Synthesis and Characterization of Bionanocomposites with Tunable

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Synthesis and Characterization of Bionanocomposites with Tunable Properties from Poly(lactic acid) and Acetylated Microfibrillated Cellulose Philippe Tingaut,* Tanja Zimmermann, and Francisco Lopez-Suevos Empa, Swiss Federal Laboratories for Materials Testing and Research, Wood Laboratory, ¨ berlandstrasse, 129 CH-8600 Du¨bendorf, Switzerland U Received October 19, 2009; Revised Manuscript Received November 27, 2009

In the present study, novel bionanocomposite materials with tunable properties were successfully prepared using a poly(lactic acid) (PLA) matrix and acetylated microfibrillated cellulose (MFC) as reinforcing agent. The acetylation of MFC was confirmed by FTIR and 13C CP-MAS NMR spectroscopies. The grafting of acetyl moieties on the cellulose surface not only prevented MFC hornification upon drying but also dramatically improved redispersibility of the powdered nanofibers in chloroform, a PLA solvent of low polarity. Moreover, we demonstrate that the properties of the resulting PLA nanocomposites could be tailored by adjusting both the acetyl content (Ac%) and the amount of MFC. These nanomaterials showed improved filler dispersion, higher thermal stability, and reduced hygroscopicity with respect to those prepared with unmodified MFC. Dynamic mechanical analysis (DMA) highlighted the reinforcing potential of both the unmodified and the acetylated MFC on the viscoelastic properties of the neat PLA. But more interesting, an increase in the PLA glass transition temperature was detected when using the 8.5% acetylated MFC at 17 wt %, indicating an improved compatibility at the fiber-matrix interface. These findings suggest that the final properties of nanocomposite materials can be controlled by adjusting the %Ac of MFC.

Introduction In line with recent environmental policies, increased attention has been paid to the development of bionanocomposite materials for several industrial applications, such as automotive, construction, or packaging. Thus, much effort has been devoted to the use of natural fibers in composite materials as an alternative to conventional inorganic fillers, traditionally used to reinforce thermoplastic matrices (i.e., glass fibers, aramid, or carbon fibers, for instance).1,2 Natural fibers present very attractive properties such as low cost, renewability, biodegradability, high specific strength and modulus, and low density.3,4 Cellulose is the most abundant biopolymer on earth and is present in a wide variety of living species, such as animals, plants, and bacteria. This linear polymer is composed of β-1,4 linked glucopyranose units, with polymer chains associated by hydrogen bonds forming bundles of fibrils, also called microfibrillar aggregates, where highly ordered regions (i.e., crystalline phases) alternate with disordered domains (i.e., amorphous phases).4 Microfibrillated cellulose (MFC) is obtained from cellulose fibers after a two-step mechanical disintegration process consisting of an initial refining step followed by a high pressure homogenization step. Developed in 1983 by Turbak et al., this technology allows the production of a network of interconnected cellulose microfibrils, with diameters from 10 to 100 nm and aspect ratios from 50 to 100.5-8 Due to its high specific strength, modulus, and aspect ratio, microfibrillated cellulose is an adequate reinforcing agent for nanocomposite applications. But to realize the targeted property improvements, the natural fibers must be homogeneously dispersed in the polymeric matrix, which is nontrivial. In general, due to their strong hydrophilic character and high aspect ratio, * To whom correspondence should be addressed. Tel.: +41 44 823 4749. Fax: +41 44 823 4007. E-mail: [email protected].

cellulose fibers at the nanometer scale (i.e., cellulose nanocrystals, also called nanowhiskers, and MFC) tend to flocculate through hydrogen bonding. Concerning cellulose nanowhiskers, stable suspensions can be obtained in water,4,9 or in very polar aprotic solvents such as DMF10-12 and DMSO.12 Nevertheless, the dispersion of hydrophilic MFC or cellulose whiskers in apolar solvents, as well as their further homogeneous incorporation in most common apolar thermoplastic polymers, is usually challenging. A non homogeneous dispersion of the filler in the polymer matrix is often obtained, thus decreasing the final mechanical properties of the nanocomposite material.13 Moreover, the hydrophilic character of MFC usually leads to irreversible fibril aggregation upon drying through a hornification process,14 during which additional hydrogen bonds are created between adjacent microfibrils, thus, preventing further dispersion in aqueous or organic solvents. For these reasons, the preparation of nanocomposite materials from MFC has been mainly restricted to water-soluble polymers,8,15-17 latexes,18 acrylic and phenol-formaldehyde resin through a fiber impregnation process,19-22 or poly(lactic acid) by specific compounding.23-25 As compared with neat matrices, the incorporation of MFC led to an increase in mechanical properties such as bending and tensile strength8,20,23-25 and Young’s modulus,8,17,21-23 as well as thermal stability.15,16,18,23-25 Moreover, recent studies have highlighted the possibility of preparing optically transparent nanocomposites by impregnating MFC with an acrylic resin.19 To expand the use of cellulose nanofibers to nonpolar environments, several attempts to decrease the nanofiber surface hydrophilicity have been reported. Recently, the use of a surfactant enabled the production of stable cellulose whisker suspensions in toluene and cyclohexane.26,27 Alternatively, the chemical modification of the surface cellulose hydroxyl groups has also been studied using alkenyl succinic anhydrides,28 silane

