- Email: [email protected]
Nanoparticle size and water diffusivity in nanocomposite agro-polymer based films
European Polymer Journal 49 (2013) 299–306
Contents lists available at SciVerse ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Nanoparticle size and water diffusivity in nanocomposite agro-polymer based films Hélène Angellier-Coussy ⇑, Emmanuelle Gastaldi, Felipe Correa Da Silva, Nathalie Gontard, Valérie Guillard
a r t i c l e
i n f o
Article history: Received 3 May 2012 Received in revised form 26 October 2012 Accepted 18 November 2012 Available online 5 December 2012 Keywords: Nanocomposite Wheat gluten Montmorillonites Size aspect ratio Modelling Water diffusivity
a b s t r a c t Nanocomposite films were prepared by a thermo-mechanical process from wheat gluten (WG) and unmodified montmorillonite (MMT). The present work aims at predicting the relationships between the decrease in relative diffusivity of liquid water and the structure of the nanocomposite films, from in situ evaluation of the MMT size aspect ratio and volume fraction, and by using a simple tortuous path mathematical model. The modelling approach first allowed predicting and validating the tortuosity effect of MMT particles on liquid water transport properties of WG-based films. In spite of a well-exfoliated nanocomposite structure, the liquid water diffusivity in WG-based films can at best be decreased by a factor of 2 with this type of clay nanoparticle, which was in agreement with the Bharadwaj model predictions. This study underlines the necessity to achieve a totally exfoliated nanocomposite structure, and above all, to select layered silicates displaying higher size aspect ratio values in order to efficiently improve water barrier properties of WG-based nanocomposite films. Furthermore, the extremely complex structure of nanocomposite materials makes detailed morphological studies often difficult, time and cost consuming. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction During the last decade, the increase in fossil energy costs and environmental concerns has resulted in the development of biodegradable materials from renewable resources for packaging applications. In this context, many studies have been devoted to the use of proteins, especially wheat gluten (WG), for the development of agromaterials. Despite the attractive properties of WG-based materials for food packaging applications [1], an obstacle to their extensive use is their important per se reactivity if compared
⇑ Corresponding author. Tel.: +33 (0)4 67 14 33 62; fax: +33 (0)4 67 14 49 90. E-mail addresses: [email protected] (H. AngellierCoussy), [email protected] (E. Gastaldi), felipecorre@hotmail. com (F.C. Da Silva), [email protected] (N. Gontard), guillard@ univ-montp2.fr (V. Guillard). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.11.006
to most conventional petrochemical-based plastics. WG-based films are known to be water-sensitive due to the hydrophilic nature of many amino acids constituting their primary structure and to the substantial and necessary amount of hydrophilic plasticizer (glycerol, water) required to impart thermo-processability and film flexibility [2–5]. Such a water sensitivity is revealed by an important swelling when they are immersed in liquid water and by a high water adsorption and poor water barrier properties in high moisture conditions [6,7]. Improving water resistance is one of the main critical issues in the development of WG-based materials for sustainable food packaging applications. A recent work has shown that the water sensitivity and water transport properties could be slightly modulated by increasing the cross-linking of the wheat gluten proteins network through a heat treatment [8]. Another route that has received considerable attention in the last 10 years for
MACROMOLECULAR NANOTECHNOLOGY
Unité Mixte de Recherche «Ingénierie des Agropolymères et Technologies Emergentes», INRA/ENSA.M/UMII/CIRAD, Université Montpellier II, CC023, pl. E Bataillon, 34095 Montpellier, Cedex, France
MACROMOLECULAR NANOTECHNOLOGY
300
H. Angellier-Coussy et al. / European Polymer Journal 49 (2013) 299–306
improving barrier properties of polymers is the introduction of impermeable lamellar nanofillers [9–11]. Montmorillonite (MMT), one of the clay minerals the most used as polymer filler, is available as micron-size tactoïds, consisting of several hundred individual platy particles held together by electrostatic forces, each plate-like layer being about 1 nm thick and 50–100 nm in lateral dimensions. In the case of agropolymer-based materials, the development of nanocomposite structures promises the extension of their use as bio-packaging since it helps to improve gas barrier properties [11] and notably water resistance [12]. Clay content, clay nature (pristine or organoclays) as well as clay dispersion state (size aspect ratio and orientation) are the main parameters that influence transport properties. Complete delamination and dispersion of the nanoclays within the matrix (exfoliation) is generally considered to be at the origin of a tortuosity effect and diffusivity decrease. As far as the particles act as impermeable obstacles, the presence of filler introduces a longer diffusive path for the penetrant. Several mathematical models have been proposed to predict this effect. The model of Nielson [13] was proved to well describe the reduction of the relative permeability by considering the particle volume fraction and aspect ratio as well as the fact that particles are oriented parallel to the surface of the film [14]. This model was further improved by Bharadwaj to also consider the possible random orientation of nanoparticles [15]. Other modifications have been proposed to include in the model either the polydispersity of aspect ratio values or also the presence of interfacial regions [14]. Whatever the model used, the interest of modelling is to be able to predict the decrease in diffusivity or permeability data from the main structural characteristics of the nanocomposite (volume fraction and size aspect ratio of nanoparticles). However, they have been developed and assessed mainly on synthetic polymers [14] and they have been only scarcely applied to the wide field of agropolymer based structures and never on wheat gluten-based nanocomposite materials. The present work proposes to focus on nanocomposite agro-polymer based materials by studying the impact of MMT volume fraction, in situ particle size aspect ratio and orientation (evaluated through TGA, WAXS and TEM image analysis) on liquid water diffusivity (from gravimetric water uptake analysis) of wheat gluten based films. The Bharadwaj mathematical model [15] was assessed for predicting the liquid water diffusivity from the main structural characteristics of nanocomposite films, and vice versa. In order to validate the model, two MMT volume fractions were tested, including the higher extreme point of validity conditions. The finalised objective was not to develop a new food packaging material but to deeply investigate the relationship between the establishment of a nanocomposite structure and liquid water transfer in agromaterials. 2. Experimental section 2.1. Materials Vital wheat gluten (designated WG) was provided by Amylum (Aalst, Belgium). Its protein content was 77%
(dry matter) according to the manufacturer. WG contained 3.6% of proteins insoluble in sodium dodecyl sulfate (SDS). Anhydrous glycerol (Fluka, purity P98%) was used as a plasticizer. Sodium montmorillonite without organic modification (designated MMT) was supplied by Süd-Chemie (Germany) under the reference NanofilÒ EXM757. MMT was characterised by a cation exchange capacity (CEC) of 80 meq 100 g1, a specific weight of 2.6 g cm3, a pH of 9.3 at 100 g L1 (20 °C) and mean lateral dimensions of about 80 nm.
2.2. Film preparation WG and MMT powders were mixed with plasticizer (glycerol and water) in a two blade counter-rotating batch mixer turning at a 3:2 differential speed (Plasticorder W50, Brabender, Duisburs, Germany) connected to a computer interface and controller unit (PL2000, Brabender, Duisburs, Germany). First the glycerol/water mixture was introduced into the mixer, then the WG/ MMT mixture was added. The mixing chamber (volume: 50 cm3) was filled with a constant total mass of 50 g. Mixing was carried out at a speed of 100 rpm for 25 min. The mixing chamber was regulated at Treg = 15 °C using a cryostat (Lauda RC 20) and water circulation in the double chamber of the mixer. After mixing, materials were pressed at 150 bar for 5 min at 120 °C between two Teflon-coated plates, using a heated hydraulic press (PLM 10 T, Techmo, Nazelles, France). A Teflon frame was placed between the two plates in order to control the thickness of the films (about 300 lm). Two MMT contents were considered, around 5 and 20 wt%. The true filler contents were determined by thermogravimetric analysis. The amounts of raw materials were calculated by considering a MMT weight content relative to the total weight of the material (50 g), a glycerol content (37.2 wt.%) relative to the amount of WG (i.e. 37.2 g glycerol/100 g gluten) and a ratio of plasticizer (water + glycerol)/total weight equal to 35 wt.%. So high plasticizers contents were required to allow the processing of WGbased nanocomposites with high MMT contents up to 20 wt% using a counter rotating batch mixer due to the torque limitation of the device. Indeed, in the case of high filler contents, the viscosity of the mix increases a lot due to the establishment of strong interactions between wheat gluten and unmodified MMT particles (high affinity between the two constituents already highlighted in previous works), inducing an increase in the shearing and the mix temperature, thus inducing the cross-linking of proteins. Water has been chosen as a ‘‘processing’’ plasticizer (in addition to glycerol) to limit these phenomena. It is worth noting that water is then evaporated during the thermoforming step. The apparent density of the dry WG-based films was calculated at 20 °C from the ratio of the weight of dry matter (drying of films over P2O5) to the corresponding volume of total material (3 replicates). Thickness of films was measured using a micrometre (Braive Instruments, Chécy, France) (10 replicates) in a dry state.
