Effect of Nanoclay Hydration on Barrier Properties ... - ACS Publications

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Effect of Nanoclay Hydration on Barrier Properties of PLA/ Montmorillonite Based Nanocomposites Nadine Tenn,†,‡,§ Nadège Follain,*,†,‡,§ Jérémie Soulestin,∥,⊥ Raphael̈ Crétois,†,‡,§ Serge Bourbigot,⊥,# and Stéphane Marais†,‡,§ †

Normandie Univ, France Université of Rouen, Laboratoire Polymères, Biopolymères et Surfaces, Bd. Maurice de Broglie, F-76821 Mont Saint Aignan Cedex, France § CNRS, UMR 6270 & FR 3038, F-76821 Mont Saint Aignan Cedex, France ∥ Department of Polymers and Composites Technology & Mechanical Engineering, Ecole des Mines de Douai, F-59508 Douai Cedex, France ⊥ Université Lille Nord de France, F-59000 Lille, France # UMET, CNRS UMR 8207, École Nationale Supérieure de Chimie de Lille, F-59652 Villeneuve d’Ascq, France ‡

ABSTRACT: Incorporating Cloisite 30B (C30B) in poly(lactic acid) (PLA) matrix was investigated as functions of the content and of the hydration state of nanoclays. Two series of PLA based nanocomposites were prepared by melt compounding using nanoclays in hydrated state (undried C30B) or preliminary dried (dried C30B). Their structure was characterized by transmission electron microscopy (TEM) observations and X-ray diffraction (XRD) measurements which highlighted the coexistence of exfoliated, intercalated, and aggregated structures. Rheological measurements put forward better degrees of dispersion and of exfoliation for nanocomposites containing undried C30B. From differential scanning calorimetry (DSC) measurements, a slight change in crystallinity was measured owing to the nucleating effect induced by the nanoclays. The transport properties were analyzed from permeation and sorption kinetics. A significant improvement of the water and oxygen barrier properties was obtained, especially for nanocomposites with undried C30B, while a reduction in diffusion was evidenced. This peculiar behavior was correlated to the presence of water molecules included in C30B contributing to a better dispersion and orientation of the nanoplatelets into the PLA matrix.

1. INTRODUCTION

glass, or paper).1 In this context, many investigations were

Appearing as an alternative to petroleum-based polymers, the biodegradable polymers and their industrial development have attracted much attention in recent years due to the emerging environmental concern for short-term applications such as food packaging. However, in addition to interesting properties inherent to biobased films like biodegradability and production from renewable resources, polymer films present barrier properties less effective than traditional materials (metal,

conducted for developing new food packaging materials with

© XXXX American Chemical Society

high performances based on biobased polymers2−4 without food safety and quality. Received: February 12, 2013 Revised: April 12, 2013

A

dx.doi.org/10.1021/jp401546t | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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al.24 An improvement of the mechanical properties with an enhancement of the crystallization, and a decrease of the oxygen permeability of about 10% to 65% were also observed by Ray et al.25 The authors explained this result as being due to the planar orientation taken by nanoclays induced by compression molding conditions.7,14,26,27 For increasing PLA/nanoclay interfacial regions, the two step extrusion master-batch processing was applied for preparing two series of PLA/C30B nanocomposites. The effects of nanoclay content and its hydration state on the quality of nanoclay dispersion into the PLA matrix were evaluated from usual physical characterizations. The present work is aimed at investigating transport properties of PLA based nanocomposites: water sorption, water permeation, and dioxygen permeation in presence of moisture were performed. A detailed evaluation of the permeation kinetics was proposed, and a simulation of experimental data based on a geometrical approach relative to the tortuosity concept was conducted. The water vapor sorption results were interpreted using the appropriate mathematical model and discussed from the analysis of the diffusion coefficients as functions of hydration state and of content of nanoclays to understand the PLA/ nanoclays interactions.

For the packaging industry, an essential prerequirement concerns the barrier properties exhibited by polymer based films. For that, one of the most relevant ways is to insert inorganic clays into the polymer matrix5−10 which lengthens by tortuosity the diffusion pathways taken by the diffusing molecules such as water or gas molecules, and hence reduces the material permeability9,11−13 and improves the shelf life of polymer based packages. Usually, the elaboration of polymer matrix/nanoclays nanocomposites is also associated with improvement of mechanical and thermal properties of polymer matrix7,14,15 even at low nanoparticle content. It is a general observation that the tortuosity effect induced by nanoclays is directly related to the dispersion state of the nanoclay platelets in the polymer matrix and by extension to the amount of matrix/nanoclay interfacial regions. This last point is relative to the degree of nanoclay delamination into the polymer matrix which is clearly impacted by the chemical modification in nanoclay surface16 for improving affinity with polymer, and by the processing conditions of nanocomposites.17,18 Also, different parameters such as the content and the aspect ratio exhibited by the dispersed nanoparticles, the nature of diffusing molecules and the free volume on material19 play a role in the nanoclay delamination. The coexistence of different nanocomposite structures is classically found: agglomerated, intercalated, and exfoliated structures.8,20 Besides, the hydration state of nanoclays before nanocomposite preparation should not be neglected because residual water molecules can act as plasticizer, promoting the nanoplatelet individualization, and therefore favoring an exfoliated structure within nanocomposite. In some cases, the changes in the crystallinity and the platelet orientation into the polymer matrix are also key factors that can affect the barrier performance of the nanocomposites.11,21,22 In the literature, processing conditions were suggested to prepare nanocomposite films with a peculiar barrier behavior: (i) for PA12/C30B nanocomposites,17 different levels of nanoclay dispersion were adjusted as a function of screw rotation speed; (ii) preprocessing by sonication of highly filled polyurethane nanocomposites with various content of Cloisite nanoclays was considered by Herrera-Alonso et al.23 conducting to an improvement of barrier properties which is explained by the high aspect ratio of clay entities (∼100); and (iii) a two step extrusion master-batch processing of PA6/C30B nanocomposites was selected by Alix et al.,18 a high exfoliation level of oriented nanoclays into PA6 matrix which contributes to a large reduction in nitrogen permeability and water permeability. Among biodegradable polymers which are subject to industrial development,1 the poly(lactic acid) (PLA) is considered because of its high transparency, excellent printability, low-temperature stability,1 and low toxicity.7 Besides, PLA was labeled as a GRAS material (Generally Recognized as Safe) by the FDA which ensures its application in food-contact items.1 To overcome its limited gas barrier performance preventing wider industrial packaging application,14 different types of inorganic clays were considered including layered silicates, carbon nanotubes, hydroxyapatite, aluminum hydroxide, and so forth.7 In most works concerning PLA based nanocomposites, the most commonly used nanosized filler is montmorillonite, being organomodified by cationic exchange for improving interfacial properties.15 Incorporating organomodified nanoparticles (Dellite 72T and Dellit E 43B) into PLA matrix contributed to generate partially intercalated and exfoliated structures, as reported by Feijoo et

2. EXPERIMENTAL SECTION 2.1. Materials. PLA pellets were composed of 4% D and 96% L (referenced as PLE 005 from NaturePlast (France)) and characterized by a melting temperature of about 150 °C and a glass transition temperature about 64 °C. Before nanocomposite preparation, the PLA pellets were dried at 80 °C in a vacuum oven for 12 h. Before incorporation into the PLA matrix, the organomodified montmorillonite clays, Cloisite 30B (C30B) supplied by Southern Clay Products (Rockwood Additives), were used either as-received or water-free by a preliminary drying step (80 °C in a vacuum oven for 12h). 2.2. Preparation of Nanocomposite Films. Two series of PLA/C30B nanocomposite films with various contents (0 up to 20 th. wt %) were prepared (Table 1). The first series noted Table 1. Number-Average Molecular Weight (Mn) and Weight-Average Molecular Weight (Mw) of the PLA Matrix and the PLA/C30B Nanocomposites samples

Mw (g/mol)

Mn (g/mol)

PLA PLA/5%C30B-d PLA/10%C30B-d PLA/15%C30B-d PLA/20%C30B-d PLA/5%C30B-h PLA/10%C30B-h PLA/15%C30B-h PLA/20%C30B-h

65 000 38 400 43 200 39 600 33 000 50 000 30 200 25 300 24 800

32 400 18 900 18 800 18 800 14 400 25 000 14 100 8900 10 100

PLA/C30B-h was prepared using C30B as received (i.e., in partially hydrated state), and the second series noted PLA/ C30B-d with C30B dried before incorporation. Based on the two step extrusion master-batch processing, the first step consisted in preparing the master-batch containing PLA with 20 th. wt % of C30B by melt compounding using a Haake polylab OS-rheodrive 7 twin screw extruder (rotation speed = 150 rpm, flow rate = 0.56 kg·h−1). A temperature profile between 210 and 170 °C was applied from the feed zone to the die zone of B



