Structure and Properties of Blend Films Prepared from Castor Oil

Nov 1, 2008 - We successfully prepared a series of transparent blend films from castor oil-based polyurethane (PU) and p-phenylene diamine soy protein...
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Ind. Eng. Chem. Res. 2008, 47, 9330–9336

MATERIALS AND INTERFACES Structure and Properties of Blend Films Prepared from Castor Oil-Based Polyurethane/Soy Protein Derivative Dagang Liu,*,† Huafeng Tian,‡ Lina Zhang,‡ and Peter R. Chang§ College of Forestry, South China Agricultural UniVersity, Guangzhou 510642, People’s Republic of China, Department of Chemistry, Wuhan UniVersity, Wuhan 430072, People’s Republic of China, and Bioproducts and Bioprocesses National Science Program, Agriculture and Agri-Food Canada, Saskatoon, Saskatchewan, S7N 0X2, Canada

We successfully prepared a series of transparent blend films from castor oil-based polyurethane (PU) and p-phenylene diamine soy protein (PDSP). The miscibility, morphology, and properties of the blend films were investigated with Fourier transform infrared spectroscopy, differential scanning calorimetry, dynamic mechanical thermal analysis, scanning electron microscopy, moisture adsorption, thermal degradation, and tensile testing. The results revealed that the PDSP exhibited certain miscibility with PU varied its content from 10 to 80 wt % and also showed the strong hydrogen-bond and chemical cross-linking interactions lied between PU and PDSP. With an increase in the PU content, the elongation at break, thermal stability, and water resistance were improved whereas the tensile strength and Young’s modulus decreased. It is worth noting that modified soy protein could be blended with hydrophobic polyurethane to obtain the blend films having good mechanical properties and optical transmittance. Introduction Polymeric materials from natural, renewable substances have attracted a lot of attention in recent years.1,2 The advantages of these polymeric starting materials include their low cost, ready availability, renewability, as well as the possible compostibility and biodegradability of the resultant polymer materials after their targeted use.3 Soy protein is one of the most promising materials because it is renewable and biodegradable, and it can be used as Bioplastic, fiber, and adhesive, etc. However, the moisture sensitivity, brittleness, and poor processibility of the protein limit its application. Typically, plasticizing, blending, derivatizaton, and graft polymerization are used to improve the performance of soy protein.4-9 However, the miscibility between soy protein and hydrophobic synthetic polymers is very poor due to different distinct polarity; hence, the ideal composite with good performance is difficult to obtain. Therefore, it is important to search for a new oil-dissolved soy protein derivative which can be homogenously codissolved in the organic solvent with other polymeric materials. To the best of our knowledge, no oildissolved soy protein derivative has yet been reported. Castor oil is readily available as a major product from castor seeds. Castor oil-based polyurethane is a useful, versatile material and widely used as an individual polymer possessing network structure because of its good flexibility and elasticity.10,11 In addition, castor oil-based polyurethane always shows good miscibility with natural polymers and their derivatives, which can form semi-interpenetrating polymer networks (semi-IPN) through the cross-linking of castor oil-based PU in the presence of the polymer during thermal curing. A series of polyurethane (PU)/natural polymer derivatives semi-IPN materials, such as * To whom correspondence should be addressed. Tel.: +86-2085280256. Fax: +86-20-85280256. E-mail: [email protected]. † South China Agricultural University. ‡ Wuhan University. § Agriculture and Agri-Food Canada.