10.1021/bm901186u  2010 American Chemical Society Published on Web 12/21/2009

Bionanocomposites with Tunable Properties

reagents,29-33 isocyanates,28,34 poly(ε-caprolactone),35 acetic anhydride,36-40 or vinyl acetate.41 Stable dispersions of cellulose whiskers could therefore be obtained in solvents of various polarities, such as acetone, chloroform, tetrahydrofuran, or toluene.30,31,36,41-43 Despite a lack of information concerning the dispersion quality of chemically modified MFC in organic solvents, in nanocomposites, however, an improvement of the adhesion at the fiber-matrix interface was generally noted.34,35 Among the potential chemical pathways that can be envisaged to decrease MFC hydrophilicity, the acetylation reaction is strongly promising. Hence, in addition to heat and water sorption resistance of the cellulose nanofibers,44 the adjustment of the acetyl content (Ac%) should improve the chemical affinity between the nanofibers and the nonpolar solvents, resulting in better dispersibility in apolar polymer matrices during further processing. Despite a tremendous amount of work on macroscopic cellulosic substrates, few studies report the use of acetylated nanofibers in bionanocomposite applications.13 Recent studies on bacterial cellulose (BC) nanofibers revealed that acetylated BC, compared to unmodified BC, increased hydrophobicity and transparency of acrylic resin nanocomposites.37,45 Polylactic acid (PLA) is a hydrophobic biopolymer, soluble in chloroform, and is considered biodegradable and biocompostable. The basic building block of PLA is lactic acid, which can be obtained either by classical chemical synthesis or by carbohydrate fermentation. High molecular weight PLA can be obtained through different methods, such as direct condensation polymerization, azeotropic dehydrative condensation, or polymerization through lactide formation. This thermoplastic polymer shows high strength and stiffness and has, thus, gained strong interests in several fields of applications, such as food packaging, automotive, or medicine.46,47 Its reinforcement with natural fibers has already been reported in the literature,48-50 but few studies report the use of cellulose nanofibers, such as MFC or nanowhiskers, as filler reinforcement in PLA matrices.23,24,49,51 Moreover, the same limitations have often been pointed out, such as aggregation of the filler and a lack of compatibility at the fiber-matrix interface.23,49 To our knowledge, the reinforcement of PLA with acetylated nanofibers has not been reported so far in the literature. (The terms microfibrils and nanofibers will now refer to microfibrillated cellulose (MFC).) Accordingly, in this study, we propose the synthesis and characterization of novel bionanocomposites with tunable properties from PLA and acetylated MFC. The surface modification of MFC with acetic anhydride (AA) is envisaged to decrease the natural nanofibers hydrophilicity and enhance their compatibility with the PLA matrix. Chemical grafting of hydrophobic acetyl moieties is expected to prevent MFC hornification upon drying and improve nanofibers redispersibility in PLA solvents of low polarity, such as chloroform. Thus, PLA bionanocomposite materials with enhanced properties (i.e., transparency, hygroscopicity and mechanical properties) are expected, with respect to those prepared with unmodified MFC. In the first part of this manuscript, the acetylation reaction is monitored spectroscopically, and the impact of the acetyl content (%Ac) on the properties of MFC is discussed. We also document for the first time the influence of the %Ac on the dispersion behaviors of the acetylated MFC in chloroform. In a second part, a PLA matrix is reinforced with the chemically modified nanofibers, and we highlighted the possibility of tailoring the resulting nanocomposites properties through a careful control of %Ac. In addition, the complete analysis of the nanocomposites’ viscoelastic properties gives an insight into the intera-

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tions between acetylated MFC and PLA, which has not been reported so far.

