Organic-Inorganic Hybrid Films Fabricated from Cellulose Fibers and

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Organic-Inorganic Hybrid Films Fabricated from Cellulose Fibers and Imogolite Nanotubes Linlin Li, Wei Ma, Akihiko Takada, Nobuhisa Takayama, and Atsushi Takahara Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.9b00881 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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Organic-Inorganic Hybrid Films Fabricated from Cellulose Fibers

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and Imogolite Nanotubes

3 Linlin Li1, Wei Ma2, Akihiko Takada3, Nobuhisa Takayama1, Atsushi Takahara*1,2,3

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Japan.

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744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

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Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395,

International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University,

Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku,

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Fukuoka 819-0395, Japan

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Abstract

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Owing to the increasing environmental awareness, nanocellulose/ natural clay composites with

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improved mechanical performance have attracted growing interest due to their eco-friendly

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properties. In this study, hybrid films composed of cellulose fibers (CFs) and imogolite

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nanotubes (natural aluminosilicate nanotubes) were fabricated. We mainly studied the structure,

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density and properties of the hybrid materials. For that, the hybrid materials were characterized

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by scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform

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infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), thermogravimetric

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analysis (TGA), dynamic mechanical analysis (DMA), rheological test, and wide angle X-ray

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diffraction (WAXD). The mechanical properties of the hybrid materials were measured by a

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tensile test, which demonstrated that the mechanical properties of the hybrid films were 1

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considerably improved by the addition of imogolite up to 1 wt%; meanwhile, the thermal-

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mechanical properties of the hybrid film were also enhanced.

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Introduction

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Nanosized tubular clays, such as halloysite and imogolite, have attracted considerable

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attention in recent years, 1 not only because of their unique one-dimensional tubular structures

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but also their eco-friendly properties. 2-3 Among the tubular clay family, imogolite, an important

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component of volcanic soil, with the general formula (OH)3Al2O3SiOH,4 has been extensively

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studied since it was discovered in Kyushu, Japan.5 Scheme 1a shows the structure of an

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imogolite nanotube. Typically, imogolite forms a single-walled nanotube with an external

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diameter of ca. 2 nm and internal diameter of ca. 1 nm. The length of imogolite ranges from

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several hundred nanometers to micrometers. Benefiting from its asymmetric outer and inner

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surface chemistry, imogolite can be modified easily because its outer surface is composed of

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Al2-μOH, while the inner surface is composed of Si-OH.5 In deionized water, imogolite is prone

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to aggregation due to its rigid structure and high surface energy,6 which bestows it with a water-

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adsorbing ability.

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Benefiting from the unique structure of imogolite, various potential applications, such as

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gas adsorption and catalysis, have been discussed.7-8 Moreover, the addition of imogolite to

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polymer matrices, such as polyvinyl alcohol (PVA) and polyvinyl chloride (PVC) is expected

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to improve the material’s mechanical properties, as has been reported in previous studies.9-16

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This mechanical strength enhancement mainly stems from the high aspect ratio and large

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specific surface area of imogolite. For instance, in our previous study, a polymer brush was first

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grafted from the external surface of imogolite nanotubes and then a polymer/imogolite 2


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nanocomposite was fabricated.10-11 As a result, the mechanical properties of the composite were

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considerably improved. Thus, the innovative development of green composites with excellent

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mechanical strengths is highly desirable.17

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Compared with the traditional polymeric matrices, cellulose nanofibers (CNF), owing to

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their high strength and stiffness, biodegradability and renewability, have gained increasing

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attention.18-19 In particular, the application of CNF in polymer reinforcement has been proved

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to be successful.20-22 In recent developments, CNF/clay composites have been shown to be

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promising because of their good mechanical strength and toughness.23-26 Indeed, as both CNF

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and clay are low-cost materials and are eco-friendly, they exhibit a great potential in

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nanocomposites. For instance, Berglund et al. prepared nanofibrillated cellulose

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(NFC)/montmorillonite (MTM) nanopaper with fire retardancy and gas barrier functions.23

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However, the mechanical properties, such as the tensile strength and elongation at break, of the

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nanopaper were lower than those of neat NFC nanopapers. This is probably because of the poor

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interaction between the NCF and MTM.27 Different from MTM, imogolite may interact with

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CFs strongly because there are a considerable number of hydroxyl groups on both of the

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surfaces, which may lead to the formation of hydrogen bonds. Improved mechanical properties

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of CFs/imogolite nanocomposites thus can be expected.

