Plasticized Waxy Maize Starch: Effect of Polyols and Relative Humidity

The plasticizing effect of different polyols such as glycerol, xylitol, sorbitol, and maltitol on waxy maize starch was investigated. The concentratio...
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Biomacromolecules 2002, 3, 1101-1108

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Plasticized Waxy Maize Starch: Effect of Polyols and Relative Humidity on Material Properties Aji P. Mathew and Alain Dufresne* Centre des Recherches sur les Macromole´ cules Ve´ ge´ tales (CERMAV-CNRS), Universite´ Joseph Fourier, BP 53, F-38041 Grenoble Cedex, France Received May 29, 2002

The plasticizing effect of different polyols such as glycerol, xylitol, sorbitol, and maltitol on waxy maize starch was investigated. The concentration of plasticizer was fixed at 33 wt % (dry basis of starch). The structure and mechanical performance of resulting films conditioned at different relative humidity levels were studied in detail. The effect of the plasticizer on the glass-rubber transition temperature (Tg) and crystallinity was characterized using differential scanning calorimetry. It was found that Tg decreases with increasing moisture content and decreasing molecular weight of the plasticizer. The water resistance of starch increased steadily with the molecular weight of the plasticizer and was directly proportional to the ratio of the end to total hydroxyl groups. As the molecular weight of the plasticizer increased, the brittleness of the dry system increased. However, the use of high molecular plasticizer allowed good mechanical properties of the moist material to be obtained in terms of both stiffness and elongation at break. Introduction Literature surveys have shown that starch is a particularly potentially interesting biodegradable material due to its low cost and renewability.1-3 Moreover, the use of starch in the plastics industry 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. In fact, starch is not truly a thermoplastic like most of the synthetic polymers. It occurs as partially crystalline, water-insoluble granules that may be destructurized at high temperatures under pressure and shear in the presence of a diluent, usually water. When dried, the resulting product is glassy at room temperature and its glassrubber transition temperature (Tg) is higher than its degradation temperature. Starch materials can be used more effectively, if the temperature range corresponding to the rubbery plateau is enlarged to include ambient temperatures. Plasticizers are generally added to convert starch into a so-called thermoplastic starch and to extrude or mold an object.4,5 Plasticizers can be defined as low molecular weight substances that are incorporated into the polymer matrix to increase the film flexibility and proccessibility. They increase the free volume or molecular mobility of polymers by reducing the H-bonding between the polymer chains. The dimension, shape, and water affinity affect the plasticizing property of the plasticizer. Glycerol and water are the most commonly used plasticizers for starch. The two main drawbacks to the more extensive use of starch as a thermoplastic are the sensitivity of its mechanical properties to fluctuations in water content and its aging * To whom correspondence [email protected].

should

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through crystallization. This last event, know as retrogradation, results from the reassociation during storage of amorphous gelatinized starch or starch with a low degree of ordering into a more ordered state. Both phenomena are affected by the choice of the plasticizer. A broad range of plasticizers has been used in experimental studies. Thermoplastic starches have been prepared using glycerol,6-12 sorbitol,13 urea,6,14 sodium lactate,14 dimethyl sulfoxide,15 and low molecular weight sugars.16 In the present study thermoplastic starch films were prepared from waxy maize starch and various possible plasticizers. Poly(ethylene glycol) (PEG 200 and PEG 4000) and polypropylene glycol (PPG 400) were used as possible plasticizers. However, after casting and water evaporation, the samples became brittle and opaque, showing that these components were ineffective as a plasticizer for starch. Phase separation was clearly evidenced. Similar results were reported for PEG 200 by Lourdin et al.14 In addition, for PEG 4000 and PPG 400 plasticized starch, the dry film was oily. These inhomogeneous materials were not considered as suitable materials for further studies. Efficient plasticizers adopted for the study were glycerol, xylitol, sorbitol, and maltitol. The samples were homogeneous and displayed good mechanical cohesion. The structure and properties of resulting films were investigated in different moisture conditions and correlated with the chemical structure of the plasticizer. Experimental Section Materials. Waxy maize starch (almost pure amylopectin, amylose content is lower than 1%) was kindly supplied by Roquette S.A. (Lestrem, France). The plasticizers used were glycerol (Prolabo, 98% purity), xylitol, sorbitol, and maltitol (these three plasticizers were kindly supplied by Roquette).