Experimental Section Materials. Microfibrillated cellulose (MFC) was supplied by Borregaard Industries Ltd., Norway, as a water suspension with a solid content of 3.27 wt %. MFC was obtained by disintegration of bleached sulfite wood pulp in a homogenizer, leading to microfibrils with diameters in the range of 10 to 100 nm. Acetic anhydride (AA), pyridine, and sodium hydroxide (NaOH) were purchased from Fluka (Switzerland). Dimethylformamide (DMF), chloroform (CHCl3), toluene, ethanol, and acetone were purchased from Roth. Hydrochloric acid (HCl) was purchased from Merck (Germany). All chemicals were reagent grade and used as received without further purification. Poly(lactic acid), Ingeo PLA 2002D, NatureWorks, was supplied by Polykemi (Sweden). Deionized water was used in all experiments. Chemical Modification of MFC. Acetylation reactions were performed under a standard set of conditions, in a three-necked roundbottomed flask equipped with a condenser and a mechanical stirrer (RW 16 basic, Ika Werke). Reactions were conducted at 105 °C under a nitrogen flow. Each homogenization/redispersion step was carried out using a homogenizer (Ultra-Turrax T25 basic, Ika-Verke). All centrifugation operations were conducted at 3500 rpm for 15 min. In a typical reaction, 100 g of a 3.27% (w/w) MFC water suspension (i.e., about 0.0605 mol total hydroxyl groups) was first solvent exchanged with DMF by three centrifugations and redispersion operations. After the last centrifugation step, the MFC suspension in DMF was introduced in a beaker and 160 mL of DMF was added. The mixture was homogenized for 30 s, after which 123.5 g of AA (1.21 mol) and 4.78 g of pyridine (0.061 mol) were added. The whole suspension was finally homogenized for an additional 30 s and introduced in the reaction flask. Different reaction times were investigated, from 2 to 4320 min (i.e., 72 h). At the end of the reaction, the suspension was centrifuged. To eliminate all nonbonded chemicals (i.e., unreacted compounds and byproduct formed), the modified material was subsequently washed three times with a toluene/ethanol/acetone mixture (4/1/1 by vol), each washing including a homogenization and centrifugation step. The acetyl content was determined using a standard saponification procedure and a FTIR calibration curve (see Supporting Information). Infrared Spectroscopy (FTIR-ATR). Infrared spectra of dried unmodified and acetylated MFC (i.e., dried at 105 °C for 14 h) were recorded using a FTS 6000 spectrometer (Portmann Instruments AG, Biel-Benken, Switzerland). For each sample, the diamond crystal of an attenuated total reflectance (ATR) accessory was brought into contact with the area to be analyzed. The contact area was about 2 mm2, and a torque of 10 cN · m was used to ensure the same pressure on each sample. All spectra were recorded between 4000 and 600 cm-1, with a resolution of 4 cm-1 and 32 scans. 13 C CP-MAS NMR Spectroscopy. Solid state 13C CP-MAS (crosspolarization-magic angle spinning) NMR spectra of unmodified and acetylated MFC were performed at room temperature on a Bruker Avance 400 NMR spectrometer (Bruker Biospin AG, Fallanden, Switzerland), using a MAS rate of 3 kHz, a contact time of 3 ms, at a frequency of 100.61 MHz for 13C NMR. Samples were packed in MAS 7 mm diameter zirconia rotors. All the spectra were run for 12 h (15000 scans). X-ray Diffraction Analysis. Wide-angle X-ray diffraction (WAXD) patterns were collected on a Panalytical X’Pert Pro Materials Research diffractometer working in reflection mode, from 2Θ ) 5° to 2Θ ) 60°. Cu KR radiation (λ ) 0.15418 nm) was generated at a voltage of 45 kV and a current of 40 mA and was monochromated with a diffracted beam monochromator. Unmodified and acetylated MFC were pressed to form pellets and the surface was analyzed. All spectra were normalized to the 004 lattice at 2Θ ) 34°, which was not affected by the chemical modification.40

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Figure 1. Acetylation of MFC with acetic anhydride (AA).

Scanning Electron Microscopy (SEM). SEM experiments were conducted using a Jeol 6300F scanning electron microscope using an acceleration voltage (EHT) of 5 kV and a working distance of 16 mm. A 0.07% (w/w) suspension of unmodified and acetylated MFC in CHCl3 was sonicated twice at room temperature for 15 min. One droplet of the suspension was then deposited on a freshly cleaved mica surface and then sputter coated with a 8 nm layer of platinum. Thermogravimetric Analysis (TGA). TGA experiments were conducted on a TGA 7 thermogravimetric analyzer (Perkin-Elmer). Each sample was heated from 30 to 600 °C at 20 °C min-1 under dry helium gas. Each sample was analyzed twice. Dispersion Tests. The behavior of unmodified and acetylated MFC in suspension was examined in CHCl3, a solvent for PLA. Dispersion tests were carried out with solvent exchanged and dried-redispersed nanofibers at a concentration of 0.07% (w/w). Unmodified MFC was first solvent exchanged from water to acetone and then to CHCl3. Acetylated MFC was directly solvent exchanged to CHCl3 after reaction. In addition, the unmodified and acetylated MFC nanofibers were dried in a ventilated oven at 60 °C under continuous stirring and then redispersed in CHCl3. All suspensions were sonicated three times for 15 min at room temperature and allowed to stand for 24 h before a photograph was taken. Nanocomposite Preparation. PLA was first dissolved in CHCl3 at room temperature for 24 h (2% w/w). The unmodified and acetylated MFC suspensions in CHCl3 were then prepared using solvent exchanged nanofibers. The desired amount of filler was then added in the PLA solution, and the mixture was homogenized for 2 min (Ultra-Turrax T25 basic, Ika-Werke). The suspension was then degassed three times and then poured into Teflon molds, where the films were obtained by solvent evaporation at room temperature. Nanocomposite films containing 0, 2.5, 5, 10, and 17 wt % of filler were prepared. Dynamic Mechanical Analysis (DMA). DMA experiments were carried out in tensile mode using a RSA III from TA Instruments (Delaware, U.S.A.) under dry air. To erase any trace of solvent, samples were first heated from room temperature to 100 at 10 °C/min, 1 Hz of frequency, and 0.01% of strain, and this temperature was kept for 10 min. Then dynamic cooling scans were conducted from 100 to 30 °C at 2 °C/min, 1 Hz, and 0.1%. of strain. During the DMA experiment, the static load was kept at 15% of the dynamic load. Rectangular films with dimensions of 20 × 6 × 0.2 mm3 were analyzed, and three replicates were averaged to characterize each sample. Differential Scanning Calorimetry (DSC). DSC measurements were performed with a DSC 7 differential scanning calorimeter (PerkinElmer). Each sample was heated from 30 to 200 °C at 20 °C min-1 under dry nitrogen gas. The glass transition temperature (Tg) was measured at half-height of the specific heat increment at the glass transition temperature. Each sample was analyzed twice. Water Sorption Tests. Water sorption tests were performed on neat PLA and nanocomposite films reinforced with unmodified and acetylated MFC at 10 and 17 wt % filler contents. All films were conditioned at 25 °C and a relative humidity of 97%, in a desiccator containing a saturated potassium sulfate solution. Three replicates were used and averaged for each sample.