H. Angellier-Coussy et al. / European Polymer Journal 49 (2013) 299–306
2.3.1. Filler content The true weight filler fractions were determined by thermogravimetric analysis. Measurements were performed using a TGA Q50 device from TA Instruments, with a heating ramp of 40 °C min1 from room temperature to 950 °C under air (60 mL/min). Approximately 10 mg of sample was heated in an open platinum crucible. The exact weight filler fraction (W) was calculating from the respective inorganic residue at 925 °C of the pristine MMT, the neat WG matrix and the composite as:
W¼
Rcomposite Rmatrix RMMT Rmatrix
ð1Þ
where Rcomposite, Rmatrix, RMMT are the respective inorganic residues of the composite materials, the neat WG matrix and the MMT. The standard deviation value was calculated as follows [16]:
rW
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u0rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi12 0rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u u 2 2 uB ðrRcomposite Þ2 þ ðrRmatrix Þ2 C B ðrRMMT Þ þ ðrRmatrix Þ C uB C C B ¼ u@ þ@ A A t Rcomposite Rmatrix RMMT Rmatrix
aparticle ¼ L=t
ð2Þ
where rRi is the standard deviation value of the inorganic residue Ri. Then, the exact volume filler fractions were calculated using the following equation: W dMMT þ d1W dMMT matrix W
rV
n ¼ ðtparticle þ d001 t platelet Þ=d001
ð6Þ
where t particle is the thickness of each particle measured by TEM coupled with image analysis, d001 is the interlayer distance measured by XRD, and tplatelet is the thickness of each individual MMT platelet, which was estimated around 0.94 nm [17].
ð3Þ
where dMMT is the density value of MMT (2.6 g cm3, data given by the supplier) and dmatrix the density value of the WG neat matrix (1.13 g cm3, measured using a gravimetric method). The standard deviation value of volume filler fraction was given by: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 0qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi12 u 2 u r =d ðrW =dMMT Þ2 þ ðrW =dmatrix Þ2 u W MMT A / ¼t þ@ W=dMMT W=dMMT þ ð1 WÞ=dmatrix
ð5Þ
The number of platelets per particle (n) was calculated using the formula:
W
/¼
of edges) in order to obtain black and white images and was used to measure the thickness of each particle and deduce the number of tactoïds per particle. The second method, already described by Fornes et al. [17], was used to measure the curve length of the particles and allowed to eliminate possible influences of the gray background of the image. This method consisted in first printing TEM images (25 16.5 cm2), then placing a transparent sheet over the print and finally manually tracing with a fine (0.2 mm) felt tip pen the dispersed platelets and/or agglomerates. The resulting transparency was scanned in high resolution and processed by Image J using the same tools as in the first method (application of filter and function of edge demarcation). The two methods gave similar results, but the latter was finally faster due to low contrasted images that make the numerical image analysis difficult. Image analysis was conducted on micrographs obtained by TEM at a magnification of 80000 using the Image J software (version 1.42q). The particle size aspect ratio was defined by:
ð4Þ
2.3.2. Transmission electron microscopy (TEM) and image analysis Film samples were initially fixed in glutaraldehyde 2.5% (v/v), dehydrated in an ethanol gradient, then impregnated in propylene oxide and finally embedded in epoxy resin epon-812 substitute, (Electron Microscopy Science, England). After 3 days of incubation at 60 °C, ultra-thin sections of 70 nm were cut with an ultramicrotome diamond and mounted on 100 mesh grids covered by a collodion film. Samples were examined with a Jeol JEM-1200EX II TEM (Jeol Ltd., Tokyo, Japan) using magnification of 50 K and operating at 75 kV. Two different methods were used for image analysis in order to measure the particle curve length (L), the particle thickness (t) and the orientation of the tactoïds. The first method consisted in directly treating numerical TEM micrographs using tools of Image J (filters and demarcation
2.3.3. Wide-angle X-ray diffraction (WAXS) analysis Pristine montmorillonite and WG-based films were characterised by X-ray diffraction at room temperature and humidity using a Philips X’Pert MPD h 2h diffractometer operating with the Cu Ka radiation (k = 1.5418 Å) and a nickel filter. 2.3.4. Water uptake and diffusivity Water uptake was determined gravimetrically at room temperature using circular specimens (diameter of 25 mm). Samples were stored on silicagel prior to testing (assuming that the RH value reached with silicagel was close to zero). After being weighted using a four-digit balance, dried samples were immersed in distilled water (volume of 50 mL), then removed at specific intervals and weighted after having carefully removed the excess of water using tissue paper. At least five replicates of the same formulation were analysed. Water uptake values were corrected by considering the glycerol losses occurring during immersion. The sorption phenomena was assumed to follow Fickian kinetics and the water sorption curve was consequently modelled as follows [18]: " # 1 Mt 8 X 1 ð2n þ 1Þ2 p2 ¼1 2 exp D t M1 p n¼0 ð2n þ 1Þ2 4L2
ð7Þ
where Mt is the quantity of water entering in the sample at time t, M 1 is the quantity of water in the sample at infinite
MACROMOLECULAR NANOTECHNOLOGY
2.3. Characterization
301
302
H. Angellier-Coussy et al. / European Polymer Journal 49 (2013) 299–306
time, L is the half thickness of the sample and D is the effective water diffusivity (supposed constant). Effective water diffusivity was identified from the experimental sorption curve by minimizing the root mean square deviations between simulated and experimental results using the Levenberg–Marquardt procedure [19]. Then the ratio of the effective diffusivity within the composite to the diffusivity within the neat matrix, Dcomposite/Dmatrix, was calculated (at least 5 replicates) and discussed. The standard deviation value on Dcomposite/Dmatrix ratio was given by
rx
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 rx1 rx2 ¼ þ x x1 x2
MACROMOLECULAR NANOTECHNOLOGY
with
ð8Þ
rxi the standard deviation value on xi and x ¼ x1 =x2 .
2.3.5. Size aspect ratio identification The mean size aspect ratio of MMT particles and their distribution were tentatively determined from the experimental values of Dcomposite/Dmatrix and volume filler fractions (U) by using Eq. (11). In order to study the impact of standard deviation values of the input parameters (Dcomposite/Dmatrix and /) on the distribution of calculated values of size aspect ratio, a Monte Carlo sampling was applied to the two input parameters in order to determine the 95% confidence interval on the calculated value of size aspect ratio, as explained elsewhere [20,21]. Briefly, m cou~ c =D ~ m ) and / ple of theoretical values of Dcomposite/Dmatrix (D are generated using a Matlab routine (The Mathworks, Inc., Natick, MA, USA) by:
~ ¼ X þ rX d X
ð9Þ
~ the theoretical generated value, X is the real meawith X sured value, rX the standard deviation on the measured value, and d a random number whose elements are normally distributed with mean 0 and variance 1. Each theoretical ~ c =D ~ m ) is then used to ~ D couple of theoretical values (/; determine an alpha value from Eq. (11). It is then checked that the calculated alpha values are normally distributed through Anderson–Darling test carried out with Matlab software at 95% confidence level. If test confirmed normality, the results were expressed as mean value of alpha associated with its standard deviation. If test did not confirm normality, confidence interval at 95% level was estimated from 95% of the m values. In this work, m was taken equal to 2000.
diffusivity in WG-based nanocomposite films could be achieved based on the use of the mathematical model of Bharadwaj [15]:
Dcomposite 1 ¼ Dmatrix 1 þ a2 /ð23ÞðS þ 12Þ
ð11Þ
Pn ai (i.e. with a ¼ i¼1 where a and ai = Li/ti, where Li and ti are n the length and the thickness of each particle or tactoïd) and / are respectively the average values of size aspect ratio and volume fraction of particles. S is an order parameter defined as:
S¼
1 ð3 cos2 h 1Þ 2
ð12Þ
where h represents the angle between the direction of preferred orientation and the surface of the film. The orientation parameter S ranges from a value of 0.5 for a system where the long axis of the filler is oriented parallel to the flux direction (no tortuosity), to a value of 1, where it is oriented perpendicular to the flux direction (maximum tortuosity), a value of 0 representing random orientation. Eq. (11) shows that water diffusivity of nanocomposite films should decrease with the increase of the volume fraction and the aspect ratio of MMT particles, for a given orientation (S = 1 in Fig. 1a). Considering a size aspect ratio of 80 (theoretical value given by the supplier corresponding roughly to the highest value reachable in case of complete exfoliation of MMT particles), a filler content of 5 wt.% (conventional filler content in nanocomposite science), and an orientation of the long axis of the filler parallel to the surface of the film (maximum tortuosity, S = 1), the Baradwaj model predicts a decrease in water diffusivity by a factor of about 2. Now considering a filler content of 20 wt.%, i.e. around 10 vol.%, which corresponds to the validity limit of the Bharadwaj model, a decrease by a factor of about 4 is predicted (Fig. 1). Wheat gluten-based nanocomposite films were prepared using a thermo-mechanical process by considering two montmorillonite weight contents (5 wt% and
3. Results and discussion If MMT particles are considered to act as impermeable obstacles in a polymer matrix, the diffusing molecule, here liquid water, has to follow a more tortuous path to go from one side to the opposite side of the composite film. The diffusivity of liquid water can be expressed as:
Dcomposite 1 ¼ Dmatrix s
ð10Þ
where Dmatrix is the diffusion coefficient in the neat matrix, Dcomposite the diffusion coefficient in the nanocomposite, and s the tortuosity. A prediction of the liquid water
Fig. 1. Prediction of Bharadwaj’s model for the relative liquid water diffusivity as a function of MMT volume fraction compared to experimental data (S = 1 in all cases).
