Rheology, Crystallization, and Biodegradability of Blends Based on

Apr 9, 2009 - Tests using a Haake torque rheometer and a high-pressure capillary rheometer both indicated that chemically modified PBS with a relative...
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Ind. Eng. Chem. Res. 2009, 48, 4817–4825

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MATERIALS AND INTERFACES Rheology, Crystallization, and Biodegradability of Blends Based on Soy Protein and Chemically Modified Poly(butylene succinate) Yi-Dong Li, Jian-Bing Zeng, Wen-Da Li, Ke-Ke Yang, Xiu-Li Wang, and Yu-Zhong Wang*

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Center for Degradable and Flame-Retardant Polymeric Materials (ERCPM-MoE), College of Chemistry, State Key Laboratory of Polymer Materials Engineering, Sichuan UniVersity, Chengdu 610064, China

In our previous study, we found that modification of poly(butylene succinate) (PBS) by introducing urethane and isocyanate groups is an effective method for improving the mechanical properties and water resistance of soy-protein-based bioplastics. This study presents the effects of the structure and content of PBS components on the rheology, crystallization, and biodegradability of soy protein/PBS blends. Tests using a Haake torque rheometer and a high-pressure capillary rheometer both indicated that chemically modified PBS with a relatively low molecular weight can improve the flowability of the blend. According to differential scanning calorimeter measurements, the crystallization ability of the PBS component in a blend with improved compatibility is enhanced markedly compared with that of the pure PBS, and the nonisothermal crystallization behavior could be described by the Jeziorny model. A combination of the weight losses of the samples with the characterization of the surface microstructure shows that both the structure and the content of PBS influence the biodegradation rate of the blend in a compost medium. 1. Introduction With growing environmental awareness throughout the world and the imminent petroleum crisis, the design and use of environmentally friendly materials based on renewable resources have attracted more attention in recent years. As one of the most important types of natural biopolymers, soy products such as soy protein isolate, soy whole flour, and soy dregs have been considered as substitutes for petroleum polymers.1,2 Soy protein consists of complex macromolecules containing 20 different amino acids3 with strong intra- and intermolecular interactions. Therefore, soy-based plastics require sufficient plasticizing agents to reduce the melt viscosity during common processing operations, such as extrusion and injection molding. The hydrogen bonds between plasticizers and the protein screen noncovalent interactions among protein chains and play an important role in determining molecular motion in soy protein materials. Well-known plasticizers for soy protein include water,4-6 glycerol,7-9 ethylene glycol,10 and sorbitol.11 Water serves as an effective plasticizer during the melt processing of soy protein. However, high volatility during processing and storage because of its low melting point restricts the application of water. On the other hand, glycerol, a high-performance plasticizer for starch12 and cellulose derivatives,13 has also been commonly applied to both improve the processability and avoid the brittleness of soy protein plastics. New evidence for the intermolecular interactions and phase structures of glycerolplasticized soy protein have recently been reported.14 In addition, some new plasticizers containing amide groups are also used in soy protein plastics because of their hydrogen-bonding interaction with protein molecules, such as acetamide15 and anionic waterborne polyurethane (WPU) synthesized by Wang and Zhang.16 Generally, the use of a plasticizer enhances the elongation of soy protein but significantly reduces its strength and modulus. * To whom correspondence should be addressed. Tel./Fax: +8628-85410259. E-mail: [email protected].

Moreover, soy-based materials are water-sensitive because of the high polarity and hydrophilicity of soy protein molecules. These drawbacks limit the applications of soy protein as a useful material. Chemical17 and radiation cross-linking18 are useful to improve its strength and water resistance, through the formation of a polymer network, an increase in molecular weight, and reductions of both solubility and elasticity.19 Nevertheless, the use of cross-linking is limited, considering the resulting toxicity, biodegradation, and processability. On the other hand, blending is an effective way to improve polymers properties,20,21 such as flowability, mechanical performance and water resistance. Currently, the polyesters used in blends with soy plastics include polycaprolactone,22-24 poly(lactic acid),25 poly(butylene succinate-co-adipate),26 and poly(tetramethylene adipate-co-terephthalate).27 For these blends, the compatibility should be improved by adding a third component (compatibilizer) or by applying reactive extrusion. Otherwise, the poor interfacial adhesion between two distinct phases usually results in inferior physical properties. To address these problems, we have synthesized modified poly(butylene succinate) (PBS) containing urethane and free isocyanate (NCO) groups and then prepared soy protein isolate (SPI)/PBS blends with improved compatibility.28 This approach was found to be effective for improving the mechanical properties and water resistance of molded SPI plastics through the hydrogen-bonding interactions of the additive or the reaction between NCO groups and the functional side groups of SPI. In the present work, our objective was to evaluate the effects of the content and structure of the modified-PBS component on the rheological properties, crystallization, and biodegradability of soy-based plastics. 2. Experimental Section 2.1. Materials. Soy protein isolate (SPI) with a protein content of 90% (dry basis) was purchased from Chengdu Protein Food Company. 1,4-Butanediol and toluene-2,4-diisocyanate