Results and Discussion Acetylation of Microfibrillated Cellulose (MFC). The acetylation of MFC was performed using a large excess of acetic anhydride (AA; Figure 1). Figure 2 presents the FTIR-ATR

spectra of chemically modified MFC samples. The characteristic vibrations of the grafted acetyl groups were easily identified, namely, the carbonyl stretching vibration at 1740 cm-1 (νCdO), the methyl in-plane bending at 1370 cm-1 (δC-H), and the C-O stretching at 1231 cm-1 (νC-O). The intensity of these bands increased gradually with the acetyl content (Ac%), indicating that nanofibers were increasingly modified. The kinetics of the reaction was monitored by plotting the evolution of the acetyl content (Ac%) against reaction time (Figure 3), where the Ac% was obtained from FTIR spectra through the use of a calibration curve (see Supporting Information). As shown in the inset plot, the Ac% increased rapidly during the first 60 min, suggesting that the reaction occurred primarily on easily accessible surface hydroxyl groups (OH). After 60 min, the reaction rate gradually slowed down up to 500 min, with an Ac% ranging from 7.5 to 10%. This behavior was associated with the modification of less accessible surface OH groups, which may be located deeper in the nanofibers, as well as steric hindrance induced by grafted acetyl groups on fiber surfaces, thus, affecting the reaction rate. At this stage of the reaction, diffusion phenomena of the AA reagent from the surface to the core of the nanofibers have a strong impact on the reaction rate, as has been reported for cellulose modified with acetic anhydride.52 From 500 to 900 min, an additional increase in the acetyl content was observed (10-17%), indicating that other available hydroxyl groups (which once again might be located deeper in the nanofibers) were increasingly modified. Above 900 min, a plateau was reached and no more modification of MFC was observed up to 4320 min. Finally, longer reaction times would be required to modify the remaining unreacted MFC hydroxyl groups. The acetylation of MFC was also confirmed with 13C crosspolarization with magic-angle spinning nuclear magnetic resonance spectroscopy (13C CP-MAS NMR; Figure 4). The spectrum of unmodified MFC displayed typical signals from cellulose that were assigned as follows: C1 (105 ppm), C4 crystalline (89 ppm), C4 amorphous (84 ppm), C2/C3/C5 (72 and 75 ppm), C6 crystalline (65 ppm), and C6 amorphous (63 ppm). After acetylation, the characteristic signals of the grafted moieties emerged at 172 and 21 ppm, corresponding to the carbons of the carbonyl and methyl groups, respectively (carbons R and β, respectively, according to the nomenclature in Figure 2). The intensity of these signals increased with the acetyl content of MFC samples, corroborating the FTIR results. Moreover, additional changes in signals intensities were detected between 110 and 60 ppm. Upon acetylation, a decrease in the C1 intensity at 105 ppm was observed together with the progressive appearance of a shoulder at 102 ppm. In the literature, it has been reported that the acylation of sugars results in a downfield shift of the peak corresponding to the Oacetylated carbon and a concomitant upfield shift of the carbon corresponding to the adjacent carbon.53 Therefore, the shoulder at 102 ppm was associated with the upfield shift of the C1 after acetylation of the OH at the C2 position. In the same time, the intensity of the C6 amorphous at 63 ppm progressively decreased with the acetyl content, showing that OH groups at this position



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Figure 2. FTIR absorbance spectra of acetylated MFC with acetyl contents of 0 (unmodified MFC), 1.5, 4.5, 8.5, 13, and 17%.

