Ind. Eng. Chem. Res. 2008, 47, 9389–9395
9389
Structure and Properties of Soy Protein Plastics with ε-Caprolactone/Glycerol as Binary Plasticizers Pu Chen,† Huafeng Tian,† Lina Zhang,*,† and Peter R. Chang‡,§ Department of Chemistry, Wuhan UniVersity, Wuhan 430072, China; BioProducts and Bioprocesses National Science Program, Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon, SK S7N 0X2, Canada; and Department of Agricultural and Bioresource Engineering, UniVersity of Saskatchewan, Saskatoon, SK S7N 5A9, Canada
We successfully prepared a series of soy protein isolate (SPI) plastics with ε-caprolactone (CL)/glycerol binary plasticizers via extrusion and compression-molding. The chemical reactions among SPI, CL, and glycerol as well as the influence of CL/glycerol content on the microstructure, thermal degradation, and mechanical properties have been investigated using Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), thermogravimetric analysis-Fourier transform infrared spectroscopy (TGA-FTIR), and mechanical tests. The results of FTIR, SEM, DSC, and DMTA revealed that CL reacted with protein and glycerol molecules under high-temperature, high-shear, and high-pressure conditions. When the CL content was low (less than 25 wt %), the CL added to the protein matrix was dispersed mainly in the glycerol-rich domains and reacted with glycerol. However, at a higher concentration, the CL predominated in the protein-rich domains and reacted with protein molecules. The chemical reactions led to a significant increase in glass transition and mechanical relaxation temperatures of the glycerol-rich and protein-rich domains. Accordingly, the chemical reactions retarded the volatilization of glycerol and the release of NH3 and CO2 and elevated the tensile strength, Young’s modulus, and the water resistance of the soy protein plastic sheets. Introduction Glycerol has a high boiling point and good stability and is regarded as one of the most efficient plasticizers for soy protein plastics. Glycerol-plasticized soy protein possesses good processing properties and mechanical performance.1,2 However, glycerol is susceptible to water and environmental humidity, leading to poor water resistance and high moisture sensitivity of the obtained plastics. It is therefore important to search for a hydrophobic plasticizer that also has good compatibility with soy protein. Among the diverse ways to improve the water resistance of soy protein plastics, soy protein/polymer blends (composites) show particular advantages.3 It has been reported that soy protein/polyether based waterborne polyurethane (WPU) plastics have a tensile strength of 18 MPa in the dry state and 7 MPa when wet.4 In addition, soy protein based plastics prepared by blending soy protein isolate (SPI), soy flour (SF), or soy dreg (SG) with castor oil based polyurethane (PU) prepolymers show good mechanical performance. Furthermore, some exhibited water resistance parameters as high as 0.55 (Rσ ) σwet/σdry).5 The results obtained from characterization of soy protein/PU blends suggest that the PU component acted mainly as a plasticizer. Polycaprolactone (PCL) is a nontoxic, hydrophobic polymer with good biological degradability, which has been used extensively in the biomedical field. SPI/PCL plastics with high tensile strength and water resistance have been reported.6 The results suggest that the PCL and SPI have improved compatibility with the addition of methylene diphenyl diisocyanate (MDI). Although the blending of SPI with WPU, PU, or PCL significantly improves the water resistance and mechanical properties of soy protein plastics, toxic diisocyanates, * Corresponding author. Tel: +86-27-87219274; Fax: +86-2768754067. E-mail:
[email protected]. † Wuhan University. ‡ Agriculture and Agri-Food Canada. § University of Saskatchewan.
