**1. Introduction**

Nowadays, the use of renewable materials instead of plastics is essential since the proliferation of plastics in the environment has been known to create various health and ecological problems. An example of extensive use of plastics is packaging. Recently, packaging was identified as an essential element to address the key challenge of sustainable food consumption [1]. When a food product is thrown away, the packaging is also discarded, leading to an additional environmental burden. Thus, petroleum-based packaging materials need to be replaced with renewable materials.

Chitosan, the second most abundant material after cellulose on Earth, is renewable, biodegradable, biocompatible, non-toxic and capable of transporting antioxidants [2–4]. Chitosan has been investigated for various applications including drug delivery, artificial skin, wound-dressing and biomedical and pharmaceutical applications [5]; contact lens and, water filtration [6]; food packaging [7–9]; and fruit preservations such as tomatoes, carrots and raw shrimps [10–13]. Chitosan does not cause any intrinsic food contamination such as phthalate leaching, thus has emerged as suitable alternatives for commercial plastics. Chitosan can be formed as a nanofiber. Chitosan nanofiber (ChNF) is beneficial to various applications since it has high aspect ratio, good chemical/physical interaction and flexibility. Most ChNFs can be fabricated by electro-spinning process including dissolving and purifying steps [14]. Instead of dissolving, ChNF can be isolated from its raw materials by physical methods, for example,

by using supermasscolloider and high water jet pressure, above 200 MPa [15,16]. ChNF is considered for many applications such as removal of Arsenate [17], biomedical applications [18,19] and filtration membranes [14,20].

Cellulose, the most abundant polymer on Earth, has been used for long time. However, it has become a very interesting subject in recent years due to its possibility for substituting petroleum-based materials, and it is readily available around the world. Cellulose has been used for a wide variety of applications such as paper, packaging, composites, textiles, biomedical and pharmaceutical applications [21,22]. Cellulose nanofiber (CNF) is a nano-sized fiber in the range of ten to a couple hundred nanometers. Since CNF has merits over cellulose nanocrystals, its market is remarkably increasing for various applications [23]. It can be prepared mainly by mechanical and chemical methods. 2,2,6,6-tetramethylpiperidine-1-oxylradical-oxidation (TEMPO-oxidation) is a chemical method to extract CNF from various cellulose resources [24,25].

Cellulose and chitosan have been studied for food packaging materials [3,4,21,22]. It is well-known that chitosan has antibacterial, antioxidant and good food preservation properties. Cellulose has also been used as a food packaging material for long time. Early studies explored cellulose–chitosan composites for food packaging materials [26,27]. However, the blending of CNFs and ChNF has not been employed in any advance research, which could be applicable for food packaging. Thus, in this research, two types of nanofibers, namely CNF and ChNF, were blended for a potential active food packaging material. IThe ChNF was isolated using a physical treatment, so-called, aqueous counter collision (ACC) method [28]. CNF was also prepared from softwood pulp using a combination of chemical method, TEMPO-oxidation, and the ACC method to further decrease its size. We intended to distinguish the ChNF and CNF size to blend them with different morphologies. The prepared ChNF and CNF were directly blended to prepare ChNF–CNF composites. The advantages of blending CNF and ChNF are simple and benign preparation. Furthermore, by distributing various sizes of ChNF and CNF, physical and functional properties of the composite can be controlled. Active food packaging or smart packaging for food products refers to packaging that has functionalities in protecting the products. Those functionalities include preserving freshness and antimicrobial activity. Previous studies have reported that chitosan exhibits the functionalities suitable for active food packaging, for example antioxidant behavior and antimicrobial activity [3,7,10]. Thus, owing to functionalities of chitosan, the ChNF–CNF composite can be an active food packaging material.

To evaluate the morphology of CNF and ChNF, several techniques are available, for example scanning electron microscope (SEM), atomic force microscope (AFM), transmission electron microscope (TEM) and particle size analyzer. The size distributions of CNF and ChNF are very broad depending on the isolation methods. ChNF prepared by electrospinning exhibited a diameter ranging from 70 to 330 nm [18]; 260 nm with beads [29]; from 128 to 153 nm without beads [17]; and between 3 nm and few microns, at which the large diameter of nanofiber was due to self-assembly of ChNF [30]. The ChNF produced by grinder and high-pressure homogenizer yielded around 88 nm in diameter [31]. When chitosan nanoparticles were prepared by dissolving then slowly precipitating them in sodium tripolyphosphate solution, the chitosan nanoparticles exhibited a diameter of around 164 nm [32]. In the case of CNF, the TEMPO-oxidized CNF exhibited its width between 3 nm and few microns in length [25]. Its morphology was also investigated by AFM and, after centrifugal fractionation, the average width of the CNF was reduced to 2.0 ± 0.6 nm [33]. Depending on the treatment conditions, not only the size distribution, but also the physical properties including the thermal properties and crystallinity index can be varied. To the best of our knowledge, there has not been any research focused on the conversion of chitosan to ChNF by using ACC method. Furthermore, no research has been attempted to explore ChNF and CNF composites applicable for active food packaging. ChNF–CNF composites can be easily prepared just by blending ChNF and CNF.

Therefore, in this paper, we investigated the effect of ACC treatment conditions on the properties of ChNF, and prepared ChNF–CNF composites by simply blending two nanofibers, which can be applicable for an active food packaging material. The prepared ChNF–CNF composites as well as ChNF were characterized in terms of morphology, hygroscopic behavior and chemical interaction, as well as thermal, optical, mechanical and antioxidant properties.
