Cellulose Whiskers versus Microfibrils: Influence of the Nature of the

Biomacromolecules 2009, 10, 425–432

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Cellulose Whiskers versus Microfibrils: Influence of the Nature of the Nanoparticle and its Surface Functionalization on the Thermal and Mechanical Properties of Nanocomposites Gilberto Siqueira, Julien Bras, and Alain Dufresne* Grenoble Institute of Technology (INP), The International School of Paper, Print Media and Biomaterials (PAGORA), 461 rue de la Papeterie, BP 65 - F-38402 Saint Martin d′He`res Cedex, France Received October 21, 2008; Revised Manuscript Received November 28, 2008

In the present work, nanowhiskers and microfibrillated cellulose (MFC) both extracted from sisal were used to reinforce polycaprolactone (PCL). We report the influence of the nanoparticle’s nature on the mechanical and thermal properties of the ensuing nanocomposites. The surface of both the nanoparticles was chemically modified to improve their compatibilization with the polymeric matrix. N-Octadecyl isocyanate (C18H37NCO) was used as the grafting agent. PCL nanocomposite films reinforced with sisal whiskers or MFC (raw or chemically modified) were prepared by film casting. The thermal behavior (Tg, Tm, Tc, and degree of crystallinity) and the mechanical properties of the nanocomposites in both the linear and the nonlinear range were determined using differential scanning calorimetry (DSC), dynamical mechanical analysis (DMA), and tensile tests, respectively. Significant differences were reported according to the nature of the nanoparticle and amount of nanofillers used as reinforcement. It was also proved that the chemical treatment clearly improves the ultimate properties of the nanocomposites.

Introduction Nowadays there is a simultaneous and growing interest in developing biobased products and innovative process technologies that can reduce the dependence on fossil fuel and move to a sustainable materials basis.1,2 In parallel, researchers have focused their works on the processing of nanocomposites (materials with nanosized reinforcement) to enhance mechanical properties.3,4 Similarly to traditional microcomposites, nanocomposites use a matrix where the nanosized reinforcement elements are dispersed. The reinforcement is currently considered as a nanoparticle when at least one of its dimensions is lower than 100 nm. This particular feature provides nanocomposites unique and outstanding properties never found in conventional composites.4,5 There are several possibilities to obtain nanoelements from renewable resources depending on their biological origin (e.g., cellulose, starch, chitin). We have focused our work on cellulose nanoparticles. Besides the low cost of the raw material, the use of cellulose nanocrystals as a reinforcing phase in nanocomposites has numerous well-known advantages.2,5-10 However, considering application as reinforcement, cellulose nanoparticles can present some disadvantages, for example, high moisture absorption, poor wetability, incompatibility with most of polymeric matrices, and limitation of processing temperature. Indeed, lignocellulosic materials start to degrade near 220 °C, restricting the type of matrix that can be used in association with natural fillers.5,11 Biobased nanocomposites are the next generation of materials for the future.1,12 Polycaprolactone (PCL) is a striking candidate as a matrix due to its biodegradability properties. PCL is a semicrystalline polymer with a glass transition temperature around -60 °C and a melting temperature of 59-60 °C. Its * To whom correspondence [email protected].

should

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addressed.

E-mail:

physical properties and commercial availability make it very attractive as a substitute material for nondegradable polymers. Cellulose nanoparticles can be extracted from lignocellulosic fibers and are available all around the World, with some of them being abundant in tropical countries.13 Sisal fibers are obtained from the leaves of the AgaVe sisalana14 being one of the most widely used natural fibers and easily cultivated. It can be easily found in India, Brazil, and Tanzania, the two formers being considered as the main producing countries.13 Most studies found in the literature about biocomposites use cellulose as simple “filler”, which in some cases contributes to enhance the rigidity, but mostly embrittles the polymer. In the present work, cellulose was used as a nanostructured highperformance constituent, in the form of nanofibers (whiskers or microfibrilated cellulose-MFC). Thus, the potential and hierarchical structure of cellulose is totally exploited. The study of cellulosic nanoparticles as a reinforcing phase in nanocomposite films started 20 years ago,15 and since this time, a huge amount of literature has been devoted to nanocellulose and is becoming a topical subject, as revealed from the abundant literature. Different descriptors of these nanoparticles are used, including whiskers, monocrystals, and nanocrystals. These crystallites have also often been referred to in literature as microfibrils, microcrystals, or microcrystallites, despite their nanoscale dimensions. The term “whiskers” is used to designate elongated crystalline rod-like nanoparticles, whereas the designation “microfibrils” should be used to designate long flexible nanoparticles consisting of alternating crystalline and amorphous strings. The obtaining of the former involves a specific step for the digestion of amorphous cellulosic domains, generally acid hydrolysis, whereas the latter is obtained from a mechanical treatment. Although abundantly studied separately, no systematic comparison between these two nanoparticles has been reported, except a short communication in 2004.16 The aim of the present study was to evaluate the thermomechanical behavior of nanocomposite films obtained from PCL

