**1. Introduction**

Cellulose is the most abundant renewable natural resource and thus has attracted the interest of researchers around the world [1]. Native cellulose consists of amorphous and crystalline regions; when subjected to appropriate treatments such as mechanical, chemical, or enzymatic methods; nanocellulose (NC) can be obtained by breaking down amorphous regions [2]. In recent years; NC has become one of the most promising nanomaterials and has attracted increasing attention in the field of nanocomposites because of its appealing intrinsic properties which include its high specific surface area; high aspect ratio; low density; high chemical reactivity; high tensile strength; and high Young's modulus [3–6].

Multiple technologies have been developed to prepare NC from cellulosic materials, including ultrasonication [7], high-pressure homogenization [8], steam explosion treatment [9], high-speed grinding [10], acid hydrolysis [11], TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) oxidation [12], and combined treatments [13]. The morphology and properties of NC particles determine their application properties and are affected by preparation methods [14]. Recently, there has been increasing interest in the production of NC from microcrystalline cellulose (MCC) because MCC has a high cellulose content, which is affected by its pectin, hemicellulose, lignin, and other lignocellulosic components [15]. The main drawback of acid hydrolysis, ultrasonication, and high-pressure homogenization methods using MCC is their low NC yield [16–18]. Several studies have attempted to increase the NC yield from MCC; however, their methods were limited by complicated pretreatment steps, high-energy consumption, long treatment times, and high water consumption [18–20].

Nanocellulose has been widely used as a reinforcing agent for various advanced composite applications because of its unique advantages [21], which include improving the mechanical and water-vapor barrier properties of starch films [22]. However, NC particles tend to agglomerate via van der Waals forces and hydrogen bonding during film preparation because a large number of hydroxyl groups are present on the NC surface, which restricts the advantages they offer. The dispersion of NC in starch matrices and the interfacial adhesion between NC and starch are widely accepted as the critical factors determining the reinforcement effect of NC in starch [14]; the advantages offered by NC—i.e., large aspect ratio, high modulus, and large surface area to interact with the starch matrix—can only be fully realized when NC is homogeneously distributed in the starch matrix.

Surface modification of NC to improve its dispersion and compatibility with polymer matrices has been widely studied and different surface modification methods have been reported, such as etherification [23], esterification [24], ionic interaction [25], silylation [26], and oxidation [27]. Most of the previous studies modified the NC directly. However, surface modification of NC is complicated and has limited effectiveness because of the highly aggregated structure of NC [28]. Therefore, it was difficult to prepare NC with a modified group. Alkaline solutions are known to swell cellulose samples. Alkali-swelling can disrupt hydrogen bonding between microfibrils in pulp fibers and could facilitate nanofibrillation [29]. This suggests that pre-swelling can be conducive to nanofibrillation of the MCC. This has positive significance for resource conservation and environmental protection. Thus, the preparation of NC from previously swelling and modified MCC is a new strategy that warrants study.

In order to efficiently obtain modified NC with high dispersibility in starch film, a new preparation process has been developed. Microcrystalline cellulose was swelling by sodium hydroxide solution and first modified with 3-chloro-2-hydroxypropyl trimethylammonium chloride. Then, cationic NCs were prepared from previously modified MCC by acid hydrolysis, high-pressure homogenization, and high-intensity ultrasonication. The yield, zeta potential, average particle size, dispersion stability, morphology, crystallinity, chemical structure, and thermal stability of the NC particles were studied. Finally, the effects of the NC preparation methods on the physico-chemical properties of starch nanocomposites were investigated.