Figure 3. Evolution of acetyl content (%Ac) as a function of reaction time. The inset represents a magnification of the region between 0 and 520 min reaction time.

were also modified. However, the modification of the OH groups at the C3 position could not be clearly confirmed, as upfield and downfield movements overlap with other signals in the 68-80 ppm region. Additional signals of spinning side bands from cellulose and acetyl moieties were also detected and directly attributed on the spectra. X-ray Diffraction Analysis. The impact of chemical modification on the fiber crystal structure was further evaluated using wide-angle X-ray diffraction (WAXD) analysis (Figure 5). Unmodified MFC displays the typical X-ray diffraction pattern of native cellulose (cellulose I), with characteristics diffraction peaks at 2Θ angles of 14.8, 16.6, 22.3, and 34.4° (101, 101j, 002, and 004 lattices, respectively).40 The intensity of the diffraction peak at 2Θ ) 34° was not affected by the chemical modification of MFC and was chosen to normalize all spectra.40,41 The diffraction pattern of acetylated MFC did not significantly differ from that of unmodified MFC up to an acetyl content of 4.5%. This result indicates that the original structure of MFC

Figure 4. 13C CP-MAS NMR spectra of acetylated MFC with acetyl contents of 0 (unmodified MFC), 1.5, 4.5, 8.5, 13, and 17% (*spinning side bands).

was maintained up to this level of modification and is in agreement with the modification of easily accessible surface hydroxyl groups as stated earlier. For higher acetate contents, however, the intensity of the peaks at 2Θ ) 14.8, 16.6, and 22.3° decreased gradually with an increase in acetyl content from 8.5 to 17%, thus, indicating that acetyl groups were increasingly grafted in the inner crystalline regions of the nanofibers. This behavior correlates well with the second increase in reaction rate observed in Figure 3 (i.e., 500 min of reaction time), thus, indicating that new hydroxyl groups located deeper in the nanofibers became accessible to the acetic

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Figure 5. WAXD spectra of acetylated MFC with acetyl contents of 0 (unmodified MFC), 1.5, 4.5, 8.5, 13, and 17%.

anhydride molecules. In the 2Θ range from 5 to 10°, a broad reflection peak was observed in all acetylated MFC samples and was commensurate with the acetyl content of the nanofibers. This peak was associated with the presence of cellulose triacetate moieties on the surface of nanofibers.38 Nevertheless, the original pattern of cellulose I could still be clearly observed even at high Ac%, indicating that most of the fiber crystalline structure was preserved. Consequently, the reinforcing potential of these modified nanofibers was not dramatically reduced prior to further processing with the PLA matrix. Scanning Electron Microscopy (SEM) Analysis. The evolution of MFC morphology upon acetylation was examined using scanning electron microscopy (SEM) (Figure 6). The SEM image of unmodified MFC (i.e., %Ac ) 0%) confirmed the presence of microfibrils with diameters in the range of 10-100 nm, bundles of these, as well as unfibrillated fibers with a diameter close to the micrometer size. No significant difference in fiber morphology could be detected between acetylated and nonacetylated fibers, indicating that the network structure was preserved even at high %Ac. This result is in agreement with a limited alteration of the MFC crystalline structure upon acetylation, as stated earlier for the WAXD results. Nevertheless, the acetylated fibers with an acetate content above 1.5% appeared to be more packed than the other ones, this aggregating effect being commensurate with the acetyl content. This behavior might arise from the increasing hydrophobic character of MFC with acetylation, leading to the agglomeration of the nanofibers when deposited on the hydrophilic mica surface. Thermogravimetric Analysis (TGA). The impact of acetyl content on the thermal stability of MFC was evaluated by TGA. The thermograms of acetylated MFC with %Ac ranging from 0 to 17% are presented in Figure 7, where the 5% weight loss temperatures (T5wt%) are listed. At 5% weight loss, a T5wt% of 290 °C was measured for the unmodified MFC but was systematically shifted to higher temperatures for acetylated samples. T5wt% increased gradually with the acetyl content, up to 10.5%, for which a temperature of 345 °C was measured.

