Cellulose Nanofibrils Multilayer Films with Good

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Robust Guar Gum/Cellulose Nanofibrils Multilayer Films with Good Barrier Properties Lei Dai, Zhu Long, Jie Chen, Xingye An, Dong Cheng, Avik Khan, and Yonghao Ni ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14471 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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ACS Applied Materials & Interfaces

Robust Guar Gum/Cellulose Nanofibrils Multilayer Films with Good Barrier Properties Lei Dai, † Zhu Long, ‡ Jie Chen, ‡ Xingye An, †, § Dong Cheng, †, § Avik Khan, † and Yonghao Ni* † †

Department of Chemical Engineering, University of New Brunswick, Fredericton, NB E3B

5A3, Canada ‡

Key Laboratory of Eco-textiles, Ministry of Education, Jiangnan University, Wuxi 214122,

China §

Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science & Technology, Tianjin

300457, China ABSTRACT: The pursuit of sustainable functional materials requires development of materials based on renewable resources and efficient fabrication methods. Hereby, we fabricated allpolysaccharides multilayer films using cationic guar gum (CGG) and anionic cellulose nanofibrils (i.e. TEMPO-oxidized cellulose nanofibrils, TOCNs) through a layer-by-layer casting method. This technique is based on alternate depositions of oppositely charged water-based CGG and TOCNs onto laminated films. The resultant polyelectrolyte multilayer films were transparent, ductile and strong. More importantly, the self-standing films exhibited excellent gas (water vapor and oxygen) and oil barrier performances. Another outstanding feature of these resultant films was their resistance to various organic solvents including methanol, acetone, N, N-dimethylacetamide (DMAc) and tetrahydrofuran (THF). The proposed film fabrication process is environmentally benign, cost-effective and easy to scale-up. The developed 1 ACS Paragon Plus Environment


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CGG/TOCNs multilayer films can be used as a renewable material for industrial applications such as packaging. KEYWORDS: cationic guar gum, cellulose nanofibrils, multilayer, gas barrier, oil barrier

INTRODUCTION Thin films with transparency, flexibility, mechanical strength and barrier (gas and oil barrier) properties are of great importance to various applications, such as flexible electronics and packaging

1-4

. Generally, synthetic polymers are used to improve the barrier properties of films,

resulting in serious recycling and environmental challenges

5-7

. It is of commercial interest to

develop sustainable, functional materials with a competitive edge over synthetic polymers, using renewable/bio-based materials and easy to adopt methods renewability/sustainability,

biodegradability,

8, 9

biocompatibility,

. Biopolymers, due to their thermostability,

etc.,

are

considered ideal candidates to replace synthetic polymers for various applications 10-12. Guar gum (GG), which is extracted from the endosperm of guar seeds, is a commercial biopolymer 11, 13. It consists of mannose units with alternating lateral branched galactose groups, with a 2:1 ratio of mannose to galactose 14. Owing to its low cost and renewable nature, GG has found a large number of industrial applications12, 15, 16. Furthermore, it is a suitable polymer to fabricate biodegradable films because of its water solubility, film forming ability and high viscosity at low concentration 17, 18. Mikkonen et al. 19 reported that the light transmittance of GG films was 89.8% at 550 nm. Other valuable features include its water retention and water blocking capacity 20, 21, for instance, GG is commonly used in the explosives industry as a water sealer to keep explosives dry properties

22

15, 21

. However, GG films exhibit relatively poor mechanical

, and their tensile strength is in the range of 20-50 MPa 2 ACS Paragon Plus Environment

23-25

. There is a great

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incentive to improve the physicochemical properties of the GG based films by combining with other biopolymers. Cellulose nanofibril (also known as nanofibrillated cellulose, NFC) is a cellulose derivative that has become the focus of a lot of research work largely due to its “green” features and many desirable properties

26

. NFC films are widely reported to exhibit excellent mechanical strength

and oxygen barrier properties and could be considered as a material of choice to substitute fossilbased synthetic polymers for various commercial applications

1, 27-31

. It can also act as an

reinforcing agent for other biopolymeric materials such as starch 32, hemicelluloses 34

33

and chitin

and it has excellent synergism with other biopolymers. The layer-by-layer (LBL) assembly technique has been quite popular for the fabrication of

films due to a number of advantages, namely: 1) the film thickness can be controlled at nanolevel, with an almost defect-free structure; 2) it is very versatile, and can accommodate various components; and 3) unique physicochemical properties can be imparted to the resultant films 3436

. As a matter of fact, the LBL technology can be done via different approaches, such as casting,

immersion, spinning and spraying

35

. For example, Fukushima et al.

