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Feng, Y., Li, J., Wang, B., Tian, X., Chen, K., Zeng, J., Xu, J., and Gao, W. (2018). "Novel nanofibrillated cellulose/chitin whisker hybrid nanocomposites and their use for mechanical performance enhancements," BioRes. 13(2), 3030-3044.

Abstract

The aim of this work was to demonstrate the production potential of a combination of the two materials cellulose nanofibrils (CNF) and chitin nanowhisker (CNW) using wheat straw and chitin. CNF and CNW were prepared from TEMPO-oxidation and acid hydrolysis prior to high-pressure homogenization. The zeta potential results indicated the differences in the suspended mechanism for the CNF and CNW dispersions. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) revealed that the chemical composition and crystal unit changes during the chemical separation process. Hybrid CNF/CNW films were prepared via casting and evaporation. The films had better mechanical properties, which were ascribed to multivalent physical interactions between CNF and CNW. However, increasing the CNW content up to 50% negatively affected the mechanical properties of the hybrid films. In addition, all films showed high transparency and excellent flexibility. These results indicated that the interaction between CNF and CNW effectively enhanced the mechanical performance of films.


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Novel Nanofibrillated Cellulose/Chitin Whisker Hybrid Nanocomposites and their Use for Mechanical Performance Enhancements

Yucheng Feng,† Jinpeng Li, Bin Wang,* Xiaojun Tian, Kefu Chen, Jinsong Zeng, Jun Xu, and Wenhua Gao

The aim of this work was to demonstrate the production potential of a combination of the two materials cellulose nanofibrils (CNF) and chitin nanowhisker (CNW) using wheat straw and chitin. CNF and CNW were prepared from TEMPO-oxidation and acid hydrolysis prior to high-pressure homogenization. The zeta potential results indicated the differences in the suspended mechanism for the CNF and CNW dispersions. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) revealed that the chemical composition and crystal unit changes during the chemical separation process. Hybrid CNF/CNW films were prepared via casting and evaporation. The films had better mechanical properties, which were ascribed to multivalent physical interactions between CNF and CNW. However, increasing the CNW content up to 50% negatively affected the mechanical properties of the hybrid films. In addition, all films showed high transparency and excellent flexibility. These results indicated that the interaction between CNF and CNW effectively enhanced the mechanical performance of films.

Keywords: Cellulose nanofibrils; Chitin nanowhisker; Nanocomposites; Mechanical properties

Contact information: Plant micro/nano fiber Research Center, State Key Laboratory of Pulp and Paper Engineering, School of Light Industry and Engineering, South China University of Technology, Guangzhou 510640, China; *Corresponding author: febwang@scut.edu.cn. † Contributed equally to this work.

INTRODUCTION

Agricultural and marine wastes are two of the least utilized classes of renewable resources in China, which is a big agricultural country (Chandra et al. 2012). The isolation of cellulose and chitin from renewable resources such as crop and shellfish residues are beneficial to industries and the environment due to their large quantities of raw materials and renewability (Shen et al. 2016). In recent decades, growing concerns for preserving the environment have resulted in an increase in the use of environmentally friendly materials to produce industrial products. Now a promising area is to more effectively use chitin, which is the next most available polysaccharide after cellulose and hemicellulose. In addition, chitin and cellulose are all renewable, biodegradable, and non-toxic.

Cellulose nanofibrils (CNF) are nanomaterials with widths on the nanometer scale and lengths often in the range 1 μm to 5 μm (Khalil et al. 2012). These nanomaterials have the advantageous properties of cellulose, including renewability, degradability, and worldwide availability. Therefore, nanomaterials show great potential for applications in many fields. Mechanical, enzymatic/mechanical, and chemical/mechanical methods have been applied as a means of isolating CNF from different resources (Huang et al. 2013; Fang et al.2014). CNF has been used as a component in various fields including electronic devices, tissue engineering, and energy storage (Zheng et al. 2013; Li et al. 2014). Although these materials possess excellent mechanical properties, the tightly packed random network structure shows typically brittle failure as cracks (Malho et al. 2014). Hence, some cross-linked polymers such as phenolic resins, epoxy, and melamine formaldehyde were mixed with CNF in the following work (Ansari et al. 2014; Nair et al. 2017; Diop et al. 2017). The consequent problems could occur due to rigid and brittle covalent bonds, which lead to losses in toughness (Ansari et al. 2014).

Chitin is a linear polysaccharide constituted of β-(1, 4)-linked N-acetylglucosamine (GlcNAc) units. Chitin nanowhiskers (CNW) have a highly uniform structure with lateral dimensions from 2.5 nm to 25 nm. The positive charge on the surface of chitin can react with negatively charged cellulose by static adsorption. Owing to the good film-forming performance, nano-scale CNW has a strong interaction with cellulose (Kumar et al. 2017).

