Relationship between Young's Modulus and Film Architecture in

Article pubs.acs.org/Langmuir

Relationship between Young’s Modulus and Film Architecture in Cellulose Nanofibril-Based Multilayered Thin Films Firas Azzam,*,†,‡ Laurent Chaunier,† Céline Moreau,† Denis Lourdin,† Patricia Bertoncini,‡ and Bernard Cathala*,† †

BIA, INRA, 44300 Nantes, France Institut des Matériaux Jean Rouxel (IMN), UMR 6502, CNRS-Université de Nantes, 44322 Nantes, France

‡

S Supporting Information *

ABSTRACT: Young’s moduli of cellulose nanofibril (CNF)− poly(allylamine hydrochloride) (PAH) multilayered thin films were measured using strain-induced elastic buckling instability for mechanical measurements (SIEBIMM) and the quantitative nanomechanical mapping technique (PF-QNM). To establish the relationship between structure and mechanical properties, three types of films with various architectures were built using the layer-by-layer method by changing the ionic strength of the dipping solution. Both methods demonstrate that the architecture of a film has a strong impact on its mechanical properties even though the film has similar cellulose content, emphasizing the role of the architecture. Films with lower porosity (Φair = 0.34) and a more intricate network display the highest Young’s moduli (9.3 GPa), whereas others with higher and similar porosity (Φair = 0.46−0.48) present lower Young’s moduli (4.0−5.0 GPa). PF-QNM measurements indicate a reverse ranking that is probably indicative of the surface composition of the films.

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INTRODUCTION Cellulose is a natural resource that has been extensively used throughout human history and is probably one of the main sources of renewable carbon available. Over the last two decades, researchers and manufacturers have focused on nanocellulose (NC) because of its great potential to produce renewable materials that are expected to offer improved functionalities, including biodegradability, enhanced biocompatibility, and decreased environmental impact. New NC-based materials obtained from cellulose nanofibrils (CNF) and cellulose nanocrystals (CNC) have thus been developed and are endowed with extraordinary properties.1−5 Among the various NC-based materials, thin films and coatings have drawn much attention because these materials can act as functional devices on their own6,7 as well as useful models for understanding the organization of raw materials (plant cell walls, cellulosic fibers, and composites).8−13 The reasons for using NC to build biobased coatings or thin films are numerous and include their light weight, their renewability, and their low toxicity as well as their high mechanical strength due to their semicrystalline structure and their high aspect ratio. The control and optimal use of the mechanical properties of NC in thin films make it necessary to establish the relationship between structure and performance. However, it is not always possible to determine the mechanical properties of thin films using standard approaches, especially when the films are not thick enough to be self-supported or when the adhesion between the film and the substrate is considerable. Thus, some © XXXX American Chemical Society

techniques that rely on different physical principles have been used to evaluate the mechanical properties of nanoscale thin films. SIEBIMM (strain-induced elastic buckling instability for mechanical measurements) was developed to study the mechanical properties of thin films. The method is based on the deposition of a thin coating on an elastic lower modulus substrate, usually poly(dimethylsiloxane) (PDMS), and the application of plane compression.14 The film will then buckle, and this buckling will be used to calculate Young’s modulus (E). This method has encountered some limitations because of difficulties with some systems that cannot be assembled on neat PDMS and that require an anchoring layer that has to be considered in the mechanical model as an individual layer with specific mechanical properties. Thus, Nolte et al.15 proposed a two-plate buckling technique that can be used when the multilayer film is assembled on activated PDMS substrates that form a glassy polymeric film with contrasted mechanical properties compared to bulk materials. Atomic force microscopy (AFM) was also used to perform nanoindentation experiments with a colloidal probe or a sharp nanometerscale pyramidal tip as an indenter.16−18 PeakForce quantitative nanomechanical mapping (QNM), an extension of the pulsedforce mode based on the periodic oscillation of an AFM cantilever, was recently developed.19−21 This technique enables Received: January 6, 2017 Revised: April 2, 2017 Published: April 13, 2017 A

