Introduction

General-purpose petroleum-based packaging materials, such as polyethylene (PE) and polypropylene (PP), are still the most commonly used on Earth today due to their remarkable characteristics, including low cost, availability, excellent optical, mechanical, and barrier properties and resistance to water. However, because of their long-time degradation process, they become environmental pollutants after disposal (Petrou et al. 2012; Salmanian et al. 2019). Therefore, considerable efforts have been directed toward producing packaging materials from biopolymers of natural resources, which have the advantages over synthetic polymers because of their comparable cost, availability, renewability, production from reproducible resources, and easy degradation in natural environments (Taherimehr et al. 2021). There has been a great deal of reported studies based on proteins, lipids, and polysaccharides, which are natural sources used as packaging materials (Soliman et al. 2007; Gouveia et al. 2019; Milani and Nemati 2022). However, due to the relatively high gas permeability of biopolymer films, their application as gas barrier materials is limited (Yao et al. 2010; Pan et al. 2015; Zou et al. 2023). In order to meet certain high requirements for gas barrier materials in food packaging, pharmaceutical packaging, and electronic encapsulation, a significant number of composites composed of biopolymers and two-dimensional (2D) nanosheets have been designed and prepared (Sanchez et al. 2011; Yadollahi et al. 2014; Xu et al. 2016; Yu et al. 2017; Wang et al. 2018b; Ahmed et al. 2021). For instance, clay and graphene have been hybridized into a polymer matrix to obtain high-performance gas barrier materials, e.g., with a low oxygen transmission rate (Yadollahi et al. 2014; Xu et al. 2016; Yu et al. 2017; Wang et al. 2018a, b; Ahmed et al. 2021).

Carboxymethyl cellulose (CMC), a water-soluble anionic linear polysaccharide derived from cellulose, is one of the most widely used biopolymers in food packaging (Zhang et al. 2021), food processing (Chen et al. 2000; Shebis et al. 2022) and pharmaceutical industries (Markovic et al. 2021) due to its excellent film formability, low cost, availability, non-toxicity, biocompatibility, hydrophilicity and stable internal network structure properties (Yadav et al. 2013; Kono 2014; Wang and Rhim 2017). However, CMC is seldom used alone in these applications but is always hybridized with other agents, like polymers and nanomaterials, to enhance its performance with more functionalization and application. For example, Zhang et al. (2021) displayed cellulose nanofibers were incorporated into CMC films to increase the tensile strength by 21 times. Di Filippo et al. (2021) mixed snail mucus with CMC to enhance the elongation and adhesion strength of CMC film, as well as the barrier property for water vapor. Yadav et al. (2013) reported that graphene oxide (GO), a kind of 2D nanosheet material, was used to fabricate a nanocomposite film by simply mixing the solution of CMC and GO and casting, whose characterization showed a significant improvement in the tensile strength and Young’s modulus. All these examples demonstrate that the properties of CMC can be enhanced by incorporation with other fillers or additives.

Layered double hydroxides (LDH) are a large class of widely investigated 2D layered materials, whose formula can be generally represented as [M1−xIIMxIII(OH)2]x+(An−)x/n·mH2O (MII and MIII are divalent and trivalent metals, respectively, An− is the interlayer anion) (Yu et al. 2019). Due to the highly flexible, large surface area, tunable chemical composition, and physical properties, various LDHs have been extensively explored for application as nano-fillers in polymer nanocomposites, catalysts, anion exchange materials, and fire retardants (Yu et al. 2017). Until now, a few reports about the nanocomposites, whose main components are CMC and LDHs, have been published (Kang et al. 2009; Yadollahi and Namazi 2013; Yadollahi et al. 2014; Wang et al. 2018a). For instance, Kang et al. (2009) presented that CMC/LDH composites were prepared through the coassembly of LDH nanosheets (LDH-NSs) (noting the thickness to be 0.3 μm) with CMC, showing improved thermal stability compared with that of CMC alone. Subsequently, Yadollahi’s group employed two different methods to prepare CMC/LDH composites: one through coprecipitation for intercalation of CMC into hydrotalcite-like anionic clays and the other via a solution casting method (Yadollahi and Namazi 2013; Yadollahi et al. 2014). Only in Yadollahi's later work were the water vapor permeability and transparency of CMC/LDH composites characterized. Recently, Wang et al. reported (2018a) that LDH-NSs were synthesized by exfoliating MgAl(NO3)-LDH nanoplates in formamide, and (LDH/CMC)n films were fabricated by a layer-by-layer deposition technique. It is worth noting that Kang et al. did not prepare the composite film or characterize the barrier properties and the optical behavior. Besides, Yadollahi et al. did not characterize the barrier properties or the optical behavior of the nanocomposite in their earlier work, but they focused on water vapor barrier properties and the optical behavior in their later work. In contrast, Wang et al. focused on barrier properties in detail and showed results on transparency of CMC/LDH coated polypropylene.

