Hydrophobic, Sustainable, High-Barrier Regenerated Cellulose Film via a Simple One-Step Silylation Reaction

With the increasing importance of environmental protection, high-performance biopolymer films have received considerable attention as effective alternatives to petroleum-based polymer films. In this study, we developed hydrophobic regenerated cellulose (RC) films with good barrier properties through a simple gas–solid reaction via the chemical vapor deposition of alkyltrichlorosilane. RC films were employed to construct a biodegradable, free-standing substrate matrix, and methyltrichlorosilane (MTS) was used as a hydrophobic coating material to control the wettability and improve the barrier properties of the final films. MTS readily coupled with hydroxyl groups on the RC surface through a condensation reaction. We demonstrated that the MTS-modified RC (MTS/RC) films were optically transparent, mechanically strong, and hydrophobic. In particular, the obtained MTS/RC films exhibited a low oxygen transmission rate of 3 cm3/m2 per day and a low water vapor transmission rate of 41 g/m2 per day, which are superior to those of other hydrophobic biopolymer films.


Introduction
Petroleum-derived synthetic polymers, such as polystyrene (PS), polyethylene (PE), and polyethylene terephthalate (PET), have been the main sources of various polymer films because they are easily processable, low-cost, durable, and highly scalable materials [1][2][3]. Polymer films have numerous industrial applications, including in the food packaging industry. However, these plastic films, which exhibit poor degradability and are difficult to recycle, have given rise to significant disposal and pollution issues, which threaten human health and the environment [4][5][6]. In particular, the accumulation of traditional food packaging films made from nondegradable materials is proving to be a serious issue for the environment. To solve this problem, many efforts have been dedicated to developing natural biopolymer films that are biodegradable, sustainable, and possess good mechanical and barrier properties [7][8][9]. Despite the fact that biopolymer films provide an alternative solution to traditional food packaging films and have attracted considerable attention, they are characterized by several intrinsic disadvantages, including their low mechanical strength and poor barrier properties, which limit their use in commercial applications [10][11][12]. In particular, excellent gas and water vapor barrier properties are highly desirable for food packaging applications to ensure food safety.
Among the different nature-derived polymers, cellulose is the most abundant renewable natural polymer on Earth and is a promising biopolymer for various applications owing to its biodegradability, low cost, mechanical strength, and chemical stability [13][14][15]. Over the past decades, nanocellulose and RC materials have been extensively utilized oped methyltrichlorosilane (MTS)-modified RC (MTS/RC) films through a highly efficient and low-cost gas-solid reaction process with MTS, addressing the specific challenges of oxygen/water vapor permeability. The reaction of vaporous MTS with an RC surface exhibiting numerous hydroxyl groups resulted in the formation of hydrophobic RC films. Our experiments reveal that hydrophobic MTS/RC films exhibit considerably better barrier properties than pristine RC films without surface modification. To the best of our knowledge, no previous works have reported organic-chlorosilane-modified RC films with good barrier properties. Overall, the simple approach described here can enhance our ability to create hydrophobic biopolymer films with excellent barrier properties.

Characterization and Measurements
Field-emission scanning electron microscopy (FE-SEM, AURIGA, Zeiss, Oberkochen, Germany) was used to investigate the surface morphology of the pristine RC film as well as that of the RC/MTS films. FE-SEM (JEOL, JSM-7610F plus, Tokyo, Japan) equipped with EDS was used to perform the elemental mapping and energy-dispersive spectroscopy (EDS) characterization. The mechanical strength of the films was evaluated using a texture analyzer (CT3 25 K, Brookfield, WI, USA) with a 20 g load cell at a speed of 0.5 mm s −1 . Before measuring mechanical strength, the samples were dried in an oven at 60 • C to perfectly remove residual moisture. The size of the samples was 30 mm in length and 10 mm in width. Seven samples of each film were tested in order to determine the tensile strength and Young's modulus. All the mechanical strength measurement were performed at room temperature (23 • C) and 10% relative humidity (RH). An ultravioletvisible (UV-vis) spectrophotometer (Neo-S2117, Neogen, Chicor, Korea) was employed to determine the transmittance of the films. The Fourier transform infrared (FT-IR) spectra of the pristine RC film, as well as the RC/MTS-10, RC/MTS-30, and RC/MTS-60 films, were acquired using an FT-IR spectrometer (Bruker Corporation, ALPHA II, Ettlingen, Germany) in ATR mode. X-ray photoelectron spectroscopy (XPS) was conducted using a K-alpha (Thermo scientific Inc., Oxford, UK) to investigate the chemical composition. Photoelectrons were generated by a monochromatic Al X-ray source (Al Kα line = 1486.6 eV) with a spot diameter of 400 µm at room temperature and a pressure of 4.8 × 10 −7 Pa. The survey spectra were collected over the energy range of 0-900 eV. Origin Pro (version 8.5) software was utilized to analyze raw data. The binding energy was calibrated using the C1s at the 284.8 eV peak.

