Effect of Ionic Strength on Porosity, Swelling, and Optical Properties

Article pubs.acs.org/Langmuir

Cellulose Nanofibril-Based Multilayered Thin Films: Effect of Ionic Strength on Porosity, Swelling, and Optical Properties Firas Azzam,*,† Céline Moreau,† Fabrice Cousin,‡ Alain Menelle,‡ Hervé Bizot,† and Bernard Cathala*,† †

UR1268 Biopolymères Interactions Assemblages, INRA, 44316 Nantes, France Laboratoire Léon Brillouin, CEA-CNRS Saclay, 91191 Gif sur Yvette, France

‡

S Supporting Information *

ABSTRACT: TEMPO-oxidized cellulose nanofibrils (CNF) and synthetic poly(allylamine) hydrochloride (PAH) were used to build multilayered thin films via the dipping-assisted layer-by-layer technique. We used the ionic strength, in both CNF suspension and PAH solution, as a key parameter to control the structure of the films. Three systems with different ionic strength parameters were investigated. We studied the growth of the films and their surface morphology by ellipsometry and AFM and investigated their porosity and swelling behavior using neutron reflectivity. Our results showed that the PAH conformation is a determining factor not only for film growth but also for structural properties: with salt-free PAH solution where chains have extended conformation, the resulting films have lower porosity and higher swelling ratios, compared to the ones made using high ionic strength (1 M) PAH solution, where chains have a coiled conformation. The slight aggregation of CNF, induced by adding a small amount of salt (12 mM), has less influence on film growth and porosity, whereas it has a greater impact on swelling. The origin of these differences is discussed. The structure of the films obtained was linked to their optical properties and, in particular, to their antireflective character.

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blocks to produce multilayered thin films, a considerable literature has developed concerning this field. The cellulose compartment was formed by either anionic CNCs or CNFs,9 whereas the other one was generally formed by a cationic polyelectrolyte such as poly(allylamine hydrochloride) (PAH), 10−15 polyethylenimine (PEI), 10,16−22 or poly(diallyldimethylammonium chloride) (PDDA).7,10,23,24 Natural polymers were used as well, either chemically modified or not, including xyloglucan,25−29 cationic xylan,30 chitosan,31 chitin,32 collagen,33 lignin,34,35 and cationic CNFs.19,24,36 Numerous studies have focused on the influence of experimental parameters on growth patterns, including the deposition procedure (dipping vs spin-coating),13 concentrations of components,12,17,26 adsorption time,25,31 and ionic strength.10,12 The variation of these parameters makes it possible to tune the fine architecture of the films and thus influences the final functional properties of the assemblies. For example, Cranston et al.13 demonstrated that the appearance of structural colors is determined by the type of deposition. However, very few of these studies focused on quantifying and controlling the porosity of the multilayered films, which is nevertheless a key factor in determining characteristics such as optical properties. Podsiadlo et al. showed the antireflective properties of tunicate

INTRODUCTION Current environmental concerns make it necessary to focus on new strategies that meet the requirements of sustainable development. In the materials field, the design of natural renewable resource-based materials offers opportunities for replacing those based on fossil resources. In this context, cellulose nanoparticles appear to be attractive building blocks to produce high performance nanomaterials. These nanoparticles, either cellulose nanocrystals (CNC) or cellulose nanofibers (CNF), are crystalline rods/wires with a nanometric section. They present amazing properties such as low density, high aspect ratio and important mechanical properties, which make them relevant in various applications.1,2 In the past decade, cellulose nanoparticles have been increasingly used for the design of thin film coatings by using the layer-by-layer (LbL) approach. The latter was suggested in 19663 and developed in the early 1990s.4,5 It consists of building a multilayered film on a substrate by alternating the assembly of two components that present attractive interactions. Polyelectrolytes were mainly used in the beginning, thus making electrostatic interactions the main driving forces of the assembly. Since then, different compounds have been used, including small molecules, biomacromolecules, and colloids, and different interactions came into play such as hydrogen bonding, hydrophobic interactions, covalent linkages, etc.6 Since the pioneering works of Podsiadlo et al.7 and Cranston et al.,8 who used cotton cellulose nanocrystals as building © XXXX American Chemical Society

Received: April 11, 2014 Revised: June 18, 2014

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dx.doi.org/10.1021/la501408r | Langmuir XXXX, XXX, XXX−XXX