10.1021/ie801718f CCC: $40.75  2009 American Chemical Society Published on Web 04/09/2009

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Scheme 1. Schematic of Preparation (a) OH-Terminated PBS Prepolymer, (b) PBS-T10.25, (c) PBS-T20.25, and (d) PBS-T0

(TDI) were obtained from Kelong Chemical Reagent Factory (Chengdu, China) and purified by distillation under reduced pressure before use. The succinic acid and glycerol used in this work were of industrial grade and were used without further purification. All other chemicals of reagent grade were purchased from Kelong Chemical Factory (Chengdu, China) and used as received. 2.2. Modification of PBS. Hydroxyl-terminated PBS prepolymer was synthesized by esterification and successive polycondensation of 1.2 mol of butanediol with 1 mol of succinic acid in the presence of tetrabutyl titanate as the catalyst in a 500 mL three-necked flask fitted with a Dean-Stark water separator, a mechanical stirrer, and an inlet of dry nitrogen (Scheme 1a). 29 The intrinsic viscosity and number-average molecular weight of the prepolymer were 0.25 dL/g and 5400 g/mol, respectively. Analytical details are provided in our previous report.28 The obtained PBS diol and a predetermined amount of TDI were placed in a two-necked flask that was placed under vacuum and purged with nitrogen three times. The reaction proceeded at 160 °C for 1 h with strong mechanical stirring, and the product was cooled in a silica gel desiccator before use. The NCO/OH molar ratio was fixed at 1 and 1.5, giving polyesters denoted as PBS-T10.25 and PBS-T20.25, respectively, where the subscript designation represented the intrinsic viscosity of the hydroxyl-terminated PBS prepolymer (Scheme 1b and c). Analogously, the polyester synthesized directly by polycondensation is denoted as PBS-T0 (Scheme 1d). The intrinsic viscosities of PBS-T0, PBS-T10.25, and PBST20.25 were 1.5, 1.9, and 0.5 dL/g, respectively. 2.3. Preparation of SPI/PBS Blends. SPI (60 parts) was premixed thoroughly with a solution of glycerol (30 parts), water (8.5 parts), and sodium sulfite (1.5 parts) using a kitchen mixer for 30 min, and the resulting solution was left overnight to reach equilibration. This premixture was melt-blended with different structures of PBS using an intensive mixer (Haake Rheocord 90, Karlsruhe, Germany) at 130 °C and 60 rpm for 10 min. The resulting soy-based bioplastic was collected and pelletized. For the SPI/PBS-T20.25 blends, the effect of the polyester content

