Morphological Investigation of Nanocomposites from Sorbitol

Biomacromolecules 2002, 3, 609-617

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Morphological Investigation of Nanocomposites from Sorbitol Plasticized Starch and Tunicin Whiskers Aji P. Mathew 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 Received December 20, 2001; Revised Manuscript Received February 13, 2002

Nanocomposites were prepared from waxy maize starch plasticized with sorbitol as the matrix and a stable aqueous suspension of tunicin whiskerssan animal cellulosesas the reinforcing phase. The composites were conditioned at different relative humidity levels. The conditioned films were characterized using scanning electron microscopy, differential scanning calorimetry, water uptake experiments, and wide-angle X-ray scattering studies. Contrarily to our previous report concerning tunicin whisker filled glycerol plasticized starch nanocomposites (Macromolecules 2000, 33, 8344), the present system exhibited a single glassrubber transition, and no evidence of transcrystallization of amylopectin on cellulose whisker surfaces and resultant antiplasticizing effects were observed. It was found that the glass-rubber transition temperature of the plasticized amylopectin matrix first increases up a whiskers content around 10-15 wt % and then decreases. A significant increase in crystallinity was observed in the composites by increasing either moisture content or whiskers content. Introduction Nowadays efficient and environmentally friendly products are developed from starch, taking advantage of its ability to form aqueous solutions and the adhesive and film-forming properties.1,2 The abundant supply, low cost, renewability, biodegradability, and the ease of chemical modifications make starch an attractive material for modern technologies.3,4 In addition to these advantages, the use of starch, or any product of an agricultural source, in plastic materials would reduce dependence on synthetic polymers made from imported oil and offers socioeconomic benefits because it generates rural jobs and a nonfood agricultural-based economy. The addition of plasticizers to starch is accepted as the means for lowering the glass-rubber transition temperature (Tg) below the decomposition temperature and making it more flexible. Glycerol and water are the most widely used plasticizers.5,6 A number of studies on the effects of plasticizer on starch have been carried out with the aim of producing useful thermoplastic materials. Shrogen et al.7 have used a combination of urea and glycols for plasticizing corn starch. The influence of various plasticizer concentrations on the Tg of starch materials was studied by Lourdin and co-workers.8 They have pointed out that the efficiency of the plasticizer is governed by its ability to form favorable interactions (probably hydrogen bonds) with starch. By itself, starch is a poor choice as a replacement for any plastic. The hydrophilic nature of thermoplastic starches makes them susceptible to moisture attack and resultant changes in dimensional stability and mechanical properties. In addition, retrogradation and crystallization of the mobile * To whom correspondence [email protected]).

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starch chains lead to an undesired change in thermomechanical properties. The use of reinforcing materials in the starch matrix is an effective method to obtain high-performance starch products.9,10 More recently, there is an increased use of cellulosic fibers as the load-bearing constituent in developing new and inexpensive biodegradable materials.11 Cellulose microfibrils extracted from byproducts have gained particular attention as possible reinforcing agents in composites due to their high modulus and high aspect ratio.12-14 Acid hydrolysis of cellulosic materials can lead to highly crystalline well-defined rods, called whiskers. The use of these model cellulosic fibers allows the understanding of the behavior exhibited by lignocellulosic-based composites owing to their well-defined dimensions. Therefore tunicin, the cellulose extracted from tunicatesa sea animalsis a material of choice, since it is highly crystalline and can lead to the preparation of high aspect ratio whiskers. The preparation and properties of starch/tunicin whisker nanocomposites plasticized with glycerol were reported earlier.15,16 Glycerol is the most commonly used plasticizer in thermoplastic starch materials. In this previous study an accumulation of glycerol on the cellulose whiskers surface was observed.15 It gave rise to an antiplasticization effect in this system at high filler loading and relative humidity level. This accumulation of plasticizer in the cellulose/amylopectin interfacial zones improves the ability of amylopectin chains to crystallize leading to the formation of a possible transcrystalline zone around the whiskers.15 The coating of the tunicin whiskers by a soft plasticizer-rich interface resulted in poor mechanical properties of the composites.16 In the present study, composites were obtained from waxy maize starch and an aqueous suspension of tunicin whiskers using 33 wt % sorbitol as the plasticizer. This study is an