such as TDI and MDI, must be employed during processing which definitely restricts industrial production and wider applications of the resulting soy protein plastics. ε-Caprolactone (CL) is the monomer of PCL and can perform ring-opening polymerization at high temperature after initiating with -OH, -NH2, or -COOH.7-9 Interestingly, the complexity of the amino acid composition of soy protein molecular chains could also offer active side groups, including those mentioned above, for reaction. Furthermore, the reaction of glycerol and CL is well proven, and the glycerides coming from the reaction have lower water affinity than glycerol.10 Such properties of soy protein, CL, and glycerol construct a premise to prepare a novel soy protein plastic with lower glycerol concentrations and higher water resistance via in situ reactions among the three components during processing. This work focuses on the preparation and characterization of soy protein plastics with CL/ glycerol binary plasticizers. The chemical reactions and interactions among SPI, CL, and glycerol as well as the influence of CL and glycerol content on the microstructure, thermal degradation, and mechanical properties have been investigated using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), thermogravimetric analysis-Fourier transform infrared spectroscopy (TGA-FTIR), mechanical and water-uptake tests. Experimental Section Materials. Commercial soy protein isolate (SPI) (protein content 92%) was purchased from Dupont-Yunmeng Protein Technology Co. Ltd. (Yunmeng, China). The weight-average molecular weight (Mw) of SPI was determined by multiangle laser light scattering instrument (MALLS, DAWN DSP, Wyatt Technology Co.) equipped with a He-Ne laser (λ ) 632.8 nm) to be 2.05 × 105.11 ε-Caprolactone was purchased from Sigma-
10.1021/ie800371f CCC: $40.75 2008 American Chemical Society Published on Web 11/08/2008
9390 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
Aldrich with a purity of greater than 99% and was used without any further purification. Glycerol (analytical grade) and CH2Cl2 were purchased from Shanghai Chemical Co. (Shanghai, China). SPI and glycerol were desiccated at 80 °C for 24 h before use. Preparation of Soy Protein Plastics with CL/Glycerol. CL and glycerol with selected ratios were mechanically stirred to prepare the homogeneous CL/glycerol plasticizer solutions. CL/ glycerol plasticizer (4 g) and SPI (6 g) were homogenized in a high-speed mixer (HR1704, PHILIPS Ltd., Zhuhai, China) for 5 min and were then extruded using a single-screw extruder (PolyDrive with Rheomex R252, ThermoHaake, Germany) with a diameter of 19.1 mm and a length/diameter ratio of 25:1. The screw rotation speed was 20 rpm, and the temperature profile along the extruder barrel was 90, 120, and 140 °C (from feed zone to exit). The extruding process was repeated three times. Subsequently, the mixtures were homogenized in the abovementioned high-speed mixer. The pulverized mixtures were immediately compression-molded at 140 °C and 20 MPa and then air-cooled to room temperature under constant pressure at a cooling rate of about 3 °C min-1 before removal from the mold. The resultant sheets were coded as C-0, C-1, C-2, C-3, C-4, C-5, C-6, C-7, and C-8 with the CL/glycerol ratios in the plasticizer varying from 0:8, 1:7, 2:6, 3:5, 4:4, 5:3, 6:2, 7:1, and 8:0. Characterization. Fourier transform infrared spectroscopy (FTIR) was recorded on a Nicolet 5700 spectrometer in the range of 400-4000 cm-1 using a KBr pellet method with a resolution of 4 cm-1. Scanning electron microscopy (SEM) images were obtained on an S-570 microscope (Hitachi, Japan) at an accelerating voltage of 20 kV. To observe the distribution of CL in the protein matrix, the sheets were extracted in CH2Cl2 for 1 week to dissolve all of the CL based products not attached to soy protein molecules. The extracted sheets were frozen in liquid nitrogen and snapped immediately after vacuum drying for 48 h, and then the cross sections of the sheets were coated with gold. Differential scanning calorimetry (DSC) tests were conducted on a differential scanning calorimeter (DSC-204, Netzsch Co., Germany) equipped with a liquid-nitrogen-cooling system, calibrated with an indium standard (Tf ) 156.6 °C). The sample in the capsule was quenched to -80 °C and then heated to 150 °C under a nitrogen atmosphere at a heating rate of 20 °C min-1. Dynamic mechanical thermal analysis (DMTA) was carried out using a DMTA-V dynamic mechanical analyzer (Rheometric Scientific Co.) at a frequency of 1 Hz in the tensile mode. The temperature ranged from -100 to 250 °C, and the heating rate was 5 °C min-1. The specimen was a thin rectangular strip (10 mm × 10 mm × 0.3 mm). TGA-FTIR analysis of the specimen was obtained on a TG209 instrument (NETZSCH Co., Germany) under a nitrogen atmosphere from 25 to 600 °C at a heating rate of 10 °C min-1. The TGA instrument was combined with an infrared spectrometer (VECTOR22, BRUKER, Germany) with a scanning rate of 8 Hz. The thermal degradation behavior in air was analyzed on a STA 499C instrument (NETZSCH Co., Germany) from room temperature to 800 °C at a heating rate of 10 °C min-1. The water uptake (WU) of the sheets at relative humidity (RH) of 75% was calculated as WU (%) )
Mt - M0 × 100 M0
(1)
where Mt and M0 are the weight of the sample after exposure to 75% RH for t min and the initial weight of the sample, respectively. The C-series sheets were dried for 2 weeks in a desiccator with P2O5 to provide 0% RH condition. The dried
Figure 1. FTIR spectra of C-0, C-4, and C-8 sheets.