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extruder barrel. The second step of processing concerned the dilution of the master-batch in order to obtain nanocomposites with different nanoclay contents. Extrudated under rod form, the nanocomposites were therefore granulated into pellets. Finally, for obtaining nanocomposite films with 150−200 μm controlled thickness, the pellets were compression molded by using a melt press (SCAMEX, France) as follows: 10 min at 185 °C under atmospheric pressure with 20 min at 185 °C under 50 bar. The unfilled PLA film was obtained using a similar processing for overcoming the effect of melt blending conditions. Before experimental measurements, the nanocomposite films were stored under vacuum over P2O5 powder at 25 °C. 2.3. Structure Characterization. 2.3.1. GPC Measurements. GPC measurements were performed using a Varian PLGPC 50 Plus apparatus with a refractometer as detection system and a PL gel column (porosity of 5 μm) at 300 K. Dichloromethane was used as the mobile phase with a solvent flow rate of 1 mL·min−1. After eliminating nanoclays by Soxhlet extraction and filtration of samples dissolved in solvent, the elution profiles were recorded. The average molecular weights were calculated by means of a calibration curve obtained with polystyrene standards range from 2360 to 143 400 g·mol−1. 2.3.2. Rheological Measurements. Oscillatory shear rheological analyses were carried out with a rotational rheometer (Advanced Rheometrics Expansion System (MARS III, ThermoFisher Scientific)) using a plate and plate geometry (35 mm diameter and 1 mm thickness) at 180 °C. Frequency sweeps with an angular velocity (ω) between 0.1 and 100 rad·s−1 were performed in the linear viscoelastic regime at strain of 0.1%. Samples were equilibrated during 10 min before measurement. The complex viscosity η* curves were fitted between 0.1 and 1 rad·s−1 using a power law to calculate the n factor reflecting the degree of nanoclay dispersion into a material. 2.3.3. X-ray Diffraction (XRD). XRD analysis was carried out with an X-ray diffraction D8 Advance diffractometer from Bruker with a Cobalt source (λ = 1.78897 Å) operating at 35 kV and 40 mA. The 2θ diffraction diagrams were recorded at 25 °C in the 2°−10° range. 2.3.4. Transmission Electronic Microscopy (TEM). The nanocomposite films were ultramicrotomed with a diamond knife on a Leica ultracut UCT microtome at room temperature to give section thickness of 70 nm. Sections were transferred to Cu grids of 200 mesh. Bright-field TEM images of nanocomposites were obtained at 300 kV under low dose conditions with a FEI Technai G2 20 electron microscope using an Orius Gatan CCD camera. The films were sampled by taking several images of various magnifications over 2−3 sections per grid to ensure that the TEM observation was based on a representative region of the sample. 2.3.5. Differential Scanning Calorimetry (DSC). The film samples (stored in vacuum over P2O5 during at least 2 weeks for eliminating moisture sorption effects) were encapsulated in standard DSC aluminum alloy pan. After calibration,29 the DSC measurements were carried out with a TA Instrument apparatus (DSC 2920) using an oscillation amplitude of 0.318 °C and an oscillation period of 60 s at a heating rate of 2 °C·min−1. Nitrogen was used as drying gas (70 mL·min−1). The crystallinity index (Xc) was determined from the following equation:

χc =

ΔH × 100 ΔHm° (1 − φ)

(1)

where ϕ is the C30B mass fraction, ΔH = ΣΔHm − ΣΔHc is the difference between the melting enthalpy and the crystallization enthalpy, and ΔH°m is the theoretical value of the melting enthalpy of a 100% crystalline PLA (93 J·g−1).30 2.4. Transport Properties. 2.4.1. Water Permeation Measurements. The water permeation measurements were carried out at controlled temperature (25 °C) by using a homebuilt apparatus, as previously detailed.31 The water permeation flux noted J was recorded at the dry interface of the tested sample, as previously reported.16,32,33 The permeability coefficient noted P was deduced from the stationary flux Jst by P=

Jst L Δp

(2)

where L is the thickness of the nanocomposite film and Δp the pressure difference across the film. P is usually expressed in Barrer units (1 barrer =10−10 cm3·(STP)·cm·cm−2·s−1·cmHg−1). The diffusion coefficient D was determined from the transient regime of the water permeation flux by assuming that its value was constant during the permeation course. Taking into account a Fickian behavior, two distinct diffusion coefficients noted DI (= L2 × 0.091/tI) and DL (= L2/6tL) were calculated at two different time of the permeation process (designed as tI and tL) corresponding to the inflection point I (jI = 0.24, tI = 0.091) and to the time-lag point L (jL = 0.24, tL = 0.091) of the theoretical flux curve,34 respectively, as detailed in the literature.16,32 2.4.2. Water Sorption Measurements. 2.4.2.1. Water Vapor Sorption. The water vapor sorption measurements were performed using an automatic gravimetric dynamic vapor sorption system (Surface Measurement System, U.K.) equipped with an electronic Cahn D200 microbalance (mass resolution of 0.1 μg) and enclosed in a temperature controller (25 °C), as previously detailed.21 The equilibrium water mass gain noted ΔMeq was calculated from eq 3 and expressed in percent of grams of sorbed water per gram of film. Thereafter, the relationship between the equilibrium water mass gain and the applied water activity was described through a sorption isotherm. meq − md ΔMeq = × 100 md (3) where meq is the mass of the sample at equilibrium state and md is the dry mass of the sample. Regarding the diffusion coefficient D calculated from the analysis of sorption kinetics, the Fick diffusion laws which are quantitative mathematical equations were applied to interpretate the sorbed water molecules−sample interactions. Two diffusion coefficients (D1 and D2) were calculated considering the time required for reaching the equilibrium water mass gain. For short time, that corresponds to the first-half sorption, the diffusion coefficient noted D1 was determined for the water mass gain lower than 50%, while for longer time, that is the second-half sorption, the diffusion coefficient noted D2 was obtained for the mass gain higher than 50%.21 2.4.2.2. Liquid Water Sorption. The liquid water sorption measurements were carried out at 25 °C by immersing the sample (preliminary dried) in distilled water which was C



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regularly weighted after being blotted dry with a lint-free paper. From sorption kinetics, the water concentration at equilibrium state was then calculated by meq Ceq = ρ m 0 M H 2O m (4) where Ceq is the water concentration at equilibrium state (mmol·cm−3), meq is the water mass at equilibrium state (mg), m0 is the sample mass at dry state (mg), MH2O is the molar mass of water, and ρm is the density of PLA (1.25 g·cm−3). 2.4.3. Oxygen Permeation Measurements. An Oxtran Model 2/21 (Lippke, Germany) equipped with a coulometric sensor was used for measuring the oxygen permeability of films at 23 °C. From the oxygen transmission rate detected by the sensor, the oxygen permeability coefficient was calculated: PO2 =

OTR × L = 0.1375 × OTR × L Δp

(5)

where PO2 is the permeability (Barrer) and Δp is the difference in oxygen pressure between both sides of the tested film and OTR the oxygen transmission rate (cm3·m−2·day−1). The permeation cell consists of two compartments separated by the film with a surface of 5 cm2 exposed to gas flux. Initially, the system was flushed with a carrier gas (N2/H2 mixture (95%/5%), Air Liquide) for removing residual water. The oxygen gas (99.99% of purity, Air Liquide) was introduced in the upstream compartment. All measurements were at least duplicated and performed under atmospheric pressure. The impact of four relative humidities (0, 50, 75, and 90% RH) on the oxygen permeability exhibited by the nanocomposite films was determined by using a humidifier system enclosed in the permeation device.