on the properties of the blend material was investigated by changing the content of PBS-T20.25 from 30 to 40%, whereas for the other SPI/PBS blends, the SPI/PBS weight ratio was fixed at 70/30. 2.4. Rheological Analysis. A Haake torque rheometer was used to measure the melt flow behavior of thermoplastic SPI and SPI/PBS. Quantitative information for the melt flow of the samples could be obtained by recording the torque and temperature as functions of time during processing. When sufficiently blended, the solid powders turn into a melt fluid state, and the obtained equilibrium torque and temperature are related to the apparent viscosity of the material under process conditions, thus indirectly reflecting the flow behavior. The rheological properties of the samples were also assessed using a high-pressure capillary rheometer (Rheograph 2003, Go¨ttfert, Buchen, Germany) with a 1-mm capillary die and a length-to-diameter ratio of 10. Measurements were carried out at 135, 140, 145, and 150 °C under a shear rate ranging from 10 to 3000 s-1. The Rabinowitsch correction was applied to account for the influence of shear thinning in the calculation of the shear rate and corresponding viscosity, and the Bagley correction, corresponding to the adjustment for excess pressure drop at the die entrance, was applied by using three capillaries with the same radius but different length/radius ratios. A simple mathematical expression describing the relationship between viscosity and shear rate is (1) η ) Kγ˙ n-1 where the consistency (K) corresponds to the viscosity value for a shear rate γ˙ of 1 s-1 and the power-law index (n) characterizes the deviation from the Newtonian behavior. 2.5. Thermal Analysis and Crystallization. Thermal analysis was performed using a TA Q20 differential scanning calorimeter (DSC) with the samples in a sealed aluminum pan. The equipment was calibrated with indium and tin standards. All samples were first kept at 140 °C for 5 min to eliminate the thermal history and then cooled to -50 °C at a cooling rate of 10 °C/min. Subsequently, the samples were again heated to 140

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Figure 1. Equilibrium torque (TOe) and equilibrium temperature (TEe) values of different SPI/PBS blends at 60 rpm and 130 °C.

°C at the same rate. Relevant transitions and enthalpies were determined from the cooling curve and the second heating curve. The nonisothermal crystallization behavior of some blends was also analyzed by DSC. The samples were crystallized at different cooling rates (from 5 to 20 °C/min) after elimination of the thermal history, and the heat flows during crystallization were recorded. 2.6. Biodegradability. The biodegradability of the materials was characterized by the weight loss of sheets (1 × 1 × 0.08 cm3), with an initial weight of about 0.35 g, in a composting medium prepared by blending courtyard-waste, paper, food, and other materials according to standard ASTM D5338 at a temperature of 58 °C. The weight ratio of sample to compost was fixed at 1:10. The moisture content of the as-prepared compost was 60%, and the C/N ratio was 20:1. After a designated time, the degraded sheets were removed from the compost; wiped clean with a soft, dry paper tissue, and then dried in a vacuum at 50 °C for 24 h before being weighed. The percentage weight loss (W) of the degraded sheets was calculated as W (%) )

W b - Wa × 100% Wb

(2)

where Wb and Wa denote the weights of the dried sheets before and after being buried in compost, respectively. 2.7. Surface Microstructure Characterization. The surface microstructures of the blends were characterized by scanning electron microscopy (SEM) (JSM-5900LV, JEOL, Tokyo, Japan) at an accelerating voltage of 20 kV. All surfaces were dried and sputter-coated with gold prior to examination. The coated samples were observed to identify the surface topographical changes due to microbial and hydrolytic degradation during composting. 3. Results and Discussion 3.1. Rheological Properties. The rheological study was aimed at evaluating the effects of the structure and content of PBS on the flow properties of the materials. 3.1.1. Torque Rheology. Figure 1 shows the effects of the PBS structure and content on the equilibrium torque (TOe) and the equilibrium temperature (TEe) of the blends at 60 rpm and 130 °C. The TEe of thermoplastic SPI was found to be much higher than the initial process temperature (130 °C), as a result of shear heating during melt processing. It is known that soy protein can degrade at high temperature, which would be

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Figure 2. Plot of apparent viscosity vs shear rate for SPI and blends at 140 °C. Table 1. Power-Law Model Parameters of SPI and Blends at 140 °C sample

consistency K (Pa · sn)

n

correlation coefficient

SPI SPI/PBS-T10.25 (70/30) SPI/PBS-T20.25 (70/30) SPI/PBS-T20.25 (60/40)

