Moisture-Permeable, Humidity-Enhanced Gas Barrier Films Based on

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Applications of Polymer, Composite, and Coating Materials

Moisture-Permeable, Humidity-Enhanced Gas Barrier Films Based on Organic/Inorganic Multilayers Jiajie Wang, Xiaozhi Xu, Jian Zhang, Mengting Chen, Siyuan Dong, Jingbin Han, and Min Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09740 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 29, 2018

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ACS Applied Materials & Interfaces

Moisture-Permeable, Humidity-Enhanced Gas Barrier Films Based on Organic/Inorganic Multilayers Jiajie Wang, Xiaozhi Xu, Jian Zhang, Mengting Chen, Siyuan Dong, Jingbin Han* and Min Wei

State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China

Author Information ∗ Corresponding author. Phone: +86-10-64412131. Fax: +86-10-64425385. E-mail: [email protected].

Keywords: hybrid films, layered double hydroxide, moisture permselectivity, gas barrier, humidity response.

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Abstract: Gas barrier films with water-vapor-permeability have exhibited broad application prospects in gas separation and dehumidification. Herein, multilayer films comprised of layered double hydroxides (LDH) nanosheets and carboxymethyl cellulose sodium (CMC) were fabricated via layer-by-layer assembly. The resulting (LDH/CMC)n films show excellent gas barrier properties, which are ascribed to the significantly increased pathway for gas permeation originating from the large aspect ratio and high orientation of two-dimensional LDH nanosheets. Unlike traditional gas barrier films with nonselective blocking effect for various gases (including water vapor), the (LDH/CMC)n films exhibit unusual moisture permselective property. The moisture-permeable property was related with the hygroscopicity of CMC and hydrophilicity of LDH, which can enrich the water molecules from the surroundings and aggrandize the osmotic pressure for water vapor, resulting in an uncommon improvement of water vapor transmission. It is interesting to find that the (LDH/CMC)n films exhibit enhanced gas (O2, CO2, CH4 and N2) barrier properties upon treatment in a humid environment, due to the formation of hydrogen bonds between the infiltrated water molecules and hydrophilic groups in CMC, thus padding the interstitial space of the CMC molecular chains and increasing the gas transmission path. The reduction of free volume and extension of the gas transmission path further enhance the gas barrier properties of (LDH/CMC)n films. Moreover, the (LDH/CMC)n films represent the water vapor permselective property in mixed gas (including O2, CO2, CH4, N2 and water vapor), while maintaining the barrier for other gases, which can be potentially applied in air dehydration and dehumidification of natural gas.

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1. Introduction Gas barrier films are widely applied in pharmaceuticals packaging, food preservation, vacuum technology, encapsulation of organic light-emitting diode, etc.1−5 To meet the requirements of high barrier, organic-inorganic hybrid films were constructed by incorporating two-dimensional (2D) inorganic plates, including graphene/graphene oxides, montmorillonite or zirconium phosphate, into polymer matrix.6−10 The platiness and large-size of inorganic components with highly-oriented arrangement extend the gas pathway through the films, improving the barrier properties of the composites, which is well-known as “tortuous pathway”. However, it is worth noting that the water vapor, as a negative factor, normally induces a plasticization effect of the barrier films, by increasing the flexibility and interchain spacing of polymer chains, and thus weakens their gas barrier properties.11,12 Layered double hydroxides (LDHs) are a type of representative inorganic clay. They are generally expressed by the formula [M2+1−xM3+x(OH)2](An−)x/n·mH2O, where M2+ and M3+ represent divalent and trivalent metals cations, respectively; An− is a counter anion.13−17 Due to their intriguing characteristics, such as abundant hydrogen bonds, 2D layered structure, large aspect ratio and environmental friendliness, LDHs have been widely studied in the construction of organic-inorganic functional materials with enhanced mechanical properties,18,19 photo-thermal stability20,21 and photoelectrical performance.22−24 In particular, our previous studies have reported the design and fabrication of LDH/polymer composite films with excellent gas barrier properties for O2, 3