10.1021/bm0101769 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/13/2002

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attempt to nullify the antiplasticizing effect observed for glycerol plasticized starch-based composites.15 The choice of sorbitol was made on the basis of its higher molecular weight (182 g‚mol-1) compared to glycerol (92 g‚mol-1). One of the expected effects is the lower molecular mobility of sorbitol, which should hinder its diffusivity resulting in antiplasticization. The effect of relative humidity and filler loading on the Tg, morphology, water uptake, and crystallinity of the system was studied in detail. The mechanical performance of the composites was also studied and will be reported in a forthcoming paper.17 Experimental Section Starch Matrix. Waxy maize starch (almost pure amylopectin, amylose content is lower than 1%) and sorbitol were kindly supplied by Roquette S.A. (Lestrem, France). Starch and sorbitol were first mixed and dispersed in water. The mixture contained 10 wt % of a waxy maize starch, 5 wt %, sorbitol and 85 wt % water. This ratio is similar to our previous work with glycerol as plasticizer.15,16 The gelatinization of starch was performed in a stirred autoclave reactor operating at 160 °C for 5 min. The determination of the disappearance of ghosts was carried out by optical microscopy, and the absence of starch degradation was checked by visual inspection of film appearance, degradation leading to a tanning of resulting films. After mixing, the suspension was degassed under vacuum in order to remove the remaining air and cast in a Teflon mold stored at 70 °C under vacuum to allow water evaporation. Cellulose Whiskers. Cellulose whiskers were extracted from tunicate (a sea animal). Colloidal suspensions of whiskers in water were prepared as described elsewhere.18-20 Mantles of tunicate were first cut into small fragments that were deproteinized by three successive bleaching treatments. The bleached mantle (the tunicin) was then disintegrated in water with a Waring blender. The resulting aqueous tunicin suspension was mixed with H2SO4 to reach a final acid/water concentration of 55 wt %. Hydrolysis conditions were 60 °C for 20 min under strong stirring. The suspension was neutralized and washed with water. The dispersion of cellulose whiskers was completed by two successive ultrasonic treatments during 3 min each. The suspension did not sediment or flocculate as a consequence of surface sulfate groups created during the sulfuric acid treatment.19 It is constituted of individual cellulose fragments consisting of slender parallelepiped rods that have a broad distribution in size.15 These fragments have a length ranging from 500 nm up to 1-2 µm and they are almost 10 nm in width. The average aspect ratio (L/d, L being the length and d the diameter) of these whiskers was estimated to be close to 70.20 Film Processing. The starting products (starch + sorbitol + water + tunicin whiskers suspension) were mixed in different ratios to obtain composite films with a homogeneous dispersion. The sorbitol content was fixed at 33 wt % (dry basis of starch matrix). The cellulose whiskers content was varied between 0 and 25 wt % (cellulose/starch + sorbitol). Similar processing conditions as those described for the unfilled starch matrix were used to gelatinize starch and to process nanocomposite films.

Mathew and Dufresne

Film Conditioning. The structure and therefore the properties of starch materials are strongly related to the water content.8,21-25 The moisture content of the nanocomposite films was achieved by conditioning the samples at room temperature in desiccators at controlled humidities containing saturated salt solutions. Six relative humidity (RH) conditions were used, namely, 0, 31, 43, 58, 75, and 98%. Conditioning was achieved for at least 2 weeks to ensure the equilibration of the water content in the films with that of the atmosphere (stabilization of the sample weight). Microscopy. Scanning electron microscopy (SEM) was performed to investigate the morphology of the nanocomposite films with a JEOL JSM-6100 instrument. The specimens were frozen under liquid nitrogen and then fractured, mounted, coated with gold/palladium on a JEOL JFC-1100E ion sputter coater, and observed. SEM micrographs were obtained using 7 kV secondary electrons. Differential Scanning Calorimetry (DSC). Differential scanning calorimetry (DSC) was performed with a PerkinElmer DSC7 fitted with a cooler system using liquid nitrogen. Conditioned samples were placed in pressure-tight DSC cells, and at least two individual measurements were carried out to ensure perfect reliability of measurements. Each sample was heated from -110 to +250 °C at a heating rate of 10 °C/min. The melting temperature (Tm) was taken as the peak temperature of the melting endotherm while the glass transition temperature (Tg) was taken as the inflection point of the specific heat increment at the glass-rubber transition. X-ray Diffraction. Wide-angle X-ray scattering (WAXS) patterns were measured in reflection mode with a diffractometer using a static detector (SIEMENS D500). Conditioned films were mounted on a poly(methyl methacrylate) (PMMA) hollow support and sealed with thin aluminum foil in order to preserve moisture conditions during the experiment as explained elsewhere.21 Samples were exposed for a period of 20 s for each angle of incidence using a Cu KR1 X-ray source with a wavelength of 1.5406 Å operating at 40 kV and 20 mA. The angle of incidence was varied between 8° and 40° by steps of 0.05°. Periodical distance (d) of the main peaks were calculated according to Bragg’s law. Water Uptake. For all compositions the kinetics of water absorption was determined. The specimens used were thin rectangular strips with dimensions of 10 mm × 10 mm × 1 mm. The film thickness 2L was therefore supposed to be thin enough, so that the molecular diffusion was considered to be one-dimensional. Samples were first dried overnight at 100 °C. After the samples were weighed, they were conditioned at room temperature in a desiccator containing copper sulfate (CuSO4‚5H2O) to ensure a RH ratio of 98%. The samples were removed at specific intervals (t) and weighed (Mt) up to an equilibrium value (M∞). The water content or water uptake of the samples was calculated by dividing the gain in weight (Mt - M0) by the initial weight (M0). The diffusion coefficient was determined from the initial slope of the plot of (Mt - M0)/M∞ as a function of (t/L2)1/2. Details of the calculation can be found elsewhere.15

Morphological Investigation of Nanocomposites

Biomacromolecules, Vol. 3, No. 3, 2002 611

Figure 1. Scanning electron micrographs of the fractured surfaces of (a) unfilled sorbitol plasticized starch matrix and related composites filled with (b) 5 wt %, (c) 15 wt %, and (d) 25 wt % tunicin whiskers.

