Article pubs.acs.org/Biomac
Epoxy Monomers Derived from Tung Oil Fatty Acids and Its Regulable Thermosets Cured in Two Synergistic Ways Kun Huang,†,‡ Zengshe Liu,*,§ Jinwen Zhang,*,∥ Shouhai Li,† Mei Li,† Jianling Xia,† and Yonghong Zhou† †
Institute of Chemical Industry of Forestry Products, CAF; Institute of Forest New Technology, CAF; National Engineering Lab for Biomass Chemical Utilization; Key Lab on Forest Chemical Engineering, SFA; and Key Lab of Biomass Energy and Material, Nanjing, Jiangsu 210042, China § Bio-Oils Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, U.S. Department of Agriculture, 1815 North University Street, Peoria, Illinois 61604, United States ∥ Composite Materials and Engineering Center, Washington State University, Pullman, Washington 99164, United States S Supporting Information *
ABSTRACT: A novel biobased epoxy monomer with conjugated double bonds, glycidyl ester of eleostearic acid (GEEA) was synthesized from tung oil fatty acids and characterized by 1H and 13C NMR. Differential scanning calorimeter analysis (DSC) and Fourier transform infrared spectroscopy (FT-IR) were utilized to investigate the curing process of GEEA with dienophiles and anhydrides. DSC indicated that GEEA could cross-link with both dienophiles and anhydrides through Diels−Alder reaction and epoxy/anhydride ring-opening reaction. Furthermore, Diels−Alder cross-link was much more active than the ring-opening of epoxy and anhydride in the curing process. FT-IR also revealed that GEEA successively reacted with dienophiles and anhydrides in both cross-linking methods. Dynamic mechanical analysis and mechanical tensile testing were used to study the thermal and mechanical properties of GEEA cured by maleic anhydride, nadic methyl anhydride and 1,1′-(methylenedi-4,1-phenylene)bismaleimide. Due to the independence between the curing agents, dienophile and anhydride, a series of thermosetting polymers with various properties could be obtained by adjusting the composition of these two curing agents.
1. INTRODUCTION Epoxy resins are important thermosetting resins because of their high stiffness, excellent thermal resistance, and processability properties. The most common and important class of epoxy resins is formed from reacting epichlorohydrin (ECH) with bisphenol A to form diglycidyl ethers of bisphenol A. The rigid aromatic structure of bisphenol A in the epoxy resin causes higher performance. However, bisphenol A is an endocrine disruptor that mimics estrogen and may lead to negative health effects.1,2 Its application has been strictly limited in many countries. Therefore, it is of great interest for the resin manufacturers and end users to explore low toxicity substitutes, © 2014 American Chemical Society
particularly from renewable alternatives that have comparable properties to the bisphenol A type epoxies. Plant oils are major agricultural commodities providing 129 million metric tonnes annually.3 About 15% of this production is used as industrial feedstocks.4 Currently, the most popular biobased epoxy monomers might be derived from plant oils, such as epoxidized vegetable oil5−7 and so on. Epoxidized plant oils such as epoxidized soybean oils have an average of four Received: November 8, 2013 Revised: January 24, 2014 Published: February 2, 2014 837
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Scheme 1. Synthesis Route to GEEA and the Thermosetting Network Structures of GEEA-MA and GEEA-NMA-BMI
modern organic chemistry. In macromolecular chemistry, it has been successfully applied to the construction of different kinds of poly adducts.27−29 The DA reaction is effective, versatile, and selective, which also fullfils most of the requirement for the “click” polymer chemistry.30 DA reactions could also be used to make thermosetting polymers. Thermally reversible thermosets are synthesized based on furan-functionalized polyketones cross-linked with (methylene-di-p-phenylene) bis-maleimide by Broekhuis et al.31 Reversible DA reactions between furan and bismaleimide may be initiated above 120 °C, which makes the thermosetting material self-repairable and recyclable. Shibata et al. studied biobased thermosetting resins composed of tung oil (TO) and bismaleimide (BMI) with a CC ratio of TO/BMI from 1/1 to 1/4. Shibata et al. revealed that the DA reaction preferentially occurred with CC ratio of TO/BMI at 2/1, and that ene reaction and other reactions such as radical homo and copolymerization gradually increased with decreasing the CC ratio of TO/PMI. However, increased BMI resulted in higher curing temperature (150−200 °C), which limited the application of the resins.32 So far, the DA reaction as a cocrosslinking strategy has never been introduced into epoxy resins. In this work, the synthesis of a glycidyl ester of eleostearic acid (GEEA) from tung oil fatty acids was reported. The GEEA was cross-linked with the mixture of anhydrides and dienophiles through both the epoxy ring-open reaction and the DA reaction. Curing agents, maleic anhydride (MA), nadic methyl anhydride (NMA), and 1,1′-(methylenedi-4,1phenylene)bismaleimide (BMI), were used to prepare two different thermosetting networks, as seen in Scheme 1.