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However, a lower degradation temperature was observed for the highest Ac%, which might be associated to the significant decrease in crystallinity observed for this sample.54 A similar behavior was observed for the evolution of the main degradation temperature with the acetyl content. While a temperature of 384 °C was measured for acetylated samples up to an acetyl content of 1.5%, a temperature of 395 °C was obtained for all the other samples. Therefore, these results clearly demonstrate the improvement of MFC thermal stability upon acetylation55 and are essential to understand the thermal behavior of bionanocomposites prepared with these nanofibers (see PLA Bionanocomposites Reinforced with Unmodified and Acetylated MFC). Dispersion Tests in Chloroform (CHCl3). The dispersion behavior of unmodified and acetylated MFC was then examined in CHCl3, a PLA solvent of low polarity (Figure 8). These informations have not been documented so far in the literature, and constitute a prerequisite to further understand the dispersion behavior of acetylated MFC in PLA nanocomposites through a solvent-casting procedure. Two sets of experiments were conducted, using either solvent exchanged or dried-redispersed nanofibers (see Experimental Section). Stable suspensions were obtained using solvent exchanged nanofibers, but the quality of the dispersion depended on MFC acetyl content (Figure 8A). Identical suspensions were obtained up to a %Ac of 1.5%, showing suspensions with slight flocculation. At higher %Ac, the suspensions became increasingly homogeneous. This behavior might be associated with the increasing amount of acetyl moieties on the cellulose surface, thus, limiting the interactions between adjacent microfibrils. This surface blocking effect was further examined by studying the dispersion of dried nanofibers (Figure 8B). Unmodified MFC could not be dispersed, as a result of strong hornification during drying. The suspensions of acetylated MFC showed varying degrees of stability, which were dependent on %Ac. Stable suspensions were observed for acetylated MFC with a %Ac above 1.5%, the quality of the suspensions being similar to the one described in Figure 8A. However, a total flocculation was observed for an MFC with a %Ac of 1.5%, which was associated with an incomplete acetylation of the nanofibers surface. These innovative results confirm that the acetyl groups prevent the interactions between nanofibers and suggest that a minimum of acetate content is required to efficiently disperse dried MFC in CHCl3. This finding is of great interest from an economical point of view, as the acetylation treatment would provide powdered MFC, reducing significantly the transportation costs. PLA Bionanocomposites Reinforced with Unmodified and Acetylated MFC. PLA bionanocomposites with tunable properties were prepared using the unmodified and acetylated MFC (with Ac% ranging from 0 to 17%), with MFC contents from 2.5 to 17 wt %. The resulting nanocomposites were characterized in terms of filler dispersion, water sorption, and thermal analysis (DMA, TGA, and DSC). Filler Dispersion. The quality of filler dispersion in the PLA matrix was visually evaluated using transparency of the resulting films, of identical thickness (0.11 ( 0.01 mm), as an indicator of good dispersibility. First of all, similar differences were observed among all filler contents tested. Therefore, only photographs corresponding to nanocomposite films reinforced with 10 wt % MFC are presented in Figure 9. As compared with the neat PLA film (Figure 9A), nanocomposite films were less translucent, but different behaviors were obtained depending on the %Ac. The addition of unmodified MFC led to a film off-white in color, with the appearance of MFC aggregates as white dots all over the film (Figure 9B).



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Figure 6. SEM images of unmodified MFC (0%) and acetylated MFC with acetyl contents of 1.5, 4.5, 8.5, 13, and 17%.

When acetylated MFC was used, the presence of aggregates progressively vanished with an increase of Ac%, and the films became increasingly translucent (Figure 9C-E). These observations suggest that the MFC dispersion in the PLA film was significantly improved by the surface grafting of acetate groups, and correlate with the dispersion tests results. Water Sorption Tests. Despite a hydrophobic character, PLA is sensitive to moisture, which can result in certain conditions in polymer hydrolysis self-catalyzed by carboxylic acid end groups of PLA chains.56 The impact of the %Ac on the nanocomposites hygroscopicity was thus evaluated, and moisture sorption tests were performed on bionanocomposites conditioned at 97% R.H, 25 °C. Figure 10 presents the evolution of water uptake of PLA films reinforced with 10 (Figure 10A) and 17 wt % (Figure 10B) of acetylated MFC, as a function of Ac%. The same moisture sorption pattern was observed in all samples, including neat PLA film. After a relatively fast moisture uptake during the first 24 h, the water sorption slowed down, leading gradually to a plateau. Concerning the neat PLA film, a moisture uptake of 0.43% was measured after 1 month experiment (i.e., 834 h), confirming its hydrophobic character. The addition of

Figure 7. TGA thermograms for acetylated MFC with acetyl contents of 0 (unmodified MFC), 1.5, 4.5, 8.5, 13, and 17%.

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Figure 8. Suspensions in chloroform of acetylated MFC (0.07% (w/w)) with acetyl contents of 0 (unmodified MFC), 1.5, 4.5, 8.5, 13, and 17%. (A) and (B) stands for solvent exchanged and dried samples, respectively. Photographs were taken 24 h after sonication.

Figure 9. Photograph of a neat PLA film (A) and nanocomposite PLA films reinforced with 10 wt % acetylated MFC with %Ac of 0 (B), 3.5 (C), 8.5 (D), and 17% (E).

unmodified and acetylated MFC increased the moisture uptake of nanocomposite materials respect to the neat PLA and was commensurate with the weight fraction of nanofibers inside the film. Nevertheless, for a given filler content, the moisture sorption decreased as the acetyl content increased, thus, confirming the reduction in hydrophilic character of the MFC upon acetylation. The water sorption of PLA nanocomposites can therefore be adjusted with the Ac%. This finding is strongly interesting, as it should allow the control of the biodegradation rate or dimensional stability of such bionanocomposites. Dynamic Mechanical Analysis. Figures 11 and 12 present the storage modulus (E′) and tan δ for all nanocomposites prepared in this work. Specifically, the influences of the type of microfibril