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washed with water by filtration. The washed pulp was suspended in water again and treated mechanically with a blender (Waring Commercial Blender) for 10 min at 12 000 rpm and then with an ULTRA-TURRAX homogenizer (Hudolph Instruments, Germany) for two cycles (4 min, 20 000 rpm) in 100 mL batches. Batches of 250 mL were then sonicated for 4 min, using an ultrasonic homogenizer (Qsonica sonicator, Delta Labo, Avignon, France) with an output power of 300 W (probe tip diameter: 12.7 mm). Finally, the suspension was centrifuged for 30 min at 14 000 rpm. The supernatant was recovered, dialyzed for 10 days, and filtered on 4 μm Whatman filters. The CNF obtained had a cross section of 3−4 nm and a length of approximately 0.5−1 μm (Figure S1, Supporting Information), with a carboxylic acid surface charge of 1.1 mmol g−1 (Figure S2, Supporting Information). Multilayered Film Preparation. Multilayered films were deposited on silicon wafers as solid substrate, cleaned beforehand for 30 min in a mixture of H2O2/H2SO4 (70/30, v/v), thoroughly rinsed in Millipore water, and finally dried under a nitrogen stream. PAH-CNF films were built on the basis of the conventional LbL dipping method. The silicon wafers were alternately immersed in the respective solutions of PAH (4 g L−1, pH 5.0, containing 0 or 1 M NaCl) and CNF suspension (0.8 g L−1, pH 5.0, containing 0, 6, or 12 mM NaCl) for 1 min. After each immersion step, the substrates were rinsed in pure water (three manual baths), and the surfaces were then dried under a nitrogen stream. The dipping sequence was repeated until an n-bilayer film was formed, with one bilayer defined as a single deposit of a PAH and CNF layer. The thin films obtained were homogeneous with colors that depended on their thicknesses (Figure S4, Supporting Information). Transmission Electron Microscopy (TEM). CNF suspensions in water were deposited on freshly glow-discharged carbon-coated electron microscope grids (200 mesh grid, Delta Microscopies, France), and excess water was removed by blotting. The sample was then immediately negatively stained with uranyl acetate solution (2%, w/v) for 2 min and dried after blotting at 40 °C just before observation. The grids were observed with a Jeol JEM 1230 TEM at 80 kV. Dynamic Light Scattering. DLS experiments were carried out with a Malvern NanoZS instrument. All measurements were made at a temperature of 25 °C with a detection angle of 173°. The hydrodynamic diameter was obtained from the analysis of the correlation function using Malvern DTS software. Ellipsometry. The thickness of multilayered films was evaluated using a variable-angle spectroscopic ellipsometer (M-2000U; J.A. Woollam, Lincoln, NE). The ellipsometric angles, Δ and Ψ, were acquired over the spectroscopic range of 250−1000 nm at three angles of incidence: 65°, 70°, and 75°. Optical modeling and data analysis were performed using the CompleteEASE software package (J.A. Woollam Co., Inc.) with a three-layer model consisting of the Si(100) substrate layer, a thin SiO2 layer, and the single Cauchy layer that describes the multilayered (CNF-PAH) film. The Cauchy parameters for each sample were used to model the ellipsometry data from the multilayered films to obtain their respective thicknesses. An average thickness value was obtained from the measurement of at least six spots per film. Atomic Force Microscopy. Surface observation of films deposited onto silicon wafers was carried out by atomic force microscopy (AFM) by means of an Innova AFM (Bruker). The images were collected in tapping mode under ambient air conditions (temperature and relative humidity) using a monolithic silicon tip (TESPA, Bruker; spring constant k = 42 N/m; frequency: f 0 = 320 kHz). The three multilayer thin films examined had almost the same thickness of approximately 30 nm. Image processing was performed with the WSxM 5.0 software. Roughness was determined from the root-mean-square (RMS) value over a scan area of 10 × 10 μm2. Neutron Reflectivity. Specular neutron reflectivity experiments were carried out at the Orphée reactor on the EROS time-of-flight reflectometer at the Laboratoire Léon Brillouin (CEA Saclay, France). A broad momentum transfer (Q) range from 0.008 to 0.1 Å−1 was obtained by collecting data at two fixed angles with a neutron white

cellulose nanocrystal-based multilayered films for the first time. Even if the porosity was not evaluated, the authors demonstrated that the highly porous architecture strongly depends on film thickness.16 The porosity was calculated by Lee et al.37 for TiO2/SiO2 multilayered thin films using a technique based on ellipsometry. The authors linked the calculated values to the optical properties and superhydrophilicity of the coatings. Harris et al.38 reported the permeability of films, which is linked to the porosity, as a function of ionic strength for PAH/PAA multilayered films. Fery et al.39 used scanning force microscopy and cyclic voltammetry to follow changes in porosity induced by salt. Recently, Dodoo et al.40 used neutron reflectivity to study the influence of ionic strength and the type of ions on the swelling of polyelectrolyte multilayers of poly(sodium 4styrenesulfonate) (PSS) and PDDA. By comparing the differences in thicknesses upon swelling, as well as the differences in the scattering length densities, they distinguished two types of water: “void water”, which fills the empty space between the polyelectrolytes, and “swelling water”, which is involved in swelling. The total water content in different cellulose thin films was quantified by Kittle et al. using quartz crystal microbalance with dissipation monitoring (QCM-D) with a solvent exchange procedure.41 In this work, we investigated the relationship between the conformations of PAH chains and the aggregation state of CNF, on the one hand, and the porosity of PAH−CNF multilayered systems, on the other. The aggregation pattern of CNF and the conformations of the cationic polyelectrolytes were both tuned by the variation of the ionic strength of the dipping solution. This was found to have a striking effect on film growth and surface morphology. Using neutron reflectivity measurements, we demonstrated that film porosity, as well as swelling behavior, depends on the PAH conformation and the CNF aggregation state, both of which are controlled by ionic strength. The close relationship between the porosity and the functionality of the films obtained was illustrated by the optical properties, particularly the antireflective character. Controlling the porosity of these films could make them useful for many applications like controlled permeability systems and release.