10.1021/bm801193d CCC: $40.75  2009 American Chemical Society Published on Web 12/29/2008

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and cellulosic nanoparticles from sisal focusing on the differences induced by the nature of the nanoparticle (whiskers vs microfibril) and its surface chemical modification.

Experimental Section Materials. Native sisal fibers (AgaVe sisalana), originating from northeast Brazil, were purchased in Mariana (Minas Gerais, Brazil). Poly(caprolactone) (Mn ) 42500 g · mol-1, Mw ) 65000 g · mol-1), sulphuric acid (g95 wt %), and N-octadecyl isocyanate were obtained from Aldrich. Ethanol, acetone, chloroform, toluene, and dichloromethane were purchased from Chimie-Plus. Cellulose Whiskers. Sisal fibers were cut with a FRITSCH Pulverisette mill, until fine particulate fibers were obtained. Then the fibers were treated with a 4 wt % NaOH solution at 80 °C for 2 h under mechanical stirring. This treatment was done three times to purify cellulose by removing other constituents present in the fibers. After each treatment, fibers were filtered and washed with distilled water until the alkali was completely eliminated. A subsequent bleaching treatment was carried out to bleach the fibers. The solution used in this treatment consisted of equal parts of acetate buffer, aqueous chlorite (1.7 wt % in water), and distilled water. The bleaching treatment was performed at 80 °C for 4 h under mechanical stirring and was repeated four times. After each treatment, the fibers were filtered and washed with distilled water. Acid hydrolysis was achieved at 50 °C with 65 wt % sulfuric acid (preheated), for about 40 min, under mechanical stirring. The fiber content during all these chemical treatments was in the range 4-6 wt %. The suspension was diluted with ice cubs to stop the reaction and washed until neutrality by successive centrifugations at 10000 rpm at 10 °C for 10 min each step and dialyzed against distillated water, in the sequence. Afterward the sisal whiskers suspension was homogenized by using an Ultra Turax T25 homogenizer for 5 min and filtered using glass filter No. 1. Some drops of chloroform were added to the whiskers suspension which was stored at 4 °C. Microfibrillated Cellulose (MFC). A 2.0% (wt/v) suspension of bleached sisal MFC was pumped through a microfluidizer processor (Model M-110 EH-30). The slurry was passed through the valves that applied a high pressure. Size reduction of products occurs into Interaction Chamber (IXC) using cellules of different sizes (400 and 200 µm). Pumping cycles were varied to optimize the fibrillation process. Surface Chemical Modification. The surface chemical modification of cellulosic nanoparticles was performed in toluene. To avoid the drying of the nanoparticles that undoubtedly should lead to a strong aggregation process we used never dried cellulose whiskers or MFC. A solvent exchange procedure from water to toluene was used. For that, an aqueous suspension with the desired amount of cellulose nanoparticles (1 wt %) was solvent exchanged to acetone and then to dry toluene by several successive centrifugations and redispersion operations. Sonication was performed after each solvent exchange step to avoid aggregation. In a three-necked round-bottomed flask, equipped with a reflux condenser, 3 g of a whisker or MFC suspension in toluene and 100 mL of toluene were added. The system was kept in a nitrogen atmosphere. An excess of n-octadecyl isocyanate (16.9 g) was added drop by drop when the temperature of the system reached 90 °C. The temperature was then increased up to 110 °C and it was kept in this condition for 30 min. The modified whiskers were filtered and washed with ethanol to remove amines formed during the reaction and the isocyanates that did not react. Afterward, the modified materials were washed with ethanol and centrifuged four times at 10000 rpm and 10 °C for 15 min each step. The final step consisted in changing the solvent of the modified nanofibers (whiskers or MFC) from ethanol to dichloromethane, which was the solvent used for the film preparation. Details of the grafting procedure and characterization of the grafted nanoparticles are reported elsewhere.17 The values of the degree of substitution have been estimated from elemental analysis for both whiskers and MFC.17 A degree of substitution of DS ) 0.07 and DS