(unmodified and acetylated), the Ac% of MFC and the filler content on the viscoelastic properties of the neat PLA are evaluated. First of all, the reproducibility of the response was very good, as demonstrated by the low standard deviation observed for each sample type. Also, recall that all DMA samples were initially heated from 30 to 100 at 10 °C/min and then annealed for 10 min before the dynamic cooling scans were collected. Spanning from low to high temperatures, three different regions could be identified: the glassy state (below 45 °C), the glass transition (approximately between 45 and 80 °C), and the rubbery plateau (approximately between 80 and 100 °C). The neat PLA (cross open symbols) exhibited a glass transition (Tg) at 63.8 °C, obtained from the tan δ curves (Figure 11B and Table 1). Through this transition the neat PLA softened



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Figure 10. Moisture sorption curves of neat PLA and PLA nanocomposites reinforced with 10 wt % (A) and 17 wt % (B) of nanofibers, using acetylated MFC with an acetyl content of 0, 3.5, 8.5, and 17%. All samples were conditioned at 97% R.H and 25 °C.

as shown by the reduction of E′ by more than 1 order of magnitude. Finally, in the rubbery region, the neat PLA became extremely soft from 90 to 100 °C. Consequently, the static force could not be maintained above 15% of the dynamic force, leading to sample buckling and, thus, resulting in unreliable data that were removed. As it can be seen in Figures 11 and 12, in general, the presence of the MFC had a strong influence on the viscoelastic response of the resulting nanocomposites in the whole temperature region. In the glassy region (see inset graph of Figure 11A), the storage modulus gradually increased with increasing contents of the unmodified and 8.5% acetylated MFC, and no significant differences were found between the two types of MFC at each filler content. Interestingly, a significantly higher reinforcement was obtained for nanocomposites reinforced with 10 and especially 17 wt % MFC, as demonstrated by the larger increase in E′ in respect to the neat PLA (Figure 11A, Figure 12A,C). This reinforcing effect might result from the formation of a stiff hydrogen bonded cellulose network above a percolation threshold.15,34 Finally, at the same filler content (i.e., 10 and 17%), the Ac% did not significantly alter the reinforcement of nanocomposites (Figure 12A,C). In the PLA glass transition, the drop in storage modulus and the decrease in damping intensity (tan δ) were gradually reduced with increasing filler content (Figure 11A,B), regardless of type of MFC. This suggested that the presence of the MFC promoted segmental restrictions of the PLA chains leading to the increase in E′ and decrease in tan δ intensity. Also, the presence of the MFC altered the PLA glass transition temperature. In general, the reinforced nanocomposites showed lower Tg than the neat PLA (Table 1 or Figures 11B and 12B,D). This could be an indication of a

Figure 11. Temperature dependence of the storage modulus E′ (A) and tan δ (B) of neat PLA (open cross) and PLA-based nanocomposites reinforced with unmodified (open symbols) and acetylated MFC (Ac% ) 8.5%; plain symbols), at filler contents of 2.5 (square), 5 (triangle), 10 (lozenge), and 17 (circle) wt %.

poor fiber-matrix interaction, resulting in an earlier softening of the material. However and most interesting, the neat PLA Tg was increased to 66.4 °C for the 8.5% acetylated MFC at 17 wt %, which can be attributed to an improved compatibility between the bulk PLA and the modified MFC. Surprisingly, the lowest Tg value was obtained when using the 17% acetylated MFC, for which the best chemical affinity toward the hydrophobic PLA matrix would be expected, at 17 wt % filler content (Figure 11B or Table 1). These findings suggest that a combination of the Ac%, which will adjust the chemical affinity of the microfibrils to the matrix, and the filler content are essential to achieve a good polymer-matrix interaction. In the rubbery region, an important reinforcing effect was observed due to the presence of the MFC. As expected, this reinforcement was more pronounced as the filler content increased (Figures 11A and 12A,B). Interestingly, nanocomposites prepared with acetylated MFC at 10 and 17 wt % filler contents showed a slightly but significantly lower storage modulus than those prepared with the unmodified MFC (Figure 12A,B). This difference might be attributed to a partial prevention of hydrogen bonding between adjacent nanofibers due to the presence of acetyl groups on the cellulose surface, resulting in softer nanocomposites.

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Figure 12. Temperature dependence of the storage modulus (E′) and tan δ of neat PLA (open cross) and PLA-based nanocomposites reinforced with 10 (A, B) and 17 (C, D) wt % MFC, at different Ac%: 0 (0), 3.5 (∆), 8.5 (b), and 17% (O). Table 1. Thermal Characteristics of PLA Nanocomposites Reinforced with Unmodified and Acetylated MFCa DMAb

DSCc

TGAc

filler

Ac%

filler content (wt %)

Tg (°C)

∆Hm (J/g)

χc (%)

T5wt% (°C)

none unmodified MFC

0 0

3.5

63.8 (( 0.0) 62.9 (( 0.0) 62.8 (( 0.1) 61.4 (( 0.0) 62.8 (( 0.0) 61.8 (( 0.0) 62.3 (( 0.0) 61.9 (( 0.0) 61.8 (( 0.0) 62.3 (( 0.0) 66.4 (( 0.0) 61.8 (( 0.0) 61.3 (( 0.0)