N2, CO2, He, etc.25−28 The common characteristic of these films is that their gas barrier mechanism is all based on the “tortuous pathway”, which creates a nonselective physical barrier for all kinds of gases without selectivity. However, in some especial fields, a permselective film for certain gases, while keeping a barrier for other gases, is required. For instance, in natural gas extraction and air dehydration, it is desirable that the films simultaneously display water-vapor-permeability and gas barrier properties, to remove the moisture and produce dry natural gas or air. However, such films have rarely been previously reported. In this study, organic-inorganic films were fabricated with a layer-by-layer (LBL) technique, using LDH nanosheets and carboxymethyl cellulose sodium (CMC) as building blocks (Scheme 1). The large-sized LDH nanosheets with high orientation play the obstructive role, endowing the (LDH/CMC)n films prominent gas barrier properties for O2, CO2, N2 and CH4. Additionally, the vast hydrophilic groups in CMC and large amount of hydroxyls in LDH give the (LDH/CMC)n films the unique permeability for water vapor. Furthermore, with the increase of the ambient humidity, the gas barrier properties of the (LDH/CMC)n films are obviously improved, instead of deteriorated. They rarely overcome the negative plasticization effect for organic-inorganic composite films, which show significant potential for extending the applications of gas barrier films.

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Scheme 1. Schematic illustration of the fabrication of (LDH/CMC)n films.

2. Experimental section 2.1 Reagents and Materials. Poly(diallyldimethylammonium chloride) (PDDA), poly(sodium-p-styrenesulfonate) (PSS), and CMC with molecular weight of ~100,000, ~70,000 and ~90,000 Da., respectively, were purchased from Aladdin (Beijing, China). Polypropylene (PP) substrates with thickness of 120 µm were obtained from J&K Scientific Ltd (Beijing, China). The following analytical grade chemicals: urea, Mg(NO3)2·6H2O, Al(NO3)3·9H2O, NaNO3, HNO3, H2SO4, acetone and ethanol were used without further purification. Deionized water was used in all the experimental processes. 2.2 Synthesis of single-layer LDH nanosheets. Single-layer MgAl-LDH nanosheets was prepared by delamination of bulk LDH nanoplates in formamide, using an approach similar to that used in previous reports.29,30 First, the MgAl(CO3)-LDH nanoplates were synthesized by a 5



hydrothermal method. Briefly, the Al(NO3)3·9H2O, Mg(NO3)2·6H2O and urea were dissolved in 100 ml of deionized water at a concentration of 0.05, 0.1 and 0.5 M, respectively. The mixed solution was transferred into a stainless-steel autoclave with a Teflon lining and hydrothermally aged at 110 °C for 24 h. After cooling, the precipitate of MgAl(CO3)-LDH nanoplates was washed several times with deionized water and ethanol, and dried at room temperature. Then the interlayered CO32− were exchanged by NO3− ions through the salt-acid treatment of MgAl(CO3)-LDH, in order to facilitate the delamination. The sample of 1 g MgAl(CO3)-LDH nanoplates was added into 1 L of aqueous salt-acid solution (0.0045 mol HNO3 and 1.5 mol NaNO3) and stirred for 24 h in a nitrogen atmosphere. The MgAl(NO3)-LDH nanoplates were collected by centrifugation and washing with water and ethanol, and dried in a vacuum. Afterwards, samples of 0.1 g MgAl(NO3)-LDH nanoplates were placed into 100 ml of formamide for 48 h with stirring. Ultimately, the delaminated single-layer MgAl-LDH nanosheets with aspect ratio of 50−2000 were successfully prepared. 2.3 Fabrication of the (LDH/CMC)n films and control samples. The LBL deposition technique was adopted to fabricate the (LDH/CMC)n films. A silicon wafer and a quartz glass were chosen as substrate for scanning electron microscopy/atomic force microscopy (SEM/AFM) and ultraviolet-visible (UV-vis) spectra analysis, respectively. The samples for oxygen and water vapor transmission rate measurements were assembled on the PP substrate. To make the substrate surface hydrophilic, we washed all the substrates with acetone, alcohol and water, in that order, for 15 min each. The LBL process was implemented by the circulations consisting of the following two steps: (a) the substrates 6