20 wt.%) in order to valid the predictions of Bharadwaj model. To allow this prediction, the true volume filler contents were evaluated through TGA measurements and were exactly 2.2 and 10.8 vol.%, respectively. Experimentally, a reduction of only a factor of 1.3 and 1.95 was respectively obtained for filler contents of 2.2 and 10.8 vol.% (Dmatrix = 5.8 1011 m2 s1, Dcomposite = 4.6 and 3.0 1011 m2 s1 for films filled with 2.2 and 10.8 vol.% of MMT particles), which was far lower that the predicted reduction (Fig. 1). It is worth noting that all WG-based nanocomposite films were transparent and displayed the same yellow–brown colour as the neat WG matrix, letting presuming that most of MMT nanoparticles were homogeneously dispersed within the WG matrix, as already reported by Angellier-Coussy et al. [12]. However, this discrepancy between predicted and experimental values underlines that the exfoliation of MMT particles was not complete, resulting in a particle size aspect ratio lower than the forecast value of 80. As a consequence, this study highlights the necessity to rigorously determine MMT particle size aspect ratio in order to predict transfer properties in nanocomposite films. The dispersion state of MMT particles within the WGbased matrix was evaluated by X-ray diffraction analysis and TEM observations coupled with image analysis. Fig. 2 displays the typical WAXS patterns recorded for pristine MMT, the neat WG matrix, and the WG/MMT nanocomposites elaborated with two different clay loadings (2.2 and 10.8 vol.%), for 2h angle values comprised between 3° and 25°. The diffraction pattern of pristine MMT was first characterised by a group of peaks around 20° corresponding to the crystallographic planes of the clay. Their presence is important since it demonstrated that the WAXS analysis was sufficiently sensitive to detect the presence of MMT in the nanocomposite. An intense diffraction peak was also noted at 2h = 7.7°, corresponding to a basal interlayer spacing value (d001) of 1.14 nm. The neat WG-based matrix displayed a typical amorphous structure characterised by two very broad peaks centred around 2h = 8° and 2h = 20°. After incorporating MMT particles, the d001 peak was no longer observed, leading to suppose that the clays were rather well exfoliated within the WG matrix. However, a small intensity peak characteristic of intercalated layered silicates could be masked by the WG peak centred around 8°. To deepen the structural characterization obtained by WAXS analysis, morphological parameters were determined using image analysis of TEM pictures (Table 1). Most of the clay sheets appeared well dispersed and globally oriented parallel to the film surface due to the pressure effect (i.e. 5 min at 150 bars) applied during the thermoforming process (Fig. 3), even if their alignment cannot be considered as perfect. We can thus reasonably consider that the nanoparticles are all orientated perpendicularly to the flux direction in the modelling approach (S = 1 in Eq. (11)). WG-based nanocomposites displayed a coexistence of individual platelets and non-fully exfoliated tactoïds, as confirmed by image analysis indicating an average number of platelets per particle of 4–5 whatever the filler content (Table 1). However, this value was probably over estimated since in the absence of a d001 peak characteristic of an
303
Fig. 2. WAXS patterns of the (a) MMT particles, (b) the neat WG-based matrix and WG-based films filled with (c) 2.2 and (d) 10.8 vol.% of MMT.
intercalated structure (Fig. 2), a d001 value of 1.14 nm (corresponding to the inter-lamellar distance of pristine MMT) was considered to calculate n in Eq. (6). For comparison, the average number of platelets per particle was also calculated in considering the maximum interlamellar distance detected by wide-angle X-ray analysis (2h = 3°) i.e. 2.94 nm as d001 value, leading to lower values of n (2–3 platelets per stack) (Table 1). Average values of size aspect ratio were 17–23 for 10.8 and 2.2 vol.%, respectively. This extent of exfoliation is similar than those obtained for synthetic polymer based nanocomposites such as nylon 6-MMT nanocomposite membranes [22]. exp ) Based on the experimental mean size aspect ratio (a calculated for the two filler contents investigated in exp ¼ 23) or (/ = 0.108; the present study (/ = 0.022; a a exp ¼ 17), a relative decrease in water diffusivity by factors of 1.25 and 1.9 was respectively predicted by Eq. (11). These predicted values were very closed to the experimental data obtained (respectively 1.3 and 1.95). We can conclude that the liquid water diffusivity within WG-based
MACROMOLECULAR NANOTECHNOLOGY
H. Angellier-Coussy et al. / European Polymer Journal 49 (2013) 299–306
304
H. Angellier-Coussy et al. / European Polymer Journal 49 (2013) 299–306
Table 1 Results of particle image analysis.
* **
Number of analysed particle (N)
Average particle length (l) (nm)
Average particle thickness (t) (nm)
Number average platelets per particle )* (n
Number average platelets per particle )** (n
Average aspect )*** ratio (a
2.2 10.8
278 308
79 ± 32 72 ± 30
4.7 ± 3.7 5.9 ± 4.3
4.2 ± 3.2 5.3 ± 3.8
2.3 ± 1.2 2.7 ± 1.5
23 ± 16 17 ± 12
Calculated using d001 = 1.14 nm, which corresponded to the interlamellar distance of pristine MMT. Calculated using d001 = 2.94 nm, which corresponded to the maximum interlamellar distance detected by wide-angle X-ray analysis (2h = 3°). P a ¼ ðL=tÞ=N.
MACROMOLECULAR NANOTECHNOLOGY
***
MMT content (vol.%)
Fig. 3. TEM micrographs of WG-based films filled with 2.2 and 10.8 vol.% of MMT.