10.1021/bm020065p CCC: $22.00 © 2002 American Chemical Society Published on Web 07/18/2002

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Table 1. Main Chemical and Physical Properties of the Plasticizers Used

carbon number molecular weight melting point (°C) heat stability (°C) hygroscopy Tg (°C)

glycerol

xylitol

sorbitol

maltitol

3 92 20 >160 high -75

5 152 94 >160 high -27

6 182 100 >160 median 0

12 344 157 >160 median 45

Figure 1. Chemical structure of (a) glycerol, (b) xylitol, (c) sorbitol, and (d) maltitol.

These products display increasing dimensions and molecular weights. The main physical and chemical properties of the plasticizers are given in Table 1. The chemical structure of these plasticizers is shown in Figure 1 for comparison. It is worth noting that maltitol may not be fully comparable to the other plasticizers due to its different molecular structure. Film Processing. Starch and plasticizer were first mixed and dispersed in water. The mixture contained 10 wt % of waxy maize starch, 5 wt % plasticizer, and 85 wt % water. These ratios were the average values found in the literature for the processing of thermoplastic starch17-23 and are similar to our previous works.10,24,25 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. Film Conditioning. The structure and therefore the properties of starch materials are strongly related to the water content.14,26-30 The moisture content of the 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, 35, 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).

Thermogravimetric Analysis. Thermogravimetric analysis was used to accurately determine the water content of the films conditioned at different relative humidities. The measurements were achieved with a Perkin-Elmer TGA7 instrument. A few milligrams of the sample was heated from room temperature up to 130 °C at 5 °C/min under nitrogen flow (flow rate 20 mL/min). The temperature was subsequently stabilized for 1 h. The loss of weight, ascribed to the water content, was measured for different water activities (water activity ) % conditioning relative humidity/100) of the samples. Scanning Electron Microscopy. Scanning electron microscopy (SEM) was performed to investigate the morphology of the plasticized 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. Differential scanning calorimetry (DSC) was performed with a Perkin-Elmer 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. Water Uptake. For all samples the kinetics of water absorption was determined. The specimens used were thin rectangular strips with dimensions of 10 mm × 10 mm × 1 mm. The films 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 weighing, 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.10 Tensile Tests. The nonlinear mechanical behavior of the plasticized starch materials was analyzed using an Instron 4301 testing machine in tensile mode, with a load cell of 100 N capacity. The specimen was a thin rectangular strip (∼30 × 5 × 1 mm), conditioned at the relative humidities mentioned above. The gap between pneumatic jaws at the start of each test was adjusted to 20 mm. The stress-strain curves of conditioned samples were obtained at room temperature at a strain rate d/dt ) 8.3 × 10-3 s-1 (cross-head speed ) 10 mm‚min-1). The true strain  can be determined by  ) ln(L/L0), where L and L0 are the length during the test and the length at zero time, respectively. The true stress σ was calculated by σ ) F/S, where F is the applied load and S is the cross-sectional area. S was

Plasticized Waxy Maize Starch

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Figure 2. Scanning electron micrographs of the fractured surface of (a) glycerol, (b) xylitol, (c) sorbitol, and (d) maltitol plasticized starch.

determined assuming that the total volume of the sample remained constant, so that S ) S0L0/L, where S0 is the initial cross-sectional area. Stress versus strain curves were plotted, and the tensile or Young’s modulus (E) was measured from the slope of the low strain region in the vicinity of σ )  ) 0 ([dσ/d]f0). Ultimate mechanical properties were also characterized. The true ultimate stress, or true stress at break, σb ) Fb/S, where Fb is the applied load at break, was reported for each tested sample. Ultimate elongation was characterized by the true ultimate strain, or true strain at break, b ) ln[1 + (∆Lb/ L0)], where ∆Lb is the elongation at break. Mechanical tensile data were averaged over at least three specimens. Results and Discussion Morphological Characterization of Plasticized Starch. The examination of the fractured surface of plasticized starch films was carried out using SEM. Panels a, b, c, and d of Figure 2 show the fractured surface of waxy maize starch films plasticized with glycerol, xylitol, sorbitol, and maltitol, respectively. It is observed that the surface of glycerol plasticized material (Figure 2a) is rough. It could be ascribed to the heterogeneous nature of this sample that was found in an earlier study10 to be composed of glycerol- and amylopectin-rich domains. The fractured surface of xylitol plasticized system (Figure 2b) appears different with a rough structured surface. On the contrary, for sorbitol (Figure 2c) and maltitol (Figure 2d) plasticized waxy maize starch, the surface is perfectly smooth. It is worth noting that the