nitrokonjac glucomannan, benzyl konjac glucomannan, and benzyl starch/PU, have been studied, indicating improved mechanical properties.12-14 In our laboratory, waterborne polyurethane was introduced into soy protein as a plasticizer to prepare WPU/SPI blends, but tensile strength was lower than 20 MPa, and water adsorption was relatively high.15 In this work, we tried to modify soy protein by p-phenylene diamine (PPD) to achieve an oil-soluble soy protein derivate PDSP and further attempted to introduce flexible hydrophobic castor oil-based polyurethane into this SPI derivative to obtain a series of blend films by varying their component contents. The miscibility between components and relationship of structure and performance for the blend were studied. We hope to provide valuable information about new proteinous blend films having water resistance and good mechanical properties. Experimental Section Materials. Commercial soy protein isolate (SPI) was purchased from DuPont-Yunmeng Protein Technology Co. Ltd. (Yunmeng, China). The weight-average molecular weight (Mw) of SPI was 2.05 × 105.16 4,4′-Diphenylmethane-diisoyanate (MDI) was purchased from Sigma-Aldrich. Castor oil with 4.49% hydroxyl group and a hydroxyl value of 163 was vaccuum dried at 110 °C for 2 h before use. N,N-Dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) were dried with 4 Å molecular sieves for 1 week. p-Phenylene diamine dihydrochloride and other reagents were of analytical grade and used without any further purification. Preparation of PDSP. SPI (5.0 g) was stirred in water (150 mL) for 120 min to obtain a slurry, and the pH was adjusted to 10 with 1.0 M NaOH aqueous solution. The slurry was further stirred for another 120 min at 50 °C. The insoluble fraction was removed through filtration. After cooling to room temperature, an aqueous alkaline soy protein solution was obtained. DMSO (250 mL) was then added to alkaline soy protein solution

10.1021/ie8009632 CCC: $40.75  2008 American Chemical Society Published on Web 11/01/2008

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Figure 1. Scheme of the preparation of modified SPI (PDSP).

(2.0 g) and stirred for 30 min to get a suspension. A 50 mL amount of p-phenylene diamine dihydrochloride (1.81 g, 10.0 mmol) aqueous solution was added dropwise to the SPI suspension with vigorous stirring overnight. Acetone was then added into the resultant suspension to precipitate modified SPI. The precipitate was then separated by centrifuging with a speed of 9000 rpm at 15 °C for 10 min and then collected and washed with acetone 3 times and deionized water 3 times. The obtained precipitate was vacuum dried to get a pure derivate which was coded as PDSP. The PDSP powder was dissolved in the mixed solvent of DMF and DMSO for further solution casting. The synthesis reaction is shown in Figure 1. The content of p-phenylene diamine was determined as followed: PDSP (20 mg) was hydrolyzed in 4.0 mL of a 6.0 N HCl solution for 24 h at 110 °C.17 The hydrolyzed product was diluted to 100 mL with deionized water. To 1.0 mL of the diluted solution was added 1.0 mL of water, 1.0 mL of 1.0 N sodium hydroxide, and 1.0 mL of sodium nitrite/sodium molybdate stock solution (100 g of NaNO2 and 100 g of Na2MoO4 in 1.0 L of deionized water). Absorbance of the resulting solution was measured at 500 nm using an ultravioletvisible spectrometer. The concentration of PDD was then determined using pure PDD solution as a standard. The linear relationship between the absorbance and the concentration was used to construct a PDD standard curve and calculated the content of PDD in the derivative of PDSP.18 The content of PDD averaged over five specimens was 3.40 wt % by calculation. Preparation of UDP Blend Films. A four-necked flask was fitted with a nitrogen inlet tube, mechanical stirrer, thermometer, and pressure-equalizing dropping funnel. MDI (50 g) was poured into the flask and then heated at 40 °C. Under a nitrogen atmosphere, 69 g of castor oil was added dropwise to the flask, and the dropping was completed within 50 min. Then, stirring was continued for 2 h to obtain the PU prepolymer. PU prepolymer was mixed with PDSP of a desired mass and 1,4butanediol as a chain extender (the amount being adjusted to give a total [NCO]/[OH] ratio of 1) were dissolved in DMSO at room temperature. The resulting mixture was given a solid content of about 10 wt % by adding DMF and then cast onto a glass plate. They were cured at 50 °C for 48 h to obtain transparent PU/PDSP films. The PU/PDSP films with a thickness of about 100 µm were peeled off and coded as UDP5, UDP 10, UDP20, UDP30, UDP40, UDP 50, UDP60, UDP70, and UDP80 depending on the PU contents which varied from 5%