37

reported a dry actuator

that was fabricated using the LBL casting procedure. Other researchers also adopted this technique to prepare multilayer composites

38

. It was reported that the LBL-based multiwall

carbon nanotubes films showed much better tensile strength than those made by solution casting, and melt-mixing methods 39. In this research work, we have used cationic guar gum (CGG, guar gum grafted with (hydroxypropyl trimethylammonium chloride) and anionic cellulose nanofibrils to prepare multilayer films based on the LBL casting technique. The anionic cellulose nanofibrils were

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prepared by using the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation method, and were termed as TEMPO-oxidized cellulose nanofibrils (TOCNs). The obtained CGG/TOCNs multilayer films were characterized using ultraviolet-visible (UV-Vis) spectrometry, atomic force microscopy (AFM), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Additionally, the tensile strength, elongation at break (%), oxygen and water vapor barrier, oil barrier, as well as solvent resistance properties of the films were also examined.

EXPERIMENTAL SECTION Materials. Cationic guar gum (CGG), with a degree of substitution of 0.17, was kindly provided by a mill in Wuxi, Jiangsu Province, China. The viscosity of the 1 wt.% CGG solution was about 1300 mPa·s (at 25 °C). The preparation of TEMPO-oxidized cellulose nanofibrils (TOCNs) was carried out following a procedure described in the literature dissolving

pulp

was

dispersed

in

deionized

water

followed

by

24

. Briefly, a

oxidation

using

TEMPO/NaBr/NaClO system (the reaction scheme was shown in Figure S1 in the Supporting Information). The carboxyl group content of the resultant oxidized cellulose was 1.34 mmol/g. The TOCNs sample was then obtained using ultrasound treatment (Sonicator Q1375, Qsonica, USA). TEM images of TOCNs were given in Figure S2: the majority of the sample was in the range of 5-25 nm in width while the length was several micrometers. All other chemicals were bought from Sigma Aldrich and were used without further purification. Film Fabrication. CGG was dissolved in deionized water to prepare 0.5 wt.% solution. The concentration of TOCNs suspension was 0.5 wt.% as well. Their visual appearance is provided in the supporting information (Figure S3). The CGG/TOCNs multilayer films were prepared 4 ACS Paragon Plus Environment

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based on the layer-by-layer casting technique. A 10 g of 0.5 wt.% CGG solution was first cast on a petri dish, forming the first layer. Subsequently, another 10 g of 0.5 wt.% TOCNs suspension was cast on top of the previous CGG layer. By repeating this process, CGG/TOCNs multilayer films consisting of 4 layers or 8 layers of CGG/TOCNs were prepared. All films were easily peeled off from the supporting petri dish after they were dried in an oven (60 °C). The corresponding films with 4 layers were denoted as (CGG/TOCNs)4 while the ones with 8 layers were denoted as (CGG/TOCNs)8. The 100% CGG control consisted of 160 g of 0.5 wt.% CGG solution. All the films were stored in a conditioning room (23 °C and 50% RH) for 3 days before further characterization. Film characterization. The apparent film density was calculated by dividing the mass with the sample dimensions. The film thickness was determined as the average of 5 random positions of each sample using a thickness meter (MP0, Fischer, Germany). Mechanical properties including tensile strength and elongation at break were determined on a BZ2.5/TNIS Zwick Material Tester (Zwick, Germany) at room temperature. The samples were cut into rectangular strips of 10 mm × 100 mm. The initial grip separation of the machine was set at 50 mm and the specimens were loaded at a constant cross-head speed of 50 mm/min. At least 5 specimens were tested and the average value was reported. Fourier transform infrared spectroscopy (FTIR) was carried out on a Nicolet 6700 FTIR spectrometer (Thermo Scientific, USA) at a resolution of 4 cm-1 in the range of 400-4000 cm-1. The samples were ground with potassium bromide (KBr), and subsequently pressed into pellets, respectively.