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Herein, in this study, we prepared a CFs/imogolite hybrid film, and mainly studied the

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structure, mechanical properties, and thermal stability of the hybrid film. First, we synthesized

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imogolite, and then mixed it with a CFs dispersion under vigorous stirring. We then cast the

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composite in a Teflon petri dish. Composites with different weight ratios of CFs and imogolite

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were characterized by tensile testing, rheological test, WAXD, TGA, as well as FT-IR. The 3



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morphologies of these nanocomposites were observed by SEM and AFM.

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Experimental Section

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Materials

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Imogolite preparation Imogolite was synthesized according to a previously reported method.28

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The suspended nanotubes were dialyzed (cellulose dialysis membranes, molecular weight cut-

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off of 5000–8000 Da from Spectrum Labs) for one week to obtain a transparent imogolite

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solution (Figure S1). The final imogolite solution was approximately 0.3 g L-1. Milli-Q water

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(≥18.2 MΩ cm) was used throughout the study. The CFs dispersion (2 wt%, WMa-10002) was

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supplied by Sugino Machine Limited and diluted to a 0.2 wt% suspension. According to the

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information from the supplier, the cellulose fibers coming from raw pulp material are produced

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by the water jet method and no further treatment. Based on the XPS spectra of neat cellulose

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fibers, a high-resolution scans of C1s (Figure S2), there is a weak peak that attributes to carbonyl

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groups. Even for that, we cannot name this CFs used in this work as the TEMPO-oxidized CNF,

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though it has been extensively studied in recent years owing to its outstanding mechanical

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properties and relatively easy functionalization.29-30 However, it is well known that from the

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practical application aspect, TEMPO-oxidized CNF is still challenging due to its costly

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processing and some environmental issues.

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Scheme 1 (a) The structure of imogolite nanotube and (b) the fabrication process of

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CFs/imogolite hybrid film.

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Preparation of CFs/imogolite hybrid films CFs/imogolite hybrid films with imogolite

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content of 1, 3, 5, and 10 wt% were prepared as follows. A transparent imogolite solution (0.3

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mg mL-1) containing 0.01, 0.03, 0.05, and 0.1 g of imogolite was slowly added to a 0.2 wt%

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CFs dispersion containing 0.99, 0.97, 0.95, and 0.9 g of CFs to obtain a dispersion of imogolite

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in CFs matrix. The mixture dispersions were labeled as CFs99/Imo1, CFs97/Imo3, CFs95/Imo5,

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and CFs90/Imo10 (Scheme 1b). The mixed dispersion (Figure S1) was stirred overnight.

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Thereafter, water was slowly removed at 60 °C from the mixture until the solid content of the

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mixture condensed to about 5 wt%. The concentrated mixture was then cast in a Teflon petri

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dish (37 °C) for 3 d. After that, films with a thickness in the range of 100 –120 µm were obtained

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and further dried in a vacuum at 105 °C for 24 h to eliminate the adsorbed water.

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Characterization

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FT-IR spectroscopic measurement was carried out with a Spectrum One (PerkinElmer Japan

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Co., Ltd.) with a resolution of 1.0 cm-1 at 23 °C, RH 50%. FT-IR spectra were acquired by

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averaging 64 scans in the wavenumber range from 4000 to 450 cm-1. AFM observations

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(tapping-mode) were carried out using a SPA 400 with a SPI 3800 Probe Station (Hitachi High-

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Tech Science Corporation) at 23 °C using a SI-DF40 rectangular cantilever with a spring

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constant of 33 N m-1 and a frequency of 303 kHz. SEM images of the films were observed using

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a JSM-7401F (JEOL, Inc.) with an accelerating voltage of 3 kV. The SEM was equipped with

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an energy dispersive spectrometer (EDS, Oxford Instrument, X-MAX 50 mm2). The samples

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were coated with a thin layer of osmium before observation. TGA was performed on a thermo-

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balance SII-EXSTAR 6000 TGA 6200 instrument (Hitachi High-Tech Science Corporation,

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Tokyo, Japan) at a heating rate of 10 °C min-1 under nitrogen atmosphere from 25 °C to 550 °C.