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EXPERIMENTAL SECTION

Materials. A commercial softwood bleached Kraft pulp was provided by Zellstof Stendal GmbH (Arneburg, Germany) in the form of never-dried wet fibers with 70% water content. The pulp was purified with 0.3% NaClO2 in acetate buffer at pH 4.8 and 60 °C for 2 h until the pulp became completely white. The pulp was then washed with water by filtration and stored at 4 °C. Poly(allylamine hydrochloride) (PAH; average Mw = 120 000−200 000 g mol−1) was purchased from Polysciences; 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxyl (AcNH-TEMPO), sodium chloride, sodium chlorite (NaClO2), sodium hypochlorite (NaClO), and other chemicals were purchased from Sigma-Aldrich and used without further purification. Deionized water (18.2 MΩ, Millipore Milli-Q purification system) was used for all experiments. Preparation of Cellulose Nanofibril Suspensions. Cellulose nanofibrils (CNF) were prepared following the protocol described by Saito et al.,42 with minor modifications. Briefly, cellulose (1 g) was suspended in 0.1 M sodium acetate buffer (500 mL, pH 4.8) dissolving AcNH-TEMPO (0.1 g, 0.5 mmol) and sodium chlorite (80%, 5.6 g, 50 mmol). The 2 M sodium hypochlorite solution (3 mL, 5.0 mmol) was diluted to 0.1 M with the same 0.1 M buffer used as the oxidation medium and was added in one step to the mixture. The suspension was stirred at 500 rpm and 40 °C for 48 h. After cooling the suspension to room temperature, the oxidized pulp was thoroughly B


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beam covering wavelengths from 3 to 25 Å. For the air/solid geometry, the angles were 0.93° and 2°. For measurements in solution, silicon wafers coated with multilayered films were placed in a liquid− solid cell. For this geometry, the angles chosen were 1.34° and 2.5° for the D2O/solid interface. In air/solid geometry, the incident neutron beam goes through the film side to the interface, whereas in liquid/ solid geometry, it goes through the silicon side. A standard treatment was applied to the raw data to subtract background and to obtain reflectivity curves on an absolute scale, following a procedure described in Cousin et al.43 for both geometries. The data were then analyzed using a “box” model, which consisted of dividing the thin film into a series of layers. Each layer was characterized by a finite thickness (d), scattering length density (SLD), and interfacial roughness (σ) with the neighboring layer. Theoretical curves were calculated using the optical matrix method. Details about the fitting procedure, the SLD of the different components, and the SLD profile determination can be found in the Supporting Information. UV−Vis Light Transmittance Measurements. Light transmittance measurements at normal incidence for the characterization of the thin films were performed in the 200−1000 nm range with a SPECORD S600 UV−vis spectrophotometer. The films were coated on one side of a quartz substrate over an area of 2.5 × 2.5 cm2.

governed by the repulsive interactions between particles arising from ionic groups. Aggregation is thus prevented as long as these interactions are not screened. Our goal was first to establish the stability regions for the CNF dispersion, to determine the concentration range of salt that induces partial aggregation, and to investigate the effect as those conformations on the architecture of multilayered thin films. We thus used the same approach as Fall et al.47 and followed the aggregation of cellulose nanofibrils as a function of NaCl concentration, using the dynamic light scattering technique (DLS) and transmission electron microscopy (TEM). Because DLS measures the diffusion coefficient of the particles, which is then converted into a hydrodynamic diameter (Dh) using the Stokes−Einstein equation, the size given by this technique is equivalent to the diameter of a sphere with the same diffusion coefficient as the cellulose nanofibrils. Two different cellulose concentrations were considered for the measurements: 0.1 g L−1, the one below the overlapping concentration C* (0.15 g L−1),47 and 0.25 g L−1, the one above C*. The hydrodynamic diameter of the initial suspension before modification is D0h. The hydrodynamic diameter Dih was measured after adding NaCl. The Dih/D0h ratio, qualitatively representing the evolution of the aggregation, was plotted as a function of the salt concentration (Figure 1). It is noteworthy that this ratio does

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RESULTS AND DISCUSSION In this study, the building of cellulose-based thin films was investigated. Cellulose nanofibrils (CNF) with carboxylic acid surface charges were used as anionic colloid, and poly(allylamine) hydrochloride (PAH) was used as a cationic polyelectrolyte. Dipping was used to build the films as follows: (i) charged silicon wafer substrate was immersed in cationic PAH solution; (ii) the deposited layer was then rinsed in water and dried under a nitrogen stream to remove all loosely bound polymers and to prevent any polymer contamination during (iii) the subsequent immersion in the anionic CNF suspension, followed by the same rinsing and drying steps. Our approach was designed to vary the ionic strength in order to modify the architectures of the thin films for the purpose of controlling their porosity, a key factor for the resulting properties. The influence of ionic strength was investigated for the polymers: • In the PAH solution, the increase of the ionic strength screens the interaction between the positive charges of the cationic polyelectrolyte. This results in a change in the polymer chain conformation. The scaling approach to the polyelectrolyte chain conformations in dilute suspensions is based on the assumption of the separation of different length scales and the concept of an electrostatic blob.44,45 At length scales greater than the electrostatic blob size, the electrostatic repulsions in a salt-free solution lead to elongation of the polyelectrolyte chain into an array of blobs, which results in an extended conformation. In a high ionic strength solution, the electrostatic blob size is smaller and chains adopt a Gaussian conformation. Because of the high density of charges of PAH, the concentration of salt added is high (1 M) compared to those used for screening charges of CNF.10,17,46 • In the CNF suspension, the addition of salt at a constant pH (pH = 5) will also screen the repulsive interactions between the nanofibrils, leading to the aggregation of these nanoparticles. However, in the case of CNF that are slightly charged nanoparticles, this approach is somewhat limited as well since the addition of salt at high concentrations leads to CNF precipitation,17 which in turn may limit the film’s growth. Thus, the different domains of CNF aggregation were determined as a function of ionic strength before the films were constructed. Inducing Cellulose Nanofiber Aggregation by Increasing Ionic Strength. Colloidal stability of CNF is

