**1. Introduction**

The massive use of petroleum-based plastics in disposable food packaging materials has triggered a global social concern, mainly because of the pollution derived from their synthesis and the related littering problems [1–4]. For this reason, a new model of economic activity, namely "circular economy," has emerged [5–7]. Among the different proposals of a circular economy applied to these plastics, the production of bio-based plastics from alternative feedstocks such as agro-food by-products and naturally occurring biopolymers is a strategy that is attracting the attention of researchers [8–15]. Cellulose is the most abundant polymer on Earth with an annual biomass production of about 1.5 ×

10<sup>12</sup> tons per year, being, hence, one of the most promising bio-renewable resources for reducing and replacing the huge amount of petroleum-based plastics. Cellulose shows full biodegradability in soil and seawater over short times, is lightweight and has excellent mechanical strength [16–18]. Among the vast group of cellulose materials, *all*-cellulose composites (ACCs) are a category of particular interest. These composites are materials in which both the reinforcement and the matrix are cellulose [19,20]. The use of the same material acting as matrix and reinforcement increases the compatibility between the phases and, therefore, the mechanical properties of the composite [21]. Two different procedures have been reported for the preparation of *all*-cellulose composites [20,22]. The first is a two-step method in which cellulose is partially dissolved and then regenerated in the presence of undissolved cellulose. Thus, regenerated and undissolved cellulose fractions may come from different natural origins [23]. The second procedure consists of a one-step method where the surface of cellulose is partially dissolved and regenerated in situ to create a matrix around the non-dissolved portion. The most common solvents employed in these processes are LiCl/DMAc, *N*-Methylmorpholine *N*-oxide (NMMO), NaOH, and the ionic liquid 1-butyl−3-methylimidazolium chloride [20]. Nevertheless, partial and slow cellulose dissolution and non-recyclability have limited the use of these solvents on an industrial scale. In any case, the dissolution process described above for both methods is followed by subsequent solvent removal, cellulose regeneration, and drying [20]. The result of such a process is an *all*-cellulose composite with exceptional mechanical properties [24], optical transparency [25], and improved barrier properties [22] with respect to regenerated cellulose, as well as full biodegradability [26]. *All*-cellulose composites are used in a wide range of applications such as the reinforcement of other polymers, substitution of bone and cartilage materials, the fabrication of electro-active paper, sensors and electrical displays, and the production of biodegradable food packaging materials and mulching films for agriculture [26–37].

Trifluoroacetic acid (TFA) is one of the non-aqueous derivatizing solvents for cellulose [38–44]. The dissolution of cellulose by TFA might not occur in the absence of a chemical reaction [38]: cellulose is trifluoroacetylated selectively in the C6-hydroxyl groups [39]. This derivative is readily hydrolyzed in water, water vapor, or the moisture in the air, forming amorphous and transparent cellulose films [11,38,40]. Trifluoroacetic acid is a naturally occurring organic acid and biodegradable by microbial action [38,45,46]. Moreover, it is recyclable by distillation due to its high volatility and is miscible with many organic solvents and water [11]. TFA has been recently used to fabricate bioplastics from microcrystalline cellulose and plant wastes, as well as blends of cellulose with seaweeds, silk, nylon, poly(methyl methacrylate), and poly(vinyl alcohol) [11,47–50]. When TFA is combined with trifluoroacetic acid anhydride (TFAA), a reactive mixture that allows the acylation of cellulose and cellulose derivatives with carboxylic acids is generated [51,52].

In this work, we prepared *all*-cellulose nanocomposites with potential application in food packaging by using a simple method consisting of dissolving cellulose in a trifluoroacetic acid:trifluoroacetic anhydride (2:1, v:v) mixture and subsequent addition of different cellulose nanofibers dispersed in chloroform. The effect of the solvent in the nanocelluloses was investigated. In addition, the influence of different percentages of these nanofillers on the morphology, optical, mechanical, thermal, and hydrodynamic properties of the nanocomposites was assessed. Furthermore, a model was developed to analyze the mechanical properties.

#### **2. Materials and Methods**