epoxies per triglyceride; however, the epoxy groups of epoxidized oils are internal oxiranes that were much less reactive than the terminal epoxy groups of glycidyl ethers or esters. The curing temperature of these internal epoxy groups with an anhydride could be as high as 200 °C even in the presence of a catalyst.8 Generally, for commercial epoxy resins, the curing temperature of polyamines with terminal epoxy group ranges between 30 and 80 °C, the postcured temperature could be set as 110 °C.9 Due to the large steric hindrance, the curing temperature of epoxidized oil and polyamines should be higher than 130 °C so that the cured materials could obtain the best properties.10 The low reactivity of the internal oxirane may prevent the complete cure of the epoxy network.11 Thus, use of epoxidized plant oils leads to poorly cross-linked materials with unsatisfactory thermal or mechanical properties.12,13 To overcome these defects, several rigid epoxy monomers have been developed, such as isosorbides,14 gallic acids,15 and catechins,16 but these biobased feedstocks are relatively expensive. Epoxy resins with high glass transition temperatures were also developed from other cheaper resources, such as lignins17−20 and rosins.21,22 However, lignin based epoxy resins have slow curing rates and unstable properties due to the low mobility of the macromolecular species and complex structures.17,18 The rosin based epoxy resins usually exhibit satisfactory thermal and mechanical properties,21 while its rigid hydrogenated phenanthrene ring structure induces severe brittleness.22 Liu et al.23 recently reported that an itaconic acid (IA) based epoxy resin with curable double bonds (EIA) was synthesized by the esterification reaction between IA and ECH. Compared with bisphenol A epoxy resins, EIA synergistically cured in a free radical way and had comparable or better mechanical, thermal, and adhesive properties. Tung oil fatty acids contain about 80% eleostearic acid (EA) with three conjugated double bonds.24 Tung oil is a very important dry oil. Tung oil fatty acids react with epoxy resins to form epoxy resin esters that can dry in the air like dry oils do.25 It is worthy to note that Tung oil fatty acids can also undergo Diels−Alder (DA) reaction easily with a dienophile.26 The classical DA reaction is one of the most useful reactions in
2. EXPERIMENTAL SECTION 2.1. Materials. TO was obtained from the Institute of Chemical Industry of Forestry Products (Nanjing, China). Nadic methyl anhydride (99.4%) was obtained from Electron Microscopy Sciences (Hatfield, PA, U.S.A.), and epichlorohydrin (99%), sodium hydroxide (97+%), maleic anhydride (99%), 1,1′-(methylenedi-4,1-phenylene) bismaleimide (95%), benzyltriethylammonium chloride (97%), and 2ethyl-4-methylimidazole (99+%) were obtained from Sigma-Aldrich (St. Louis, MO, U.S.A.). Materials were used as received. 838
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2.2. Synthesis. 2.2.1. Preparation of EA. EA was prepared and purified in accordance with ref 33. Typically, TO (200 g) was stirred by refluxing with 60 g potassium hydroxide, 50 mL of water, and 500 mL of 95% ethanol at 90 °C for 0.5 h. After cooling, the soap was acidified in a separatory funnel with 725 mL of 2 N hydrochloric acid. The liberated fatty acids were separated from the aqueous phase and dissolved without further treatment in 1000 mL of 95% ethanol. This solution was kept for 24 h at −20 °C to promote the formation of acid crystals. The very light-colored crystals were filtered and washed with about 75 mL of cold 95% ethanol, followed