nanocomposite materials can be well predicted using a simple tortuous path model (Fig. 1). It is worth noting that a detailed structural analysis using TEM analysis allowing the evaluation of the aspect ratio is required for each filler content since the dispersion state strongly depends on the filler content. It is well-known that TEM analysis has some inherent limitations due to the fact that it projects threedimensional (3D) objects onto a two-dimensional (2D) plane, resulting in TEM cross sections that do not necessarily represent the actual size of all the platelets [17]. Furthermore, an ideal image would exhibit sharp transitions from black to white, what is not the case in reality,
necessitating the conversion of the original gray-scale TEM micrograph into a black and white image using a semi-automated approach [17], this step being inevitably accompanied by errors. However, the present work highlights that even if image analysis of TEM pictures is marred with a certain uncertainty, it remains a good tool to estimate quantitatively the clay dispersion state. Results also indicate that the liquid water diffusivity could be decreased at the most by a factor of 2 (in case of a maximum 10.8 vol.% of MMT particles in the WG based network). With the aim of substituting petrochemical plastics by bio-based materials, similar barrier properties need to be achieved. This means that water permeability of agro-based films should be reduced by a factor of 10–100 if large applications in the food packaging sector are targeted. For example the water vapour permeability of WG based films for a 0–100% RH difference ranges from 5 1012 mol m1 s1 Pa1 to 6.2 1011 mol m1 s1 Pa1, depending on the film preparation process [23,24], whereas it is equal to 0.05 1012 mol m1 s1 Pa1 in case of synthetic films such as low-density polyethylene [25]. Based on the presented modelling approach, it can already be concluded that such a reduction factor is not feasible with this type of MMT nanoparticles within processable range of filler content. This underlines the utility of the modelling approach and leads to the conclusion that next researches must focus on the use of nanoparticles displaying higher size aspect ratio values (higher than 100 when dispersed within the polymer matrix) in order to sufficiently improve water barrier properties of WG based films. Another interest of the modelisation of structure/mass transfer properties relationships would be the possibility to estimate the mean size aspect ratio of MMT particles, as well as its polydispersity, based on the knowledge of both the experimental volume filler fraction and the liquid water diffusivity values and their respective uncertainty. Indeed, the quantitative determination of the size aspect ratio values of MMT particles dispersed within complex and sensitive agro-polymer based materials by TEM image analysis is often complex, time and cost consuming. One can wonder if only two estimations of water transport properties (one for the neat matrix and one for the nanocomposite) would be sufficient to estimate an apparent value of the mean size aspect ratio with a satisfactory standard deviation. The final objective would be to modelise other mass transfer properties, which would be more
H. Angellier-Coussy et al. / European Polymer Journal 49 (2013) 299–306
Frequence (%)
a
15
2.2 vol% = 21 Simulated
10
5
0 0
Frequence (%)
b
20
40
60
15
80
100
10.8 vol% = 17 Simulated
10
4. Conclusion As expected, the volume filler content, the size aspect ratio and the orientation of MMT particles are key parameters to modulate liquid water transport properties through a tortuosity effect in WG-based films. If the orientation is often subjected to the thermomechanical process used to prepare WG-based nanocomposite films, it is possible to play on both the filler content and the aspect ratio of MMT particles. Liquid water diffusivity values in nanocomposite materials were well predicted using a model based on the tortuosity effect, such as the Bharadwaj model. For this purpose, an accurate determination of both the true volume filler content and the size aspect ratio of MMT particles is required for each formulation. In agreement with the Bharadwaj model predictions, the liquid water diffusivity can only be decreased by a factor of 2 in our best case (10.8 vol.% of MMT particles) in spite of a well-exfoliated nanocomposite structure. It must be pointed out that this value of 10.8% MMT is completely unrealistic for further commercial application. Consequently, from a commercial point of view, we can conclude that for food packaging application, the decrease in water sensitivity achieved by using commonly used filler contents (about 2 vol.%) would be totally insufficient. Moreover, before commercial application safety issue raises due to the use of nanoparticles that may migrate from the food contact material into the food product. This is definitively out of the scope of this study but should be addressed in further work for any intended application of nanocomposite materials in the food packaging field. This study underlines the necessity to combine two strategies to efficiently improve water barrier properties of WGbased nanocomposite films: (i) the selection of layered silicates displaying high size aspect ratio values and (ii) the achievement of a totally exfoliated nanocomposite structure. Although TEM and X-ray diffraction provide means for observing nano-structures, the extremely complex morphology of this kind of materials has made detailed morphological studies time and cost consuming. This study highlights that the characterization of transport properties, and in the present case the evaluation of liquid water diffusivity, combined with a modelling approach can be a quite successful route in elucidating structural aspects, at least to evaluate apparent value of the mean size aspect ratio for the modelisation of further mass transfer properties. Acknowledgements
5
We thank Chantal Cazevieille from CRIC INSERM (Montpellier, France) for TEM analysis. 0 0
20
40
60
80
100
alpha Fig. 4. Predicted (curves) and experimental (bars) probability frequencies of the size aspect ratio values obtained for filler contents of (a) 2.2 vol.% and (b) 10.8 vol.%. Predicted values were obtained by using Eq. (11) and a Monte Carlo procedure with average values of Dcomposite/Dmatrix and / and their respective standard deviation as input parameters.
References [1] Guilbert S, Gontard N, Morel MH, Chalier P, Micard V, Redl A. Formation and properties of wheat gluten films and coatings. In: Gennadios A, editor. Protein-based films and coatings. CRC Press; 2002. p. 69–122. [2] Gontard N, Ring S. Edible wheat gluten film: influence of water content on glass transition temperature. J Agric Food Chem 1996;44(11):3474–8.
MACROMOLECULAR NANOTECHNOLOGY
complex or longer to characterise, for example the diffusivity of molecules with higher molecular weight than water. In the present study, the volume filler content and the liquid water diffusivity are characterised by standard deviation values that have to be taken into account when they are used as input parameters in Eq. (11) (/ = 0.0215 ± 0.0023 with Dcomposite/Dmatrix = 0.786 ± 0.033, and / = 0.1078 ± 0.0006 with Dcomposite/Dmatrix = 0.513 ± 0.104). The impact of uncertainty propagation on the distribution of size aspect ratio of MMT particles was studied for the two filler contents through Monte Carlo simulations, considering that both Dcomposite/Dmatrix ratios and volume filler contents were normally distributed with a variance r2x . The resulting simulated distributions are gathered with the experimental data on Fig. 4. In both cases, Gaussian-like curves were obtained for the simulated distributions with a narrow peak around a mean value that ¼ 26 matched very well with experimental one (a ¼ 19 (16–40) vs 23 for 2.2 vol.% of MMT particles and a (7–42) vs 17 for 10.8 vol.% of MMT particles).
305
MACROMOLECULAR NANOTECHNOLOGY
306
H. Angellier-Coussy et al. / European Polymer Journal 49 (2013) 299–306
[3] Pouplin M, Redl A, Gontard N. Glass transition of wheat gluten plasticized with water, glycerol, or sorbitol. J Agric Food Chem 1999;47(2):538–43. [4] Pommet M, Redl A, Guilbert S, Morel MH. Intrinsic influence of various plasticizers on functional properties and reactivity of wheat gluten thermoplastic materials. J Cereal Sci 2005;42(1):81–91. [5] Mangavel C, Barbot J, Gueguen J, Popineau Y. Molecular determinants of the influence of hydrophilic plasticizers on the mechanical properties of cast wheat gluten films. J Agric Food Chem 2003;51(5):1447–52. [6] Tunc S, Angellier H, Cahyana Y, Chalier P, Gontard N, Gastaldi E. Functional properties of wheat gluten/montmorillonite nanocomposite films processed by casting. J Membr Sci 2007;289(1–2):159–68. [7] Domenek S, Brendel L, Morel MH, Guilbert S. Swelling behaviour and structural characteristics of wheat gluten polypeptide films. Biomacromolecules 2004;5(3):1002–8. [8] Angellier-Coussy H, Gastaldi E, Gontard N, Guillard V. Influence of processing temperature on the water vapour transport properties of wheat gluten based agromaterials. Ind Crops Prod 2011;33(2): 457–61. [9] Giannelis EP. Polymer layered silicate nanocomposites. Adv Mater 1996;8(1):29–35. [10] Alexandre M, Dubois P. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater Sci Eng R Rep 2000;28(1–2):1–63. [11] Ray SS, Bousmina M. Biodegradable polymers and their layered silicate nano composites: in greening the 21st century materials world. Prog Mater Sci 2005;50(8):962–1079. [12] Angellier-Coussy H, Torres-Giner S, Morel MH, Gontard N, Gastaldi E. Functional properties of thermoformed wheat gluten/ montmorillonite materials with respect to formulation and processing conditions. J Appl Polym Sci 2008;107(1):487–96. [13] Nielsen LE. Models for the permeability of filled polymer systems. J Macromol Sci – Phys 1967;1:929–42.
[14] Choudalakis G, Gotsis AD. Permeability of polymer/clay nanocomposites: a review. Eur Polym J 2009;45(4):967–84. [15] Bharadwaj RK. Modeling the barrier properties of polymer-layered silicate nanocomposites. Macromolecules 2001;34(26):9189–92. [16] Skoog DA, West DM, Holler FJ. Les erreurs aléatoires dans les analyses Chimie Analytique. 7th ed. DeBoeck Université; 1997. p. 21–44. [17] Fornes TD, Paul DR. Modeling properties of nylon 6/clay nanocomposites using composite theories. Polymer 2003;44(17): 4993–5013. [18] Crank J. The mathematics of diffusion. New York (NY): Oxford University Press; 1980. [19] Gill EP, Murray W, Wright MH. Practical optimization. London: Academic Press; 1981. p. 401. [20] Penicaud C, Peyron S, Bohuon P, Gontard N, Guillard V. Ascorbic acid in food: development of a rapid analysis technique and application to diffusivity determination. Food Res Int 2010;43(3):838–847. [21] Hessler JP. The use of Monte Carlo simulations to evaluate kinetic data and analytic approximations. Int J Chem Kinet 1997;29(11): 803–17. [22] Picard E, Gerard JF, Espuche E. Water transport properties of polyamide 6 based nanocomposites prepared by melt blending: on the importance of the clay dispersion state on the water transport properties at high water activity. J Membr Sci 2008;313(1– 2):284–95. [23] Gontard N, Guilbert S, Cuq JL. Water and glycerol as plasticizers affect mechanical and water–vapor barrier properties of an edible wheat gluten film. J Food Sci 1993;58(1):206–11. [24] Pommet M, Redl A, Morel MH, Guilbert S. Study of wheat gluten plasticization with fatty acids. Polymer 2003;44(1):115–22. [25] Myers AW, Meyer JA, Rogers CE, Stannet V, Szwarc M. Studies in the gas and vapour permeability of plastic films and coated papers. Tappi J 1961;45:58–67.