Siqueira et al. ) 0.09 were found for whiskers and MFC, respectively. This result takes into account the whole hydroxyl groups, whereas the experimental conditions allow grafting only the surface OH groups. The grafting was sufficient for this application. This grafting is important to better understand the changes in compatibility and dispersion of these nanoparticles. Nanocomposite Films Preparation. The polycaprolactone (PCL 42500 g · mol-1) was first dissolved in dichloromethane at room temperature for 20 h (0.036 g · L-1). Different amounts of whiskers or MFC were used to prepare the nanocomposite films, namely, 0, 3, 6, 9, and 12 wt % on the dry basis. The maximum of 12 wt % is generally enough to level off the properties of the materials. The corresponding amounts of nanoparticles in suspension in dichloromethane were mixed and magnetically stirred for 6 h with the PCL solution. The suspensions were sonicated for 2 min before being cast in Teflon molds, where the films were obtained by solvent evaporation at room temperature. Characterizations. Samples for transmission electron microscopy (TEM) were observed with a Philips CM200 transmission electron microscope using an acceleration voltage of 80 kV. A drop of diluted suspension of sisal whiskers was deposited on a carbon-coated grid. The samples were stained with a 2 wt % solution of uranyl acetate. A field emission scanning electron microscope (SEM), model Quanta 200 FEI, with accelerating voltage of 12.5 kV was used to study sisal MFC surfaces’ topography. The samples were mounted onto a substrate with carbon tape and coated with a thin layer of gold. Optical microscopy observations were performed using an optical microscope Olympus BH-2 in transmission mode. Differential scanning calorimetry (DSC) experiments were carried out with a DSC Q100 differential calorimeter (TA Instruments) fitted with a manual liquid nitrogen cooling system. The samples were placed in hermetically closed DSC devices. The heating and cooling rates were 10 °C · min-1 from -100 to 100 °C and from 100 to -100 °C, respectively, in a N2 atmosphere. Sample weights were between 6 mg and 8.5 mg. Tensile tests were carried out with a RSA3 (TA Instruments, U.S.A.) equipment with a 100 N load cell. Measurements were performed with a cross head speed of 10 mm · min-1 at 25 °C. The samples were prepared by cutting strips of the films 20 mm long and the distance between jaws was 10 mm, whereas the width and the thickness of the samples were measured before each measurement. The initial strain rate was therefore ε˙ ) 1.67 × 10-2 s-1. Five samples were used to characterize each nanocomposite. Dynamical mechanical analysis (DMA) of the nanocomposite films was carried out using a RSA3 (TA Instruments, USA) equipment working in tensile mode. The measurements were performed at a constant frequency of 1 Hz, strain amplitude of 0.05%, in the temperature range from -100 to 100 °C, a heating rate of 5 °C · min-1 and a distance between jaws of 10 mm. The width of the samples varied from 3 to 5 mm, which were measured before each analysis. Two samples were used to characterize each nanocomposite.

Results and Discussion Morphological Analyses. TEM micrographs of sisal whiskers reported in Figure 1 show the homogeneity and nanometric dimensions of sisal whiskers. The length and diameter of sisal nanocrystals were determined by using digital image analysis (ImageJ). The geometric average length and diameter were around 215 nm ( 67 nm and 5 nm ( 1.5 nm, respectively. A minimum of 421 and 205 measurements were used to determine both the length and the diameter, respectively, of sisal whiskers. These dimensions are in agreement with the results found by Garcia de Rodriguez et al.18 However, the length of our whiskers is lower compared to the one reported by Garcia de Rodriguez18 (250 nm), resulting in a lower aspect ratio (L/d ) 43 compared to 6018). It is worth noting that sisal plants were not ground at the same place and under the same conditions (Brazil vs India).

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Figure 1. Transmission electron micrographs of sisal whiskers.

Figure 2. SEM image sisal MFC (A) and optical microscopy image of sisal MFC (B).

Figure 3. Visual examination of sisal whiskers (A) and microfibrillated cellulose (B) in different liquids: (i) water, (ii) acetone, and (iii) dichloromethane.

