, at an ECH-to-AGU molar ratio of 0.86 and an ethanol concentration of 75 wt% (Table S1, sample DC34). For comparison, the PC cellulose film exhibited a slightly higher σb of 101.4 MPa but lower E and εb of 3.1 GPa and 24%, respectively, which were caused by strong hydrogen bond interactions and chain entanglements among cellulose chains. In addition, the CC cellulose film also demonstrated lower mechanical properties than the DC3-4 cellulose film with σb, E, and εb values of 85.4 MPa, 3.1 GPa and 25%, respectively. Remarkably, the DC3-4 cellulose films exhibited a maximum work of fracture (Wf) of 24.7 MJ/m3, which was approximately 1.4 times and 1.5 times higher, respectively, than that of the PC and CC cellulose films, demonstrating a significant improvement by the incorporation of the chemical covalent crosslinking by ECH, cellulose II crystallites, and hydrogen bonding interactions and chain entanglements between self-assembled cellulose nanofibrils in the toughness of the DC cellulose films.32


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Figure 5. (a, b) Tensile stress-strain curves of cellulose films prepared with different ECHto-AGU molar ratios (a) and ethanol concentrations (b). (c, d) Tensile stress-strain curves of the DC3-4 cellulose films at different relative humidities (c) and during loadingunloading cycles with varying maximum stretching strain (d). The relative humidity was 72%. Additionally, upon absorbing water, the mechanical properties of the rewetted CC cellulose film dramatically decreased and were much lower than those of the rewetted DC3-4 cellulose film. This difference was most likely due to the low crystallinity and high swellability of the CC cellulose film (Figure S5) and indicates that the incorporation of chemically and physically crosslinked structures and chain entanglements plays an important role in the mechanical properties of rewetted DC cellulose films.32 We also examined the moisture-responsive mechanical behavior of


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the DC3-4 cellulose films by exposing them to different relative humidities in a humidified chamber (Figure 5c). The water content of the DC3-4 cellulose film increased from 11% to 24% as the relative humidity increased from 30% to 98%. As expected, the σb and E of the DC3-4 cellulose film decreased from 108.7 MPa to 52.8 MPa and from 4.4 GPa to 1.5 GPa, respectively, whereas εb increased from 18% to 38% (Table S1). However, the Wf of the DC3-4 cellulose film increased from 16.1 MJ/m3 at 30% relative humidity to 24.7 MJ/m3 at 72% relative humidity and then decreased to 13.9 MJ/m3 at 98% relative humidity, which suggested that water molecules act as a plasticizer to enhance the mobility and flexibility of the cellulose chains and may improve the toughness of the DC cellulose films by competitive hydrogen bonding.43 We further evaluated the mechanical properties of the PC, DC3-4 and CC cellulose films under loading-unloading cycles with varying maximum stretching (Figure 5d; Figure S6). The stress of the DC3-4 cellulose film increased with the applied strain up to 25% and demonstrated more remarkable hysteresis (energy dissipation) and larger permanent deformation under loading-unloading cycles than the other films. Therefore, the incorporation of chemically and physically crosslinked structures afforded DC cellulose films with high stiffness, high toughness and more effective energy dissipation relative to the PC and CC cellulose films. The DC structure in cellulose films not only leads to stress whitening behaviors but also significantly enhances the mechanical properties of the DC cellulose films. We further examined the effect of the orientation on the nanofibrillar structure and mechanical properties of predrawn DC cellulose films via Route 2 (Figure 6). The polarization microscopy observations under polarized light showed that the predrawn DC cellulose film demonstrated obvious birefringent behavior, suggesting nanoscale order is generated in the nanofibrillar structures at different tension regions during hydrogel uniaxial stretching and the final drying step (Figure 6a; Figure S7).44 In


