Crab Shell Chitin Whiskers Reinforced Natural Rubber

Biomacromolecules 2003, 4, 1835-1842

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Crab Shell Chitin Whiskers Reinforced Natural Rubber Nanocomposites. 3. Effect of Chemical Modification of Chitin Whiskers Kalaprasad Gopalan Nair and Alain Dufresne* Centre de Recherches sur les Macromole´ cules Ve´ ge´ tales (CERMAV-CNRS), Universite´ Joseph Fourier, BP 53, F 38041 Grenoble Cedex 9, France

Alessandro Gandini and Mohamed Naceur Belgacem Ecole Francaise de Papeterie et des Industries Graphiques (INPG), BP 65, 38402 St Martin d’He` res, France Received July 22, 2003; Revised Manuscript Received September 3, 2003

The purpose of this study was to chemically modify the surface of chitin whiskers and to investigate the effect of the incorporation of these modified whiskers into a natural rubber (NR) matrix on the properties of the ensuing nanocomposite. Different chemical coupling agents were tested, namely, phenyl isocyanate (PI), alkenyl succinic anhydride (ASA) (Accosize 18 from American Cyanamid), and 3-isopropenyl-R,R′dimethylbenzyl isocyanate (TMI). The extent of chemical modification was evaluated by Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), and surface energy analysis. After chemical modification, nanocomposite films were obtained using a toluene natural rubber solution in which the whiskers were dispersed. Their mechanical properties were found to be inferior to those of unmodified chitin/NR composites presented in our previous study. In fact, even though there is an increase in fillermatrix interaction as a result of chemical modification of the chitin whiskers, this does not contribute to the improvement in the mechanical properties of the resulting nanocomposite. It is concluded that this loss of performance is due to the partial destruction of the three-dimensional network of chitin whiskers assumed to be present in the unmodified composites. Introduction Over the past few years, much effort has been devoted to the use of microcrystals obtained from natural cellulose and cellulose derivatives as reinforcing agents in several polymeric matrixes.1-12 The advantages of natural fillers are their low density, renewable character, and biodegradability associated with the highly specific properties of nanoparticles. Compared with cellulose, its animal amide counterpart, chitin, extremely abundant in nature, has received much less attention, although in recent years considerable interest has been devoted to biomaterials based on it and on its aminoderivative chitosan. The dispersion and liquid crystalline behavior of cellulose microcrystals or microfibrilles have been studied mostly in an aqueous medium,13 although some reports deal with the use of nonpolar organic solvent.14-17 The chemical modification of the surface of cellulose fibers has been studied with the aim of improving their interfacial compatibility with various polymeric matrixes, that is, to enhance the mechanical properties of the ensuing composite.18-21 Unmodified chitin whiskers, obtained from squid pen,22 Riftia tubes,23 and crab shells,24,25 have been tested as the reinforcing elements in matrixes such as styrene and butyl

acrylate copolymers,22 poly(caprolactone),23 and natural rubber (NR).24,25 The present investigation describes the surface chemical modification of chitin whiskers with various reagents and their incorporation into natural rubber to obtain composite materials with improved mechanical properties. The second objective of this surface chemical modification was to broaden the number of possible polymer matrixes and to allow the processing of nanocomposite materials from an organic solvent solution instead of using aqueous suspensions. Experimental Section Materials. The details concerning the preparation and characterization of chitin whiskers and natural rubber were described previously.24,25 The chitin whiskers consisted of slender parallelepiped rods with an average length of about 240 nm and an aspect ratio close to 16, which yields a specific surface of 180 m2 g-1. The reagents used to modify the chitin whiskers were (1) a commercial alkenyl succinic anhydride (ASA, Accosize 18 from American Cynamid) consisting of a mixture of oligomers of different sizes centered around C18 (Mn ) 300), bearing the general structure

* To whom correspondence should be addressed. E-mail: [email protected]. Present address: Ecole Francaise de Papeterie et des Industries Graphiques (INPG), BP 65, 38402 St. Martin d’He`res, France 10.1021/bm030058g CCC: $25.00 © 2003 American Chemical Society Published on Web 10/07/2003

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Biomacromolecules, Vol. 4, No. 6, 2003

(2) phenyl isocyanate (PI, Aldrich)

and (3) isopropenyl-R,R′-dimethylbenzyl isocyanate (TMI, Aldrich)