Figure 1. Dih/D0h ratio vs NaCl concentration (Dih: hydrodynamic diameter measured at different NaCl concentrations; D0h: hydrodynamic diameter at 0 mM NaCl concentration). The inset displays the aggregation state of CNF as observed by TEM at 0, 8, and 20 mM of NaCl.

not in any case represent a quantification of the aggregation number. In order to make sure of the validity of Dih values, the correlation functions were also plotted as a function of delay time for different ionic strength suspensions (Figure S3, Supporting Information). At 0 mM of salt, this ratio is equal to 1, and the cellulose nanofibrils are very well dispersed and individualized, as shown by the corresponding TEM image. An increase of the ratio is observed with the increase of salt concentration, accompanied by a shift of the monoexponential correlation function to a longer delay time (Figure S3, Supporting Information), indicating an increase of D hi compared to D0h, which implies that an aggregation has occurred for both of the CNF concentrations studied. For CNaCl > 16 mM, Dih/D0h > 2. Transmission microscopy revealed the formation of compact, large size aggregates. The problem of C


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using such a suspension to build multilayered films is the risk of obtaining a very rough surface. We therefore decided to use two salt concentrations, 6 and 12 mM, for which 1.5 < Dih/D0h < 2. In this salt concentration range, the CNF suspension is still stable, even if some aggregates are formed. The latter seem to be smaller and less compact, as shown by the TEM micrograph for CNF suspension at CNaCl = 8 mM. These results are in good agreement with those obtained by Fall et al.,47 even if their study was more exhaustive and combined the effects of the surface charge density and pH, in addition to the effect of the ionic strength. Recently, Fukuzumi et al. investigated the dispersion stability of a 0.1 wt % TEMPO-oxidized cellulose nanofibril suspension as a function of NaCl salt concentrations.48 They found that severe aggregations, accompanied by a loss of suspension fluidity, appear after adding 50 mM of salt. Thus, three different types of multilayer films were built on the basis of these parameters. Using CNF suspensions without any added salt and PAH solutions with or without salt, the corresponding films are referred to as (PAH 1 M−CNF 0 mM)n and (PAH 0 M−CNF 0 mM)n, respectively, where n is the number of bilayers. When a small amount of NaCl was added to the CNF suspension to reach a concentration of i mM and a PAH solution with 1 M of NaCl was used, the film obtained is referred to as (PAH 1 M−CNF i mM)n. Effect of Ionic Strength on Film Growth. The effect of ionic strength in the anionic CNF and cationic polyelectrolyte on the construction of thin films was studied. First, the influence of NaCl concentration (0 and 1 M) in PAH solution on film thickness was evaluated (Figure 2). As can be seen,

amount of CNF is adsorbed in the case of PAH solution with 1 M NaCl, forming a thicker layer. This difference is attributed to the change in PAH chain conformation, which is in good agreement with previous results obtained in our group by Moreau et al.12 using negatively charged cellulose nanocrystals. It is therefore hypothesized that in the case of high salt content, and due to charge screening of the ionic groups, a less extended conformation of PAH chains is adopted upon adsorption. Thus, as a result, PAH chains can form more densely charged layers with more polymer loops, offering a higher density of charges per surface unit, making an adsorption of a cellulose nanocrystal double layer possible. In the case of the absence of ionic strength, PAH chains are in an extended conformation. Moreover, electrostatic repulsions between chains are not screened, and the density of polymer on the surface is lower, leading to the adsorption of only one cellulose nanocrystal layer. For the same system, Jean et al.11 already observed a double-layer packing using a high ionic strength PAH solution. They proposed that the adsorption of the first monolayer is driven by electrostatic interactions between the nanoparticles and PAH, while the adsorption of the second layer is related to the gain of entropy associated with the counterion release with water molecules. In our case, the thickness increment per bilayer is 5−6-fold higher with salt than without in the PAH solution, i.e., a thickness that corresponds to that of a five-layer packing. However, the comparison between the two systems is difficult because of major differences between the two nanoparticles in terms of thickness and flexibility; nevertheless, the driving forces of the different adsorption processes should remain the same. In a similar system where carboxymethylated cellulose nanofibrils were used, Wagberg et al.10 reported the same effect when they added large amounts of salt to a linear polymer (PAH) and when they used a strongly charged branched polymer (PEI). In a subsequent step, the influence of the limited aggregation of CNF due to the addition of a small amount of NaCl on the film growth was evaluated. The thickness increment per bilayer, which is approximately 20.7 nm for films when CNF suspension is devoid of salt (PAH 1 M−CNF 0 mM)n, decreases slightly to 19.1 and 18.4 nm for (PAH 1 M−CNF 6 mM)n and (PAH 1 M−CNF 12 mM)n films, respectively. In fact, in the ionic strength range that we used, although a small aggregation of the cellulose nanofibrils occurred in solution, the charge density of these nanoparticles did not drastically decrease and the film growth was therefore not affected. In comparison, Wagberg et al.10 calculated the interaction energy as a function of the separation distance between two CNF rods dispersed in water. Their results showed that, for particles with a diameter of 5 nm and pH 7, the repulsive potential barrier slightly decreases from 20 to 15 kT after adding 10 mM of salt. This could explain the minor change observed in the film growth. Surface Topography by AFM. Surfaces of films obtained in the different conditions of aggregation were imaged with a top layer consisting of cellulose nanofibrils (Figure 3). For the three films, a porous structure can be observed. Such a structure was already observed in CNF and CNC-PEI10,16 and in CNF− Chitin systems.32 The RMS roughness calculated from the film built with no salt in PAH solution (Figure 3a) was approximately 2.6 nm, a value close to the CNF width. This value is consistent with the thickness growth illustrated in Figure 2 that showed that each bilayer is formed by one layer of CNF in these conditions. When PAH solution contains 1 M of