The LDH-NSs used in this work were synthesized using Yu’s method (Yu et al. 2019), which is presented as safer and more environmentally friendly compared with those employed by Wang et al. (2018a) and others. As reported, two-layer MgAl-LDH-NSs are obtained by reconstruction of calcined Mg2Al-CO3-LDH in glycine solution in Yu’s method, while single-layer MgAl-LDH-NSs were prepared by delamination of bulk MgAl(NO3)-LDH nanoplates in formamide in Wang’s procedure. Comparing the two methods, we consider that the LDH-NSs prepared through Yu’s method could potentially expand LDH-NS’s application in food and pharmaceutical packaging, such as for edible coating. CMC/LDH-NS nanocomposites, in this work, are prepared through a simple casting/evaporation procedure involving the mixed liquid containing CMC and LDH-NS as shown in Scheme 1. The resulting CMC/LDH-NS films exhibit a significant improvement in gas barrier properties compared with CMC and maintain a high transparency. These excellent characteristics would guarantee the practical application of CMC/LDH-NS in food and pharmaceutical packaging and electronic encapsulation.

Scheme 1
scheme 1

Schematic of preparing process of the nanocomposite film

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Experimental

Materials

The original platelet Mg2Al-LDH sample (CAS No.: 11097–59-9) is purchased from Shanghai Acmec Biochemical Co., Ltd. Carboxymethyl Cellulose (CMC) (MW: 250000 g·mol−1, DS: 0.7, viscosity: 1500–3100 mPa·s), glycine (≥ 99.0%), and other chemicals used are of analytical grade and used as received. Deionized water is used throughout the experimental procedures.

Layered double hydroxide nanosheet (LDH-NS) preparation

The preparation of LDH-NS is based on Yu's method (Yu et al. 2019) with a slight modification. The purchased Mg2Al-LDH (5.00 g) is calcined at 450 °C for 12 h and about 2.73 g of calcined LDH (named LDO) is obtained. Typically, 2.73 g of LDO, 4.10 g of glycine and 28.00 g of distilled water are mixed in an autoclave and react at 100 °C for 48 h to obtain a semi-transparent gel, which is then dispersed in distilled water. The obtained suspension is then purified by dialysis with distilled water for 5 d. The purified LDH nanosheet (named LDH-NS) suspension is stored in the refrigerator for further use and an image of LDH-NS suspensions is shown in Fig. S1.

After dialysis, the collected LDH-NS suspension is partially dried at 100 °C in an oven for 2 h to determine the solid content for nanocomposite films preparation.

Preparation of CMC/LDH-NS nanocomposite films

CMC-based nanocomposite films are prepared by solution casting as described by Ahmed et al. (2017) with some modifications. The process is displayed in Scheme 1. CMC is dissolved in the LDH-NS suspension to obtain CMC and LDH-NS mixtures with mass ratios of LDH-NS:CMC of 1:100, 15:100 and 50:100. The mixtures are cast onto polystyrene (PS) petri-dish (12 cm × 12 cm) and dried at 50℃ for 24 h in an oven to obtain dried films. The dried films are peeled and stored at laboratory conditions in polyethylene valve bags. The obtained film samples, with ratios of LDH-NS:CMC of 1:100, 15:100 and 50:100 are named CMC/LDH-1, CMC/LDH-15 and CMC/LDH-50, respectively.