DOI: 10.1021/acs.langmuir.7b00049 Langmuir XXXX, XXX, XXX−XXX

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the associated force−distance curve to be measured and, consequently, the local stiffness of the films. The two methods can provide some complementary information because SIEBIMM evaluates the entire film whereas QNM gives more local information. SIEBIMM and QNM techniques have seldom been used for the investigation of mechanical properties of thin films composed of nanocellulose. The SIEBIMM technique was first used several years ago by Wågberg’s team at KTH.22,23 On the basis of measurements made on polyelectrolyte−nanocellulose multilayers, the authors demonstrated that the mechanical properties are highly humidity-dependent.23,24 The SIEBIMM technique was also used to investigate the mechanical properties of poly(vinylamine) (PVAm)−CNF and cationic starch-containing thin films.22,25 However, these pioneering works also pointed out the limitations of the method. For instance, the growth of the film on PDMS requires uncharged PDMS to avoid the formation of a glassy PDMS layer that has to be taken into account in the mechanical model. The consequence of this is that the growth of the film on uncharged PDMS is different from the growth usually observed on charged substrates such as activated silicon wafers.23 This point limits the comparison between different architectures and thus makes it difficult to understand the relationship between the film structures and the mechanical properties. For instance, neutron reflectivity that is a very powerful method for investigating the inner structure of the film cannot be implemented on PDMS substrates. QNM was used to investigate the local mechanical properties of a cellulose nanofiber/polyelectrolyte multilayered thin film in combination with the SIEBIMM technique in Cranston’s work and led to the conclusion that thin films have mechanical behavior closer to their individual components rather than to the bulk materials.23 The development of nanocellulose-based thin films can be achieved by the so-called layer-by-layer (LbL) adsorption method that consists of the repeated alternate assembly of polymers or nanoparticles that present attractive interactions.26−29 This method is highly versatile, and it is now acknowledged that any change in the experimental parameters can modify the architecture of a film, inducing variations in its mechanical properties. For example, Merindol et al.30 showed that Young’s modulus and strain percentage at break of PVAm−CNF multilayered freestanding films built by the dipping-assisted LbL method could be modulated via the architecture of films that are tailored through the change in the pH of the dipping solution. In a previous study, we prepared and characterized PAH− CNF multilayered films with different organizations that were obtained by varying the ionic strength in LbL dipping solutions. These changes finely tuned the composition, the architecture, and the optical properties of the films.31 Thus, these previously studied structures with different but finely controlled architectures are case studies in evaluating the role of the organization of building blocks on the properties of nanocellulose-based thin films. In our study, the SIEBIMM technique with the modified two-plate model is used to investigate the mechanical properties of the different films. Because the SIEBIMM technique addresses the entire range of film properties, we also used PeakForce QNM to obtain average surface Young’s moduli as a complementary technique. The results are discussed in light of variations in the internal structure and the surface composition of the films.