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were immersed into a single-layer MgAl-LDH colloidal suspension (1.0 g/L) for 10 min, washed with water and air dried; (b) the coated substrates were dipped into a CMC solution (2 wt%) for 10 min, washed with water and air dried. The circulation was repeated n times to prepare the (LDH/CMC)n films. For the comparison study, the (LDH/PSS)n and (PDDA/CMC)n films were fabricated using the same LBL approach. The pristine CMC film was obtained by spraying the CMC solution (2 wt%) on the substrates with a spray gun. 2.4 Characterization. The X-ray diffraction (XRD) measurements were performed with a Rigaku XRD-6000 diffractometer (Rigaku Corp., Tokyo, Japan), using Cu Kα radiation (λ = 0.1542 nm) at 40 kV and 30 mA. The morphology was examined by AFM using a NanoScope IIIa microscope (Veeco Instruments Santa Barbara, CA, USA) in the tapping mode, by field emission SEM (FESEM) with a Zeiss Supra 55 field emission scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany), with 20 kV accelerating voltage and by high-resolution transmission electron microscopy (HRTEM) using a JEOL JEM-2010 transmission electron microscope (JEOL Ltd., Tokyo, Japan) operated at 200 kV. The

UV-vis absorption spectra were recorded on a Shimadzu

U-3000

spectrophotometer (Shimadzu Corp., Kyoto, Japan) in the range of 190−400 nm. Thermogravimetric analysis (TGA) was performed with a HCT-1 differential thermal gravimetric analyzer (Beijing Henven Scientific Instrument Factory, Beijing, China). The particle size distribution of LDH nanoplates and Zeta potential of LDH colloidal suspension (1.0 g/L in formamide) were measured with a Malvern Mastersizer 2000 7



analyzer

(Malvern

Instruments

Ltd.,

Malvern,

UK).

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A

Vector

22

(Bruker)

spectrophotometer with 2 cm–1 resolution was used to collect the Fourier transform infrared (FT-IR) spectra. The gas (O2, N2, CO2, and CH4) and water vapor transmission rates were measured using a VAC-V2 and W3/060 gas transmission rate testing system (Labthink Instruments Co., Ltd., Jinan, China). The gas transmission rates were tested at 23 °C and 0% relative humidity, unless otherwise indicated. The content of each component for the mixed gas (H2O, CO2, O2, CH4 and N2) was obtained by using an Orsat gas analyzer (see details in the Supporting Information, Figure S1). The water contact angle (WCA) test was conducted using a DSA100 drop shape analysis system (KRüSS GmbH Company, Hamburg, Germany). The thickness of the film was measured with a CHY-C2A pachymeter (Labthink Instruments Co., Ltd.). The positron lifetime measurements were performed using a fast-slow coincidence ORTEC system (ORTEC AMETEK, Oak Ridge, TN, USA) at room temperature, with a time resolution of 187 ps.

3. Results and discussion The XRD patterns of the MgAl(CO3)-LDH (top line) and MgAl(NO3)-LDH (bottom line) nanoplates are shown in Figure 1a. A series of reflections, namely [003], [006], [012], [015], [018], [110] and [113], are displayed with no impurity peaks present, illustrating the well-defined and highly crystalline LDH phase. Compared with the MgAl(CO3)-LDH, the MgAl(NO3)-LDH exhibits a distinct reflection shift of the (003) plane from 12.2° to 10.2°, which proves that the interlayered CO32− has been successfully replaced by NO3−. 8


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The SEM image (Figure 1b) shows the regularly hexagonal MgAl(NO3)-LDH with a good uniformity and a lateral size of 2−3 µm. The particle size analysis (Figure S2) shows a centralized distribution with average size of ~2.4 µm, which is consistent with the SEM observation. Based on the classical Nielsen model and a theory latterly modified by Bharadwaj,31−33 the aspect ratio and orientation of inorganic are the key factors to determine the gas transmission properties of the organic/inorganic composite films, i.e., the larger aspect ratio and the higher orientation of the 2D inorganic plates are favorable to the barrier properties. In order to obtain LDH nanosheets with large aspect ratio, the bulk MgAl(NO3−)-LDH nanoplates were stirred in formamide to be delaminated into single-layer LDH nanosheets. Taping mode AFM image (Figure 1c and 1d) reveals a thickness of ~0.80 nm for LDH nanosheets, which was regarded as the sum of the thickness of a single-layer LDH nanosheet (0.48 nm) and an adsorbed monolayer (0.3 nm) of formamide molecules.29,30,34,35 The obvious side-incident light beam in the LDH nanosheets suspension (Figure S3) displays the Tyndall light scattering effect, illustrating the successful exfoliation of the LDH nanoplates. The Zeta potential of LDH nanosheets colloidal suspension (1 g/L in formamide) is 8.2 mV (Figure S4), indicating a positively charged surface, which means the LDH nanosheets can be assembled with polyanions via electrostatic interaction. The colloidal suspension has excellent transparency and stability for at least one month, which makes it an ideal building block for the fabrication of organic-inorganic hybrid films. 9



