Preparation process of cellulose film. Figure 1a shows the fabrication process and formation mechanism of the cellulose films. Cellulose cotton pulp was dispersed in 87 wt% NMMO aqueous solution at 100 oC under mechanical stirring. The oxygen atom of NMMO molecular can form hydrogen bonds with the hydroxyl of cellulose to destroy the crystalline structure of cellulose and results in a clear homogeneous solution where the polymer chains are molecularly isolated(Sayyed et al., 2019). After high-speed centrifugation to remove bubbles in the solution, the cellulose solution is coated on a glass plate, and finally immersed in a water bath. During in the regeneration process, the NMMO molecular diffused from the NMMO-cellulose system into water, then the cellulose chains aggregated together to form hydrogel. The resulting hydrogel was washed with water to remove residual chemicals, and then fixed on a polymethyl methacrylate (PMMA) plate to dry at room temperature, then a flexible and transparent cellulose film was obtained (Fig. 1b).
Characteristics of CL-N series films. As shown in Fig. 2a, the CL-N series film has satisfactory transparency, the transmittance of all film is above 80% at the wavelength range of 490-780nm. Expressly, CL-N0 film has the highest transmittance (87% at 550 nm), this transparency is comparable with that of commercial regenerated films (Wan et al., 2021). At the same time, the content of NMMO in the coagulation bath have a certain effect on the transparency of the cellulose film that transmittance of the film decreased with the increase of the NMMO content in the coagulation bath, which can be explained by the film formation mechanism of the phase inversion (Ilyas et al., 2021; Cheng et al., 1995; Cheng et al., 1994). As the concentration of the coagulation bath increases, the delay time of the system increases, and the polymer concentration at the film/coagulation bath interface decreases. The film surface layer is more likely to undergo phase separation, such that the porous cellulose hydrogel is formed (Reuvers, van den Berg, et al., 1987; Reuvers & Smolders, 1987). When light travels through such a structure, multiple reflections and refractions will occur, and part of the light will even be depleted in the end. The macroscopic manifestation is that the light transmittance of the film decreases (Shahbazi et al., 2019; Gobrecht et al., 2015; Tang et al., 2008). Figure 2b shows the FTIR spectrum of the CL-N series films. Both CL-N0 and other films show characteristic peaks of cellulose, E.g. 3442 cm− 1(O-H, v), 2902 cm− 1(C-H, vas), 1374 cm− 1(C-O-H, δ), and 897 cm− 1(C-O-C at the β-(1–4)-glycosidic linkage, vas)(Shandilya et al., 2020). The characteristic peaks of NMMO at 2935 cm− 1(C-H, v), 1448 cm− 1(C-H2, δ), 1116 cm− 1(C-O-C, v), and 858 cm− 1(C-O-C, δ) are not observed in the spectra, suggesting that there is no chemical reaction occurred between the NMMO and cellulose(Yu Zhang et al., 2020). Figure 2c shows the TG and DTG test curves of the CL-N series film. The decomposition temperature of all samples is almost the same, including that the use of different concentrations of NMMO in coagulation bath does not affect the thermal stability of the films. Figure 2d shows the X-ray diffraction images of cellulose film of CL-N0, CL-N2.5, CL-N5, CL-N7.5, CL-N10, respectively. According to the method of Zheng, Bingbing et al.(B. Zheng et al., 2017), The relative crystallinity calculated by Jade 6.0 software is found to be about 24.23%, 26.86%, 27.60%, 54.34%, and 62.67% for CL-N0, CL-N2.5, CL-N5, CL-7.5, CL-N10 film, respectively. These results indicating that the NMMO in coagulation bath is helpful for the formation of cellulose crystals. The NMMO concentration difference between cellulose solution and coagulation bath, reduced with the increase of NMMO content, the double diffusion rate between the solvent and the nonsolvent decreased, thus the cellulose molecules have enough time to rearrange and crystallize (Ichwan et al., 2012; Yaopeng Zhang, 2002; Bang et al., 1999; van de Witte et al., 1996).