19451 69762 10520 3120

0.39 0.33 0.45 0.60

0.9916 0.9753 0.9978 0.9996

disadvantageous to the properties of the products. As expected, the addition of PBS-T20.25 to soy protein decreased both TOe and TEe markedly. Moreover, TOe further decreased with increasing PBS content, suggesting the enhanced flowability of the soy protein material with increasing content of polyester. It is worth noting that the structure of PBS prominently influenced the properties of the blends. Blending with PBS-T10.25 increased TOe and TEe of the product compared with the values for pure thermoplastic SPI, because of the high apparent viscosity of the high-molecular-weight polyester. In addition, an enhanced processing temperature could lead to degradation of the protein and an increase of the costs. 3.1.2. Capillary Rheology. The apparent viscosities of SPI and the SPI/PBS blends measured in a capillary rheometer at 140 °C are shown in Figure 2. The measurements for the blends were readily recorded in the shear rate range of 10-3000 s-1. However, the testing region for SPI could not exceed 1000 s-1 with the same amount of sample. Thus, the melts of the blends exhibited much more stability under high shear. When 30% PBS-T20.25 was introduced into the SPI, the viscosity of the blend decreased compared with that of the SPI, and the higher the polyester content, the lower the viscosity. However, the viscosity increased significantly after blending of PBS-T10.25 at a given shear rate and temperature, meaning that this blend was expected to be processed at higher temperature. These results were consistent with the conclusions of the torque rheology tests. This difference was mainly caused by the various molecular weights of the polyesters. Obviously, the interactions between molecules become stronger as the molecular weight increases. Consequently, more entangled points should exist, and the apparent viscosity of the blend melts should become higher. Therefore, only PBS-T20.25 with a relatively lower molecular weight could improve the flowability of the blend. On the other hand, the power-law index (n) and consistency (K) at 140 °C can be derived from the experimental data according to eq 1 linking the viscosity to the shear rate. The results obtained (Table 1) demonstrate the shear thinning and thermoplastic nature of SPI and all of the blends, as well as their ability to flow under certain conditions. It is clear that the molecular weight and polyester content also influenced the power-law index. Incorporation of PBS with a high molecular

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Figure 3. Effect of temperature on the viscosities of thermoplastic (a) SPI and (b) SPI/PBS-T20.25 (70/30). Table 2. Relevant Transitions and Enthalpies from Figure 5 sample

Tc (°C)

∆Hca (J/g)

tmaxb (min)

Tg (°C)

Tm (°C)

∆Hma (J/g)

SPI/PBS-T0 SPI/PBS-T10.25 SPI/PBS-T20.25 PBS-T0 PBS-T10.25 PBS-T20.25

74.7 78.1 79.7 79.4 58.7 60.4

54.5 51.2 60.4 50.8 46.7 56.9

1.19 0.82 0.85 1.70 1.88 2.15

-32.1 -26.1 -25.5 -27.4 -26.9 -26.0

112.4 109.6 109.9 110.3 111.7 112.3

62.3 51.9 55.6 61.4 41.1 60.4

a Values normalized to the amount of the polyester phase. b tmax ) (To - Tp)/Φ, where To and Tp are the onset and peak temperatures of crystallization, respectively, and Φ is the cooling rate.

Table 3. Characteristic Parameters of SPI/PBS-T20.25 for Different Nonisothermal Crystallization Processes Figure 4. Viscous activation energies of SPI and SPI/PBS-T20.25 (70/30) at various shear rates.

weight resulted in a more pronounced shear-thinning tendency, because of the large number of entangled points among the long molecular chains. In contrast, the non-Newtonian behavior was weaker after the incorporation of PBS-T20.25, and the value of n increased with increasing PBS-T20.25 content, showing that the mobility of the macromolecules was improved after blending with PBS of relatively low molecular weight. The relationships of the apparent viscosities of SPI and SPI/ PBS-T20.25 (70/30) with shear rate at different temperatures are illustrated in Figure 3. The apparent viscosity of the SPI melts decreased sharply with increasing temperature. Therefore, a higher processing temperature might be required for the melt processing of SPI, which must be balanced against the possible onset of degradation of the material. However, the apparent viscosity of the blends is less sensitive to temperature than is that of SPI. Therefore, it is preferable to increase the flowability of SPI/PBS-T20.25 by adjusting the shear rate. The effect of temperature on the apparent viscosity may be modeled by an Arrhenius-type equation, which could be expressed by: ln η ) ln A +

∆Eη RT

(3)

where T is the temperature in Kelvin, ∆Eη is the viscous activation energy, and R is the universal gas constant. The viscous flow activation energies of SPI and PBS-T20.25 obtained from linear regression on semilogarithmic plots of apparent viscosity versus temperature are shown in Figure 4. Usually, the viscous activation energy represents the temperature sensitivity of the apparent viscosity. Thermoplastic SPI had a much higher ∆Eη value than the blend, showing that adjusting the

sample

SPI/PBS-T20.25 (70/30) -1

Φ (°C · min ) Tc (°C) ∆Hc (J · g-1) ∆t (min) ∆t1/2 (min) tmaxa (min)