Physical adsorption is a feasible way to form composite materials and retain their mechanical properties (Qiu et al. 2016). The CNF-based materials were prepared using polysaccharide carbohydrates and proteins, such as chitosan, chitin, and silk, which consist of various kinds of chemical bonds that can bond with the cellulose surface by physical or chemical interactions (Singh et al. 2013; Naseri et al. 2014). Due to physical adsorption and complementary properties of both cellulose and chitosan (e.g., hydrogen bonding between amide and hydroxyl groups, and electrostatic attraction between the positively charged and the negatively charged groups), some hybrid composites have been prepared and used in different areas, such as anti-microbial and oxygen barrier materials (Singh et al. 2013; Kingkaew et al. 2014; Mututuvari and Tran 2014). Toivonen provided a new method to prepare a CNF/chitosan nanopaper, which showed excellent mechanical properties under a wet moist environment (Toivonen et al. 2015). However, the agglomeration of cellulose/chitosan composites has been found to occur, especially at a higher level of chitosan loading; this led to a decrease in mechanical properties (Abdul et al. 2016).

In this study, chitin nanowhiskers were used to reinforce the CNF nanofilms. First, CNF was mixed with CNW to form homogeneous and aqueous mixtures. The chemical bonds within the intramolecular and intermolecular of CNF and CNW were investigated by Fourier transform infra-red (FTIR) spectra. Additionally, the mixed film was easily cast and highly transparent. Its mechanical and thermodynamic properties were evaluated.

EXPERIMENTAL

Materials

Wheat straw was provided by Shandong Tranlin Group Co., Ltd. (Shandong, China). Cellulosic fiber (α-cellulose ≥ 95%) was obtain from wheat straw (Supplementary information 1, S1). α–Chitin was purchased from Yuan Ye BIO Ltd. (Shanghai, China). Analytical grade 2, 2, 6, 6-tetramethy-1piperidinyloxy (TEMPO) and sodium hypochlorite were used as received. Water at 18.2 MΩ was purified with a Millipore Milli Q system (Millipore Direct Q5, Merck, USA). All other chemicals were analytical reagent grade obtained from Sinopharm Chemical Reagent Co. (Shanghai, China) and used without further purification.

Preparation and characterization of CNF and CNW

Cellulosic fiber was oxidized under the TEMPO/NaClO/NaBr system at pH 10.2 to minimize hydrolysis of the polymeric structure. More detailed information is described in S2 (Supplementary information 2 in the Appendix). The carboxylate content of cellulose was investigated by conductometric titration (Supplementary information 3, S3) and the conductometric titration curve is shown in Fig. S1(a). Viscosity of nanofibers and the degree of polymerization (DP) was determined by S4 (Supplementary information 4).

Chitin nanowhiskers were prepared according to Li et al. (2015). Chitin was hydrolyzed under acidic conditions and then partially deacetylated in a NaOH solution (Supplementary information 5, S5).The C2 amino content of CNW was determined by conductometric titration (Supplementary information 6, S6), and the conductometric titration curve are depicted in Fig. S1(b). Molecular weight and degree of deacetylation of CNW were determined by S7 (Supplementary information 7, S7).

Preparation of mixtures of CNF /CNW and film casting

The films of CNF and CNW were mixed and dispersed in different mass ratios and formed by a casting method (Supplementary information 8, S8). The films were marked as CNF-1 (pure CNF-1 film), CNF-2 (pure CNF-2 film), CNF-5 (pure CNF-5 film), CNW (pure CNW film), MIX-1 (CNF-2/CNW=70:30), MIX-2 (CNF–2/CNW=50:50) and MIX-3 (CNF2/CNW=30:70).

Methods

Morphological analysis

Morphological features of CNF and CNW were determined by transmission electron microscopy (TEM). A homogeneous solution was obtained by sonication. Dilute CNF and CNW suspensions were dropped onto glow-discharged carbon-coated TEM grids and negatively stained with 2 wt.% uranyl acetate for 5 min. The samples were observed on TEM (Jeol, JEM-2100F, Tokyo, Japan).

To complement the TEM images, the hybrid film was observed by atomic force microscopy (AFM) with a multimode scanning probe microscope (MultiMode-8, Bruker Corporation, Billerica, MA) that was operated in tapping mode.

Zeta potential

The zeta potential of CNF and CNW solutions were measured using a Zeta sizer Nano ZS (Malvern Instruments, Malvern, UK). Cellulosic fiber and chitin whisker dispersions (0.30 wt.%) were previously dispersed with homogenizer for 5 min at 12000 rpm/min.