Figure 2. Thickness of thin films measured by ellipsometry vs number of bilayers deposited.

regardless of the conditions studied, a linear growth of PAH− CNF films is observed, indicating that a regular process of construction has occurred in all cases. However, results showed significant differences depending on construction parameters: for the same number of bilayers n, (PAH 1 M−CNF 0 mM)n films are almost 10 times thicker than (PAH 0 M−CNF 0 mM)n. In the first case, the thickness increment per bilayer is approximately 20 nm and around 3.5 nm in the second case. Since the lateral dimensions of CNF are between 3 and 4 nm, it can be assumed that in the case of salt-free PAH only a single layer of CNF is adsorbed on the PAH layer, while a greater D


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Figure 3. Tapping-mode AFM height images (at left) over an area of 10 × 10 μm2 of (a) (PAH 0 M−CNF 0 mM)10 film, (b) (PAH 1 M−CNF 0 mM)3 film, and (c) (PAH 1 M−CNF 12 mM)3 film and the corresponding profiles (d, e, and f) (at right).

salt, the RMS roughness of the resulting film increases to 7 nm (Figure 3b). The reason for this increase in roughness is probably linked to the structure of a given bilayer within such architecture, since we have shown that in this case it is formed by many CNF layers. Figure 3c reveals some small aggregates on the surface of the film produced with 12 mM of salt in CNF suspension. These aggregates appear more clearly on the images of areas of 50 × 50 μm2 (Figure S5c, Supporting

Information). Their relatively small size did not lead to any drastic increase of the RMS roughness, which was approximately 8.4 nm. Swelling and Porosity by Neutron Reflectivity. Neutron reflectivity (NR), a powerful nondestructive technique that provides information about the thickness and composition in the direction perpendicular to the reflecting surface, was used to probe the internal structure of our cellulose-based thin films E


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Figure 4. Neutron reflectivity curves in a dry (a) and swelled (b) state for three different films: (PAH 1 M−CNF 0 mM)4 (green), (PAH 0 M−CNF 0 mM)14 (red), and (PAH 1 M−CNF 12 mM)4 (blue). The solid lines are best fits obtained from the model described in the text.

Table 1. Parameters Deduced from NR Fits in the Dry and Swollen Statea

a

film

(PAH 1 M−CNF 0 mM)4



ddry film by NR/nm (ddry film by ellipsometry/nm) av roughness/nm dswollen film by NR/nm SLDdry film/× 10−6 Å−2 SLDswollen film/× 10−6 Å−2 SLDpolymers/× 10−6 Å−2 Φswelling Φair Φcellulose/(Φcellulose + ΦPAH)

52.3 (47 ± 2.3) 9.2 86.2 1.25 5.11 2.04 0.39 0.48 0.9

44.6 (41.6 ± 3.1) 3.2 93.1 1.22 4.94 1.84 0.52 0.34 0.78

42.8 (45.8 ± 2.9) 3.5 89.5 0.91 5.05 1.90 0.52 0.46 0.82

d: film thickness; SLD: scattering length density (×10−6 Å−2); Φ: volume fraction.

roughness at the interface (the film air interface in the dry state and the film−D2O interface in water, respectively). The thicknesses of the swollen films (86, 93, and 89 nm) deduced from the fits are much larger than the ones in the dry state (52, 44, and 42 nm) and are almost twice as large for the latter two cases. On the basis of the difference of thicknesses between the two states, the swelling ratio of film in heavy water, Φswelling, can be calculated for each film (Table 1) as follows:

built with different ionic strength parameters. Three different types of multilayered films were probed: (PAH 1 M−CNF 0 mM)4, (PAH 0 M−CNF 0 mM)14, and (PAH 1 M−CNF 12 mM)4 films. The number of bilayers was chosen in order to have similar thicknesses so as to easily compare these films to each other. Experiments were carried out in a dry state and a swollen state where the swelling liquid is heavy water (D2O). Reflectivity curves from each film (Figure 4a,b) were fitted using a “box” model, which consisted of dividing the thin film into a series of layers (details of the fitting procedure are provided in the Supporting Information, Figure S6). For each state, three main parameters were deduced from the fitting of the reflectivity curves (NR profiles are given in Figure S7 of the Supporting Information): the finite thickness of the film (d), its average scattering length density (SLD), and its average interfacial roughness (σ). Results are summarized in Table 1. First, we noticed that in the dry state, thickness values (ddry film) deduced from fits were in good agreement with those calculated from ellipsometry measurements. Furthermore, the average roughness obtained from the fits is higher for the films built with a high ionic strength solution of PAH (9.2 nm) compared to the one for films devoid of salt (3.5 nm), in agreement with surface roughness values obtained by AFM. Second, a comparison of the swelling behavior for the three different samples was possible. After immersion in heavy water, the amplitudes of spectra fringes decreased and were shifted toward low Q (with respect to Qc), indicating both an overall increase of the layer thickness upon swelling and of the