Figure 1. Diffractograms obtained for Cloisite 30B (C30B) nanoclay: (a) the PLA/C30B-d nanocomposites and (b) the PLA/C30B-h nanocomposites.

can observe that the intensity of this diffraction peak increases with the increase of the C30B content, as noted by other authors.6,32 Finally, the XRD measurements reveal the coexistence of intercalated and aggregated structures in both series. The hydration state of nanoclays (C30B-d and C30B-h) seems to not affect the dispersion of nanofillers in the PLA matrix. This point will be thereafter discussed under TEM and rheological considerations. Additional support for evaluating the degree of dispersion of nanoclays into the PLA matrix is based on the TEM observations. The TEM images taken from a representative region of the nanocomposite films are presented in Figure 2. One can indicate (i) a change in the dispersion state of the C30B into the PLA matrix as a function of C30B content and (ii) a dispersion impacted by the hydratation state of the C30B. The PLA/5%C30B-d nanocomposite presents an aggregated structure with a random dispersion of small tactoids (Figure 2a), whereas the coexistence of aggregated, intercalated, and exfoliated structures is exhibited by the PLA/5%C30B-h nanocomposite (Figure 2b). At higher nanoclay content, an exfoliated structure with small tactoids reflecting a higher homogeneity in the nanoclay dispersion is observed for the PLA/10%C30B-d nanocomposite (Figure 2c). When incorporating hydrated C30B, a better intercalation of the C30B platelets is noted (Figure 2d). Besides, a certain alignment of the nanoplatelets parallel to the film surface (in the flow direction18,35) can be observed because favored by the two step extrusion master-batch processing. This orientation is induced by shearing forces generated during the

3. RESULTS AND DISCUSSION 3.1. Dispersion and Structure of PLA/C30B Nanocomposites. The determination of the dispersion state of the C30B in the PLA matrix is essential to analyze the properties exhibited by the nanocomposites. For that, the observation by transmission electronic microscopy (TEM) and the X-ray diffraction (XRD) measurements were performed on the PLA matrix and its nanocomposites. Concerning the XRD measurements, the evolution of the basal spacing d001 evidences the dispersion state of the C30B in the PLA matrix. The intercalation of the nanoclay platelets by the polymer chains induces an increase of the interlayer distance which shifts the silicate diffraction peak to lower 2θ angles. The diffraction peak of the C30B is centered on 2θ = 5.6° (Figure 1), corresponding to an inter-reticular distance of 1.8 nm according to the Bragg relation. Concerning both PLA/C30B nanocomposites (Figure 1), two diffraction peaks are observed with only a slight variation in the inter-reticular distances due certainly to the presence of residual water molecules inside nanoclays for PLA/C30B-h nanocomposites: the first peak is centered on 2θ = 5.9°, found to be close to the peak characteristic of C30B, and the second one on 2θ = 3°. The first peak conforms to the formation of an aggregated structure in the nanocomposites by virtue of a lack of compatibility between nanofillers and PLA polymer, as recently reported by Rhim et al.12 The second peak located at 2θ = 3° (corresponding to an inter-reticular distance of 3.5 nm) evidences the intercalation of nanoclays in the PLA matrix. One D



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Figure 2. TEM images of PLA/C30B nanocomposites: (a) PLA/5%C30B-d, (b) PLA/5%C30B-h, (c) PLA/10%C30B-d, (d) PLA/10%C30B-h, (e) PLA/15%C30B-d, (f) PLA/15%C30B-h, (g) PLA/20%C30B-d, and (h) PLA/20%C30B-h.

extrusion processing18 and clearly evidenced by the TEM image of the PLA/15%C30B-h nanocomposite (Figure 2f), whereas it is not as obvious for the PLA/15%C30B-d nanocomposite (Figure 2e). At the highest nanoclay content (20 th. wt %), an aggregated structure is again obtained (Figure 2h), and even more by incorporating dried C30B (Figure 2g), suggesting that at this high content the dispersion is less accomplished. This is consistent with a lack of space in bulk PLA volume for

achieving the nanoclay delamination, as reported in the literature.6,9,22,36 The implementation of these structures is attributed to the hydrogen bond interactions between the hydroxyl groups of the C30B surfactant with the carbonyl groups of PLA chains.32,37 For PLA/C30B-h nanocomposites, it can be noted that the nanoclay orientation is still present for the nanoclay content range tested, which is not the case for PLA/C30B-d nanocomposite (Figure 2h). The nanoclay E



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within the interlayer spacing of undried C30B contributing to enlarge the gap between nanoplatelets and probably facilitating the PLA chain intercalation and the nanoclay delamination. For evaluating the impact of nanoclays on the PLA polymer backbone during the melt processing, GPC measurements were performed. From Table 1, one can see a continuous decrease of the average molecular weights of PLA chains with respect to its initial value. This result indicates an alteration of the PLA chain lengths with the increase in C30B content. Such a behavior is not really surprising, since a decrease in polymer Mw and Mn values is generally observed after a melt processing41 relative to its inherent mechanical steps. It is a general agreement that the degradation of the polyester chains via hydrolysis reactions takes place preferentially in the amorphous region of the matrix,42 contributing to an expected increase in the crystallinity of PLA polymer, as observed from DSC results. Moreover, it is noteworthy that the most significant degradation in terms of molecular weight decrease is assigned to the PLA/C30B-h nanocomposites, revealing the role played by water molecules included in C30B structure able to react with ester groups along the PLA chains.43,44 3.2. Thermal Properties of PLA/C30B Nanocomposites. Considering the nucleating effects induced by nanoclays45 which change the crystallinity of polymer films, the impact of the nanoclay incorporation into the PLA matrix was investigated using DSC measurements. In fact, the crystalline fraction of a material acting as physical barrier to the diffusing molecules could contribute to affect the transport properties. The crystalline structure of the PLA polymer is slightly modified with the incorporation of C30B, as attested by Figure 4. As a first insight, the presence of an endothermic peak at around 56 °C overlapping the heat flow signal assigned to the glass transition temperature can be observed. This endothermic peak is related to the physical aging effect of the processed nanocomposites owing to the structural relaxation of the polymer chains. This phenomenon originates from the polymer molecular motions that rearrange the polymer chains in a more favorable energetic configuration.46 One can consider that the storage of film samples is at the origin of these relaxation phenomena.32 Colomines et al.47 also observed a relaxation peak during a first heating cycle of DSC protocol which disappears during a second heating cycle; they attributed this result to the change in polymer structure order. Because of the overlapping of the signal assigned to the glass transition of PLA polymer, the change in its transition induced by the C30B nanoclays is hardly determined. In fact, the incorporation of nanofillers in a polymer matrix usually causes an increase in the glass transition temperature explained by a confinement effect which reduces the macromolecular chain mobility. Nevertheless, several studies showing a decrease of the glass transition temperature has been reported in the literature resulting from either the plasticization effect of the nanoclay surfactant10,14,32,37 or the degradation effect generated by the nanofillers during the melt processing.22 For all the films, an exothermic crystallization peak was observed at around 96 °C (Figure 4) associated with a double melting endotherm with two well-defined melting peaks at around 140 and 150 °C. These two endothermic peaks are attributed to the melting of different crystalline forms of PLA polymer which differ in terms of size, crystallite layer thickness, and degree of perfection. The higher the melting temperature

orientation and the presence of more or less associated or aggregated nanoplatelets are highlighted according to the nanocomposite formulation considered. In terms of barrier properties, one cannot neglect the barrier behavior of these associated or aggregated nanoplatelets which can complicate the diffusion pathways taken by the diffusing molecules in generating more tortuosity. Rheological measurements were performed on PLA/C30B nanocomposites for evaluating the nanoplatelet dispersion and exfoliation degrees at macroscopic scale. It is well-admitted that the rheological behavior of a polymer material is significantly influenced by the nanoclay exfoliation in the low frequency region: it is therefore possible to obtain a semiquantitative evaluation of this dispersion. The quantification derived from the measurement of the slope of the complex viscosity η*/ angular velocity ω curves in the low frequency region38−40 which were fitted using a power law as follows:

η* = Aωn

(6)

The n factor is considered as an indicator of the nanoclay dispersion degree. Without nanoclay platelets, the n factor tends to 0 reflecting a constant viscosity of the material conforms to a Newtonian plateau. With nanoclays, the n factor increases and especially when the exfoliation degree is improved. At same nanoclay content, a higher n factor indicates a better nanoclay exfoliation into the polymer matrix. The evolution of the n factor as a function of the nanoclay content is presented in Figure 3. The PLA/C30B-h nanocomposites are

Figure 3. Evolution of n factor as a function of nanoclay content for PLA/C30B-h nanocomposites and PLA/C30B-d nanocomposites (the lines serve to guide the eyes).