Cross-section morphology of CL-N series films. Based on the picture in Fig. 3, it was found that the film is clearly divided into two areas with different densities and the pore size is decrease when the NMMO content in coagulation bath temperature increases from 0 (water only) to 10 wt % NMMO. The formation of two areas in films is due to the unlike exchange rate of different part in coagulation bath. On the side where the solution is in contact with PMMA plate, the film is denser since there is less chance of contact with the coagulation bath and the slower exchange rate compare with another side. As the concentration of the coagulation bath to rise, the exchange rate between another side and the side in contact with PMMA plate to decrease, and finally a uniform and dense film is formed. The formation of a pore can be explained by Laity, P. R and Muhammad Ichwan’s, work (Ichwan & Son, 2012; Laity et al., 2002). In P.R. Laity's work, the coagulation process of film was summarized into four route (phase separation occurs in the Ⅰ) vitrification region, Ⅱ) the metastable region between the binodal and spinodal lines, Ⅲ) the unstable region, Ⅳ) metastable region) according to the locations on the phase diagram where the phase separation takes place and the corresponding morphologies. This film is considered as result of the nucleation growth mechanism as demonstrated by routes Ⅱ above for the slow solidification process and the uniform dense structure as shown in Fig. 3b-f. When the cellulose/NMMO aqueous solution is immersed into NMMO/H2O (coagulation bath), liquid–liquid phase separation occurs between cellulose solution and coagulation bath. Then, as when entering the metastable region between binodal and spinodal line, nuclei are formed in the polymer solution, these nuclei grow into droplets until their growth is stopped by the solidification of the surrounding polymer solutions. Finally, the pore structure of films was formed (Lee et al., 1999). In this case, nuclei have enough time to grow up due to the low exchange rate of NMMO and water and finally form homogenous and dense film. Therefore, addition of NMMO to coagulation bath will slower the diffusion rate and allow nuclei to grow homogenously to yield denser and smaller pore size of film structure with the content of NMMO increase.
Mechanical property of cellulose films under different preparation conditions. In this study, the effects of different coagulation bath concentrations, coagulation bath temperature, and glycerin on the strength of the film were discussed. The mechanical strength test results of all films as show in Fig. 4a-c. and Fig. 4a shows results of the mechanical test of CL-N series film, which is regenerated under different concentrations of NMMO in coagulation bath. With the increase of NMMO content from 0–10%, the tensile strength of the cellulose film increased from 106.1MPa to 149.5MPa, higher than the regeneration cellulose film fabricated in alkali/urea system (Fig. 4d, Table 1)(Shi et al., 2019). This is because the cellulose film was prepared with different concentrations of the coagulation bath results in different degrees of crystallinity (Fig. 2d). It is worth mentioning that the post-treatment of glycerin and the regeneration temperature of the film also have some influence on its mechanical properties. As shown in Fig. 4b, the tensile strength of the CL-G series film, which is treated with different glycerin concentrations, will decrease. Compared with CL-G0 film, the tensile strength of CL-G2 film has dropped to 61.5MPa, a decrease of 0.6 times, but the elongation at break of CL-G0 film, is 4.3%, and the CL-G2 film is 7.3%, an increase of nearly 0.6 times, similar with the chitin films (B. Duan et al., 2014). This can be explained that the glycerol molecule act as a plasticizer in the cellulose film for its small molecule relative to the cellulose molecule. Specifically, the glycerol molecule penetrates into the gaps between the cellulose molecules when the cellulose hydrogel film is soaking in the glycerin solution, which weaken the cellulose molecular chain, and made the cellulose molecule chains easier to slip (Jianqing et al., 2008). Therefore, the tensile strength of the cellulose film treated with glycerol will decrease while the elongation at