Fourier transform infrared spectroscopy (FTIR) measurement

The FTIR spectra for CNF, CNW, and film-samples were recorded with a Nexus 470 spectrophotometer (Thermo Nicolet, Madison, WI, USA). The samples were mixed and ground with KBr for IR measurements within the frequency range of 4000 cm-1 to 400 cm-1.

X-ray diffraction

CNF, CNW, and CNF/CNW films were X-rayed using a D8 Advance diffractometer (Bruker, Daltonik GMBH, Bremen, Germany) with CuKα filtered radiation (λ = 0.154 nm) and was operated at 40 kV with a current density of 40 mA. The scanning range (2θ) was from 5° to 45° at a scan rate of 2°/s and a step width of 0.02°.

The crystal width was determined by the Scherrer equation (Eq. 1),

 (1)

where K is a constant equal to 0.9, λ is the wavelength of the radiation, θ is the diffraction angle, and β is the full width at half-maximum intensity. According to the previous work (Wada et al. 2001), the Z value was determined to discriminate the Iα and Iβdominant structures, given by Eq. 2,

 (2)

where d1 and d2 are the d-spacings marked in XRD profile.

Mechanical test

The films were cut into 10 mm × 80 mm strips. The mechanical tests were performed with an Instron 5565 testing machine (Inston, Canton, MA, USA) using 0.1 kN power sensor at a crosshead rate of 10 mm/min in a constant temperature and humidity facilities. Film thicknesses were measured with a thickness tester (L&W, SE51, Stockholm, Sweden), and the thickness of each film was determined as an average of ten measurements.

Thermogravimetric analysis (TGA)

The TGA of films was performed with a NETZSCH TG 209 Jupiter Thermal Analysis System (TA Instruments, New Castle, DE, USA) thermogravimetric analyzer under an air atmosphere (50 mL/min). The samples were heated from 25 °C to 650 °C at a heating rate of 10 °C/min.

Porosity

The film porosity was determined by measuring the wet film weight in deionized water overnight at room temperature and the dry membrane was weighted after drying in a vacuum oven at 50 °C for 24 h (Supplementary information 9, S9).

RESULTS AND DISCUSSION

Preparation and Characterization of CNFs and CNW

The degree of polymerization (DP) of the CNF presented a descending trend according to the viscosity data shown in Table 1.The reduction of DP in TEMPO-oxidized cellulose was consistent with Zhang et al. (2012). When the amount of NaClO was raised to 9 mmol/g, the DP decreased by approximately 25%.

The carboxylate content of the TEMPO-oxidized fiber was investigated with the addition of different amounts of NaClO. The carboxylate content of CNF was remarkably increased from 0.59 mmol/g to 0.95 mmol/g at 1.5 and 9.0 mmol/g of NaClO, respectively. The change in zeta potential from -32.9 mV to -48.1 mV indicated that more carboxylate was formed on the fiber surface. Therefore, in the oxidation process, hypochlorite promotes the conversion from alcoholic hydroxyls to carboxyls in cellulose units (Zhang et al. 2016).

All the prepared CNF and CNW were easily suspended in aqueous solution to achieve a well-dispersed suspension. The zeta potential of CNF and CNW suspensions were then detected, and the results were different, as shown in Table 1. Therefore, the nano-dispersion mechanisms of TEMPO-oxidized CNF and acid hydrolyzed CNW were different. CNF dispersed in water by anionic C6-carboxylated groups, which were formed by TEMPO oxidation on the glucose unit surface in CNF (Zhang et al. 2012). CNW dispersed in water by cationic C2-ammonuiun salt group on the N-acetylglucosamine units surface in CNW. The dispersed behaviors of the suspensions, when considered separately, were caused by charge repulsions between the positive electrical charges on CNW and the negative electrical charges on CNF.

Table 1. Physical and Crystal Structures for CNF and CNW with different Chemical Treatments

Note: Amount of hypochlorite added: a: 1.5 mmol/g; b: 3.0 mmol/g; c: 9.0 mmol/g

The viscosity average molecular weights (Mv) and degree of deacetylation (DD) of raw chitin were 8.41 × 105 and 16.1%. After the acid hydrolysis and N-deacetylation reaction, Mof CNW was reduced to 1.15 × 104 and DD was raised to 54.16%. This indicated that the amorphous regions of chitin were destroyed in the acid hydrolysis process, and the molecular chains of chitin were broken into smaller chains. The DD values in the present work were less than the referenced values (Kaya et al. 2016). In addition, the partial deacetylation caused more intensive protonation and amino groups exposed on the surface of CNW (Li et al. 2016). Then higher values of positive zeta potential were shown in aqueous solution after N-deacetylation treatment, which is more conducive to get a stable dispersion system (Li et al. 2016).