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EXPERIMENTAL SECTION

Materials. Softwood bleached Kraft pulp (70% water content) was furnished by Zellstof Stendal GmbH (Arneburg, Germany). Prior to utilization, it was bleached with 0.3% NaClO2 (acetate buffer pH 4.8, 60 °C, 2 h). Poly(allylamine hydrochloride) (PAH, average Mw = 120 000−200 000 g·mol−1) was obtained from Polysciences (Baden Germany), poly(dimethylsiloxane) (PDMS, RTV615) was obtained from Elecoproduit (Gennevilliers, France), and 4-acetamido-2,2,6,6tetramethylpiperidine-1-oxyl (AcNH-TEMPO), sodium chloride (NaCl), sodium chlorite (NaClO2), sodium hypochlorite (NaClO), and the other chemicals were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used without further purification. CNF Preparation. Cellulose nanofibrils (CNF) were prepared according to the protocol of Saito et al.32 with minor modifications. Cellulose (1 g) was suspended in 0.1 M sodium acetate buffer (500 mL, pH 4.8), AcNH-TEMPO (0.1 g, 0.5 mmol), and NaClO2 (80%, 5.6 g, 50 mmol). The 2 M NaClO solution (3 mL, 5.0 mmol) was diluted to 0.1 M with 0.1 acetate buffer (500 mL, pH 4.8) and was added in one step to the mixture. The suspension was stirred at 500 rpm and 40 °C for 48 h. The suspension was cooled to room temperature and thoroughly washed with water by filtration. It was resuspended in water and treated with a blender (Waring Commercial Blender, USA) for 10 min at 12 000 rpm, followed by ULTRATURRAX homogenizer treatment (Heidolph Instruments, Germany, 2 × 4 min, 20 000 rpm). Therefore, the samples were sonicated for 4 min using an ultrasonic homogenizer (Qsonica sonicators, Delta Labo, Avignon, France, power 300 W, probe tip diameter 12.7 mm). The suspension was then centrifuged for 30 min at 14 000 rpm. The supernatant was dialyzed for 10 days. The CNF cross-section of the fibers measured by AFM is about 3−4 nm, and the length is approximately 0.5−1 μm. The carboxylic acid surface charge was measured by conductometric titration and is equal to 1.1 mmol·g−1.31 Multilayered Film Preparation. Silicon wafers and PDMS substrates were used for multilayer deposition. In the case of silicon, the wafers were cleaned for 30 min in a mixture of H2SO4/H2O2 (70/ 30 v/v), rinsed in ultrapure water, and dried under nitrogen. PDMS substrates were prepared following the protocol described by Cranston et al.23 by mixing the PDMS and the curing agent (90/10) under thorough stirring. The mixture was placed in a Petri dish with a silicon wafer at the base and then placed in a desiccator under vacuum for 2 h to remove air bubbles. The PDMS was then cured in the oven at 60 °C for 2 h. After cooling, the 5-mm-thick PDMS was cut into 4 × 1 cm2 strips, rinsed with water and ethanol, and dried under a nitrogen stream. The PDMS strip was activated for 3 min in a plasma cleaner (Harrick Plasma, Ithaca, NY, USA) at high intensity, creating a glassy silicon layer on the PDMS surface. PAH−CNF films were built by the LbL dipping method, as previously described.31 The silicon wafers or PDMS substrates were alternately immersed in solutions of PAH (4 g·L−1, pH 5.0, without or with 1 M NaCl) and CNF suspension (0.8 g·L−1, pH 5.0, without or with 12 mM NaCl), respectively, for 1 min. After each layer deposition, the films were rinsed three times in water and dried under nitrogen. The dipping was repeated until an n-bilayer film was formed, with one bilayer defined as a deposit of PAH and CNF layers. For PDMS substrates, all measurements were made on the smoothest side that was in contact with the silicon wafer. Ellipsometry. The thickness of multilayered films was determined using a variable-angle spectroscopic ellipsometer (M-2000U, J.A. Woollam, Lincoln, NE, USA). The ellipsometric angles, Δ and Ψ, were measured from 250 to 1000 nm at three angles of incidence: 65, 70, and 75°. Data treatments were performed using the CompleteEASE software package (J.A. Woollam Co., Inc., Lincoln, NE, USA). For films deposited on a silicon substrate, a three-layer model was used. It consisted of a Si(100) substrate layer, a thin SiO2 layer, and the single Cauchy layer that describes the multilayered (PAH−CNF) film. The latter assumes that the real part of the refractive index, n, can be described by n(λ) = A + B/λ2 + C/λ4, where A, B, and C are constants and λ is the light wavelength. For films deposited on PDMS, the Si and SiO2 layers were replaced by a Cauchy substrate layer. Parameters A, B, B


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To solve eq 1, κ was replaced by a value of 1. Equation 1 then yielded a refined value of E̅ CNF−PAH, which was used to calculate a new value of κ using eq 3, and the process was iterated until E̅ CNF−PAH no longer changed. The standard deviation (Sx) that accounted for errors in thickness, wavelength, and the substrate Young’s modulus was used to calculate confidence intervals Δx as error bars, with Δx = Sxt/(N)1/2, where the t value is the Student’s t distribution at a confidence level of 95% for N − 1 degrees of freedom. Atomic Force Microscopy. A Multimode 8 AFM (Bruker Nano Surfaces Division, Santa Barbara, CA, USA) was used in PeakForce QNM mode to record the surface topology and elastic modulus maps. All measurements were made under ambient conditions at room temperature and a relative humidity of 43%. All quantitative measurements were performed using standard RTESPA probes (Bruker). The spring constant of cantilevers was determined using the thermal method and was 38.8 N/m for RTESPA microlevers. Data processing was performed using the commercial Nanoscope Analysis software (Bruker AXS Corporation, Santa Barbara, CA, USA), and modulus value maps were calculated according to the Derjaguin, Muller, and Toporov (DMT) model.33 WSxM 5.0 software was used to determine the root-mean-square (rms) surface roughness from a 5 × 5 μm2 scan area.