Fabrication of the RC Films
To prepare an 8 wt% cellulose solution, 4 g of cellulose was dissolved in 50 g of an aqueous solution composed of 4.6 wt% LiOH, 15 wt% urea, and 80.4 wt% deionized water (DIW) while stirring for 1 h. The prepared cellulose solution was immediately and rapidly frozen using liquid nitrogen. After the complete dissolution of the cellulose, it was thawed and centrifuged at 4000 rpm for 3 min to remove air bubbles from the dissolved cellulose solution. The prepared cellulose solution was poured into a glass mold (9 cm × 9 cm with a thickness of 1 mm) and immersed in an ethanol bath for 2 h to obtain an RC hydrogel. After 2 h, the obtained RC hydrogel was washed with excessive DIW for 12 h to remove the residual reagent. The RC hydrogel was dried at 40 • C and 90% RH in thermo-hygrostat to obtain the RC films.

Preparation of the Hydrophobic MTS-Modified RC Films
To fabricate the hydrophobic RC films, a vial containing the MTS solution (1 mL) and the dried RC film (7 cm × 7 cm with a thickness of 70 µm) was placed in a desiccator. The desiccator was then placed in an oven at 60 • C to vaporize the MTS. The gas-solid reaction was performed for 10, 30, or 60 min. Here, the MTS-modified RC films are referred to as MTS/RC-10, MTS/RC-30, and MTS/RC-60, respectively. Finally, the RC/MTS films were subsequently dried in a fume hood for 2 h to remove the unreacted silane as well as the by-product, hydrochloric acid, before use [40][41][42].

Contact Angle Measurements
The contact angles of pristine RC and MTS/RC films were measured using a contact angle analyzer (Phoenix 300, SEO, Suwon, Republic of Korea) at room temperature (23 • C) using the static method. A 10 µL DIW drop was placed on the surface of the pristine RC and MTS/RC films. Each sample was fixed on a microslide glass with 3M tape to prevent movement of the flat surface during wetting. Immediately afterward, the contact angle image was acquired using a charge-coupled device (CCD) camera. The contact angle measurement for each film was repeated at least three times.

Oxygen Transmission Rate (OTR) and Water Vapor Transmission Rate (WVTR)
To evaluate the oxygen barrier properties, the oxygen transmission rate (OTR) was measured using an oxygen permeation analyzer (OX-TRAN Model 2/21, Mocon, Brooklyn Park, MN, USA) in the range of 0.05-2000 ccm −3 day −1 , according to the ASTM D 3895 standard. The pristine RC and MTS-modified RC films (sample area: 1 cm 2 ) were mounted on an aluminum mask, and the OTR measurements were carried out under conditions of 23 • C and 0% RH. One side of the film was exposed to nitrogen gas, and the opposite side was exposed to flowing nitrogen gas (10 sccm). The amount of permeated oxygen was measured for 30 min using a detector when the transmission reached a steady state. Fifteen cycles were repeated, and it took 10 h to complete one test. The WVTR was measured following the ASTM E96 standard. Before measuring WVTR, calcium chloride (CaCl 2 ) was baked on a hot plate at 120 • C for 6 h to perfectly remove residual the water existing in CaCl 2 . Additionally, baked CaCl 2 was poured into a snap ring vial with a diameter of 12 mm, and the cap of the vial was sealed with filter paper, the pristine RC film, and the MTS/RC films. The sealed vials were stored at 23 • C and 50% RH for 24 h, and the weight of CaCl 2 was measured to calculate the WVTR. Five samples of each film were tested in order to determine the WVTR.