examination of sorbitol and maltitol plasticized samples was problematic since after exposure to the electron probe for about 1 min, the surface became blistered. In Figure 2d, a crack is observed showing a marked brittleness of maltitol plasticized starch at low temperature. Differential Scanning Calorimetry. DSC measurements were performed on plasticized starch films conditioned at various relative moisture contents. DSC thermograms of plasticized starch films conditioned at 43% RH are shown in Figure 3. As reported previously,10 glycerol plasticized materials displayed two distinct ill-defined specific heat increments. The temperatures associated with the midpoints of these two calorimetric transitions are reported in Table 2 and plotted in Figure 4a as a function of the water content. The water content of the samples conditioned at different RH levels was determined from thermogravimetric analysis. The experimental data are collected in Table 2. The two specific heat increments observed for glycerol plasticized starch were assigned to the partial miscibility of glycerol and starch. They were associated with the glass-rubber transition of glycerol-rich (low-temperature transition) and amylopectin-rich (high-temperature transition) phases. A similar phase-separated structure was also observed by others.7,31 Such a phase separation did not occur, or at least was not observed in DSC experiments, in xylitol, sorbitol, or maltitol plasticized starch (see Table 2 and Figure 4a). This should lead to a better efficiency of the plasticizer because of its higher dispersion level and higher interaction possibility with

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Figure 3. DSC thermograms of plasticized waxy maize starch films conditioned at 43% RH. The nature of the plasticizer is indicated in the figure for each thermogram. Table 2. Water Content Determined from TGA Experiments and Temperatures of the Calorimetric Transitions of Plasticized Starch Using Data Obtained from the DSC Curves: Glass-Rubber Transition Temperature (Tg), Melting Temperature (Tm), and Associated Heat of Fusion (∆Hm) RH water plasticizer (%) content (%) glycerol

xylitol

sorbitol

maltitol

Tg (°C)a

Tm (°C) ∆Hm (J/g)

0 35 43 58 75

2.8 7.7 15.9 16.9 23.6

-53.5 26.5 -47.5 27.0 -82.5 1.0 -86.5 -1.5 -97.5 -13.0

0 31 43 58 75

3 7.7 11.5 15.6 22.5

12.5 -40.0 -40.0 -41.0 -54.5

146.5 145.0

505 835

0 31 43 58 75

3.5 7.7 10.4 15.3 23.5

13 -6.5 -27.5 -30.5 -51.5

146.5 143.0 145.0

486 507 827

0 31 43 58 75

2.5 7.7 8.6 14.8 23.5

28.5 19.5 4.5 -5.0 -42.0

159.0 155.5 136.0 139.0

66 426 493 602

132.5 157.0 156.0

496 614 887

a For glycerol plasticized starch, the low T is associated to glycerolg rich domains and the high Tg is associated to amylopectin-rich domains.

amylopectin chains. Actually, the molecular weight of sorbitol, for example, 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 xylitol, sorbitol, or maltitol. In addition, the higher mobility of glycerol can also favor the phase separation.

Figure 4. Glass-rubber transition temperatures associated with the midpoints of the transitions versus (a) water content for glycerol (b), xylitol (O), sorbitol (9), and maltitol (0) plasticized maize starch and (b) molecular weight of the plasticizer for plasticized maize starch conditioned at 0% RH (0), 31% RH (b), 43% RH (O), 58% RH (2), and 75% RH (∆). The solid lines correspond to the best linear fits.