Figure 2 shows SEM image (A) and optical microscopy image (B) of sisal MFC. The diameter of microfibrilated cellulose from sisal were determined by digital image analysis (ImageJ) of SEM micrographs. The average diameter was about 52 nm ( 15 nm showing that microfibrils bundles were obtained. A minimum of 50 measurements were performed for its determination. Nonetheless it was not possible to determine the avegare length of sisal MFC by SEM microscopy analysis. Indeed, when a drop of MFC suspension was dried on the substrate prepared for SEM analysis, a film formed which hindered the observation of MFC’s length. Films Preparation. PCL-based films reinforced with various fractions of either raw or chemically modified sisal nanoparticles have been prepared. Figure 3A shows the dispersion state of unmodified whiskers in different organic solvents, namely, water

(i), acetone (ii), and dichloromethane (iii). The dispersion state of unmodified MFC in various liquids can be observed in Figure 3B. We clearly observe that MFC’s suspension remains homogeneous when substituting water for acetone (Figure 3B, (i) and (ii), respectively). However, because the polarity of the solvent decreased, the solvent exchange step became more difficult. So, MFC were not homogeneously dispersed in dichloromethane (Figure 3C, (iii)). Therefore, the preparation of unmodified MFC filled PCL films by casting from a dichloromethane suspension was not possible. The difference in the dispersibility between whiskers and MFC could be related to their only differences, that is, the possibility of entanglement and the presence of residual pectins at the surface of MFC.19 The existence and the characterization of residual hemicelluloses at the surface of MFC were reported

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Figure 4. Typical stress-strain curves obtained from tensile tests for PCL-based nanocomposite films reinforced with 3 wt % (A) and 12 wt % (B) sisal whiskers: unmodified (O) and chemically modified (∆). The behavior of the neat PCL matrix (9) is added as reference. The inset is an expanded view of the low strain region.

elsewhere.20 It was found that around 2% of hemicelluloses were present at the surface. Tensile Tests. The nonlinear mechanical behavior of PCLbased nanocomposite films was characterized by tensile tests performed at room temperature. Typical stress-strain curves obtained from tensile tests for PCL-based nanocomposite films are shown in Figure 4. These figures clearly show the influence of the grafting of cellulosic whiskers on the mechanical behavior of the nanocomposite films. Young’s modulus values were analyzed from the initial slope of the tensile curves, as detailed in the expanded views. Figure 4A shows that the film reinforced with 3 wt % of modified sisal whiskers displays higher tensile modulus, strength, and strain at break, compared to its unmodified filler counterpart. It clearly shows the positive impact of the surface chemical modification of the whiskers on the mechanical behavior of the nanocomposite films. Moreover, compared to the neat matrix, the addition of 3 wt % of modified nanoparticles allows enhancing the tensile modulus without detrimental effect on the ultimate mechanical properties (strength and strain at break) contrarily to unmodified nanoparticles. At this loading level, the nanocomposite films display a ductile and rubber-like behavior similar to the PCL matrix and typical of a semicrystalline thermoplastic material tested above its glass transition temperature. On the contrary, highly filled composites (12 wt %, Figure 4B) show a completely different behavior. No plastic deformation was observed and their extreme brittleness is clearly observed. The strain at break is extremely low whether the surface of the nanoparticle was chemically modified or not. A similar behavior has been reported for MFC reinforced poly(styrene-co-butyl acrylate) by Dalmas et al.21 The influence of the morphology of the cellulosic nanoparticle (whiskers vs MFC) on the tensile mechanical behavior of the nanocomposite film is shown in Figure 5. The addition of 3 wt % of chemically modified cellulosic nanoparticles induces an increase of the tensile modulus regardless their nature, as can be seen in Figure 6A. However, the ultimate mechanical properties differ depending on the nature of the filler. Modified sisal whiskers allow keeping a higher level of elongation at break for the PCL film, compared to modified MFC. However, the value of the strength is higher for modified MFC nanocomposites than for modified whiskers nanocomposites. Figure 6 (A) shows the evolution of the Young’s modulus determined from the initial slope of the stress-strain curves as a function of the nanoparticle content. Nominal data were used

Figure 5. Typical stress-strain curves obtained from tensile tests for PCL-based nanocomposite films reinforced with 3 wt % modified sisal whiskers (O) and 3 wt % modified MFC (∆). The behavior of the neat PCL matrix (9) is added as reference.