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The light transmittance of the films was measured within the UV-Visible region (200-800 nm) on a GENESYS 10 UV-Vis spectrophotometer (Thermo Fisher Scientific Inc.) using air for the background correction. Atomic Force Microscope (AFM) imaging was applied to characterize the surface morphology and roughness of the films. The MFP-3D AFM (Asylum Research, USA) operating in tapping mode under ambient air conditions was used. The surface and cross-section of the films were also characterized with JEOL 6400 scanning electron microscope with a 15 kV acceleration voltage. Particularly, the films were treated/fractured in liquid nitrogen for a cross-section observation. The obtained pieces were fixed on a sample holder with fracture area facing upward. All samples were sputter-coated directly with a layer of gold before imaging. Water vapor transmission rate (WVTR) of the films was determined gravimetrically. A saturated sodium chloride (NaCl) aqueous solution was used to establish 75% RH circumstance inside a test chamber. The film specimen was sealed on a circular dish (diameter of 60 mm) filled with 15 g oven dried calcium chloride (CaCl2) that can maintain 0% RH in the circular dish. The circular dish was kept in the test chamber and weighed every 12 hours for 3 days. The amount of water vapor penetrated through the film was determined by the weight increase of the circular dish. The WVTR in (g/(m2·24h·atm)) was calculated according to the equation below. Three replicates were made for each film and the average value is shown.

WVTR g/ · 24h · atm) ) =

24 ℎ) − ) ℎ m )

The oxygen transmission rate (OTR) of the films was measured with a Labthink VAC-V1 apparatus (Labthink, China) at 23 °C and (40 ± 1) % RH. The sample size was 38.48 cm2 and the


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partial pressure of oxygen was 0.1 MPa. Three measurements were carried out for each sample and the average value was reported. The water contact angle of each film was measured with an Attension Theta Optical Tensiometer (Biolin Scientific, Finland) at ambient condition. A drop of water was deposited on the film surface, and then images and videos of the water droplet were captured and recorded. Oil resistance property was tested according to the method reported by Osterberg et al.

40

A

Sudan II dyed 100% pure canola oil was taken as the oil model. The density of canola oil is 0.917 g/mL while its viscosity at 25 °C is 57 mPa·s. A circular filter paper with a diameter of 5 cm was immersed in the dyed oil for 30 s to saturate it. The stained filter paper was then put on top of the film sample, and another blank filter paper (as a stain absorber) was placed underneath the testing film. The whole stack was pressed between two stainless steel plates and maintained in an oven at 60 °C for 4 h. At the end of the test period, the assembly was removed to check the stained spots on the stain absorber. N,N-Dimethylacetamide (DMAc), water, tetrahydrofuran (THF), acetone and methanol were used to test the films’ solvent resistance and absorption. Pieces of films with the size of 10 × 50 mm were immersed in solvent for 48 h, and after a certain time they were photographed to show their solvent resistance performance. The solvent absorption of the films was determined by immersing the film strips in solvent and reweighing them periodically. The solvent absorption ratio (%) was calculated using the following equation: Solvent absorption ratio %) = [./ − .0 )/.0 ] × 100%



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Where, wt is the film weight after being immersed in solvent for a certain time and w0 is the initial film weight.