Contents lists available at SciVerse ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Nanoparticle size and water diffusivity in nanocomposite agro-polymer based films Hélène Angellier-Coussy ⇑, Emmanuelle Gastaldi, Felipe Correa Da Silva, Nathalie Gontard, Valérie Guillard
a r t i c l e
i n f o
Article history: Received 3 May 2012 Received in revised form 26 October 2012 Accepted 18 November 2012 Available online 5 December 2012 Keywords: Nanocomposite Wheat gluten Montmorillonites Size aspect ratio Modelling Water diffusivity
a b s t r a c t Nanocomposite films were prepared by a thermo-mechanical process from wheat gluten (WG) and unmodified montmorillonite (MMT). The present work aims at predicting the relationships between the decrease in relative diffusivity of liquid water and the structure of the nanocomposite films, from in situ evaluation of the MMT size aspect ratio and volume fraction, and by using a simple tortuous path mathematical model. The modelling approach first allowed predicting and validating the tortuosity effect of MMT particles on liquid water transport properties of WG-based films. In spite of a well-exfoliated nanocomposite structure, the liquid water diffusivity in WG-based films can at best be decreased by a factor of 2 with this type of clay nanoparticle, which was in agreement with the Bharadwaj model predictions. This study underlines the necessity to achieve a totally exfoliated nanocomposite structure, and above all, to select layered silicates displaying higher size aspect ratio values in order to efficiently improve water barrier properties of WG-based nanocomposite films. Furthermore, the extremely complex structure of nanocomposite materials makes detailed morphological studies often difficult, time and cost consuming. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction During the last decade, the increase in fossil energy costs and environmental concerns has resulted in the development of biodegradable materials from renewable resources for packaging applications. In this context, many studies have been devoted to the use of proteins, especially wheat gluten (WG), for the development of agromaterials. Despite the attractive properties of WG-based materials for food packaging applications [1], an obstacle to their extensive use is their important per se reactivity if compared
⇑ Corresponding author. Tel.: +33 (0)4 67 14 33 62; fax: +33 (0)4 67 14 49 90. E-mail addresses: [email protected] (H. AngellierCoussy), [email protected] (E. Gastaldi), felipecorre@hotmail. com (F.C. Da Silva), [email protected] (N. Gontard), guillard@ univ-montp2.fr (V. Guillard). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.11.006
to most conventional petrochemical-based plastics. WG-based films are known to be water-sensitive due to the hydrophilic nature of many amino acids constituting their primary structure and to the substantial and necessary amount of hydrophilic plasticizer (glycerol, water) required to impart thermo-processability and film flexibility [2–5]. Such a water sensitivity is revealed by an important swelling when they are immersed in liquid water and by a high water adsorption and poor water barrier properties in high moisture conditions [6,7]. Improving water resistance is one of the main critical issues in the development of WG-based materials for sustainable food packaging applications. A recent work has shown that the water sensitivity and water transport properties could be slightly modulated by increasing the cross-linking of the wheat gluten proteins network through a heat treatment [8]. Another route that has received considerable attention in the last 10 years for
MACROMOLECULAR NANOTECHNOLOGY
Unité Mixte de Recherche «Ingénierie des Agropolymères et Technologies Emergentes», INRA/ENSA.M/UMII/CIRAD, Université Montpellier II, CC023, pl. E Bataillon, 34095 Montpellier, Cedex, France
MACROMOLECULAR NANOTECHNOLOGY
300
H. Angellier-Coussy et al. / European Polymer Journal 49 (2013) 299–306
improving barrier properties of polymers is the introduction of impermeable lamellar nanofillers [9–11]. Montmorillonite (MMT), one of the clay minerals the most used as polymer filler, is available as micron-size tactoïds, consisting of several hundred individual platy particles held together by electrostatic forces, each plate-like layer being about 1 nm thick and 50–100 nm in lateral dimensions. In the case of agropolymer-based materials, the development of nanocomposite structures promises the extension of their use as bio-packaging since it helps to improve gas barrier properties [11] and notably water resistance [12]. Clay content, clay nature (pristine or organoclays) as well as clay dispersion state (size aspect ratio and orientation) are the main parameters that influence transport properties. Complete delamination and dispersion of the nanoclays within the matrix (exfoliation) is generally considered to be at the origin of a tortuosity effect and diffusivity decrease. As far as the particles act as impermeable obstacles, the presence of filler introduces a longer diffusive path for the penetrant. Several mathematical models have been proposed to predict this effect. The model of Nielson [13] was proved to well describe the reduction of the relative permeability by considering the particle volume fraction and aspect ratio as well as the fact that particles are oriented parallel to the surface of the film [14]. This model was further improved by Bharadwaj to also consider the possible random orientation of nanoparticles [15]. Other modifications have been proposed to include in the model either the polydispersity of aspect ratio values or also the presence of interfacial regions [14]. Whatever the model used, the interest of modelling is to be able to predict the decrease in diffusivity or permeability data from the main structural characteristics of the nanocomposite (volume fraction and size aspect ratio of nanoparticles). However, they have been developed and assessed mainly on synthetic polymers [14] and they have been only scarcely applied to the wide field of agropolymer based structures and never on wheat gluten-based nanocomposite materials. The present work proposes to focus on nanocomposite agro-polymer based materials by studying the impact of MMT volume fraction, in situ particle size aspect ratio and orientation (evaluated through TGA, WAXS and TEM image analysis) on liquid water diffusivity (from gravimetric water uptake analysis) of wheat gluten based films. The Bharadwaj mathematical model [15] was assessed for predicting the liquid water diffusivity from the main structural characteristics of nanocomposite films, and vice versa. In order to validate the model, two MMT volume fractions were tested, including the higher extreme point of validity conditions. The finalised objective was not to develop a new food packaging material but to deeply investigate the relationship between the establishment of a nanocomposite structure and liquid water transfer in agromaterials. 2. Experimental section 2.1. Materials Vital wheat gluten (designated WG) was provided by Amylum (Aalst, Belgium). Its protein content was 77%
(dry matter) according to the manufacturer. WG contained 3.6% of proteins insoluble in sodium dodecyl sulfate (SDS). Anhydrous glycerol (Fluka, purity P98%) was used as a plasticizer. Sodium montmorillonite without organic modification (designated MMT) was supplied by Süd-Chemie (Germany) under the reference NanofilÒ EXM757. MMT was characterised by a cation exchange capacity (CEC) of 80 meq 100 g1, a specific weight of 2.6 g cm3, a pH of 9.3 at 100 g L1 (20 °C) and mean lateral dimensions of about 80 nm.
2.2. Film preparation WG and MMT powders were mixed with plasticizer (glycerol and water) in a two blade counter-rotating batch mixer turning at a 3:2 differential speed (Plasticorder W50, Brabender, Duisburs, Germany) connected to a computer interface and controller unit (PL2000, Brabender, Duisburs, Germany). First the glycerol/water mixture was introduced into the mixer, then the WG/ MMT mixture was added. The mixing chamber (volume: 50 cm3) was filled with a constant total mass of 50 g. Mixing was carried out at a speed of 100 rpm for 25 min. The mixing chamber was regulated at Treg = 15 °C using a cryostat (Lauda RC 20) and water circulation in the double chamber of the mixer. After mixing, materials were pressed at 150 bar for 5 min at 120 °C between two Teflon-coated plates, using a heated hydraulic press (PLM 10 T, Techmo, Nazelles, France). A Teflon frame was placed between the two plates in order to control the thickness of the films (about 300 lm). Two MMT contents were considered, around 5 and 20 wt%. The true filler contents were determined by thermogravimetric analysis. The amounts of raw materials were calculated by considering a MMT weight content relative to the total weight of the material (50 g), a glycerol content (37.2 wt.%) relative to the amount of WG (i.e. 37.2 g glycerol/100 g gluten) and a ratio of plasticizer (water + glycerol)/total weight equal to 35 wt.%. So high plasticizers contents were required to allow the processing of WGbased nanocomposites with high MMT contents up to 20 wt% using a counter rotating batch mixer due to the torque limitation of the device. Indeed, in the case of high filler contents, the viscosity of the mix increases a lot due to the establishment of strong interactions between wheat gluten and unmodified MMT particles (high affinity between the two constituents already highlighted in previous works), inducing an increase in the shearing and the mix temperature, thus inducing the cross-linking of proteins. Water has been chosen as a ‘‘processing’’ plasticizer (in addition to glycerol) to limit these phenomena. It is worth noting that water is then evaporated during the thermoforming step. The apparent density of the dry WG-based films was calculated at 20 °C from the ratio of the weight of dry matter (drying of films over P2O5) to the corresponding volume of total material (3 replicates). Thickness of films was measured using a micrometre (Braive Instruments, Chécy, France) (10 replicates) in a dry state.
