## 1 Introduction  
Cellulose nanocrystals (CNCs) surface chemistry and shape may be utterly different, depending on the type of the extraction and the raw materials used. Generally, CNCs extraction consists of an acidic or oxidative hydrolysis of cellulosic sources (cotton linters, vegetal by-products, wood pulp, etc.). As reported in many papers, CNCs extracted by hydrochloric acid (HCI) are uncharged [1], while those obtained by sulfuric acid (H\({}_{2}\)SO\({}_{4}\)) contain sulfate half ester charges [2; 3; 4], and those extracted by 2,2,6,6-Tetramethylpiperidin-1-yloxyl (TEMPO) and ammonium persulfate (APS) contain carboxylic charges on their surface [5; 6]. It bears mentioning that the superficial chemical charges dictate the dispersibility of the CNCs in solvent or water [7; 8] and the strength of adhesion during the interaction CNCs/polymer [9]. Due to their many positive features, like tiny size and high crystallinity, CNCs may be implemented in food packaging materials, in particular as coatings on plastic films, with a consequent enhancement of gas barrier properties [10; 11; 12]. In addition, it has been shown that in dry conditions, with a thickness of about 1 \(\upmu\)m, CNCs coatings displayed an oxygen barrier higher than that obtained from 3 \(\upmu\)m to 4 \(\upmu\)m thickness of common synthetic barriers, such as Ethylene vinyl alcohol copolymer (EVOH) and Polyvinylidene chloride homopolymer (PVDC), currently used for the extension of oxidation sensitive foods [13]. However, it has also been revealed that the interaction polymer/CNCs is not the same for all polymers, and therefore investigation must be conducted to establish a broad knowledge on the polymers and CNCs chemistry for being ascertained by their chemical affinity and adhesion throughout the coating process. Recent studies have demonstrated that the relevance of the CNCs barrier against gases are lost in humid atmospheres, with a subsequent increase in permeation of gases [14; 15; 16]. From the reported findings, CNCs, being very hydrophilic and hygroscopic materials, tend to integrate water, followed by a gradual disentanglement of their crystalline lattice, which then facilitates the gases to cross the coated plastic films even abruptly [16]. Several attempts have been made in order to alleviate that inconvenience by producing more hydrophobic CNCs through their surface chemical modifications, such as esterification, carbamation, silylation, amidation and surfactant adsorption [17; 18; 19; 20]. It is likely that esterification represents one of the most versatile and widespread functionalization pathways of cellulose nanocrystals. Most esterification reactions, such as solvent-free esterification [21; 22], Fischer esterification [23] and transesterification usually use a chemical compound for grafting [24], which most often severely impacts the morphology and the crystallinity of the modified CNCs [25]. For keeping unblemished the crystallinity of the esterified CNCs and lowering the degree of substitution, in this paper, in situ esterification was promoted between crystals (CNCs-CNCs) without a grafting agent, using APS not only as an oxidizing agent, but also as a catalyst for the esterification. It is important to underline that, for reasons of convenience; CNCs from two different sources have been used to perform the esterification: CNCs\({}_{\text{SOSH}}\) from wood pulp were purchased, while CNC\({}_{\text{COOH}}\) from cotton linters were produced in our laboratory. Even though some researchers successfully implemented hydrophobic CNCs, the gas barrier properties of the coated polymers with those modified crystals were lost or slightly improved, but not at acceptable and relevant standards for being applied in packaging [26; 27; 28].