Results and Discussion Morphology of Nanocomposites. The examination of the fractured surface of sorbitol plasticized starch/tunicin whisker composites was carried out using a scanning electron microscope. Figure 1a shows the SEM of unfilled plasticized starch films. The surface is rather smooth. The examination of this sample was problematic since after exposure to the electron probe for about 1 min, the surface becomes blistered. Panels B-D of Figure 1 show the fractured surfaces of the composites filled with 5, 15, and 25 wt % of tunicin whiskers. The tunicin whiskers appear as white dots, which are distributed evenly throughout the plasticized starch matrix. As the filler loading increases from 5 up to 25 wt %, the composites exhibit an increase in the concentration of whiskers on the fractured surface. A uniform distribution of whiskers in the matrix is clearly seen for all the compositions. Such an even and uniform distribution of filler in the matrix is essential for obtaining optimum mechanical performances.26,27 Sorbitol crystallites were sometimes observed at the surface of the film (not shown), most probably due to an excess of sorbitol within the material. Blistering of the surface after exposure to the electron probe was not observed for the composites. Thermal Analysis. DSC studies of unfilled and filled plasticized starch materials, conditioned at various RH conditions, were performed.

Figure 2. DSC thermograms of unfilled sorbitol plasticized waxy maize starch films for different moisture contents. The relative humidity conditions are indicated in the figure.

Starch/Sorbitol Films. Figure 2 shows the DSC curves of sorbitol plasticized starch conditioned at 0% up to 98% RH. In all the curves a single ill-defined (at least for this heat capacity scale) specific heat increment was observed at low temperature. Extended views (not shown) of the DSC traces were performed to precisely analyze these events. It

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Table 1. Temperatures of the Calorimetric Transitions of Tunicin Whisker Filled Sorbitol Plasticized Starch Using Data Obtained from the DSC Curves: Glass-Rubber Transition Temperature (Tg) and Associated Heat Increment (∆Cp), Melting Temperature (Tm), and Associated Heat of Fusion (∆Hm) RH (%)

whisker content (wt %)

Tg (°C)

∆Cp (J‚g-1‚K-1)

Tm (°C)

∆ Hm (J‚g-1)

0

0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25

13.2 16.3 18.7 17.9 13.5 10.5 -6.3 3.5 2.9 14.5 11.5 7.3 -27.6 -30.3 -28.6 -30.8 -29.3 -31.4 -30.3 -31.5 -32.0 -25.1 -29.5 -34.5 -51.5 -50.3 -47.0 -51.9 -53.6 -52.5 -64.7 -61.6 -56.5 -49.2 -52.7 -54.5

0.39 0.50 0.24 0.33 0.44 0.29 0.52 0.26 0.32 0.23 0.40 0.21 0.50 0.30 0.28 0.31 0.28 0.30 0.25 0.39 0.37 0.34 0.49 0.52 0.56 0.44 0.43 0.56 0.43 0.60 0.48 0.68 0.67 0.41 0.48 0.39

156.1 154.1 152.0 148.3 146.5 142.6 149.2 139.0 143.5 143.2 142.8 140.0 141.4 137.6 132.7 138.1 145.2 128.6 133.4 127.4 130.3 133.9 123.1 124.8 135.6 126.3 123.8 130.1

59 108 129 136 486 444 750 750 892 883 687 548 819 760 1143 991 827 1129 1074 1089 1068 855 951 978 926 863 1074 879

31

43

58

75

98

corresponded to the glass-rubber transition (Tg) of the material. The values of Tg are collected in Table 1. In our previous work on glycerol plasticized starch, the presence of two Tg values was reported.15 They were ascribed to the existence of two phases in the plasticized matrix. The low temperature transition was assigned to Tg of glycerol-rich domains and the high temperature one to Tg of amylopectinrich domains. It appears that such a phase separation does not occur in the sorbitol plasticized starch or at least is not observed in DSC experiments. This should lead to a better efficiency of the plasticizer. Actually, the molecular weight of sorbitol is about twice that of glycerol. Therefore, for a given composition, a higher amount of glycerol is able to interact with starch. As a result, a lower amount of glycerol is most probably necessary to occupy all accessible sites of the monomeric units. The phase separation between starch and plasticizer can therefore be reached for a lower plasticizer content for glycerol compared to sorbitol. In addition, the higher mobility of glycerol can also favor the phase separation.

Figure 3. Glass-rubber transition temperatures associated with the midpoints of the transitions versus relative humidity for sorbitol plasticized waxy maize starch filled with 0 (b), 5 (O), 10 (9), 15 (0), 20 ([), and 25 wt % (4) tunicin whiskers.

