Highly Functional Unsaturated Ester Macromonomer Derived from

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High-functionality Unsaturated Ester Macromonomer Derived from Soybean Oil: Synthesis and Copolymerization with Styrene Chengguo Liu, Yan Dai, Yun Hu, qianqian shang, Guodong Feng, Jing Zhou, and Yonghong Zhou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.6b00700 • Publication Date (Web): 01 Jul 2016 Downloaded from http://pubs.acs.org on July 8, 2016

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ACS Sustainable Chemistry & Engineering

High-functionality Unsaturated Ester Macromonomer Derived from Soybean Oil: Synthesis and Copolymerization with Styrene Chengguo Liu, *,†,‡ Yan Dai,† Yun Hu,† Qianqian Shang,† Guodong Feng,† Jing Zhou,† Yonghong Zhou,*,†

†

Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry

(CAF); National Engineering Lab. for Biomass Chemical Utilization; Key Lab. on Forest Chemical Engineering, State Forestry Administration; Key Lab. of Biomass Energy and Material, Jiangsu Province; 16 Suojin North Road, Nanjing 210042, P. R. China ‡

Institute of Forest New Technology, CAF, Dongxiaofu-1 Xiangshan Road, Beijing

100091, P.R. China

* Yonghong Zhou. E-mail: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT A high-functionality unsaturated ester macromonomer was synthesized from soybean oil (SBO) and its chemical structure was confirmed by FT-IR, 1H-NMR, 13C-NMR, and gel permeation chromatography. The monomer was prepared through modifying epoxidized soybean oil (ESO) firstly with a synthesized precursor, hydroxyethyl acrylated maleate (HEAMA), then employing maleic anhydride (MA) to modify the produced ESO-HEAMA. The obtained SBO-based monomer (ESO-HEAMA-MA) possessed a C=C functionality of 6.75–8.15 per ESO. Effects of styrene concentration, feed ratio, and initiator concentration on dynamic mechanical properties of the cured bioresins were investigated carefully. Using the monomer with the highest C=C functionality, the cured resins with 20–60 wt% of styrene demonstrated crosslink densities of 5.07–9.52 (103mol)/m3, storage moduli at 25 °C of 1.32–2.16 GPa, glass transition temperatures of 69.9–114.1 °C, tensile strengths and moduli of 19.7–33.1 MPa and 1.17–2.11 GPa, respectively. Microstructural morphologies of tensile-fractured surfaces of the cured resins were studied by scanning electron microscopy. Finally, curing behaviors of the resultant resin was studied by differential scanning calorimeter. The developed eco-friendly biomaterials have potential applications in the industry of unsaturated polyester resins.

KEYWORDS: Soybean oil, Unsaturated ester, Thermosetting polymer, Synergetic modification, Unsaturated polyester resin (UPR), Structural plastic, Curing behavior

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INTRODUCTION Unsaturated polyester resins (UPRs), the blends of unsaturated polyesters synthesized through the polycondensation of unsaturated and saturated diacids with diols and vinyl comonomers (e.g. styrene), are widely employed in industrial and domestic areas.1, 2 With the current focus on exploring alternatives to petroleum and emphasis on reduced environmental impact, research is increasingly being directed at development of polymeric materials from renewable resources, such as proteins, lignin, carbohydrates, and natural oils.3 Vegetable oils are an important natural resource because of their low cost, triglyceride structures suitable for further chemical modification, and potential biodegradability.4, 5 As a consequence, alternative UPRs from vegetable oils have attracted much attention recently. An emerging trend is the development of unsaturated ester (UE) monomers from vegetable oils which can copolymerize with vinyl monomers in the same manner as petroleum-based UPRs. Numerous oil-based maleates or acrylates were already developed via chemical modifications of vegetable oils or their fatty acid (FA) derivates. 6-14 Among them, unsaturated co-esters (co-UEs) containing two types of polymerizable C=C bonds such as maleates and acrylates in one UE monomer were developed to acquire oil-based monomers with high functionality. One typical pathway to synthesis such monomer is using maleic anhydride (MA) to further modify acrylated epoxidized triglycerides, e.g. acrylated epoxidized soybean oil (AESO).14 In our group, we developed another avenue to obtain new co-UE monomers by introducing maleic groups firstly onto tung oil triglyceride and acrylic 3