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XPS was carried out on an XPS-APEX (Physical Electronics Co., Ltd.) at 1.9 ×10-9 Pa using a

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monochromatic Al-Kα X-ray source of 150 W. The zeta potentials of the imogolite and CFs in

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the NaCl solutions (1mM) were measured on ELS-Z2 (Otsuka Electronics Co., Ltd.) at 25 °C,

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and five measurements were recorded for each sample. DMA was carried out by a Rheovibron

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DDVP-01FP/25FP (A&D Co., Ltd. Tokyo, Japan) in tensile mode at a heating rate of 1 °C

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min-1 in the temperature range from -50 °C to 100 °C with a frequency of 11 Hz. The rheological

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test was carried out by a rheometer Physica MCR 101 (Anton Parr, Graz, Austria) with parallel-

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plate geometry (diameter 50 mm; gap length 1 mm) at 25 °C and the deionized water was used

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as a solvent. The concentrations of the neat CFs, neat imogolite, and CFs90/imo10 solutions

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were 0.18, 0.02 and 0.2 wt%, respectively. WAXD was carried out at the BL05XU beamline

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in SPring-8. The wavelength of the X-ray was 0.1 nm. A 981 × 1043 pixels X-ray detector

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(PILATUS1M, DECTRIS, Switzerland) was used to record the scattering patterns. The pixel

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resolution of the detector was 172 μm. The camera length was set as 117.44 mm. The scattering

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vector was calibrated using the peak positions of Cerium dioxide. The scattering vector q (nm-

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1)

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angle, respectively. Tensile testing was carried out using a testing machine (assembled by the

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JUNKEN MEDICAL Co., Ltd) in a humidity control box at 23 °C. The films were cut into

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strips with widths of 0.5 mm and lengths of 2.5 cm. The thickness of the film was measured by

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a thickness meter with a digital reader (IP-65, Mitutoyo). The tensile test was carried out at

is defined as q = (4π/λ) sin(θ/2), where λ and θ are the wavelength of the X-ray and scattering

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three different relative humidities (RH): 5%, 60%, and 100%. After they were stored in a

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humidity control box for 24 h at RH of 5%, 60%, and 100%, the samples were immediately

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subjected to a tensile test in the humidity control box with the corresponding RH. The distance

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between the grips was 15 mm, and the rate of elongation was 1 mm min-1. The results for each

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material were based on the results of at least five specimens.

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Results and Discussion

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Figure 1 shows the AFM images of the CFs and imogolite nanotubes. Even in the dilute

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CFs solution, many nanofibers can be seen to still be entangled together (Figures 1a and S2a),

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and the diameter of the CFs ranged from hundreds of nanometers to a few micrometers, which

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is much larger than that of imogolite nanotubes. The length of the CFs was about several

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micrometers, indicating its high aspect ratio. The diameters of the CFs shown in Figure S3b

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were as high as several micrometers. This wide range of diameter distributions may have an

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negative effect on the mechanical properties of the CFs. An AFM image of synthetic imogolite

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nanotubes also revealed the high aspect ratio of the nanotubes (Figure 1b), and the length of

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synthetic nanotubes in this study varied from several tens to hundreds of nanometers. As the

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diameter of imogolite was ca. 2 nm, which is much smaller than that of CFs, it would be difficult

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to distinguish the imogolite from the CFs matrix.

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Figure 1. AFM images of (a) the CFs and (b) the synthetic imogolite nanotubes. The dilute CFs

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solution and imogolite solution were spin-coated on silicon wafer. The concentrations of the

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imogolite and CFs solutions are 0.03 wt% and 0.01 wt%, respectively.

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Figure 2 shows the surface morphologies of the neat CFs and CFs90/Imo10 hybrid film.