samples were weighed on an analytical balance to obtain M0. The dried sheets were then transferred to another desiccator with saturated NaCl solution to provide 75% RH and conditioned for another 2 weeks to obtain Mt. The mechanical properties of the dry sheets (RH ) 0%) and in the wet state (RH ) 75%) were measured on a universal testing machine (CMT6503, Shenzhen SANS Test Machine Co. Ltd., Shenzhen, China) with a tensile rate of 5 mm min-1 according to ISO527-3:1995(E). Before tensile testing, the sample sheets were conditioned in two desiccators with P2O5 (RH ) 0%) and saturated NaCl solution (RH ) 75%) separately for 2 weeks. Five parallel measurements for each sample were carried out on the testing machine immediately after the sheets were removed from the desiccator. Results and Discussion Structure and Interactions of Soy Protein Plastics. Figure 1 shows FTIR spectra of C-0, C-4, and C-8 sheets. The characteristic stretching bands (1730 cm-1, υCdO; 1167 cm-1, υC-O) of CL do not appear in the spectra, indicating that CL may have been consumed by chemical reactions during the extruding and molding processes.13The broad band at 3399 cm-1 in C-0 was ascribed to the stretching absorbance of -OH and -NH2 group. Considering the abundant glycerol existing in the matrix, this band was assigned mainly to the stretching vibration of -OH groups. The peaks around 1653 and 1546 cm-1 represent the characteristic amide bands: amide I (CdO stretching) and amide II (C-N stretching).14,15 The bands around 1111 and 1044 cm-1 were ascribed mainly to the absorbance of C-OH and C-C of glycerol molecules.16 With an increase in CL content, the stretching absorbance of glycerol in C-4 shifts to 3423 cm-1 with decreasing intensity. There were new absorbance bands at 1718 and 1170 cm-1 for C-4 which may be attributed to the CdO and C-O groups on the polymerized CL of low molecular weight or the residual CL monomers.17-21 The bands around 3409 and 3315 cm-1 of C-8 may be ascribed to the stretching absorbance of -OH end groups of ring-opening polymerized CL and the -NH of soy protein, respectively. For the C-8 sheet, the relative intensity of the characteristic bands of CdO and C-O at 1718 and 1170 cm-1 increased which may indicate an increased amount of polymerized, low molecular weight CL, or residual CL monomers. The SEM images of C-0, C-2, C-4, C-6, and C-8 before and after extraction are shown in Figure 2. The cross section of C-0 shows a homogeneous and smooth morphology (Figure 2a). With an increase in CL content, the fracture becomes increasingly rougher, as shown in Figure 2b-e. This trend suggests
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9391
Figure 2. SEM images of cross sections of C-0 (a), C-2 (b), C-4 (c), C-6 (d), and C-8 (e) sheets before CH2Cl2 extraction and of C-0 (f), C-2 (g), C-4 (h), C-6 (i), and C-8 (j) after CH2Cl2 extraction.