and C were determined by scanning an uncoated piece of PDMS substrate and were 1.403, 0.0049, and 6.04 × 10−6, respectively. Six spots per film were measured to determine an average value. Buckling Experiments. Because the PDMS substrate was plasmatreated, a thin SiO2-like layer formed on the surface. Since the classic SIEBIMM model alone was no longer applicable for the calculation of the PAH−CNF film modulus, the two-plate model introduced by Nolte et al. (2006), was used instead (Figure S1). The following three equations were used

⎡ − ESiO ̅ 2 ⎢⎣ ϕSiO2 −

Eeff ̅ 4

ECNF ̅ − PAH =

(

1−

κ 2

κ=

( (

2

2

1 + ϕSiO

2

ESiO ̅ 2 ECNF ̅ − PAH ESiO ̅ 2

ECNF ̅ − PAH

κ 2

3

SiO2

⎡ λ ⎤3 Eeff ⎥ ̅ = 3Es̅ ⎢ ⎣ 2πd total ⎦

1 + ϕSiO

2

( ) + ( ) ⎤⎦ ) − (ϕ − ) κ 3 2

κ 3 2

(1)

(2)

) − 1) −1

■

(3)

RESULTS AND DISCUSSION Film Growth on PDMS Substrates. It is widely acknowledged that the thickness, chain organization, and network structure of multilayered thin films can dramatically change as a function of small variations in the deposition conditions. To investigate the effect of modifications in thin film architecture on mechanical properties, three CFN/PAH films were built by varying the ionic strength of the dipping solutions.31 Two types of films were constructed from CNF suspensions without any ionic strength (CNF 0 mM). One was combined with a PAH salt-free solution (PAH 0 M), and the other was combined with a solution containing 1 M salt (PAH 1 M); the corresponding films are referred to as (PAH 0 M− CNF 0 mM)n and (PAH 1 M−CNF 0 mM)n, respectively, where n is the number of bilayers. The third type of film was built from a CNF suspension containing a small amount of NaCl (CNF 12 mM) in order to induce a limited aggregation of CNF to avoid precipitation, and 1 M NaCl was added to the PAH solution. The films obtained are referred to as (PAH 1 M−CNF 12 mM)n. Because the PAH conformation is controlled by ionic strength, it has a deep impact on the growth of the film and its internal organization.34,35 With the salt-free PAH solution, chains have an extended conformation, and the resulting films have lower porosity compared to the ones made using a high ionic strength (1 M NaCl) PAH solution in which chains adopt a coiled conformation. The aggregation of CNF by adding salt has a smaller influence on film growth and porosity, whereas it has a greater impact on swelling. The combination of all of these growth parameters offers a panel of films with similar chemical composition but extremely different architectures (i.e., variation of the thickness per bilayer, porosity, swelling capacity, polymer−particle entanglement and interactions). LbL (PAH−CNF) films were grown on PDMS substrates, whereas the usual procedures encountered in the literature used silicon surfaces. Silicon can be chemically oxidized to provide a clean surface as well as to create charges to activate the surface in order to anchor the first polymer layer. Many polyelectrolyte systems do not grow correctly if such surface activation is omitted. This is the case for the PAH/CNF system that we used in our previous work that was dedicated to the study of