PI and ASA were used to improve the quality of the interface between the NR used as a matrix and the chitin whiskers. The TMI was used to copolymerize with the unsaturations present in NR matrix. We used these reagents because they were thoroughly studied in our laboratory, as reported before.18-20 Procedure for Chemical Modification. The chemical modification was carried out using chitin whiskers, which were obtained by freeze-drying of their colloidal suspension followed by vacuum-drying at 100 °C for 1 day. In the case of treatment with ASA, 1.5 g of whiskers was suspended in dry dioxane, and thereafter, 1.5 g of ASA was added together with 2 drops of dimethyl amino pyridine. The reaction mixture was heated to 70 °C and left under magnetic stirring for 1 week. The whiskers were then recovered by filtration through a nylon cloth, washed several times with dichloromethane, and finally vacuum-dried for 1 day. This product will be denoted as CHASA. In the case of treatment with isocyanates, these were added to the whiskers-dioxane suspension under nitrogen, together with a small amount of commercial dibutyltin dilaurate used as a catalyst. With PI, 1.2 g of this reagent was mixed with 1.5 g of chitin, and the reaction was allowed to take place for 1 week at room temperature under nitrogen. With TMI, 1.5 g of chitin was mixed with 4 g of this reagent, and the reaction was allowed to take place at 60 °C for 10 days under nitrogen. The grafting agent/whiskers ratio was calculated on the basis of the amount of the OH functions available at the surface of chitin whiskers. An excess of 10 times was systematically used. The reaction time was determined following the evolution of NCO peak in FTIR spectra. The reaction was stopped when the intensity of NCO band became constant with time. The TMI is known to have a slower reaction rate; that is why it was necessary to heat at 60 °C. These parameters were established previously and are described in detail elsewhere.18-20 After both isocyanate modifications, the whiskers were recovered by filtration and subjected to a 1-day Soxhlet extraction with dichloromethane, followed by a 1-day Soxhlet extraction with dimethyl formamide. The latter extraction was conducted to remove any residual urea formed by the reaction of the isocyanate with moisture. Finally, the modified whiskers, denoted CHPI and CHTMI, respectively, were vacuum-dried at 50 °C for 1 day. The characterization of starting and modified whiskers was carried out by Fourier transform infrared (FTIR) spectroscopy

Gopalan Nair et al.

(Perkin-Elmer Paragon 1000) and transmission electron microscopy (TEM, Philips CM200). For the latter, a droplet of a dilute suspension of chitin whiskers was deposited and allowed to dry on a carbon-coated grid. The accelerating voltage was 80 kV. Contact angle measurements were also performed. For the latter analysis, pressed, smooth pellets were prepared from the different whiskers. The apparatus used here, previously described in detail,26 provided 200 Hz image analyses of the drops from which contact angle values are calculated within (1°. From the equilibrium data related to four liquids of different polarity, namely, water, formamide, diiodomethane, and hexadecane, the dispersive and polar contributions to the surface energy of the whiskers were calculated using the classical approach proposed by Owens and Wendt.27 Nanocomposite Processing. The processing of nanocomposites was carried out as follows: The whiskers were vigorously stirred in toluene using a dispermat, and the ensuing suspensions were subsequently sonicated for 5 min. The NR latex was lyophilized, and the resulting elastomer was dispersed in toluene by prolonged stirring. The two dispersions were then mixed and stirred vigorously for 1 day, after which the solvent was removed in a rotavapor. The thick slurry was then transferred into a Teflon mold and dried in an oven at 60 °C for several days. All ensuing composite films contained 10 wt % of chitin whiskers and are denoted NCHASA, NCHPI, and NCHTMI. For some experiments, the behavior of modified whisker nanocomposites is compared to unmodified ones. The latter were obtained by water evaporation after mixing and stirring of an aqueous suspension of chitin whiskers and NR latex as described earlier.24 This reference sample is denoted NCHITIN. Nanocomposite Characterization. The characterization of the composites called upon scanning electron microscopy (SEM, JEOL JSM-6100): the samples were frozen in liquid nitrogen, fractured, mounted, coated with gold/palladium on a JEOL JFC-1100E ion sputter coater, and observed using 7 kV secondary electrons. The solubility of the composite samples was determined in toluene after 48 h immersion of a weighed thin disk (M0) at room temperature. The insoluble residue was removed, dried for 12 h at 55-60 °C, and weighed again (M′0). From the relative weight loss (RWL ) [M0 - M′0]/M0) thus calculated, the fraction of NR bound to the whiskers could thus be estimated, together with the fraction of NR dissolved in toluene. Dynamic mechanical tests were carried out with a RSA2 spectrometer from Rheometrics working in the tensile mode. The value of 0.05% for the strain magnitude was chosen to be in the domain of the linear viscoelasticity of the materials. The samples were thin rectangular strips with dimensions about 22 × 5 × 0.35 mm3. Measurements were performed in isochronal conditions at 1 Hz, and the temperature was varied between -100 and 250 °C by steps of 3 °C. The nonlinear mechanical behavior of the composites was analyzed using an Instron 4301 testing machine in the tensile mode with a load cell of 100 N capacity. The specimens had the same geometry as those used for the dynamic