for the stress and strain values. Results were reported for the neat matrix and nanocomposite films reinforced with unmodified and chemically modified whiskers, as well as chemically modified MFC extracted from sisal. The addition of raw sisal whiskers in PCL results in a global decrease of the tensile modulus of the material. It could be ascribed to the poor interfacial adhesion between the cellulosic nanoparticles (hydrophilic) and the PCL matrix (hydrophobic). However, previous studies have shown that not only the fillermatrix adhesion but also the filler-filler interactions are important when considering the reinforcing capability of cellulose whiskers.5 Indeed, it was shown that above the percolation threshold, the cellulosic nanocrystals form a stiff percolating network through hydrogen bonding that is responsible for the reinforcing effect when the nanocomposite films were obtained by a casting/ evaporation technique. For rod-like nanoparticles with an aspect ratio of 43, the percolation threshold is around 1.6 vol %, that is, around 2 wt %.5 However, in our case the liquid medium was dichloromethane and not water. This should most probably affect the dispersion of the nanoparticles in the liquid and then in the solid nanocomposite film. Therefore, the decrease of the tensile modulus is most probably ascribed to the poor dispersibility of the highly hydrophilic nanoparticles within the


Figure 6. Evolution of the Young’s modulus (A), strength (B), and strain at break (C) for PCL-based nanocomposites vs filler (whisker or MFC) content: unmodified (9) and chemically modified whiskers (O) and MFC (∆).

hydrophobic matrix rather than to the poor adhesion at the fillermatrix adhesion as already suggested elsewhere.22 The crystallization of the polymeric matrix could also affect the formation of the percolating cellulosic network. When the surface of the nanoparticles is chemically modified, the tensile modulus tends to increase. It most probably results from the compatibilizing effect of the grafted molecules that induces a better dispersion of the reinforcing phase within the matrix. Compared to the neat PCL matrix, the addition of only 3 wt % of modified sisal whiskers or modified MFC increases the tensile modulus by a factor 15 or 60%, respectively. With


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12 wt % nanoparticles, the modulus is increased by 45 and 137%, respectively. A simple visual inspection of the nanocomposite films allows estimating the level of dispersion of the nanoparticles within the polymeric matrix. Indeed, compared to the neat matrix, the nanocomposite films reinforced with unmodified sisal whiskers becomes dotted with black. When the cellulose whiskers were chemically modified the occurrence of these aggregates vanish and the appearance of the nanocomposite film becomes similar to the one of the unfilled film evidencing the higher dispersion level of the filler. Several authors21,23,24 have also reported an increase in the stiffness of polymeric films by increasing the filler content. However, none of them have compared whiskers and MFC. With these innovative results we prove that MFC has a better impact on the stiffness increase and we confirm that the reinforcing effect depends on the morphology of the filler.16 According to Dufresne et al.,19 pectins act as a binder between cellulose microfibrils, improving the mechanism of load transfer to the filler. This binding mechanism is controlled by hydrogen bonding and/or covalent connections between pectins, hemicellulose, and cellulose microfibrils. Therefore, the higher Young’s modulus observed for modified MFC-based nanocomposites could be explained, at least partially, by the presence of pectins at the surface of the nanoparticles. Pectins and hemicelluloses are not present at the surface of the whiskers because they are eliminated during the hydrolysis treatment. The evolution of the strength and strain at break for PCLbased films as a function of the filler content, presented in Figures 6B and C, confirms the differences between the two cellulose nanoparticles. Figure 6B shows that the strength is drastically reduced upon loading PCL with unmodified sisal whiskers, while both modified sisal whiskers and MFC are able to maintain the strength of PCL at a relatively higher level, especially for low contents of modified nanofillers (e.g., 3 wt %). Low difference is reported between the two kinds of modified nanoparticles, although the strength is systematically higher for MFC. However, this difference is still significant in regard with the error bars. As expected, the strain at break (Figure 6C) decreases upon filler addition. However, for low filler content (3 and 6 wt %), a clear difference is observed depending on the nature of the nanoparticle. The most brittle material is the nanocomposite film reinforced with unmodified sisal whiskers, followed by modified MFC and then modified whiskers. For higher filler content, the strain at break remains similar regardless the nature of the nanoparticle. The surface chemical modification of the nanoparticles most probably affects their dispersibility as already suggested. This poor dispersibility and the lack of intimate adhesion between the filler and the matrix lead to numerous irregularly shaped microvoids or microflaws in the composite structure. Because of these microflaws the stress transfer from the matrix to the filler is poor and the mechanical properties of the nanoparticles are not fully utilized. The brittleness of the material is accentuated by the probable aggregation of the unmodified nanoparticles that leads to the formation of weak points. The lowest elongation at break observed for modified MFC compared to modified whiskers is most probably ascribed to the possibility of entanglements of the former. Filler entanglements are not likely to occur with whiskers that occur as straight rodlike nanoparticles and then the behavior at break of the ensuing nanocomposites is mainly governed by the matrix. These results fully agree with the observations reported by Azizi Samir et al.16 in their study of unmodified sugar beet