Figure 3. DSC thermograms of C-series sheets.
that with the addition of CL a chemical reaction occurs in the protein matrix, and the phase adhesion is significantly improved. CH2Cl2 is a good solvent for CL and PCL but a poor solvent for soy protein.7-9 After extraction with CH2Cl2, soy protein shrinks, and irregular wrinkles appear on the fractures (Figure 2f-h). There is no obvious cavity structure in the protein matrix of C-0 to C-4 sheets, indicating that at those CL content levels no noticeable residues of CL monomer or homopolymerized PCL exist in the soy protein matrix after extrusion and compression-molding. Some small-dimension (100-200 nm) pores occurred in the fractures of the C-6 and C-8 sheets. These pores may be due to the loss of residual CL and its oligomers after the CH2Cl2 extraction. The SEM observation supports the results of FTIR; that is, most of CL in the C-0 to C-4 sheets participates in the chemical reactions (for example, grafting polymerization, copolymerization, etc.) with glycerol and SPI during the extrusion and compression-molding processes.
Figure 4. Dependence of tan δ and E′ on temperature for C-0 to C-8 sheets.
Glass Transition and Mechanical Relaxation Performance. The DSC thermograms of CL/glycerol plasticized soy protein plastics are displayed in Figure 3. In our previous works, two kinds of domains in glycerol-plasticized soy protein plastics, glycerol-rich and protein-rich, were reported.22,23 Two glass transitions coexist in the thermograms of C-0 to C-5, which indicates the existence of microstructures similar to glycerol plasticized soy protein plastics. For the convenience of discus-
9392 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
Figure 5. TGA and dTG curves of C-0, C-2, C-4, and C-8 in a N2 atmosphere.
Figure 6. FTIR of gas degradation products of C-0, C-2, C-4, and C-8 at about 220 °C in a N2 atmosphere.
sion, in this paper we use the structure models of glycerol plasticized soy protein plastics and the concepts of the glycerolrich and protein-rich domains. The glass transitions at low temperature (Tg1) in Figure 3 were assigned to the glycerolrich domains and those around 40 °C (Tg2) to the protein-rich domains. The endothermic peaks of residual water evaporation in C-0 and C-1 cover other thermal capacity changes between 30 and 150 °C, making the Tg2 hard to distinguish in the related thermograms. The water evaporation endothermic peaks do not exist in the thermograms of C-2 to C-8. This indicates that the addition of CL effectively eliminates the water sensitivity of the plastic sheet. Generally, Tg1 and Tg2 appear simultaneously in DSC thermograms when the CL content is low (C-0 to C-5). Tg1 increases from -51.3 to -25.6 °C with an increase in CL,
while Tg2 remains at about 40 °C. The typical glass transition at approximately -60 °C, and the crystal-melting peak of PCL was not observed.24-27 These results show that the addition of CL does not interrupt the original domain structure of the protein matrix when the CL content is low, and no new domain is formed. For C-6 and C-7, with more CL, Tg1 almost disappears, and Tg2 increases from 40 to 52.8 °C due to the decrease in glycerol content and the large amount of CL. The DSC results for the C-0 to C-7 sheets suggest that the chemical reactions among CL, glycerol and soy protein occur mainly in the glycerol-rich domain when the CL content is low. These reactions restrain the segmental motion of the soy protein molecular chain and elevate the Tg1 of the protein matrix noticeably. At a high CL content, the CL reaction occurs within the protein-rich domain which induces the increase of Tg2. In the DSC thermogram of the C-8 sheets, there are two glass transitions at 53.4 and 98.5 °C. These two glass transitions can not be attributed to the glycerol-rich and protein-rich domains because there is no glycerol present. In our previous work, we found that glycerol-free soy protein had double glass transitions at 67.1 and 132.8 °C, which implied the existence of two