where ECNF−PAH is the PAH−CNF multilayer Young’s modulus, Eeff is the effective modulus and represents the calculated modulus of the two-plate composite (SiO2 layer + PAH−CNF multilayers) as if it was a homogeneous film, in which dtotal is the film thickness, ESiO2 is Young’s modulus of the thin SiO2 layer, Es is Young’s modulus of the substrate (PDMS), and ϕSiO2 is the height fraction of the SiO2 layer with ϕSiO2 = dSiO2/dtotal. The overbars indicate reduced modulus values, where E̅ = E/(1 − υ2), with E being Young’s modulus and υ being Poisson’s ratio (υcellulose = 0.33, υPDMS = 0.5, and υSiO2 = 0.17). The effective modulus Eeff was determined using the classic SIEBIMM technique. A tensile stress testing stage (ref. TST350, Linkam Scientific Instruments, Tadworth, U.K.) equipped with a 20 N load cell and controlled with linksys32 software was used to compress PDMS substrates coated with the different films at 2.5% strain. The samples were conditioned in a closed vessel with a controlled relative humidity (43% R.H. using K2CO3-saturated solution). An optical microscope (Olympus BX51) equipped with a model XCD-SX90CR camera (Sony, Japan) was used to capture images of the resulting buckling pattern. More than 10 different regions from 5 different images and 3 similar samples (150 images) were computed using ImageJ software to determine an average buckling wavelength λ. Young’s modulus (1.50 ± 0.05 MPa) of the PDMS substrate was measured in two different ways: (a) by compression applied to each sample after the buckling experiment using the tensile stress testing stage and (b) by a classical tensile test on two specimens extracted from each PDMS-loaded Petri dish. Young’s moduli obtained from the two methods were similar to a difference of less than 2.5% between the two values. For substrates and CNF-based thin films, we used Poisson’s ratios of 0.5 and 0.33, respectively.23 Young’s modulus of the SiO2 layer was also calculated using the classic SIEBIMM technique. A PDMS strip was plasma treated for 3 min, creating the SiO2 thin layer. The thickness of this layer (4.4 ± 0.6 nm) was measured using the ellipsometry technique, as described above. The treated PDMS strip was compressed by 3% using a homemade compression device, and the surface was imaged in three different spots using atomic force microscopy (AFM, Figure S2) at a relative humidity of 43%, allowing the measurement of the average buckling wavelength λ (0.3 ± 0.1 μm). ESiO2 = 3Es

(1 − υSiO2 2) ⎡ λ ⎤ ⎢ ⎥ (1 − υs 2) ⎢⎣ 2πdSiO2 ⎥⎦

3

(4)

Young’s modulus was calculated using eq 4 and was 3.98 ± 0.20 GPa. C


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Langmuir internal film structure by neutron reflectivity.31 The SIEBIMM technique has to be implemented on an elastic substrate with a low modulus, and films have to be built on poly(methylsiloxane) (PDMS). To link the film’s internal structural organization to its mechanical properties, the growth regimes on PDMS and on silicon have to be compared to ensure that they are identical. Thus, the growth of the multilayered films on PDMS was achieved after treatment of the PDMS by air plasma and the deposition of a PAH layer. Figure 1 shows the surface

Table 1. Comparison of the Thickness Increments per Bilayer for PAH−CNF Films Grown on PDMS and Silicon Substrates Measured by Ellipsometry and Neutron Reflectivity (NR) substrates PDMS

silicon

thickness increment per bilayer (nm)