Results
In this study, the MTS/RC films were fabricated using RC as a biopolymer film substrate and MTS as a hydrophobic coating layer. Figure 1A provides a schematic of the fabrication process for the hydrophobic MTS-modified RC films via the gas-solid silylation reaction. Firstly, the cellulose solution was poured into a glass mold and soaked in an ethanol coagulation bath to form the RC hydrogels. The size of RC films can be scaled by controlling the size of the glass mold [43][44][45]. After complete drying, the RC hydrogels were transformed into RC films with a thickness of 70 µm. Next, the RC film and MTS were placed in a closed chamber. Upon heating at 60 • C, the MTS reacted with the hydroxyl groups of the RC surface to form covalently attached hydrophobic layers [46,47]. Importantly, alkyltrichlorosilanes provide a relatively fast covalent coupling reaction without prehydrolysis steps, and they can thus readily couple with the hydroxyl groups of an RC surface through a condensation reaction [48,49]. To investigate the optical, mechanical, and wetting properties of the obtained MTS/RC films as a function of the MTS deposition time, MTS/RC films with different deposition times (namely 10, 30, or 60 min) were fabricated (MTS/RC-10, MTS/RC-30, and MTS/RC-60, respectively). As shown in Figure 1B, the pristine RC film and the MTS/RC films were found to be highly without prehydrolysis steps, and they can thus readily couple with the hydroxyl groups of an RC surface through a condensation reaction [48,49]. To investigate the optical, mechanical, and wetting properties of the obtained MTS/RC films as a function of the MTS deposition time, MTS/RC films with different deposition times (namely 10, 30, or 60 min) were fabricated (MTS/RC-10, MTS/RC-30, and MTS/RC-60, respectively). As shown in Figure 1B, the pristine RC film and the MTS/RC films were found to be highly optically transparent. Interestingly, MTS/RC-60, which showed no evident thickness change compared with the pristine RC film, still exhibited optical transparency.   Figure 2A). Importantly, the surface morphology of the MTS/RC-10 film is very similar to that of the pristine RC film, suggesting that MTS did not fully cover the RC surface ( Figure 2B). However, as the MTS deposition time increased to 30 and 60 min, a different surface morphology with aggregated nanoparticles was clearly observed, probably due to the hydrolytic polycondensation of MTS ( Figure 2C,D) [50,51]. As shown in Figure 2A, MTS could mainly react with the hydroxyl groups present on the surface of the RC film, because the RC film did not have a porous structure. With increasing reaction time, the polymeric siloxane structure was created through horizontal and vertical condensation to form an aggregate morphology on the top of the RC surface ( Figure 1A) [52,53]. These results indicate that the MTS layers fully covered the RC surface when the MTS deposition time was 30 or 60 min. EDS analysis was performed to confirm the elemental composition of the RC and MTS/RC films. The element O was clearly detected on the pristine RC film due to its abundance in cellulose ( Figure 2E). Notably, the presence of Si was only found on the MTS/RC-30 and -60 films ( Figure 2F,G), indicating the successful grafting of silane groups onto the MTS/RC films.   Figure 2A). Importantly, the surface morphology of the MTS/RC-10 film is very similar to that of the pristine RC film, suggesting that MTS did not fully cover the RC surface ( Figure 2B). However, as the MTS deposition time increased to 30 and 60 min, a different surface morphology with aggregated nanoparticles was clearly observed, probably due to the hydrolytic polycondensation of MTS ( Figure 2C,D) [50,51]. As shown in Figure 2A, MTS could mainly react with the hydroxyl groups present on the surface of the RC film, because the RC film did not have a porous structure. With increasing reaction time, the polymeric siloxane structure was created through horizontal and vertical condensation to form an aggregate morphology on the top of the RC surface ( Figure 1A) [52,53]. These results indicate that the MTS layers fully covered the RC surface when the MTS deposition time was 30 or 60 min. EDS analysis was performed to confirm the elemental composition of the RC and MTS/RC films. The element O was clearly detected on the pristine RC film due to its abundance in cellulose ( Figure 2E). Notably, the presence of Si was only found on the MTS/RC-30 and -60 films ( Figure 2F,G), indicating the successful grafting of silane groups onto the MTS/RC films.