Figure 1. (a) XRD patterns of MgAl(CO3)-LDH (top line) and MgAl(NO3)-LDH (bottom line). (b) SEM image of the MgAl(NO3)-LDH nanoplates. (c) Tapping-mode AFM image and (d) height profiles of single-layer MgAl-LDH nanosheets.

The LBL technique has been approved to be an effective approach to achieve high-orientation of inorganic component in the nanoplate/polymer hybrid films,36,37 which was also used to prepare (LDH/CMC)n films. The UV-vis spectra (Figure 2a) of the as-prepared (LDH/CMC)n films show an intense absorption band at 198 nm, which is attributed to the characteristic absorption of CMC. The absorbance increases linearly (Figure 2a, inset) relative to the number of deposition cycles, which illustrates the well-proportioned and gradual growth of the multilayer films. Besides electrostatic interaction, hydrogen bonding is also supposed to exist between LDH nanosheets and CMC, because of the abundant hydroxyl groups in CMC molecules and LDH laminates. 10


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It is well-known that the electrostatic interaction is much stronger than hydrogen bond, so that we believe the electrostatic interaction should be the predominant driving force in the LBL assembly of the (LDH/CMC)n multilayers. The top-view SEM image of (LDH/CMC)30 film with a continuous and smooth surface is shown in Figure 2b. The side-view SEM image (Figure 2c) reveals a ~1060 nm thickness of the (LDH/CMC)30 film. Meanwhile, the cross-sectional TEM image (the specimen was obtained by a resin-embedding and section method) of the (LDH/CMC)30 film (Figure 2d) shows layers of LDH nanosheets as some darker lines with a distinct stratified architecture, which illustrates the good dispersion and high orientation of the LDH nanosheets. The 2D plates with perfect parallel orientation to the substrate play the best role in the gas barrier function. The transmittance spectrum of the (LDH/CMC)30 film (250−800 nm, Figure S5) shows an average light transmittance of 78.9%, indicating a good transparency. The SEM image of the scratched (LDH/CMC)30 film on the PP substrate (Figure S6) displays a clear border without obvious detachment and cracking, indicating the excellent adhesion between the (LDH/CMC)n films and PP substrate. In addition, the (LDH/CMC)30/PP film also exhibits a good mechanical properties with a yielding stress of ~30 MPa and an elastic deformation of ~10% (Figure S7).

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Figure 2. (a) UV-vis absorption spectra of the (LDH/CMC)n (n = 5−30) films (inset: the absorbance at 198 nm is plotted against the bilayer number n). (b) Top-view SEM, (c) side-view SEM and (d) side-view TEM images of the (LDH/CMC)30 film. The oxygen transmission rate (OTR) of bare PP substrate is 98.143 cm3 m−2 day−1 atm−1, as shown in Figure 3a. After coating of the (LDH/CMC)n films, the OTR undergoes a gradual decrease from 70.056 to 0.106 cm3 m−2 day−1 atm−1, with the bilayer number increasing from 5 to 30, which is indicative of the outstanding oxygen barrier properties of the (LDH/CMC)30 film. As a control sample, the pure CMC film was fabricated with nearly the same thickness as that of the (LDH/CMC)30 films via spray coating technology, and has a much inferior oxygen barrier properties with an OTR of

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Figure 3. (a) OTR of the (LDH/CMC)n films on the PP substrate (n = 0−30). The inset shows the tortuous path for oxygen transmission. (b) OTR of the PP substrate, (LDH/CMC)30, (LDH/PSS)30 and pristine CMC films. 94.584 cm3 m−2 day−1 atm−1 (Figure 3b). However, another control sample, the (LDH/PSS)30 film (thickness: 65 nm) displays an OTR of 0.114 cm3 m−2 day−1 atm−1, exhibiting a similar excellent oxygen barrier property as that of the (LDH/CMC)30 film. These results demonstrate that the LDH nanosheets play an influential role on the enhancement of the oxygen barrier property, by extending the transmission path of the gas molecules in the hybrid films.

