ARTICLE pubs.acs.org/IECR
Hydrophobic Thermoplastic Starches Modified with Polyester-Based Polyurethane Microparticles: Effects of Various Diisocyanates Yamei Leng, Yu Zhang, Xiaoxia Chen, Chao Yi, Bingbing Fan, and Qiangxian Wu* Green Polymer Lab, Polymer Science Department, College of Chemistry, Huazhong Normal University, Wuhan 430079, China
bS Supporting Information ABSTRACT: It is critical to prepare ductile and hydrophobic modified starch material in an effective and environmentally friendly way. In this work, polyurethane prepolymers (PUPs) with various isocyanates were synthesized and mixed reactively with thermoplastic starches (TPS) in an intensive mixer to prepare modified TPS. The effects of the isocyanates on the structure and properties of the modified TPS were then investigated by using scanning electron microscope (ESEM), tensile tester, and contact angle meter. Results showed that mechanical properties of the modified TPS were improved with the increase in hydrophobicity of the isocyanate. 4,40 -Methylenedi-p-phenyl diisocyanate (MDI) was hydrophobic, the NCO groups in PUPs were not easily consumed by water when modified starch was prepared, leading to a significantly increased reaction of the NCO groups with starch. As the amount of urethane bonds between starch and PUP increased, the compatibility between the two polymers was also improved, resulting in the improvement of tensile properties. Isocyanates played an important role in improving the compatibility between the starch and PUP and the properties of the modified TPS.
’ INTRODUCTION In the past decades, nonbiodegradable plastic wastes have turned into hazardous wastes which seriously pollute the environment, and the shortage of petroleum resources has become a global problem.1,2 The reality is that we are badly in need of environmentally friendly materials from natural and renewable resources as alternatives. Starch becomes a particularly interesting biodegradable substrate and has been widely used in agriculture,3 medicine, industrial foam,4 and food packaging,5 due to the properties of sustainablility, low price, excellent biodegradability, and its availability in large quantities from sources.6,7 However, starch has many disadvantages for its application as a potential biodegradable material, such as its sensitivity to water and brittle property.8 Starch modification, which involves the alteration of the physical and chemical characteristics of the native starch, can be used to overcome these disadvantages. A variety of physical treatments were used to alter starch, and these treatments included heating with or without moisture, radiation, and mechanical processing.9,10 These treatments provide improved process ability, texture, and stability.11 Chemical modification involves the introduction of functional groups into the starch molecule, resulting in markedly altered physical-chemical properties.12 Such modification of native granular starch profoundly alters their gelatinization, pasting, and retro-gradation behavior.13 Recently, polyurethane prepolymer (PUP) was widely used to chemically modify starch owing to the reaction between the isocyanate group of PUP and the hydroxyl group of starch.14 Polyurethane (PU), a unique polymeric material with a wide range of physical and chemical properties, has been extensively used in the automobile, paint, furniture, and textile industries.15 A number of studies have been carried out to modify starch with PU.14,16 However, some organic solvents such as acetone16 were used during this modification and the reaction efficiency was low r 2011 American Chemical Society
because of short reaction residence time.17 Therefore, it is crucial to prepare high-reaction efficiency modified-starch in a green way. In our laboratory, an effective and green method for preparing modified thermoplastic starch (TPS) was developed: PUP, water, and starch were reactively mixed in an intensive mixer or an extruder for preparing chemically modified TPS.1822 The results indicated that almost 100% of modified PUP was cross-linked to the starch matrix, and the final products were thermoplastics. The high efficiency of the modification was attributed to the formation of PUP microparticles with large amounts of NCO groups. Obviously, the large amounts of NCO groups increased the likelihood of reaction of PUP microparticles with starch. When compared with pure starch, the toughness and hydrophobic