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For the neat CFs, it can be seen that the nanofibers are distributed randomly and stacked

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together, forming the nanofiber network (Figure 2a). In the hybrid film, though a similar surface

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morphology could also be observed, almost no nanocavities were present, which would be

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favorable for the mechanical properties of the hybrid film. In addition, it was difficult to

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distinguish the nanotubes and nanofibers even at higher magnifications owing to the small size

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of imogolite compared with the CFs (Figure S3c). The EDS data of the CFs95/Imo5 hybrid

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film showed that imogolite was dispersed well in the CFs matrix (Figure S4b). As the content

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of imogolite further increased, the aggregation of the imogolite could be observed in the

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CFs90/Imo10 hybrid film (Figure S4c). In the cross-section images, layered structures were

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observed both in the neat CFs and hybrid film (Figure 2c, d). The larger-diameter nanofibers

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were not only outside the cross-section surface due to the nature of the nanofibers but also split

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into smaller size fibers after the tensile test, which were linked to each other (see the insert

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image in Figure 2c). Yet, interestingly, this phenomenon does not appear before the tensile test 8


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(Figure S5). This may be one of the reasons why CFs materials are generally mechanically

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strong. A similar structure was also reported in other studies.24

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Figure 2. SEM images of surface morphology of (a) CFs and (b) CFs90/Imo10, and cross-

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section after tensile test of (c) CFs film and (d) CFs90/Imo10 hybrid film.

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XPS was used to investigate the elemental information of the CFs and hybrid film (Figure

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S6). The spectra revealed that both the neat CFs film and CFs/imogolite hybrid film contained

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C and O, which were attributed to the nanofibers. In the hybrid film, there was almost no Al,

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but a weak Si peak was observed, which might have been caused by the residual siloxane when

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the imogolite was synthesized because in the synthetic process, tetraethyl orthosilicate is

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molecular outstrip the aluminum trichloride hexahydrate. Also, the synthetic imogolite solution

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are only dialyzed for one week, which may not enough to remove all the by-product, such as

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siloxane. It is highly possible that the imogolite nanotubes were embedded in the nanofiber

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matrix because imogolite nanotubes can easily penetrate the CFs matrix owing to their small

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size.

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Rheological measurements were used to investigate the properties of the neat CFs, neat

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imogolite, CFs95/Imo5 and CFs90/Imo10 hybrid solutions. All data shown in Figure 3 were

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obtained in a steady state of shear flow. In Figure 3a, within the range of low flow shear rates,

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the viscosities of all the solutions decreased with an increase in shear rate, and the slopes of all

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the three curves were almost -1 in the double logarithmic plot. This decreasing tendency of

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viscosity with shear rate is sometimes called shear thinning or thixotropy.31-33 On the other hand,

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in the range of high flow shear rates, the slopes of the viscosities approached zero, indicating

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that the solutions were behaving like Newtonian fluids. The shear thinning behaviors could be

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easier to understand by another expression shown in Figure 3b, where shear stress is plotted

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against flow shear rate. In the range of high flow shear rates, those curves (in Figure 3b)

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positively depended on shear rate. When the shear rate decreased, the slope reduced, and the

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stress values became constant for all samples at the lowest shear rate. This means that those

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solutions exhibit a yield stress, which generally implies that a structure that could sustain shear

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stress was formed in the solutions. This structure should be a kind of network formed by fibrous

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or rod-like compounds. Consequently, in the solutions, the CFs and imogolite could contact

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with each other and cross-link by attractive interactions, such as hydrogen bonding and

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electrostatic interactions. In our case, the stress was measured in a steady shear flow state;

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therefore, this yield stress corresponds to the minimum stress that can flow the system.