kinds of proteins in SPI.22 Compared to the two glass transitions in the glycerol-free soy protein, the values of those in the C-8 sheets were obviously decreased. This fact may indicate that CL can react with both of the proteins. Figure 4 shows a plot of storage tensile modulus (E′) and loss angle tangent (tan δ) as a function of temperature. At low temperature, between -80 and -50 °C, the plastic sheets with low CL content have a high E′, attributed to the antiplasticization of glycerol at low temperatures.1,2 E′ of the sheets in the temperature range of 0-50 °C is enhanced with an increase in CL content. At room temperature, C-6 and C-8 have values of E′ as high as 1.1 × 109 and 1.5 × 109 Pa, respectively. This suggests that the chemical reactions among CL, glycerol, and protein effectively improve the strength of the glycerolplasticized soy protein plastics. Tan δ reflects the dependence of the motion of the macromolecule segment on temperature. For the C-0 to C-6 sheets, with an increase of the CL content, the R1 relaxation peak on the tan δ curve, attributed to the glycerol-rich domain, becomes gradually weaker and the peak value (TR1) shifts from -46.1 to 7.2 °C. The peak value (TR2) attributed to the protein-rich domain also shifts from 76.7 to 129.0 °C. The trends of TR1 and TR2 is consistent with those of Tg1 and Tg2 and supports the postulation for the distribution of the CL reaction in the soy protein matrix. There are two relaxations in the DMTA tan δ curve for the C-8 sheet located at 18.2 and 137.6 °C, respectively. As mentioned in the DSC section, these two relaxations can be assigned to two different types of proteins in the sheets. Thermal Decomposition Behavior. The TGA and differential TGA (dTG) curves of C-0, C-2, C-4, and C-8 in a N2 atmosphere are shown in Figure 5. Protein plastics with CL/ glycerol as plasticizer have three stages of weight loss during heating. In the first stage, from room temperature to 120 °C, moisture volatilizes. Weight loss during the second stage, 120-260 °C, is attributed to evaporation of glycerol and CL oligomers. Finally, the weight loss beyond 250 °C is mainly due to the decomposition of protein. With the increase in CL content, the volatilization of water in the first step is rapidly reduced and the weight loss between 120 and 260 °C also decreases. The Tmax value of the second step on the dTG curve shifts from 204.2 to 247.2 °C. This suggests that CL reacts with glycerol and protein, leading to an improvement in the hydrophobicity and thermal stability of SPI plastics. The curves
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9393
Figure 7. FTIR spectra of gas degradation products of C-0 (a), C-2 (b), C-4 (c), and C-8 (d) at 30-600 °C in a N2 atmosphere.
Figure 8. TGA and dTG curves of C-0, C-2, C-4, C-6, and C-8 in air. Table 1. Water Uptake of Soy Protein Sheets Conditioned at 75% RH sample
C-0
C-3
C-5
C-7
water uptake/%
15.53
9.91
6.77
4.30
of C-4 and C-8 exhibit a shoulder at about 425 °C corresponding to the thermal degradation temperature of PCL,28 further confirming the existence of a reaction among CL, glycerol, and soy protein through grafting or copolymerization of soy protein. Figure 6 shows the FTIR spectra of gaseous degradation products coming from the C-0, C-2, C-4, and C-8 sheets at 220 °C. The multiple bands around 4000-3400 and 1900-1400 cm-1 are attributed to H2O. The bands at 2360 and 2340 cm-1 are characteristic of the absorbance of CO2. The one at 1735 cm-1 is ascribed to the CdO stretching vibration. The bands at 3262, 1100, and 1025 cm-1 indicate volatilization of glycerol. The absorbances at 2973 and 2869 cm-1 are assigned to the C-H vibration, and those at 970 and 830 cm-1 are attributed to NH3. The band around 670 cm-1 is correlated to the outof-plane bending vibration of )CH2.28-30 On the basis of the above results, the weight loss of the plastic sheets may be
Figure 9. Effects of CL content on mechanical properties of CL/glycerol plasticized SPI plastic sheets at RH ) 0%.