Biomacromolecules, Vol. 4, No. 6, 2003 1837

Crab Shell Chitin Whiskers Reinforced

the slope of the low-strain region in the vicinity of σ ) ) 0 ([dσ/d]f0). The ultimate mechanical properties were also characterized. The true ultimate stress or true stress at break, σb ) Fb/S, where Fb was the applied load at break, was determined for each tested sample. The true ultimate strain or true strain at break, b ) ln[1 + (∆Lb/L0)], where ∆Lb is the elongation at break, was also determined. These mechanical tensile data were averaged over at least three specimens. Results and Discussion

Figure 1. Fourier transform infrared spectrum taken for a film of chitin whiskers: (a) unmodified; (b) ASA-modified; (c) TMI-modified; (d) PImodified. Table 1. Contact Angle (θ), Polar Contribution (γps ), Nonpolar Contribution (γds ), and Surface Energy (γs) Values of Unmodified and Chemically Modified Chitin Whisker Samples θ (deg) sample water formamide diiodomethane hexadecane CHITIN

50

γps

39

28

21

20

γds

γs

32.6 52.6

CHASA

62

41

31

18

13.4 33.6 47

CHTMI

67

42

31

30

14.2 27.8 42

CHPI

84

77

60

63

8.6 16.2 24.8

mechanical tests. The gap between the pneumatic jaws at the starting of each test was adjusted to 15 mm. The stress-strain curves of the samples were obtained at room temperature at a strain rate d/dt ) 1.1 × 10-2 s-1 (cross-head speed ) 10 mm min-1). The true strain was determined by ) ln(L/L0), where L and L0 were the length at the time of the test and the length at zero time, respectively. The true stress σ was calculated by σ ) F/S, where F was the applied load and S was the cross-sectional area. S was determined assuming that the total volume of the sample remained constant, so S ) S0(L0/L), where S0 was the initial cross-sectional area. Stress versus strain curves were plotted, and the tensile or Young’s modulus (E) was measured from Scheme 1

Characterization of Modified Whiskers. The expected chemical reactions that occur in the alternative chemical modifications of chitin whiskers with isocyanates and ASA are seen in Scheme 1. Figure 1 shows the typical FTIR spectrum of unmodified chitin whiskers (curve a), together with those of whiskers modified with ASA, TMI, and PI (curves b, c, and d, respectively). The relevant differences relative to the ASA treatment are to be found in a strong increase of the peak at 2920 cm-1 associated with C-H vibration arising from the long aliphatic chain of the reagent and with the shoulder at 1715 cm-1 (carbonyl group) associated with the formation of the ester moiety. In the case of the reaction with TMI, the spectroscopic evidence for any coupling reaction is difficult to pinpoint because the resulting modified moieties contain urethane and alkenyl functions, which are totally masked by strong absorption by chitin itself. The only slight difference is the hint of a shoulder around 1710 cm-1, suggesting the presence of urethane carbonyl groups. A stronger evidence of modification was obtained with PI because the carbonyl shoulder was more clearly visible, together with the double peak at 690-750 cm-1, typical of monosubstituted aromatic structure. The determination of surface energy contributions, obtained by contact angles measurements, is given in Table 1 and clearly corroborates the occurrence of surface chemical modification. An inspection of the γps values indicated a systematic decrease, which is particularly high for the