To further confirm the introduction of MTS into the RC films, we carried out FT-IR analyses on the pristine RC film and the MTS/RC films obtained after different deposition times (Figure 3). The broad peak of the -OH stretching vibration at around 3300 cm −1 gradually disappeared with an increasing MTS deposition time, indicating that the surface hydroxyl group of cellulose reacted with MTS [31,42,54]. Compared with the pristine RC film, the MTS/RC films exhibited new peaks at 1272 and 782 cm −1 , which could be assigned to the asymmetric stretching vibrations of Si-CH 3 and the characteristic vibrations of Si-O-Si, respectively [46,[55][56][57]. In contrast to MTS/RC-30 and -60, two peaks at 1272 and 782 cm −1 were not clearly detectable for the MTS/RC-10 film, probably due to the lack of silanization. Importantly, with an increase in the deposition time of MTS, the MTS/RC To further confirm the introduction of MTS into the RC films, we carried out FT-IR analyses on the pristine RC film and the MTS/RC films obtained after different deposition times (Figure 3). The broad peak of the -OH stretching vibration at around 3300 cm −1 gradually disappeared with an increasing MTS deposition time, indicating that the surface hydroxyl group of cellulose reacted with MTS [31,42,54]. Compared with the pristine RC film, the MTS/RC films exhibited new peaks at 1272 and 782 cm −1 , which could be assigned to the asymmetric stretching vibrations of Si-CH3 and the characteristic vibrations of Si-O-Si, respectively [46,[55][56][57]. In contrast to MTS/RC-30 and -60, two peaks at 1272 and 782 cm −1 were not clearly detectable for the MTS/RC-10 film, probably due to the lack of silanization. Importantly, with an increase in the deposition time of MTS, the MTS/RC films showed more distinct peaks at 1272 and 782 cm −1 . These results indicate that a stronger silicone coating was formed on the surface of the MTS/RC-60 films. These FT-IR data are in line with the FE-SEM images reported in Figure 2.   To further confirm the introduction of MTS into the RC films, we carried out FT-IR analyses on the pristine RC film and the MTS/RC films obtained after different deposition times (Figure 3). The broad peak of the -OH stretching vibration at around 3300 cm −1 gradually disappeared with an increasing MTS deposition time, indicating that the surface hydroxyl group of cellulose reacted with MTS [31,42,54]. Compared with the pristine RC film, the MTS/RC films exhibited new peaks at 1272 and 782 cm −1 , which could be assigned to the asymmetric stretching vibrations of Si-CH3 and the characteristic vibrations of Si-O-Si, respectively [46,[55][56][57]. In contrast to MTS/RC-30 and -60, two peaks at 1272 and 782 cm −1 were not clearly detectable for the MTS/RC-10 film, probably due to the lack of silanization. Importantly, with an increase in the deposition time of MTS, the MTS/RC films showed more distinct peaks at 1272 and 782 cm −1 . These results indicate that a stronger silicone coating was formed on the surface of the MTS/RC-60 films. These FT-IR data are in line with the FE-SEM images reported in Figure 2.  The surface element compositions of pristine RC and MTS/RC films were further examined with XPS analysis (Figure 4A-D). For the pristine RC, only the peaks of O and C, which are present in cellulose, were found without any impurities ( Figure 4A). In contrast to the pristine RC film, two new peaks at 101.6 and 151.0 eV were observed in the spectrum of MTS/RC-60, which we attributed to Si 2p and Si 2s peaks. In addition, a clear peak in the high-resolution spectra XPS of Si 2p at 102.8 eV could be attributed to C-Si-O 3 ( Figure 4B) [42,58]. MTS that contains hydrolysable groups such as chloride can react with water to produce intermediate silanols, which can further react with other silanols or hydroxyl groups present on the surface of RC substrates. As a result, a silicon layer with Si-O-Si bonds, as well as monolayers with C-Si-O bonds, are formed on RC surfaces, resulting in hydrophobic and interconnected sili-con coating on the RC substrates [46]. In comparison with the pristine RC film, the high-resolution XPS spectra O 1s and C 1s of the MTS/RC-60 showed two new peaks at 532.2 eV (Si-O-Si) and 284.4 eV (C-Si-O) ( Figure 4C,D), clearly indicating the successful surface silanization on the MTS/RC-60 film [42,[58][59][60]. After being modified by MTS, the bond energy of C-O-C/C-O-H shifted to 531.8 eV ( Figure 4C) [61,62]. Additionally, the bond energy of C-C/C-H and C-O-H/C-O-C shifted to 285.0 and 286.6 eV, respectively ( Figure 4D) [60,[63][64][65][66]. XPS can be utilized to analyze the elemental composition of the surface in the range of 1-10 nm. It is important to note that the peaks of O and C present in pristine RC films were still observed on the MTS/RC-60 film ( Figure 4C,D), indicating that a thin polysiloxane layer with a thickness below 10 nm formed on the RC surface.