to 80%. After solvent evaporation, the films were dried in vacuum to a constant weight at room temperature for 3 days before characterization. Characterization. Fourier transform infrared spectra (FTIR) of the films were recorded on the spectrometer (Spectrum One, Perkin-Elmer, USA) in the range of 4000-400 cm-1 using a KBr pellet method with a resolution of 4 cm-1. Differential scanning calorimetry (DSC) measurement of the films was carried out on a DSC-204 apparatus (Netzsch Co., Germany) under a nitrogen atmosphere at a heating rate of 10 °C min-1 from -50 to +250 °C. Dynamic mechanical thermal analysis (DMTA) was carried out on a dynamic mechanical thermal analyzer (DMTA V, Rheometric Scientific Co.) in rectangular tension mode. The temperature program was run from -50 to 250 °C using a heating ramp of 5 °C min-1 at a fixed frequency of 1 Hz. Specimens with a typical size of 10 mm × 10 mm (length × width) were used. Thermogravimetric analysis (TGA) was performed on a Pyris TGA instrument (Perkin-Emer Co., USA) under a nitrogen atmosphere from 30 to 500 °C at a heating rate of 10 °C min-1. Optical transmittance (Tr) of the films with a thickness of about 0.4 mm was measured with an ultraviolet-visible spectrometer (UV-160A, Shimadzu, Japan) in the wavelength range from 800 to 400 nm. SEM images were taken on an SEM microscope (Hitachi S-570, Japan). Films were frozen in liquid nitrogen and snapped immediately, and the cross-sections of the films were coated with gold for SEM observation with an accelerating voltage of 20 kV. Photographs of the films were taken utilizing a Canon A640 camera. All films were conditioned in desiccators with P2O5 as desiccant (0% relative humidity) at room temperature to give an initial weight (W0). The moisture content of the blend film was achieved by conditioning the samples at room temperature in desiccators with CuSO4 · 5H2O-saturated aqueous solutions to provide a relative humidity (RH) of 98%. The films with similar dimensions were removed at specific intervals and weighed on a five-digit balance to give the weight Wt. The water content or water uptake (WU) of the films was calculated as19 WU(%) )

Wt - W0 × 100 W0

(1)

For rectangular specimens of 40 mm × 10 mm (length × width) at a thickness of about 0.3-0.5 mm, 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.20 The diffusion coefficient (D) can be calculated by the following equation D)

(

)

Mt - M0 2 πL2 M∞ 4t

(2)

where M∞ is the equilibrium weight of the conditioned specimen, 2L is the thickness of the specimen, and t is the time. Tensile strength (σb), elongation at break (b), and Young’s modulus (E) of the films conditioned at 98% RH were measured on a universal testing machine (CMT6503, Shenzhen SANS Test Machine Co. Ltd., Shenzhen, China) with a tensile rate of 5 mm/min according to ISO527-3: 1995 (E). An average value of five replicates of each sample was taken. Results and Discussions Structure, Morphology, and Intermolecular Interaction of UDP. Infrared spectroscopy is a powerful instrument for investigation of the hydrogen-bond behavior of PU or protein.21,22 The NH- of the urethane bond could be the proton donor, while

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Figure 3. Typical photos of the blending films (a) and optical transmittance (Tr) of UDP films at 800 nm (b). The inset figure was a typical transmittance curve of UDP film from 800 to 400 nm.