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Moreover, during flowing, this yield stress continuously breaks and reforms the network-

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structure that forms by the physical crosslinks of fibers. The yield stress could be estimated

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within a 50% error by repeated sampling and measurements. The yield stresses of the neat CFs solution and neat imogolite solution were 90 mPa and

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100 mPa, respectively. In contrast, the yield stress of the CFs/Imogolite hybrid solution was

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about 1000 mPa. This value was much larger than those of the neat solutions, indicating that

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the CFs and imogolite should have specific interactions that result in the formation of a robust

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and more durable mixture. Moreover, Considering the test error of the rheological result, the

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viscosity and yield stress of the 5%-imogolite hybrid solution is similar with that of 10%-

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imogolite hybrid solution. This result shows that even some amount of imogolite can form the

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stable structure with CFs in in the solution, and almost no changes with the imogolite contents

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further increase. Hence, we imagine that the interaction may originate from the electrostatic

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interaction between the CFs and imogolite because imogolite is known to be positively charged

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on its surface, while CFs is weakly negative charged on its surface. The zeta potential

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measurements of imogolite and CFs were thus carried out. The value of the imogolite zeta

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potential was +20 mV, while that of CFs was -11 mV, indicating that a somewhat weak

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electrostatic interaction is present between the two materials.

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Figure 3. The steady-state viscosity (a) and shear stress (b) of deionized water, neat imogolite,

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neat CFs, CFs95/Imo5, and CFs90/Imo10 hybrid solutions.

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Generally, inorganic components such as clay have neither good interactions with polar

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polymers nor adequate adhesion between them.34 Thus, surface modifications have been

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1

commonly carried out to achieve better interaction between the clay surface and polymeric

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matrix.27,

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polybutadiene and cellulose nanocrystals, Zhang et al. fabricated hybrid materials with

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excellent mechanical performance.36 However, considering the numerous hydroxyl groups on

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both the surfaces of imogolite and CFs, surface interaction between them could be strong

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without further modification. FT-IR spectroscopy was used to characterize the molecular

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interaction between CFs and imogolite. Figure 4 shows the FT-IR spectra of imogolite, CFs,

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and CFs/imogolite hybrid films. The absorption at 1055 cm-1 is attributed to the C-O-C group

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in the neat CFs,34 while absorption of observed at 936 cm-1 and 993 cm-1 are ascribed to the Si-

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O-Al in neat imogolite.9-10 In the hybrid materials, the absorption bands that belong to imogolite

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almost cannot be observed, which may suggest the presence of interaction between imogolite

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and CFs. The absorption bands at 3510 cm-1 correspond to the -OH of imogolite,9-10 and the

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absorption band at 3370 cm-1 corresponds to the -OH of cellulose I (CFs).34 For the hybrid

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films, both absorption bands at 3370 and 3510 cm-1 shift to 3410 cm-1, which may be due to the

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formation of hydrogen bonds between imogolite and CFs. The hydrogen bonding interaction is

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thus expected to bestow the hybrid film with an excellent mechanical performance.

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For instance, utilizing the hydrogen bonding between cyclodextrin-modified

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Figure 4. FT-IR spectra of neat CFs, imogolite and hybrid materials with different content of

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imogolite (1 and 5 % by weight).

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The density of a neat CFs film and CFs/imogolite hybrid films were evaluated. In this

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work, the density of the hybrid films was larger than that of the neat CFs. With the imogolite

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content raised, the density of the hybrid films was in the range of 1500–1530 kg m-3, while, the

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density of the neat CFs film was 1320 kg m-3, similar to the density of nanofibrillated

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cellulose.37 This relative low density meant that a porous structure was formed in the CFs film.

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The void contents thus existed, leading to the inferior mechanical properties for CFs films.

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Considering the theoretical densities of CFs and imogolite are 1560

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respectively, we could calculate the theoretical density of the composites and the void content

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according to this equation: Vv = ( V – mc /ρc – mI /ρI ) / V, where V is the volume of the hybrid

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film, mc is the weight of the CFs, mI is the weight of imogolite, ρI and ρe are the theoretical

14

densities of imogolite and CFs, respectively. The theoretical density of a composite can be

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calculated from the densities of its constituents and their weight fractions.

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Table 1. Calculated void content of CFs/imogolite hybrid films 13


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and 2700 kg m-3 38,


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Imogolite content / wt %

0

1

5

10

Void content / %

12.7

1.9

2.4

3.5

1

Table 1 shows the calculated void content of different CFs/imogolite hybrid films. The

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neat CFs film had the largest void content of 12.7%, which indicates that the CFs network was

3

porous. It should be noted that the voids had a crucial effect on the mechanical performance of

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the films, such as the strength and fatigue life, as the presence of a void meant that the material

5

contained defects. It has been reported that higher-density CFs films with a higher Young’s

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modulus could be fabricated by using high pressures in the fabrication process.39 In the CFs/Imo

7

hybrid film, the imogolite nanotubes could fill the void space during CFs network formation.