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the AFM observations, a minor nanofibril alignment is observed at low birefringence (position 1) and longer, oriented nanofibrils are found at high birefringence (position 4), demonstrating that the alignment and orientation of the cellulose nanofibrils increased with increasing stress and birefringence along the predrawn DC cellulose films.44 Furthermore, the orientation of cellulose nanofibrils perpendicular to predrawn DC cellulose films with different draw ratios was evaluated using two-dimensional wide-angle X-ray diffraction (2D WAXD) (Figures 6b-6f). The reflections of the 2D WAXD patterns of a pristine DC3-4 cellulose film showed a semicrystalline morphology characterized by broad Bragg reflections and an amorphous halo, indicating the presence of a moderate orientation (Figure 6b). However, the reflections of the predrawn DC3-4 cellulose films exhibited narrow equatorial arcs at higher draw ratios (Figure 6c-6f), indicating high alignment and orientation of the cellulose nanofibrils. Moreover, the 2D WAXD pattern of the predrawn DC3-4 cellulose film with a draw ratio of 1.60 exhibited equatorial belts corresponding to the (110), (110) and (020) reflections of cellulose II.45 The orientation index of the predrawn DC cellulose films gradually increased from 0.543 to 0.841 as the draw ratio increased from 1.00 to 1.60 (Table S3). In addition, the diffraction intensity of the peak at 20.2°, which stands for the (110) plane of cellulose II, increased with the increasing draw ratio, while the (110) intensity at 12.4° decreased (Figure S8), which suggested that the uniplanar orientation of the hydrophilic plane parallel to the surface was reduced.46, 47 Although the apparent crystallinity of the predrawn DC cellulose films increased slightly with the increasing draw ratio, this phenomenon might be caused by a change in the planar orientation instead of crystallization. Generally, the crystallinity of crystalline polymers increases during a drawing process. However, regenerated cellulose films are formed by a large number of hydrogen bonds, which can impede the crystalline properties. Similar results were also observed in


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cellulose/LiCl/DMAc solution systems; i.e., the planar orientation in the amorphous regions is much higher than that in the crystalline regions of the cellulose films after the drawing process.48 High orientation in amorphous regions has also been observed in highly stretched and swollen cellulose films with relatively low crystallinity obtained from an NMMO solution by uniaxial drawing.47 Therefore, amorphous regions play a crucial role in the molecular orientation and mechanical properties of predrawn DC cellulose films. According to our previous work, DC cellulose hydrogels consist of well-dispersed chemically and physically crosslinked domains. In this system, the irreversible covalent crosslinks serve as “sacrificial bonds” to efficiently disperse stress, the cellulose II crystallite hydrates act as “load carriers” to effectively absorb energy and withstand large deformations, and the percolation network of intertwined cellulose nanofibrils acts as a “scaffold” to successfully transfer mechanical stresses.32 Therefore, DC cellulose hydrogels readily orient cellulose nanofibrils and cellulose II crystallite hydrates in the deformation direction in the presence of water molecules. During the drying process under tension, the abundant water molecules in the hydrogels gradually evaporate, and the mobility of the cellulose chains and cellulose II crystallites decreases sharply due to strong hydrogen bonding interactions, resulting in the preferential alignment and orientation of the cellulose nanofibrils and cellulose II crystallites in the drawing direction.36, 49 As expected, the mechanical properties of predrawn DC cellulose films could be enhanced significantly by uniaxial drawing (Figure 6g). Note that the stress whitening phenomenon was not observed in the predrawn DC cellulose films, which is most likely due to the high alignment and orientation of the cellulose nanofibrils. Unstretched DC cellulose films possessed a high plasticity and large εb (up to 33%) that decreased to 10% at a draw ratio of 1.60, which was probably due to the residual disorder structure being aligned and oriented under stretching.44 Meanwhile, the tensile strength and Young’s modulus of the predrawn DC cellulose


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films increased from 97 to 234 MPa and from 3.9 to 9.3 GPa, respectively, as the draw ratio increased from 1.00 to 1.60, which was a significant improvement in the mechanical properties of the predrawn DC cellulose films (Table S3). Notably, the maximum Wf of the predrawn DC3-4 cellulose film was 28.2 MJ/m3 at a draw ratio of 1.15, which is the highest toughness reported for regenerated cellulose films from DMAc/LiCl (max. 25.4 MJ/m3) and NMMO solution (max. 9.4 MJ/m3), cellulose nanofibrillar films (max. 16.7 MJ/m3), and bacterial cellulose films (max. 24.7 MJ/m3).48,

50-56

These features with the birefringence, AFM images, and 2D WAXD patterns

suggest the alignment and orientation of self-assembled cellulose nanofibrils and cellulose II crystallites, especially entangled cellulose chains in amorphous regions, along the draw axis as the draw ratio increases, leading to the high tensile strength and elastic moduli. In addition, the anisotropy of the cellulose chains was enhanced under uniaxial drawing, which led to fracture of the predrawn DC cellulose films at lower applied strain.