properties of the modified starch were significantly improved. Furthermore, no organic solvents were used. Therefore, this process is an environmentally friendly way for solving the low reaction efficiency problem during modification of starch. The structure and properties of the modified starches are affected by PUPs, and the isocyanate type is a key factor that determines the structure and properties of the PUPs. Therefore, in this work, the effects of the different types of isocyanates in PUPs on the structure and properties of the modified starches were studied. In the experiment, polyurethane prepolymers were synthesized with different kinds of isocyanate and mixed reactively with the starchwater mixture to prepare modified TPS. The modified TPS were then characterized. Poly-1,4-butylene glycol adipate (PBA) was used because it falls in the category of renewable natural resources. Received: May 25, 2011 Accepted: August 25, 2011 Revised: August 7, 2011 Published: August 25, 2011 11130
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’ EXPERIMENTAL SECTION Materials. Poly-1,4-butylene glycol adipate (PBA) 4,40 methylenedi-p-phenyl diisocyanate (MDI, 98%) were purchased from Sigma-Aldrich Fine Chemicals, and 1,6-hexamethylene diisocyanate (HDI, 98%) were purchased from Sigma-Aldrich Fine Chemicals (St. Louis, MO). 2,4-Toluene diisocyanate (TDI, 98%) was purchased from Wuhan Jiangbei Chemical Co., Ltd. (Wuhan, China). Isophorone diisocyanate (IPDI,98%) was purchased from Guangzhou Jinli Chemical Co., Ltd. (Guangzhou, China). Corn starch (amylose: 2326 wt %; moisture: 13 wt %) was obtained from Wuhan Corn Starch Co., Ltd. (Wuhan, China) and used without any further pretreatments. Butyl acetate (analysis grade) was purchased from China National Pharmaceutical Group Corporation (Shanghai, China). Synthesis of PUP. The molar ratio of isocyanate to hydroxyl groups (NCO/OH) was 2.0. PBA (Mw = 2000 g mol1, 120 g) was charged into a 500 mL three-necked flask fitted with a stirrer operating at a speed of 300 rpm, an inlet, and an outlet. The system was dried in vacuum at 100 °C to remove the PBA’s moisture. After 30 min, the temperature was decreased to 60 °C, and MDI (30 g) was then charged into the flask. The translucent mixture quickly became clear. Five minutes after the addition of MDI, the mixture was stirred vigorously and reacted at 80 °C for 1 h. White PUP was obtained as a result. The PUP prepared with MDI was designated as PUMDI. With the same procedure, a series of PUPs (PUHDI, PUTDI, and PUIPDI) with different isocyanates (HDI, TDI, and IPDI, respectively) were prepared. Synthesis of Modified TPS. Anhydrous corn starch (45.8 g), PUMDI (11.5 g), and water (15.8 g added water and the moisture content of the starch) were charged into an intensive mixer (SU-70, Changzhou Suyan Science and Technology Co., Ltd. Changzhou city, China) and mixed reactively at 90 °C with a stirrer speed of 100 rpm. After 20 min, a white modified starch was obtained. The modified TPS was equilibrated in a sealed plastic bag for one day before use. The starch modified with the PUMDI was designated as CPM, representing corn starch modified with PUMDI. Following the same procedure, PUHDI, PUTDI PUIPDI and PBA diol were used to prepare CPH, CPT, CPI and CPB respectively. The weight of PU to the total weight of dry starch and PUP in all modified starches was maintained at 20 wt %. Without the addition of PUP, native corn starch was also processed and assigned as TPS. Preparation of Sample Sheets by Compression Molding. Wet modified thermoplastic starches were compression-molded using a hot press (R-5001 model, Wuhan Qien Science & Technology Co., Ltd. Wuhan, China) equipped with a water cooling system. The molding time, temperature and pressure were 5 min, 95 °C, and 40 MPa, respectively. The wet sheets were cut into a dumbbell-like sheet (5A type) according to GB/T1040-2006. The length of the dumbbell-like sheet was 75 mm, with a width of 4 mm at the narrowest section. The sheets were equilibrated at 60% RH for at least 2 weeks. Moisture Content Measurement. Small parts (about 1 g) were cut from the molded starch sheets, weighed, and dried at 110 °C. After 6 h, the sample was weighed again to calculate the moisture content in the sample using the following equation:
moisture content ð%Þ ¼ ðW 1 W 2 Þ=W 1 100
ð1Þ
where W1 and W2 represented the weight of starch sheets before and after drying, respectively.