attributed to the vaporization of glycerol and the rupture of peptide bonds. The main components of the gaseous degradation products are H2O, CO2, NH3, and some compounds with carboxyl groups as well as some saturated and unsaturated alkanes. The absorbance of the CL monomer was not found in all of the profiles, suggesting that there should be no noticeable CL monomer left in the sample sheets. Figure 7 shows the FTIR profiles of C-0, C-2, C-4, and C-8 over the entire degradation period. The plastics with different CL content have similar gaseous degradation products such as H2O, glycerol, CO2, NH3, carboxyl groups, and so on. However, the addition of CL retards the release of CO2 and NH3 and moves the volatilization of glycerol to a higher temperature. In the second degradation stage from 120 to 260 °C, the relative absorbance intensity of the gaseous products, as determined by FTIR, decreased with the addition of CL. This suggests that CL can react with SPI and glycerol and elevate the thermal stability of the resulting sheets.
9394 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
30 to 40 wt % (C-6 to C-8), the εb and σb improved simultaneously. The E and σb of the C-8 sheet with a CL content of 40 wt % reached 195.7 and 14.0 MPa, respectively, and its εb was around 98.0%. This result suggests that the addition of CL plays a key role in the enhancement of the water resistance of soy protein plastics. Conclusion
Figure 10. Effects of CL content on mechanical properties of CL/glycerol plasticized SPI plastic sheets at RH ) 75%.
The degradation behaviors of C-0, C-2, C-4, and C-8 in air are shown in Figure 8. Similar to that in N2, the degradation of SPI plastics in air has three stages: weight loss in the first stage (room temperature to 120 °C) attributed to the evaporation of residual moisture; the second stage (120-260 °C) was the evaporation of glycerol and the degradation of some proteins; the final stage (above 260 °C) due mainly to the degradation of SPI. With an increase in CL, the moisture content in the first stage and the weight loss in the second stage decreased obviously, but the weight loss in the third stage increased. Tmax shifted from 217.0 to 247.8 °C. These results support a conclusion similar to those in the N2 atmosphere, that adding CL to the soy protein and glycerol is beneficial to improvements in thermal stability. Water Uptake and Mechanical Properties. Water uptake of the soy protein sheets conditioned at a RH of 75% is shown in Table 1. With an increase in CL in the sheets, the water uptake decreased gradually and the sheets exhibited better water resistance. CL is hydrophobic in nature; therefore, introduction of CL should improve the water resistance of the resulting sheets. Figure 9 shows the mechanical properties of the CL/glycerol plasticized soy protein sheets at RH ) 0%. As the CL content of the sheets increased from 0 to 5 wt % (C-0 and C-1), the σb and E of the plastic sheets decreased, whereas εb increased from 67.6% to 146.77%. This indicates that CL may plasticize SPI in glycerol-rich domains with a relatively high affinity with glycerol. With the increase in CL content from 10 to 30 wt % (C-2 to C-6), σb and E increased from 8.3 and 67.2 MPa to 33.0 and 344.5 MPa, respectively, but the εb decreased from 177.7% to 3.5%. This result is consistent with the results obtained from the DSC and DMTA studies that indicated that the glycerol-rich domains gradually disappear in this range of CL content. This further confirms that CL takes part in the chemical reaction with glycerol and protein. A further increase in CL content had little influence on the σb and εb, but the E value increased to 492.1 MPa. Figure 10 shows the effects of CL content on the mechanical properties of the CL/glycerol plasticized SPI plastic sheets at RH ) 75%. Compared to the mechanical properties of the sheets at RH ) 0%, moisture uptake leads to a sharp increase in εb and a decrease in E and σb. With the CL content less than 25 wt % (C-0 to C-5), variations in CL have little influence on the E, εb, and σb of the humid sheets. Interestingly, as the CL content increased from