Figure 4a shows the plot of the glass-rubber transition temperature as a function of the water content for starch plasticized with the different plasticizers. It can be seen that the transition shifts more or less linearly to lower temperatures with increasing moisture content whatever the plasticizer may be. This is ascribed to the well-known plasticizing effect of water. For instance, it decreases from 13 °C for the 0% RH conditioned sorbitol plasticized starch film down to -51 °C for the 75% RH conditioned sample. Moreover, the plasticizing effect of water can give rise to another phenomenon. The rearrangement of amorphous starch chains in the presence of moisture during storage was reported by scientists (see, for example, ref 22). This phenomenon known as retrogradation is favored by the plasticization effect of water. During crystalline domains formation by amylopectin chains, the plasticizer may get squeezed out of the crystalline regions making the ratio of plasticizer in the amorphous regions higher. As a result plasticization of amorphous regions increases and their Tg decreases accordingly. The single Tg values measured for xylitol, sorbitol, or maltitol plasticized material range between the two Tg values reported for glycerol plasticized waxy maize starch. In addition, it is observed that the efficiency of the plasticizer decreases with its molecular weight. Figure 4b shows the evolution of Tg for xylitol, sorbitol, and maltitol plasticized starch films conditioned at 0, 31, 43, 58, and 75% RH versus

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Plasticized Waxy Maize Starch

the molecular weight of the plasticizer. The Tg values of glycerol plasticized materials were not include in Figure 4b owing to the complexity of this event for this sample (presence of two Tg values). Tg is found to increase linearly with the molecular weight of the plasticizer. A possible explanation can be found through the number of effective amylopectin/plasticizer interactions. As the molecular weight of the plasticizer increases, the amount of plasticizer molecules interacting with the amylopectin chains decreases and the plasticizing effect is lower for the same plasticizer content. Accordingly, the Tg tends to increase with the increase in plasticizer molecular weight. At the same time, as the Tg of the material decreases, the amylopectin chains are more mobile. This increasing mobility of amylopectin chains leads to a higher crystallization during storage and conditioning. The crystalline domains act as cross-links and restrict the relaxation of the amorphous amylopectin regions. This leads to an increase in Tg value. It was observed from DSC measurements that for low water content (up to 31-35% RH) the thermograms had a flat shape indicating amorphous nature of the starch when plasticized with glycerol10 and sorbitol.25 Similarly, xylitol plasticized maize starch was fully amorphous in dry atmosphere (up to 43% RH). At 31% RH maltitol plasticized starch exhibits a small endothermal peak indicating crystallinity. The melting temperature (Tm) and associated heat of fusion (∆Hm) are collected in Table 2. The effect of plasticizer on crystallinity is complex and involves mainly two opposite factors. When the plasticizing effect increases, i.e., when the mobility of amorphous chains increases and Tg value decreases, the mobility of polymer chains favors the formation of crystalline domains. Considering this phenomenon, the crystallization of starch should decrease with increasing the molecular weight of the plasticizer. At the same time, decreasing the molecular weight of the plasticizer leads to an increase of the amount of amylopectin/ plasticizer interactions and therefore to a decrease of the possible interactions between the amylopectin chains. These restricted interactions hinder the formation of crystalline domains from amorphous amylopectin chains. Considering the latter phenomenon the crystallization of starch should decrease with decreasing the molecular weight of the plasticizer. It seems that the latter phenomenon is predominant in a dry atmosphere. By increase of moisture content, the magnitude of the endothermal peak observed for maltitol plasticized starch increases showing that the reorganization and crystallization of amylopectin chains are favored by the plasticizing effect of water. This can be explained based on the crystallinity developed by retrogradation of amylopectin starch during storage. The crystallization of starch occurs also for other plasticized systems, between 43 and 58% RH depending on the plasticizer (see Table 2). In a highly moist atmosphere, the degree of crystallinity increases with decreasing the molecular weight of the plasticizer. This means that the mobility of amorphous chains is mainly involved in the crystallization. This phenomenon is most likely also linked to the higher hydrophilic nature of low molecular weight compounds.