break will increase. At the same time, the glycerin concentration used to treat the cellulose hydrogel should not be too large. Experiments show that when the glycerin concentration reaches 5 %, the corresponding tensile strength of the CL-G5 film drops to an unfavorable value. Similar to the effect of glycerin on the films, the increase in regeneration temperature also reduces the tensile strength of the film to a certain extent. Figure 4c is a stress-strain curve for testing the mechanical properties of CL-T0, CL-T20, CL-T40, and CL-T60 films. The tensile strength of the film decreases only when the temperature rises from 0 oC to 20 oC. Compared with the CL-G series of film, the elongation at break of CL-T0, CL-T20, CL-T40, and CL-T60 film decreases correspondingly with the increase of regeneration temperature, this is because as the regeneration temperature increases, the thermal movement of molecules intensifies, the diffusion speed of NMMO molecules in the coagulation bath is accelerated, and the precipitation and reorganization of cellulose molecules are also accelerated, which is not conducive to the formation and growth of cellulose crystals (Yaopeng et al., 2002), and because of the loss of the lubrication effect of the glycerin molecule, the elongation at break of the cellulose film has also decreased. In general, increasing the concentration of coagulation bath or temperature will reduce the strength of the cellulose film, while the addition of plasticizers (such as glycerin) can significantly increase the toughness of the films. Therefore, the strength of the film can be adjusted by changing concentration and temperature of coagulation bath during the preparation of films. At the same time, plasticizer can be added to adjust the toughness during the post-treatment of films. Finally, considering the amount of plasticizer, concentration and temperature of coagulation bath, cellulose films with different strength characteristics can be obtained.
Table 1
Mechanical property of CL-N series film compared with other regenerated cellulose films.
Solvent | Classification | Stress (MPa) | Strain (%) | Thickness (um) | Ref |
NMMO | Organic solvents | 106.3-149.5 | 4.1-6.0 | 16 ± 2 | This work |
NMMO | Organic solvents | 82.3 ± 10.6 | 10.7 ± 2.0 | 30 | (Kim et al., 2011; Mao et al., 2010; Khare et al., 2007) |
64.9 ± 18.3 | 6.5 ± 1.5 | 14 ± 0.5 |
73.1 ± 10.1 | 15.8 ± 1.3 | 25 |
CA(cellulose acetate) | Acid | 60.1 ± 11.6 | 13.1 ± 8.0 | / | (Khare et al., 2007) |
[DBNH][OAc] | IL (Ionic Liquids) | 188 | 2.9 | / | (Niu et al., 2021) |
1-Allyl-3-methylimidazolium chloride ([Amim][Cl]) | IL (Ionic Liquids) | 160 | 20 | / | (Wan et al., 2021; X. Zheng et al., 2019; Yan et al., 2016) |
152 ± 30 | / | / |
208 ± 7.1 | 5.5 ± 0.9 | / |
1-ethyl-3-methylimidazolium acetate [Emim][Ac] | IL (Ionic Liquids) | 100–110 | 7.5-8 | / | (Pang et al., 2015) |
Benzyl trimethyl ammonium hydroxide (BzMe3NOH) | Alkali | 158.3 ± 0.7 | 21.6 ± 1.9 | | (Y. Wang et al., 2019) |
NaOH/urea | Alkali/urea | 129 ± 10 | 1.44 | / | (Liu T, 2012) |
90 ± 5 | 1.85 |
97 ± 3 | 3.53 |
100 ± 5 | 2.96 |
89 ± 4 | 3.27 |
105 ± 6 | 8.14 |
NaOH/urea | Alkali/urea | 87 | 9–10 | 20 | (H. Qi et al., 2009) |
LiOH/urea | Alkali/urea | 86.9 | 6–7 | / | (Shi et al., 2019; Liu et al., 2019; C. Wang et al., 2018) |
50–60 | 4–5 | / |
66.5 | 16.5 | / |
Solvent resistance test of CL-N0. To prove chemical stability of the cellulose film, the solvent resistance test of CL-N0 film was carried out and the results shows in Fig. 5. Even after immersing in acid (pH = 1), alkali (pH = 14) and other solvency for 24 hours, it can still maintain its original shape. It is shows that the cellulose film is chemically inert to a variety of solvents, this is because the hydrogen bonds between the cellulose molecular chains that make up films have strong interactions. Hence it is very difficult for common solvents to destroy this structure. This feature can ensure that the film remains stable in complex chemical environment. In fact, composite films based on cellulose and chitosan films with similar structures have the same excellent performance in solvent resistance experiments (Shi et al., 2019; Zhu et al., 2019). In these works, films in different solvents only show swelling and its geometry has not been destroyed.