5 83.3 62.0 4.98 1.46 3.84

10 79.7 60.4 2.99 0.72 2.33

15 76.2 58.2 2.06 0.64 1.58

20 73.8 58.0 1.60 0.56 1.19

SPI/PBS-T20.25 (60/40) 5 82.9 60.7 4.04 1.18 2.89

10 79.7 58.3 2.63 0.70 1.98

15 76.0 56.0 1.64 0.55 1.12

20 72.6 55.1 1.14 0.44 0.71

a tmax) (To - Tp)/Φ, where To and Tp are the onset and peak temperatures of crystallization, respectively, and Φ is the cooling rate.

temperature was a very effective way of regulating the flowability of SPI, which agrees with the preceding results. On the other hand, the ∆Eη value of SPI increased with increasing shear rate. This phenomenon is contrary to the behavior of SPI/PBS-T20.25 and general polymers. The flow activity energy is determined by the transition resistance of chain segments. For most polymers, the resistance becomes lower at higher shear rate, because the disentanglement of the molecular chains is stronger than the entanglement, which lead to a reduced ∆Eη. However, the abnormal relationship of ∆Eη to shear rate for SPI might be due to chemical cross-linking caused by gels among the protein macromolecules.30 The first heat treatment of SPI in the intensive mixer led to some thermosetting gels without solubility and melting points, which resulted in much worse mechanical properties after secondary processing.28 Such gels could not disentangle or become orientated even under high shear rates. However, the ∆Eη value of SPI/PBS-T20.25 decreased with increasing shear rate, showing that the formation of gels was restricted by the incorporation of PBS. 3.2. Crystallization. 3.2.1. Crystallization and Thermal Behaviors of SPI/PBS (70/30) Blends and PBS with Various Structures during Dynamic DSC Scans. Figure 5a,b shows the cooling and heating thermograms for different kinds of blends and polyesters. Table 2 provides a summary of the

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Figure 5. DSC thermograms of different blends and polyesters: (a) cooling scan, (b) heating scan.

Figure 6. Crystallization exotherms of SPI/PBS-T20.25: (a) 70/30, (b) 60/40 (at cooling rates ranging from 5 to 20 °C/min).

Figure 7. Development of relative crystallinity of SPI/PBS-T20.25 with temperature for nonisothermal crystallization: (a) 70/30 and (b) 60/40.

DSC results from two scans, revealing the glass transition, melting point, and crystallization status of the PBS component. As compared to directly synthesized PBS-T0, the crystallization of PBS containing urethane groups occurred at much lower temperature and required more time for the fastest crystallization (tmax), showing a depression of crystallization after the introduction of urethane groups. This was attributed to the lower regularity and flexibility of the polymer chains. Moreover, both the glass temperature (Tg) and the melting temperature (Tm) of the modified PBS increased slightly in comparison with those of PBS-T0, because of the rigid

benzene rings. For the SPI/PBS blends with improved compatibility, the crystallization temperature (Tc) of the PBS component increased markedly, and tmax dropped sharply to less than half that of the neat polyester. In addition, the PBS in the blends showed a higher change in enthalpy upon crystallization (∆Hc) than that of the pure polyester employed (Table 2). These results indicate that soy protein induced and accelerated the crystallization of PBS, which favors solidification and molding for extrudate during melt processing. The same phenomenon has been observed for soy protein and polylactide blends.23 On the contrary, the PBS-T0 showed

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Figure 8. Development of relative crystallinity of SPI/PBS-T20.25 with time for nonisothermal crystallization: (a) 70/30 and (b) 60/40.