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reaction with PI. Given the structure of the three coupling agents, this was to be expected because of their relative lack of polar contribution. The highest reactivity of PI is also logical because, on one hand, the reactivity of isocyanates with OH groups is higher than that of anhydride and, on the other hand, aromatic NCO groups are more reactive than aliphatic ones. In conclusion, the three surface modifications were shown to occur, albeit with different extents. It is important to underline that even with “complete surface conversion”, the actual global percentage of coupling remains low (less than 10%) in this context precisely because the conditions chosen did not allow reactions to take place within the bulk of the whiskers. This explains why the contact angle measurements (surface characterization) gave much more convincing evidence than the infrared spectra (bulk transmission characterization). Unmodified chitin whiskers cannot be dispersed in toluene because they immediately aggregate. Attempts were made to disperse modified whiskers in toluene. The formation of stable dispersions suggests the nonpolar nature of the whiskers after surface chemical modification. However, none of these toluene dispersions except CHASA exhibits a colloidal behavior when observed in polarized light between crossed polars. It was observed that CHASA partially displays a colloidal behavior in toluene. The colloidal nature of unmodified whiskers in water medium is due to the presence of positive charges (NH3+) on the surface of whiskers, resulting from the protonation of the whiskers.28 In toluene, the electrostatic repulsion between ASA modified chitin whiskers is ensured by the presence of ASA grafted at the surface of the chitin nanoparticles. The carboxyl group (-COOH) formed as a result of esterification coupling during ASA modification may positively affect the colloidal stability of whiskers in toluene. The possibility of hydrogen bonding between whiskers is also diminished as a result of modification, and this will prevent the rapid agglomeration of microcrystals. After isocyanate treatment, the electrostatic repulsion between whiskers is completely destroyed because of the comparatively high reactivity of NCO group, and this will lead to the formation of aggregates and phase-separated dispersion in toluene. Figure 2 shows a TEM micrograph of whiskers before and after modification. The average length and width of unmodified chitin whiskers (panel a) were estimated to be around 240 and 15 nm, respectively.24 The aspect ratio of the chitin fragments was therefore around 16. After surface chemical modification with ASA and PI, the appearance of the chitin fragments changes. They seem to be entangled, and individual whiskers are difficult to observe. This binding agent is most probably the chemical coupling agent used for chemical modification of the chitin fragments. However, it seems that these TEM micrographs reveal the absence of major morphological changes associated with the various treatments applied. Morphology of Nanocomposites. The cryofractured surface of unmodified and modified chitin whiskers/NR nanocomposites were examined with the help of SEM. The SEM micrographs are presented in Figure 3. Figure 3a shows the

Gopalan Nair et al.

Figure 2. Transmission electron micrographs of a dilute suspension of (a) unmodified, (b) ASA-modified, and (c) PI-modified chitin whiskers.

SEM of NR reinforced with 10 wt % unmodified chitin whiskers. The chitin whiskers appear as white dots and are distributed uniformly throughout the matrix. This uniform distribution of chitin whiskers was assumed to be due to the formation of a rigid chitin-chitin network and was well explained in our previous study.24,25 Figure 3b displays the SEM cryofractured surface of NCHASA. The white dots revealing the presence of chitin whiskers in unmodified composites cannot be seen in NCHASA because of their poor dispersion in the NR matrix. Large smooth unfilled domains are clearly evidenced (indicated by arrows). The unmodified composites were prepared by mixing NR latex and aqueous suspension of whiskers, whereas the NCHASA was prepared by mixing toluene dispersions of solid NR and whiskers. That is, in aqueous medium, a better colloidal dispersion can be achieved because of the high electrostatic repulsion between the whiskers, but in toluene medium, these interactions between the modified chitin whiskers are practically



Figure 4. Photographs of 10 wt % modified chitin whiskers filled NR films before and just after swelling in toluene for 24 h at room temperature (25 °C). The diameter of all samples before swelling was d0 ) 7.5 mm. Table 2. Diameter (d) of 10 wt % Chitin Whiskers/NR Nanocomposite Disks Immersed for 24 h in Toluenea sample

d (mm)

diameter variation (%)

NCHITINb NCHASA NCHTMI NCHPI

12.5 15.9 19.8 19.7

67 112 163 162

a The initial sample diameter (before swelling) was d ) 7.5 mm, and 0 the diameter variation was determined by (d - d0)/d0. bData corresponding to the unmodified whiskers based composite are added for comparison and were obtained from ref 24.