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Figure 7. Evolution of the logarithm of the storage tensile modulus (log E′) vs temperature at 1 Hz for PCL-based nanocomposites films reinforced with modified sisal whiskers (A) or modified MFC (B): 0 wt % (9), 3 wt % (O), 6 wt % (+), 9 wt % (×), and 12 wt % (∆).

cellulose microfibrils reinforced poly(styrene-co-butyl acrylate). They submitted cellulose microfibrils prepared from sugar beet to different hydrolysis conditions in order to obtain nanofillers with various entanglements characteristics. They observed a decrease of both the strength and tensile modulus, and an increase of the strain at break when the hydrolysis strength increases, that is, when decreasing the possibility of filler entanglement. To conclude on the nonlinear mechanical behavior of sisal nanoparticles reinforced PCL, the surface chemical modification has a positive impact on both the stiffness and ductility of the films. Moreover, the use of MFC instead of whiskers allows obtaining stiffer but more brittle nanocomposite films. Dynamic Mechanical Behavior. Figure 7 shows the evolution of the logarithm of the storage tensile modulus as a function of temperature for modified sisal whiskers (panel A) and MFC (panel B) reinforced PCL films. Modulus values have been normalized at low temperature. Between the main relaxation around -60 °C associated with the glass transition and the irreversible polymeric chain flow around 50 °C due to the melting of the PCL matrix, the storage modulus is increased upon filler addition. This effect is more significant with MFC in agreement with tensile tests. Also, it is observed that the irreversible flow of the polymeric matrix upon heating occurs at lower temperature for MFC reinforced PCL films compared to cellulose whiskers reinforced

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films. This effect is ascribed to differences in the melting behavior of the two sets of nanocomposites, as for instance the melting temperature and degree of crystallinity, as it will be outlined in the next section. Thermal Characterization. The thermal characterization of PCL-based nanocomposite films was carried out using DSC measurements. From the analysis of DSC traces, the glass-rubber transition temperature (Tg), the melting temperature (Tm), associated heat of fusion (∆Hm), degree of crystallinity (χc), and crystallization temperature (Tc) were obtained for the unfilled PCL film, and nanocomposite materials reinforced with either unmodified and modified whiskers, and modified MFC. The resulting experimental data are listed in Table 1. For all nanocomposites, the crystallization temperature is significantly increased (around 10-12 °C) compared to the neat matrix. For unmodified cellulosic whiskers, no evolution upon filler loading is observed, whereas it is found to globally slightly increase for modified nanoparticles. Moreover, the Tc increase is more pronounced for a given whiskers content. The filler most probably acts as a nucleating agent for the crystallization of PCL. Panaitescu et al.25 also reported an increase of 10 °C of the crystallization temperature for maleic anhydride modified polypropylene (MAPP) composites reinforced with 20 wt % of MFC or for polypropylene (PP) filled with 30 wt % of microcrystalline cellulose. They also observed that the increase in the crystallization temperature was more important in the presence of filler-matrix compatibilizer. A similar result was reported for MAPP-compatibilized palm tree fibers reinforced PP.26 The degree of crystallinity of PCL is almost not influenced by the presence of chemically modified sisal MFC. It remains around 50%. Abdelmouleh et al.27 did not observed any significant effect of lignocellulosic fibers on the degree of crystallinity of the low density polyethylene (LDPE) matrix, even after fiber modification with different silane agents. Yao et al.28 analyzed the effect of the type of fiber and fiber content on the degree of crystallinity of high density polyethylene (HDPE)-based composites and no significant effect was reported even for fiber content as high as 50 wt %. On the contrary, the presence of sisal whiskers to the PCL matrix seems to significantly increase the degree of crystallinity of the host matrix, regardless their modification state. It is increased from 51 wt % for the neat matrix to values close to 60 wt % for nanocomposite films regardless the filler content, except for the composite reinforced with 3 wt % of unmodified whiskers. This enhancement of the crystallinity of the PCL matrix probably results, at least partially, in the improvement of the stiffness for these nanocomposites reported previously. The origin of this difference between cellulose whiskers and MFC on the crystallization behavior of PCL is under investigation and results will be published shortly. An increase of the crystallinity was also reported for residual lignocellulosic flour from spruce and ground olive stone reinforced poly(hydroxybutyrate-co-valerate) (PHBV).29 Reinsch and Kelley30 reported that short wood fibers acted as nucleating sites for the crystallization of PHBV and enhanced its crystallization rate. It was suggested by Luo and Netravali31 that Reinsch and Kelley’s results may have been influenced by the lignin of wood fiber. Then, despite the surface chemical modification of MFC, residual hemicellulose and pectin at the surface may affect the crystallization capability of PCL. A significant increase in crystallinity of sorbitol plasticized starch32 was reported when increasing cellulose whiskers content. This phenomenon was ascribed to an anchoring effect