A series of SPI plastics with CL/glycerol as binary plasticizers were successfully prepared through extrusion and compressionmolding. The results obtained from FTIR, SEM, DSC, and DMTA show that the chemical reaction of CL with soy protein and glycerol could be realized at high temperature, high shear, and high pressure, leading to better compatibility among the three main components. When the CL content was low (less than 25 wt %), the added CL was distributed in the glycerolrich domains, resulting in a reaction with glycerol. However, at the higher content, CL was located mainly in the proteinrich domains and reacted with the protein molecules. The addition of CL led to a significant rise in the glass transition and the mechanical relaxation temperatures of the glycerol-rich and protein-rich domains. Accordingly, the thermal stability of protein plastics was improved, which was related to the retarded volatilization of glycerol and release of NH3 and CO2. Furthermore, the addition of CL increased the tensile strength, Young’s modulus, and the water resistance of the soy protein plastic sheets. Acknowledgment This work was supported by a grant from the National Natural Science Foundation of China (20474048) and the Key Laboratory of Cellulose Chemistry, Guangzhou Institute of Chemistry, Chinese Academy of Sciences. The authors are thankful to Center for Electron Microscopy of Wuhan University. Literature Cited (1) Zhang, J.; Mungara, P.; Jane, J. Mechanical and thermal properties of extruded soy protein. Polymer 2001, 42, 2569–2578. (2) Tummala, P.; Liu, W.; Drzal, W. T.; Mohanty, A. K.; Misra, M. Influence of Plasticizers on Thermal and Mechanical Properties and Morphology of Soy-Based Bioplastics. Ind. Eng. Chem. Res. 2006, 45, 7491–7496. (3) Rhim, J.-W.; Mohanty, K. A.; Singh, S. P.; Ng, P. K. W. Preparation and Properties of Biodegradable Multilayer Films Based on Soy Protein Isolate and Poly(lactide). Ind. Eng. Chem. Res. 2006, 45, 3059–3066. (4) Wang, N.; Zhang, L. Preparation and characterization of soy protein plastics plasticized with waterborne polyurethane. Polym. Int. 2005, 54, 233–239. (5) Chen, Y.; Zhang, L.; Du, L. Structure and properties of composites compression-molded from polyurethane prepolymer and various soy products. Ind. Eng. Chem. Res. 2003, 42, 6786–6794. (6) Zhong, Z. K.; Sun, X. S. Properties of soy protein isolate/ polycaprolactone blends compatibilized by methylene diphenyl diisocyanate. Polymer 2001, 42, 6961–6969. (7) Liu, J.; Liu, L. Ring-opening polymerization of ε-caprolactone initiated by natural amino acids. Macromolecules 2004, 37, 2674–2676. (8) Yu, Z.; Liu, L.; Zhuo, R. Microwave-improved polymerization of ε-caprolactone initiated by carboxylic acids. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 13–21. (9) Yu, Z.; Liu, L. Effect of microwave energy on chain propagation of poly (ε-caprolactone) in benzoic acid-initiated ring opening polymerization of ε-caprolactone. Eur. Polym. J. 2004, 40, 2213–2220. (10) Pitet, L. M.; Hait, S. B.; Lanyk, T. J.; Knauss, D. M. Linear and Branched Architectures from the Polymerization of Lactide with Glycidol. Macromolecules 2007, 40, 2327–2334. (11) Wu, Q.; Zhang, L. Effects of the Molecular Weight on the Properties of Thermoplastics Prepared from Soy Protein Isolate. J. Appl. Polym. Sci. 2001, 82, 3373–3380.