Film conditioning and thickness measurements

All films are conditioned at 25 ± 1 °C and 50 ± 2% RH, in a chamber containing a saturated solution of calcium nitrate tetrahydrate for 24 h before tests. Film thickness is measured at least five times with a digital micrometer at a random position, and the mean value is employed.

Light transmittance and transparency values

Films are cut into 1.0 cm × 5.0 cm pieces. The percent transmittance (%T) of the films is measured from 200 to 800 nm wavelengths using a double beam UV–vis spectrophotometer (UV-2450, Shimadzu Co., Ltd., Japan). The transparency value (TV) of the films is calculated following Eq. (1) (Arfat et al. 2017):

$$TV=\frac{-\text{log}\ {T}_{600}}{x}$$
(1)

where T600 is the fractional transmittance at 600 nm and x is the film thickness (mm). A higher transparency value represents lower transparency of the film.

Tensile properties

Tensile properties, including tensile strength (TS) and elongation at break (EB), are determined with an electronic universal testing machine (UTM4104, Jinan Labthink Instruments Co., Ltd., China) according to the Chinese standard method (GB 1040.3–2006). The strip dimensions, initial grip separation and crosshead speed are 10 mm × 100 mm, 52 mm, and 25 mm/min, respectively. The TS (MPa) is calculated by dividing the peak force (N) by the cross-section area (m2) of the film. Similarly, the EB (%) is measured by dividing the change in length (m) achieved at break by the film’s original length (m).

Water vapor permeability

Water vapor permeability (WVP) is measured by a Water Vapor Test System (PERME® W3/060, Jinan Labthink Instruments Co., Ltd., China) with the gravimetric cup method according to the ASTM E96-05 method at 25 ± 1℃. The film sample is sealed above a test dish containing distilled water. The test dishes are then placed in a desiccator with molecular sieves. The weight loss is recorded every 2 h at 70% relative humidity (RH) for 10 h. Each film is determined three times and the results are presented as an average.

Oxygen permeability

The values of Oxygen permeability (OP) of films are recorded by an Oxygen Transmission Rate Test System (C230H, Jinan Labthink Instruments Co., Ltd., China) with a differential-pressure method according to ASTM-3985 at 23 °C and 50% relative humidity (RH). The OP values of film samples are recorded by the system.

Fourier transform-infrared spectroscopy

Fourier Transform infrared (FT-IR) spectra for the nanocomposite films are recorded with a FT-IR spectrophotometer with an ATR (attenuated total reflectance) accessory (Model Spectrun 100 IR PerkinElmer, USA). The spectrum is taken in the wavelength range of 4000–600 cm−1 and a resolution of 1 cm−1.

X-ray diffraction

All X-ray diffraction (XRD) measurements of samples are carried out using an X-Ray Powder Diffractometer (Model D8 Advance XRD Brucker, Germany) operating with Cu-Kα radiation (λ = 1.5406 Å). Samples are exposed to the X-ray beam with the X-ray generator running at 40 kV and 40 mA. Scattered radiation is detected at ambient temperature in the angular region (2θ) of 5–80° at a rate of 0.02°/min.

Thermal stability analysis

The thermal properties of the film samples are determined by thermogravimetric analysis (TGA) (TA 550, USA). About 10 mg of film sample is cut into small pieces and placed into a sample pan, which is then equilibrated at 30 °C for 5 min and heated to 700 °C at a rate of 10 °C·min−1. The whole process occurs under a nitrogen flow of 40 mL·min−1.

Scanning electron microscopy

Micromorphology of samples is taken by scanning electron microscopy (SEM) (Model SU8020 FESEM Hitachi, Japan) with a testing voltage of 15 kV and current of 10 mA. A group of film samples is frozen in liquid nitrogen, and immediately fractured with tweezers. Another group of film samples is torn naturally. Samples treated from these two groups are used to observe the cross-section morphology. For the LDH-NS samples, LDH-NS suspension is dropped onto silicon pellet and dried for further use. A sputter coater (Model MC1000 Hitachi, Japan) is used to pre-coat conductive platinum (Pt) onto the film surfaces before the micrographs are observed.