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groups subsequently.15 Generally, the resulting co-UE thermosets indeed showed better stiffness and thermal properties such as glass transition temperature (Tg) than those of original UE ones due to their higher crosslink densities.14, 16 Especially for the MA-modified AESO (MAESO) monomer with a C=C functionality of around 5.2 per ESO, its resulting copolymers with styrene reached the tensile strengths of 41–44 MPa. The positive results motivate us to synthesis monomers with higher C=C functionality which may lead to superior mechanical and thermal properties than those of MAESO. In this study, a new co-UE monomer was synthesized from epoxidized soybean oil (ESO). The co-UE monomer was synthesized in three steps (Scheme 1): firstly HEA and MA reacted to produce an unsaturated precursor, hydroxyethyl acrylated maleate (HEAMA); then the HEAMA was utilized to modify ESO and ESO-HEAMA monomer was obtained; finally MA was employed to further modify ESO-HEAMA and ESO-HEAMA-MA monomer were prepared. The final macromonomer would possess a theoretical C=C functionality of 9–12 per ESO molecule which is very high in all the reported oil-based monomers for UPR use. The objective of this work was to assess the performance of the obtained high-functionality macromonomer when copolymerizing with styrene. Therefore, the thermal and mechanical properties of the cured co-UE/styrene resins were carefully investigated. In addition, microstructural morphologies were studied by scanning electron microscopy (SEM) to explore structure–property relationship of the resulting thermosets. Curing behaviors were investigated by differential scanning calorimetry (DSC) to understand the curing 4


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process of the as-prepared soybean oil (SBO)-based resins.

Scheme 1 Synthesis of (b) ESO-HEAMA and (c) ESO-HEAMA-MA monomers from (a) epoxidized soybean oil (ESO).

EXPERIMENTAL SECTION Materials. ESO and tert-butyl peroxybenzoate (t-BPB, ≥98%) initiator were obtained from Shanghai Aladdin Chemistry Co., Ltd. (China). The ESO was functionalized with approximately 4.23 epoxy groups per triglyceride which should have an average molecular weight of 950 g/mol. HEA (≥97%) was purchased from Macklin Chemical Reagent Co., Ltd. (China). Styrene (≥99%) and MA (≥99.5%) were obtained from Nanjing Chemical Reagent Co., Ltd. (China). N,N-Dimethyl benzyl amine (≥98%) was obtained from Tianjin Chemical Reagent Institute Co., Ltd. (China). Triphenyl phosphine (≥98%) and hydroquinone (≥99%) were obtained 5



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from Guoyaojituan Chemical Reagent Co., Ltd. (China). Tetrabutyl titanate (≥98.5%) and phosphoric acid (≥85%) were obtained from Chengdu Kelong Chemical Reagent Co., Ltd. (China). p-Toluenesulfonic acid monohydrate was obtained from Shanghai Titan Scientific Co., Ltd. (China). The styrene, HEA, and N,N-dimethyl benzyl amine were dried by molecular sieves for at least one week before use.

Synthesis of HEAMA. About 58.1 g (0.5 mol) of HEA, 49.0 g (0.5 mol) of MA, and 0.11 g of hydroquinone were placed together in a 250 mL four-neck round-bottom flask equipped with a mechanical stirrer, a thermometer, a nitrogen (N2) gas inlet, and a refluxing condenser with an anhydrous calcium chloride drier. The mixture was heated to 70 °C by an oil bath and agitated at this temperature until the MA completely melted. Then the mixture was heated to 90 °C under N2 atmosphere and agitated at this temperature for 5 h. The HEAMA product was a light yellow and transparent liquid when cooled naturally to room temperature.