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To perform a comparison study, a pure organic (PDDA/CMC)30 film was obtained by the LBL technique as well. The cross-sectional TEM image (Figure S8) shows a homogeneous distribution of the organic component, with no clear layered structure, indicating a random orientation. Based on the above analysis results, it is concluded that the high orientation of LDH nanosheets, together with their large aspect ratio, significantly increase the oxygen transmission path, leading to a suppressed permeation of oxygen and thus an outstanding oxygen barrier property. The interfacial interaction between building blocks is very important to the fabrication and performance of the hybrid films. The carboxymethyl cellulose sodium can be ionized into carboxymethyl cellulose anions and sodium cations pairs. The carboxymethyl cellulose anions were assembled with positively charged LDH nanosheets by electrostatic attraction. Besides, the Fourier transform infrared spectroscopy (FT-IR) spectra indicate the presence of hydrogen bonding (C=O…H−O−) between CMC and LDH (see details in the Supporting Information, Figure S9). To investigate the influence of interracial property on the performance of hybrid films, carboxymethyl cellulose (Na-free) was used to assemble with LDH nanosheets. It was found that the obtained film was rather rough on the surface, with a large proportion of uncovered surface, even though the bilayer number (n) reaches 30. The existence of naked substrate is ascribed to the week interfacial interaction between nonionized carboxymethyl cellulose and LDH nanosheets. The incomplete coated substrate exhibits poor gas barrier properties, because the gas molecules can transmit across the naked area in an easier manner. Therefore, the 14


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electrostatic interaction between the LDH and carboxymethyl cellulose sodium plays a key role in the film fabrication process and thus the gas barrier properties of the resulting multilayer films. To study the versatility of the (LDH/CMC)n film, its barrier effect for other kinds of gases was investigated. In view of their universality and significance, CO2, N2 and CH4 were selected as the probe molecules, as they are the main components of air or natural gas. The data presented in Figure 4 reveals that the CO2, N2 and CH4 transmission rates of the (LDH/CMC)30 film are sharply decreased by 99.99, 99.97 and 99.96%, respectively, compared with those of the PP substrate, which further illustrates the excellent and broad gas barrier properties of the (LDH/CMC)n film. These findings demonstrate that the tortuous barrier mechanism associated with the large aspect ratio and high orientation of LDH nanosheets is also applicable to other gas molecules.

Figure 4. The gas transmission rate of the (LDH/CMC)30 film for CO2, N2 and CH4.

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The water vapor transmission rate (WVTR) of (LDH/CMC)n films with various bilayer number (n = 0−30) at 100% relative humidity are shown in Figure 5a. The WVTR of the bare PP substrate is 1.95 cm3 m−2 day−1 atm−1. Unlike the barrier effect of the (LDH/CMC)n films for oxygen, the same films do not hinder the transmission of water vapor, which, on the contrary, exhibits a clear enhancement of the WVTR from 2.03 to 2.32 cm3 m−2 day−1 atm−1 upon increasing n from 5 to 30. Compared with the WVTR for the bare PP substrate, that of the (LDH/CMC)30 film is enhanced by ~19%. In view of the inherently low WVTR of the PP substrate, we believe that the WVTR of the whole film (substrate + coating) can be further improved if other materials with higher moisture permeability

were

selected

as

substrate.

The

interesting

discovery

of

the

water-vapor-permeable and oxygen barrier properties of the (LDH/CMC)n films motivate us to investigate the underlying structure-property relationship.