Φswelling =

dswollen film − ddry film dswollen film

(1)

where ddry film and dswollen film are the thickness of the film in the dry and swollen state, respectively. Interestingly, the three films did not swell in the same way: (PAH 1 M−CNF 0 mM)4 film seems to swell less than (PAH 0 M−CNF 0 mM)14 film, with swelling ratios of 0.39 and 0.52, respectively. The explanation for this difference comes from the different architecture of the two films. In the first case, the coiled conformation of PAH chains due to high salt content forms a thick layer with more polymer loops and tails, leading to a high surface three-dimensional network. In this case, the PAH layer probably offers many possible interactions with the subsequent adsorbed thick layer formed by cross-linked CNF nanowires. This architecture may limit their mobility upon swelling, resulting in a low swelling ratio. On the other hand, the absence of ionic strength favors an extended conformation of PAH chains. Thus, a thinner CNF layer is adsorbed, with less F


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Once Φpolymers is calculated, SLDpolymers can be calculated using eq 2. The porosities (Φair) of the different films are reported in Table 1. It appears that the (PAH 1 M−CNF 0 mM)4 film, with 48% of air volume fraction, is more porous than the (PAH 0 M−CNF 0 mM)14 film (34%). Again, this could be attributed to the architecture of the films. In the first case, the numerous cross-linking points of the adsorbed CNF layer facilitate the formation of voids. In contrast, in the second case, this phenomenon seems to be less favored because of the flat adsorbed conformation of PAH devoid of salt, where only one layer of CNF is adsorbed. In a recent review, Volodkin et al.52 summarized the properties of polyelectrolyte multilayers associated with different parameters in preparation conditions. They suggested that at low ionic strength a strong polyion−polyion complexation occurs within the multilayered films, which slows down the mobility of chains and thereby favors the formation of a large air/void fraction in the dry state but reduces the swelling ability of films in water. At high ionic strength, interactions between polyions and ions are favored, strongly increasing the chain mobility and resulting in the formation of a small void/air fraction along with a strong swelling of films in water. Our results for PAH−CNF systems as a function of ionic strength are the contrary of such trends. This could be attributed to differences between the CNFs and other synthetic polyelectrolytes. Indeed, in the case of PAH solution with high ionic strength where a thick layer of CNF is adsorbed, the semiflexible nature of these nanofibers, contrary to the highly flexible polyelectrolyte chains, combined with the possible hydrogen bonding between them, gives rise to cross-linking between CNF. That phenomenon makes the films more porous and reduces the mobility of the nanofibers and, consequently, the swelling of the films. In the case of PAH solution devoid of salt, less cross-linking occurs in the thin CNF layer adsorbed. The nanofiber mobility is then enhanced in these salt-free films, resulting in a decrease of porosity and an increase of swelling. Compared to the film with no salt in the CNF suspension, the (PAH 1 M − CNF 12 mM)4 film has a very similar porosity value (46%). Even if the small aggregation of cellulose nanofibrils induced by the small amount of salt added influences the swelling behavior, it seems that it does not have a significant impact on the porosity. Obviously, a full understanding of the system would be contingent on a study of the effect of film thickness on this porosity. The calculation of SLDpolymers (Table 1) is useful to evaluate the volume fraction ratio of cellulose and PAH. Considering that SLDcellulose and SLDPAH are equal to 2.1 and 0.55, respectively, the ratio Φcellulose/(Φcellulose + ΦPAH) for (PAH 1 M−CNF 0 mM)4, (PAH 0 M−CNF 0 mM)14, and (PAH 1 M−CNF 12 mM)4 are 0.9, 0.78, and 0.82, respectively. These high cellulose ratio values are not surprising in view of the difference in size and charge density between CNF and PAH. Furthermore, it seems that the swelling ratio closely depends on these values since the greatest swelling is obtained for films that have the lowest cellulose volume fraction (higher PAH volume fraction). It is therefore logical with regard to the PAH high charge density compared to CNF, which resulted in more solvation and, consequently, more pronounced swelling. Optical Properties. The optical properties, i.e., antireflective properties, were examined after deposition of the different films on a quartz substrate. Transmittance measurements of PAH−CNF films with 4 and 14 bilayers were