H. Angellier-Coussy et al. / European Polymer Journal 49 (2013) 299–306
2.3.1. Filler content The true weight filler fractions were determined by thermogravimetric analysis. Measurements were performed using a TGA Q50 device from TA Instruments, with a heating ramp of 40 °C min1 from room temperature to 950 °C under air (60 mL/min). Approximately 10 mg of sample was heated in an open platinum crucible. The exact weight filler fraction (W) was calculating from the respective inorganic residue at 925 °C of the pristine MMT, the neat WG matrix and the composite as:
W¼
Rcomposite Rmatrix RMMT Rmatrix
ð1Þ
where Rcomposite, Rmatrix, RMMT are the respective inorganic residues of the composite materials, the neat WG matrix and the MMT. The standard deviation value was calculated as follows [16]:
rW
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u0rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi12 0rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u u 2 2 uB ðrRcomposite Þ2 þ ðrRmatrix Þ2 C B ðrRMMT Þ þ ðrRmatrix Þ C uB C C B ¼ u@ þ@ A A t Rcomposite Rmatrix RMMT Rmatrix
aparticle ¼ L=t
ð2Þ
where rRi is the standard deviation value of the inorganic residue Ri. Then, the exact volume filler fractions were calculated using the following equation: W dMMT þ d1W dMMT matrix W
rV
n ¼ ðtparticle þ d001 t platelet Þ=d001
ð6Þ
where t particle is the thickness of each particle measured by TEM coupled with image analysis, d001 is the interlayer distance measured by XRD, and tplatelet is the thickness of each individual MMT platelet, which was estimated around 0.94 nm [17].
ð3Þ
where dMMT is the density value of MMT (2.6 g cm3, data given by the supplier) and dmatrix the density value of the WG neat matrix (1.13 g cm3, measured using a gravimetric method). The standard deviation value of volume filler fraction was given by: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 0qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi12 u 2 u r =d ðrW =dMMT Þ2 þ ðrW =dmatrix Þ2 u W MMT A / ¼t þ@ W=dMMT W=dMMT þ ð1 WÞ=dmatrix
ð5Þ
The number of platelets per particle (n) was calculated using the formula:
W
/¼
of edges) in order to obtain black and white images and was used to measure the thickness of each particle and deduce the number of tactoïds per particle. The second method, already described by Fornes et al. [17], was used to measure the curve length of the particles and allowed to eliminate possible influences of the gray background of the image. This method consisted in first printing TEM images (25 16.5 cm2), then placing a transparent sheet over the print and finally manually tracing with a fine (0.2 mm) felt tip pen the dispersed platelets and/or agglomerates. The resulting transparency was scanned in high resolution and processed by Image J using the same tools as in the first method (application of filter and function of edge demarcation). The two methods gave similar results, but the latter was finally faster due to low contrasted images that make the numerical image analysis difficult. Image analysis was conducted on micrographs obtained by TEM at a magnification of 80000 using the Image J software (version 1.42q). The particle size aspect ratio was defined by:
ð4Þ
2.3.2. Transmission electron microscopy (TEM) and image analysis Film samples were initially fixed in glutaraldehyde 2.5% (v/v), dehydrated in an ethanol gradient, then impregnated in propylene oxide and finally embedded in epoxy resin epon-812 substitute, (Electron Microscopy Science, England). After 3 days of incubation at 60 °C, ultra-thin sections of 70 nm were cut with an ultramicrotome diamond and mounted on 100 mesh grids covered by a collodion film. Samples were examined with a Jeol JEM-1200EX II TEM (Jeol Ltd., Tokyo, Japan) using magnification of 50 K and operating at 75 kV. Two different methods were used for image analysis in order to measure the particle curve length (L), the particle thickness (t) and the orientation of the tactoïds. The first method consisted in directly treating numerical TEM micrographs using tools of Image J (filters and demarcation
2.3.3. Wide-angle X-ray diffraction (WAXS) analysis Pristine montmorillonite and WG-based films were characterised by X-ray diffraction at room temperature and humidity using a Philips X’Pert MPD h 2h diffractometer operating with the Cu Ka radiation (k = 1.5418 Å) and a nickel filter. 2.3.4. Water uptake and diffusivity Water uptake was determined gravimetrically at room temperature using circular specimens (diameter of 25 mm). Samples were stored on silicagel prior to testing (assuming that the RH value reached with silicagel was close to zero). After being weighted using a four-digit balance, dried samples were immersed in distilled water (volume of 50 mL), then removed at specific intervals and weighted after having carefully removed the excess of water using tissue paper. At least five replicates of the same formulation were analysed. Water uptake values were corrected by considering the glycerol losses occurring during immersion. The sorption phenomena was assumed to follow Fickian kinetics and the water sorption curve was consequently modelled as follows [18]: " # 1 Mt 8 X 1 ð2n þ 1Þ2 p2 ¼1 2 exp D t M1 p n¼0 ð2n þ 1Þ2 4L2
ð7Þ
where Mt is the quantity of water entering in the sample at time t, M 1 is the quantity of water in the sample at infinite
MACROMOLECULAR NANOTECHNOLOGY
2.3. Characterization
301
302
H. Angellier-Coussy et al. / European Polymer Journal 49 (2013) 299–306
time, L is the half thickness of the sample and D is the effective water diffusivity (supposed constant). Effective water diffusivity was identified from the experimental sorption curve by minimizing the root mean square deviations between simulated and experimental results using the Levenberg–Marquardt procedure [19]. Then the ratio of the effective diffusivity within the composite to the diffusivity within the neat matrix, Dcomposite/Dmatrix, was calculated (at least 5 replicates) and discussed. The standard deviation value on Dcomposite/Dmatrix ratio was given by
rx
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 rx1 rx2 ¼ þ x x1 x2
MACROMOLECULAR NANOTECHNOLOGY
with
ð8Þ
rxi the standard deviation value on xi and x ¼ x1 =x2 .
2.3.5. Size aspect ratio identification The mean size aspect ratio of MMT particles and their distribution were tentatively determined from the experimental values of Dcomposite/Dmatrix and volume filler fractions (U) by using Eq. (11). In order to study the impact of standard deviation values of the input parameters (Dcomposite/Dmatrix and /) on the distribution of calculated values of size aspect ratio, a Monte Carlo sampling was applied to the two input parameters in order to determine the 95% confidence interval on the calculated value of size aspect ratio, as explained elsewhere [20,21]. Briefly, m cou~ c =D ~ m ) and / ple of theoretical values of Dcomposite/Dmatrix (D are generated using a Matlab routine (The Mathworks, Inc., Natick, MA, USA) by:
~ ¼ X þ rX d X
ð9Þ
~ the theoretical generated value, X is the real meawith X sured value, rX the standard deviation on the measured value, and d a random number whose elements are normally distributed with mean 0 and variance 1. Each theoretical ~ c =D ~ m ) is then used to ~ D couple of theoretical values (/; determine an alpha value from Eq. (11). It is then checked that the calculated alpha values are normally distributed through Anderson–Darling test carried out with Matlab software at 95% confidence level. If test confirmed normality, the results were expressed as mean value of alpha associated with its standard deviation. If test did not confirm normality, confidence interval at 95% level was estimated from 95% of the m values. In this work, m was taken equal to 2000.
diffusivity in WG-based nanocomposite films could be achieved based on the use of the mathematical model of Bharadwaj [15]:
Dcomposite 1 ¼ Dmatrix 1 þ a2 /ð23ÞðS þ 12Þ
ð11Þ
Pn ai (i.e. with a ¼ i¼1 where a and ai = Li/ti, where Li and ti are n the length and the thickness of each particle or tactoïd) and / are respectively the average values of size aspect ratio and volume fraction of particles. S is an order parameter defined as:
S¼
1 ð3 cos2 h 1Þ 2
ð12Þ
where h represents the angle between the direction of preferred orientation and the surface of the film. The orientation parameter S ranges from a value of 0.5 for a system where the long axis of the filler is oriented parallel to the flux direction (no tortuosity), to a value of 1, where it is oriented perpendicular to the flux direction (maximum tortuosity), a value of 0 representing random orientation. Eq. (11) shows that water diffusivity of nanocomposite films should decrease with the increase of the volume fraction and the aspect ratio of MMT particles, for a given orientation (S = 1 in Fig. 1a). Considering a size aspect ratio of 80 (theoretical value given by the supplier corresponding roughly to the highest value reachable in case of complete exfoliation of MMT particles), a filler content of 5 wt.% (conventional filler content in nanocomposite science), and an orientation of the long axis of the filler parallel to the surface of the film (maximum tortuosity, S = 1), the Baradwaj model predicts a decrease in water diffusivity by a factor of about 2. Now considering a filler content of 20 wt.%, i.e. around 10 vol.%, which corresponds to the validity limit of the Bharadwaj model, a decrease by a factor of about 4 is predicted (Fig. 1). Wheat gluten-based nanocomposite films were prepared using a thermo-mechanical process by considering two montmorillonite weight contents (5 wt% and
3. Results and discussion If MMT particles are considered to act as impermeable obstacles in a polymer matrix, the diffusing molecule, here liquid water, has to follow a more tortuous path to go from one side to the opposite side of the composite film. The diffusivity of liquid water can be expressed as:
Dcomposite 1 ¼ Dmatrix s
ð10Þ
where Dmatrix is the diffusion coefficient in the neat matrix, Dcomposite the diffusion coefficient in the nanocomposite, and s the tortuosity. A prediction of the liquid water
Fig. 1. Prediction of Bharadwaj’s model for the relative liquid water diffusivity as a function of MMT volume fraction compared to experimental data (S = 1 in all cases).