Transmission electron microscope

Transmission electron microscopy (TEM) images are recorded on a transmission electron microscope (Model FEI Tecnai G2 F30, USA) by placing drops of LDH-NS suspension and composite film solution on a copper grid with an accelerating voltage of 300 kV.

Statistical analysis

All experiments in this study were conducted at least in triplicate, and all data are expressed as mean ± standard deviation. One-way analysis of variance (ANOVA) was utilized to assess significant differences among the data of different groups. Statistically significant differences were determined by Duncan’s test, with the confidence interval of 0.05 considered significant.

Results and discussion

Micromorphology of LDH-NS

The preparation of LDH-NSs is conducted according to the description in Yu’s work with a few modifications (Yu et al. 2019). The morphology of synthesized LDH-NS is characterized with SEM and TEM, and the obtained images are shown in Fig. 1. Similar morphology, as that shown in the publication by Yu et. al., can be observed in Fig. 1. In the SEM images (Fig. 1a and b), LDH-NSs are found to have diameters of about a few hundred nanometers and stack one on top of another, which might be caused by the aggregation of LDH-NSs during the drying process of the suspension on the silicon pellet. Interestingly, in Fig. 1b, some LDH-NSs are discovered standing upright, allowing the thickness to be estimated at a few nanometers. In Fig. 1c and d, the TEM images of LDH-NSs show diameters distributed around 200 nm, and a well-defined shape of LDH-NS platelets, instead of random irregular fragments, can be observed. These results demonstrate that the LDH-NS reported by Yu et al. has been successfully synthesized in our work.

Fig. 1
figure 1

SEM images (a, b) and TEM images (c, d) of LDH-NS suspensions at various scales. The inset in (c) represents the diameters measured by TEM

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Characterization of nanocomposite films

Morphology characterization

During the preparation of CMC/LDH-NS nanocomposite films, the mixture suspensions of CMC and LDH-NS are prepared by dissolving CMC in LDH-NS suspensions. The mixture suspension prepared is homogenous and stable, which can be stored for a few days (as shown in Fig. S2). The obtained films, peeled off from the PS petri-dish, are transparent (details will be presented in a later section) with a smooth surface. To investigate the interaction between LDH-NS and CMC, and the distribution of LDH-NS in CMC, the micromorphology of the suspension mixture of LDH-NS and CMC and the cross-sections of nanocomposite films are characterized with TEM and SEM. The resulting images are shown in Fig. 2. For recording the TEM images, mixture suspensions of CMC and LDH-NS with low concentration are added onto the carbon-coated copper grid and dried for TEM testing. These images in Fig. 2a, b, and c present the real state of CMC macromolecules and LDH-NS in the film samples, as the preparation procedures of both TEM samples and film samples are similar (casting vs. drying). As observed in Fig. 2a, b, and c, most of the LDH-NS are well distributed in the CMC solution, with no obvious aggregations, and very few LDH-NSs are placed in a vertical direction, with most of other platelets (indicated by the yellow arrows).

Fig. 2
figure 2

(a), (b) and (c) are TEM images of samples of the mixture solution of CMC and LDH-NS suspensions of CMC/LDH-1, CMC/LDH-15, and CMC/LDH-50, respectively; (CK1), (a1), (b1), and (c1) are SEM images of cross-sections, fractured with tweezers after freezing with liquid nitrogen; (CK2) (a2), (b2), and (c2) are SEM images of cross-sections, torn naturally with fingers. Insets present the samples at another scale. The images in the same row share the same scale

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The micromorphology of the cross-section of film samples is presented with SEM images in the second row (fractured after freezing with liquid nitrogen) and third row (torn naturally with fingers) of Fig. 2, which intuitively and truly reflects the interface interaction of LDH-NS and CMC in the films. The cross-sections of all film samples fractured after liquid nitrogen freezing show a smooth and compact surface, while the nanocomposite films display some white lines inside, which could be the LDH-NSs clamped by the CMC polymers. The white lines increase from left to right (in the second row of Fig. 2), which might be due to the increasing concentration of LDH-NS in the CMC solution. In the third row of Fig. 2, it is evident that the LDH-NSs are piled up in the CMC polymers. As the ratio of LDH-NS:CMC increases, the cross-section of films becomes rougher. All the SEM results demonstrate that the LDH-NS are well-aligned layer by layer in the CMC film matrix. These well-aligned LDH-NSs with an impermeable property (Yu et al. 2019) create a torture pathway for gas molecules attempting to pass through the film, thereby enhancing the barrier character of the nanocomposite film, as illustrated in Scheme 1.