RESULTS AND DISCUSSION The main objective of this work was to fabricate smooth, uniform, strong multilayer films with good barrier properties using CGG and TOCNs. The film fabrication process is shown in Scheme 1. Our hypothesis was that the lamination of CGG and TOCNs through the layer-bylayer casting method could effectively retain the properties of both these two materials. TOCNs can form dense and compact films with superior mechanical strength and excellent oxygen barrier properties 40. Meanwhile, CGG also has good film forming feature and the resultant CGG films should exhibit some unique properties, such as water retention capability

41

. In addition,

this LBL casting method could also take advantage of electrostatic attractions between CGG and TOCNs. The strong electrostatic interactions between quaternary ammonium groups of CGG and carboxyl groups of TOCNs, would significantly densify the film structure and subsequently enhance the performance (such as strength and barrier properties) of the consequent CGG/TOCNs multilayer films. Furthermore, CGG and TOCNs could also form inter-molecular hydrogen bonding since they both have abundant hydroxyl groups, which would further increase their interactions and compatibility. Formation of interpenetrating networking could further strengthen the interactions between the two components, resulting in enhanced mechanical properties of the films. All these chemical/physical interactions could also restrict the chain movement and weaken the plasticization effect brought by water. Scaling up the LBL process is crucial for commercial applications

34, 35

and this LBL casting process is readily scalable. 8

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Therefore, we believe not only the laminated CGG/TOCNs multilayer films prepared in this method should have good overall performance, but also the present work can be an example for scaling up the LBL technology.

Scheme 1. Preparation of CGG/TOCNs multilayer films and their interactions. The obtained CGG/TOCNs multilayer films were glossy and flexible (Figure S4). The thickness, density, moisture content and mechanical properties of the self-standing films were characterized and listed in Table 1. It was found that the increase of the thickness of the films was proportional to the increase in number of layers. The (CGG/TOCNs)4 films (with 4 layers) had a thickness of 51.5 ± 0.9 µm while the (CGG/TOCNs)8 counterpart (with 8 layers) had a thickness of 104.8 ± 1.1 µm, which supports the conclusion that the film formation process was uniform and repeatable. The variation of density and moisture of films were essentially the same. In addition to resulting in the transparent and smooth films, this layer-by-layer casting 9 ACS Paragon Plus Environment


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fabrication method also led to tough films. In particular, as indicated in Table 1, the resultant CGG/TOCNs films exhibited much higher tensile strength (81.4 MPa) than the control CGG films (41.0 MPa). The mechanical strength of the resultant composite films was higher than that of commercially available cellulose-based materials such as cellulose acetate (56.2 MPa) nanocrystalline cellulose films (61 MPa)

26

42

and

, but similar to those of microfibrillated cellulose

films (82.2-90.3 MPa) 43. It should be noted that the results of mechanical properties are strongly affected by their testing environment (such as relative humidity and temperature). Cellulose nanofibrils have often been used to enhance the strength of biomaterials, such as xylan-rich hemicellulose films 33. Thus, the presence of TOCNs could partially explain the high mechanical strength of the CGG/TOCNs multilayer films obtained in this work. Besides, strong interaction and good compatibility between CGG and TOCNs could have led to uniform stress transfer through the film, resulting in high mechanical strength. Ho et al. 44 noted the beneficial effect of the electrostatic interactions of opposite charges in composite systems to the mechanical properties. As shown in the FTIR spectra (Figure S5), the band at 1490 cm-1 for CGG was assigned to quaternary ammonium cations, which was red-shifted to 1471 cm-1 in the CGG/TOCNs multilayer film due to their interactions with carboxylate groups of the TOCNs. The band at 1421 cm-1 in TOCNs was due to the -COO- symmetric stretching vibration, which was also shifted to 1418 cm-1 in the multilayer film sample. The band at 1380 cm-1 of the CGG sample is due to C-N stretching vibration, which was still evident in the CGG/TOCNs multilayer film. The band at 870 cm-1, due to skeletal stretching vibration of CGG, was again present in the CGG/TOCNs multilayer film. The band at 1610 cm-1 for TOCNs was due to the C=O stretching of carboxylate groups. The band at 1650 cm-1 for the CGG sample was attributed to quaternary 10 ACS Paragon Plus Environment

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ammonium cations. For the CGG/TOCNs multilayer film, the characteristic band was at 1625 cm-1, which may be explained by 1) the electrostatic interactions between carboxylate groups of TOCNs and quaternary ammonium cations of CGG; 2) overlapping of their characteristic bands. Table 1. Fundamental and Mechanical Properties of the control and CGG/TOCNs Multilayer Films

