Construction of TOCNF/NRL high barrier composite film
The general design idea diagram of TOCNF/NRL composite membrane is shown in Scheme 1. The design is based on the idea of replacing plastics with biomass-based composites, and the composite film material with low cost and excellent barrier performance is prepared by vacuum filtration of nanocellulose fibrils (TOCNF) and NRL prepared by TEMPO oxidation method. TOCNF has the advantages of high strength, nano-scale and excellent film formation. NRL has very high elongation at break and excellent hydrophobic properties, which can improve the water resistance and elasticity of the composite film (Sharma et al. 2020). Interfacial compatibility is an important factor affecting the mechanical and barrier properties of polymer composites. Zeta potential shows that TOCNF mixed with NRL has good dispersion stability (Figure S1). The vacuum filtration into the film enables the TOCNF to be arranged in parallel stacks, reducing the free volume of the film. This structure can lengthen the transport path of water vapor and oxygen, which helps to improve the barrier performance of the film, as has been demonstrated in previous studies (Scheme1b) (Wu et al. 2022).
The process of vacuum filtration into a film, TOCNF gradually deposited onto the surface of the filter membrane, forming a dense structure with parallel stacking, and the hydrogen bonding between cellulose and cellulose provided high mechanical strength to the film. During the drying process, the TOCNF formed a three-dimensional interwoven network with each other. NRL is granular in aqueous solution with an average particle size of 362.5 nm (Figure S2c). With the loss of water, NRL particles undergo deformation after contacting each other. When the capillary force between the NRL particles exceeds the repulsive force of the bilayer between them, the outer membrane of the particles breaks and the particles start to fuse, after which the polyisoprene molecular chains inside the particles start to diffuse and entangle, thus forming a uniform, continuous and mechanically strong whole (Rippel et al. 2003; Steward et al. 2000; Tanaka and Tarachiwin 2009; Wei et al. 2020). The whole formed by NRL fills the gap of the TOCNF interleaved network. Thus the SEM image of the film clearly demonstrates the dense and homogeneous surface (Figure S3). The phospholipid-protein among the NRL particles after film formation has been confirmed to be present as a cross-linking point in the network structure of rubber molecular chains (Wu et al. 2017). The force between TOCNF and NRL measured by AFM demonstrated that the protein-phospholipid layer of NRL adheres to the TOCNF surface by cohesive forces (Figure S4). In this process, the tail of the cis-isoprene extends outward in air, allowing the TOCNF to acquire hydrophobic properties (Chhajed et al. 2023). Finally, a biomass-based composite film with high water resistance and high barrier properties was obtained, and strawberry preservation experiments were conducted to prove its good preservation effect.
Mechanical Properties
First, the mechanical properties of the film in the dry state are analyzed. Pure NRL film has excellent flexibility and ductility, but it also has the very obvious disadvantage of low tensile strength and Young's modulus (Sharma et al. 2020). Table 1 shows that pure TOCNF film (C100N0) has high tensile strength and Young's modulus (68.40 MPa and 6.90 GPa). However, it also has the problem of high brittleness and poor flexibility, and the elongation at break of the film is only 1.37%. Based on this, the TOCNF/NRL composite film was designed and prepared in order to coordinate the tensile strength and flexibility of the film. When the proportion of NRL increases, the maximum tensile strength and Young's modulus of the films gradually decreased, while the corresponding elongation at break gradually increased until the tensile strength, Young's modulus, and elongation at break of C10N90 were 2.42 MPa, 0.034 GPa, and 100.52%, respectively. This pattern coincides with the studies of other researchers (Sharma et al. 2020). The addition of NRL increased the elongation at break of the films, probably because the highly ductile NRL filled the interwoven pore structure of TOCNF and covered the surface of TOCNF, acting as a binder. At the same time, TOCNF and the non-rubber components in NRL can promote the strain-induced crystallization behavior of NRL, which further improves the elongation at break and mechanical strength of the films (Kakubo et al. 2000; Tosaka et al. 2004; Trabelsi et al. 2003). In addition, many other studies have shown that the toughening mechanism of TOCNF/polymer composite films mainly lies in the strong interaction between TOCNF and polymer (Forti et al. 2021; Yu et al. 2017; Zhao et al. 2022), while Figure S4 shows that there is an interaction force between TOCNF and NRL, indicating that the latex of NRL can improve the toughness of TOCNF composite films.