Figure 9. Crystallization rates of SPI/PBS-T20.25 at different cooling rates.

a lower Tc in the immiscible blend, providing no nucleation effect of SPI because of complete phase separation. The crystallization rate of PBS was high, and no cold crystallization was observed during the second heating scan for each blend and PBS. However, less perfect crystals, owing to the good dispersion of PBS in the soy protein and strong interactions between two phases, resulted in decreased melting peaks for both SPI/PBS-T10.25 and SPI/PBS-T20.25. 3.2.2. Crystallization Behaviors of SPI/PBS-T20.25 Blends with Various Ratios at Different Cooling Rates by DSC. It is

necessary to understand the nonisothermal crystallization behavior because most processing techniques are actually conducted under nonisothermal conditions. For the SPI/PBS-T20.25 blends with promoted crystallization, the effect of PBS content on nonisothermal crystallization from the melt at four different cooling rates ranging from 5 to 20 °C/min was also addressed and are illustrated in Figure 6a,b. Double crystallization peaks with some overlap were observed for SPI/PBS-T20.25 (70/30), independent of the cooling rate. According to our earlier results,28 at least 22.8% of the polyester, which was calculated by chloroform extraction, reacted with SPI. The mobility of the grafted PBS was inevitably restricted by the chain entanglement of the quite long protein molecules, leading to a decreased Tc compared with that of the ungrafted polyester. However, the mobility restriction was weakened by the decrease of the soy protein content, and the cooling scans for SPI/PBS-T20.25 (60/40) showed only one crystallization peak. By all appearances, the exothermic peak widened and shifted to lower temperature with increasing cooling rate. The PBS component could have a longer time to form crystals at a lower cooling rate, so the crystallization peak appears at higher temperature. Conversely, the motion of the PBS chains could not follow the change of temperature at a higher cooling rate, which leads to a lower Tc as seen in Table 3. The curves of relative crystallinity (Xt) as a function of temperature are shown in Figure 7. Here, Xt was calculated by integration of the exothermic peak during cooling process according to the equation

Figure 10. Avrami plots of log[-ln(1 - Xt)] versus log t at different cooling rates for (a) SPI/PBS-T20.25 (70/30) and (b) SPI/PBS-T20.25 (60/40).

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Table 4. Nonisothermal Crystallization Kinetic Parameters Based on Jeziorny Theory SPI/PBS-T20.25 (70/30)

SPI/PBS-T20.25 (60/40)

Φ (°C · min-1)

n

log K

Kn

correlation coefficient

n

log K

Kn

correlation coefficient

5 10 15 20

3.29 3.10 3. 20 3.56

-6.51 -5.23 -5.18 -5.47

0.050 0.30 0.45 0.53

0.9984 0.9981 0.9995 0.9984

3.34 2.93 3.04 3.18

-6.39 -4.97 -4.83 -4.71

0.053 0.32 0.48 0.58

0.9976 0.9957 0.9977 0.9967

∫ X) ∫

T

T0

t

(dHc ⁄ dT) dT

T∞

T0

(4) (dHc ⁄ dT) dT

where To and T∞ are the temperatures at the onset and end of crystallization, respectively. All curves exhibit a reverse sigmoid shape, indicating the fast primary crystallization during the early stage and slow secondary crystallization in the later stage. Figure 8 with a horizontal time scale could be transformed from the temperature axis using the equation t)

T0 - T Φ

(5)

where T is the temperature at crystallization time t and Φ is the cooling rate. In addition, the values of ∆t and ∆t1/2 presented in Table 3, meaning the times needed for crystallinity to reach 100% and 50% conversion, respectively, can be directly obtained from Figure 8. Obviously, both ∆t and ∆t1/2 decreased,

Figure 11. Effect of composting time on the weight loss of the SPI/PBS blends.

suggesting that the crystallization rate was enhanced with increasing cooling rate. On the other hand, the reciprocal value of tmax, i.e., 1/ tmax, can also be used to describe the nonisothermal crystallization rates at various cooling rates. It was found that the crystallization rate increased linearly with increasing cooling rate (Figure 9). This change might be caused by the impingement of growing crystallites. First, the crystals could grow freely in a space of amorphous polymer prior to impingement. Then, further crystallization could take place in the interlamellar regions and could result in a much lower rate after the impingement of adjacent crystals.31,32 When the cooling rate is lower, the polymers have enough time to produce more crystals, so crystal growth will be much more difficult in the following process, which leads to a lower rate of the overall crystallization. Moreover, the decrease of soy protein content in the blend also accelerated the crystallization at the same cooling rate. As indicated in the previous discussion, the phase morphologies of the blends exhibited unique cell structures with relatively uniform holes with quite small dimensions after being extracted by solvent.28 The size of the PBS phase increased with increasing polyester content. Therefore, more PBS molecular chains inside the dispersed drops have higher crystallization rates and weaker mobility restrictions by protein. On the other hand, the ∆Hc values decreased with the increase of both the cooling rate and the PBS content. This means that the orderly arrangement of PBS molecules was restricted to a certain extent, because of the lack of sufficient time for the crystals to fully grow and perfect their morphology. 3.2.3. Nonisothermal Crystallization Kinetics of the Blends. The method proposed by Jeziorny was used to describe the crystallization process for the blends. Jeziorny theory is based on the Avrami model, which is the most common approach for describing overall isothermal crystallization kinetics. Therefore, it is adequately corrected by considering a constant temperature change, and the kinetics of nonisothermal crystallization is given by