characterized by n factors higher than the ones for PLA/C30Bd nanocomposites, whatever the nanoclay content is. From a macroscopic point of view, this result is attributed to a better dispersion level and a higher exfoliation degree. Moreover, it is noteworthy that the evolution of the n factor versus nanoclay content is different for the two series of nanocomposites. For the PLA/C30B-d nanocomposites, increasing nanoclay content tends to improve the dispersion of nanoclay platelets within polymer matrix, whereas for the PLA/C30B-h nanocomposites the n factor tends to decrease, revealing a noticeable decrease in the nanoclay dispersion and exfoliation degrees. These results confirm the conclusion stated from XRD measurements and TEM observations: the incorporation of undried C30B into the PLA matrix reduces the formation of aggregated platelets. This could be explained by the presence of water molecules included F



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Figure 4. Thermograms of (a) the PLA/C30B-d nanocomposites and (b) the PLA/C30B-h nanocomposites.

temperatures. The incorporation of nanoplatelets into the PLA matrix thus causes the formation of crystallite defects. Comparing now both series, one can estimate that the incorporation of undried C30B into the PLA matrix (PLA/ C30B-h nanocomposites) causes a more pronounced impact on thermal properties of PLA in comparison with the incorporation of dried C30B into the PLA matrix (PLA/C30B-d nanocomposites). The DSC thermograms of PLA/C30B-h nanocomposites (Figure 4b) exhibit a constant decrease of crystallization and melting temperatures, whatever the filler content used. That is not the case for PLA/C30B-d nanocomposite serie (Figure 4a) which is characterized by an effect reversed according to the nanoclay content (i.e., >15 th. wt %): at high nanoclay content, an increase of the temperatures associated with crystallization and melting phenomena is measured (Figure 4a). One can consider that the nanofiller-induced nucleating effect is hindered, which lowers the crystallization rate, and therefore a more ordered crystalline fraction is obtained, as just above-mentioned. This difference can be explained by the contribution of water molecules present in undried nanoclays which improves the dispersion of C30B in the PLA matrix: a better delamination and orientation of C30B was observed, as highlighted by TEM

is, the higher the degree of perfection of the crystalline structure is. Concerning the nanocomposites, the temperatures assigned to the endothermic and exothermic peaks are shifted to lower values after C30B incorporation with a variation in the peak intensity. It is worth noting that the peak magnitude is relative to the amount of the organic phase, that is, the PLA matrix. In that way, the decrease of the crystallization and melting peak intensities results from the increase of the inorganic nanofiller content in the nanocomposites. The decrease of the crystallization temperature may be attributed to the nucleating effect of the C30B in the PLA matrix which increases the crystallization rate of the ordered structures present in the nanocomposites and induces less perfect crystallites. Similar results have been recently reported by Fukushima et al.6 for two types of nanocomposites based on PLA/5%C30B (that is Cloisite 30B) and PLA/5%NAN804 (that is Nanofil 804). In comparison with the PLA matrix, the decrease of the two melting temperatures for the nanocomposites indicates an easier melting of crystalline phases arising from the nucleating effect of the C30B. Indeed, the increase of the crystallinity rate due to this nanofiller-induced nucleating effect would lead to the formation of less ordered crystals melting at lower G



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corresponding to the difference between the melting enthalpy and the crystallization enthalpy). An increase of the crystallinity degree is observed with the incorporation of C30B up to 15 th. wt % into the PLA matrix. This effect is caused by the nucleating effect of the nanofillers which generates a faster crystallization of polymer crystalline fraction and can be also attributed to the alteration of the PLA chains, in a lesser extent. Conversely, at higher content, the crystallinity degree is decreased. This result is clearly consistent with the disorder induced by C30B and especially by the presence of more associated or aggregated nanoplatelets into the PLA matrix due to a lack of space in bulk PLA polymer volume for obtaining the nanoclay delamination. This phenomenon has been also observed by Gamez-Perez et al.22 with PLA/OMMT nanocomposites: the crystallinity degree is found to be increased with 0.5% of OMMT and decreased with 2.5% of OMMT. Some authors detected an increase of the crystallinity when increasing the nanoclay content,45,48 while others considered that the incorporation of nanoclays induces very few effects on the crystallinity degree; in this latter case, the crystalline structure can only be affected in terms of layer thickness and size.9,10 According to Wu and Wu,36 the crystallinity degree is significantly decreased with the incorporation of nanoclays in the PLA matrix as a result of an increase in the disorder and not owing to a reduction in the crystalline phase fraction. For Zaidi et al.,15 the decrease in the crystallinity as a function of nanoclay content for PLA/C30B systems is due to the reduction of the macromolecular chain arrangement with the increase of the filler content.

observations. For the second series containing dried C30B, at high nanoclay content, more associated nanoplatelets within the nanocomposites are observed (Figure 2) which lead to a more organized crystalline structure and therefore impact the resulting thermal temperatures. The presence of more ordered crystallites shifts the thermal phenomena to higher temperatures. For the highest nanoclay content, the limited dispersion of nanofillers has to compact the structure by hindering the exfoliation of C30B and has thus to reduce its nucleating effect. The partial aggregation of nanoclays (evidenced by XRD measurements) induces more ordered crystalline structures (decrease of less perfect crystalline structures) which melt at higher temperatures (Figure 4). Concerning the crystallinity degree, the values calculated from eq 1 are presented in Table 2 (with ΔH values Table 2. Crystallinity Degree (χc) and ΔH (expressed in J·g−1) for the PLA Matrix and the PLA/C30B Nanocomposites samples

ΔH (J·g‑1)

χc (%)


2.5 3.3 5.2 6.2 3.9 3.0 6.3 7.8 3.2

2.7 3.7 6.2 7.9 5.1 3.5 7.9 10.7 4.6

Table 3. Water Permeation Parameters for the PLA Matrix and the PLA/C30B-d and PLA/C30B-h Nanocomposites

PLA

PLA/5%C30B -d

PLA/10%C30B-d

PLA/15%C30B-d

PLA/20%C30B-d

PLA/5%C30B-h

PLA/10%C30B-h

PLA/15%C30B-h

PLA/20%C30B-h

a

sample no.

P (Barrera)

1 2 avg 1 2 avg 1 2 avg 1 2 avg 1 2 avg 1 2 avg 1 2 avg 1 2 avg 1 2 avg

1957 1956 1957 ± 1 1760 1604 1682 ± 110 1224 1221 1223 ± 2 593 595 594 ± 1 685 684 683 ± 1 1193 1166 1180 ± 19 331 377 354 ± 33 99 97 98 ± 1 277 321 299 ± 31

D0 × 1010 (cm2·s‑1)

DI × 1010 (cm2·s‑1)

DL × 1010 (cm2·s‑1)

⟨D⟩ × 1010 (cm2·s‑1)

36.0 35.5

132 163

190 242

384 504

3.7 4.1

6.6 10.0

36.3 38.9

98 87

134 114

258 209

3.2 2.8

4.3 3.4

16.6 24.8

84 62

130 83

266 156

4.2 3.0

8.5 3.6

39.3 18.1

41 64

42 90

44 183

0.2 3.6

0.2 8.9

10.8 33.0

50 47

68 57

154 82

4.1 1.6

8.7 1.8

36 22

112 90

145 136

227 270

2.1 3.9

3.8 8.5

25

41 70

48 72

83

2.1 −1.1

5.0 −13

12 16

23 20

30 22

52 31

2.4 1.2

12.0 3.6

10 2

41 6

58 8

123 15

3.9 3.2

16.3 1.4

γCeq

γ (cm3·mmol‑1)

IF (%)

14

38

70

65

40

82

95

85

1 Barrer = 1010 cm3(STP)·cm·cm−2·s−1·cmHg−1. H



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Figure 5. Variation of the reduced flux (JL) as a function of the reduced time (tL−2) for the PLA matrix, for (a) the PLA/C30B-d nanocomposites and (b) the PLA/C30B-h nanocomposites.