trum of MTS/RC-60, which we attributed to Si 2p and Si 2s peaks. In addition, a clear peak in the high-resolution spectra XPS of Si 2p at 102.8 eV could be attributed to C-Si-O3 (Figure 4B) [42,58]. MTS that contains hydrolysable groups such as chloride can react with water to produce intermediate silanols, which can further react with other silanols or hydroxyl groups present on the surface of RC substrates. As a result, a silicon layer with Si-O-Si bonds, as well as monolayers with C-Si-O bonds, are formed on RC surfaces, resulting in hydrophobic and interconnected silicon coating on the RC substrates [46]. In comparison with the pristine RC film, the high-resolution XPS spectra O 1s and C 1s of the MTS/RC-60 showed two new peaks at 532.2 eV (Si-O-Si) and 284.4 eV (C-Si-O) ( Figure  4C,D), clearly indicating the successful surface silanization on the MTS/RC-60 film [42,[58][59][60]. After being modified by MTS, the bond energy of C-O-C/C-O-H shifted to 531.8 eV ( Figure 4C) [61,62]. Additionally, the bond energy of C-C/C-H and C-O-H/C-O-C shifted to 285.0 and 286.6 eV, respectively ( Figure 4D) [60,[63][64][65][66]. XPS can be utilized to analyze the elemental composition of the surface in the range of 1-10 nm. It is important to note that the peaks of O and C present in pristine RC films were still observed on the MTS/RC-60 film ( Figure 4C,D), indicating that a thin polysiloxane layer with a thickness below 10 nm formed on the RC surface. We next examined the effects of the MTS deposition time on the wettability of the obtained MTS/RC films by measuring the water contact angle. The water contact angle was 58.9 ± 0.86° for the pristine RC film. After MTS modification, the water contact angles were found to be 64.4 ± 1.5, 75.2 ± 1.0, and 83.2 ± 1.3° for the RC/MTS-10, RC/MTS-30, and RC/MTS-60 films, respectively ( Figure 5A). These results suggest that the presence of methyl groups in MTS endows RC films with hydrophobicity [47,[67][68][69][70][71]. To clearly visualize We next examined the effects of the MTS deposition time on the wettability of the obtained MTS/RC films by measuring the water contact angle. The water contact angle was 58.9 ± 0.86 • for the pristine RC film. After MTS modification, the water contact angles were found to be 64.4 ± 1.5, 75.2 ± 1.0, and 83.2 ± 1.3 • for the RC/MTS-10, RC/MTS-30, and RC/MTS-60 films, respectively ( Figure 5A). These results suggest that the presence of methyl groups in MTS endows RC films with hydrophobicity [47,[67][68][69][70][71]. To clearly visualize the difference in wettability between various films, we dropped a colored aqueous solution on the pristine RC film and the MTS/RC films ( Figure 5B). As expected, the MTS/RC-30 and MTS/RC-60 films exhibited a higher hydrophobicity than the pristine RC film and the MTS/RC-10 film.