interaction points and cross-linking than in the previous case, which makes the swelling greater. In the third case, where the film is made by PAH solution with 1 M NaCl and CNF suspension with 12 mM NaCl, the swelling ratio (0.52) is higher than the one in the first case and identical to the one in the second case, indicating a pronounced swelling behavior. This behavior could be explained by the fact that in this architecture the deposition of small open aggregates preformed in volume leads to a less entangled adsorbed CNF layer. It should be noted that the swelling experiment is done in pure desalted water. It is then possible that the resulting decrease of the ionic strength leads to the partial dissociation of a part of the CNF aggregates, yielding a higher swelling ratio. Many studies in the literature focused on the swelling of cellulose thin films. However, the latter were composed of cellulose only, and the comparison with PAH−CNF films is not exhaustive.41,49−51 Another parameter that should be considered and that could explain the difference in swelling behavior between the three systems, which will be discussed later, is the composition of the films. Third, the porosity of the thin film is important information that can be obtained by comparing the scattering length densities of the films in dry (SLD dry film ) and wet (SLDswollen film) states (Table 1). The porosity represents the volume fraction of void/air in the film. Upon swelling, this fraction is filled with heavy water, which does not contribute to the swelling of the film but exclusively to the change of the scattering length density. When the chains are swelled, a second fraction of water is incorporated and contributes to the thickness change. The proper evaluation of the film thicknesses in both the dry and swelled state, combined with the determination of the scattering length densities, makes it possible to calculate the air volume fraction (porosity) in the dry state, which will be discussed in relation to the experimental conditions of film growth. In the dry state, the SLD of the film can be expressed as SLDdry film = ΦpolymersSLDpolymers + (1 − Φpolymers)SLDair = ΦpolymersSLDpolymers

(2)

where Φpolymers is the total volume fraction of both CNF and PAH and SLDair = 0. SLDpolymers is a mean value that depends on the respective contents of cellulose and PAH within the film and which may vary from one architecture to another because there is a rinsing step in the preparation. It is unknown beforehand but can be extracted from the comparison of dry measurements and swollen measurements. In fact, in the swollen state, the SLD of the film is expressed by SLDswollen film = (1 − Φswelling )[ΦpolymersSLDpolymers + (1 − Φpolymers)SLDD2O] + Φswelling SLDD2O

(3)

Combining (2) and (3), we obtain Φpolymers, regardless of the value of SLDpolymers: Φpolymers =

SLDdry film SLDD2O

−

SLDswollen film − Φswelling SLDD2O (1 − Φswelling )SLDD2O

+1 (4)

Consequently, Φair can be calculated by Φair = 1 − Φpolymers

(5) G


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of quartz, a coating should then have a refractive index of approximately 1.26. (PAH 0 M−CNF 0 mM)14 film contains about 34% of void and 66% of polymers (cellulose and PAH). Since the corresponding cellulose ratio was calculated (Table 1) and refractive indices of cellulose and PAH are 1.55 and 1.42, respectively, the resulting nc is approximately 1.36. In the case of (PAH 1 M−CNF 0 mM)4 film, the void percentage is about 48%, and the same way to calculate nc gives us a value of 1.28, which is very similar to the value obtained with the relationship mentioned below. Many studies treated the antireflective coating using the layer-by-layer technique that includes cellulose-based polymers.16,32 To our knowledge, this is the first time that antireflectiveness is directly related to the quantified porosity of the thin films. We wanted to identify the effect of ionic strength in the CNF suspension on the optical properties as well. As shown in Figure 6, (PAH 1 M−CNF 12 mM)n films show antireflective properties. If we compare the latter to (PAH 1 M−CNF 0 mM)n films, two main differences can be observed. Concerning the interference fringes, the maximum undergoes a small blueshift after adding 12 mM of NaCl, which could be attributed to the small decrease in thickness since the wavelength for which the transmitted light is the maximum depends on the thickness of the coating expressed by ddry = λ/4nc. The second difference is the increase of the transmitted light after adding the 12 mM of salt, revealing better antireflective properties. This result is surprising if we consider NR results that show no difference in porosity after adding the salt. This can perhaps be explained by the fact that adding the small amount of salt does not drastically change the porosity of the film but the size of the pores instead. Investigations using other techniques to obtain pore size distribution could be useful to clarify this point. On the basis of these results, Figure 7 represents illustrations of the different film architectures and their corresponding properties.

performed over the 200−1000 nm range (Figure 5). We observed that films built from PAH solution with 1 M NaCl

Figure 5. UV−vis light transmittance spectra of an uncoated quartz slide (black curve) and coated slide with (PAH 1 M−CNF 0 mM)4 (green), (PAH 1 M−CNF 0 mM)14 (blue), (PAH 0 M−CNF 0 mM)4 (pink), and (PAH 0 M−CNF 0 mM)14 (red).

show antireflective properties (AR): transmitted light is higher by about 5−6% compared to the bare quartz substrate. For a large number of bilayers (n = 14), the spectra show some fringes due to an interference phenomenon. However, the films constructed with PAH solution devoid of salt did not show such a property. The particularly interesting comparison is between spectra from quartz-coated (PAH 0 M−CNF 0 mM)14 and quartz coated (PAH 1 M−CNF 0 mM)4, which have approximately the same thicknesses. The transmitted light for the quartz coated (PAH 0 M − CNF 0 mM)n films is lower than the one for uncoated quartz, regardless of the number of bilayers (4 or 14). This behavior could be attributed to the difference in film porosity (Φair) calculated from NR data, which was higher for films with 1 M NaCl. In fact, to obtain antireflective properties, the refractive index of the coating should satisfy the relationship nc = (nans)1/2, where nc, na, and ns are the refractive indices of the coating, the air, and the substrate, respectively. In the case