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In addition, while hydrogen bonding interactions form in both neat CFs and hybrid films,

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electrostatic interaction also exist in hybrid films. In this case, the electrostatic interaction

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probably supports the low void content film formation because the CFs and imogolite not only

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can be stacked tightly owing to the attraction but also may weaken the electrostatic repulsion

12

between the cellulose fibers. Because of these factors, composites have a lower void content

13

than neat CFs films (Table 1).

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Figure 5. WAXD patterns of neat CFs, CFs95/Imo5 and CFs90/Imo10 hybrid films. X-ray diffractions of the neat CFs, imogolite, and CFs/Imo hybrid film were

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investigated (Figure 5 and S5). In dry conditions, the diffractions of imogolite exhibit four

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peaks at q = 2.9, 4.0, 6.7, 9.6 nm-1 (Figure S7a), indicating that formation of typical imogolite

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bundles.11, 40 These four peaks were also observed in our previous work.10 The neat CFs exhibits

4

a broad peak at q =11.6 nm-1 that represents the amorphous part of the nanofibers.41 The sharp

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diffraction peaks at 15.9 and 24.5 nm-1 can be attributed to the crystalline parts of CFs, and

6

indicate the presence of a cellulose I structure.41-42 For the CFs90/Imo10 hybrid film, CFs and

7

imogolite peaks can be observed. The peaks at q = 2.9 and 6.7 nm-1 (the weak shoulder peak at

8

4.0 nm-1) are attributed to the bundles of imogolite nanotubes (Figure S7b), indicating the

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aggregation of imogolite nanotubes. This is because imogolite nanotubes have a strong

10

tendency to form bundles. For the CFs95/Imo5 hybrid film, the diffraction peaks that are

11

attributed to imogolite are not observed but are appeared in the CFs90/imo10 hybrid film. No

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imogolite diffraction peaks in 5wt% imogolite hybrid film might be due to the low amount of

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imogolite content, and heterogeneous samples. The other factor may attribute to the weak

14

scattering contrast. At 5% the imogolite are intimately linked inside the fibers because of their

15

interactions, the electronic density contrast may be reduced leading to a weak scattering. At 10%

16

imogolite hybrid film, however, many imogolite nanotubes outside the cellulose would have an

17

increase scattering contrast. Therefore, we cannot confirm that imogolite bundles formed

18

though the absence of imogolite diffraction peaks in the CFs95/Imo5 hybrid film.43-44 As the

19

imogolite content reaches 10 wt%, the imogolite bundles form, meaning that imogolite

20

inevitably aggregates in the CFs matrices.

15



Page 16 of 28

1 2

Figure 6. (a) TGA data and (b) corresponding differential thermogravimetric analysis (DTG)

3

for neat CFs and CFs/imogolite composites. The heating rate was 10 oC min-1 under a nitrogen

4

atmosphere with a nitrogen flow rate of 250 mL min-1.

5

Table 2 Temperatures of degradation (temperature at weight loss of 10 wt%) and maximum

6

decomposition-rate of neat CFs and CFs/imogolite hybrid materials, derived from derivative of

7

weight change with temperatures. Sample (oC)

Degradation temperature Peak temperature (oC)

Neat CFs

CFs99/Imo1

CFs97/Imo3

CFs95/Imo5

285 337

284 334

275 320

271 317

8

TG-DTG was carried out to investigate the thermal stability of CFs and the hybrid films

9

as shown in Figure 6a and 6b. The degradation temperature was defined as the temperature at

10

which the weight loss was 10 wt%. Both the CFs and the hybrid films were thermally stable

11

below 250 °C, and only the adsorbed water was lost. As the temperature was further increased,

12

all the samples started to degrade. It should be noted that the thermal stability of the hybrid

13

films was inferior to that of the CFs film. The degradation temperature and DTG peak