Figure 3 shows the plot of the temperature associated with the midpoint of this calorimetric transition as a function of RH level (filled circles). It can be seen that the transition shifts to lower temperatures with increasing moisture content. It decreases from 13 °C for the 0% RH conditioned sample down to -65 °C for the 98% RH conditioned matrix. These Tg values range between the two Tg values reported for glycerol plasticized waxy maize starch.15 In Figure 2, at low RH level (below 31%) the flat shape of the curve is indicative of the amorphous state of the material. As the moisture content increases (above 31%), an endothermal peak arises around 145 °C. It is observed at 146.5 °C for 43% RH and shifts to lower temperatures (∼123 °C) as the RH increases to 98%. This transition is considered to be due to the melting of water-induced crystalline amylopectin domains. However, it is noticeable that for 43, 58, and 75% RH the endothermic peak is found at ∼145 °C and only at 98% RH a sharp decrease to ∼125 °C is observed. The melting temperatures (Tm) and the enthalpies of fusion (∆Hm) are collected in Table 1. It is worth noting that ∆Hm values were calculated per gram of plasticized starch matrix. Therefore, ∆Hm was obtained from the ratio of the apparent enthalpy and of the weight fraction of plasticized starch in the composites. For glycerol plasticized waxy maize starch, an increase of Tm was observed when the RH increased from 43% to 58%.15 For higher moisture content (up to 75% RH), it tended to stabilize. This phenomenon was ascribed to the formation of larger crystal domains with increasing water content, due to an increased mobility of the amorphous chains. The stabilization of Tm for higher moisture level corresponded to a decrease of this mobility due to the formation of crystalline zones. These two effects should also compete with each other in a sorbitol plasticized system. From 43 to 75% RH, it seems that these two effects compensate since Tm remains constant. For the moister sample (98% RH), an effective plasticization is reported because Tm decreases. On the other hand, the molecular weight of the plasticizer should also play a role on the crystallinity of starch. The number of amylopectin/plasticizer links for a given composition should



Figure 4. DSC thermograms of 15 wt % filled tunicin whiskers/sorbitol plasticized waxy maize starch composites for different moisture contents. The relative humidity conditions are indicated in the figure.

Figure 5. Glass-rubber transition temperatures versus whiskers content for tunicin whiskers/sorbitol plasticized waxy maize starch composites conditioned at 0 (b), 31 (O), 43 (9), 58 (0), 75 ([), and 98% RH (4). Solid lines serve to guide the eye.

be less in the case of sorbitol compared to glycerol. It is observed (Table 1) that ∆Hm, and therefore the degree of crystallinity, increases as the moisture content increases. It is ascribed to the well-known water-induced crystallization of starch (retrogradation). Tunicin Whiskers/Sorbitol Plasticized Starch Composites. The DSC curves of all the composite samples exhibited a single glass-rubber transition, as observed in the case of plasticized starch matrix. Figure 4 shows the DSC curves of samples filled with 15 wt % whiskers at various RH conditions ranging from 0 to 98% RH. It is observed that Tg decreases with moisture content. The glass-rubber transition of amylopectin chains in 15 wt % filled systems decreases from 17.9 °C at 0% RH down to -49 °C at 98% RH. This effective plasticization of the composites with increased water content will lead to the conclusion that the sorbitol/water plasticizing system is dispersed evenly in the composite. The values of Tg for all compositions are plotted as a function of relative humidity level in Figure 3. It can be seen that the glass transition shifts to lower temperatures with increasing moisture content, irrespective of whisker loading. Similarly to the unfilled starch matrix, an endothermal peak ascribed to the melting of amylopectin is observed in moist atmosphere (Figure 4). The main difference with the unfilled matrix is the presence of a melting endotherm for the composite conditioned at 31% RH. This means that tunicin whiskers, even at low moisture content for which the plasticized starch cannot crystallize, induce the crystallization of amylopectin. It is observed that for a given RH level the crystallinity of the matrix increases significantly with the whisker content (Table 1). For example, ∆Hm is around 56.5 J‚g-1 for the 5 wt % filled composite conditioned at 31% RH, and it is increased up to 136.5 J‚g-1 for the 25 wt % filled composite at the same RH level. The unfilled matrix was found to be fully amorphous in these conditions. This phenomenon can be most probably ascribed to an anchoring effect of the cellulosic filler, tunicin whiskers probably acting as nucleating agents for starch and favoring its crystallization. In addition, it is worth noticing that none