Synthesis of ESO-HEAMA and ESO-HEAMA-MA. The synthesis of ESO-HEAMA was carried out under different catalysts, feed ratios, and reaction temperatures (Table 1 and Table S1). Typically, approximate 50.0 g (0.0526 mol) of ESO, 43.35 g (0.202 mol) of HEAMA, 0.94 g of N,N-dimethyl benzyl amine, 0.188 g of hydroquinone were placed together in a 250 mL four-neck round-bottom flask equipped with a mechanical stirrer, a thermometer, a N2 gas inlet, and a refluxing condenser with an anhydrous calcium chloride drier. The mixture was heated to 6


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100 °C by an oil bath and agitated at this temperature for 4 h. At the end of the reaction, the reaction temperature was changed to 90 °C and 5.16–15.48 g (0.0526– 0.158 mol) of MA was added into the reaction mixture in order to synthesize different MA-modified ESO-HEAMA monomers (Table 2). The mixture reacted at 90 °C for 1 h. Finally, styrene in 20–60% of the total resin weight and hydroquinone in 0.1% of the styrene weight were added and agitated at 90 °C for 1 h. The obtained ESO-HEAMA-MA/styrene resins were light yellow and transparent liquids when cooled naturally to room temperature. The SBO content in the resulting bioresins ranged approximately from 17.8% to 35.7%.

Curing of ESO-HEAMA-MA Resins. The obtained ESO-HEAMA-MA/styrene resins were blended with specified amounts of t-BPB initiator for 30 min, then degassed under reduced pressure, and finally poured into a polytetrafluoroethylene mold with a specified size suitable for DMA and mechanical tests. The resins were cured at 120 °C for 3 h and postcured at 150 °C for 2 h.

Characterization. Acid values (Av) of the samples was determined according to the procedures specified in GB/T 2895–1982. Typically, about 0.5 g of the product was dissolved into 50 mL of toluene/ethanol (50:50 v/v) solution. Then the solution was titrated by a KOH/ethonol solution with an accurate concentration determined by potassium acid phthalate standard. The acid value of the sample was calculated using the following equation 7



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Av =

56 ⋅ ∆V ⋅ C KOH 1000 ⋅ ∆m

(1)

where ∆V and CKOH represented the consumed volume and initial concentration of KOH solution, respectively; ∆m was the resin weight. Viscosities (Vs) of the obtained liquid resins were measured by a NDJ-1 rotational viscometer from Shanghai Pingxuankeji Instrument Corporation (China). GPC measurements were performed using a HPLC system together with a 2414 refractive index (RI) detector (Waters Corporation, USA). The Waters Styragel HR1 and HR2 (300 mm×7.8 mm) columns were used for separation and were maintained at 35 °C. The flow rate of tetrahydrofuran (THF) was 1.0 mL/min. The products were dissolved with THF in a concentration of 15–25 mg/mL. The calibration curve was determined by a series of narrowly-distributed polystyrene standards with known molar masses of 580–19600 and then employed to determine relative molar masses of the analyzed samples. Spectra of FT-IR were recorded on a Nicolet iS10 IR spectrometer (Thermo-Fisher Corporation, USA) in a scanning range of 650–4000 cm-1 and with a resolution of 4 cm-1. Peaks of the obtained spectra were labeled using OMNIC software (Thermo Electron Corporation, USA). Spectra of 1H-NMR and 13C-NMR were recorded on a DRX-300 Advance NMR spectrometer (Bruker Corporation, Germany) and CDCl3 was used as a solvent. MestReNova software from Mestrelab Research S. L. (Santiago de Compostela, Spain) was utilized to analyze the NMR data. Dynamic mechanical analysis (DMA) of the cured polymer samples were carried out within three-point bending geometry on a Q800 solids analyzer (TA Corporation, USA) with an oscillating frequency of 1 Hz. The temperature was swept at a rate of 8