14

temperatures are listed in Table 2. It can be seen that both the degradation and the peak

15

temperatures decreased with an increase in the imogolite content, indicating that the hybrid

16

films had an inferior thermal stability. This thermal stability of the hybrid film was different 16


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Biomacromolecules

1

from those of other clay/CFs composites. For instance, in the case of sepiolite /CFs hybrid

2

materials, the temperatures increased with clay content.29 The neat CFs generally has a low

3

thermal-expansion coefficient and two degradation stages, thermal cracking (50–350 °C), and

4

carbonization (400–500 °C). Imogolite, which is similar with halloysite,45 exhibits mainly two

5

weight loss steps (Figure S8). The first step is from 30 to 180 °C, the second step is from 180

6

to 500 °C. The first step was attributed to the loss of adsorbed water, while the second one

7

corresponds to the dehydroxylation of imogolite.46 The TGA curves demonstrated that the

8

degradation temperature of CFs/imogolite hybrid materials were lower than that of the neat CFs

9

film. The decreased thermal stabilities of the CFs were possibly caused by the catalytic effect

10

of the acidic metal group of imogolite. The aluminum groups may act as the catalyst for CFs

11

thermal degradation, as levoglucosan was produced in the first degradation stage for CFs.47 For

12

imogolite, a thermal collapse phase formed, leading to more acidic sites than on normal

13

nanotubes,8 which could have accelerated the degradation of CFs.

14

To characterize the thermal-mechanical property of the hybrid film, DMA was carried

15

out. Figure 7 shows both of the storage modulus of the CFs film and CFs95/Imo5 hybrid film

16

decrease with the increase in temperature. Compared with the neat CFs film, the storage

17

modulus of the hybrid film is higher over the entire temperature range, which may be attributed

18

to the good dispersion of imogolite in the CFs matrix. Furthermore, in the CFs/imogolite system,

19

the formation of a robust and durable structure may also contribute to the superior mechanical

20

performance. It should be noted that this modulus is also much better than that of

21

polymer/imogolite hybrid materials due to the better thermal stability of the CFs.48 Similar

22

phenomena have also been reported. For instance, Gabr et al. fabricated a bentonite (another 17



1

kind of clay)/nanocellulose composite, the storage modulus of the hybrid material decreased

2

with an increase in temperature, and they ascribed the decreased modulus to the α-relaxation,

3

which is the glass–rubber transition of nanocellulose.24

4 5

Figure 7. Storage modulus and mechanical tan δ as the function of temperature for neat CFs

6

film and CFs95/Imo5 hybrid film.

7

The stress–strain curves from the tensile tests performed at different RH are presented in

8

Figure 8. The tensile strength, elongation at break, and Young’s modulus are listed in Table S1.

9

At a low RH of 5% (Figure 8 a, c), the tensile strength increased when imogolite was introduced.

10

For instance, the strength increased from 168 MPa for neat CFs to 193 MPa for the 5 wt%

11

imogolite/CFs hybrid material. Additionally, the elongation at break improved considerably

12

from 5.7% for the neat CFs to 7.9% for the 5 wt% imogolite/CFs hybrid material. The Young’s

13

modulus also increased with an increase in imogolite content (Table S1). Therefore, imogolite

14

can reinforce CFs films. Indeed, imogolite also improved the mechanical performance of the

15

PVA films as the mechanical performance of the PVA/imogolite was considerably higher than

16

that of the PVA films.48 As the EDS data shows, the 5 wt% imogolite content can disperse well 18


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Biomacromolecules

1

in the CFs matrices. For the 10 wt% imogolite content, bundles of imogolite form, indicating

2

that dispersion in the CFs matrices was inadequate. This could be a factor that affects the

3

mechanical performance of the material; it is also worth noting that the interactions and the

4

robust mixture that formed in the CFs/imogolite system also benefit its mechanical result.

5 6

Figure 8. The tensile stress-strain curves of CFs and CFs/imogolite hybrid materials with

7

various amounts of imogolite at (a) RH 5%, (b) RH 60%, and corresponding strength, ultimate

8

elongation at RH 5% (c), RH 60% (d).