checked from two specimens per material: the PLA matrix and its resulting nanocomposites. To compare all water permeation curves by overcoming the thickness effect, the JL versus tL−2 curve is reported (Figure 5) corrersponding to the variation of the reduced water permeation fluxes as a function of the reduced time scale. Concerning the PLA/C30B-d nanocomposites (Figure 5a), with the increase of the nanoclay content, a decrease of the water permeability P (deduced from the steady state of the permeation curve) associated with an increase of the delay time of the diffusion is observed. This barrier effect is clearly linked to the tortuosity effect which increases the diffusion pathway of the water molecules in the PLA matrix.9,12 This decrease of the water permeability is combined with a moisture resistance found to be increased to 70% for 15% mass fraction of C30B. Surprisingly, for nanoclay content higher than 15 th. wt %, the tendency is slightly reversed. A slight increase of the water permeability (7%) and also the enhanced diffusivity are shown in comparison with the PLA/15%C30B-d nanocomposite. This

Eventually, by taking into account the error measurements, one can estimate that the nanoclay-induced nucleating effect contributes to the increase in the crystallinity degree for both series of PLA/C30B nanocomposites. And, at high nanoclay content, this effect is hindered due to the limitation in nanoclay dispersion. 3.3. Transport Properties of PLA/C30B Nanocomposites. Several works studied the effect of the incorporation of nanoparticles in the PLA matrix on the transport properties. In most cases, the improvement of the barrier properties is related to the tortuosity effect created by the presence of nanoclay platelets in the polymer matrix. These barrier properties depend mainly on the dispersion state of the nanoclay platelets and their orientation in the polymer matrix. 3.3.1. Water Permeation. 3.3.1.1. Effects on Water Permeation Parameters. To study the influence of the C30B nanoclays on the water transport properties of the PLA matrix, the water permeation parameters have been determined and given in Table 3. The reproducibility of measurements has been I



Article

Concerning the plasticization phenomenon, at this stage of discussion, it is difficult to determine a tendency from γCeq or γ coefficients. Concerning the PLA/C30B-h nanocomposites (prepared with undried C30B nanoclays), a strong decrease of the stationary flux with the increase of the C30B content is obtained, as shown in Figure 5b. Both series of PLA based nanocomposites exhibit the same water behavior: reduction of the water permeability with a decrease of the water diffusivity. Besides, with increasing the nanofiller content, a decrease in the water diffusion coefficients (that are D0, DI, DL, and ⟨D⟩) is obtained knowing that the plasticization effect induced by water still occurs in all nanocomposites (Table 3). For the highest C30B content, an increase in the water permeability is also obtained. As observed for the PLA/20%C30B-d nanocomposite, despite the tortuosity effect induced by nanoclays, the occurrence of the percolation phenomenon could be at the origin of the increase of permaebility. Nevertheless, although the barrier properties were improved compared with the unfilled PLA matrix, the reduction of the PLA chain length cannot be neglected since a slight increase of water permeability is measured in the case of the highest loaded nanocomposites (Table 3) displaying the lowest molecular weights (Table 1). Indeed, it is noteworthy that the highest reincrease of permeability is obtained for the PLA/20%C30B-h nanocomposite characterized by the highest reduction of molecular weight. By comparing now both series of nanocomposites containing similar nanoclay contents, it can be noticed that a better barrier improvement is obtained for PLA/C30B-h nanocomposites: the best improvement factor (IF) is found to be 95% with the PLA/15%C30B-h nanocomposite (Table 3). Despite this very significant water barrier effect, the plasticization phenomenon exerted by sorbed water is still present since the diffusivity is maintained not constant (the value of DL is higher than that of DI). The difference in the barrier properties between the two series of nanocomposites would be relative to the quality of dispersion of the nanoclay platelets and their orientation within the PLA matrix. Indeed, the degree of exfoliation of undried C30B can be estimated to be higher in the PLA matrix than that in the nanocomposites prepared with dried C30B, as shown by TEM observations. The presence of water molecules in nanoclays would enhance their individualization. Further, an orientation of the C30B platelets into the PLA matrix is highlighted from the TEM images (Figure 2), and this orientation is estimated more pronounced in the longitudinal direction of the extrusion processing of nanocomposites with undried C30B platelets compared with dried C30B platelets. In other words, the nanoplatelets are dispersed and oriented in the direction perpendicular to the water flux. In view of the water permeation results, one can consider that the combination of the dispersion state with the orientation of the C30B platelets leads to a high improvement of the barrier properties. A similar tendency has also been reported by Thellen et al.9 suggesting that the improvement of the barrier properties depends mainly on the effect of the dispersion state, the aspect ratio, and the orientation of nanoclays. 3.3.1.2. Modeling Using Geometrical Approach. Although the barrier effect in nanocomposite is generally attributed to the tortuosity phenomenon, other factors affecting the barrier properties cannot be neglected. These factors can be evidenced through the evolution of the relative permeability as a function

difference in water permeation behavior was also observed in the work on PLA12/montmorillonite nanocomposites carried out by Alexandre et al.17 The authors considered that the barrier effect is caused essentially by the tortuosity effect; they explained the rise in water permeability by the presence of specific interactions between water molecules and the nanocomposite with high nanoclay content, in particular with the C30B nanoclays which exhibit a hydrophilic character in spite of the presence of the surfactant (hydrophobic nature). These interactions can lead to the percolation phenomenon that occurs usually in highly loaded nanocomposites.17,28 This threshold phenomenon appears with the formation of nanoclay network or with a cluster of structures created by aggregation. This network contributes to an enhancement of the diffusion of the water molecules by taking a preferential diffusing pathway in the polymer matrix.17,28 Despite the fact that the water permeability of the PLA/20%C30B-d nanocomposite again increases, the improvement of the water barrier property remains significant of about 65%. According to Thellen et al.,9 the large reduction (50%) of the water permeability obtained for PLA/MLS 25A materials (that is montmorillonite layered silicate) is due to the presence of water clusters in nanocomposites that reduce the water diffusivity. Rhim et al.12 showed the reduction of the water vapor permeability of 6% and 36% for PLA/Cloisite 30B and PLA/Cloisite 20A nanocomposites, respectively. They explained this difference in permeability by the higher hydrophobic character of Cloisite 20A and its better compatibility with PLA matrix, in comparison with Cloisite 30B. In terms of water diffusivity, whatever the nanocomposite film is, the value of DL is higher than that of DI (Table 3), which means that the diffusivity is not constant but increases with the water concentration during the permeation process.49 This concentration-dependence diffusion coefficient is usually attributed to the plasticization effect of water50 and can be described by the well-known exponential law: D = D0 expγC

(7)

where D0 is the limit diffusion coefficient at nil concentration, γ is the plasticization factor, and C is the local concentration of sorbed water. In our case, all permeation curves have been well fitted by using this exponential law of D reflecting the dependence of diffusivity with the sorbed water concentration (not shown here). It was then possible to deduce the plasticization coefficient γCeq (for the water concentration Ceq taken from the stationary state), γ, D0, and the mean integral diffusion coefficient ⟨D⟩.17 Permeation parameters gathered in Table 3 show a noticeable decrease of the water diffusivity (D0, DI, DL, and ⟨D⟩) when the filler content increases. In addition to the tortuosity effects, the permeation results agreed well with the change in crystallinity. Indeed, with the C30B content, the variation of the permeability and diffusion coefficients is similar to that of the amorphous fraction in the resulting nanocomposites. Nevertheless, although the crystallinity degree is not a predominant factor in the barrier properties because of its low variation with the nanoclay content, we must keep in mind that it contributes to the tortuosity effect since the crystalline phase is estimated to behave as physical barrier. For the nanocomposite with the highest C30B content (PLA/20% C30B-d nanocomposite), a percolation phenomenon due to the aggregated structure of nanoclays is estimated to be the relevant factor for explaining the reincrease of the permability. J



Article

Table 4. Orientation Factor Values Calculated for the Aspect Ratio Values Resulting from the Two Dispersion States optimal exfoliation

moderate exfoliation

nanocomposites

αexp

ϕNa+

oNa+

RSS

ϕC30B

oC30B

RSS

PLA/5%C30B-d PLA/10%C30B-d PLA/15%C30B-d PLA/20%C30B-d PLA/5%C30B-h PLA/10%C30B-h PLA/15%C30B-h PLA/20%C30B-h

62 71 65 72 77 103 105 124

0.016 0.034 0.052 0.070 0.016 0.034 0.052 0.070

0.0 0.2 1.0 0.5 1.0 0.9 1.0 0.3

0.0004 0.00002 0.0026 0.00001 0.009 0.00007 0.00004 0.000002

0.032 0.066 0.100 0.136 0.032 0.066 0.100 0.136

0.0 0.0 0.4 0.0 0.6 0.2 0.2 0.0

0.018 0.097 0.000001 0.0005 0.00001 0.0002 0.000004 0.0030

perpendicular to the flux direction, Table 5). From data gathered in Table 4, for a given aspect ratio αexp, the value of o

of C30B volume fraction and its modeling from geometrical approaches such as Maxwell, Nielsen,16 and Bharadwaj11 models. In these models, different parameters are considered: the aspect ratio, the volume fraction of the impermeable phases, and the orientation of the nanoclay platelets. In the present study, the Bharadwaj model11 is considered more appropriate since it includes the influence of the three aforementioned parameters according to the following equation: Prelative =