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3 °C/min. The samples were cuboid within a size of 40×10×2 mm3. Duplicated tests of each sample were performed in order to make sure the reproducibility of data. Thermogravimetric analysis (TGA) of the cured polymer samples were carried out on a STA 409PC thermogravimetry instrument (Netzsch Corporation, Germany) at a heating rate of 15 °C/min. The oven temperature was ranging from 35 to 600 °C under N2 gas at a flow rate of 100 mL/min. A Diamond differential scanning calorimeter (PerkinElmer Corporation, USA) was employed to analyze Tgs of the cured bioresins. Before the measurements, cured resin samples were ground into powders and dried at 40 °C under reduced pressure for 2 days. The measurements were conducted from -20 to 180 °C at a scan rate of 20 °C/min under N2. Two heating and cooling cycles were performed and Tg is determined as the inflection point of the second heating trace. Tensile properties of the cured polymer materials was evaluated by a SANS7 CMT-4304 universal tester from Shenzhen Xinsansi Jiliang instrument corporation (China) with the gauge length of 50 mm at a draw speed of 5.0 mm/min. Specimens within a size of 80×10×2 mm3 were evaluated. At least five specimens were tested for each resin sample to determine the average value. SEM measurements of the tensile-fractured samples were performed using an S-3400N scanning electron microscope (HITACHI Corporation, Japan). The surface of the fractured samples after stretching by the universal tester was coated with gold prior to SEM observation. The curing behaviors were also investigated by the above DSC. The resin samples for analysis were the residual parts of the prepared liquid resins for the DMA test. 5–10 mg of the samples was sealed in a 40 µL aluminum crucible and immediately tested 9



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by DSC. The sample was scanned from 25 to 220 °C at the heating rates of 5, 10, 15, and 20 °C/min, respectively.

RESULTS AND DISCUSSION Synthesis and Characterization of ESO-HEAMA and ESO-HEAMA-MA. The synthesis of ESO-HEAMA-MA macromonomer involved three steps (Scheme 1). The first step was to synthesize the precursor HEAMA, which was expected to have a C=C functionality of 2. The reaction was monitored by the Av value (Figure S1). The reaction readily proceeded without any catalyst. After a reaction time of 5 h, the Av of the product reached a stable value of 243.6 mg KOH/g, which was smaller than the theoretical value of 261.3 mg KOH/g. The reason for this can be attributed to the sublimation of MA. The second step is to prepare ESO-HEAMA monomer. The reaction was monitored by conversion of HEAMA which was estimated using the ratio of Av values of the mixture in the real time and the initial time. Before other experimental conditions were determined, several catalysts were investigated firstly to find a suitable catalyst for the synthesis of ESO-HEAMA. The results were listed in Table S1. N,N-Dimethyl benzyl amine was selected for the synthesizing process based on its comprehensive performance. With this catalyst the conversion approached a stable value at the reaction time of 4 h and the system didn’t reach gelation. Table 1 shows the effects of stoichiometric ratio of reactants, reaction temperature, and reaction time on the conversion of HEAMA. The ultimate conversion of HEAMA increased as the growth of the ratio of epoxy to carboxyl groups. Under the 10


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Table 1. Reactions of ESO and HEAMA, conversion of HEAMA, and introduced C=C functionality per ESO Molar ratioa

Molar ratio

Conversionb %

Temperature

NC=Cc

Entry

a

(ESO/HEAMA)

(Epoxy/Carboxyl)

1

1.00/4.23

2

o

C

1h

3h

4h

1.00/1.00

100

64.9

69.7

70.5

/

1.00/3.84

1.10/1.00

100

65.9

73.9

74.9

6.24

3

1.00/3.52

1.20/1.00

100

68.4

75.6

76.6

5.71

4

1.00/3.02

1.40/1.00

100

69.5

79.2

Gel.

/

5

1.00/3.84

1.10/1.00

90

62.9

71.2

72.5

/

6

1.00/3.84

1.10/1.00

110

68.8

Gel.

/

/

Molar ratio of epoxy group on ESO to carboxyl group on HEAMA. b Conversion of HEAMA. c Introduced C=C functionality per ESO molecule.