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Figure 6. (a) Birefringence and corresponding AFM topography images (top) and phase contrast images (bottom) of the predrawn DC3-4 cellulose film at different positions with a draw ratio of 1.60. The scale bars are 500 nm. (b-f) 2D WAXD patterns of the predrawn DC3-4 cellulose films with draw ratios of b) 1.00, c) 1.15, d) 1.30, e) 1.45, and f) 1.60. (g) Tensile stress-strain curves of the corresponding predrawn DC3-4 cellulose films with different draw ratios from 1.00 to 1.60. The thermal mechanical properties of DC cellulose films with different ECH-to-AGU molar ratios, ethanol concentrations, draw ratios and relative humidity were also evaluated by dynamic mechanical analysis (DMA) (Figure S9). The storage modulus of the DC cellulose films gradually decreased from -100 °C to 250 °C (Figure S9a and b), which is typical of semicrystalline polymers in the glassy state but is still higher than that of commercially available synthetic polymers, such


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as polyethylene (PE) and polypropylene (PP).57 Moreover, the storage modulus of the DC cellulose films gradually increased with the decrease in the relative humidity and increase the draw ratio (Figure S9c and d), which was consistent with the mechanical properties of the DC cellulose films summarized in Tables S2 and S3. Additionally, the DC cellulose films exhibited moderate thermal stability with a maximum decomposition temperature of approximately 326 °C, which is between that of the PC and CC cellulose films (Figure S10). Furthermore, the DC cellulose films exhibited extremely low oxygen permeability ranging from 0.00010 to 0.00015 Barrer, which is much lower than that of most commercially available packaging materials, such as cellulose acetate (CA), PE, poly(vinyl chloride) (PVC) and polyethylene terephthalate (PET) (Supporting Information Table S4).52,

58-60

Therefore, these DC cellulose films with excellent thermomechanical properties,

moderate thermal stability and extremely low oxygen permeability can potentially be applied for uses in battery separators, flexible electronics and food packaging. CONCLUSIONS High-strength and toughness DC cellulose films were created by two different sequential dryingdrawing and drawing-drying procedures. For the first time, we observed the stress whitening phenomena in the DC cellulose films. The ECH-to-AGU molar ratio, ethanol concentration and relative humidity are critical parameters that influence the microstructure and stress whitening of these DC cellulose films, and the cavities that form due to the heterogeneous structure of the chemically and physically crosslinked domains are responsible for the stress whitening phenomenon in DC cellulose films. Specifically, upon predrawing, the alignment and orientation of the cellulose nanofibrils, cellulose II crystallites and entangled cellulose chains impart the DC cellulose films with extremely high strength and high toughness. Additionally, the DC cellulose films had excellent thermomechanical properties, moderate thermal stability and extremely low


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oxygen permeability. We expect that these films can be utilized as promising candidates for applications, including food and drug packaging, battery separators and biodegradable flexible electronics.

ASSOCIATED CONTENT Supporting Information. The supporting information are available free of charge. Physical properties of PC, DC and CC cellulose films with different ECH-to-AGU molar ratios, ethanol concentration, relative humidity and draw ratios; oxygen permeability, FT-IR spectra, XRD curves, representative AFM height retraces of PC, DC and CC cellulose films; selected in situ USAXS patterns of DC cellulose films with different ethanol concentrations and relative humidity; tensile stress-strain curves of PC and CC cellulose films during loading-unloading cycles; photographs of the DC cellulose films under optical and polarized light; DMA temperature sweeps for the DC cellulose films; Thermal gravity analysis curves of PC, DC and CC cellulose films (PDF) Videos S1-S3 (ZIP) AUTHOR INFORMATION Corresponding Author *[email protected]. ORCID Ying Lu: 0000-0001-5012-6160


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Yongfeng Men: 0000-0003-3277-2227 Lina Zhang: 0000-0003-3890-8690 Jie Cai: 0000-0002-0660-4740 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was funded by the National Natural Science Foundation of China (51373125, 21422405) and the Major Program of National Natural Science Foundation of China (21334005). The authors thank to the facility support of the Wuhan Morning Light Plan of Youth Science and Technology (2017050304010312), the Special Fund for the Development of Strategic Emerging Industries of Shenzhen City of China (JCYJ20170818112409808), and Fundamental Research Funds for the Central Universities (2042017kf0195, 2042015kf0259, 2042017 KF0227).

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Table of content

Synopsis: Double-crosslinked strategy was used to fabricate cellulose films with unique stress whitening phenomena and high toughness.


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Unique Stress Whitening and High-toughness Double-crosslinked

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