Chemical Structural Analysis CNF and CNW

FTIR was employed to investigate the functional group, which further explained the correlation of chemical structure and dispersion characteristics of CNF and CNW suspensions, as shown in Fig. 1.

The absorption peak of CNW in the region around 3250 cm-1 was an indication of the stretching of -OH groups. The vibration absorption peaks ranging from 2900 cm-1 to 2800 cm-1 were anti-symmetric and symmetric -CH stretching vibrations (Li et al. 2015). The vibration absorption peak at 3460 cm-1 was attributed to the N-H stretching vibration, which resulted in a positive zeta potential of CNW in suspension liquid.

The FTIR spectra of CNF with different oxidation degrees are depicted in Fig. 1a. The spectra exhibited similar characteristic absorption peaks. The absorption peak in the range of 3650 cm-1 to 3000 cm-1 was assigned to the stretching of O-H, and the peak at 1600 cm-1 was attributed to the C=O stretching vibration of COO. The presence of the COO group indicated a negative surface charge on CNF surface in suspension liquid.

Moreover, a comparison of the intensity of the stretching vibration band at 1600 cm-1 was executed by the normalization of the FTIR spectrum (Fig. 1b). Due to the higher oxidation degree, the peak at 1600 cm-1 attributed to carboxylate group in CNF-5, was stronger than CNF-2 and CNF-1.

Fig. 1. (a) FTIR spectra of CNW and CNF (CNF-1, CNF-2, and CNF-5), (b) the spectra were normalized so that the peak heights of the bands were equal, (c) XRD profile of CNF, and (d) Chitin and CNW

XRD Analysis

The X-ray diffraction profiles of CNF and CNW were analyzed, and results are shown in Fig. 1 (c) and (d). The diffractions of CNF at 22.6o is related with (200) planes, which reveals a typical cellulose I crystalline form (Chen et al. 2011). The crystal width, Z value (to discriminate the Iα and Iβ dominant structures) and degree of crystallinity (CrI) were determined according to S10 (Supplementary information 10).

A positive value manifested Iα structure and a negative value represented Iβ structure. The Z value of -5.59 indicated that the prepared CNF belongs to Iβ crystal structure. The crystal length and width of CNF1 were 9.1 and 4.3 nm, determined from the (004) and (200) diffraction peaks, respectively, and the results are listed in Table 1. The aspect ratio of the cellulosic crystals was 2.1, which was lower than that of the wood cell wall (Marko et al. 2008).

Similarly, the crystal sizes of chitin were 8.4 and 5.7 nm, which was determined from the diffraction peaks at 9.6° and 19.6°, as shown in Fig. 1(d). The crystal sizes of CNW slightly decreased to 7.7 and 4.4 nm after acid hydrolysis. However, the CrI value of CNW increased after acid hydrolysis, which was from 0.44 to 0.52. A possible explanation is that hydrolysis mostly occurred on the amorphous region of α-chitin.

Morphological Analysis

Figure 2 shows TEM images of CNFs and CNW. It was revealed that nanofibers could easily form lateral aggregates due to their large specific surface area. Carboxylate groups on the surface of CNFs promoted the conversion of fibers into individual fibers with uniform lateral size ranging from 10 nm to 15 nm. The diameter of CNF-5 sample was smaller than CNF2 and CNF-1. The results indicated that TEMPO oxidation efficiently promoted cellulose nanofibrillation to individual nanofibrils (Liimatainen et al. 2012). Fig. 2(c) shows that more severely oxidized fibrils have smaller diameter and smaller size of CNF, which tends to form more transparent suspension, as shown in Fig. 2(e).

Fig. 2. TEM images of (a) CNF-1, (b) CNF-2, (c) CNF-5, (d) CNW, and (e) the suspension of CNF

Preparation and Characterization of Films

The goal was to prepare a hybrid nanofilms with the positively charged CNW and negatively charged CNF to improve the performances of the nanocomposites. Figure 3a shows the photographs of CNF-2, CNW, and MIX-2 films. There were obvious differences in transmittance of the films with different composition ratios, as shown in Fig. 3b. The films of CNF (CNF-5, CNF-2, and CNF-1) showed high transparencies in the range of 400 nm to 800 nm, which were between 60% and 80%.

Table 2. Physical and Mechanical Properties of the Films from CNF and CNW with Different Proportions

Note: Test Conditions: a: films were kept at 23 °C and relative humidity of 50% ± 1% for 24 h; b: films were kept at 23 °C and relative humidity of 98% ± 1% for 24 h.