Fabrication of highly flexible nanochitin film and its composite film with anionic polysaccharide
Introduction
Chitin is a representative structural material present in the exoskeletons of crustaceans, shellfish, and insects (Muzzarelli, 2011; Muzzarelli et al., 2014), and is composed of β(1→4)-linked N-acetyl-d-glucosamine repeating units (Kurita, 2006; Pillai et al., 2009; Rinaudo, 2006). Although chitin is an abundant biomass resource with comparable availability to cellulose, it is unutilized in practical applications due to the poor feasibility and processability of its stiff fibrous crystalline structure comprising numerous intra- and intermolecular hydrogen bonds. In response, research regarding the fabrication of chitin-based functional materials has gained popularity in recent years. Nanofibrillation is an efficient approach for the functionalization of chitin, and involves the fabrication of nanocrystals and nanofibers (Ifuku, 2012, Ifuku, 2014; Ifuku & Saimoto, 2012; Muzzarelli et al., 2014; Rolandi & Rolandi, 2014; Zhang & Rolandi, 2017). For example, a simple bottom-up approach for the fabrication of chitin nanofibers (ChNFs) has been developed, where a dispersion of self-assembled ChNFs with a width of ca. 20 to 60 nm and length of several hundred nanometers can be easily obtained via the regeneration from a chitin ion gel with an ionic liquid, namely 1-allyl-3-methylimidazolium bromide (AMIMBr), in methanol under ultrasonication (Kadokawa et al., 2011; Tajiri et al., 2013). During this process, AMIMBr forms an ion gel with chitin during heating of the mixture (Prasad et al., 2009). Isolation of the resulting ChNFs from the methanol dispersion via filtration produces a ChNF film with a highly entangled nanofiber morphology. However, this film exhibited relatively brittle properties, where the elongation at break was ca. 1% to 2% under tensile mode. This was attributed to insufficient entanglement with some vacancies among such fibers with relatively large sizes and low aspect ratios. Further, a paper-like chitin sheet comprising longer ChNFs was fabricated via regeneration from a solution of chitin in a deep eutectic solvent of 1-allyl-3-methylimidazolium chloride and thiourea under appropriate conditions (Kadokawa, Idenoue, & Yamamoto, 2020). This sheet exhibited better mechanical properties than the previously developed ChNF film, with an elongation at break of 8.0% and tensile strength of 10.3 MPa, but remained unsuitable for application as a practical soft material. Another ChNF sheet has been reported via mechanical grinding of a native chitin source according to a top-down approach, where stronger properties were achieved, such as an elongation at break of ca. 7.5% and tensile strength of ca. 45 MPa (Ifuku et al., 2011; Ifuku & Saimoto, 2012; Shams et al., 2010). Overall, the above previous studies have led to inflexible nanochitin films and sheets with a low elongation at break, regardless of their fabrication method.
A previous study reported that the above self-assembled ChNFs have a bundle-like morphology, and are constructed via the assembly of thinner fibrils (Kadokawa et al., 2019). Thus, the self-assembly of chitin molecules leads to the formation of thin fibrils during regeneration from an ion gel, which further hierarchically assemble to form ChNFs. Alkaline treatment of the self-assembled ChNF film in aqueous NaOH partially generates amino groups on the ChNFs via partial deacetylation (PDA-ChNFs) (Fig. 1(a)) (Kadokawa et al., 2013; Kadokawa, Egashira, & Yamamoto, 2018; Kadokawa, Obama, et al., 2018). The PDA-ChNF film can be converted to a cationic ammonium structure via protonation in acidic aqueous media (e.g., aqueous formic acid), which readily redisperses the ChNFs via electrostatic repulsion (Kadokawa, Noguchi, et al., 2020). The self-assembled cationic ChNFs (ca. 20–60 nm in width), as well as the other cationic ChNFs with amidinium groups (ca. 100 nm in width) prepared by top-down approach from a native chitin source, followed by chemical derivatization (Kadokawa et al., 2011), were used as reinforcing agents by forming composites with anionic polysaccharide (xanthan gum) hydrogels via electrostatic interaction (Kadokawa, Noguchi, et al., 2020; Kawano et al., 2019). The cationic amidinium ChNFs were also incorporated into composite sheets with alginate, another anionic polysaccharide (Sato et al., 2016). However, the composition ratio of ChNFs to alginate was quite low, and no obvious compositing effect for enhanced properties was observed. This was probably due to the significant difference in the scale levels of the ChNFs (tens – hundreds of nanometers) and alginate (molecular scale).