XRD analysis

The crystal structure of the nanocomposite films is analyzed through XRD, with results spanning from 5° to 80° (2θ) shown in Fig. 3. The XRD spectrum of pure CMC shows a broad diffraction peak at 2θ = 21.6°, indicating an amorphous structure is present in the CMC films (Yadav et al. 2013; Li et al. 2020). For the patterns of LDH-NS, the (003), (006), and (009) diffraction peaks are detected at 2θ = 11.7°, 23.5°, and 35.7°, respectively, suggesting the highly crystalline layered structure of LDH-NS (Li et al. 2021). These diffraction peaks can also be observed in the spectra of CMC/LDH-1, CMC/LDH-15, and CMC/LDH-50, suggesting that the process of preparing the nanocomposite films has little impact on the structure of LDH-NS and is consistent with the study of Lu et al. (2016). Moreover, as the addition of LDH-NS increases, the intensity of diffraction peaks in the nanocomposite film also increases, indicating that the addition of LDH is beneficial for improving the crystallinity of the nanocomposite film.

Fig. 3
figure 3

XRD patterns of dried LDH-NS, pure CMC films, and different CMC/LDH-NS nanocomposite films

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FTIR spectra analysis

The FT-IR spectra of dried LDH-NS, pure CMC films, and different CMC/LDH-NS nanocomposite films are shown in Fig. 4. In the spectrum of CMC, a broad band around 3332 cm−1 is assigned to the stretching vibration of hydroxyl group (O–H), related to intermolecular and intramolecular hydrogen bonds (Yadollahi et al. 2014), which can be observed. A red shift attributed to the O–H group of CMC is noted upon the addition of LDH-NS, from 3332 cm−1 in CMC to 3276 cm−1, 3279 cm−1, and 3278 cm−1 in CMC/LDH-1, CMC/LDH-15, and CMC/LDH-50, respectively, which implies the formation of intermolecular hydrogen bonds between CMC and LDH-NS. The characteristic peak of LDH-NS attributed to ν(CO32−) (possibly due to the CO2 contamination from air during the synthesis process), appears at 1359 cm−1 (Aşçı 2017; Yang et al. 2021), which is also observed in the spectra of CMC/LDH-15 and CMC/LDH-50. However, the peak at 1359 cm−1 is not found in the spectrum of CMC/LDH-1, which may be due to the low percentage of LDH-NS in the nanocomposite. These results indicate the existence of molecular interaction between CMC and LDH-NS in these nanocomposites.

Fig. 4
figure 4

Spectra of FTIR of dried LDH-NS, pure CMC films, and different CMC/LDH-NS nanocomposite films. The right image is a zoomed-in section within the dashed box of the left image

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Tensile properties

The tensile strength (TS) and elongation at break (EB) are critical indicators of the tensile properties of films and are determined through tensile testing. The results for CMC/LDH-NS nanocomposite films are depicted in Fig. 5. The TS values improve with the addition of LDH-NS to the nanocomposites, with CMC/LDH-50 displaying the most significant increase (20.27%). The increase in TS can be attributed to the uniform dispersion of LDH-NS and hydrogen bonding interactions between LDH-NS and CMC (Li et al. 2021). The EB for CMC/LDH-1 shows the highest value, while those of the other two nanocomposite films decrease. The variation in EB can be ascribed to the addition of a small percentage of LDH-NS potentially reducing the entanglement of CMC macromolecular chains, but an excessive amount of LDH-NS might intensify the lamellar barrier effect of the nanosheets, which could restrict the segmental motion of CMC molecular chains in the nanocomposite films (Zhou et al. 2017).