of the DSC curves exhibit any shoulder on the lowtemperature side of the melt endotherm as in the case of the glycerol plasticized system.15 This shoulder was ascribed to the formation of a possible distorted transcrystalline zone around the whiskers. Figure 5 shows the plot of Tg of the plasticized starch matrix as a function of whisker content at different RH levels. It is observed that for most of the samples, Tg first increases up to a whisker content around 10-15 wt % and then decreases. The increase of Tg upon whisker addition can be well understood taking into account the concomitant increase in the crystallinity of the starch matrix with tunicin content. This restricted mobility of amorphous amylopectin chains results from the physical cross-links induce by the crystallization. The increase in Tg up to 15 wt % whiskers addition is therefore the direct outcome of the decrease in the flexibility of amylopectin chains in the presence of both stiff crystalline whiskers and crystalline amylopectin domains. At higher loading level, this trend is not clear and an unexpected inverse dependence with the whiskers addition is observed. Above 15 wt % whiskers a decrease of Tg is reported, showing that amylopectin chains undergo glassrubber transition at a lower temperature. A possible explanation could be that during crystallization sorbitol is most probably ejected, at least partially, from the crystalline domains. This ejection of the main plasticizer from the crystalline zones results in an increase of its concentration in the amorphous part of the matrix. This phenomenon which occurs also at low whiskers content should compete with the increase of Tg upon whiskers addition and becomes most probably predominant at high loading level inducing a decrease of Tg. Wide-Angle X-ray Scattering. The nanocomposites were characterized by WAXS to study the crystallinity developed based on whisker loading and moisture content. Sorbitol Plasticized Starch. The wide-angle X-ray diffraction pattern of sorbitol is shown in Figure 6. The crystalline nature of sorbitol is clearly evidenced with many

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Figure 6. Wide-angle X-ray diffraction patterns of sorbitol and tunicin whiskers film.

Figure 7. Wide-angle X-ray diffraction patterns of sorbitol plasticized waxy maize starch for various moisture contents. The relative humidity conditions are indicated in the figure.

diffraction peaks. The diffractograms of unfilled sorbitol plasticized starch conditioned at 43 and up to 98% RH are presented in Figure 7. The diffractograms of low moisture content films (0 and 31% RH) are not shown since these films were found to be fully amorphous from DSC measurements. In WAXS experiments, they displayed a broad hump located around 2θ ) 20° with some low intensity diffraction peaks ascribed to sorbitol. It is characteristic of the amorphous phase of the sample. The presence of the weak diffraction peaks means that almost pure sorbitol domains or clusters occur within the material, in agreement with SEM observations. At higher moisture content (Figure 7), the amorphous character of the film progressively lessens and the diffraction peaks become predominant (water-induced crystallization). The B-starch structure is known to be characterized by strong diffraction peaks located around 2θ ) 17°, 22.1°, and 23.8°.15,28 These peaks are weakly defined in Figure 7, since some diffraction peaks ascribed to sorbitol superimposed to the diffractograms.

Mathew and Dufresne

Figure 8. Wide-angle X-ray diffraction patterns of 75% RH conditioned tunicin whiskers/sorbitol plasticized waxy maize starch composites. The tunicin whiskers contents are indicated in the figure.

Tunicin Whiskers/Starch Composites. The WAXS of the composites were studied as a function of the filler content and moisture level. Diffractograms of moist (75% RH) nanocomposite materials are shown in Figure 8. Diffraction patterns of tunicin whiskers films are added as a reference in Figure 6. The diffractograms of the various cellulose/starch composites consist of superimposition of the diffractograms of the two parent components (sorbitol plasticized starch and tunicin whiskers) balanced by the composition. For the unfilled matrix, three ill-defined peaks are observed at 2θ ) 18.5°, 22.7°, and 24.5°. They are typical of B-starch structure, but their angular position seems to be slightly affected by sorbitol. The tunicin whiskers are having a highly crystalline structure with well-defined peaks at 2θ ) 14.6°, 16.4°, and 22.7° and the d values associated with these peaks are 6.06, 5.40, and 3.91 Å, respectively. They are typical of cellulose I. The relative magnitude of these peaks depends on the whisker orientation in the film. As the whiskers content in the film increases, the peaks corresponding to cellulose become more significant. It is worth noting that no evidence of any additional peak is observed contrarily to what was reported for glycerol plasticized starch based nanocomposites.15 The occurrence of this extra peak (at 2θ ) 21.15°) was ascribed to the accumulation of the plasticizers (glycerol and water) in the cellulose/amylopectin interfacial zones, leading to the formation of a possible transcrystalline zone around the whiskers. Therefore, we can conclude that in sorbitol plasticized starch composites there is no evidence for transcrystallization. This is supported by the DSC results also. Contrarily to what was reported from DSC measurements, no significant evolution of the crystallinity is observed upon whisker addition. This is due to difficulties associated with the deconvolution of diffraction peaks from the amorphous hump and with the determination of a degree of crystallinity from WAXS experiments. To confirm the good superimposition of the diffractograms of the parent component for the composites, a modeling of the X-ray diffractograms was performed from the combina-


Figure 9. Experimental and predicted wide-angle X-ray diffraction patterns of the 25 wt % filled tunicin whiskers/sorbitol plasticized waxy maize starch composite conditioned at 75% RH.