Table 3. Water Uptake at Equilibrium and Water Diffusion Coefficients in Plasticized Waxy Maize Starch Conditioned at 98% RH

plasticizer

water uptake at equilibrium (wt %)

water diffusion coefficient (cm2/s × 108)

glycerol xylitol sorbitol maltitol

62 42 40 27

0.18 14.6 10.1 7.0

The melting temperatures obtained from DSC experiments are reported in Table 2. This temperature is directly linked to the size of the crystalline domains. As the size of the crystallites increases, Tm increases. A different evolution of Tm with moisture content is observed depending on the molecular weight of the plasticizer. For low molecular weight plasticizer (glycerol), the temperature position of the endothermal peak first increases with water content, from ∼130 °C for the sample conditioned at 43% RH up to ∼155 °C for the sample conditioned at 58% RH. At increasing moisture content, it tends to stabilize. For higher molecular weight plasticizer (maltitol), the inverse dependence is reported. The Tm value decreases from ∼160 °C for the sample conditioned at 31% RH up to ∼135-140 °C for the sample conditioned at 75% RH. For medium molecular weight plasticizers (xylitol and sorbitol) Tm is roughly constant whatever the water content may be. This plasticizer-nature-dependent evolution of Tm upon water content is most probably the result of two effects competing with each other. Indeed, the reorganization and crystallization of the amylopectin molecules are favored by the plasticization effect induced by water. At sufficiently high water contents, the amylopectin is thought to form inter- and intramolecular double helices. When the water content increases, larger crystal domains can form and therefore higher Tm values are reported as a result of increased mobility of amorphous chains. At the same time, increasing the crystallinity of the amylopectin lowers the mobility of amorphous chains, resulting in a reinforcement of the network by the formation of physical cross-links and a stabilization of the retrogradation phenomenon. Water Uptake Studies. As one of the major drawbacks in the use of starch systems is its water absorption tendency, any improvement in water resistance is highly important. The water uptake study of the plasticized starch films conditioned at 98% RH was carried out. It was observed that each sample absorbed water during the experiment. As reported previously for similar systems,10,25 the kinetics of absorption was fast at lower times (t < 100 h) and decreased at extended times. The equilibrium swelling, or maximum relative water uptake, or water uptake at equilibrium corresponds to the plateau value observed for times higher than 200 h. These values are collected in Table 3 and plotted in Figure 5a as a function of the molecular weight of the plasticizer. A continuous decrease of the water uptake at equilibrium with the molecular weight of the plasticizer is observed. It decreases from 62 wt % for the glycerol plasticized film down to 27 wt % for the maltitol plasticized sample. An explanation can be proposed based on the chemical structure of the plasticizers.

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Figure 6. Evolution of the water diffusion coefficient for plasticized waxy maize starch versus glass-rubber transition temperature of the material. The solid line serves to guide the eye, and results are the average values of triplicates. Table 4. Mechanical Properties of Plasticized Starch Using Data Obtained from Tensile Tests: Tensile Modulus, E, True Strength, σB, and True Ultimate Strain, B

Figure 5. Maximum relative water uptake or water uptake at equilibrium, during conditioning at 98% RH for plasticized waxy maize starch versus (a) molecular weight and (b) ratio of end to total hydroxyl groups of the plasticizer. The solid line serves to guide the eye in Figure 4a and corresponds to the best linear fit in Figure 4b.

The amount of end hydroxyl groups, which are expected to be more accessible to water, is all the higher as the molecular weight of the plasticizer is low. Figure 5b displays the evolution of the water uptake at equilibrium as a function of the ratio of end to total hydroxyl groups of the plasticizer. A linear evolution is observed. The water diffusivity or diffusion coefficient, D, of the plasticized starch samples was determined as described in the Experimental Section. The D values are collected in Table 3. It is observed that the water diffusion coefficient decreases with molecular weight of the sample except for the glycerol plasticized starch. The D value associated to the latter material is the lowest one (around 1 × 10-9 cm2‚s-1) compared to the other plasticized systems (around 1 × 10-7 cm2‚s-1). The glycerol plasticized material was found to be composed of glycerol-rich domains dispersed in an amylopectin-rich continuous phase.10,24 The high Tg continuous phase is therefore expected to be in direct contact with the atmosphere and most probably restricts the diffusion of water due to its lower molecular mobility. Indeed, the diffusion coefficient corresponds to the aptitude of water to diffuse through the material. It depends on material properties such as glass-rubber transition temperature and degree of crystallinity, which both affect the specific volume of the material and molecular mobility at room temperature. The evolution of the water diffusion coefficient versus glass-rubber

plasticizer

RH (%)