Figure 2. FTIR spectra of PU, UDP, and PDSP films.

the proton receptor could be -CdO of SPI; -CdO of PU soft and hard segments also could be the proton receptor, and the donor is -NH- on SPI. FT-IR spectra of PU, UDP, and PDSP films are displayed in Figure 2. Bands centered at approximately 1731 cm-1 are assigned to stretching vibration of free -CdO groups.23-26 The relative intensity of the free CdO band decreases with an increase of the PDSP content, indicating that the fraction of the free carbonyl of PU decreases. Broad absorption peaks centered at 3360 cm-1 are attributed to the stretching vibration of -NH of PU. The peaks shift to a higher wavenumber with an increase of PDSP content (UDP80 3365 cm-1, UDP50 3370 cm-1, UDP30 3375 cm-1, UDP10 3379 cm-1). This is due to a cross-linking or hydrogen-bonding interaction of the free -NH group fraction in the UDP films. In other words, the original inter- and intramolecular hydrogen bonds involved in PU are hindered or weakened in the UDP films as a result of the interpenetration and entanglements between PU networks and PDSP macromolecules. The characteristic absorbing peak at about 1650 cm-1 is attributed to a stretching vibration of CdO of amide I, which shifts to a higher

wavenumber with an increase of PU content (PDSP 1646 cm-1, UDP10 1647 cm-1, UDP30 1658 cm-1, UDP50 1664 cm-1). When the PU content is more than 50%, the peaks of amide I and -CdO are combined into one strong peak. According to the harmonic oscillator model, a relatively high peak frequency difference of -NH of PU and CdO of proteinous amide I responds to the strong hydrogen-bonding interaction.27 It is obvious that a strong hydrogen-bonding or even cross-linking effect exists between PDSP and PU. Figure 3a shows a typical transparent and flexible film of the UDP blending films. Optical transmittance (Tr) of UDP films at 800 nm is shown in Figure 3b, and a typical transmittance curve of UDP films from 800 to 400 nm is inserted. All films have a similar transmittance curve with some variations in wavenumber. With the increase of wavenumber, the transmittance increases gradually and all the films have their highest transparence at a wavenumber of 800 nm. Exceeding of 80% of optical transmittance for all films at 800 nm suggests that PU and PDSP have a relatively good compatibility and the blend films have a homogeneous structure although a microstructure separation as demonstrated in DSC and DMTA tests later. SEM images of cross-sections of PU, UDP, and PDSP films are shown in Figure 4. Many parallel compact layered structure or sharp cracks appeared in PDSP (Figure 4a) are typical characteristics of rigid brittle fracture. With addition of PU, UDP10 (Figure 4b) shows the decreasing brittle fracture area as well as the sum of cracks, which suggests a strong interaction between the two components. When the content of PU increases further, there was no obvious phase separation between the hydrophobic polyurethane and hydrophilic soy protein and the interphase of the two components was ambiguous. In fact, in our system rigid PDSP can disperse into PU continuous phase to form a complex network as shown in Figure 4c, 4d, and 4e.

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Figure 4. SEM images of the cross-sections of PU, UDP and PDSP films: (a) PDSP; (b) UDP10; (c) UDP30; (d) UDP50; (e) UDP80; (f) PU. Table 1. Results from DSC and DMTA of the PDSP, PU, and UDP Films DMA samples PDSP UDP5 UDP10 UDP20 UDP30 UDP40 UDP50 UDP60 UDP70 UDP80 PU Figure 5. DSC thermograms of PDSP, PU, and UDP films.

To some extent, the blending films exhibit a tight structure on the whole, indicating certain miscibility. DSC thermograms of PU, UDP, and PDSP films are shown in Figure 5, and the corresponding data are summarized in Table 1. The glass-transition temperatures (Tg) of PU and PDSP are 22.3 and 192.3 °C, coded as Tg1 and Tg2, respectively. Tg1 values of UDP films are lower than that of pure PU and decrease with an increase in the content of PDSP. This may be the reason that incorporation of PDSP partly broke the intramolecular hydrogen bond, hindered forming PU perfect cross-linking networks, and hence improved the flexibility and mobility of PU molecular segment.28-31 Meanwhile, the corresponding heat capacity (∆Cp1) shows the same decreasing trend as that of Tg1. Tg of PDSP is about 200 °C, which suggests that the rigid benzene ring introduced into soy protein makes the increasing stiffness of the molecular chains inflexible. However, all Tg2 of