Sample ID

Sample Description

Thickness (µm)

Apparent Density (g/cm3)

Control

100% CGG

152.0 ± 4.5

0.820

(CGG/TOCNs)4

4 layers of CGG/TOCNs

51.5 ± 0.9

(CGG/TOCNs)8

8 layers of CGG/TOCNs

104.8 ± 1.1

Moisture

Tensile Strength (MPa)

Elongation at Break (%)

16.4 ± 0.6

41.0 ± 3.5

6.4 ± 0.1

0.989

15.3 ± 0.4

81.4 ± 2.0

4.9 ± 0.1

0.972

15.7 ± 0.1

82.1 ± 4.3

4.7 ± 0.2

Content (%)

The CGG/TOCNs films appeared highly transparent, which could be seen from the insert in Figure 1. Furthermore, the optical transmittance of the films was determined with a UV−Vis spectrophotometer and the results are depicted in Figure 1. The light transmittance of the films increased sharply at around 400 nm. These films (4 or 8 layers of CGG/TOCNs) possessed remarkable optical transparency in the visible light spectrum (390−750 nm). For instance, the light transmittance at 600 nm for (CGG/TOCNs)4 film was 84.4%. The transparency of (CGG/TOCNs)8 films was only slightly lower than that of (CGG/TOCNs)4 films. The CGG/TOCNs assembly might include some fibril aggregates, leading to light scattering, and likewise, the thicker films might contain more fibril aggregates that resulted in lower light transmittance. 11 ACS Paragon Plus Environment


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Figure 1. Light transmittance results of (CGG/TOCNs)4 and (CGG/TOCNs)8 (picture shown in the insert) films. The CGG/TOCNs multilayer films were flat and smooth without wrinkle upon drying, which is in contrast to the NFC films, thanks to the presence of CGG. The film surfaces only had nanometer scale roughness and exhibited a dense structure, which can be seen from AFM images (Figure 2a and 2b). As illustrated in the TEM picture of TOCNs (Figure S2), a good fibrillation of cellulose fibers was obtained. However, the TOCNs suspension was used without further purification process after its preparation, thus containing some thick fibrils. The majority of them were in the range of 5-25 nm in width while a small number of thick fibrils were in the range of 50-100 nm in width. The results indicated that the presence of a small number of thick fibrils had negligible effect on the dense and smooth structure. Osterberg et al. 40 also concluded that small parts of thicker fibrils would not deteriorate the dense and smooth structure formed by cellulose


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nanofibrils in their study of NFC films. In addition, SEM was also used to study the microstructure of the films (Figure 2c). The SEM image of CGG side (Figure 2c-1) indicates that it had good smoothness while the image of the TOCNs side (Figure 2c-2) shows a network structure formed by randomly oriented fibrils. Figure 2c-3 (cross-section of the films) demonstrated a high level of composition compatibility and uniformity, and dense and compact structure of the films. In fact, it is difficult to differentiate the boundary between each layer inside the films. The numerous hydroxyl groups from CGG and TOCNs may have provided good miscibility with each other by the formation of hydrogen bonds. Thus, the presence of a high level of intermixing and interpenetration between CGG and TOCNs interface could be concluded, thanks to the strong electrostatic interactions and abundant hydrogen bonds between the layers.



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Figure 2. Surface morphology and cross-section structure of CGG/TOCNs multilayer films. a and b: AFM tapping-mode of CGG/TOCNs multilayer films (a-1: height, a-2: 3D images of CGG side; b-1 and b-2: height and 3D images of TOCNs side). c: SEM characterization of the CGG/TOCNs multilayer films (Surface characterization: c-1: CGG side, c-2: TOCNs side; Cross-section characterization: c-3). The Water vapor barrier property of the control (only CGG), (CGG/TOCNs)4 and (CGG/TOCNs)8 films were evaluated to determine the influence of structure and thickness on films’ gas barrier properties (Figure 3). The control films exhibited a WVTR of 60.47 g/(m2·24h·atm). It is noted that the thickness of the control film was 152.0 µm. Whereas, the


