**4. Cellulose network and cellulose interactions**

#### **4.1 Cellulose fibrillar network**

The cellulose microfibrils and their derivatives are responsible for the formation of a cellulose polymer network as seen in film structures, where they interact with other polymeric materials and doped nanoparticles through electrostatic forces, hydrogen bonds, Van der Waals attraction as illustrated in **Figure 2**. The nature of these interactions influences the strength and morphology of cellulose-based films. For example, the presence of similar charged substitutes on fiber surfaces may create repulsive forces to result in porous structure while opposite charged groups on fibril units might cause dense and smooth film surfaces [54]. In addition to this, high dispersibility of cellulose fibrillar matrices and strong hydrogen bonds with other polar moieties through hydroxyl groups are important for the strength of the cellulose-based films. For example, nanocellulose films with high fibrillar network and extensive hydrogen bonding demonstrate low porosity and high oxygen and moisture barrier [55]. Overall, these interactions support a good film framework important for desirable mechanical and gas barrier features. This section explains and exemplifies the nature and importance of cellulose interactions with a focus on physicochemical and mechanical properties of film materials.

#### **4.2 Cellulose-protein interactions**

Native cellulose is a nonionic polymer with reduced interaction capacity without pretreatment. Cellulose derivatives can show interfacial activity and electrostatic

#### **Figure 2.**

*A schematic illustration of cellulose interactions with polymers and nanoparticles.*

potential to interact with proteins via electrostatic interaction, hydrogen bonding or hydrophobic attraction. Electrostatic interactions between protein and cellulose derivatives (e.g., negatively charged methylcellulose and carboxymethyl cellulose) can provide advanced control over the characteristics of cellulose films to improve their mechanical and barrier properties. For example, a recent research showed that cellulose nanocrystals from a bacterial source can participate in protein gels to improve mechanical properties (tensile strength) and as well as oxygen and moisture barrier abilities of nanocomposite biodegradable packaging films [56]. Another study showed that the intermolecular interactions between fish gelatin and crystalline regions of cellulose microfibrils significantly increased the tensile strength and reduced the water vapor permeability of biodegradable fish gelatin films [57]. In addition to this, the physicochemical properties (e.g., pH, temperature, ionic strength, concentration, etc.) of solutions of cellulosic polymers and proteins determine the extent of their interactions. Among these parameters, pH played the most critical role since it affects the surface charges, wettability, density and conformational state of dispersed systems [58]. For example, at lower pH (pH 3), soy protein isolate (SPI) and TEMPO oxidized bacterial cellulose interacted via electrostatic attractions and formed a positively charged noncovalent complex that displayed good creaming stability and elastic gel texture. On the other hand, at higher pH (pH 9), cellulose-protein interaction shifted from strong electrostatic attraction to repulsion [59]. Moreover, at pH 4 negatively charged CMC can form a complex with positively charged pea protein to form insoluble complexes. At lower pH values, the interaction between CMC and protein increases with strong electrostatic repulsion, high viscosity, and steric hindrance [60]. In addition, the presence of small molecules, such as salts or sugars, may alter the nature of interactions. For example, ionic liquids, which are molten salts with organic cations and anions, show significant effects on cellulose-protein complexes [61]. Anions may form hydrogen bonds with hydroxyl groups found in cellulose or protein and disrupt the naturally occurring hydrogen bond between these two polymers. Moreover, cations in ionic liquids may interact with ether oxygen atoms or CH groups in native cellulose or derivatives [62]. It was shown that chloride ion associated with hydroxyl groups in cellulose and resulted in weakening intermolecular hydrogen bonds between protein-cellulose complex extracted from peanut leaf. These interactions reduced the tensile strength of prepared films [63].