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Figure 5. (a) Water vapor transmission rate of the (LDH/CMC)n film on the PP substrate (n = 0−30). (b) Water contact angle of the (LDH/CMC)30 film. (c) The thermogravimetric analysis of the (LDH/CMC)30 film treated at different humidity. (d) Water vapor transmission rate of the PP substrate, (LDH/CMC)30, (LDH/PSS)30 and pristine CMC films.

It is well established that the hydrophilic/hydrophobic property of the film is closely related with its water vapor permeability. The surface wettability of the (LDH/CMC)30 film was measured by the WCA test. The data presented in Figure 5b reveals that the (LDH/CMC)30 film has a WCA of 25.1°, confirming the hydrophilicity of the film surface. Such a hydrophilicity is attributed to the good wettability of the LDH and CMC with WCA of 19.4° and 32.6°, respectively (Figure S10). The water absorbing property of the (LDH/CMC)n film was studied to further understand the interaction between the film and water moisture. As shown in Figure 5c, after treatment for 2 h, at 100% relative humidity, 17



the wet (LDH/CMC)30 film shows a large weight loss (~70%) at 100 ℃ (black line). In contrast, the dry (LDH/CMC)30 film does not exhibit obvious weight loss (red line). The large water absorbing capability of the (LDH/CMC)30 film is related with the molecular structure of CMC (Figure S11), whose large number of carboxyl and hydroxyl groups form a channel for water molecules movement with low resistance and ensure a facile water absorption process. According to the above results, the reasons for water vapor transmission promotion by (LDH/CMC)n films are based on the following three aspects. First, the hydrophilic surface of (LDH/CMC)n films facilitates the adsorption of water molecules, offering an unimpeded access. Second, the abundant hydrophilic groups in the interior of the film unclog the channel for the movement of water molecules. Third, the large water absorbing capacity of the (LDH/CMC)n film increases the osmotic pressure between the two sides of the substrate, which accelerates the transmission of water vapor across the substrate. In addition, the solvent effect is a favourable factor for the movement of water molecules in hybrid films.38,39 Based on the solvent effect, the Na+ of CMC in the films is solvated by surrounding water molecule, forming a hydration shell.40 The abundant hydrophilic groups and Na+ hydration shells construct continuous water molecule channels with low resistance and ensure a facile water molecule transmission process. Meanwhile, the water molecules are connected with each other by hydrogen bonds and can form large water clusters during the transport process.41,42 The structure breaking

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effect43 of Na+ changes the large water clusters into small water clusters, which further enhances the diffusion and penetration of water molecules in the continuous channels. To further confirm the above conjecture, the WVTR test was performed on various control samples to conduct a comparison study (Figure 5d). The pristine CMC film displays a larger WVTR than the (LDH/CMC)30 film and PP substrate, which indicates that the CMC component promotes water vapor transmission across the hybrid film. However, the (LDH/PSS)30 film shows a lower WVTR than the (LDH/CMC)30 film, even smaller than that of the PP substrate, demonstrating the barrier effect of the LDH nanosheets for water vapor, similar to the hindering effect for oxygen. Accordingly, the combination of the functionalities of gas barrier and water vapor permeability could be realized in one single film, via the rational design and ingenious fabrication of organic-inorganic hybrid films. Here we defined a relative permeability factor (F) to describe the selective transmission properties of the (LDH/CMC)n films, which can be represented as follows： F=

WVTR GTR

where GTR is the gas (O2, CO2, N2 and CH4) transmission rate, and WVTR is the water vapor transmission rate. Apparently, if the F value is smaller than 1, it is more advantageous for O2, CO2, N2 and CH4 transmission, whereas a bigger F value than 1 means that the transmission is selective water vapor. The difference between the F value and 1 is positively related to the permselectivity. The relative permeability factor of the bare PP substrate for H2O/O2, H2O/CO2,

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H2O/N2 and H2O/CH4 is 0.199, 0.006, 0.046 and 0.011, respectively (Figure 6a), showing a faster GTR than WVTR. The higher GTR than WVTR of organic films has also been observed in other polymer films, including PET and PE, due to their low- or the non-polarity of their molecular chains.44−46 However, after coating of the (LDH/CMC)30 film, the F value sharply increases to 21.877, 19.496, 193.333 and 36.825, respectively, indicating a prominently faster water vapor than gas transmission. Compared with that of the pure PP substrate, the F value for the (LDH/CMC)30 film increases by hundreds (or thousands) of times, which indicates an excellent selective transmission properties for water vapor.