9

At a higher RH of 60% (Figures 8b, 8d), the tensile strength decreased, but the

10

elongation was larger than that at the low RH. At an RH of 60%, the strength continuously

11

increased with an increase in imogolite content, from 115 MPa for the neat CFs to 137 MPa for

12

the 10 wt% imogolite/CFs hybrid material. Meanwhile, the elongation at break value was much

13

larger than that at an RH of 5%. The maximum elongation was achieved when the imogolite

14

content was 1 wt%. The enhanced elongation at an RH of 60% compared with that at an RH of

15

5% was due to the existence of plastic deformation, which was related to the reorganization of

19



1

the CFs network. Perhaps by water molecules, individual fibrils de-bonding from each other.49

2

By means of the inelastic neutron scattering method, a recent study demonstrated that, in high

3

humidity conditions, most of the water molecules would accumulate, forming a cluster in the

4

amorphous cellulose domains,50 which is unfavorable for the formation of a hydrogen-bond

5

network in cellulose (CFs). Therefore, the mechanical properties were inferior to those at low

6

humidity conditions. The strain-hardening tendency in the post-yield region indicated some

7

orientation of the CFs. As the RH was further increased to 100%, both the neat CFs and hybrid

8

materials exhibited much inferior mechanical properties, i.e., both materials were fragile (data

9

not shown here).

10

At a low RH (5%), the random CFs network in the film without imogolite was held

11

together mostly by physical interactions between the fibril surfaces. As the imogolite was

12

introduced, benefiting from the interactions between them, the CFs and imogolite stacked

13

tightly. This led not only to an increase in the density of the hybrid film but also the sufficiency

14

for effective load transfer; thus, the strength increased. As the amount of imogolite was further

15

increased (10 wt%), the strength of the film decreased because of the inevitable aggregation of

16

imogolite (imogolite bundles), which had a negative effect on the films’ strength and elongation.

17

When the RH was increased further, the water acted as a plasticizer, weakening the hydrogen

18

bonds between CFs and imogolite because the water formed strong hydrogen bonds with the

19

polar carbohydrates and broke the fiber–fiber interaction.51

20

Conclusion

21

CFs/imogolite nanotube hybrid films fabricated by a casting method were prepared. The

22

rheological data reveals that the electrostatic attraction between the CFs and imogolite in 20


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Biomacromolecules

1

solution results in the formation of a robust and durable structure. The densities of the neat CFs

2

and CFs/imogolite hybrid films demonstrate that the void content in CFs/imogolite hybrid films

3

is lower than that in the neat CFs because of the interactions between CFs and imogolite. The

4

EDS reveals that imogolite may be well-dispersed in the CFs matrix when its content is 5 wt%;

5

however, aggregation occurs as the imogolite content further increases (10 wt%), which also

6

can confirm by WAXD data. The thermal stability of the films decreased as imogolite was

7

introduced, which was possibly caused by a catalytic effect. The mechanical properties

8

measured at different RHs revealed that imogolite could reinforce CFs films.

9

Supporting Information

10

The Supporting Information is available free of charge on the ACS Publications website at DOI:

11

.

12

Photographs of imogolite, CFs and their mixed solutions,

13

image of neat CFs and SEM images of the surface of neat CFs and hybrid film, SEM and EDS

14

elemental map of neat CFs and hybrids, SEM images of cross-section before tensile test of neat CFs

15

film and hybrid film., XPS spectra of neat CFs and hybrid film, WAXD profile of CFs and hybrids,

16

TGA curves of imogolite nanotube, table of mechanical properties of neat CFs, and

17

films(PDF).

18

Acknowledgments

19

The authors acknowledge the financial support of JSPS Grant-in-aid for Scientific Research

20

(A) (Grant No. 26248053, 17H01221) and JSPS A3 Project. WAXD measurements were

21

conducted on the BL05XU beamline in SPring-8 with the approval of the Impact project. We

22

thank Dr. Taiki Hoshino for experimental assistance in BL05XU.

C1S spectra of neat CFs,

21


AFM phase

hybrid


1

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Organic-Inorganic Hybrid Films Fabricated from Cellulose Fibers and

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