E (MPa)

σB (MPa)

glycerol

35 43 58 75

30 12 38 18

0.76 1.13 0.23 0.76

xylitol

0 31 43 58 75

110 77 59 93 91

7.46 4.33 5.20 4.68 3.86

5.3 8.8 5.8 6.4 12.0

sorbitol

31 43 58 75

88 159 112 48

9.2 4.39 4.9 3.49

4.0 10.3 13.3 15.9

maltitol

58 75

126 118

4.03 3.54

5.0 22.0

B (%) 141 18 14 10

transition temperature of the plasticized material is plotted in Figure 6. It is worth noting that for glycerol plasticized starch the Tg value of amylopectin-rich domains (high Tg value) was used because it is characteristic of the material in direct contact with the atmosphere. It is observed that D decreases continuously as Tg increases. Tensile Tests. The tensile mechanical behavior of the plasticized starch films was analyzed at room temperature as a function of the nature of the plasticizer and conditioning atmosphere. The tensile modulus, tensile strength, and elongation at break of the plasticized starch films were determined from the plot of the true stress versus true strain as described in the Experimental Section. The results are collected in Table 4. In the case of glycerol and xylitol plasticized samples, the films were soft enough at room temperature to conduct tensile tests with ease. In the case of sorbitol- and maltitol-based systems, the samples were not tested when conditioned at low RH level, because of the brittleness of the films. The tests were carried out only starting at 31% RH for sorbitol and 58% RH for maltitol. The increase of brittleness of the materials when the molecular weight of the plasticizer increases was most probably due, on the one hand, to the possibility of

Plasticized Waxy Maize Starch

crystallization at lower moisture content and, on the other hand, to the increase in Tg. The SEM examination of fractured surfaces also revealed the brittleness of sorbitol and maltitol plasticized materials (Figure 2c,d). The evolution of the tensile modulus is difficult to interpret taking either the moisture content as the variable parameter for a given plasticizer or the plasticizer characteristics as the variable parameter for a given RH level. It is worth noting that at room temperature most of the drier plasticized starch materials are in the glass-rubber transition zone, as measured by DSC (see Table 2). The modulus value is therefore strongly sensitive to any temperature fluctuation. The Young’s modulus values do not exhibit a definite trend but show a global tendency to increase as the molecular weight of the plasticizer increases. It could be ascribed to the higher Tg of high molecular weight plasticizer systems. In addition, because the moist starch films contain both amorphous and crystalline regions, the measured moduli are average values reflecting the contributions of each phase. Crystalline domains, whose ratio tends to increase in moist conditions, act as both filler and cross-links on the mechanical properties. At the same time, the Tg of amorphous domains decreases with increasing water content resulting in a softening of this part of the material. It leads to an inverse tendency on the stiffness of the material. The measured moduli reflect therefore a compromise between the development of crystalline domains and the softening of amorphous domains at high moisture content. High molecular weight plasticizer systems display both a higher Tg value and ability to crystallize in drier atmosphere, and it is quite normal that they have a higher tensile modulus. Similar observations can be done for the evolution of the tensile strength or stress at break. However, it can be seen for xylitol and sorbitol plasticized starch films that σB tends to roughly continuously decrease with increasing water content. The drier (35% RH conditioned) glycerol plasticized sample was found to be fully amorphous (Table 2). It displays a typical rubberlike behavior with a high strain at break (∼140%). As the water content increases, starch is likely to crystallize and the elongation at break strongly decreases (up to ∼18% at 43% RH). The relatively low elongation at break and relatively low modulus of moist glycerol plasticized starch are an indication of the weak cohesion of the material. Compared to glycerol, other plasticizers cause a dramatic decrease of the elongation at break for dry samples. In addition, it was specified previously that the brittleness of the maltitol plasticized samples at low RH levels affects the tensile performance adversely, and only at higher RH levels was the system sufficiently plasticized to get a good tensile performance. As the water content increases, B continuously increases. For instance it increases from 4% up to about 16% by increasing the RH level from 31% to 75% for sorbitol plasticized material. In addition, the elongation at break increases with the molecular weight of the plasticizer for moister samples (75% RH conditioned). It increases from 10% for the glycerol-based material up to 22% for the maltitol plasticized material. This could be due to the lowering of the degree of crystallinity of the material

