Engineering cellulose nanopaper with water resistant, antibacterial, and improved barrier properties by impregnation of chitosan and the followed halogenation
Graphical abstract
Introduction
Over the past two decades, cellulose nanofibrils (CNFs) have received a great deal of attention due to their outstanding physiochemical properties such as high mechanical strength, large specific surface area, high aspect ratio, low density, tunable surface chemistry, as well as good biodegradability and biocompatibility (Du et al., 2016; Lv et al., 2019). In light of these fascinating properties, CNFs have broad application prospect as reinforcing fillers (Du et al., 2017; Kargarzadeh et al., 2018), rheology modifiers (Li, Wu, Moon, Hubbe, & Bortner, 2021; Liu et al., 2017), biomedical materials (Du et al., 2019; Liu, Du, et al., 2020), and flexible electronics (Fu et al., 2021; Zhao, Zhu, et al., 2020), among others. Recently, cellulose nanopaper (CNP), which is made from CNFs by a self-assembly process such as vacuum filtration or casting (Miao et al., 2020), has attracted extensive interest from researchers because of its advantages such as high thermal stability, low thermal expansion coefficient (<10 ppm/K), tunable optical properties and superior mechanical properties (Fang et al., 2019). Recent studies have demonstrated that CNP has great potential for applications in many interdisciplinary fields such as energy storage electrodes and separators (Chen & Hu, 2018; Chen et al., 2018; Liu, Du, Zheng, Xu, et al., 2021), EMI shielding materials (Cao et al., 2018; Parit et al., 2020), electronic and photonic devices (Dias et al., 2020; Yang et al., 2020), etc.
It should be pointed out that the mechanical properties of CNP are sensitive to moisture due to the hydrophilic nature of cellulose, which significantly limits the practical application of CNP in high-moisture environments. To address this issue, several hydrophobic modification approaches such as physical adsorption, esterification, and polymer grafting have been developed in recent years (Sun et al., 2021). However, most of these modification processes are time-consuming, low efficient and involved in hazardous substances, which show less possibility for commercial application (Wang et al., 2018). Therefore, developing facile and environmentally friendly methodologies to improve the water resistance of CNP is highly desired.
In recent years, chitosan (CS) has emerged a renewable and sustainable polysaccharide with diverse potential applications such as food packaging materials, adsorbents for wastewater treatment, biomedical materials due to its low cost, nontoxicity, biocompatibility, as well as good film-forming and gelling properties (Bakshi et al., 2020). It is worth noting that CS shows good water resistance in neutral and alkaline environments, and it can form strong hydrogen bonding with cellulose due to the presence of abundant surface hydroxyl and amino groups (Mao et al., 2019). Therefore, some attempts have been made to improve the wet strength of cellulose paper or CNP by introducing CS as coatings or additives (H.P.S. et al., 2016). Blending and Layer-by-Layer (LbL) assembly have been considered as two prevalent methods for this purpose. For example, Toivonen et al. (2015) reported the preparation of CNF/CS film by blending CNF suspension and CS solution, followed by a casting process. It was found that the CNF/CS (80/20 w/w) film showed an ultimate wet strength up to 100 MPa. Zhao, Wang, et al. (2020) systematically compared the mechanical, optical, and barrier properties of CNF/CS films produced by two different methods (blending vs. LbL). Results indicated that visible agglomeration occurred in the blending process, resulting in decreased mechanical strength, increased opacity, and inferior barrier property. While the agglomeration can be efficiently avoided in the LbL assembly process, which enabled the formation of compacted structure of CNF/CS films with higher mechanical strength and better barrier property. Although LbL assembly technique shows many advantages and has been used for development of various functional materials (Wagberg & Erlandsson, 2020), large-scale processing of CNF and CS by LbL assembly seems to be challenging. Thus, developing simple and effective method for the preparation of CNF/CS films is highly desired.