by vacuum drying. Both dark and light colored crystals were isolated for a total mass of about 142 g. 2.2.2. Synthesis of GEEA. To a 50 mL flask, equipped with reflux condenser, magnetic stirrer, and thermometer were charged with 2.78 g (10 mmol) EA, 9.25 g (100 mmol) epichlorohydrin, and 0.012 g (0.05 mmol) benzyltriethyl ammonium chloride. The reaction temperature was raised to 117 °C and the reaction continued at that temperature for 2 h. After the mixture was cooled to 60 °C, 0.40 g (10 mmol) sodium hydroxide pellets was added, and the pellets soon turned into tiny particles in suspension. The mixture was stirred at 60 °C for 3 h and then filtered. The filtrate was distilled under vacuum to recycle the excess epichlorohydrin, 3.13 g yellowish liquid was obtained. The crude product was purified using a silica gel column (ethyl acetate/hexane = 1:10 v/v) to receive 2.95 g pure colorless liquid (yield: 88% relative to EA) with an epoxide equivalent weight of 334 g/mol. More purified GEEA were obtained according to above method for analysis and property test. The viscosity (25 °C) of the purified GEEA was 47.9 mPa·s. 2.3. Preparation of Test Specimens. For giving consideration to the ratio of dienes and dienophiles, epoxy group and anhydride group were maintained in a molar ratio of 1:1. 2-Ethyl-4-methylimidazole (EMI) was used as the catalyst and added at 0.5 wt % on the basis of total weight of the anhydride and epoxy. The ingredients were mixed at 90 °C, and then the mixture was charged in a steel mold preheated at 120 °C. As for GEEA/NMA/BMI system, the solid BMI was first dissolved in NMA at 100 °C to form a clear solution, then the solution was blended with GEEA. The mold for tensile test is based on ASTM D 638 Type V. The dimensions of the mold for dynamic mechanical analysis (DMA) was 65 × 13 × 3 mm. Curing was performed at 120 °C for 2 h and then at 160 °C for 4 h. The cured specimens were carefully removed from the mold and examined for tensile test and DMA. 2.4. Characterizations. 1H and 13C NMR spectra of the compounds in deuterated chloroform (CDCl3) were recorded with a Bruker 400 MHz spectrometer (Bruker, Rheinstetten, Germany) at room temperature. FT-IR spectra were recorded using a Thermo Nicolet Nexus 470 spectrometer (Madison, WI, U.S.A.). Visicosity was examined by TA ARES-G2 rheometer at 25 °C. The sample was loaded in a 25 mm steel parallel plate with a gap of 500 μm and swept from shear rate 100 to 0.1 s−1 at 25 °C. Curing behavior was studied by differential scanning calorimetry (DSC) using a DSC 2910 (TA) instrument (New Castle, DE, U.S.A.). Approximately 5−10 mg of each sample was weighed and sealed in 40 μL aluminum crucible and the curing on DSC was performed in the nitrogen atmosphere immediately. DSC analysis for each sample was repeated twice. Dynamic mechanical analysis (DMA) was conducted by using a DMA Q800 (TA) instruments (New Castle, DE, U.S.A.) in a dual cantilever mode with an oscillating frequency of 1 Hz. The temperature was swept from 25 to 165 °C at 3 °C/min. For each sample, duplicated tests were performed in order to ensure the reproducibility of data. Tg was determined as the temperature at the maximum of the tan δ versus temperature curve. Tensile properties were measured using an Instron 4201 equipped with a 1 kN electronic load cell according to ASTM D 638 type V. The tests were conducted at a crosshead speed of 10 mm/min. All samples were conditioned at 50% humidity and 23 °C for 2 days prior to tensile testing. Five replicates were tested for each sample to obtain an average value.