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multilayer films, (CGG/TOCNs)4 and (CGG/TOCNs)8 exhibited a WVTR of 121.88 g/(m2·24h·atm) (for a film thickness of 51.5 µm) and 68.52 g/(m2·24h·atm) (for a film thickness of 104.8 µm), respectively. Therefore, if one would compare the WVTR value at the same thickness, one would draw the conclusion that the CGG/TOCNs multilayer films presented much improved resistance against water vapor. The underlying explanations are: 1) the compact structure of the film; 2) the crystalline part of cellulose nanofibrils; 3) the water vapor barrier property of guar gum. The enhanced water vapor barrier performance could partially be the result of the compact structure due to the strong interaction and formation of polyelectrolyte complexes (PECs) with cationic CGG and anionic TOCNs

45

. Ho et al.

44

also noted that ionic

interaction played an important role in water vapor barrier properties of the composites made of cationic nanofibrillated cellulose and anionic silicates. Crystallinity is an important factor affecting the permeability of the nanocomposite films as water vapor predominantly diffuses through the amorphous areas of polymer. The increase in deposited layers may have led to an increase in tortuosity in the films, hence the reduction in WVTR 46. These results also indicated that (CGG/TOCNs)8 films had better water vapor barrier performance than that of the pristine nanofibrillated cellulose films (600 g/(m2·day) with a similar film thickness

40

. Typically, NFC

films have relatively poor water vapor barrier property especially under high relative humidity (RH)

44

. It was reported that the formation of water clusters under high RH (˃ 70%) could

significantly increase the solubility of water vapor, which would lead to increased water vapor penetration

47

. The incorporation of CGG markedly improved the water vapor barrier

performance of the films. GG although intrinsically hydrophilic, is not hygroscopic

15, 48

,

considering the fact that GG is used as a water sealer in explosive industries to protect the explosives from getting wet 15, 21.



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Interestingly, the CGG/TOCNs films also exhibited good oxygen barrier performance. For (CGG/TOCNs)4 films, the OTR was 7.77 (cm3/(m2·24h·atm)) while that of (CGG/TOCNs)8 films was 5.56 (cm3/(m2·24h·atm)). NFC films have a low OTR due to the high degree of crystallinity of the fibrils and the dense structure of the films

49, 50

. Both the high crystallinity of

nanofibrils and the closed compact nanofibrils network will lead to decreased free volume, subsequently decreasing the oxygen permeability 40. From these results, it can be concluded that all these CGG/TOCNs films inherited the very good oxygen barrier property from TOCNs films. For the CGG/TOCNs films, strong electrostatic interactions, abundant intermolecular hydrogen bonds and the interpenetrating networks are responsible for the high oxygen barrier capacity. Particularly, the strong interactions between CGG and TOCNs could effectively restrict the movement of polymer chains, so as to prevent the oxygen permeation under the humid environment. Aulin et al.

1

reported similar results in their study of anionic NFC and cationic

polyethyleneimine multilayer films.

Figure 3. Gas transmission rate of CGG/TOCNs multilayer films and control (a: WVTR; b: OTR). In general, polysaccharide-based films exhibit good barrier properties against oil due to their high cohesive energy density 2. Figure 4 illustrates the oil barrier performance of the resultant 16 ACS Paragon Plus Environment

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CGG/TOCNs multilayer films. As shown in Figure 4a, after 4 h treatment in oven under 60 °C, the blank filter paper (the stain absorber) covered by the film was kept from the oil and maintained totally clean, which confirmed the excellent oil barrier ability of the film. It was surprising to see that the testing film was clean when wiped with wet tissue, indicating the film was totally impermeable to oil (Figure 4b). These results support the conclusion that the films possess superior barrier property against oil. In order to explain this phenomenon, the following explanations can be offered (Figure 4c): 1) the compact and dense structure of film, 2) the presence of water layer (due to hydration) that could prevent the oil from dissolving in the film, 3) the impermeability against oil of the crystalline part of cellulose nanofibrils, 4) the oil resistance property of guar gum itself. The good oil resistance property of the control film (CGG only) was also verified, and shown in Figure S6. Aulin et al.

