Sustainable Preparation of Cellulose Nanofibrils and Their Applications for Multifunctional Nanocomposites

Abstract

Cellulose nanofibrils (CNFs), which are commonly produced from lignocellulosic biomass, have received increasing attention in recent years due to their unique properties, such as high specific surface area (up to 600 m2/g), high elastic modulus (29-36 GPa), high thermal stability (being stable against temperatures up to 200-300 °C), as well as renewability and biodegradability. These unique properties make CNFs as promising building blocks for the manufacture of many sustainable nanomaterials. However, sustainable and low-cost production of CNFs is still challenging, which limits their large-scale applications. As a kind of agro-industrial wastes, paper mill sludge (PMS) has posed serious environmental and economic challenges for disposal due to the more stringent regulations and diminishing land availability recently. Instead of the traditional landfill or burning treatment strategies, PMS, usually with negative cost, could be used as a cheap and sustainable feedstock for producing CNFs. Thus, the main objectives of this research are to valorize PMS into value-added CNFs through a sustainable approach and to develop multifunctional nanocomposites based on the PMS-derived CNFs. In the first project (Chapter 2), a sustainable approach was demonstrated to efficiently convert PMS to CNFs by formic acid (FA) hydrolysis pretreatment and the followed microfluidization. It was found that the mild FA hydrolysis (at 95 ºC for 3-6 h) pretreatment could hydrolyze most of hemicellulose, swell and break down the cellulose fibers, and the collected cellulosic solid residue with a high yield (over 75%) could be further converted to CNFs with relatively low-intensity microfluidization (only two passes). Notably, more than 90% FA in the collected supernatant can be recovered through a simple vacuum distillation process. Moreover, cellulose nanopaper (CNP) was prepared from the CNFs suspension via a simple vacuum filtration approach. The obtained CNP exhibited good mechanical properties with the maximum tensile strength and toughness of 106.4 MPa and 6.62 MJ/m3, respectively. In light of the advantages of superior mechanical properties, high thermal stability, low thermal expansion coefficient, tunable optical properties, etc., CNP has been considered as a promising material with great application potential in diverse fields. However, the hydrophilic nature of CNP significantly limits its practical application. In the second project (Chapter 3), a facile and sustainable approach was demonstrated to functionalize the CNP obtained in Chapter 2 by impregnation of chitosan (CS) and the followed halogenation. The mechanical strength of the functionalized CNP was improved at both dry and wet conditions, especially the wet tensile strength was increased by 512.6%. Meanwhile, both the transparency and barrier properties were significantly enhanced. Importantly, part of the amino groups on CS can be transformed into N-halamines during the halogenation process, which endowed the chlorinated CNP/CS with excellent antibacterial performance against both S. aureus and E. coli. The functionalized CNP with water resistance, high transparency, excellent antibacterial and barrier properties shows great application potential in the field of advanced packaging materials. Polypyrrole (PPy) with hydrophobic and conductive nature has been investigated as a promising conducting polymer with numerous potential applications. In order to further improve the water resistance of the PMS-derived CNP and introduce new features to expand its potential applications, in the third project (Chapter 4), PPy was further introduced into the CNP/CS through a facile in situ polymerization process. Results indicate that the obtained CNP/CS/PPy showed excellent water resistance with the wet tensile strength up to 80 MPa, which doubled the value of CNP/CS and was more than 10 times higher than that of the pure CNP. Intriguingly, the functionalized CNP/CS/PPy exhibited a high conductivity of 6.5 S cm-1 and good antibacterial activity towards S. aureus and E. coli. In addition, the EMI shielding application of the CNP/CS/PPy was further examined which showed a high shielding performance around 18 dB. Considering the multifunctional properties, the CNP/CS/PPy may find applications in a variety of high-tech fields such as medical packaging, electronic packaging, EMI shielding, and so forth. As another promising conducting polymer, PEDOT:PSS has been widely used as the electrode materials for supercapacitors. However, processing PEDOT:PSS bulk films with good flexibility and mechanical stability is still challenging. In the fourth project (Chapter 5), we investigated the feasibility of using the PMS-derived CNFs as building blocks for the fabrication of PEDOT:PSS based flexible electrodes. Instead of using the costly commercial PEDOT:PSS, we developed a facile and low-cost strategy for the fabrication of mechanically strong and conductive PEDOT:PSS/CNF nanopaper (denoted as PEDOT:PSS/CNP). Firstly, well-dispersed PEDOT:PSS/CNF aqueous suspension was prepared through an in situ polymerization process, in which the CNFs were coated with conductive PEDOT:PSS. Afterwards, flexible PEDOT:PSS/CNP was fabricated from the PEDOT:PSS/CNF suspension by a vacuum filtration approach and the followed DMSO treatment. Results indicated that the as-prepared PEDOT:PSS/CNP showed high tensile strength (70 MPa) and electrical conductivity (66.67 S/cm), which can be directly used as the electrodes for flexible supercapacitors. It was found that the assembled supercapacitor exhibited high areal specific capacitance of 888.7 mF cm-2, high areal energy density of 30.86 μWh cm-2, and remarkable cycling stability (95.8% capacitance retention after 10,000 charge/discharge cycles), which was among the best electrical performance reported for PEDOT:PSS based supercapacitors. In summary, this research established sustainable and economically feasible approaches to valorize the PMS into CNFs and their functional nanocomposites. To the best of our knowledge, this work is among the very few pioneering studies that explore the sustainable production of CNFs from agro-industrial wastes. Also, the findings in the development and application of the CNFs based nanocomposites will provide many sustainable alternatives for the traditional packaging materials, EMI shielding materials, and energy storage electrode materials. It is expected that this research will contribute to the revival of the pulp and paper industry in the United States by enabling various biomass-derived products on the market in the near future.