TR1 (°C)

TR2 (°C)

DSC Tg1 (°C)

Cp1 (J g-1 K-1)

16.4 12.0 15.9 16.6 17.7 17.9 18.8 21.8 21.9 22.3

0.26 0.30 0.30 0.39 0.41 0.42 0.47 0. 51 0.52 0.53

201.1 32.3 25.3 25.5

206.0 209.0 202.7

29.8

197.1

36.6

195.4

40.7

Tg2 (°C)

Cp2 (J g-1 K-1)

192.3 193.9 201.8 195.7 199.2 203.7 216.7 216.0 193.2 193.0

0.73 0.63 0.57 0.46 0.42 0.19 0.19 0.13 0.08 0.06

UDP are higher than Tg of PDSP, and ∆Cp2 of UDP are lower than that of PDSP, which shift to low values with decreasing PDSP content in UDP films. The reason for this may be that the chemical bonding interactions caused by functional groups (NCO, COOH, etc.) of PU are higher than intermolecular hydrogen bonding. Therefore, free motion of protein molecular chain segments was restrained, which results in an increase of Tg2. The intermolecular interactions between PU and PDSP brought about different altering trends of Tg because PU is at its soft elastic state while rigid PDSP stays at its hard glass state. To some extent the chain motion of PDSP dispersed in PU continuous matrix is restricted. Dynamic mechanical analysis is one of the most effective means to characterize the molecular chain segment motion, intermolecular interaction, and miscibility of polymer blends.32,33 The storage modulus (E′) and mechanical loss factor (tan δ) for the UDP films as a function of temperature are plotted in Figures 6 and 7, respectively, and the peak positions of loss

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Figure 6. Temperature dependence of storage (E′) for the PDSP, PU, and UDP films.

Figure 8. Tensile strength (σb), elongation at break (εb), and Young’s modulus (0) as a function of PU content for the PU and UDP films.

Figure 7. Temperature dependence of mechanical loss factor (tan δ) for the PDSP, PU, and UDP films.

factor tan δ curves are summarized in Table 1. The storage modulus of PDSP is nearly 1.0 GPa (-50 to 0 °C), which is much higher than that of PU, indicating the hard nature of the PDSP. Therefore, E′ of UDP has been improved obviously by increasing the content of PDSP. The improvement of rigidity suggests that PDSP has a strong reinforcement effect on protein films. E′ curves of the UDP films exhibit two distinctive drops in the stiffness at lower and higher temperatures, which are assigned to major segmental motions (R relaxation) of PU and PDSP, respectively. It indicates that a certain extent of phase separation took place in the materials. For UDP samples storage modulus decreases with increasing temperature. When the temperature is higher than TR1, a rubber plateau appears in the E′ curves. Usually R relaxation corresponds to the glass transition of polymer and can provide useful information to analyze molecular motions. Two transitions at low and high temperature correspond to glass transitions of PU and PDSP, respectively, and are coded as TR1 and TR2 as shown in Figure 7. TR1 of UDP samples (except UDP10) gradually shifts to low temperature as the content of polyurethane elevates, which partly coincides with the DSC results. Simultaneously, TR2 tends to increase at the first stage (no more than 20%) and then decreases with increasing PU content (more than 20%). From DMTA curves the location and shape of the relaxation peak can provide information about the degree of freedom or order of the segment motions. For example, the intensity of the R2 transition increases with increasing PDSP content, whereas the R1 transition decreases. The fact is ascribed to the elevated amount of mobile units of PDSP participating in the relaxation process. Performance of UDP Films. Mechanical properties of PU and UDP films are shown in Figure 8. Pure PDSP films were

Figure 9. TG and DTG thermograms of PDSP, UDP, and PU films conditioned at 0% RH.