As can be observed in Figure 6A, the transmittance of the MTS/RC-10 and MTS/RC-30 films remained almost completely unchanged compared with that of the pristine RC film. The transmittance of the MTS/RC films at 550 nm decreased from 88.7% to 87.0% as the MTS deposition time was increased to 30 min. Though the transmittance of the MTS/RC-60 film at 550 nm was determined to be 72.8%, this film still preserved optical transparency. It is important to note that the optical transmittance of MTS/RC films was comparable to that of previously reported biopolymer (chitosan, PLA, etc.)-based films [72][73][74][75]. Recently, Polymers 2023, 15, 1901 8 of 13 Makarov et al. reported N-methylmorpholine-N-oxide (NMMO) surface treatment based on the partial dissolution of cellulose and its coagulation for modification of a paper surface [30]. In general, surface-modified RC films exhibit uniform surface morphology as well as high transmittance compared with surface-modified papers, probably due to a perfect dissolution and regeneration process. As shown in Figure 6B-D, the mechanical properties of the pristine RC film and the MTS/RC films were investigated by conducting tensile stress-strain measurements as a function of the MTS deposition time. The pristine RC film exhibited the highest tensile strength (182. 6 MPa) among all the tested films. The tensile strength of the MTS/RC-30 and -60 films was found to be considerably lower, namely 146.4 and 125.9 MPa, respectively [76,77]. The decrease in tensile strength may have been due to the hydrochloric acid produced as a by-product of silanization. The acidic degradation of the cellulose chains can cause a decrease in MTS/RC film strength [51,78,79]. However, the pristine RC film and the MTS/RC films exhibited a nearly identical Young's modulus (2.3 ± 0.4 GPa). It is important to note that the mechanical strength of the MTS/RC films is superior to those of previously reported nanocellulose-based hydrophobic films (48.8-103.7 MPa), PLA-based hydrophobic films (~63.2 MPa), and hydrophobic paper composites (16)(17)(18)(19)(20) [51,53,80,81]. the difference in wettability between various films, we dropped a colored aqueous solution on the pristine RC film and the MTS/RC films ( Figure 5B). As expected, the MTS/RC-30 and MTS/RC-60 films exhibited a higher hydrophobicity than the pristine RC film and the MTS/RC-10 film. As can be observed in Figure 6A, the transmittance of the MTS/RC-10 and MTS/RC-30 films remained almost completely unchanged compared with that of the pristine RC film. The transmittance of the MTS/RC films at 550 nm decreased from 88.7% to 87.0% as the MTS deposition time was increased to 30 min. Though the transmittance of the MTS/RC-60 film at 550 nm was determined to be 72.8%, this film still preserved optical transparency. It is important to note that the optical transmittance of MTS/RC films was comparable to that of previously reported biopolymer (chitosan, PLA, etc.)-based films [72][73][74][75]. Recently, Makarov et al. reported N-methylmorpholine-N-oxide (NMMO) surface treatment based on the partial dissolution of cellulose and its coagulation for modification of a paper surface [30]. In general, surface-modified RC films exhibit uniform surface morphology as well as high transmittance compared with surface-modified papers, probably due to a perfect dissolution and regeneration process. As shown in Figure 6B-D, the mechanical properties of the pristine RC film and the MTS/RC films were investigated by conducting tensile stress-strain measurements as a function of the MTS deposition time. The pristine RC film exhibited the highest tensile strength (182. 