Figure 12. SEM images of the surfaces for the SPI/PBS blend sheets after degradation for (a-c) 1 and (d-f) 2 weeks.

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log[-ln(1 - Xt)] ) log K + n log t

(6)

log Kn ) log K ⁄ Φ

(7)

where t is the time; Xt is the relative degree of crystallinity calculated by eq 4 mentioned above; Φ is the cooling rate; n is the Avrami exponent; and K and Kn are the crystallization rate constants before and after correction, respectively. Figure 10 shows Avrami plots of log[-ln(1 - Xt)] versus log t for different cooling rates. Except for a secondary crystallization at higher degrees of crystallinity, each curve shows good linearity. The correlation coefficients indicate that the Jeziorny model is suitable for characterizing the nonisothermal crystallization of these blends. The values of n determined from the slopes and the corrected Kn values are listed in Table 4. The Avrami exponent has a mean value of 3.29 for SPI/PBS-T20.25 (70/30) and 3.12 for SPI/PBST20.25 (60/40), suggesting a heterogeneous nucleation with spherical growth geometry regardless of the SPI/PBS ratio. The value of Kn increased with increasing cooling rate and PBS content, showing that the crystallization was accelerated, in agreement with the result discussed above. 3.3. Biodegradability. The effect of composting time on the weight loss of the SPI/PBS-T20.25 (70/30), SPI/PBS-T20.25 (60/40), and SPI/PBS-T10.25 (70/30) blends is presented in Figure 11. The residual SPI/PBS-T20.25 (70/30) and SPI/PBST10.25 (70/30) blends were powdery in texture and too small in quantity to be retrieved completely after 70 days. As indicated in Figure 11, all blends showed a high initial weight loss, as a result of the leaching of glycerol from the blends. Usually, low-molecular-weight plasticizers, such as glycerol and glycol, are easily dissolved in water and removed in the moist environment. After the initial weight loss, microbial activity (enzymatic degradation) and hydrolysis became the dominant factors, resulting in a higher rate of biodegradation and continued weight loss. Similar results were reported previously by Lodha et al.30 Obviously, SPI/PBS-T20.25 (70/30) showed a higher degradation rate than SPI/PBS-T20.25 (60/40), because of the relatively low biodegradability of PBS. Moreover, increasing the molecular weight of PBS led to a drop in the degradation rate. These results were further confirmed by microscopic surface characterization of the blends after degradation in the compost for 1 and 2 weeks (Figure 12). After 1 week of composting, the majority surface of the SPI/PBS-T20.25 (70/ 30) had been digested away by the microbes (Figure 12a). However, the blend containing 40% PBS-T20.25 showed inconspicuous biodegradation with tiny holes (Figure 12c). In the case of SPI/PBS-T10.25 (70/30), the shallow crevices and rough surface that developed (Figure 12b) confirmed the medium biodegradation rate measured by the weight loss. After 2 weeks of composting, a similar sequence of biodegradation rates was observed. The interior portions of SPI/ PBS-T20.25 (70/30) with a large number of deep pits were exposed because the surface was completely digested (Figure 12d). For SPI/PBS-T20.25 (60/40), visible cracks could be observed, and the surface became rougher (Figure 12f). The SEM image of SPI/PBS-T10.25 (70/30) (Figure 12e) is similar to Figure 12a, demonstrating that more time is needed to achieve similar levels of degradation and damage. 4. Conclusions The structure and content of the PBS component have an obvious effect on the rheology, crystallization, and biodegradability of SPI/PBS blends. The equilibrium torque, equilibrium

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ReceiVed for reView November 11, 2008 ReVised manuscript receiVed February 23, 2009 Accepted March 13, 2009 IE801718F