Fig. 5
figure 5

(a) Tensile strength and (b) elongation at break of pure CMC films and different CMC/LDH-NS nanocomposite films

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Thermal stability analysis

Thermogravimetric analysis (TGA) is employed to investigate the thermal stability of the prepared CMC/LDH-NS nanocomposite films, compared with that of CMC and LDH-NS, with results illustrated in Fig. 6. In the TGA curve for CMC, similar results are observed as in the work of Yadollahi et al. (2014), with water evaporation before 200 °C, followed by decarboxylation and decomposition of CMC and the formation of carbonaceous char from 200 to 600 °C (the maximum degradation occurring around 285 °C). The TGA curve for LDH-NS presents similar results to that of Yu’s work (Yu et al. 2019), featuring the evaporation of physisorbed water molecules, dihydroxylation, and collapse of the layered structure. The TGA curves of the nanocomposite films differ significantly from both components. Before 200 °C, the weight loss is primarily due to moisture evaporation (Kang et al. 2021; Yadollahi et al. 2014). During this period, the TGA curves of nanocomposites samples present similar trends to CMC but vary in speed, which could be ascribed to the different content of water and hydrogen bonds between the components. The slower reducing speed (as observed in CMC/LDH-50) indicates a higher content of hydrogen bonds, consistent with the deductions made in FTIR analysis. The period of weight loss from 200 to 320 °C in the nanocomposites is mainly due to the degradation of the CMC. With an increase in LDH-NS content, a lower degradation initiation temperature for the nanocomposites is observed in this period. This phenomenon can be attributed to the higher content of LDH-NS, whose weight loss continues during this period. Above 320 °C, the weight loss of nanocomposites with a higher LDH-NS content (like CMC/LDH-50) occurs at a faster rate, which could be due to LDH-NS, with rapid weight loss from 320 to 400 °C and a weight drop from 400 to 450 °C. The rate of weight loss of residues around 700 °C in nanocomposite films with higher LDH-NS content presents a higher value, but this is not the case for CMC/LDH-1, which may be due to an anomaly caused by thermal degradation. All these results suggest that the addition of LDH-NS can improve the thermal stability of CMC and may involve more than simple mixing, indicating molecular interaction between the different components in the nanocomposites.

Fig. 6
figure 6

TGA curves of LDH-NS, pure CMC films, and different CMC/LDH-NS nanocomposite films

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Gas barrier properties

Water vapor permeability (WVP) is a crucial index for assessing barrier property of a nanocomposite film. The values of WVP of the nanocomposite films are shown in Fig. 7a. Compared with pure CMC films, the values of CMC/LDH-NS nanocomposite films decrease significantly (p < 0.05), and CMC/LDH-NS-50 exhibits the lowest WVP values at 1.43 × 10–12 g·cm·cm−2·s−1·Pa−1, which is a 51% reduction compared with that of CMC. A lower WVP indicates better water vapor barrier performance of the films (Riaz et al. 2020). The impact of LDH-NS on the variation trends of oxygen permeability (OP) differs from that of WVP, as shown in Fig. 7b. The OP values of the nanocomposite films decrease significantly with an increase in LDH-NS addition. A reduction of two orders of magnitude in the OP value with the addition of 15% LDH-NS [from 2862 × 10–16 cm3·cm·cm−2·s−1·cmHg−1 for CMC to 11.37 × 10–16 cm3·cm·cm−2·s−1·cmHg−1 for CMC/LDH-15, resulting in a reduction of about 252 times] can be noted. The lowest OP value for CMC/LDH-15 displays as 0.0201 cm3 m−2 d−1, highlighting the excellent oxygen barrier property of the nanocomposite film.

Fig. 7
figure 7

Barrier properties of the pure CMC and different CMC/LDH-NS nanocomposite films, including (a) water vapor permeability (WVP) and (b) oxygen permeability (OP)