For packaging materials, there is frequent exposure to wet environments. Therefore, it is crucial to study the wet tensile properties of the films. The wet tensile strength of pure TOCNF film (C100N0) is only 2.52 MPa, and the wet strength retention is only 3.68%. This is because the film is saturated with water and the hydrogen bonding between the cellulose is broken to some extent. The wet tensile strength of C50N50 film was 15.87 MPa, and the wet strength retention rate was 71.69%. The wet strength retention of C70N30 and C90N10 films reached 79.98% and 95.87%, but their wet strengths were lower than that of C50N50 films due to their lower dry tensile strengths.
Table 1
Maximum tensile strength, elongation at break, Young's modulus, and wet strength retention of films
Sample Name | Elongation at Break (%) | Young’s modulus (GPa) | Tensile strength (MPa) | Wet strength retention rate (%) |
Dry | Wet | Dry | Wet | Dry | Wet |
C100N0 | 1.37 | 2.38 | 6.90 | 0.10 | 68.40 | 2.52 | 3.68 |
C90N10 | 1.91 | 5.96 | 5.40 | 0.12 | 49.10 | 4.54 | 9.24 |
C70N30 | 3.47 | 6.69 | 3.46 | 0.13 | 40.80 | 6.87 | 16.84 |
C50N50 | 12.70 | 12.62 | 2.14 | 1.09 | 22.14 | 15.87 | 71.69 |
C30N70 | 88.84 | 45.10 | 0.88 | 0.26 | 9.01 | 7.21 | 79.98 |
C10N90 | 100.52 | 89.34 | 0.04 | 0.08 | 2.42 | 2.32 | 95.87 |
Water Resistance Of The Film
Evaluate the wettability of a film surface by measuring the water contact angle of the film. The variation of contact angle for a certain time is shown in Fig. 2a (Digital photos see Figure S5). According to the surface wettability principle, hydrophobicity and surface rough structure are the keys to improve the hydrophobic properties of the films (Chen et al. 2019). From Fig. 2a, the small contact angle of C100N0 is due to the high hydrophilicity of nanocellulose, while the water contact angle of the film gradually increases with the increase of NRL content from C100N0 to C50N50, indicating that the hydrophobicity of the film improves during this process. The FTIR plots also showed a shift of the adsorbed water molecule peak with increasing NRL content, which laterally indicates the improved hydrophobicity of NRL (Figure S6). The water contact angle of the C50N50 to C90N10 films showed a slight decrease, which may be due to the accumulation of hydrophilic proteins and phospholipids on the film surface due to the high NRL content. The SEM image of the film clearly demonstrates the gradual increase and increase of bumps on the surface of the film from C50N50 to C90N10 (Figure S3a). Meanwhile, the AFM 3D morphology of the film shows a gradual increase in surface roughness from C50N50 to C90N10 films (Figure S3b). Both data demonstrate the accumulation of protein-phospholipids on the surface of the films. It is worth mentioning that the water contact angle of the pure nanocellulose (C100N0) film decreased by 11° within two minutes, and the film was gradually infiltrated by water. In contrast, after adding NRL (C70N30 to C90N10), the water contact angle do not change significantly within two minutes, indicating that NRL hindered the penetration of water and also showed the stability of the hydrophobic properties of NRL, which is consistent with the previous studies (Adibi et al. 2022).
MC indicates the total free volume of water molecules in the film network. WS of the film indicates the integrity of the film under aqueous conditions. SD indicates the ability of the film to absorb exudates or water when used as a biomaterial or food coating. Figure 2 (b, c, d) shows that the MC, WS, and SD of the films gradually decrease from C100N0 to C10N90 due to the addition of NRL, which reduces the hydrophilic sites such as hydroxyl, carboxyl, and amino groups in the films, thus reducing the adsorption capacity of the films to water molecules (Valenzuela et al. 2013). Moreover, at a certain total film mass, the proportion of NRL in the film increases and the TOCNF decreases, resulting in an increase in the hydrophobic properties of the film. Meanwhile, NRL forms an isoprene coating on the surface of TOCNF during the drying process (Chhajed et al. 2023), which reduces the hydrophilicity of TOCNF. In addition, the dense network structure of the film has the potential to impede the penetration of water molecules, thus improving the water resistance of the film. These data above indicate that the addition of NRL results in films with excellent water barrier and water resistance.