29.7 (( 0.3) 28.2 (( 0.3) 26.2 (( 0.4) 24.2 (( 0.1) 21.6 (( 0.8) 26.4 (( 1.3 25.8 (( 0.8) 30.6 (( 1.5) 27.6 (( 0.8) 24.5 (( 0.1) 23.9 (( 0.7) 23.5 (( 0.1) 23.2 (( 0.7)

31.9 (( 0.3) 31.1 (( 0.4) 29.7 (( 0.4) 28.9 (( 0.2) 28.1 (( 1.1) 31.5 (( 1.6) 33.5 (( 1.1) 33.8 (( 1.7) 31.3 (( 0.9) 29.3 (( 0.2) 31.1 ((1.0) 28.1 (( 0.1) 30.1 (( 0.8)

335 (( 2.1)

acetylated MFC

0 2.5 5 10 17.3 10 17.3 2.5 5 10 17.3 10 17.3

8.5

17

299 (( 0.1) 316 (( 2.0)

338 (( 0.0) 338 (( 0.7)

Glass transition temperature (Tg), enthalpy of melting (∆Hm), and degree of crystallinity (χc). χc ) ∆Hm/w; ∆Hm°, where ∆Hm° is the enthalpy of melting for a 100% crystalline PLA (∆Hm° ) 93 J/g),49 and w is the weight fraction of the polymeric matrix in the nanocomposite. b Tg extracted from tan δ peak and is the average of three replicates. c Experiment conducted on nanocomposite films after DMA analysis. a

Finally, the crystallinity and thermal stability of the analyzed DMA samples were further evaluated by DSC and TGA, respectively. As it can be observed in Table 1, in general, the addition of unmodified or acetylated MFC did not significantly modify the PLA crystallinity. Consequently, in the present study, crystallinity did not seem to play a key role in the nanocomposite reinforcement. However, the acetylation of MFC dramatically increased the thermal stability of the resulting nanocomposites, with respect to those prepared with the unmodified MFC (Table 1). Hence, the onset degradation temperature (at the 5% weight loss) was increased from 299 to a maximum of 338 °C, corroborating the improved thermal stability of MFC upon acetylation as previously discussed (Figure 7). Therefore, the

thermal degradation properties of such biomaterials can be adjusted with the %Ac. This relationship has not been extensively reported so far in the literature, which constitutes an additional asset of such nanomaterials.

Conclusions In the present work, novel bionanocomposites were synthesized from PLA and acetylated MFC, with the objective of demonstrating that the final nanocomposite properties could be tailored through a careful control of %Ac. MFC with different Ac% were successfully prepared, as demonstrated by FTIR and 13C CP MAS NMR spectroscopy, to


decrease the hydrophilicity of the nanofibers and to enhance their compatibility with the PLA matrix. It was shown that Ac% above 4.5% promoted significant changes in the MFC crystalline structure (WAXD experiments) and, more interesting, prevented hornication upon drying. This was possible because the grafted acetyl groups allowed reduced hydrogen bonding between cellulose microfibrils. Moreover, we demonstrated that the resulting powdered nanofibers were not only more thermally stable but also easily redispersed in chloroform, a PLA solvent of low polarity, leading to very stable suspensions. This finding constitutes a great industrial interest because the acetylated MFC can be stored in dry form, improving its durability against bacteria and reducing transportation costs. PLA bionanocomposites were prepared using acetylated MFC as reinforcing agent, through a solvent casting approach in chloroform. We showed that MFC with increasing Ac% provided more translucent nanocomposites with reduced hygroscopicity and improved thermal stability versus unmodified MFC, and all these properties could be fine-tuned through an accurate control of %Ac. The DMA experiments showed the potential of MFC to enhance the viscoelastic properties on the neat PLA in the temperature range studied and also provide an insight into the interactions between PLA and acetylated MFC. Regardless of the microfibril type, increasing amounts of MFC provided higher storage modulus and lower damping intensities than the neat PLA. In general, the presence of the MFC slightly reduced the neat PLA glass transition temperature. However and most interestingly, the neat PLA Tg was significantly increased when the 8.5% acetylated MFC were used at 17 wt %, reflecting a strong fiber-matrix interaction. This finding suggests that compatibility between the MFC and the PLA matrix can be enhanced by adjusting both the Ac% and filler content. We conclude that the redispersibility of dried acetylated MFC in a nonpolar solvent, such as chloroform, is very promising from an economical point of view, and opens up new opportunities for the development of novel nanocomposites materials using nonpolar matrices. The accurate control of the Ac% shows promise as a straightforward method for the design of nanocomposites with targeted properties, and especially in application where the control of thermal, hygroscopic, and mechanical properties is of particular interest. Acknowledgment. This work was financially supported by the European Commission under the Contract No. 214660 (Call Identifier: FP7-NMP-2007 Large1, SustainComp, project: http://www.sustaincomp.eu). Daniel Rentsch, Beatrice Fischer. and Urs Gfeller are acknowledged for their help with NMR, DSC, and WAXD experiments, respectively. Supporting Information Available. Determination of acetyl content and FTIR calibration curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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BM901186U

Synthesis and Characterization of Bionanocomposites with Tunable

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