NCO Content. NCO content was measured using the 2-dibutylamine method reported by Cong et al.23 PUP (about 3 g) was put in conical flask, and anhydrous methylbenzene (20 mL) was added to dissolve the sample. Then 2-dibutylamine-methylbenzene (10 mL) was added. After the mixture was allowed to stand for 30 min, isopropyl alcohol (40 mL) was added. Bromocresol green was used as indicator, and hydrochloric acid was the standard solution for titration in the determination. When the solution changed from blue to yellow, titration ended. Three replicates and a blank test were done. The NCO content of PUP was calculated as follow:
W NCO ¼ ðV 0 V 1 Þ c 42=1000m
ð2Þ
where V0 and V1 represent the volume of hydrochloric acid consumed in the blank test and titration of samples, respectively, c represents the concentration of hydrochloric acid, m is the mass of sample, and 42 represents the molar weight of the NCO group. Tensile Test. Mechanical properties were measured using a tensile tester (6P-TS 2000S, Shenzhen GaoPin Test Machine Co. Ltd.) with a strain rate of 5 mm min1. The distance between the two clamps was 40 mm. The strength at break (σb, MPa) and elongation at break (εb, %) of the sheets were recorded. Five duplications were carried out. Reaction Ratio. A quantitative method was used to analyze the reaction ratio of modified starch. CPM (5 g) and water (250 g) were cooked in a beaker (500 mL) at 95 °C for 1 h to obtain starch dispersion, and then butyl acetate (20 g) was added to separate the nonreactive PU component in the CPM. After mixing for 30 min, the aqueous system was left to stand at 5 °C for 12 h to obtain a clear butyl acetate layer and water layer. The water layer was roughly taken out of the beaker using a pipet, and the residue in the beaker was washed 4 times with water. The washed residue (containing nonreactive PU) was concentrated, dried, and weighed. As a control, anhydrous unmodified starch (4 g) was extracted using the above waterbutyl acetate solvent mixture. To know the efficiency of this analysis method, PUMDI power (1 g) and anhydrous unmodified starch (4 g) were mixed and extracted using the same procedure as that for CPM. The “CS-SR” and “CPM-SR” represent the solid residue in butyl acetate layer of the CS sample and of CPM sample, respectively. As analyzed by our previous work,22 CPM-SR contained corn protein as well as free polyurethane components, and CS-SR contained protein, therefore the differences between the weight of solid residue in CS-SR and CPM-SR was the weight of nonreactive PU in modified TPS. The reaction ratio of modified starch was calculated as follows: reaction ratio ð%Þ ¼ 100 ðW L W 1 Þ=W 2 100
ð3Þ
where WL represented the weight of solid residue in butyl acetate layer for modified starches, W1 was the weight of solid residue in butyl acetate layer for anhydrous unmodified starches, and W2 was the weight of PU used to modify starch in theory (1 g). Three duplications were carried out. Contact Angle (CA). Surface hydrophobicity and wettability of sheets were estimated from the contact angle (CA) measurement with a contact angle meter (Contact Angle System OCA20, Germany). Water was used as the testing liquid in this work; 1 μL of testing liquid was deposited on the solid sheet surface. The contact angles were measured with a CCD camera and processed using an image analysis video card which calculated the contact angle automatically using an image analysis setup. 11131
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Table 1. Viscosity of CS, TPS, CPM, and CP at Different Temperature viscosity value (cp) at different temperatures (°C) samples
60 °C
70 °C
80 °C
90 °C
CS
24.4 ( 1.2
37.6 ( 1.0
510.1 ( 63.3
4355.7 ( 87.4
TPS
6.3 ( 1.3
10.8 ( 1.7
12.7 ( 1.8
27.5 ( 0.9
CPM CPH
2.8 ( 0.7 4.4 ( 0.2
2.9 ( 1.2 4.5 ( 0.0
3.4 ( 0.6 5.2 ( 0.7
5.4 ( 1.0 5.3 ( 0.4
Figure 1. FTIR spectra of TPS, CPT, PUTDI, CPB, and PBA.