Thermogravimetric analysis (TGA) was performed on a SDT Q600 TGA (TA) instruments (New Castle, DE, U.S.A.). Each sample was scanned from 25 to 600 °C under a 40 mL/min air flow and a heating rate of 20 °C/min−1.
3. RESULTS AND DISCUSSION 3.1. Characterization. According to the reference, the EA prepared should contain 98% α-EA and 2% β-EA.31 No attempt was made to separate the isomers in this study. Since the α-EA is the primary compound, the structure of α-EA is illustrated in the NMR spectrum. Figures 1 and 2 displayed the 1H and 13C
Figure 1. 1H NMR spectra of EA and GEEA.
Figure 2. 13C NMR spectra of EA and GEEA.
NMR of EA and GEEA, respectively. The chemical shift assignments and the integration ratio of the protons were also labeled in the figures. In Figure 1, the multiple peaks (6H) at 5.40, 5.72, 6.01, 6.11, 6.18, and 6.39 ppm were assigned to the six hydrogens of conjugated double bonds. The triple peaks (2H) at 2.36 ppm were attributed to the two hydrogens of number 17 carbon. The quartet peaks at 2.18 (2H) and 2.12 ppm (2H) were attributed to the hydrogens of −CH2− beside the double bonds. The peaks at 0.91 and 1.65 ppm were the 839
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included both the DA reaction and the ring-open reaction of epoxy/anhydride. It was clear that the heat of the DA reaction was much lower than that of the ring-open reaction of the epoxy group. Under the same curing condition, the peak temperature of a DSC exothermic curve is often taken as an indicator to evaluate the reactivity of the compound in curing reactions. The lower the temperature of the peak the higher the reactivity is.35,36 Based on the DSC results mentioned above, it can be concluded that the DA cross-link reaction has more reactivity than the ring-opening reaction of epoxy with anhydrides. In the absence of EMI, GEEA/MA and GEEA systems were also scanned separately. Without catalyst, the peak of the DA reaction between GEEA and MA was still around 105 °C, but the peak of the ring-opening reaction between epoxy and anhydride groups did not appear around 160 °C anymore. Without MA, there was no peak that the DA reaction occurred in the DSC curve of GEEA. For both GEEA/MA and GEEA, it was clear that exothermic peaks started around 270 °C. Generally, in the nitrogen atmosphere, the evaporation or thermal degradation lead to endothermic peaks during DSC scanning. Therefore, these exothermic peaks were probably caused by the self-polymerization of epoxy groups from GEEA at high temperatures without catalyst. It is worthy to note that although NMA is a substituted anhydride compound its double bond does not connect directly to the carboxyl group. That means that the NMA is not a dienophile for eleostearic acids, it will not involve the D−A reaction. This suggestion has been confirmed by performing two independent control DSC experiments including NMA/GEEA and BMI/GEEA systems without catalyst (EMI) as shown in the Supporting Information. To gain further knowledge of chemical information during the curing process, the FT-IR spectral changes of GEEA/MA at different stages were illustrated in Figure 4. Figure 4(I) is the spectra of GEEA/MA mixture before cure. The weak peak at 3114 cm−1 was due to the C−H stretching of MA. The strong peaks observed at 1055 and 840 cm−1 were assigned to the double bond C−H bending of MA. The stronger peak at 887 cm−1 was the CC stretching assignment of MA. The typical anhydride CO stretching of MA were found at 1849 and 1776 cm−1.37 The strong band at 993 cm−1 is attributed to the conjugated double bonds of GEEA.38 The characteristic peak of the epoxy group was observed at 908 and 