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9395 (12) Chen, P.; Zhang, L.; Cao, F. Effects of moisture on glass transition and microstructure of glycerol-plasticized soy protein. Macromol. Biosci 2005, 5, 872–880. (13) Xie, J. Applications of Infrared Spectrum in Organic Chemistry and Pharmaceutical Chemistry: Science Press: Beijing, China,1987. (14) Nahar, S.; Tajmir-Riahi, H. A. Complexation of heavy metal cations Hg, Cd, and Pb with proteins of PSII: evidence for metal-sulfur binding and protein conformational transition by FTIR spectroscopy. J. Colloid Interface Sci. 1996, 178, 648–656. (15) MacDonald, G. M.; Barry, B. A. Difference FT-IR study of a novel biochemical preparation of photosystem II. Biochemistry 1992, 31, 9848– 9856. (16) Lu, Y.; Deng, Z. Applied Infrared Spectrum Analysis; Publishing House of Electronics and Industry: Beijing, China, 1989. (17) Tripathy, A. R.; MacKnight, W. J.; Kukureka, S. N. In-situ copolymerization of cyclic poly(butylene terephthalate) oligomers and ε-caprolactone. Macromolecules 2004, 37, 6793–6800. (18) Jiang, H.; Wu, P.; Yang, Y. Variable temperature FTIR study of poly (ethylene-co-vinyl alcohol)-graft-poly (ε-caprolactone). Biomacromolecules 2003, 4, 1343–1347. (19) Rong, G.; Deng, M.; Deng, C.; Tang, Z.; Piao, L.; Chen, X.; Jiang, C. Synthesis of poly(ε-caprolactone)-b-poly(γ-benzyl-L-glutamic acid) block copolymer using amino organic calcium catalyst. Biomacromolecules 2003, 4, 1800–1804. (20) Chan, S.; Kuo, S.; Chang, F. Synthesis of the organic/inorganic hybrid star polymers and their inclusion complexes with cyclodextrins. Macromolecules 2005, 38, 3099–3107. (21) Zheng, G.; Sto¨ver, D. H. Grafting of poly(ε-caprolactone) and poly(ε-caprolactone- block-(dimethylaminoethyl methacrylate)) from polymer microspheres by ring-opening polymerization and ATRP. Macromolecules 2003, 36, 7439–7445. (22) Chen, P.; Zhang, L. New evidences of glass transitions and microstructures of soy protein plasticized with glycerol. Macromol. Biosci. 2005, 5, 237–245.
(23) Chen, P.; Zhang, L. Interaction and properties of highly exfoliated soy protein/montmorillonite nanocomposites. Biomacromolecules 2006, 7, 1700–1706. (24) Scandola, M.; Ceccorulli, G. Cocrystallization of random copolymers of ω-pentadecalactone and ε-caprolactone synthesized by lipase catalysis. Biomacromolecules 2005, 6, 902–907. (25) Castelletto, V.; Hamley, I. W. Crystallization in poly(L-lactide)b-poly(ε-caprolactone) double crystalline diblock copolymers: a study using X-ray scattering, differential scanning calorimetry, and polarized optical microscopy. Macromolecules 2005, 38, 463–472. (26) Herrera, D.; Zamora, J.; Bello, A.; Grimau, M.; Laredo, E.; Mu¨ller, A. J.; Lodge, T. P. Miscibility and crystallization in polycarbonate/poly(εcaprolactone) blends: application of the self-concentration model. Macromolecules 2005, 38, 5109–5117. (27) Zhu, B.; Yoshie, N.; Asakawa, N.; Inoue, Y. Partial phase segregation in strongly hydrogen-bonded and miscible blends. Macromolecules 2004, 37, 3257–3266. (28) Persenaire, O.; Alexandre, M.; Dege´e, P.; Dubois, P. Mechanisms and kinetics of thermal degradation of poly(ε-caprolactone). Biomacromolecules 2001, 2, 288–294. (29) Schmidt, V.; Giacomelli, C.; Soldi, V. Thermal stability of films by soy protein isolate-sodium dodecyl sulfate. Polym. Degrad. Stab. 2005, 87, 25–31. (30) Moharram, M. A.; Abd-El-Nour, K. N.; Abd-El-Raof, G. Infrared spectra and dielectric properties of thermally treated soybean proteins. Polym. Degrad. Stab. 1994, 45, 429–434.
ReceiVed for reView March 6, 2008 ReVised manuscript receiVed September 3, 2008 Accepted October 7, 2008 IE800371F