The present study investigated the disintegration of the assembled bundles in PDA-ChNF films to obtain individual thin fibril materials, which are referred to as scaled-down ChNFs (SD-ChNFs). The PDA-ChNF film was treated in 1.0 mol/L aqueous acetic acid at room temperature with ultrasonication using a homogenizer at 20 kHz and 400 W for 10 min. Scanning electron microscopy (SEM) and transition electron microscopy (TEM) revealed efficient disintegration via electrostatic repulsion to give a SD-ChNF aqueous dispersion (Fig. 1(b)). The SD-ChNFs were isolated from the dispersion via filtration to form a film comprising more highly condensed SD-ChNFs than the parent ChNFs. This afforded the SD-ChNF film excellent flexibility that has not been previous reported for a nanochitin film or sheet. Furthermore, a composite material of SD-ChNFs with ι-carrageenan, a sulfated polysaccharide, was produced via multi-point ionic cross-linking (Fig. 1(c)). The ι-carrageenan paired well with the SD-ChNFs, and a significantly enhanced flexibility was achieved.
Section snippets
Materials
Chitin powder from crab shell was purchased from Wako Pure Chemicals, Tokyo, Japan. i-Carrageenan was purchased from Sigma-Aldrich, Darmstadt, Germany. An ionic liquid, AMIMBr, was prepared by reaction of 1-methylimidazole with 3-bromo-1-propene according to the method modified from the literature procedure (Zhao et al., 2005). Other reagents and solvents were used as commercially received.
Preparation of partially deacetylated chitin nanofiber (PDA-ChNF) film
A mixture of chitin (0.120 g, 0.59 mmol) with AMIMBr (1.00 g, 4.92 mmol) was allowed to stand at room
Results and discussion
A PDA-ChNF film (degree of deacetylation = 20.0%, determined by 1H NMR analysis after acidic hydrolysis in DCl/D2O) was produced via nanostructured regeneration from a chitin/AMIMBr ion gel using methanol, followed by partial deacetylation at 80 °C for 6 h in 30 wt% aqueous NaOH (Fig. 1(a)). An SEM image of the resulting PDA-ChNF film observed to retain nanofiber morphology (Fig. 2(b)). As previously reported, the resulting film was redispersed by electrostatic repulsion in acidic aqueous media
Conclusions
In this study, a SD-ChNF film with superior flexibility was fabricated, where tensile testing revealed a higher tensile strength and elongation at break than those of a PDA-ChNF film. The SD-ChNFs, furthermore, were effectively composited with ι-carrageenan via multi-point ionic cross-linking, where the composite film exhibited a further enhancement in flexibility. The findings of this study demonstrated that the scaling down of PDA-ChNFs to SD-ChNFs enhanced the mechanical properties and
Credit author statement
Kazuya Yamamoto and Jun-ichi Kadokawa conceived the project, designed the experiments, directed the research, and wrote the manuscript. Takuya Hashiguchi performed the experiments and calculations. All authors discussed the results and edited the manuscript.
Declaration of competing interest
The authors declare no competing financial interests.
Acknowledgment
This work was supported by Izumi Science and Technology Foundation (2020-J-018). We would like to thank Editage (www.editage.com) for English language editing.
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