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CONCLUSIONS In this study, multilayered thin films using TEMPO-oxidized cellulose nanofibers and poly(allylamine) hydrochloride (PAH) were built using the dipping-assisted layer-by-layer technique. Particular attention was focused on the influence of ionic strength on film properties. Our results showed that with high ionic strength in PAH solution, where chains adopt random coil

Figure 6. UV−vis light transmittance spectra of an uncoated quartz slide (black curve) and coated slide with (PAH 1 M−CNF 0 mM) film (green) and (PAH 1 M−CNF 12 mM) film (blue), of eight and ten bilayers (a and b, respectively). H


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Figure 7. Illustration of different (PAH−CNF) film architectures and their corresponding properties.

conformation, the corresponding thin films have high porosity, weak swelling, and an antireflective character. In contrast, an extended chain conformation at low ionic strength leads to less porous films with strong swelling and no antireflective properties. The aggregation state of cellulose nanofibers in solution prior to the deposit, which can be modified by the addition of salt in the suspension, seems to influence both the swelling and the optical properties of the films, but has a lower impact on the porosity. On the basis of these results, this work describes a simple way to control the porosity of cellulosebased thin films.

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(2) Eichhorn, S. J.; Dufresne, A.; Aranguren, M.; Marcovich, N. E.; Capadona, J. R.; Rowan, S. J.; Weder, C.; Thielemans, W.; Roman, M.; Renneckar, S.; et al. Review: Current International Research into Cellulose Nanofibres and Nanocomposites. J. Mater. Sci. 2010, 45, 1− 33. (3) Iler, R. K. Multilayers of Colloidal Particles. J. Colloid Interface Sci. 1966, 21, 569−594. (4) Decher, G.; Hong, J. D.; Schmitt, J. Buildup of Ultrathin Multilayer Films by a Self-Assembly Process: III. Consecutively Alternating Adsorption of Anionic and Cationic Polyelectrolytes on Charged Surfaces. Thin Solid Films 1992, 210−211 (Part 2), 831−835. (5) Decher, G. Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science 1997, 277, 1232−1237. (6) Decher, G.; Schlenoff, J. B. Multilayer Thin Films: Sequential Assembly of Nanocomposites Materials; Wiley-VCH: Weinheim, 2003. (7) Podsiadlo, P.; Choi, S.-Y.; Shim, B.; Lee, J.; Cuddihy, M.; Kotov, N. A. Molecularly Engineered Nanocomposites: Layer-by-Layer Assembly of Cellulose Nanocrystals. Biomacromolecules 2005, 6, 2914−2918. (8) Cranston, E. D.; Gray, D. G. Formation of Cellulose-Based Electrostatic Layer-by-Layer Films in a Magnetic Field. Sci. Technol. Adv. Mater. 2006, 7, 319. (9) Martin, C.; Jean, B. Nanocellulose/Polymer Multilayered Thin Films: Tunable Architectures towards Tailored Physical Properties. Nord. Pulp Pap. Res. J. 2014, 29, 19−30. (10) Wagberg, L.; Decher, G.; Norgren, M.; Lindstrom, T.; Ankerfors, M.; Axnas, K. The Build-Up of Polyelectrolyte Multilayers of Microfibrillated Cellulose and Cationic Polyelectrolytes. Langmuir 2008, 24, 784−795. (11) Jean, B.; Dubreuil, F.; Heux, L.; Cousin, F. Structural Details of Cellulose Nanocrystals/Polyelectrolytes Multilayers Probed by Neutron Reflectivity and AFM. Langmuir 2008, 24, 3452−3458. (12) Moreau, C.; Beury, N.; Delorme, N.; Cathala, B. Tuning the Architecture of Cellulose Nanocrystal−Poly(allylamine Hydrochloride) Multilayered Thin Films: Influence of Dipping Parameters. Langmuir 2012, 28, 10425−10436. (13) Cranston, E. D.; Gray, D. G. Morphological and Optical Characterization of Polyelectrolyte Multilayers Incorporating Nanocrystalline Cellulose. Biomacromolecules 2006, 7, 2522−2530. (14) Cranston, E. D.; Gray, D. G. Birefringence in Spin-Coated Films Containing Cellulose Nanocrystals. Colloids Surf., A 2008, 325, 44−51. (15) Cranston, E. D.; Gray, D. G.; Rutland, M. W. Direct Surface Force Measurements of Polyelectrolyte Multilayer Films Containing Nanocrystalline Cellulose. Langmuir 2010, 26, 17190−17197.

ASSOCIATED CONTENT

S Supporting Information *

TEM micrographs of CNF, their conductometric titration curve, DLS correlation functions at different ionic strengths, photos and AFM images of PAH−CNF films, details of the NR data fitting procedure, and NR profiles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail fi[email protected] (F.A.). *E-mail [email protected] (B.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the ANR (French National Research Agency) through the Pitbulles Program. The authors acknowledge the BIBS platform of INRA AngersNantes for access to its microscopy facilities. We also thank Nadège Beury and Emilie Perrin for their excellent technical support for the AFM and TEM images, respectively, and Mélanie Marquis for her fruitful discussions.

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REFERENCES

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Effect of Ionic Strength on Porosity, Swelling, and Optical Properties

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