ellipsometry

ellipsometry

NR

(PAH 1 M−CNF 0 mM)n (PAH 1 M−CNF 12 mM)n (PAH 0 M−CNF 0 mM)n

11.75 11.25 2.45

11.87 11.25 2.95

13.1 10.5 3.2

to the conformation of PAH chains. Indeed, in the case of saltfree PAH, chains are in an extended conformation and a thin layer of CNF (3−4 nm) is adsorbed on the PAH layer, whereas a greater amount of CNF is adsorbed in the case of the PAH solution with 1 M NaCl due to the random coil conformation of PAH chains.34 In the case of adsorption of the CNF suspension containing a small amount of salt (12 mM), the film growth was not significantly affected, even if the repulsive interactions between nanofibers slightly decreased as a result of ionic strength, and it was assumed that limited aggregation occurred. The thickness increments per bilayer of PDMS substratecoated films are in the same range as the thickness increments per bilayer measured on the silicon substrates (Table 1). In previous studies, the adsorption of multilayer films consisting of ((PAH or PEI)−CNF) films was found to be half the amount adsorbed on non-plasma-treated silicon substrates.23 This discrepancy could be attributed to the different preparation methods of PDMS surfaces and demonstrated the effect of the anchoring layer on the film growth. Plasma treatment is thus a key prerequisite of our work that allows us to discuss architectural features in relation to mechanical properties. However, the oxygen plasma treatment induces the formation of a glassy PDMS layer at the surface of the PDMS film/ substrate. The behavior of this layer is mechanically different from both neat PDMS and PAH−CNF films and thus has to be considered in the mechanical model by applying the two-plate buckling approach.15 To conclude, it can be assumed that adsorption processes remain the same, regardless of the substrate (PDMS or silicon), and that mechanical measurements can be achieved with the activated PDMS and discussed in relation to structural data. Buckling Experiments. To determine Young’s modulus of the different films, we applied the SIEBIMM technique to the films deposited on the PDMS substrate. The films were compressed using a micromechanical device. The measurements were made under controlled relative humidity (43%). Periodic and sinusoidal patterns that appeared over the whole surface could be observed by optical microscopy (Figure 3). The wavelengths, λ, corresponding to the distance between two light (or dark) bands and determined from the microscope images were between 1 and 6 μm. As expected, the buckling wavelength for the same film system increased with the film thickness. For example, in the case of (PAH 1 M−CNF 0 mM)n films, λ shifts from 2.24 to 3.98 and 5.7 μm for four, six and eight bilayers, respectively. λ and the total film thickness made it possible to calculate an effective Young’s modulus for the films that consist of a thin silicon oxide layer and the CNFbased multilayered film using eq 3. The modulus of the PDMS oxidized layer was determined (Experimental Section and Supporting Information), and the two-plate model from eq 1

Figure 1. Tapping-mode AFM height image over an area of 5 × 5 μm2 for (PAH 1 M−CNF 0 mM)4 on PDMS.

topography of the (PAH 1 M−CNF 0 mM)4 film deposited on PDMS. A smooth surface and densely packed CNF can be seen. No modification was observed compared to films deposited on silicon substrates. Figure 2 reveals the thickness

Figure 2. Thickness of (PAH 1 M−CNF 0 mM)n (black squares), (PAH 1 M−CNF 12 mM)n (blue circles), and (PAH 0 M−CNF 0 mM)n (red triangles) films measured by ellipsometry vs the number of bilayers (PDMS substrate).

of the three different types of films deposited on PDMS substrates measured by ellipsometry as a function of the number of bilayers. Linear growth was observed for the three types of films, meaning that the expected adsorption processes occurred. As is the case for growth on silicon, a significant difference was detected between the films built without and with salt (0 and 1 M NaCl) in PAH solution, where the thickness increment per bilayer shifts from approximately 3 to 13 nm (Table 1). The same pattern was observed on a silicon wafer in previous studies.23,31,35 This difference was attributed D


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Figure 3. Optical microscopy at 40× magnification (43% R.H.) for (PAH 1 M−CNF 0 mM)n films undergoing a compressive strain of 2% for a number of bilayers n equal to 4 (a), 6 (b), and 8 (c).

therefore enabled the calculation of the multilayered film modulus that was plotted as a function of the thickness (Figure 4). When the thickness is less than 20−30 nm, the films are not