6 MPa) among all the tested films. The tensile strength of the MTS/RC-30 and -60 films was found to be considerably lower, namely 146.4 and 125.9 MPa, respectively [76,77]. The decrease in tensile strength may have been due to the hydrochloric acid produced as a by-product of silanization. The acidic degradation of the cellulose chains can cause a decrease in MTS/RC film strength [51,78,79]. However, the pristine RC film and the MTS/RC films exhibited a nearly identical Young's modulus (2.3 ± 0.4 GPa). It is important to note that the mechanical strength of the MTS/RC films is superior to those of previously reported nanocellulose-based hydrophobic films (  To examine the effect of the MTS coating on the barrier properties, we measured the OTR and WVTR of the filter paper, the pristine RC film, and the MTS/RC films ( Figure 7A,B). The pristine RC film exhibited an OTR of 11.3 ± 1.8 cm 3 /m 2 per day, while the MTS/RC-10, MTS/RC-30, and MTS/RC-60 films exhibited an OTR of 4.1 ± 0.9, 3 ± 0.7, and 2.3 ± 0.6 cm 3 /m 2 per day, respectively. The WVTR values of the filter paper and the pristine RC film were 1152.8 and 86.8 g/m 2 per day at 23 • C and 50% RH, respectively. On the other hand, the MTS/RC-10, MTS/RC-30, and MTS/RC-60 films exhibited WVTR values of 52.1 ± 3.1, 41.7 ± 2.1, and 32.1 ± 1.3 g/m 2 per day, respectively. These findings suggest that the MTS coating on the RC films could reduce the OTR and WVTR of the pristine RC film by 80% and 63%, respectively. The low OTR and WVTR of the MTS/RC films could be attributed to the hydrophobic MTS layer as well as its compact structure on the surface of the RC film [82,83]. These data are in line with the contact angle results presented in Figure 5. It is worth noting that the MTS/RC films have considerably superior barrier properties compared with those of previously reported natural polymer (CNF, starch, PLA, Natureflex, polysaccharide composite, etc.)-based films ( Figure 7C) [84][85][86][87][88][89]. The oxygen and moisture barrier performance of the obtained MTS/RC films meets the packaging requirements for bakery products, fruits, and vegetables ( Figure 7D). To examine the effect of the MTS coating on the barrier properties, we measured the OTR and WVTR of the filter paper, the pristine RC film, and the MTS/RC films ( Figure  7A,B). The pristine RC film exhibited an OTR of 11.3 ± 1.8 cm 3 /m 2 per day, while the MTS/RC-10, MTS/RC-30, and MTS/RC-60 films exhibited an OTR of 4.1 ± 0.9, 3 ± 0.7, and 2.3 ± 0.6 cm 3 /m 2 per day, respectively. The WVTR values of the filter paper and the pristine RC film were 1152.8 and 86.8 g/m 2 per day at 23 ℃ and 50% RH, respectively. On the other hand, the MTS/RC-10, MTS/RC-30, and MTS/RC-60 films exhibited WVTR values of 52.1 ± 3.1, 41.7 ± 2.1, and 32.1 ± 1.3 g/m 2 per day, respectively. These findings suggest that the MTS coating on the RC films could reduce the OTR and WVTR of the pristine RC film by 80% and 63%, respectively. The low OTR and WVTR of the MTS/RC films could be attributed to the hydrophobic MTS layer as well as its compact structure on the surface of the RC film [82,83]. These data are in line with the contact angle results presented in Figure  5. It is worth noting that the MTS/RC films have considerably superior barrier properties compared with those of previously reported natural polymer (CNF, starch, PLA, Natureflex, polysaccharide composite, etc.)-based films ( Figure 7C) [84][85][86][87][88][89]. The oxygen and moisture barrier performance of the obtained MTS/RC films meets the packaging requirements for bakery products, fruits, and vegetables ( Figure 7D).

Conclusions
In summary, we utilized a one-step CVD method to fabricate a high-performance hydrophobic bioplastic film that is optically transparent, mechanically strong, and has a good barrier performance. The interconnected silicon coating of both a silicon layer and a

Conclusions
In summary, we utilized a one-step CVD method to fabricate a high-performance hydrophobic bioplastic film that is optically transparent, mechanically strong, and has a good barrier performance. The interconnected silicon coating of both a silicon layer and a monolayer deposited on an RC surface via a gas-solid reaction led to the enhancement in hydrophobicity and barrier properties. The obtained MTS/RC films exhibited good mechanical properties as well as good oxygen/water vapor barrier performance. Notably, the hydrophobic MTS/RC-30 film possessed an optical transparency of 87% at 550 nm, a high tensile strength of 146 MPa, a low OTR of 3 cm 3 /m 2 per day and a low WVTR of 41 g/m 2 per day, which are superior to those of other hydrophobic biopolymer films reported in previous studies. These findings may be valuable for the production of highperformance natural polymer-based films.