20 wt.%) in order to valid the predictions of Bharadwaj model. To allow this prediction, the true volume filler contents were evaluated through TGA measurements and were exactly 2.2 and 10.8 vol.%, respectively. Experimentally, a reduction of only a factor of 1.3 and 1.95 was respectively obtained for filler contents of 2.2 and 10.8 vol.% (Dmatrix = 5.8 1011 m2 s1, Dcomposite = 4.6 and 3.0 1011 m2 s1 for films filled with 2.2 and 10.8 vol.% of MMT particles), which was far lower that the predicted reduction (Fig. 1). It is worth noting that all WG-based nanocomposite films were transparent and displayed the same yellow–brown colour as the neat WG matrix, letting presuming that most of MMT nanoparticles were homogeneously dispersed within the WG matrix, as already reported by Angellier-Coussy et al. [12]. However, this discrepancy between predicted and experimental values underlines that the exfoliation of MMT particles was not complete, resulting in a particle size aspect ratio lower than the forecast value of 80. As a consequence, this study highlights the necessity to rigorously determine MMT particle size aspect ratio in order to predict transfer properties in nanocomposite films. The dispersion state of MMT particles within the WGbased matrix was evaluated by X-ray diffraction analysis and TEM observations coupled with image analysis. Fig. 2 displays the typical WAXS patterns recorded for pristine MMT, the neat WG matrix, and the WG/MMT nanocomposites elaborated with two different clay loadings (2.2 and 10.8 vol.%), for 2h angle values comprised between 3° and 25°. The diffraction pattern of pristine MMT was first characterised by a group of peaks around 20° corresponding to the crystallographic planes of the clay. Their presence is important since it demonstrated that the WAXS analysis was sufficiently sensitive to detect the presence of MMT in the nanocomposite. An intense diffraction peak was also noted at 2h = 7.7°, corresponding to a basal interlayer spacing value (d001) of 1.14 nm. The neat WG-based matrix displayed a typical amorphous structure characterised by two very broad peaks centred around 2h = 8° and 2h = 20°. After incorporating MMT particles, the d001 peak was no longer observed, leading to suppose that the clays were rather well exfoliated within the WG matrix. However, a small intensity peak characteristic of intercalated layered silicates could be masked by the WG peak centred around 8°. To deepen the structural characterization obtained by WAXS analysis, morphological parameters were determined using image analysis of TEM pictures (Table 1). Most of the clay sheets appeared well dispersed and globally oriented parallel to the film surface due to the pressure effect (i.e. 5 min at 150 bars) applied during the thermoforming process (Fig. 3), even if their alignment cannot be considered as perfect. We can thus reasonably consider that the nanoparticles are all orientated perpendicularly to the flux direction in the modelling approach (S = 1 in Eq. (11)). WG-based nanocomposites displayed a coexistence of individual platelets and non-fully exfoliated tactoïds, as confirmed by image analysis indicating an average number of platelets per particle of 4–5 whatever the filler content (Table 1). However, this value was probably over estimated since in the absence of a d001 peak characteristic of an
303
Fig. 2. WAXS patterns of the (a) MMT particles, (b) the neat WG-based matrix and WG-based films filled with (c) 2.2 and (d) 10.8 vol.% of MMT.
intercalated structure (Fig. 2), a d001 value of 1.14 nm (corresponding to the inter-lamellar distance of pristine MMT) was considered to calculate n in Eq. (6). For comparison, the average number of platelets per particle was also calculated in considering the maximum interlamellar distance detected by wide-angle X-ray analysis (2h = 3°) i.e. 2.94 nm as d001 value, leading to lower values of n (2–3 platelets per stack) (Table 1). Average values of size aspect ratio were 17–23 for 10.8 and 2.2 vol.%, respectively. This extent of exfoliation is similar than those obtained for synthetic polymer based nanocomposites such as nylon 6-MMT nanocomposite membranes [22]. exp ) Based on the experimental mean size aspect ratio (a calculated for the two filler contents investigated in exp ¼ 23) or (/ = 0.108; the present study (/ = 0.022; a a exp ¼ 17), a relative decrease in water diffusivity by factors of 1.25 and 1.9 was respectively predicted by Eq. (11). These predicted values were very closed to the experimental data obtained (respectively 1.3 and 1.95). We can conclude that the liquid water diffusivity within WG-based
MACROMOLECULAR NANOTECHNOLOGY
H. Angellier-Coussy et al. / European Polymer Journal 49 (2013) 299–306
304
H. Angellier-Coussy et al. / European Polymer Journal 49 (2013) 299–306
Table 1 Results of particle image analysis.
* **
Number of analysed particle (N)
Average particle length (l) (nm)
Average particle thickness (t) (nm)
Number average platelets per particle )* (n
Number average platelets per particle )** (n
Average aspect )*** ratio (a
2.2 10.8
278 308
79 ± 32 72 ± 30
4.7 ± 3.7 5.9 ± 4.3
4.2 ± 3.2 5.3 ± 3.8
2.3 ± 1.2 2.7 ± 1.5
23 ± 16 17 ± 12
Calculated using d001 = 1.14 nm, which corresponded to the interlamellar distance of pristine MMT. Calculated using d001 = 2.94 nm, which corresponded to the maximum interlamellar distance detected by wide-angle X-ray analysis (2h = 3°). P a ¼ ðL=tÞ=N.
MACROMOLECULAR NANOTECHNOLOGY
***
MMT content (vol.%)
Fig. 3. TEM micrographs of WG-based films filled with 2.2 and 10.8 vol.% of MMT.