Figure 3. Scanning electron micrographs of the cryofractured surfaces of 10 wt % (a) unmodified, (b) ASA-modified, and (c) PImodified chitin whiskers filled NR films.

absent or weak in nature. Figure 3c is SEM of fracture surface of isocyanate-treated nanocomposite (NCHPI). The nonuniform dispersion of chitin whiskers is clearer for this sample, and aggregated chitin whiskers can be observed (indicated by arrows). Because the isocynates are highly reactive compared to ASA, the electrostatic interactions between chitin whiskers were made negligible thereby causing the complete destruction of colloidal stability of the whiskers in toluene, which ultimately leads to the aggregation of chitin whiskers in the NR matrix. Swelling Behavior. After chemical modification, the extent of the interactions between the modified whiskers and the NR matrix and the possibility of chitin-chitin network formation in the matrix were determined by measuring the swelling ratio or diameter variation of the samples before and after swelling in toluene, as explained in the Experimental Section. Figure 4 shows photographs of 10 wt % chitin whiskers/NR nanocomposites before and 24 h after swelling in toluene. These photographs were recorded with a Nikon COOLPIX885 digital camera. It can be seen that all of the samples swelled after immersion in toluene.

However, the extent of swelling is least for NCHASA film, compared to NCHTMI and NCHPI samples. The swelling of these two samples are almost the same, as observed in Figure 4. These observations were quantified by measuring the diameter of the disk after 24 h of swelling in toluene. Results are reported in Table 2. The data corresponding to the NR film filled with 10 wt % unmodified chitin whiskers24 are included for comparison. The diameter of NCHASA is increased by 112% (67% was observed for unmodified composites),24 whereas for NCHTMI and NCHPI, the increase was around 160% on swelling. In our previous study,24 it was concluded that the formation of a stiff hydrogen-bonded whiskers network within the NR matrix hinders the swelling of the elastomer. It is clear from both Table 2 and Figure 4 that the swelling of modified chitin whiskers/NR nanocomposites is much higher than the one of unmodified based materials. This could be ascribed to the lower interactions between modified whiskers, part of the surface hydroxyl groups being substituted, and the lower dispersion level revealed by SEM observations. By comparison of ASA- and isocyanate-modified whiskers, it seems that this effect is more pronounced for the latter. The bound rubber content and fraction of NR dissolved after immersion in toluene was also measured in this study. The relative weight loss (RWL) and fraction of bonded

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Table 3. Relative Weight Loss (RWL) and Fraction of Bound Matrix (FBM) of Chitin Whiskers/NR Nanocomposite Films Immersed for 3 days in Toluene sample

RWL (%)

FBM (%)

NR matrix NCHITINa NCHASA NCHTMI NCHPI

77.6 11.8 36.6 32.8 23.6

0 36.6 33.2 37.1 46.2

a Data corresponding to the unmodified whiskers based composite are added for comparison and were obtained from ref 24.

matrix (FBM) values of composites after 3 days of immersion in toluene are collected in Table 3. The fraction of bonded matrix (FBM) was determined from RWL data. For the calculation, it was assumed that the insoluble part of the unfilled matrix (1 - RWL0) should be present in the composites balanced by the matrix content (1 - wF, wF being the filler weight fraction). The “sol” fraction (% “sol”) of each sample is therefore the sum of three terms: % “sol” ) 1 - RWL ) wF + (1 - RWL0)(1 - wF) + FBM (1) The first one (wF) corresponds to the whiskers content, the second one [(1 - RWL0)(1 - wF)] to the insoluble part of the matrix, and the third one (FBM) to the fraction of NR in strong interaction with the surface of the chitin filler. NR used for this experiment was prepared by dissolving freezedried NR latex in toluene and evaporating the solvent. Table 3 shows that about 78% of the unfilled NR is dissolved in toluene. The dissolution of a limited amount of NR was also reported for samples prepared from NR aqueous suspensions24 and was ascribed to the fact that the experiments were performed at room temperature without any stirring. For the untreated whiskers based material, the RWL value is much lower (11.8%) than for the treated ones. It can be ascribed to the possible chitin-chitin network formed by unmodified whiskers within the NR matrix, which prevents the swelling of the material. The RWL value of NCHASA is slightly higher than that of NCHTMI, which in turn is higher than that for NCHPI composites. The low RWL values of isocyanate-treated composites are due to the stronger interaction between the whiskers and matrix for these materials. The FBM values were determined from RWL data and are also collected in Table 3. It is worth noting that FBM values correspond to the NR fraction that is entrapped within the network of chitin and on the surface of the chitin whiskers. For the unmodified whiskers based material, our previous observations24,25 suggest that the nanosized chitin fragments form a three-dimensional rigid network assumed to be governed by a percolation mechanism. The critical volume fraction of chitin whiskers at the percolation threshold was found to be 4.4 vol % (around 6.4 wt %). For the 10 wt % unmodified whiskers filled NR film, this rigid network is therefore likely to be formed and most of the FBM should originate from the entrapped NR amount. This entrapped NR amount is expected to be much lower in modified whiskers based composites because of their poorer dispersion in the matrix. However, Table 3 shows that the order of magnitude of the FBM values is similar. This could