too fragile to test; therefore, no data were obtained. The Young’s modulus and tensile strength of UDP10 are 1.0 GPa and 36 MPa, respectively, indicating typical character of rigid plastics. The modulus of UDP films decreases gradually with an increase of the PU content, which is consistent with DMTA. By adding soft PU into rigid PDSP, polyurethane behaves like a plasticizer and can toughen modified soy protein plastics. Namely, elongation at the break of the UDP films increases significantly, while the tensile strength and Young’s modulus decrease sharply. With addition of more than 80% PU, the films behave like typical elastomer with σb of approximately 10 MPa and b of approximately 250%. At room temperature PU remains at its elastic state and has high elongation at break. A low content of rigid PDSP can disperse in PU networks and act as reinforcing particles. Interestingly, mechanical properties of the UDP materials varied from plastics to elastomer when the ratio of each component was changed. TG and DTG curves of the PDSP, UDP, and PU films are shown in Figure 9. Thermal degradation of modified soy protein PDSP mainly consists of two stages from the DTG curves. Weight loss in the first stage (below 150 °C) is assigned to evaporation of residual moisture, and the second step is mainly due to degradation of SPI (from 250 to 400 °C). It is worth noting that moisture still remained in PDSP even conditioned in 0% RH, which may be ascribed to the bounding water of protein. However, compared with PDSP, the weight loss of the blend films at this stage decreases or even disappears, indicating

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water resistance of the blend materials has been significantly improved compared to pure soy protein plastics.37 Conclusion

Figure 10. Water uptake of PDSP, UDP, and PU films conditioned at 98% RH. Table 2. Equilibrium Water Uptake (WU) and Water Diffusion Coefficients (D) of PDSP, UDP, and PU Films Conditioned at 98% RH samples

water uptake (%)

D (cm2 · s-1 × 1010)

PDSP UDP10 UDP20 UDP30 UDP40 UDP50 UDP60 UDP70 UDP80 PU

42.03 20.80 16.51 15.01 12.44 9.20 7.30 4.32 2.08 0.20

7.80 3.27 1.95 1.48 1.02 0.69 0.36 0.09 0.04 0

strong interaction between PU and PDSP through chemical bond linking, as evidenced by FTIR. Thermal degradation of PU and UDP consists of three steps of weight loss: weight loss from 250 to 375 °C is attributed to decomposing of amide and amino acid, from 375 to 440 °C attributed to decomposing of the PU soft segment, and the final step around 475 °C attributed to decomposing of the PU hard segment.34 From DTG temperature at the maximum weight loss of the UDP films is higher than that of PDSP. Meanwhile, the initial decomposing temperature of UDP films is lagged, and thermal weight loss before 375 °C of the UDP films is much lower than that of PDSP. Thus, we can draw a conclusion that the thermal stability of blend films UDP is better than protein because of incorporation of castor oil-based polyurethane. Serious moisture sensitivity has restricted the applications of soy protein materials. If the water resistance was improved the range and scale of its application would be greatly enhanced. The water uptake isothermal curves of PDSP, UDP, and PU films exposed to 98% RH are shown in Figure 10. The calculated water uptake and water diffusion coefficient (D) are summarized in Table 2. From isothermal curves the uptake of UDP films are rapid in the initial zone (t < 100 h). After this, the sorption rate decreases until arriving at a plateau, corresponding to equilibrium uptake. The equilibrium water uptake and diffusion coefficient of hydrophobic castor oil-based polyurethane are almost 0%, whereas that of PDSP are about 42% and 7.8 × 10-10, respectively. It is noted that the equilibrium water uptake and water diffusion coefficient of UDP films are far lower than that of PDSP and decreases with increasing PU content. The diffusivity of water is strongly influenced by the microstructure of the materials and the water affinity of the polymer components.35,36 This can be explained that the hydrophilic groups of PDSP linked by polyurethane networks were restricted as a result of low diffusion of water. Therefore,