Fourier Transform Infrared Spectroscopy (FTIR). A FTIR spectrometer (Avator 360, Nicolet, MA) at room temperature was used. Test samples were pulverized with KBr and pressed into transparent disks for analysis. PUMDI and PUHDI were compression molded into dry thin films. All spectra of samples were recorded in transmission mode at a resolution of 4 cm1 with accumulation of eight scans. Emission Scanning Electron Microscopy (ESEM). An ESEM (FEI, Quanta 200 FEG, Netherlands) was used to observe the cross sections of fractured samples. Each sample was frozen using liquid nitrogen and then fractured using tweezers to produce cross sections. The cross sections were coated with gold and then used for ESEM observation. Viscosity Measurements. A rotary viscometer analyzer (DVII+PRO LV, Brookfield, U.S.) was used to determine viscosity properties of the starch suspension. Before the test, castor oil was used to calibrate the instrument. An aqueous suspension of starch (6%, w/w) was prepared, and the starch suspension was cooked at 60 °C with stirring. The Brookfield viscosity of the sample was measured with 100 rpm of spindle speed and a No. S61 spindle. The viscosity of dispersion samples was also tested at 70, 80, and 90 °C, respectively. The viscosity of CS dispersion at 90 °C was measured with a spindle speed of 20 rpm and a No. S62 spindle. Thermogravimetric Analysis (TGA). Testing was conducted using a thermal gravimetric analyzer (STA 449 C, Netzsch Instruments Inc. MA, USA). Approximately 10 mg of the sample was subjected to heating from 30 to 500 °C at a rate of 20 °C/min in nitrogen atmosphere. Weight loss and temperature signals were recorded.
’ RESULTS AND DISCUSSION In this work, PBA was used due to its excellent biodegradability, low cost, and availability in large quantities from industrial production. Furthermore PUP synthesized from PBA had elasticity and hydrophobicity, which could improve the toughness and hydrophobicity of starch.24 It was a good choice for modifying starch. Structure. The FTIR spectra of TPS, CPT, PUTDI, CPB and PBA were shown in Figure 1. Compared with TPS and PUTDI, a new peak for CPT at 1660 cm1 was attributed to the absorption of CdO groups in NHCOO,25 the result indicated that urethane linkages were formed between polyurethane and
Figure 2. CA patterns of TPS, CPH, CPT, CPM and CPI.
starch in CPT, suggesting a successful modification. However, compared with TPS and PBA, no new peak appeared in the spectra of CPB because no reaction took place between TPS and PBA. The results indicated that the isocyanates played an important role in preparing the modified TPS. Our results (not shown) showed the NCO content of PUMDI, PUHDI, PUTDI, and PUIPDI were 3.6% ( 0.5, 3.7% ( 0.2, 3.3% ( 0.1, and 3.7% ( 0.1, close to its theory value (3.7%, 3.6%, 3.6%, and 3.8%), respectively. The results indicated that the reaction for synthesizing PUP had reached its end point. Viscosity results of CS, TPS, CPM, and CPH are shown in Table 1. For CS dispersion, a sharp increase in viscosity was observed from 80 to 90 °C which was associated with the gelatinization of starch granules.26 The viscosity of TPS dispersion increased with the increase in temperature, which presented a similar viscosity behavior of the CS dispersion. However, the viscosity of the TPS dispersion decreased and did not significantly enhance with the increase in temperature from 80 to 90 °C as compared to CS. This could be attributed to the degradation of the starch under high crushing in an intensive mixer.27 However, for CPM and CPH, the hydrophobic polyurethane cross-linked with starch, and decreased the interaction between starch and water, the viscosity of CPM and CPH as a result decreased compared to that of TPS. CA curves of TPS, CPH, CPT, CPM, and CPI are shown in Figure 2. Because the starch was full of OH-rich macromolecules, it could form hydrogen bonds in water. When adding a droplet of distilled water, it quickly spread out on the surface of the starch, giving a low contact angle. As shown in Figure 2, a significant increase of contact angle of the droplet of water was observed after modifying the TPS with PUP. In addition, the evolution of contact angle with time slowed down. This indicated that the hydrophobic properties of the modified starch improved, and the hydrophobic PU content was responsible for this improvement. 11132
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Figure 3. SEM images of (a) CPM (8.0 KV, 3000); (b) CPM (8.0 KV, 1000); (c) CPB (8.0 KV, 3000); (d) CPB (8.0 KV, 2000).