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Figure 7. Typical true stress vs true strain curves of plasticized waxy maize starch conditioned at 75% RH (T ) 25 °C, d/dt ) 8.3 × 10-3 s-1). The nature of the plasticizer is indicated in the figure for each curve.

conditioned at 75% RH with increasing the molecular weight of the plasticizer (see Table 2). This is illustrated by typical stress versus strain curves of moist samples shown in Figure 7. The evolution of B shows that for glycerol plasticized starch the ultimate properties of the sample are mainly governed by the retrogradation phenomenon and by the water-induced crystallization of the material. The crystallization weakens the sample in moist atmosphere. In addition, compared to other plasticized systems this sample displays the highest moisture sensitivity and water uptake. The soft amorphous domains should be unable to give the necessary cohesion between crystalline domains under high tensile strain. On the contrary, for higher molecular weight plasticizers, both the retrogradation phenomenon and the water uptake, mainly by amorphous zones, are less. Indeed, DSC measurements have shown (Table 2) that the degree of crystallinity of moist samples tends to decrease with the molecular weight of the plasticizer. It was also shown that the water uptake decreases with the molecular weight of the plasticizer. Therefore, the cohesion between amorphous domains and crystalline zones remains sufficient to ensure good mechanical performances. The main phenomenon involved in the ultimate properties is the elongation of soft amorphous amylopectin chains with increasing water content. An interesting finding is that the use of the highest molecular weight under the used plasticizers, namely, maltitol, allows obtaining in moist atmosphere relatively high modulus thermoplastic starch with a relatively high elongation at break. Conclusions Thermoplastic starch films were obtained from waxy maize starch and different plasticizers. The concentration of plasticizer was fixed at 33 wt % (dry basis of starch). Among the various components used, poly(ethylene glycol) and polypropylene glycol were found to be ineffective to plasticize starch. Suitable materials were obtained with polyols, viz., glycerol, xylitol, sorbitol, and maltitol, of increasing molecular weight. The glycerol plasticized material appears as a complex heterogeneous system composed of glycerol-rich domains dispersed in an amylopectin-rich

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continuous phase. Each phase exhibits its own glass-rubber transition, for which the temperature decreases as the moisture content increases owing to the plasticizing effect of water. Other systems exhibit a single glass-rubber transition, for which the temperature also decreases with increasing water content and ranges between the two Tg values of glycerol plasticized maize starch. The Tg increases linearly with the molecular weight of the plasticizer. It is ascribed to the decrease of possible amylopectin/plasticizer interactions with increasing molecular weight of the plasticizer. The crystallization of starch is found to be favored by high molecular weight plasticizers in dry atmosphere and by low molecular weight plasticizers in moist atmosphere. The water resistance of starch increases steadily with the molecular weight of the plasticizer and is directly proportional to the ratio of end to total hydroxyl groups. The water uptake was as low as 27% for maltitol plasticized starch conditioned at 98% RH. The water diffusion coefficient decreases as the molecular weight of the plasticizer, and then the Tg of the material increases. As the molecular weight of the plasticizer increases the brittleness of the dry system increases. However, whereas the elongation at break decreases with moisture content for glycerol plasticized systems, the inverse dependence is observed for the three other plasticizers. It is found that the use of high molecular plasticizer allows obtaining good mechanical properties of the moist material in terms of both stiffness and elongation at break. It is ascribed to the lower water uptake and good cohesion between amorphous and crystalline domains in these materials.

Mathew and Dufresne

(9)

(10) (11) (12) (13) (14)

(15) (16) (17) (18) (19) (20) (21) (22)

(23)

Acknowledgment. The authors are grateful to Roquette S.A. for the supply of waxy maize starch, xylitol, sorbitol, and maltitol, and Mrs D. Dupeyre for her help in the SEM study. The authors are indebted to ADEME (Agence Franc¸ aise de l’Environnement et de la Maıˆtrise de l’Energie) for financial support (ADEME/CNRS convention #99 01 033).

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