tion of the WAXS patterns of the pure parent components. A simple mixing rule was used to build up the theoretical diffractograms. For instance, both experimental and theoretical WAXS patterns are shown in Figure 9 for highly filled nanocomposites (25 wt % tunicin whiskers) conditioned at 75% RH. The theoretical curve fits very well the experimental data, except for the cellulose diffraction peaks, for which the predicted magnitude is much higher than the experimental one. This probably results from the different orientation of tunicin whiskers within the film obtained from only cellulose and from the composite. The whiskers distribution is most likely random in the composite contrarily to the reference whiskers film which is composed of in-plane oriented filler. The WAXS patterns of composites filled with different whiskers content were also plotted for a given RH level to see the influence of this parameter for composites (not shown). Again, the peaks located around 2θ ) 17°, 22°, and 24° were representative of B-starch structure. The magnitude of these peaks evolves with moisture content due to waterinduced crystallization of amylopectin chains. The diffraction peaks typical of cellulose I were also easily identified, except for the peaks located around 16.4° and 22.7, which merged with two of the peaks ascribed to the B-starch structure. A peak observed around 2θ ) 18° corresponded to sorbitol in excess. Water Uptake. The mass of the water sorbed by the samples was traced as a function of time in the sorption kinetics studies. The samples with filler loading from 0 up to 25 wt % were exposed to a relative humidity level of 98%, and the water uptake was determined. It was observed that all the samples absorbed water. It was reported earlier that the addition of plasticizer increases the affinity toward moisture.13,29 In Figure 10 the water uptake up to equilibrium swelling is plotted vs time. It can be seen that, in all the cases irrespective of filler loading, the uptake is rapid in the initial zone (t < 100 h). After this, the sorption rate decreases leading to a plateau, corresponding to equilibrium swelling. In Figure 11, the equilibrium water uptake is plotted as a function of filler loading. The experimental data obtained elsewhere15 for glycerol plasticized starch have been added for comparison. It was found that the unfilled sorbitol


Figure 10. Water uptake during conditioning at 98% RH versus time for sorbitol plasticized waxy maize starch filled with 0 (b), 5 (O), 10 (9), 15 (0), 20 ([), and 25 wt % (4) tunicin whiskers. Solid lines serve to guide the eye.

Figure 11. Maximum relative water uptake, or water uptake at equilibrium, during conditioning at 98% RH for glycerol (b) and sorbitol (O) plasticized waxy maize starch filled with tunicin whiskers versus whiskers content. The solid line serves to guide the eye.

plasticized matrix absorbs around 39.5% water. This value is much lower than what was reported for 33 wt % glycerol plasticized waxy maize starch (62% water).15 An explanation can be proposed based on the chemical structure of both plasticizers. The chain length of sorbitol is about twice that of glycerol. Therefore in glycerol the end hydroxyl groups, which are expected to me more accessible to water, are about twice those compared to sorbitol. The water uptake of sorbitol plasticized starch composites remains roughly constant as the whiskers content increases up to 25 wt %. At high loading level the curve tends to merge with those corresponding to the glycerol plasticized systems. At low whiskers content, the matrix mainly governs the water uptake and it is therefore strongly sensitive to the nature of the plasticizer. By increasing the whiskers content, it seems that the cellulose filler mainly influences the moisture sorption, since the evolution becomes similar for both plasticizer-based materials. This can be ascribed to the formation of a cellulose network through hydrogen bonding between whiskers. Increasing the filler content results in an increase of the number of interwhisker interactions and therefore of the strength of the cellulose network. The role of the matrix becomes therefore negligible. The water diffusion coefficient, D, in the starch-based composites was estimated as described in the Experimental Section. The D values are given in Table 2. The unfilled

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Table 2. Water Diffusion Coefficients in Tunicin Whiskers/Sorbitol Plasticized Starch Composites Conditioned at 98% RH whisker content (wt %)

water diffusion coefficient (cm2/s × 108)

0 5 10 15 20 25

10.1 7.0 6.1 7.5 8.0 7.9

matrix presents the maximum diffusion coefficient (around 1 × 10-7 cm2‚s-1). This value is much higher than the one reported for glycerol plasticized starch (around 1 × 10-9 cm2‚s-1).15 This means that the water diffusivity is much easier in sorbitol plasticized starch compared to glycerol plasticized matrix despite the higher water uptake at equilibrium for the latter. This unexpected result can be ascribed to the morphology of both starch-based matrixes. The glycerol plasticized material was found to be composed of glycerol-rich domains dispersed in an amylopectin-rich continuous phase.15,16 This high Tg continuous phase is in direct contact with the atmosphere and most probably restricts the diffusion of water due to its lower molecular mobility. Moreover, the addition of dextrin, sucrose, and glucose to starch, to simulate the effect of low molecular weight carbohydrates on the diffusion of water, was found to reduce the moisture diffusivity of granular starches in proportion to their percentage.30 Glycerol is a lower molecular weight molecule than sorbitol and could bring a similar effect. Therefore, despite the fact that the water uptake is higher for the glycerol plasticized system, the kinetic of water absorption, displayed through the water diffusion coefficient, is about 100 times higher with sorbitol. On adding the whiskers (up to 10 wt %) D decreases up to 6.1 × 10-8 cm2‚s-1 and then increases gradually and stabilizes around 8 × 10-8 cm2‚s-1. These observations agree with previous reported trends of tunicin whiskers filled glycerol plasticized starch.15 At low filler loading, this phenomenon was ascribed to the presence of a threedimensional cellulose whiskers network within the matrix. It was expected to result from strong hydrogen bonds between cellulose whiskers allowed to develop during the film formation (evaporation step). This network tends to stabilize the starch matrix when it is submitted to strong moisture conditions. By increasing the whiskers content, the stiffness of this network is increased but tends to roughly stabilize. This is a satisfactory explanation for the decrease of D with the cellulose concentration up to 10 wt %. In addition, DSC measurements show that the presence of tunicin whiskers results in an increase of the degree of crystallinity of the starch-based matrix. The crystalline domains of amylopectin are most probably less permeable to water than amorphous zones. The diffusivity of water is therefore restricted in composites. Increasing the whiskers content the inverse dependence of the diffusion coefficient with the whiskers addition is observed, in agreement with glycerol plasticized starch based composites.15 No explanation was given in our previous work for this behavior. A possible explanation can be found

