## 1 Introduction  
As a renewable resource and a biodegradable raw material, cellulose derivatives are attracting more and more attention in the context of increasingly depleted oil resources and environmental protection. It is estimated that bioenergy will replace 30% of the oil source by 2025 [1]. As one of the most important materials of cellulose derivatives, cellulose nanofibers have the advantages of high hydrophilicity, high young's modulus, high strength, and large specific surface area, which provide a broader space for the development of cellulose nanofiber composites, and cellulose nanofibers are now favored by more and more industries.  
Polymer nanocomposites are composed of polymers as matrix and nanomaterials as filler. Cellulose nanofibers are a kind of excellent filler, which is widely used as an enhancement element in various materials, such as starch and cellulose matrices [2, 3]. There are three kindsof cellulose nanofibers: cellulose nanocrystal, cellulose nanofiber, and microfibrillated cellulose (MFC). Among them, MFC has a higher aspect ratio and entangled structure, which become a novel nano-reinforcing material with great potential [4; 5].  
The dimensions and morphology of MFC vary greatly, mainly depending on the degree of fibrillation and any pretreatment involved. Usually, the preparation of MFC includes enzymatic hydrolysis [6], multipass high-pressure homogenization [7], direct mechanical fibrillation [8], or a combination of the aforementioned ways. The direct mechanical fibrillation is mainly for preparation of MFC, which is achieved by a homogenizing process with high pressure for dilute slurries of cellulose pulp. The fibers are subjected to a large pressure drop with shearing and impact forces [9; 10; 11]. The principle of the physical-mechanical method is to break cellulose pulp into fibrils by mechanical forces. In addition to the high-pressure homogenization method [12; 13], other physical methods are usually adopted, including the fine grinding method [14], freezing crushing method [15; 16], and ultra-high frequency ultrasonic method [17; 18]. In our laboratory, NaOH was used to a merecieraeucalyptus kraft pulp, and then MFC was obtained by ball-milling method. Due to the strong impact force of ball-milling, the obtained nanocellulose had a large dispersion in diameter and a relatively short length [2].  
To reduce the energy consumption of the physical-mechanical method, some pretreatments for cellulose modification were developed. For example, after ionization [19] or TEMPO oxidation [20] of cellulose pulp, the electrostatic repulsion between cellulose molecules would facilitate the mechanical treatment to crack the fiber bundles and improve the stability of the MFC dispersion. For example, when the substitution degree of carboxymethylation of a cellulose pulp was 0.1, the energy consumption of following high pressure homogenization can be dropped to 500 kWh/ton [11].  
Inspired by the pretreatment and ionization of cellulose, we used cellulose carbamate (CC) as a precursor of cellulose nanofiber. CC is the intermediate of the Carboel process [21; 22], which is used to produce rayon and cellophane, and thus has received a lot of attention [23; 24]. The starting point of this study is to prepare CC from lignocellulosic of eucalyptus and investigate the preparation conditions. The intent is to loosen the bulk structure of cellulose through the carbamate reaction, and then make the amino group of CC with a positive charge under acidic conditions, so as to facilitate the following mechanical splitting. The colloid milling was used instead of high-pressure homogenization, and the nanocellulose carbamate fibers (CCNF) within 100 nm in diameter were obtained. A starch/CCNF composite film was prepared to test the enhancement of CCNF on the polymer.