to be between 18 and 50 GPa.18 The values obtained from our film surfaces are therefore probably lower as a result of the presence of noncrystalline polyelectrolytes, amorphous regions of NFC, and the influence of the PAH layers. Our finding is more close range than those determined via QNM by Cranston et al.40 that report Young’s moduli estimated by PeakForce QNM of 0.86 and 0.3 GPA for NFC and PEI, respectively. From these results and by comparison with the buckling experiment, they conclude that the thin film displays mechanical behavior closer to that of CNF and polyelectrolytes than bulk materials. Discussion. The mechanical properties of the films are discussed in relation to neutron reflectivity data (NR) reports in Table 2. E was plotted as a function of the thickness of each film (Figure 4). When the thickness is less than 20−30 nm, the moduli are relatively low compared to the ones measured on thicker films. Such a result was also observed by Nolte et al.15 on PAH−PSS systems and was attributed to different conformations of the polymer chains due to the vicinity of the substrate surface. Moreover, the measurements made at low thicknesses display a large error interval, probably resulting from the uncertainties in the thickness measurements that are greater for thinner films. At thicknesses greater than 20−30 nm, E reaches a plateau. The effect of the ionic strength of PAH solutions on film growth has been demonstrated, and this parameter also has an impact on the mechanical properties. (PAH 0 M−CNF 0 mM) films have a higher modulus (9.3 GPa) than (PAH 1 M−CNF 0 mM) films (5 GPa) and (PAH 1 M−CNF 12 mM) films (4.0 GPa). All of the films have similar cellulose volume fractions, demonstrating that the architecture plays a central role in the mechanical properties, in addition to the chemical composition. The structure obtained in the absence of salt is less porous, i.e., Φair = 0.34 vs Φair = 0.48 and 0.46 in the presence of NaCl (Table 2). Therefore, the lower porosity is probably the reason for the higher modulus obtained for (PAH 0 M−CNF 0 mM)14 films. The effect of porosity on stiffness was also suggested on thick CNF cast films.37,38 Another structural difference between the films is the PAH content, but the link between this parameter and the mechanical properties is less obvious. For (PAH 1 M−CNF 0 mM), ΦPAH was 0.05 and equal to 0.1 for (PAH 1 M−CNF 12 mM) (Table 2), whereas their Young’s modulus values (5 and 4 GPa) are similar. The films with the highest PAH content present the highest Young’s modulus (PAH 0 M−CNF 0 mM). The difference in porosity may also reflect more efficient crosslinking of the building blocks of the films, i.e., PAH and CNF, which is also relevant to a higher content of PAH. It is noteworthy that the stiffness is somehow related to the growth pattern and the distribution of the cellulose nanofibrils in the

Figure 4. Young’s modulus as calculated by the SIEBIMM technique coupled with the two-plate model for (PAH 1 M−CNF 0 mM)n (black squares), (PAH 1 M−CNF 12 mM)n (blue circles), and (PAH 0 M−CNF 0 mM)n (red triangles) films as a function of the total thickness.

as stiff and their Young’s modulus is between 1 and 3 GPa. At thicknesses greater than 20−30 nm, E reaches a maximum value for the three types of films and appears to be stable in the 30−90 nm thickness range. A similar trend was reported in previous works.15,25 This may suggest that a minimal thickness is required to obtain the film cohesion, and this limit might differ with particles’ different aspect ratios. PeakForce QNM. This technique simultaneously provides the topographical height and displays the elastic modulus on the nanometer scale. We investigated the surface mechanical properties of three films (Figure 5). The average elastic surface modulus was 2.6 GPa for (PAH 0 M−CNF 0 mM), 9.3 GPa for (PAH 1 M−CNF 0 mM)4, and 5.5 GPa for (PAH 0 M−CNF 12 mM)4 film. These values are lower than those recently reported by Panaitescu et al.,36 who determined a transverse modulus of approximately 19 GPa for cellulose nanofibers. However, in the work of Panaitescu, the fibers were prepared by acid hydrolysis and, consequently, are probably composed of the crystalline part of the cellulose, resulting in higher mechanical properties. Similarly, the transverse modulus of isolated cellulose nanocrystals measured by AFM was estimated E


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Figure 5. AFM images using the PeakForce QNM mode over an area of 1 × 1 μm2 showing the height data (left) and the modulus calculated using the DMT model (right) for (PAH 1 M−CNF 0 mM)4 (a and b), (PAH 0 M−CNF 0 mM)14 (c and d), and (PAH 1 M−CNF 12 mM)4 (e and f). The number of bilayers was chosen so that all of the films would present similar thicknesses (about 40 nm).

Table 2. Values of Young’s Moduli Calculated by SIEBIMM and PF-QNM Techniques for the Three Different (CNF−PAH) Films and the Corresponding Volume Fractions and Surface Concentrations of the Different Components for These Three Films Deduced from Neutron Reflectivity Measurements31a

a

films

(PAH 1 M−CNF 0 mM)4



Young’s modulus (by SIEBIMM) (GPa) Young’s modulus (by PF-QNM) (GPa) Φcellulose ΦPAH Φair [CNF]/[PAH]

5.0 ± 0.4 9.3 ± 2.1 0.47 0.05 0.48 22.9

9.3 ± 0.8 2.6 ± 0.7 0.51 0.15 0.34 8.15

4.0 ± 0.9 5.5 ± 0.8 0.44 0.1 0.46 10.98

The number of bilayers was chosen such that all of the films present similar thicknesses (about 40 nm).