Mathew and Dufresne

through the evolution of two opposite factors. On one hand, the increase of cellulose whiskers content leads to the formation of a denser microcrystals network. It was found that increasing the tunicin whiskers content results in a sharp increase of the mechanical properties of the composite for low loading level.20 The gain in modulus was lower for higher filler content. This should result in a decrease of the loss in diffusivity of water within the matrix. This phenomenon most probably stabilizes roughly with increasing filler loading similarly to mechanical properties. On the other hand, by increasing the whiskers content, the proportion of microcrystals positioning at the surface increases continuously. If the adhesion level between the filler and the matrix is not good, the emerging of these whiskers can create diffusion pathways which tend to increase the diffusivity of water. If such an explanation is valid, it seems that the latter effect becomes predominant around 10 wt % whiskers. Another possible explanation, which is only valid in the case of sorbitol plasticized starch based nanocomposites, is related to the evolution of Tg of the matrix. Our DSC measurements show that the addition of whiskers (up to 1015 wt %) results in an increase of Tg followed by a decrease of Tg for higher loading level. This decrease of Tg ascribed to an increase of the mobility of amorphous amylopectin chains can most probably favor the diffusivity of water molecules. Conclusions Nanocomposites were obtained from sorbitol plasticized waxy maize starch and cellulose whiskers extracted from tunicate. The unfilled sorbitol plasticized starch matrix exhibits a single glass-rubber transition which temperature decreases with increasing moisture content due to the plasticizing effect of water. This plasticizing effect induces the crystallization of amylopectin chains in a moist atmosphere. Compared to our previous study the main difference is the presence of a single glass-rubber transition for sorbitol plasticized materials instead of two distinct ones in the case of glycerol. It seems that the unfilled plasticized material is more homogeneous when using sorbitol. When tunicin whiskers are added to the plasticized starch matrix, they get homogeneously dispersed in the system. The overall crystallinity of the system increases continuously on the addition of tunicin whiskers. This phenomenon results most probably from a nucleating effect of the filler. The Tg of the plasticized starch matrix is found to increase slightly up to about 15% whiskers loading. This is ascribed to the presence of stiff crystalline whiskers and to the increase of crystallinity upon whisker addition, both resulting in a restriction of the mobility of amorphous amylopectin chains. At higher whisker content a decrease of Tg is observed. It is most probably due to an increase of the concentration of plasticizer in amorphous domains, resulting from the crystallization of starch. Both DSC and WAXS measurements show no evidence of preferential migration of plasticizers toward the cellulose and transcrystallization of amylopectin on cellulose surface contrarily to what was reported for glycerol plasticized systems. The water uptake of the composites is


found to remain roughly constant upon whiskers addition. The water diffusion coefficient decreases on adding low whisker content as a result of the formation of a stiff threedimensional cellulose network within the material and of the increase of the Tg of the starch matrix. At higher whisker content the inverse dependence is observed. It could be due to the increase of the proportion of cellulose microcrystals positioning at the surface creating diffusion pathways and to the lowering of Tg. Acknowledgment. The authors are grateful to Roquette S.A. for the supply of waxy maize starch and sorbitol, Dr. M. Paillet for helping in film fabrication and Mrs. D. Dupeyre for her help with the SEM study. The authors are indebted to ADEME (Agence Franç aise de l’Environnement et de la Maıˆtrise de l’Energie) for financial support (ADEME/CNRS convention # 99 01 033). References and Notes (1) Galliard, T. Starch properties and potential; Wiley: New York, 1987. (2) Stephen, A. M. Food polysaccharides and their applications; Marcel Dekker: New York, 1995. (3) Lacourse, N. L.; Altieri, P. A. U.S. patent, 4,863,655, 1989. (4) Gallagher, F. G.; Shin, H.; Tietz, R. F. U.S. patent 5,219,646, 1993. (5) Stevens, D. J.; Elton, G. A. H. Thermal properties of the starch water system. 1. Measurement of heat of gelatinization by differential scanning calorimetry. Starch 1971, 23, 8-11. (6) Forsell, P.; Mikkila, J.; Suortti, T.; Plasticization of barely starch with glycerol and water. Macromol. Sci.sPure Appl. Chem. 1996, 33, 703-715. (7) Shrogen, R. L.; Swanson, C. L.; Thompson, A. R. Extrudates of cornstarch with urea and glycols: structure/mechanical property relations. Starch 1992, 44, 335-338. (8) Lourdin, D.; Coignard, L.; Bizot, H.; Colonna, P. Influence of equilibrium relative humidity and plasticizer concentration on the water content and glass transition of starch materials. Polymer 1997, 38, 5401-5406. (9) Dufresne, A.; Cavaille, J. Y.; Helbert, W. New nanocomposite materials: microcrystalline starch reinforced thermoplastic. Macromolecules 1996, 29, 7624-7626. (10) Wollerdorfer, M.; Bader, H. Influence of natural fibres on the mechanical properties of biodegradable polymers. Ind. Crops Prod. 1998, 8, 105-112. (11) Mwaikambo, L. Y.; Bisanda, E. T. N. The performance of cottonkapok fabric-polyester composites. Polym. Test. 1999, 18, 181-198. (12) Dufresne A. High performance nanocomposite materials from thermoplastic matrix and polysaccharide fillers. Recent Res. DeV. Macromol. Res. 1998, 3, 455-474.