Table 5. Aspect Ratio Calculated from the Orientation Factor Values Given According to the Two Dispersion States optimal exfoliation

(1 − φ) 1+

αφ 3

(ο + 12 )

(8)

−1 ⎛ ρi (1 − m i ) ⎞ ⎟ φ = ⎜⎜1 + ρp m i ⎟⎠ ⎝

oNa+

αcalc

RSS

oC30B

αcalc

RSS

PLA/%C30B-d

0.0 0.5 1.0 0.0 0.5 1.0

128 64 43 452 226 151

0.044 0.044 0.044 0.056 0.056 0.056

0.0 0.5 1.0 0.0 0.5 1.0

60 30 20 221 110 74

0.041 0.041 0.041 0.054 0.054 0.055

PLA/%C30B-h

where Prelative is the relative permeability defined by the ratio of permeabilities Pn/Pm with n and m designing nanocomposite and matrix, respectively. α is the aspect ratio of the nanoclay platelets. o = 1/2(3cos2 θ −1) is defined as the orientation parameter of the platelets where θ is the angle between the plan of platelets and the perpendicular to the diffusive flux. ϕ (the volume fraction of impermeable phase) is calculated according to the following equation:

moderate exfoliation

nanocomposites

remains higher for the most exfoliated system. Also, from Table 5, for a given orientation o, the value of the calculated aspect ratio αcalc remains higher for the most exfoliated system. By comparing the experimental and calculated values of α, it can be deduced that (i) for the series designed PLA/C30B-d nanocomposites, the mean value of αexp = 67 is convenient with the orientation parameter o in the range 0−0.5, depending on the exfoliation state; (ii) for the series designed PLA/C30Bh nanocomposites, the mean value of αexp = 102 is convenient with the orientation parameter o in the range 0.5−1, depending on the exfoliation state. Even if the determination of αexp from TEM images is questionable and despite the coexistence of the exfoliated, intercalated, and aggregated structures in both series of nanocomposites, the PLA/C30B-h nanocomposites present a better dispersion of fillers characterized by a better individualization and orientation of the C30B platelets in comparison with the nanocomposites prepared with dried C30B (PLA/ C30B-d nanocomposites). Using the Bharadwaj model,11 the fitting of the experimental permeation data for both series of nanocomposites is presented in Figure 6. This modeling is performed on the basis of the minimal RSS and for the two exfoliation states, designed ϕNa+ and ϕC30B. It is clear that the fitting is not quite suitable: deviation between experimental data and the fitting curve. This observation suggests that one consider the variation of α and o with the filler content but also to integrate other parameters in the tortuosity concept such as the interactions between water molecules and nanoclays, the stiffness of the polymer chain at the vinicity of the nanoclays, the crystallinity degree, and the percolation phenomenon at the interface matrix/nanoclays. From our experimental information, it is premature to establish a new model which would integrate all these phenomena. 3.3.2. Water Sorption. 3.3.2.1. Water Vapor Sorption Isotherms. In order to investigate the contribution of the C30B

(8)

where mi is the weight fraction of the nanoclay considered as impermeable phase in the polymer matrix, ρi is the specific gravity of the impermeable phase, and ρp is the specific gravity of the permeable phase. The volume fraction of the impermeable phases will depend on the exfoliation state of nanoclays. Two exfoliation levels are considered and designed as follows. (i) Optimal exfoliation (ϕNa+): the dispersion of the nanoclays is well accomplished, a good exfoliation is detected, ρi corresponds to the volume fraction of Cloisite 30B without modification, that is, Cloisite Na+ (ρi = 2.86 g·cm−3). (ii) Moderate exfoliation (ϕC30B): the dispersion of the nanoclays is not really successful, the presence of aggregated nanoclays is detected so that the inorganic phase of surfactant is to be considered, and ρi corresponds to the volume fraction of Cloisite30B (ρi = 1.98 g·cm−3). However, for fitting the relative permeability, the evaluation of the aspect ratio factor α corresponding to the C30B phase dispersed in the PLA matrix is required by the Bharadwaj model (see eq 8).11 Two approaches were studied: the former is relative to the aspect ratio factor αexp directly calculated from the statistic analysis of TEM images. The orientation factor o is then deduced from the residual sum of squares RSS (Table 4). The latter consists in the determination of the average value of the calculated aspect ratio αcalc and an arbitrary value of the orientation factor o (o = 0 for a random orientation, o = 0.5 for semioriented orientation, and o = 1 for an orientation K



Article

Figure 6. Experimental data and fitting of the relative water permeability as a function of the C30B content for the PLA/C30B-d nanocomposites and the PLA/C30B-h nanocomposites.

incorporation on the sorbed water amount as functions of the content and of the hydration state of nanoclays, water vapor sorption measurements have been performed on the PLA matrix, the Cloisite 30B, and the resulting nanocomposites for several water activities. All sorption isotherms characterized by a sigmoidal shape curve (mass gain as a function of water activity) are presented in Figures 7 and 8. One can notice a

Figure 8. Water sorption isotherms of the Cloisite 30B, the PLA matrix, and the PLA/%C30B-h nanocomposites.

the PLA matrix, all the nanocomposites are characterized by a higher water sorption capacity. This result can be related to the capacity to sorb water molecules of organo-modified nanoclays, and its effect is more pronounced when increasing the C30B content. From our sorption data, one can assume that the influence of the slight variation in crystallinity (as reported in Table 2) is negligible on the water sorption in the nanocomposites. Most authors underlined this observation.51 We can consider that the incorporation of nanoclays into the PLA matrix certainly favors the entrance of water molecules within the nanocomposites, and that a fraction of water molecules are locally accumulated at the PLA/C30B interfacial regions. However, the increase of water mass gain with the increase of the C30B content seems to be in contradiction with the water permeation behavior since the water permeability is decreased.

Figure 7. Water sorption isotherms of the Cloisite 30B, the PLA matrix, and the PLA/C30B-d nanocomposites.

large difference in terms of water mass gain between the Cloisite 30B with the nanocomposites and the PLA matrix. Although the C30B is organomodified with a hydrophobic surfactant, the nanoclay sorbs more water molecules than the tested films because of the presence of water-sensitive groups in the surfactant composition such as silicate and hydroxyethylene groups. Otherwise, by comparing to the sorption behavior of L



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model: the mathematical fitting is consistent with the experimental data. The calculated parameters relative to the Park model (eq 8) are gathered in Table 6. All sorption isotherm curves of PLA/C30B-d nanocomposites have been well-fitted from this model since E values are found below 10% (Table 6), as mentioned in the literature.53 Concerning the PLA/C30B-h nanocomposites (with undried C30B), similar results are obtained (Figure 8, Table 6). One can see the decrease of AL values for the nanocomposites with the increase of C30B content in comparison with the PLA matrix, with the impact on bL values being not enough to be interpreted. The reduction of Langmuir-type sorption terms is consistent with a reduction in the accessibility to polar sorption sites. The incorporation of C30B nanoclays disrupts the sorption of water molecules in the PLA matrix at low water activities. At this low water concentration level, one can assume that an “antiplasticization” effect of water molecules takes place reflecting a strong cohesion established into polar domains through water− nanoclay hydrogen-bonding interactions. This effect is more marked when undried nanoclays have been incorporated into the PLA matrix during processing. This observation evidences the establishment of water−water interactions between the sorbed water molecules with the water molecules initially present into the nanoclays. At high water activity range (aw > 0.7), the increase of Kag and n is in good agreement with the increase of interactions between water and the nanoclays favoring the formation of water clusters. The size of water aggregates is largely increased compared with that of the PLA matrix until a constant value is reached (Table 6). Nevertheless, the n value is unchanged from certain C30B content, indicating that the size of water aggregate becomes constant. One can note a gap in n value between both series of nanocomposites: n equals 18 for PLA/ C30B-d nanocomposites and 14 for PLA/C30B-h nanocomposites. This result is consistent with the initial hydration state of nanoclays before nanocomposite preparation. Indeed, the fact that the PLA/C30B-h nanocomposites exhibit a more exfoliated structure contributes to limiting the water aggregate size. 3.3.2.3. Influence of Hydration State of Nanoclays on Sorption Properties. To investigate the influence of the hydration state of the C30B nanoclays on the water sorption properties of the PLA matrix, it is more convenient to compare the both types of nanocomposites containing the same experimental nanofiller contents by analyzing the mass gain reduced to the matrix amount according to:

In addition to the tortuosity effect induced by nanoclays, this result gives evidence that the nanoclay may retain water molecules at the matrix/C30B interface contributing to the water cluster formation within nanocomposites. This point will be thereafter discussed in view of the modeling of experimental sorption data and the evolution of water diffusion coefficients. 3.3.2.2. Modeling Using Park Model. The sigmoidal shape of the isotherm curves is convenient with the well-known Park model52 which describes the sorption behavior from three successive contributions: (1) at aw < 0.2, the Langmuir-type sorption corresponds to the sorption of water molecules on specific sites (polar functions) of the polymer matrix; (2) for intermediate water activities (0.2 < aw < 0.7), the Henry-type sorption becomes predominant and occurs when the water molecules are randomly sorbed in the polymer matrix; and (3) at the highest water activities (aw > 0.7), an exponential evolution of the mass gain is observed which corresponds to the aggregation of water molecules. This model is given from the following equation: ⎛ A ·b ⎞ C = ⎜ L L ·a w ⎟ + (KH·a w ) + (n·KHn ·K ag ·awn) ⎝ (1 − bL) ⎠

(9)

with AL being the Langmuir sorption capacity, bL being the affinity constant of Langmuir sorption, kH being the Henry constant, Kag being the aggregation constant, and n being the average water aggregate size. As depicted in Figure 9, a good fitting of the sorption isotherm of the PLA matrix is obtained by using the Park

Figure 9. Water sorption isotherms of the PLA/10%C30B-h nanocomposite and the PLA/10%C30B-d nanocomposite.

ΔMeq =

meq − md md (1 − φ)

× 100

(10)

Table 6. Parameters of the Park Model Calculated for the PLA Matrix and the PLA/C30B Nanocomposites samples

AL

bL

kH

Kag

n

E′

R2


2.97 0.92 0.83 0.46 0.35 0.94 0.003 0.21 0.05

0.06 0.02 0.06 0.05 0.07 0.09 0.18 6.07 8.80

0.72 1.05 0.08 1.10 1.23 0.96 1.13 1.18 1.50

0.07 0.09 1.10 0.13 0.18 0.11 0.12 0.20 0.32

8 13 18 18 18 14 14 14 14

4.97 1.63 9.23 6.24 6.00 9.29 8.71 4.09 7.43

0.999 0.999 0.997 0.998 0.997 0.999 0.999 0.999 0.999

M



Article

Figure 10. Variation of the diffusion coefficients, D1 and D2, as a function of water activity for the PLA matrix and the PLA/20%C30B-d nanocomposite.

Figure 11. Variation of the diffusion coefficients, D1 and D2, as a function of water activity for the PLA matrix and the PLA/20%C30B-h nanocomposite.

where ϕ is the volume fraction of impermeable phase relative to the nanoclay platelets. Doing so and assuming that the nanoclays are an impermeable phase,54,55 the sorption isotherms have been plotted in Figure 9 for nanocomposites containing about 10 th. wt % of nanoclays (the clay fraction for which the difference in the relative water permeability between two series of nanocomposites is high). It can be noticed that no significant difference is observed between the two sorption isotherms. In other words, one can estimate that the hydrated or dried state of the C30B does not change the water sorption capacity of the PLA matrix. However, one cannot neglect the nanoclay/matrix interface contribution and its impact on the molecular chain mobility due to the chain confinement effects, regardless of the nanocomposite structure, the shape, and the size of the nanoclays considered. In terms of transport properties in nanocomposites, the incorporation of Cloisite nanoclays into the PLA matrix is found at the origin of two opposite effects: (i) the increase of the affinity between nanoclays and water molecules facilitates the water sorption in the matrix; and (ii) in

the same time, the tranfer of water molecules in the PLA matrix is reduced due to the increase of the tortuous diffusion pathway and the increase of chain segment stiffness at the vicinity of nanoclays. 3.3.2.4. Water Vapor Diffusivity. To complete the study on the influence of the incorporation of C30B on the barrier properties, the diffusion coefficients have been calculated for the PLA matrix and its resulting nanocomposites. Their variation has been analyzed for each sorption kinetic in the whole range of water activity with the purpose of comparison. In the semilogarithmic scale (log D), as a function of water activity, the variation of the diffusion coefficients corresponding to the first-half sorption (D1) and the second-half sorption (D2) are depicted in Figures 10 and 11. For the sake of clarity, only the results concerning the PLA matrix and the nanocomposite with the highest C30B content have been reported in this publication. As it is shown in Figure 10, the two diffusion coefficients, D1 and D2, of the PLA/20%C30B-d nanocomposite are lower than those of the PLA matrix. These results are consistent with N



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the PLA/C30B-h nanocomposite. This effect is related to the affinity to water of undried C30B nanoclays which contains water molecules included in confined spaces. One can also consider that the degradation of PLA chains enhanced by nanofillers allow to increase the water sorption due to hydrophilic functions from hydrolyzed PLA chains. In that case, the more pronounced degradation for PLA/C30B-h nanocomposites could contribute to an additional water mass gain and hence sorb more water as for PLA/C30B-d nanocomposites. Concerning the diffusivity, at this highest water activity (aw = 1), it can be observed a decrease of the water diffusivity during the liquid water sorption process: the D1 and D2 coefficients are lower than those of the PLA matrix. Again, it may be reminded that the tortuosity effect characterizing the presence of nanoclay platelets into the material and the formation of water aggregates could be at the origin of this reduction of water mobility. Eventually, the liquid water sorption measurements are convenient with the water vapor sorption measurements in terms of water affinity and of solubility for the PLA based nanocomposites. However, unlike to the sorption process, it is clear that, in terms of barrier properties, the permeation results have shown significant differences between the behavior exhibited by the PLA/C30B-d nanocomposites and the PLA/ C30B-h nanocomposites. In that case, it is noteworthy that the barrier properties are mainly governed by the tortuosity effect which is directly linked to the quality of dispersion of C30B in the PLA matrix. And this last point is found to be improved when incorporating undried C30B nanoclays into the PLA matrix, compared with dried C30B nanoclays, which so leads to the best barrier properties. 3.3.3. Oxygen Permeation in Moisture. The oxygen permeability results obtained with both nanocomposites is given in Figure 12 considering the relative humidities applied. A decrease of the oxygen permeability is globally obtained in comparison with the PLA matrix for both nanocomposites (Figure 12), whatever the relative humidity considered. However, this decrease is not linear: at content higher than 10 wt % for PLA/C30B-d nanocomposites and 15 wt % for PLA/C30B-h nanocomposites, an increase of the oxygen permeability is observed when the relative humidity increases. By comparing both series of nanocomposites containing similar C30B contents, PLA/C30B-h nanocomposites exhibit lower oxygen permeability. This difference is maintained whatever the nanocomposite tested and the relative humidity used. Nevertheless, the barrier improvement factor (IF) calculated from our experimental data varies from 40% to 64% for the PLA/C30B-d nanocomposites (Figure 12a) and from 48% to 74% for PLA/ C30B-h nanocomposites (Figure 12b). In terms of oxygen barrier properties, it is interesting to note that the most loaded nanocomposites are the most sensitive to water and this is probably due to the hydrophilic nature of the nanoclays. One can estimate that an immobilization of water molecules occurs on the surface of the organomodified clay nanoplatelets via the establishment of hydrogen bondings. Thellen et al.9 and Sanchez-Garcia et al.57 also showed an improvement of the oxygen barrier properties about 15% to 48% for PLA based nanocomposites depending on the nanoclay content and the relative humidity. This improvement of the oxygen barrier properties was explained in the literature58,59 by using the tortuosity concept caused by the presence of impermeable phases in the polymer matrix.