Figure 6. (a) The permeation selectivity of the (LDH/CMC)30 film for O2, CO2, N2, CH4 and H2O. (b) The percentage of gas transmission for mixed gas (CO2, CH4, O2, N2 and H2O) for (LDH/CMC)n films (n = 0−30). The initial concentration of CO2, CH4, O2, N2 and H2O is 20 % for all.

Based on the selective transmission of (LDH/CMC)n films for water vapor, their moisture sieving behaviour with mixed gas was investigated. A mixed gas containing 20% (by partial pressure) of each component (including O2, CO2, N2, CH4 and water vapor) was used as probing source and sealed by the (LDH/CMC)30 film. The filtered gas was 20


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collected from the other side of the (LDH/CMC)n film, and its composition was analyzed and is summarized in Figure 6b. The data reveals that the gas filtered by the pure PP substrate (n = 0) contains only 0.25% water vapor and 99.75% of the other gases (O2, CO2, N2 and CH4). In comparison, after coating with the (LDH/CMC)n films, the water vapor content of the filtered gas rapidly increases from 34.27 to 92.28%, with n increasing from 10 to 30. The uniquely enhanced water vapor transmission properties of the (LDH/CMC)n films enables this kind of composites to be useful candidates for the selective removal of water vapor from wet gases.

Figure 7. (a) OTR of (LDH/CMC)5 and (LDH/CMC)30 films at different relative humidity. (b) Circulation of the OTR changes between 0 and 100 % relative humidity. (c) The thickness change for the (LDH/CMC)30 film at 0 and 100% relative humidity. (d) Free volume fraction of the (LDH/CMC)30 film at 0 and 100% relative humidity.

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Generally, the gas barrier properties of organic-inorganic composite (or pure polymer) films would recede under high-humidity condition.12 However, the (LDH/CMC)n film surprisingly shows an uncommonly enhanced gas barrier properties under high-humidity condition, which is in contrast to most of the reported gas barrier films. The OTR variation tendency of the (LDH/CMC)n films (n = 5 and 30) at various humidity conditions is shown in Figure 7a. With the relative humidity rising from 0 to 100%, the OTR of the (LDH/CMC)5 and (LDH/CMC)30 films decreases from 70.056 to 41.283 cm3 m−2 day−1 atm−1 and 0.106 to 0.064 cm3 m−2 day−1 atm−1, respectively. In addition, the recovery of the OTR can be achieved by decreasing the relative humidity to 0%; and this unique fluctuation of the OTR accompanied by humidity change can be repeated for several cycles (Figure 7b), indicating the effect of the high stability of the humidity-induced regulation on the gas barrier properties. To reveal the mechanism of the humidity-enhanced gas barrier properties, the internal structure change of the (LDH/CMC)n film under different humidity conditions was studied. The thickness of the (LDH/CMC)30 film at 0% relative humidity was measured with a pachymeter (Figure 7c), showing a ~1.1 µm thickness in the dry state, which is consistent with the side-view SEM image result (Figure 2d). However, upon exposure to 100% relative humidity, the film thickness increases almost three times. The increased film thickness enlarges the transmission path of the oxygen molecules, which makes the movement of the oxygen molecules more sluggish. In addition, the free volume of the (LDH/CMC)30 film measured by positron annihilation lifetime spectroscopy decreases 22


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from 23.262 to 13.074%, when the relative humidity changes from 0 to 100% (Figure 7d). This is attributed to the formation of hydrogen bonds between the vast hydrophilic groups in the (LDH/CMC)30 film and the infiltrated water molecules. The filled water molecules occupy the interstitial space of the CMC molecular chains, thus reducing the free volume of the (LDH/CMC)30 film and impeding the transmittance of oxygen molecules. Based on the above analysis, we believe that the extension of the movement path of the oxygen molecules and the reduction of the free volume for the (LDH/CMC)30 film are responsible for the enhancement of the oxygen barrier properties under high-humidity condition.