Figure 5. Logarithm of the storage tensile modulus E′ vs temperature at 1 Hz for chitin whiskers/NR composites: (b) unfilled NR matrix; (O) NCHITIN; (2) NCHASA; (4) NCHPI; (9) NCHTMI.

be an indication of comparatively stronger interactions between the whiskers and the matrix in chemically modified systems. It seems that the extent of these interactions is higher in isocyanate-treated composites. This is more pronounced for phenyl isocyanate. Dynamic Mechanical Analysis. Figure 5 shows the variation of the storage tensile modulus at 1 Hz as a function of temperature for unfilled NR matrix, together with unmodified and modified chitin/NR composites. The curve corresponding to the unfilled NR matrix is typical of fully amorphous materials with a constant E′ value around 3 × 109 Pa at low temperature (glassy state) and around 106 Pa at high temperature (rubbery state). Between these two plateaus, a sharp modulus drop corresponding to the main relaxation process associated with the glass-rubber transition is observed around -60 °C. In the terminal zone, the elastic modulus becomes lower and lower with temperature, and the experimental setup fails to measure it because of the flow of the material. The curve corresponding to the unmodified whiskers based composite (NCHITIN) is taken from our previous study25 and serves for comparison with modified ones containing the same whiskers content. A significant increase in rubbery modulus is observed. The relaxed modulus at Tg + 150 °C (∼90 °C) of the 10 wt % unmodified chitin whiskers filled NR film is more than 70 times higher than the one of the unfilled matrix. In addition to this high reinforcing effect, a significant improvement in the thermal stability of the composite is also noticed up to 220-230 °C. Normally, chitin starts to degrade at this temperature. Both effects were ascribed to the formation of a three-dimensional chitin whiskers network in the composite film.25 Above Tg, the modulus of all chemically modified whiskers based composites was found to be much lower than that of the unmodified one and more similar to that of the unfilled matrix. The thermal stability of the composites is also very much reduced after chemical modification. It could be mainly attributed to the negligible presence or absence of the rigid network of chitin as a result of chemical modification onto the whiskers. A clear hierarchy is also observed depending on the nature of the chemical coupling agent used. The ASAtreated sample displays a higher rubbery modulus the



matrix even after chemical modification of the whiskers. Even though the whisker-matrix interactions in composites is supposed to be greater in NCHTMI and NCHPI, the chance of network formation and thereby a good dispersion of whiskers in these composites is negligible compared to that in NCHASA. Conclusions

Figure 6. Typical stress vs strain curves of chemically modified chitin whiskers/NR composites (T ) 25 °C, d/dt ) 1.1 × 10-2 s-1). The nature of the sample is indicated in the figure. Table 4. Mechanical Properties of Chitin Whiskers/NR Nanocomposite Films Using Data Obtained from Tensile Tests, Tensile Modulus (E), Conventional Modulus (E100%), Stress at Break (σb), and Elongation at Break (b) σB (MPa)

B (%)

sample

E (MPa)