nanocomposite materials can be well predicted using a simple tortuous path model (Fig. 1). It is worth noting that a detailed structural analysis using TEM analysis allowing the evaluation of the aspect ratio is required for each filler content since the dispersion state strongly depends on the filler content. It is well-known that TEM analysis has some inherent limitations due to the fact that it projects threedimensional (3D) objects onto a two-dimensional (2D) plane, resulting in TEM cross sections that do not necessarily represent the actual size of all the platelets [17]. Furthermore, an ideal image would exhibit sharp transitions from black to white, what is not the case in reality,
necessitating the conversion of the original gray-scale TEM micrograph into a black and white image using a semi-automated approach [17], this step being inevitably accompanied by errors. However, the present work highlights that even if image analysis of TEM pictures is marred with a certain uncertainty, it remains a good tool to estimate quantitatively the clay dispersion state. Results also indicate that the liquid water diffusivity could be decreased at the most by a factor of 2 (in case of a maximum 10.8 vol.% of MMT particles in the WG based network). With the aim of substituting petrochemical plastics by bio-based materials, similar barrier properties need to be achieved. This means that water permeability of agro-based films should be reduced by a factor of 10–100 if large applications in the food packaging sector are targeted. For example the water vapour permeability of WG based films for a 0–100% RH difference ranges from 5 1012 mol m1 s1 Pa1 to 6.2 1011 mol m1 s1 Pa1, depending on the film preparation process [23,24], whereas it is equal to 0.05 1012 mol m1 s1 Pa1 in case of synthetic films such as low-density polyethylene [25]. Based on the presented modelling approach, it can already be concluded that such a reduction factor is not feasible with this type of MMT nanoparticles within processable range of filler content. This underlines the utility of the modelling approach and leads to the conclusion that next researches must focus on the use of nanoparticles displaying higher size aspect ratio values (higher than 100 when dispersed within the polymer matrix) in order to sufficiently improve water barrier properties of WG based films. Another interest of the modelisation of structure/mass transfer properties relationships would be the possibility to estimate the mean size aspect ratio of MMT particles, as well as its polydispersity, based on the knowledge of both the experimental volume filler fraction and the liquid water diffusivity values and their respective uncertainty. Indeed, the quantitative determination of the size aspect ratio values of MMT particles dispersed within complex and sensitive agro-polymer based materials by TEM image analysis is often complex, time and cost consuming. One can wonder if only two estimations of water transport properties (one for the neat matrix and one for the nanocomposite) would be sufficient to estimate an apparent value of the mean size aspect ratio with a satisfactory standard deviation. The final objective would be to modelise other mass transfer properties, which would be more
H. Angellier-Coussy et al. / European Polymer Journal 49 (2013) 299–306
Frequence (%)
a
15
2.2 vol% = 21 Simulated
10
5
0 0
Frequence (%)
b
20
40
60
15
80
100
10.8 vol% = 17 Simulated
10
4. Conclusion As expected, the volume filler content, the size aspect ratio and the orientation of MMT particles are key parameters to modulate liquid water transport properties through a tortuosity effect in WG-based films. If the orientation is often subjected to the thermomechanical process used to prepare WG-based nanocomposite films, it is possible to play on both the filler content and the aspect ratio of MMT particles. Liquid water diffusivity values in nanocomposite materials were well predicted using a model based on the tortuosity effect, such as the Bharadwaj model. For this purpose, an accurate determination of both the true volume filler content and the size aspect ratio of MMT particles is required for each formulation. In agreement with the Bharadwaj model predictions, the liquid water diffusivity can only be decreased by a factor of 2 in our best case (10.8 vol.% of MMT particles) in spite of a well-exfoliated nanocomposite structure. It must be pointed out that this value of 10.8% MMT is completely unrealistic for further commercial application. Consequently, from a commercial point of view, we can conclude that for food packaging application, the decrease in water sensitivity achieved by using commonly used filler contents (about 2 vol.%) would be totally insufficient. Moreover, before commercial application safety issue raises due to the use of nanoparticles that may migrate from the food contact material into the food product. This is definitively out of the scope of this study but should be addressed in further work for any intended application of nanocomposite materials in the food packaging field. This study underlines the necessity to combine two strategies to efficiently improve water barrier properties of WGbased nanocomposite films: (i) the selection of layered silicates displaying high size aspect ratio values and (ii) the achievement of a totally exfoliated nanocomposite structure. Although TEM and X-ray diffraction provide means for observing nano-structures, the extremely complex morphology of this kind of materials has made detailed morphological studies time and cost consuming. This study highlights that the characterization of transport properties, and in the present case the evaluation of liquid water diffusivity, combined with a modelling approach can be a quite successful route in elucidating structural aspects, at least to evaluate apparent value of the mean size aspect ratio for the modelisation of further mass transfer properties. Acknowledgements
5
We thank Chantal Cazevieille from CRIC INSERM (Montpellier, France) for TEM analysis. 0 0
20
40
60
80
100
alpha Fig. 4. Predicted (curves) and experimental (bars) probability frequencies of the size aspect ratio values obtained for filler contents of (a) 2.2 vol.% and (b) 10.8 vol.%. Predicted values were obtained by using Eq. (11) and a Monte Carlo procedure with average values of Dcomposite/Dmatrix and / and their respective standard deviation as input parameters.
References [1] Guilbert S, Gontard N, Morel MH, Chalier P, Micard V, Redl A. Formation and properties of wheat gluten films and coatings. In: Gennadios A, editor. Protein-based films and coatings. CRC Press; 2002. p. 69–122. [2] Gontard N, Ring S. Edible wheat gluten film: influence of water content on glass transition temperature. J Agric Food Chem 1996;44(11):3474–8.
MACROMOLECULAR NANOTECHNOLOGY
complex or longer to characterise, for example the diffusivity of molecules with higher molecular weight than water. In the present study, the volume filler content and the liquid water diffusivity are characterised by standard deviation values that have to be taken into account when they are used as input parameters in Eq. (11) (/ = 0.0215 ± 0.0023 with Dcomposite/Dmatrix = 0.786 ± 0.033, and / = 0.1078 ± 0.0006 with Dcomposite/Dmatrix = 0.513 ± 0.104). The impact of uncertainty propagation on the distribution of size aspect ratio of MMT particles was studied for the two filler contents through Monte Carlo simulations, considering that both Dcomposite/Dmatrix ratios and volume filler contents were normally distributed with a variance r2x . The resulting simulated distributions are gathered with the experimental data on Fig. 4. In both cases, Gaussian-like curves were obtained for the simulated distributions with a narrow peak around a mean value that ¼ 26 matched very well with experimental one (a ¼ 19 (16–40) vs 23 for 2.2 vol.% of MMT particles and a (7–42) vs 17 for 10.8 vol.% of MMT particles).
305
MACROMOLECULAR NANOTECHNOLOGY
306
H. Angellier-Coussy et al. / European Polymer Journal 49 (2013) 299–306
[3] Pouplin M, Redl A, Gontard N. Glass transition of wheat gluten plasticized with water, glycerol, or sorbitol. J Agric Food Chem 1999;47(2):538–43. [4] Pommet M, Redl A, Guilbert S, Morel MH. Intrinsic influence of various plasticizers on functional properties and reactivity of wheat gluten thermoplastic materials. J Cereal Sci 2005;42(1):81–91. [5] Mangavel C, Barbot J, Gueguen J, Popineau Y. Molecular determinants of the influence of hydrophilic plasticizers on the mechanical properties of cast wheat gluten films. J Agric Food Chem 2003;51(5):1447–52. [6] Tunc S, Angellier H, Cahyana Y, Chalier P, Gontard N, Gastaldi E. Functional properties of wheat gluten/montmorillonite nanocomposite films processed by casting. J Membr Sci 2007;289(1–2):159–68. [7] Domenek S, Brendel L, Morel MH, Guilbert S. Swelling behaviour and structural characteristics of wheat gluten polypeptide films. Biomacromolecules 2004;5(3):1002–8. [8] Angellier-Coussy H, Gastaldi E, Gontard N, Guillard V. Influence of processing temperature on the water vapour transport properties of wheat gluten based agromaterials. Ind Crops Prod 2011;33(2): 457–61. [9] Giannelis EP. Polymer layered silicate nanocomposites. Adv Mater 1996;8(1):29–35. [10] Alexandre M, Dubois P. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater Sci Eng R Rep 2000;28(1–2):1–63. [11] Ray SS, Bousmina M. Biodegradable polymers and their layered silicate nano composites: in greening the 21st century materials world. Prog Mater Sci 2005;50(8):962–1079. [12] Angellier-Coussy H, Torres-Giner S, Morel MH, Gontard N, Gastaldi E. Functional properties of thermoformed wheat gluten/ montmorillonite materials with respect to formulation and processing conditions. J Appl Polym Sci 2008;107(1):487–96. [13] Nielsen LE. Models for the permeability of filled polymer systems. J Macromol Sci – Phys 1967;1:929–42.
[14] Choudalakis G, Gotsis AD. Permeability of polymer/clay nanocomposites: a review. Eur Polym J 2009;45(4):967–84. [15] Bharadwaj RK. Modeling the barrier properties of polymer-layered silicate nanocomposites. Macromolecules 2001;34(26):9189–92. [16] Skoog DA, West DM, Holler FJ. Les erreurs aléatoires dans les analyses Chimie Analytique. 7th ed. DeBoeck Université; 1997. p. 21–44. [17] Fornes TD, Paul DR. Modeling properties of nylon 6/clay nanocomposites using composite theories. Polymer 2003;44(17): 4993–5013. [18] Crank J. The mathematics of diffusion. New York (NY): Oxford University Press; 1980. [19] Gill EP, Murray W, Wright MH. Practical optimization. London: Academic Press; 1981. p. 401. [20] Penicaud C, Peyron S, Bohuon P, Gontard N, Guillard V. Ascorbic acid in food: development of a rapid analysis technique and application to diffusivity determination. Food Res Int 2010;43(3):838–847. [21] Hessler JP. The use of Monte Carlo simulations to evaluate kinetic data and analytic approximations. Int J Chem Kinet 1997;29(11): 803–17. [22] Picard E, Gerard JF, Espuche E. Water transport properties of polyamide 6 based nanocomposites prepared by melt blending: on the importance of the clay dispersion state on the water transport properties at high water activity. J Membr Sci 2008;313(1– 2):284–95. [23] Gontard N, Guilbert S, Cuq JL. Water and glycerol as plasticizers affect mechanical and water–vapor barrier properties of an edible wheat gluten film. J Food Sci 1993;58(1):206–11. [24] Pommet M, Redl A, Morel MH, Guilbert S. Study of wheat gluten plasticization with fatty acids. Polymer 2003;44(1):115–22. [25] Myers AW, Meyer JA, Rogers CE, Stannet V, Szwarc M. Studies in the gas and vapour permeability of plastic films and coated papers. Tappi J 1961;45:58–67.