761 cm−1. After the curing reaction proceeded at 90 °C for 1 h, the intensities of these peaks related to double bonds all decreased owing to the DA reaction as seen in Figure 4(II). However, there was no obvious change related to the peaks of the anhydride group under these conditions. When the curing reaction proceeded further toward completion, Figure 4(III) showed that all characteristic peaks of the epoxy, anhydride, double bond of MA and conjugated double bonds of GEEA had disappeared. Clearly, the results of both DSC and FTIR showed that GEEA successively reacted with dienophiles and anhydrides by the DA reaction and then the epoxy/anhydride ring-opening reaction. It is noteworthy that these two cross-link pathways are independent of each other. Hence, there is a possibility to control the properties of thermosetting polymers by adjusting the proportion of those two cross-linkers. 3.3. DMA and Tensile Properties. Because of the independence between the DA reaction and the epoxy/ anhydride ring-opening, by varying the amounts of the two kinds of curing agents, anhydrides and dienophiles added to the
signs of methyl and the methylene of the number 16 carbon. The multiple peaks around 1.34−1.41 ppm were assigned to the hydrogens of −CH2− groups. The 1H NMR result of EA was consistent with the results of tung oil reported by Shibata et al.30 The major difference between EA and GEEA is the glycidyl ester group. The chemical shifts of five hydrogens at 2.65 (1H), 2.85 (1H), 3.21 (1H), 3.92 (1H), and 3.41 (1H) ppm clearly display the existence of glycidyl ester in Figure 1. The 13C NMR chemical shifts of 6 carbons in conjugated double bonds of EA were clearly demonstrated in Figure 2. Likewise, the glycidyl ester group made a distinction between EA and GEEA. The chemical shift of the carboxyl carbon from 178.83 to 173.48 ppm indicates the carboxyl acids transferred into an ester as EA progressed into GEEA. The chemical shifts of the three carbons in the glycidyl ester group occurred at 44.64, 49.37, and 64.73 ppm, respectively. The results were also consistent with the glycidyl ester synthesized by Cadiz.34 3.2. Curing Behavior. It is noteworthy to mention that the GEEA contains an epoxy group and three conjugated double bonds. In order to certify the curing process of the epoxy group and conjugated double bonds in the GEEA, the nonisothermal curing behaviors of GEEA cured by MA and NMA/BMI were investigated by DSC. Figure 3 shows the DSC scanning curves of EA/MA (0.5 wt % EMI), GEEA/NMA/BMI (0.5 wt % EMI), GEEA/MA (0.5 wt % EMI), GEEA/MA, and GEEA from 25 to 300 °C at 10 °C/min.
Figure 3. DSC scanning results of five different systems at 10 °C/min: EA/MA (0.5% EMI), GEEA/NMA/BMI (0.5% EMI), GEEA/MA (0.5% EMI), GEEA/MA, and GEEA.
In the presence of catalyst EMI, the heat flow for GEEA/MA and GEEA/NMA/BMI both displayed multiple exothermic peaks. For a comparison, EA/MA was selected as a model system because EA would react with MA only by the DA reaction. The scanning curve of EA/MA displayed a single exothermic peak around 105 °C. Thus, for GEEA/MA and GEEA/NMA/BMI, the peaks at a lower temperature (∼105 °C) belonged to the Diels−Alder reaction of GEEA with MA or BMI, whereas the peaks at a higher temperature (∼160 °C) were attributed to the ring-opening reaction between epoxy and anhydride groups catalyzed by EMI. The enthalpy of EA/MA was only 71.1 J/g, which was assigned to the enthalpy of DA reaction, while the enthalpy of GEEA/MA was 318.9 J/g and 840
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Figure 4. FT-IR spectrum of GEEA/MA: (I) before cure, (II) cured at 90 °C for 1 h, and (III) after cure.