In addition to wet strength, antimicrobial activity can be imparted to CNP by introducing CS, due to the presence of positively charged amino groups in the CS molecule backbone. Nevertheless, the moderate antibacterial activity of CS cannot meet the requirement of most practical applications (Wang et al., 2020). It is worth mentioning that the antimicrobial activity of CS could be significantly enhanced upon chlorine bleach treatment, in which the amino groups can be transformed into N-halamines, resulting in CS based N-halamines (usually denoted as CS-Cl) (Cao & Sun, 2008). Note that the N-halamines are compounds containing one or more nitrogen-halogen covalent bonds which is generated by halogenation of imide, amide, or amine groups (Nautiyal et al., 2018). The antimicrobial activity is attributed to the fact that positive halogens in the N-halamine structure can be transferred to appropriate receptors in the cells, eventually causing the bacteria killed (Ma et al., 2020). More importantly, after the halide atoms are consumed, the antimicrobial activity of N-halamines can be easily recovered by recharging them with halogenation agents (Nautiyal et al., 2018).
Motivated by the above discussions, the current study is aimed to functionalize CNP with CS and the followed halogenation to achieve water resistance and antibacterial property. As illustrated in Fig. 1, CNP is firstly immersed in CS solution and the CS functionalized CNP (CNP/CS) could be obtained after drying. Afterwards, halogenation of CS is performed by immersing the CNP/CS into NaClO solution, resulting in the CNP/CS-Cl. To the best of our knowledge, this is the first report on the functionalization of CNP with CS-Cl. This study could supply a facile and sustainable strategy to functionalize CNP and expand its potential applications.
Section snippets
Materials
Never-dried paper mill sludge (PMS) containing 75.6 ± 0.5% of moisture was obtained from a paper mill in USA. The chemical compositions of the PMS include 68.1 ± 0.9% cellulose, 13.5 ± 0.1% hemicellulose, 9.1 ± 0.6% extractives, 5.8 ± 0.1% ash, and less than 0.1% lignin. Acetate acid, sodium hydroxide, sodium hypochlorite aqueous solution (14.0%), potassium iodide, and sodium thiosulfate (99%) were purchased from VWR, USA. Chitosan with deacetylation degree of 85% and molecular weight of
Optimization of CS impregnation
Firstly, we optimized the influence of CS amount on the mechanical properties of the obtained CNP/CS samples by repeating the impregnation process for 1–3 times. As summarized in Table S1, the CS weight gain significantly increased from 9.17% to 31.12% as the impregnation process repeated for 1 to 3 times. Fig. 2 shows the strain-stress curves of pure CNP and CNP/CS samples and the corresponding tensile strength, Young's modulus, and toughness. Compared with pure CNP, the tensile strength
Conclusion
In summary, the current study demonstrated a facile and sustainable approach to functionalize CNP with multifunctional properties including water resistance, antibacterial ability, and improved transparency and barrier performance. After impregnating with CS, the optimized CNP/CS exhibits significantly enhanced mechanical strength with 42% and 557% increment at dry and wet conditions, respectively, and the barrier properties towards both water vapor and oxygen were prominently improved as well.
Abbreviations
- CNFs
cellulose nanofibrils
- CNP
cellulose nanopaper
- CS
chitosan
- CS-Cl
chlorinated chitosan or chitosan-based N-halamine
CRediT authorship contribution statement
Haishun Du: Conceptualization, Methodology, Investigation, Formal analysis, Data curation, Visualization, Writing – original draft. Mahesh Parit: Investigation, Writing – review & editing. Kun Liu: Investigation. Miaomiao Zhang: Investigation. Zhihua Jiang: Resources, Writing – review & editing. Tung-Shi Huang: Resources, Writing – review & editing. Xinyu Zhang: Project administration, Supervision, Writing – review & editing. Chuanling Si: Resources, Funding acquisition, Writing – review &
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (32071720), and Key Technology Research and Development Program of Tianjin (19YFZCSN00950) from Tianjin Municipal Science and Technology Bureau, P.R. China. H. Du and M. Zhang acknowledge the financial support from the China Scholarship Council (No. 201708120052, No. 201708370121).
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