The lipophilic soy protein derivative PDSP was prepared using p-phenylene diamine dihydrochloride as a modifier. Under the DMF/DMSO cosolvent system PDSP and castor oil-based polyurethane were mixed and then cast to obtain a kind of transparent film. The two components are compatible for quite a broad distribution ratio as a result of the strong hydrogen bonds or chemical cross-linking existing between PU and PDSP. When the protein component was dominant, small fractions of PU behaved as an effective toughening agent for PDSP plastics, whereas when the PU component was dominant, small fractions of protein particles could effectively reinforce PU elastomer. A series of materials with prospective performance was achieved by varying the ratio of the two components. Elongation, thermal stability, and water resistance of UDP films were greatly improved by introduction of PU. It is obvious that hydrophobic modified soy protein and introduction of castor oil-based PU could form blend films with an improved mechanical property, thermal stability, and waterproof performance. Acknowledgment We are grateful for the financial support from a grant from the Natural Science Foundation of Guangdong Province, China. Literature Cited (1) Wu, Q.; Zhang, L. Properties and structure of soy protein isolateethylene glycol sheets obtained by compression molding. Ind. Eng. Chem. Res. 2001, 40, 1879–1883. (2) Cao, X.; Dong, H.; Li, C. New nanocomposite materials reinforced with flax cellulose nanocrystals in waterborne polyurethane. Biomacromolecules 2007, 8, 899–904. (3) Kumar, R.; Liu, D.; Zhang, L. Advances in proteinous biomaterials. J. Biobased Mater. Bioenergy 2008, 2, 1–24. (4) Liu, Y.; Li, K. Chemical modification of soy protein for wood adhesives. Macromol. Rapid Commun. 2002, 23, 739–742. (5) Liu, Y.; Li, K. Modification of soy protein for wood adhesives using mussel protein as a model: the influence of a mercapto group. Macromol. Rapid Commun. 2004, 25, 1835–1838. (6) Huang, X.; Netravali, A. N. Characterization of nano-clay reinforced phytagel-modified soy protein concentrate resin. Biomacromolecules 2006, 7, 2783–2789. (7) Liu, D.; Zhang, L. Structure and properties of soy protein plasticized with acetamide. Macromol. Mater. Eng. 2006, 291, 820–828. (8) Liu, D.; Tian, H.; Zhang, L. Influence of different amides as plasticizers on properties of soy protein plastics. J. Appl. Polym. Sci. 2007, 106, 130–137. (9) Liu, D.; Tian, H.; Jia, X.; Zhang, L. Effects of calcium carbonate polymorph on the structure and properties of soy protein based nanocomposites. Macromol. Biosci. 2008, 8, 401–409. (10) Zhang, L.; Zhou, Q. Effects of molecular weight of nitrocellulose on structure and properties of polyurethane/nitrocellulose IPNs. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 1623–1631. (11) Gao, S.; Zhang, L. Molecular weight effects on properties of polyurethane/nitrokonjac glucomannan semi-interpenetrating polymer networks. Macromolecules 2001, 34, 2202–2207. (12) Lu, Y.; Zhang, L. Morphology and mechanical properties of semiinterpenetrating polymer networks from polyurethane and benzyl konjac glucomannan. Polymer 2002, 43, 3979–3986. (13) Cao, X.; Zhang, L. Effects of molecular weight on miscibility and properties of polyurethane/benzyl starch semi-interpenetrating polymer networks. Biomacromolecules 2005, 6, 671–677. (14) Zhang, L.; Zhou, J.; Huang, J.; et al. Biodegradability of regenerated cellulose films coated with polyurethane/natural polymers interpenetrating polymer networks. Ind. Eng. Chem. Res. 1999, 38, 4284–4289. (15) Wang, N.; Zhang, L. Preparation and characterization of soy protein plastics plasticized with waterborne polyurethane. Polym. Int. 2005, 54, 233–239.

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ReceiVed for reView June 18, 2008 ReVised manuscript receiVed September 8, 2008 Accepted September 19, 2008 IE8009632