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Table 1. Thermal Characteristics of PCL-Based Nanocomposites Obtained from DSC Analysisa filler

sample

Tg (°C)

Tm(°C)

∆Hm (J · g-1)

χcb

Tc (°C)

-62.0

63.4

80.7

0.51

22.8

unmodified whiskers

WU WU WU WU

3% 6% 9% 12%

-53.0 -55.8 -56.0 -59.6

65.4 65.9 65.7 64.2

80.1 86.7 90.0 85.5

0.53 0.59 0.63 0.62

32.7 33.7 33.2 32.6

modified whiskers

WM WM WM WM

3% 6% 9% 12%

-58.5 -57.6 -57.9 -57.2

65.7 64.2 64.6 64.1

94.1 91.2 88.9 86.7

0.62 0.62 0.62 0.63

33.8 33.5 33.9 34.9

modified MFC

MFCM MFCM MFCM MFCM

-56.9 -56.2 -54.4 -54.8

61.9 61.8 61.9 60.8

76.0 72.2 68.4 69.6

0.50 0.49 0.48 0.50

33.4 33.9 34.2 34.8

PCL

3% 6% 9% 12%

a Glass transition temperature (Tg), melting temperature (Tm), enthalpy of fusion (∆Hm), degree of crystallinity (χc), and temperature of crystallization (Tc). b χc ) ∆Hm/w∆Hm°, where ∆Hm° ) 157 J/g (heat of fusion for 100% crystalline PCL) and w is the weight fraction of polymeric matrix in the composite.

of the cellulosic filler, probably acting as a nucleating agent. For POE based composites the degree of crystallinity of the matrix was found to be roughly constant up to 10 wt % tunicin whiskers33,34 and to decrease for higher loading level.35 Yao et al.28 reported an increase of the degree of crystallization of HDPE, from 16 to 19%, when 30 and 50 wt % of wood fibers were added, respectively. It is worth noting that in our case the addition of only 3 wt % of modified whiskers induce an increase of the degree of crystallinity around 10%. Jimenez et al.36 studied the crystallization of PLC/clay composites and reported that the crystallization was governed by two terms, the diffusion and the nucleation. In addition, they observed that only a small amount of clay was useful to serve as nucleation agent, while a large amount seemed to hold back the transportation of polymer segments. Moreover, the dependence of the clay content on the occurrence of these effects was pointed out. Transcrystallization of PP at cellulose nanocrystal surfaces was recently evidenced and it was found to result from enhanced nucleation due to some form of epitaxy.37 It seems that the nucleating effect of cellulosic nanocrystals is mainly governed by surface chemical considerations. A decrease in the degree of crystallinity of PCL was reported when adding Riftia tubes chitin whiskers.38 It was suggested that, during crystallization, the rod-like nanoparticles are most probably first ejected and then occluded in intercrystalline domains, hindering the crystallization of the polymer. Concerning the melting point of the PCL matrix, it slightly increases, when adding sisal whiskers, from about 63 °C to 64-66 °C (Table 1). This temperature is directly related to the size of the crystalline domains. It means that the presence of the cellulose whiskers does not interfere significantly with the crystal growth, regardless their surface modification state. On the contrary, the melting point tends to decrease upon modified MFC addition. It means that the presence of MFC induces steric hindrance effects restricting the growth of crystalline PCL regions, probably resulting from entanglements between microfibrils. It results in both a lower melting point and lower degree of crystallinity. In the literature, melting temperature values were reported to be nearly independent of the filler content in plasticized starch32,39 and in POE-based materials33,34 filled with tunicin whiskers. The same observation was reported for PCL reinforced with Riftia tubes chitin whiskers38 and CAB reinforced with native bacterial cellulose whiskers.40 However, for the latter system, Tm values were found to increase when the amount of trimethylsilylated whiskers increased. The authors ascribed this