permeation experiments and confirm the tortuosity effect exerted by the nanoclays in the PLA matrix. At high water activity range, the decrease of the diffusion coefficients observed for the PLA matrix and the PLA/20%C30B-d nanocomposite is in good agreement with the water aggregation phenomenon. The immobilization of water molecules near the nanoclays reduces the water mobility and hence its diffusion due to the size of water clusters.56 As for PLA/10%C30B-d nanocomposite, a similar behavior was obtained for PLA/20%C30B-h nanocomposite (Figure 11): lower D coefficients are calculated compared with the PLA matrix highlighting the reduction in water diffusion and within the material. Nevertheless, at low water activities (aw < 0.15), one can observe a significant decrease in the diffusion coefficients which is more pronounced for the PLA/20% C30B-h nanocomposite, as attested in Figure 11. This decrease can be attributed to the “water antiplasticization” phenomenon initiated at very low sorbed water concentration, as mentioned from the water sorption isotherms. This observation again evidences the formation of water−water interactions by hydrogen-bonding. Also, this phenomenon is more marked when undried nanoclays have been incorporated into the PLA matrix (Figure 11), that is, in presence of water molecules included in the confined spaces of the nanoclays. The hydrated state of C30B nanoclays would enhance the interactions between water molecules and Cloisite via hydrogen-bonding, resulting in a greater limitation of the mobility water molecules. Otherwise, the increase of affinity between water with the specific sorption sites of Cloisite contributes to reduce the access of water molecules inside the material by tortuosity effect and hence the retention capacity of the clays. Indeed, at high water activities, if we compare with the PLA/20%C30B-d nanocomposite behavior (Figure 10), the decrease of water diffusivity for PLA/20%C30B-h nanocomposite can be estimated less pronounced and this result is convenient with the n value relative to the size of water cluster; the higher n is, the lower the diffusivity is. 3.3.2.5. Liquid Water Sorption. From liquid water sorption kinetics, the mass gain and the diffusion coefficients, D1 and D2, have been calculated for the PLA matrix and both series of PLA/C30B nanocomposites. As gathered in Table 7, in the presence of nanoclays, the water concentration in the PLA matrix is increased because of the hydrophilic nature of the Cloisite. Indeed, the water mass gain exhibited by the nanocomposites increases with the clay content. By comparing the nanocomposites containing similar C30B mass fraction, one can note that this increase in mass gain is slightly higher with Table 7. Diffusion Coefficients (D1 and D2) and the Mass Gain Deduced from Liquid Water Sorption Kinetics for the PLA Matrix and the PLA/C30B Nanocomposites samples

D1 × 1010 (cm2·s‑1)

D2 × 1010 (cm2·s‑1)

mass gain (%)


71.0 14.0 9.5 3.3 1.7 57.0 3.9 0.9 1.1

5.5 2.7 1.0 2.3 1.5 7.7 3.0 0.8 3.5

0.7 0.8 1.8 3.4 4.5 0.9 3.6 9.9 9.1 O



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the structuration state of the nanocomposite. If many factors can be considered in the tortuosity concept, the quality of dispersion, the shape, the size, and the orientation of platelets represent the key parameters. The relative oxygen permeability versus C30B content (at 0% RH) for both series of nanocomposites plotted in Figure 13 clearly shows a higher barrier improvement for PLA/C30B-h nanocomposites. Again, the experimental data have been fitted by using the mathematical equation relative to the Bharadwaj model11 for applying the same approach as for water permeation in order to compare the experimental and calculated values of α. For both series of nanocomposites, it can be noticed that the fitting of relative oxygen permeability is better than the relative water permeability one. This result is not surprising, since the Nielsen-Bharadwaj model11 based on a geometrical approach is better suited to describe the barrier effect for gas permeation. Indeed, in the case of gas molecules, the tortuosity concept can be easily applied since no interaction occurs between the diffusing gas molecules and the material, whereas for water or organic vapors other phenomena should be considered by virtue of the presence of new interactions formed during the permeation course.

4. CONCLUSION The transport properties of two series of PLA based nanocomposites were investigated by oxygen permeation and water permeation and sorption measurements. Three coexisting structures as a function of C30B content were evidenced from TEM observations and XRD measurements with a more successfully dispersed structure for PLA/C30B-h nanocomposites. Indeed, nanoclays were oriented in an arrangement such that the direction of the diffusion is normal to the direction of the nanoplatelets. In terms of water barrier properties, the water permeation process put forward an important barrier improvement up to 95% attributed essentially to the decrease of the water diffusion and to the increase of the tortuous diffusion pathways induced by impermeable nanoclays into the PLA matrix. The PLA/ C30B-h nanocomposites presented a higher water barrier effect compared with the PLA/C30B-d nanocomposites. Indeed, this decrease of the water diffusion was confirmed by the water sorption measurements, revealing a capacity to sorb water molecules of nanocomposites owing to the water affinity of the organomodified C30B and the effect relative to their hydration state. However, at high C30B contents, the water permeation reincreased, reflecting the percolation effect brought by the aggregated nanoclay platelets due to a lack of space in bulk PLA for nanofiller delamination: the preferential diffusion pathways for water molecules were thus formed. The size of water aggregates formed within the nanocomposites was also impacted by the hydration state and the content of nanoclays: an increase in water/nanoclay interactions favored the formation of water clusters restricting the water diffusivity. Nevertheless, other parameters than the dispersion state of the nanoclay within the polymer matrix, the aspect ratio, the content, and the orientation of the nanoclay platelets influencing the tortuosity should be taken into consideration since the modeling of the relative water permeability based on geometrical approach was not sufficient to well describe the experimental data. Most authors underlined the contribution of various concomitant factors such as the stiffness of the polymer chain around the nanoclay platelets, the crystallinity degree, and the percolation phenomenon, but to date no model correctly

Figure 12. Dioxygen permeability for various relative humidities (RH = 0%, 50%, 75% and 90%) for (a) the PLA/C30B-d nanocomposites and (b) the PLA/C30B-h nanocomposites;.

According to Güsev and Lusti,60 the permeablility levels that can be obtained with nanocomposites are dependent on two factors, namely, a geometric factor that reduce the permeability by increasing the diffusion pathways around the platelets and changes in the local permeability due to molecular-level transformations in the polymer matrix. Bhattacharya et al.13 attributed the increase of oxygen barrier properties of styrenebutadiene copolymer/montmorillonite based nanocomposites to the increase of the nanoclay/polymer interactions. These interactions would lead to a decrease of the free volume and of the chain segment mobility. Assuming that the permeability P is calculated as the product SD,61 these authors consider that the two transport parameters depend strongly on the homogeneity of the nanoclay dispersion. Silvestre et al.5 suggested a new model to analyze the barrier properties in the nanocomposites. This model takes into consideration the free volume around the nanoclay platelets, particularly the free volume present at the interface between the polymer and the nanoclay. Finally, as reported for water permeation, the incorporation of nanoclays in the PLA matrix contributes to a high decrease of the permeability and the improvement of oxygen barrier properties is found better when the C30B clays are not dried before their incorporation in the matrix. This result is obviously related to the better dispersion of nanoclays into the matrix since the tortuous diffusion pathway is directly connected to P



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Figure 13. Experimental data and fitting of the relative dioxygen permeability at 0% RH as a function of the C30B content for the PLA/C30B-d nanocomposites and the PLA/C30B-h nanocomposites.

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includes all the contributions affecting the transport properties. Besides, the impact of matrix/nanoclay interactions located at the interfacial region of nanocomposites appeared crucial in the analysis of transport properties. In terms of oxygen barrier properties, the best performance was also observed for the PLA/C30B-h nanocomposites because of the better dispersion, exfoliation, and orientation of the C30B nanoplatelets within the PLA matrix. The presence of water molecules included in the confined spaces within nanoclays seems to have promoted a better compatibility between nanoclays with the PLA polymer restricting the free volume at the matrix/nanoclay interface. Since no interaction occurred between the diffusing gas molecules and the nanocomposites, the diffusion properties of the interfacial region around the nanoclays were altered and the tortuosity concept was evidenced. When adding moisture during the dioxygen permeation course, the gas permeability is globally unchanged except for the highest filled nanocomposites showing a greater water affinity for which the dioxygen permeability is then slightly increased.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

ACKNOWLEDGMENTS

The authors are grateful to the French Ministry for Research and Technology for the financial support (Ph.D. fellowship of N.T.). The authors are thankful to F. Cuvilly from GPM (Institute of Material Research, France) for X-ray diffraction analysis and Séverine Bellayer from ENSCL (France) for her technical assistance during TEM observations. Q



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