true

nominal

true

nominal

NR matrix ASA TMI PI

0.96 5.5 2.5 1.7

2.36 10.3 5.5 5.5

0.34 2.4 1.55 0.88

195 150 128 184

620 352 260 530

isocyanate ones. For instance, the relaxed modulus at Tg + 150 °C (∼90 °C) of the 10 wt % ASA- and PI-modified chitin whiskers filled NR film is only 5.8 and 1.6 times higher than the one of the unfilled matrix, respectively. This result is in agreement with the swelling behavior of the materials and could be an indication of comparatively stronger interactions between isocyanate-modified whiskers and the NR matrix. The increase of the filler/matrix interactions results in a decrease of the chitin-chitin interactions responsible for the high mechanical characteristics of the unmodified whiskers filled NR film. Tensile Tests. The tensile mechanical behavior of the chemically modified chitin whiskers/NR composite films was analyzed at room temperature. Typical stress vs strain curves (nominal data) are shown in Figure 6. The samples exhibit an elastic nonlinear behavior typical of amorphous polymer at T > Tg. The stress continuously increases with the strain. The polymeric matrix is in the rubbery state, and its elasticity from entropic origin is ascribed to the presence of numerous entanglements due to high molecular weight chains. The tensile modulus, tensile strength, and elongation at break of the films were determined from the plot of the stress versus strain as described in the Experimental Section. The results are collected in Table 4. Whatever the nature of the chemical coupling agent is, a reinforcing effect is displayed through an increase of the modulus and stress at break. However, from this test also, a clear hierarchy is observed between ASA- and isocyanate-modified whiskers. The modulus and ultimate tensile strength of NCHASA is found to be higher than that of NCHTMI and NCHPI composites. The superior tensile properties of NCHASA could be attributed to the partial existence of a chitin-chitin network within the NR

The surface of chitin whiskers, prepared by acid hydrolysis of chitin from crab shells, was chemically modified using different coupling agents, namely, phenyl isocyanate (PI), alkenyl succinic anhydride (ASA), and 3-isopropenyl-R,R′dimethylbenzyl isocyanate (TMI). FTIR spectroscopy, TEM, and contact angle measurements were performed to prove the occurrence of the surface modification without any major morphological changes associated with the various treatments applied. Stable suspensions of these chemically modified chitin whiskers were obtained in toluene. Nanocomposites were prepared using a toluene natural rubber solution in which the whiskers were dispersed. After removal of the solvent, the resulting solid films were characterized by SEM, swelling experiments, and mechanical analysis, in both the linear and nonlinear range. All of the results lead to the conclusion that the various chemical treatments improve the adhesion between the filler and the matrix. However, the mechanical performances of the composites strongly decrease after the chemical modification. This loss of performance, more pronounced for the isocyanate treatments, could be due to the partial or total destruction of the three-dimensional network of chitin whiskers assumed to be present in the unmodified composites. Acknowledgment. The authors are grateful to Mrs. C. Crepeau (Technical Center, MAPA Company) for the supply of NR latex, Mrs. I. Paintrand and D. Dupeyre for their help in TEM and SEM study, respectively, and INPG-EFPG for giving all facilities to carry out this work. K. Gopalan Nair gratefully acknowledges the French Ministry of Research for its financial support. References and Notes (1) Favier, V.; Canova, G. R.; Cavaille´, J. Y.; Chanzy, H.; Dufresne, A.; Gauthier, C. Polym. AdV. Tech. 1995, 6, 351. (2) Favier, V.; Cavaille´, J. Y.; Chanzy, H. Macromolecules 1995, 28, 6365. (3) Helbert, W.; Cavaille´, J. Y.; Dufresne, A. Polym. Compos. 1996, 17, 604. (4) Dufresne, A.; Cavaille´, J. Y.; Helbert, W. Polym. Compos. 1997, 18, 198. (5) Favier, V.; Canova, G. R.; Shrivastava, S. C.; Cavaille´, J. Y. Polym. Eng. Sci. 1997, 37, 1732. (6) Chazeau, L.; Paillet, M.; Cavaille´, J. Y. J. Polym. Sci., Polym. Phys. 1999, 37, 2151. (7) Dubief, D.; Samain, E.; Dufresne, A. Macromolecules 1999, 32, 5765. (8) Dufresne, A.; Kellerhals, M. B.; Witholt, B. Macromolecules 1999, 32, 7396. (9) Dufresne, A. Compos. Interfaces 2000, 7, 53. (10) Angle`s, M. N.; Dufresne, A. Macromolecules 2000, 33, 8344. (11) Angle`s, M. N.; Dufresne, A. Macromolecules 2001, 34, 2921. (12) Mathew, A. P.; Dufresne, A. Biomacromolecules 2002, 3, 609. (13) Turbak, A. F.; Synder, F. W.; Sandberg, K. R. J. Appl. Polym. Sci, Appl. Polym. Symp. 1983, 37, 815. (14) Heux, L.; Chauve, G.; Bonini, C. Langmuir 2000, 16, 8210. (15) Cavaille´, J. Y.; Chanzy, H.; Fleury, E.; Sassi, J.-F. PCT Int. Appl. WO 97 12, 1997; p 917.

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