difference to the stronger filler-matrix interaction in the case of chemically modified whiskers. It might be also the case with our results. The glass transition temperature (Tg) of nanocomposites reinforced with either unmodified or modified cellulosic nanoparticles is systematically higher than the one of the neat matrix (Table 1). This effect is more significant with unmodified sisal whiskers. It means that the molecular mobility of amorphous PCL chains is restricted by the presence of the filler. The filler/ matrix compatibilization by surface grafting limits this phenomenon. However, it is worth noting that the value of Tg can also be affected by the degree of crystallinity of the matrix. In most studies, no modifications of Tg values were reported when increasing the amount of whiskers, regardless the nature of the polymeric matrix. This result appeared to be surprising because of the high specific area of these nanoparticles, that is, around 170 m2 · g-1, for instance, for tunicin whiskers.39 In glycerol plasticized starch based composites, peculiar effects of tunicin whiskers on the Tg of the starch-rich fraction were reported depending on moisture conditions.39 For low loading level (up to 3.2 wt %), a classical plasticization effect of water was reported. However, an antiplasticization phenomenon was observed for higher whiskers content (6.2 wt % and up). These observations were discussed according to the possible interactions between hydroxyl groups on the cellulosic surface and starch, the selective partitioning of glycerol and water in the bulk starch matrix or at whiskers surface, and the restriction of amorphous starch chain mobility in the vicinity of the starch crystallite coated filler surface. For glycerol plasticized starch reinforced with cellulose crystallites prepared from cottonseed linter,42 an increase of Tg with filler content was reported and attributed to cellulose/starch interactions. For tunicin whiskers/ sorbitol plasticized starch,32 Tgs were found to increase slightly up to about 15 wt % whiskers and to decrease for higher whiskers loading. Crystallization of amylopectin chains upon whiskers addition and migration of sorbitol molecules to the amorphous domains were proposed to explain the observed modifications. For waxy maize starch nanocrystals reinforced natural rubber, a decrease of the onset glass transition temperature with the increase of the nanoparticles content was reported.43 When a glycerol plasticized starch matrix was used, an increased temperature was reported for the main relaxation process associated with the glass-rubber transition of amylopectin-rich domains when increasing the starch nanocrystals content.44 The reduction in the molecular mobility of matrix amylopectin chains

432


for filled materials was explained by the establishment of hydrogen bonding forces between both components. A similar observation was reported for polyvinyl acetate (PVA) reinforced with sisal cellulose whiskers18 and carboxymethyl cellulose (CMC) reinforced with cotton cellulose whiskers.45

Conclusions Cellulosic nanoparticles were prepared from sisal and used to reinforce polycaprolactone (PCL). Both rod-like whiskers and long flexible “hairy” microfibrils or microfibrillated cellulose (MFC) were prepared. The surface of the nanoparticles was also chemically modified by grafting N-octadecyl isocyanate (C18H37NCO). The chemical grafting was found to improve the dispersion of nanofillers in organic solvents. The effects of the filler content, chemical grafting, and nature of the reinforcing phase were investigated. Sisal whiskers were found to induce a limited reinforcing effect because of the aggregation of the filler within the nanocomposites film prepared from dichloromethane. However, the presence of the filler increases the glass transition, crystallization, and melting temperatures as well as the degree of crystallinity of the PCL matrix, the cellulosic whiskers probably acting as nucleation sites. It was not possible to process PCL nanocomposite films reinforced with unmodified MFC because of the poor dispersibility of the latter in dichloromethane. The chemical grafting of cellulose whiskers improve their compatibility with the polymeric matrix and dispersity. The mechanical properties of ensuing nanocomposite films were really improved in terms of both stiffness and ductility. Characteristic temperatures and degree of crystallinity were also found to increase with the filler content. When comparing sisal whiskers and MFC (both chemically modified), it was found that the modulus was higher for MFC-reinforced composites, whereas the elongation at break was lower for a given loading level. The difference was most probably ascribed to the possibility of entanglements of MFC contrarily to rod-like nanoparticles. The introduction of MFC within the PCL matrix did not induce an increase of the degree of crystallinity of the matrix and its melting point tends to decrease contrarily to what was observed for sisal whiskers-reinforced films. Again, this difference was ascribed to the possibility of entanglement of MFC that tends to confine the polymeric matrix and restrict its crystallization. Although the crystallinity of MFC-based nanocomposite films was lower compared to whiskers-based composites, the stiffness of the former was higher, showing the high impact of the entanglements on the mechanical properties. Acknowledgment. The authors gratefully acknowledge ALBAN Program for the financial support (Ph.D. fellowship of G.S.), and Y. Habibi and FCBA for the support in MFC production.

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Cellulose Whiskers versus Microfibrils: Influence of the Nature of the

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