Barrier Properties
Water vapor barrier properties are one of the most important features of bio-nanocomposite films for food packaging applications. The permeation of gases through a dense membrane is divided into four main components: adsorption, dissolution, diffusion and desorption (Thomas 1833). Therefore, the permeation of water vapor through the TOCN/NRL composite membrane can also be divided into the above four parts. First water vapor is adsorbed and dissolved on the surface of the film, then liquid water diffuses through the film and finally desorbs on the other side of the film. The WVTR and WVP of 100% pure nanocellulose film (C100N0) are 22.64 g/m2·24h and 29.35×10− 10g·mm/m2·s·pa, respectively. As the NRL ratio increases, the WVP of the film gradually decreased and finally stabilization (Fig. 3a). Compared with C100N0 film, C50N50 film has lower WVTR (3.95 g/m2·24h) and WVP values (6.07×10− 10g·mm/m2·s·pa), which is owing to the fact that the addition of NRL reduces the hydrophilic properties of the film and prevents water from being adsorbed and dissolved on the surface. Meanwhile, NRL fills the voids in the TOCNF interwoven network, making the film denser and further slowing down the gas permeation rate. Furthermore, it is known from previous reports that adsorption and diffusion phenomena occur exclusively in the amorphous phase of the polymer and not in its highly crystalline region (Perumal et al. 2022). TOCNF acts as a crystalline region in the matrix due to its high crystallinity, extending the effective path of gas diffusion while reducing the mobility of the polymer chains and slowing down the gas permeation rate (Soni et al. 2016). Thus, when the NRL content is further increased, although the hydrophobicity of the film increases and water is less likely to condense and dissolve on the surface, the decrease in TOCNF reduces the effective path length for gas diffusion, so that the water vapor barrier effect of C50N50 to C90N10 films tends to be stable.
The oxygen transfer rate (OTR) is an indication of the amount of oxygen that passes through a substance in a given period of time. Oxygen transport can lead to oxidation, which can trigger a number of food changes such as deterioration of odor, color, taste and nutrients. Therefore, obtaining nanocomposite films with a high oxygen barrier can help improve food quality and extend the self-life of food products. As can be seen from Fig. 3b, C100N0 has high oxygen barrier properties due to the high crystallinity of TOCNF and the dense structure formed by vacuum filtration of the film, and previous studies have also demonstrated the high oxygen barrier properties of TOCNF films (Kwon et al. 2020; Wang et al. 2018; Wu et al. 2022). Compared with other composite films, C90N10 has the lowest OTR and OP (2.20×10− 4·cm3/m2·d·Pa and 0.89×10− 15·cm3·cm/cm2·s·Pa), which is attributable to the fact that NRL particles fill the voids between the interwoven network of TOCNF, and effectively preventing oxygen penetration into the film. At the same time, the reduction in the number of pores inside the film leads to a decrease in the number of air-TOCNF interfaces, which reduces the refraction of the optical film in the interior (Kumar et al. 2016) and effectively improves the light transmission performance of the film by 68% (Figure S7). In addition, as the proportion of NRL increases, the high crystallinity TOCNF decreases, making the transport path for gas diffusion shorter and gas diffusion faster due to the reduction of highly crystalline regions in the film matrix, so the OTR and OP values of C90N10 to C10N90 films gradually increase.
Combining the gas barrier properties, water resistance and mechanical properties of the above films, C50N50 film was selected as the final product. Compared to commercially available PE cling film, C50N50 has comparable water vapor and oxygen barrier properties (Fig. 3). And compared with some existing nanocellulose-based barrier packaging materials, the water vapor and oxygen barrier properties of C50N50 film have been better than their research results, as shown in Table 2.