4. Conclusions The (LDH/CMC)n films with permselectivity, which were fabricated by the LBL technique, show gas barrier and moisture-permeable properties. The 2D feature and high orientation of the LDH nanosheets extend the pathway of the gas molecules, leading to the first-class gas barrier properties. In addition, the (LDH/CMC)n films were found to be able to absorb water molecules from the environment, and thus increase the water osmotic pressure of the substrate, leading to an uncommon improvement for the water vapor transmission capacity. Moreover, the gas barrier properties were further enhanced under high humidity, due to the formation of hydrogen bonds between the vast hydrophilic groups in the CMC and the infiltrated water molecules, which fill the interstitial space of the CMC molecular chains and increase the thickness of the films. As a result, the reduction of the free volume and extension of the gas molecule transmission path further

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enhance the gas barrier properties of the (LDH/CMC)n films. To the best of our knowledge, it is uncommon for organic-inorganic hybrid films to improve their gas barrier properties under high humidity. Therefore, this work offers a new strategy for the design and fabrication of gas barrier film with water vapor permeability, which can be potentially used in air dehumidification and natural gas dehydration. Supporting Information Available: particle size distribution, UV-vis transmittance spectrum, Zeta potential, SEM and TEM images, water contact angle, FT-IR spectra, molecule structural formula of CMC. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements. This work was supported by the 973 Program (Grant No. 2014CB932102), the National Natural Science Foundation of China (NSFC), the Fundamental Research Funds for the Central Universities, the Young Elite Scientists Sponsorship Program by CAST, and the Beijing Nova program.

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Entry for the Table of Contents

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Scheme 1. Schematic illustration of the fabrication of (LDH/CMC)n films. 272x208mm (300 x 300 DPI)



Figure 1. (a) XRD patterns of MgAl(CO3)-LDH (top line) and MgAl(NO3)-LDH (bottom line). (b) SEM image of the MgAl(NO3)-LDH nanoplates. (c) Tapping-mode AFM image and (d) height profiles of single-layer MgAl-LDH nanosheets. 208x159mm (300 x 300 DPI)


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Figure 2. (a) UV-vis absorption spectra of the (LDH/CMC)n (n = 5−30) films (inset: the absorbance at 198 nm is plotted against the bilayer number n). (b) Top-view SEM, (c) side-view SEM and (d) side-view TEM images of the (LDH/CMC)30 film. 272x208mm (300 x 300 DPI)



Figure 3. (a) OTR of the (LDH/CMC)n films on the PP substrate (n = 0−30). The inset shows the tortuous path for oxygen transmission. (b) OTR of the PP substrate, (LDH/CMC)30, (LDH/PSS)30 and pristine CMC films. 208x289mm (300 x 300 DPI)


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Figure 4. The gas transmission rate of the (LDH/CMC)30 film for CO2, N2 and CH4. 209x148mm (300 x 300 DPI)



Figure 5. (a) Water vapor transmission rate of the (LDH/CMC)n film on the PP substrate (n = 0−30). (b) Water contact angle of the (LDH/CMC)30 film. (c) The thermogravimetric analysis of the (LDH/CMC)30 film treated at different humidity. (d) Water vapor transmission rate of the PP substrate, (LDH/CMC)30, (LDH/PSS)30 and pristine CMC films. 189x144mm (300 x 300 DPI)


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Figure 6. (a) The permeation selectivity of the (LDH/CMC)30 film for O2, CO2, N2, CH4 and H2O. (b) The percentage of gas transmission for mixed gas (CO2, CH4, O2, N2 and H2O) for (LDH/CMC)n films (n = 0−30). The initial concentration of CO2, CH4, O2, N2 and H2O is 20 % for all. 109x44mm (300 x 300 DPI)



Figure 7. (a) OTR of (LDH/CMC)5 and (LDH/CMC)30 films at different relative humidity. (b) Circulation of the OTR changes between 0 and 100 % relative humidity. (c) The thickness change for the (LDH/CMC)30 film at 0 and 100% relative humidity. (d) Free volume fraction of the (LDH/CMC)30 film at 0 and 100% relative humidity. 189x144mm (300 x 300 DPI)


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Moisture-Permeable, Humidity-Enhanced Gas Barrier Films Based on

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