51

reported that microfibrillated cellulose

coating could give superior oil barrier property to paper products due to the decreased porosity. Based the above results, it can be concluded that the CGG/TOCNs multilayer films have great potentials for oil barrier applications due to good oil resistance, as well as their renewable and biocompatible features. Such oil-resistant packaging products are green (made from natural products) so that they may be used for food processing/packaging, for example, to be used to reduce oil uptake during food frying, in this way healthier fried food with good taste can be produced 52.



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Figure 4. Oil resistance of the CGG/TOCNs multilayer film (a: after 4 h treatment in oven, the blank filter paper covered by the testing film was totally clean without any stain; b: the testing film was restored to clean without absorbing any stained oil after it was wiped with wet tissue; c: schematic illustration of oil resistance mechanism) (the (CGG/TOCNs)4 film was shown in this figure). The solvent resistance of the CGG/TOCNs multilayer films was also characterized in this study by using various solvents, such as DMAc, H2O, THF, acetone and methanol (Figure 5). It is interesting to note that no visible changes in the structure of these films were observed after even 48 h soaking in organic polar solvents, including DMAc, THF, acetone and methanol. In water (Figure 5b), the film showed some swelling yet, it still kept its shape. The good solvent resistance could be ascribed to the compact structure that hindered the solvent penetration into the films (by decreasing the permeation of solvents into the films via capillary force). Besides, CGG could also contribute to the results since it is known that GG can resist most organic 18 ACS Paragon Plus Environment

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solvents, like hydrocarbons, alcohols, esters, ketones etc.

15, 48

The good resistance performance

of 100% CGG films (as control) against organic solvents was also studied and the results were provided as Figure S7. Sharma et al.

53

found that GG-based gels had good solvent resistance

against acetone, N,N-dimethylformamide, and dimethyl sulfoxide. Furthermore, partial hornification of the cellulose nanofibrils during the films drying process and the intimate contacting of nanofibril networks can also lead to the good solvent resistance, as noted by Osterberg et al. 40 when preparing NFC films. The quantitative solvent uptake test was also implemented using (CGG/TOCNs)4 films and the results are listed in Table S1. For instance, the film absorbed 7.93% acetone after 48 h immersing, which was much lower than that of NFC films (71% after 18 h) reported in literature 40

. As reported, GG is a solvent resistant film former

48

. Ji et al.

54

observed that guar gum

particles only swelled a little in the polar solvents (alcohols and acetone). The excellent resistance to solvents is a key feature of these CGG/TOCNs multilayer films, which will enhance their potential applications.



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Figure 5. Solvent resistance of CGG/TOCNs multilayer films during different time periods (a: DMAc; b: H2O; c: THF; d: Acetone; e: Methanol) ((CGG/TOCNs)4 films were used in this test).

CONCLUSION A new generation of all-polysaccharides multilayer films was prepared using cationic guar gum (CGG) and TEMPO-mediated oxidized cellulose nanofibrils (TOCNs) based on a facile and straightforward layer-by-layer (LBL) casting approach. The resultant CGG/TOCNs films showed good barrier and solvent resistance properties. They also exhibited dense and compact structures, the product consisting of 8 layers of CGG/TOCNs had low water vapor (68.52 g/(m2·24h·atm)) and oxygen (5.56 (cm3/m2·24h·atm)) transmission rates. Moreover, the films exhibited excellent barrier capacity against oil, and their shapes were maintained even after 48 h of soaking in organic solvent. Also, such a LBL casting process can readily be scaled up, which has significant


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practical applications. The biocompatible robust films can be potentially used as a green packaging material, or adopted as a platform for functional materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is free of charge. Schematic of TEMPO-mediated oxidation of cellulose, TEM image of TOCNs, photographs of CGG and TOCNs suspension, as well as the films, FTIR, solvent resistance and absorption data (PDF file), and videos of tensile testing process (AVI files).

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge the financial support from the Canada Research Chairs Program.

REFERENCES



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GRAPHIC ABSTRACT


Cellulose Nanofibrils Multilayer Films with Good

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