Table 2
This study compared with other studies
Category | Thickness (µm) | WVP (10− 10g·mm/m2·s·Pa ) | OP (10− 15·cm3·cm/cm2·s·Pa) | References |
CS/ZB | 133 | 11.2 | 14.64 | (Zhang et al. 2022) |
CS/BC | 48 | 19.48 | — | (Liu et al. 2023) |
PSO/PE | 118 | 28 | 112 | (Popović et al. 2021) |
PLA-ChNF/CNC | 7.7 | 64 | 20 | (Satam et al. 2018) |
TOCNF/NRL | 41 | 6.07 | 3.11 | This Work |
Note: CS is chitosan; ZB is pebble bilayer; BC is bacterial cellulose; PSO is pumpkin seed oil; PLA-ChNF/CNC is a composite film of PLA, chitin and nanocellulose crystals. |
Strawberry Preservation Performance
Integrating the mechanical properties, gas barrier properties, TG(Fig S8) and water resistance of TOCNF/NRL composite films, we selected C50N50 films for freshness preservation experiments for the blank group (CK) and C50N50 group, respectively.
Figures 4a-c represent the variation of firmness, color, and total soluble solids content of strawberries. Firmness is an important sensory indicator during storage of fruits and vegetables. The firmness of strawberries during postharvest storage is usually influenced by physicochemical changes that continue to ripen even after harvest, resulting in softening of the fruit (Ahouagi et al. 2020). Changes in strawberry brightness (the smaller the value of L*, the darker the object) are mainly related to the increase in strawberry ripeness and oxidation by oxidative enzymes after harvesting, and later darkening is associated with microbial infection (Liu et al. 2021). The content of strawberry soluble solids is an important indicator of the sugar content of strawberry fruit, and soluble solids are the main object consumed during respiration (Wang et al. 2019). The firmness, brightness and total soluble solids of the strawberries in all groups gradually decreased overall during storage. Among them, the total soluble solids of strawberries showed a slight increase on the second day, probably due to the accumulation of sugar. However, the three indicators decreased slowly in the C50N50 group, while they decreased rapidly in the CK group. This is because the C50N50 packaging film has a good water vapor and oxygen barrier, which reduces water loss from strawberries, avoids oxidation, reduces respiration, and reduces the chance of fungal infection.
Figures 4d-f represent the variation of decay index, relative conductivity, and weight loss of strawberries. The decay index is a weighted average of the fruit decay grades, the higher the decay index the more severe the fruit decay. Relative conductivity can reflect the degree of fruit cell membrane damage. During post-ripening aging of fruit and vegetable tissues or when subjected to adverse environmental stress, the functional activity and integrity of the cytoplasmic membrane decreased, membrane permeability increased, leakage of intracellular electrolytes to the outside occurred, and the relative conductivity became large (Xu et al. 2022). The decay index, relative conductivity, and weight loss of strawberries gradually increased in all groups during storage. However, the increasing trend in the C50N50 group was significantly lower than that in the CK group, which was due to the fact that the C50N50 packaging film hindered the transmission of water vapor and oxygen barrier, reduced water loss and microbial reproduction, and protected the strawberries from destruction.
Figure 4g represents the changes in visual appearance of strawberries during storage. On day 0, each group of strawberries shows a fresh appearance and no wrinkles are observed on the surface of the strawberries due to the sufficient moisture content at this time. With the extension of storage time, the strawberries in each group showed gradual ripening until they started to rot, and the strawberries in the CK group showed a significantly faster rate from gradual ripening to rot than those in the C50N50 group. The strawberries in the CK group showed a few signs of decay on the surface at the beginning of Day 1. On day 3, there was a large area of decay and mold on the surface, and the strawberries lost their edibility. On day 7, the strawberries in the CK group are already crumpled due to dehydration and the surface is completely occupied by mold. In contrast, the strawberries in the C50N50 group still do not show signs of extensive decay on the surface until Day 7, fully demonstrating the freshness preservation effect of the C50N50 film. It shows that C50N50 film has greater practical application value in the field of food packaging.