Biomacromolecules, Vol. 3, No. 3, 2002 617 (13) Dufresne, A.; Dupeyre, D.; Vignon, M. R. Cellulose microfibrils from potato tuber cells: processing and characterization of starch-cellulose microfibrils composites. J. Appl. Polym. Sci. 2000, 76, 2080-2092. (14) Dufresne, A.; Vignon, M. R. Improvement of starch film performance using cellulose microfibrils. Macromolecules 1998, 31, 2693-2696. (15) Angle`s, M. N.; Dufresne, A. Plasticized starch/tunicin whiskers nanocomposites. 1. Structural analysis. Macromolecules 2000, 33, 8344-8353. (16) Angle`s, M. N.; Dufresne, A. Plasticized starch/tunicin whiskers nanocomposites. 2. Mechanical behavior. Macromolecules 2001, 34, 2921-2931. (17) Mathew, A. P.; Dufresne, A. In preparation. (18) Wise, L. E.; Murphy, M.; D’Addiecco, A. A. Chlorite holocellulose, its fractonation and bearing on summative wood analysis and on studies on the hemicelluloses. Paper Trade J. 1946, 122, 35-43. (19) Marchessault, R. H.; Morehead, F. F.; Walter, N. M. Liquid crystal systems from fibrillar polysaccharides. Nature 1959, 184, 632-633. (20) Favier, V.; Cavaille´, J. Y.; Chanzy, H. Polymer nanocomposites reinforced by cellulose whiskers. Macromolecules 1995, 28, 63656367. (21) Buleón, A.; Bizot, H.; Delage, M. M.; Pontoire, B. Comparison of the X-ray diffraction patterns and sorption properties of the hydrolyzed starches of potato, wrinkled and smooth pea, broad bean and wheat. Carbohydr. Polym. 1987, 7, 461-482. (22) Trommsdorff, U.; Tomka, I. Structure of amorphous starch. 1. An atomistic model and X-ray scattering study. Macromolecules 1995, 28, 6128-6137. (23) Trommsdorff, U.; Tomka, I. Structure of amorphous starch. 2. Molecular interactions with water. Macromolecules 1995, 28, 61386150. (24) Bizot, H.; Le Bail, P.; Leroux, B.; Davy, J.; Roger, P.; Buleón, A. Calorimetric evaluation of the glass transition in hydrated, linear and branched polyanhydroglucose compounds. Carbohydr. Polym. 1997, 32, 33-50. (25) Butler, M. F.; Cameron, R. E. A study of the molecular relaxations in solid starch using dielectric spectroscopy. Polymer 2000, 41, 2249-2263. (26) George, J.; Bhagawan, S. S.; Prabhakaran, N.; Thomas, S. Short pineapple leaf fibre reinforce low-density polyethylene composites. J. Appl. Polym. Sci. 1995, 57, 843-854. (27) Felix, J. M.; Gatenholm, P. Effect of transcrystalline morphology on interfacial adhesion in cellulose/propylene composites. J. Mater. Sci. 1994, 29, 3043-3049. (28) Van Soest, J. J. G.; Hulleman, S. H. D.; de Wit, D.; Vliegenthart, J. F. G. Cristallinity in starch bioplastics. Ind. Crops Prod. 1996, 5, 11-22. (29) Gontard, N.; Guilbert, S.; Cuq, J.-L. Water and glycerol as plasticizers affect mechanical and water vapor barrier properties of an edible wheat gluten film. J. Food Sci. 1993, 58, 206-211. (30) Marousis, S. N.; Karathanos, V. T.; Saravacos, G. D. Effect of sugars on the water diffusivity in hydrated granular starch. J. Food Sci. 1989, 54, 1496-1500.

BM0101769

Morphological Investigation of Nanocomposites from Sorbitol

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