#### **4.3 Cellulose-polysaccharide interactions**

The interactions between cellulose and polysaccharides, such as starch, glucomannan, pullulan occur between unsubstituted chains via hydrogen bonding. Anionic cellulose derivatives, such as CMC and CNC can interact with charged polysaccharides, such as sodium alginate or chitosan to form hydrogel structures by intermolecular inclusion interaction. These interactions occur via hydrogen bond or ionic crosslinking and enhances mechanical performances For example, chitosan-carboxymethyl cellulose interaction creates polyelectrolyte multilayer films that includes oppositely charged layers of polyelectrolytes [64]. The electrostatic interaction between these two polymers results in binding of the oppositely charged polymers on the film surface. This brings about improved strength in composite film. Another study showed that intermolecular interaction between konjac glucomannan and pullulan and natural cellulose nanofibrils yields strong crosslinks to improve the flexibility and elasticity of films and reduce the water vapor permeability [65]. Besides, strong hydrogen bonds between hydroxyl groups of cellulose nanofibrils and hydroxyl or carboxyl groups of sodium alginate increased the cohesiveness and water resistance of the biopolymer films [66]. The authors in the reference [67] examined the influence of molecular interactions between microcrystalline cellulose (MCC) and propylene glycol alginate-agar polymeric mixture and their effect on mechanical, physical and barrier properties of microcrystalline cellulose gum edible films. The addition of 4% MCC was sufficient to reduce the water vapor permeability and enhance the tensile strength of the film due to inter and intramolecular hydrogen bonds. Interestingly, incorporation of cellulose resulted in reduced intramolecular interactions between polymers and caused less compact network and hydrophobicity on the film surfaces. A study focused on the effect of electrostatic interactions between cellulose nanofiber and alginate or chitosan in their films [68]. It was highlighted that cellulose-polysaccharide interaction mechanisms led to an increase in tensile strength and water vapor resistance of the polysaccharide films. Another study modified CMC films with xanthan gum, and flaxseed gum to improve the physical and mechanical properties (e.g., water vapor resistance, tensile strength, elasticity), and reduce the weight loss and increase shelf life of mango [69].

#### **4.4 Cellulose-nanoparticle interactions**

Packaging films prepared from native cellulose or derivatives, such as cellulose nanocrystals or CMC, can be doped with nanoparticles to provide them strength, stability, ultraviolet barrier, optical or antimicrobial functionality. For example, paramagnetic iron oxide nanoparticles coating to cellulose nanocrystals was shown to improve the thermostability of the films [70]. Another study focused on the preparation of cellulose nanocrystal films incorporated silver (Ag) [71]. The films showed ultraviolet barrier property and reduced water vapor permeation. Furthermore, calcium hydroxide nanoparticles can support the crosslinking between polysaccharide and cellulose nanofiber by diffusing into the matrix and attaching with ionic bonds. This enables to increase in the opacity and thermal stability of multilayer

packaging films. Photocatalytic TiO2–Ag nanoparticles can be added to provide antimicrobial and photocatalytic activity to CMC based films. In addition, they also increased the tensile strength of the films related to increased electrostatic attractions between hydroxyl groups and O∙Ti∙O bonds between CMC and nanoparticles [72]. In another study, ZnO-loaded cellulose acetate film was obtained by ionic interaction between zinc and oxygen atoms in cellulose acetate [73]. These films exhibited antibacterial action against *E. coli* and moisture barrier characteristics. Recently, Amini et al. [74] created nano-composite packaging films using cotton cellulose and hydrophobic polycaprolactone (PCL) dissolved in an ionic liquid containing zinc oxide nanoparticles. The intermolecular hydrogen bond between cellulose fibers, PCL and zinc oxide resulted in promising film properties (i.e., improved oxygen barrier property, good tensile energy absorption, elasticity, and surface smoothness). The authors mentioned that nanocomposite film may serve as a packaging film for peanuts and meat based on physical and mechanical characteristics. An interesting study mentioned that CMC films prepared with the addition of cobalt nanosheets show strong antimicrobial activity against *E. coli and S.s aureus* without affecting mechanical properties of the films [75].