Table 2. Formulations and Tensile Properties of Molded TPS Sheets Tested properties of molded sheetsb
formulations for modification samples
a
anhydrous starch (g)
PUP (g)
water (g)a
σb (MPa)
εb (%)
toughness (J/m3)
moisture content (%)
TPS
60.0
0
24.0
38.1 ( 2.7
0.9 ( 0.1
18.8 ( 2.5
9.6 ( 0.7
CPM CPT
45.8 45.8
11.5 11.5
22.7 22.7
37.5 ( 1.3 35.4 ( 0.7
2.9 ( 0.7 1.9 ( 0.1
45.3 ( 4.2 29.7 ( 2.7
10.1 ( 1.3 10.1 ( 0.3
33.3 ( 2.9
1.5 ( 0.3
26.4 ( 3.7
10.7 ( 0.2
CPI
45.8
11.5
22.7
CPH
45.8
11.5
22.7
Total weight of added water and moisture in samples. b Molded sheets subjected for tensile test.
The results of increasing the hydrophobic property for the modified TPS were in agreement with those from the viscosity analysis. The ESEM images of the cross sections of the CPM and CPB sheets are shown in Figure 3. Usually, a smooth and sharp surface in the ESEM image is associated with a brittle material such as native starch material.28 However, the ESEM images of the cross sections of the CPM and CPB sheets revealed rough surface. The area marked with the triangle symbol (4) in Figure 3a was assigned to starch matrix, and the granule marked with the box (0) was associated with the PU-rich dispersion phase. Figure 3a showed that PUMDI was partially embedded into the starch matrix, indicating that the PU dispersion and starch matrix were partially compatible. However, compared with CPM, the cross
section of the CPB sheets revealed many holes, which were marked with the circles (O) in Figure 3c. These holes were formed when the PBA was easily disengaged from the TPS while using liquid nitrogen in the preparation of the cross section of CPB. The phenomenon indicated that the interaction between PBA and starch was weak. Analysis by FTIR indicated that there was no reaction between the starch and PBA, therefore, isocyanates in modified TPS were coupling agents which improved the compatibility between the starch and PBA. Mechanical Properties. The data of mechanical properties of the TPS molds were shown in Table 2. In contrast with nonmodified TPS, the mechanical properties of modified TPS did not improve significantly as the ESEM analysis showed that 11133
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Industrial & Engineering Chemistry Research the PU particle and starch were partially compatible. From CPM to CPI, the mechanical properties decreased, and the data of the mechanical properties of CPH could not be obtained. The key factor determining the enhancement of the mechanical properties of the modified TPS could be attributed to the hydrophilicity of isocyanates. The hydrophilic isocyanates could be more easily consumed by water when preparing modified starch, resulting in a decreased level of reaction of the NCO content with starch. As analyzed in our previous work,22 when the NCO content of the PUPs was increased, the amount of urethane bonds between starch and PU increased too. In addition, the compatibility between the starch and PUP was also improved resulting in the improvement of its tensile properties. The interaction between water and isocyanates can be proven in the following
Figure 4. FTIR spectra of PUHDI and PUMDI. PUMDI and PUHDI sheets were both marinated in water for 0, 10, and 20 min, respectively. The strength of NCO groups of PUMDI and PUHDI at different times was recorded by FTIR.
Figure 5. Image of PUHDI and PUMDI sheets in the water at 20 min.
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experiment. In Figure 4, the PUMDI and PUHDI sheets were both immersed in water for 0, 10, and 20 min, consecutively. The FTIR spectra recorded the NCO contents of PUMDI and PUHDI at the above-mentioned times. As shown in Figure 4, the NCO contents of hydrophilic PUHDI decreased rapidly from 0 to 20 min, however, the NCO groups of PUMDI did not change significantly. As shown in the Figure 5, the PUHDI sheet was nearly dissolved when the time reached the 20 min mark, indicating the strong interaction between PUHDI and water. However the PUMDI sheet did not change obviously, showing the stability of PUMDI in water. Therefore, the hydrophilicity of isocyanates was a key factor that affected the mechanical properties of modified TPS. In our previous study,19 castor-oil-based PU (COPU) was used to modify starch, the interphase between PU microparticles and the starch matrix was continuous and dense, indicating good compatibility. And the breaking elongation of modified starch containing 20% PU was 21.7%. However, the CPM showed lower breaking elongation (εb = 2.9%). This may be due to the castor oil being hydrophobic while the PBA was partially