Cytotoxicity and biological compatibility evaluation of CL-N series films. In this work, the in vitro cytotoxicity and biocompatibility tests were carried out by a direct contact of NIH-3T3 cells with CL-N series films for 1d and 3d. The cell proliferation was evaluated by CCK-8 assay and the live-dead cells were stained by Calcein-AM / PI kit. In Fig. 6a, it can be found that the cell viability of all groups is higher than 95 % compared to that of control group both in 1d and 3d culture incubation. This result revealed that the CL-N series films had no side effect and cytotoxicity on cell proliferation. This conclusion was further evidenced by fluorescence microscope images (Fig. 6b), in which NIH-3T3 cells was live–dead stained with the green color standing for live cells and the red color standing for dead cells. It’s shown that there are no obvious dead cells in both CL-N series films group and control groups, the cell number all had a fast increase from 1d to 3d incubation and the morphology are all similar fibroblast-like, indicating that cells could be alive and continuously grew with good cell adhesion in the existence of CL-N series films, confirming the good biocompatibility of the cellulose films prepared by NMMO, which is consistent with the chemical crosslinked cellulose film(Shi et al., 2019). Therefore, it’s providing experimental support for potential application of cellulose in food preservation and textiles field.
The moisture-conducting and air barrier properties of the cellulose film. Air barrier and breathable materials have been widely used in warm cotton clothing, wound dressings, medical dressings and food packaging (Yue et al., 2021; Zhou et al., 2021; Patil et al., 2020; Tehrani-Bagha et al., 2019; Hu et al., 2006). In order to measure the air and moisture-conducting character of the cellulose film, CL-G2 film was used as a representative sample to air barrier and moisture-conducting function test, the results are shown in Fig. 7a. In the apple experiment, with the extension of the test time, it can be observed that the color of apple of the CL-G2 film group and the PE film group was still bright, while the apples in the blank group have become dull and brown due to oxidation. In the PE group, the flesh is slightly yellowed due to loss of water. This shows that the fiber prepared in the laboratory has the same function of isolating oxygen as the PE film on the market. The test results of the film moisture-conducting character are shown in Fig. 7a. With the extension of the test time, the silica gel in the blank group and the CL-G2 group began to fade, while the color of the silica gel in the PE film group hardly changed. At the same time, the quality change of apple and silica gel in the sample bottle also proved this, in Fig. 7b, it can be clearly observed that the quality of apple and silica gel in the blank group and CL-G2 group has changed, while the PE group remains stable. As show in Fig. 7c, gas such as oxygen and carbon dioxide cannot pass through the film, because of the dense surface. But due to the hydrophilicity of cellulose film, water molecules can form hydrogen bonds with the cellulose chains and conduct from one side (high relative humidity) of the film to another side (low relative humidity). Finally, carbon dioxide is accumulated on one side, so the film has a certain carbon dioxide control function. In food storage, gas permeable membrane with suitable H2O permeation could maintain an atmosphere with low O2 concentration and high CO2 concentration, which restrains plants physiological metabolism to a low degree, inhibits the proliferation of bacteria, and finally effectively extend the storage time of food (Zhou et al., 2021). And as textile, the film can be used as the middle layer to compound with other fabrics to prepare moisture-permeable and wind-proof work clothes.