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The improvement of the gas barrier capabilities of the nanocomposite films by adding LDH-NSs can be explained as follows: (i) the structure formed by LDH-NSs in CMC matrix resembles that of a brick wall, as shown in Scheme 1 and the cross-section images of SEM in Fig. 2, creating a tortuous pathway for the lateral diffusion of gas molecules and thereby lengthening the path of water vapor or oxygen molecules’ movement; (ii) the orientation force between polar molecules of H2O and polar groups on nanocomposites, and the induction force between non-polar molecules of O2 and polar groups on nanocomposites, would also hinder the movement of these small gas molecules (Fukuzumi et al. 2009); (iii) the formation of hydrogen bonds between LDH-NS and CMC impedes the penetration and diffusion of water molecules in the membrane matrix (Ghanbarzadeh et al. 2013; Fathi Achachlouei and Zahedi 2018); (iv) the improved crystallinity of the nanocomposite film, as proved in XRD characterization, makes the penetration and diffusion of water molecules more difficult. It is noteworthy that the high content of LDH-NS (50%) in films causes an increase in OP value but remain lower than that of pure CMC film, which could be attributed to the agglomeration of LDH-NS at high content in the CMC matrix and the creation of porosity in the nanocomposite films (Du et al. 2021).

Light transmittance and transparency

The optical properties of the nanocomposite film are closely related to its degree of homogeneity, compatibility between components, and the internal microstructure (Cheng et al. 2015; Chu et al. 2020). Poor compatibility between two components typically leads to reduced light transmittance of nanocomposite films due to light reflection or scattering at the phase interface (Chu et al. 2020). The optical transparency of the prepared films in this study is evaluated with a UV − vis spectrophotometer by acquiring light transmittance through the films as a function of wavelength (Fig. 8a). The transmittance values of pure CMC film, CMC/LDH-1, CMC/LDH-15, and CMC/LDH-50 nanocomposite film at 600 nm are recorded around 87.6%, 85.8%, 81%, and 79.8%, respectively. As known, a lower transparency value (TV) indicates higher transparency of films. All nanocomposite films display low TVs of 1.69, 1.75, 2.47, and 2.51 for CMC, CMC/LDH-1, CMC/LDH-15, and CMC/LDH-50, respectively. In the study by Arfat et al. (2017), a fish skin gelatin film showed a low TV of 1.45, but after adding even 0.5% Ag–Cu nanoparticles, the TV increased to 5.55. It is observed that, compared to pure CMC, the transparency of nanocomposites films experiences a slight decrease with an increase of LDH content. However, even for the highest content of LDH-NS (50%), films can maintain satisfactory transparency (Fig. 8b). This decrease in transparency can be attributed to the presence of some clustered LDH-NS with increasing LDH content, which is associated with uneven dispersion of LDH suspension and can be corroborated by the apparent morphology of the film. In summary, the excellent transparent property of CMC/LDH-NS composite films indicates a high degree of homogeneity and great compatibility between components, making them suitable candidates for application in food and pharmaceutical packaging and electronic encapsulation.

Fig. 8
figure 8

(a) UV absorption spectra of the pure CMC and different CMC/LDH-NS nanocomposite films; (b) photographs of the pure CMC films and different CMC/LDH-NS nanocomposite films

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Conclusions

In this work, CMC/LDH-NS nanocomposite films with high gas barrier properties, including water vapor and oxygen gas, and high transparency are prepared using a straightforward mixing and casting method from CMC solution and LDH-NS suspension. We systematically investigate and explain the effect of different ratio of LDH-NS on the nanocomposite films, as well as the mechanism behind the high gas barrier properties and high transparency, through characterizing the micromorphology, XRD and FTIR. The enhanced gas barrier characteristic is primarily due to the tortuous pathway, the improved crystallinity, and the existing of plenty of hydrogen bonding formed in the CMC film after the addition of 2D nanosheets of LDH-NS, synthesized via a non-toxic exfoliation method in aqueous amino acid solution. The high transparency is ascribed to the high degree of homogeneity and compatibility between CMC and LDH-NS. The nanocomposite films of CMC/LDH-NS show 252-fold reduction in oxygen permeability and 51% reduction in water vapor permeability compared with that of the CMC film. The transmittance values of the nanocomposite films at 600 nm can still maintain about 80% even when the mass ratio of LDH-NS:CMC is 50%. Moreover, the tensile properties and thermal stability of the nanocomposite films are also evaluated. These outstanding characteristics indicate a promising practical application of CMC/LDH-NS in food and pharmaceutical packaging and electronic encapsulation.