GEEA, then the thermosetting properties of the GEEA polymers can be altered. The typical samples with three molar ratios were prepared and studied by DMA and tensile test, they were (a) GEEA/MA = 1:1, (b) GEEA/NMA/BMI = 1:1:0.5, and (c) GEEA/NMA/BMI = 1:1:0.25, respectively. It is not possible to calculate the cross-link density by the rubbery modulus measured by DMA at Tg + 50 °C.39 Because the rubber elasticity theory assumes a Gaussian distribution of chains40 and this is certainly not the case in this series containing three species: aromatic (BMI), aliphatic (GEEA, MA), and cycloaliphatic (NMA) units. Therefore, in this study, the average molecular weight between two cross-link points (Mc) of these thermosets were calculated based on the equation below to obtain the cross-link densities of the thermosets: Mc =
W n
where W is the molecular weight per mole monomer mixture and n is the cross-link point per mole monomer mixture. The DA reaction was assumed as a single cross-link point, while epoxy/anhydride ring-opening was assumed as two cross-link points. For example, the molecular weight of 1 mol GEEA/MA mixture is the sum relative molecular weight of GEEA and MA. This mixture has one DA cross-link point and two epoxy/ anhydride ring-opening cross-link points. Figure 5 shows the temperature dependence of the storage modulus (E′) and loss factor (tan δ) for samples a−c from 25 to 165 °C. The results of Mc, E′ at 25 °C and glass transition temperature (Tg) for samples a−c were listed in Table 1. GEEA/MA system has the highest Tg (106 °C) among the three samples, which is mainly due to the highest cross-link density. The Diels−Alder reaction is an annulation curing process, which generates cyclohexenyl moieties in the network. The cyclohexenyl moieties definitely contribute to the stiffness of the polymer. However, the aliphatic nature of the GEEA and MA segments still gives a relatively lower E′ at 25 °C compared to the other two samples. Although sample b has a lower crosslink density compared with sample a, it still has high Tg (103 °C) and much higher storage modulus, which were attributed to the rigid cycloaliphatic and aromatic imide segments of NMA and BMI in the cured polymer backbone. As for sample c, the amount of BMI was only half calculated by stoichiometry,
Figure 5. Temperature dependence of the storage modulus and loss factor for samples of different composition molar ratios: (a) GEEA/ MA = 1:1, (b) GEEA/NMA/BMI = 1:1:0.5, and (c) GEEA/NMA/ BMI = 1:1:0.25.
Table 1. Results of Mc, E′ at 25 °C, and Glass Transition Temperature (Tg) for Samples a−c sample
composition (molar ratio)
a b c
GEEA/MA = 1:1 GEEA/NMA/BMI = 1:1:0.5 GEEA/NMA/BMI = 1:1:0.25
Mc (g)
E′ at 25 °C (MPa)
Tg (°C)
144 230 241
1429 2092 1595
106 103 75
and caused the Tg to drop to 75 °C, but the E′ at 25 °C is still higher than that of sample a. The intensity and broadness of tan δ are sensitive to the amplitude and homogeneity of the 841
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Table 2. Results of Tensile Properties for Samples a−c sample
composition (molar ratio)
tensile strength (MPa)
Young’s modulus (MPa)
strain %
a b c
GEEA/MA = 1:1 GEEA/NMA/BMI = 1:1:0.5 GEEA/NMA/BMI = 1:1:0.25
42.4 ± 1.1 57.5 ± 3.6 48.7 ± 6.2
1949.1 ± 152.5 2442.6 ± 348.4 2331.6 ± 235.9
3.1 ± 0.5 2.8 ± 0.7 2.6 ± 0.6
macromolecular chain motions.39 The decreased cross-link density of sample c increased the damping capability that led to the taller and more narrow tanδ peak than those of sample a and b. Higher cross-link densities of samples a and b gave rise to more restrictive chain motion, which led to the reduction of damping capability resulting in lower and broader tan δ peak.41,42 The tensile strength, Young’s modulus, and tensile strain of samples a−c are presented in Table 2. It shows tensile strength and Young’s modulus are affected by both cross-link density and the structure of curing agents. Though GEEA/MA polymer system has the highest cross-link density (smaller Mc value), flexible