The importance of the fiber distribution on the mechanical properties has already been pointed out by Merindol et al.30 In their study, the authors built micrometer-thick multilayered thin films composed of anionic nanofibrillated cellulose and cationic poly(vinylamine) (PVam) at different pH values. They reported a higher Young’s modulus for PVAm−CNF films where the pH values of the PVAm solution were 10 and 11 (17 GPa) compared to the modulus found for the films where the pH values were 8 and 9 (12 GPa). This was attributed to the

films.39 This probably reflects the construction mode. When the PAH solution is salt-free, PAH and CNF are adsorbed in an extended conformation, inducing a lower thickness increment per bilayer (3 nm/bilayer). This may result in the formation of a higher density of bonds between PAH and CNF and, consequently, the formation of a denser layer. When the PAH solution has a higher ionic strength, the adsorption of PAH occurs as a random coil conformation, and CNF is subsequently deposited in a thicker layer (15 nm/bilayer). F


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increase in the PVAm content due to the reduction of the polymer PVAm charge density, inducing a random coil conformation. The increase in PVAm content creates more bridges between PVAm and fibrils, inducing a more effective reinforcing effect. In our case, the higher density of the film obtained when PAH chains adopt an extended conformation probably reflects the maximization of the polymer packing on the surface and, consequently, a greater number of bridges that induce higher moduli. The PF-QNM technique was also used to probe the surface mechanical properties of the films. The trends found with buckling measurements were reversed, i.e., (PAH 1 M−CNF 0 mM)4 films had higher average moduli than (PAH 0 M−CNF 0 mM)14 films. However, QNM quantifies the mechanical properties of individual components that are on the surface and are therefore not obviously directly linked to QNM and buckling results. To gain further insight and possible explanations for this difference, we calculated the ratio of surface concentrations of CNF and PAH in the three films using neutron reflectivity data.31 The results reported in Table 2 show that the surface concentration ratio between CNF and PAH ([CNF]/[PAH]) is not identical for the three films. Indeed, the (PAH 1 M−CNF 0 mM)4 film exhibits the highest ratio (22.9), followed by the (PAH 1 M−CNF 12 mM)4 film (10.98) and (PAH 0 M−CNF 0 mM)14 (8.15). Interestingly, the order also corresponds to the one for Young’s moduli calculated using the PF-QNM technique; i.e., the film with the highest ratio ([CNF]/[PAH]) exhibits the highest Young’s modulus, measured by QNM. In fact, the higher the surface concentration ratio, the greater the quantity of CNF on the surface, compared to PAH. Because CNF is thought to be significantly stiffer than polyelectrolytes, as demonstrated by the QNM measurement by Cranston et al.,40 it is a matter of course that the highest ratio corresponds to the highest modulus. Moreover, the highest ratios are obtained when the PAH is adsorbed in a random coil conformation, and it is thus possible that some loops of PAH pass through the NFC layer or form some enriched PAH zones, which might be the reason that the modulus is lower.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00049. Schematic representation of the composition of the films built on PDMS substrate and AFM image of plasmatreated PDMS (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: fi[email protected]. *E-mail: [email protected]. ORCID

Bernard Cathala: 0000-0002-3844-872X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the MATIERES project and the Région Pays de la Loire. We also thank Hervé Bizot and Joelle Davy for their fruitful discussions and excellent technical support.

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REFERENCES

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CONCLUSIONS

The aim of this work was to use the SIEBIMM and PeakForce QNM techniques to study the mechanical properties of different PAH−CNF multilayered thin films that differed in their structure and composition. Results show that the films with lower porosity have a higher Young’s modulus and vice versa. The variation in porosity reflects the architecture and the growth mode of the films. When the PAH chains have an extended conformation, i.e., salt-free solution, the growth is slow with a low value of the thickness increment per bilayer. Thus, the CNF is well distributed in the film and closely linked to PAH chains, resulting in a higher modulus compared to the case of fast growth. Young’s modulus measured by the PeakForce QNM technique reflects the surface composition of the films. This study illustrates how a simple modification on the nanometric scale, i.e., polyelectrolyte chain conformation, could have important consequences on the macroscopic mechanical properties of CNF-based multilayered films and describes a simple way to control these properties. G


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Relationship between Young's Modulus and Film Architecture in

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