hydrophilic, and there was less NCO content of COPU being absorbed by water in preparing modified TPS, resulting in a more flexible CP20. Of course, more experimental data should be used to support this analysis in our future work. Thermal Properties. Figure 6 presents the TGA experimental results for TPS, CPM, and CPB. TPS showed two major weight loss phases. The first phase represented the evaporation/dehydration that began immediately after the temperature increased and ended at around 100 °C. The percentage weight loss in this phase was dependent on the moisture content of the starch sample. The second weight loss phase corresponded to thermal decomposition of starch mar, which commenced at around 300 °C.29 Two stages were observed during the decomposition of CPM and CPB samples. From Figure 6, it could be observed that the remaining residue of CPB approached the value of TPS, indicating that the PBA had no effect on the starch, which may indicate the incompatibility between PBA and the starch. However, when compared with TPS and CPB, the remaining residue and degradation temperature of CPM both increased indicating the improvement of the thermal stability, which was due to the interaction between PUMDI and starch. Reaction Ratio. In this work, the reaction ratio is defined as the weight percentage of the PU linked to the starch, to the total weight of PUP added into the starch. In traditional starch modifications,30 the reaction ratio of modifier was generally low.17 However, the reaction ratio of PUP for modified CPM was 97.5 ( 0.2%. In our previous work,19 the reaction ratio of
Figure 6. TGA experimental results of TPS, CPM, and CPB. 11134
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Industrial & Engineering Chemistry Research castor-oil-based PUP for preparing modified TPS was 98.8%. These results indicated the high efficiency of modified TPS by our method.
’ CONCLUSIONS PU prepolymers were successfully synthesized using PBA diols with various isocyanates, then subjected for preparing modified TPS in an intensive mixer. The results showed that the isocyanates were coupling agents that improved the compatibility between starch and PBA. Compared with pure starch material, mechanical properties and hydrophobicity of the modified TPS were improved, due to the interaction between starch and polyurethane microparticles. With the increased of the hydrophilicity of isocyanates in PU, the tensile properties of modified TPS decreased. It was better to use hydrophobic polymer and isocyanates to prepare hydrophobic PUP, and we will further modify starch in our future work. ’ ASSOCIATED CONTENT
bS
Supporting Information. Photo of 5 wt % CS and CPT aqueous solutions; photos of PUTDI powder + CS and CPT independently mixed with waterbutyl acetate. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel./Fax: +86-27-67867953. E-mail: greenpolymerlab@yahoo. com. Address: College of Chemistry, Huazhong Normal University, Wuhan, China, 430079.
’ ACKNOWLEDGMENT We appreciate the financial support from the National Natural Science Foundation of China under Grant No. 50803024. ’ REFERENCES (1) Long, Y.; Deana, K.; Li, L. Biodegradable Polymer Blends and Composites from Renewable Resources; Wiley: New York, 2008. (2) Wu, Q. X.; Zhang, L. N. Properties and structure of soy protein isolateethylene glycol sheets obtained by compression molding. Ind. Eng. Chem. Res. 2001, 40, 1879. (3) Arvanitoyannis, I. S. Totally and partially biodegradable polymer blends based on natural and synthetic macromolecules: Preparation, physical properties, and potential. J. Macromol. Sci. Rev. Macromol. Chem. Phys. 1999, 39, 205. (4) Alfani, R.; Iannace, S.; Nicolais, L. Synthesis and characterization of starch-based polyurethane foams. J. Appl. Polym. Sci. 1998, 68, 739. (5) Guilbert, S.; Gontard, N. Edible and biodegradable food packaging. In Food Packaging Materials; Ackerman, P., Jaegerstad, M., Ohlsson, T., Eds.; Royal Society of Chemistry: Oxford, U.K., 1995; p 159. (6) Satyanarayama, D.; Chaterji, P. R. Biodegradable polymers: Challenges and strategies. Macromol. Sci. Rev. Macromol. Chem. Phys. 1993, 33, 349. (7) Wool, P. R. The science and engineering of polymer composite degradation. In Degradable Polymers; Scott, G., Gilead, D., Eds.; Chapman & Hall: London, 1995; p 207. (8) Ellis, R. P.; Cochrane, M. P.; Dale, M. F. P.; Duffus, C. M.; Lynn, A.; Morrison, I. M.; Prentice, R. D. M.; Swanston, J. S.; Tiller, S. A. Preparation of starch-based polyurethane films and their mechanical properties. J. Sci. Food Agric. 1998, 77, 289.
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