polymer chains in the aliphatic structure would decrease the rigidity of the polymer chain, and hence leads to the reduction of the tensile strength and modulus compared to other two systems. It is similar to the case that the addition of soft matters like rubber and polyurethane to epoxy resins generally results in the reduction of tensile strength and modulus.43,44 Comparing samples b and c shows sample b has better tensile properties than sample c because of the higher cross-link density. Although cross-link densities of both samples b and c are lower than that of sample a, the rigidity of NMA and BMI segments offsets the negative contribution by the lower cross-link density to their strength and modulus. Therefore, samples b and c have higher tensile strength and modulus than sample a.45 3.4. Thermal Stability. Figure 6 shows the TGA results for cured samples of different composition molar ratios: (a) GEEA/MA = 1:1, (b) GEEA/NMA/BMI = 1:1:0.5, and (c) GEEA/NMA/BMI = 1:1:0.25. The temperatures at which 5% weight loss (T5%) of a, b, and c are 386.5, 386.0, and 354.5 °C, respectively. Tan et al. reported that the cured epoxidized
soybean oil with higher cross-link density showed better thermal stabilities.46 Therefore, the decrease in thermal stability from samples a−c was due to the differences of cross-link densities between samples a, b, and c. This result showed a good agreement with the data of Mc, which was collected in Table 1. The char yields at 590 °C of samples b, c, and a are 26.3, 18.7, and 9.7%, respectively. The char yield is inversely proportional to the hydrogen content of the polymers.47 The bismaleimide group has low hydrogen content which resulted in the higher char yields of samples b and c. The aliphatic structure of sample a caused much lower char yield. The different curing agents could be utilized to regulate the thermal stability of the cured GEEA materials.
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CONCLUSIONS Glycidyl ester of eleostearic acid (GEEA) was successfully synthesized from fatty acids of tung oil. GEEA could be cured synergistically with both anhydrides and dienophiles through the epoxy/anhydride ring-opening reaction and the Diels− Alder reaction. The Diels−Alder reaction tends to be more reactive and gives off less heat than that of an epoxy/anhydride ring-opening reaction. Because these two curing methods are independent of each other, anhydrides and dienophiles with different structures or special properties could be used as curing agents in order to adjust the structure and the cross-link densities of the resultant thermosetting polymers.
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ASSOCIATED CONTENT
* Supporting Information S
Additional supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*Phone: 309-681-6104. Fax: 309-681-6524. E-mail: kevin.liu@ ars.usda.gov (Z.L.). *Phone: 509-335-8723. Fax: 509-335-5077. E-mail: jwzhang@ wsu.edu (J.Z.). Notes
The authors declare no competing financial interest. ‡ K.H. is a visiting student at Washington State University.
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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (Nos. 31000280 and 31170544) and the National Agricultural Scientific and Technological Achievements Project of China (No. 2011GB24320017). The authors also acknowledge Mr. Daniel A. Knetzer for technical help.
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REFERENCES
(1) Feldman, D. Endocrinology 1997, 138, 1777−1779. (2) Sonnenschein, C.; Soto, A. M. J. Steroid Biochem. Mol. Biol. 1998, 65, 143−150. (3) Carlsson, A. S. Biochimie 2009, 91, 665−670.
Figure 6. Temperature dependence of the weight percent for cured samples of different composition molar ratios: (a) GEEA/MA = 1:1, (b) GEEA/NMA/BMI = 1:1:0.5, and (c) GEEA/NMA/BMI = 1:1:0.25. 842
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Biomacromolecules
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dx.doi.org/10.1021/bm4018929 | Biomacromolecules 2014, 15, 837−843