ESO/HEAMA ratio of 1.00/3.84 and 1.00/3.52, the conversion reached 74.9% and 76.6%. The introduced C=C functionality (NC=C) per ESO molecule was estimated by 1

H-NMR, which would be discussed later in the structural characterization. The

determined NC=C value was 6.24 and 5.71 for the feed ratio of 1.00/3.84 and 1.00/3.52, respectively. Considering the balance of conversion and NC=C data, the ratio of 1.00/3.84 and the reaction temperature of 100 °C were suitable for the synthesis of ESO-HEAMA. Besides, the occurring of gelation was found when the feed ratio of ESO:HEAMA was 1:3.02 or when the reaction temperature was 110 °C. The reason for this was probably attributed to the residual unreacted MA and HEA in HEAMA 11



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precursor, as evident in 1H-NMR spectrum of HEAMA (Figure S2). Under the effect of N,N-dimethyl benzyl amine, the small quantities of MA and HEA probably cause crosslinking reaction or oligomerization of epoxy groups on ESO,17 as illustrated in Scheme 2. When the lowest feed ratio of 1:3.02 was used, the amount of epoxy groups on ESO was excess too much over carboxyl groups, thus leading to a more quick formation of gelation than other feed ratios. When the reaction temperature of 110 °C was employed, the higher temperature would also accelerate the as-mentioned crosslinking reaction or oligomerization to form gelation.

Scheme 2 Possible crosslinking reaction or oligomerization of epoxy groups on ESO during the synthesis of ESO-HEAMA.

The last step was to synthesis ESO-HEAMA-MA monomer in the third step. 12


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Table 2 presents feed ratios of reactants and corresponding physical characteristics of each product. The NC=C data ranged in 6.75–8.15, which were clearly higher than those of MAESO and other reported oil-based monomers.6-8, 14, 18 Based on the calculated NC=C values, the ESO/HEAMA/MA ratio of 1/3.84/2 was appropriate for the MA modification. The reaction time of 1 h was chosen in order to avoid gelation of the products. Table 2. Maleinization reaction of ESO-HEAMA and physical characteristics of the resulting products Molar ratio

Ava

Weight ratio

NC=Cb

Entry (ESO/HEAMA/MA)

(ESO-HEAMA/MA)

mgKOH/g

1

1/3.84/1

100/6.19

58.8

6.75

2

1/3.84/2

100/12.38

89.2

8.15

3

1/3.84/3

100/18.57

108.9

7.47

a

Acid value. b Introduced C=C functionality per ESO molecule.

Figure 1 shows the GPC chromatographs and their corresponding deconvolutions of ESO, ESO-HEAMA, and ESO-HEAMA-MA. The peaks of ESO-HEAMA and ESO-HEAMA-MA were much broader than that of pristine ESO and shifted to shorter retention times owing to the growth of molecular weight. With the calibration curve of standard polystyrene samples, molecular weights of the three substances were determined and listed in Table 3. Compared with ESO, the ESO-HEAMA monomer possessed much larger weight-average and number-average molar masses 13



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Figure 1. GPC curves of (a) ESO, (b) ESO-HEAMA, and (c) ESO-HEAMA-MA. Solid lines: original GPC curves; dotted lines: deconvolution of GPC curves.

Table 3. GPC results of ESO, ESO-HEAMA, and ESO-HEAMA-MA Mwa

Mnb Dc

Sample ID

a

(g/mol)

(g/mol)

ESO

1456

1209

1.20

ESO-HEAMA

11055

3807

2.90

ESO-HEAMA-MA

11606

3957

2.93

Weight-average molar mass. b Number-average molar mass. c Polydispersity index.

14


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Scheme 3 Possible condensation reaction of ESO-HEAMA with MA during the synthesis of ESO-HEAMA-MA.

(Mw and Mn) as well as wider polydispersity index (D), indicating the product was composed of several monomers, oligomers, or even polymers. After the deconvolution, the resolved peaks with Mw of 6880–38320 clearly demonstrated the formation of oligomers or polymers, which was also attributed to the occurrence of crosslinking reaction or oligomerization of epoxy groups (Scheme 2). During the synthesis of ESO-HEAMA-MA, the used MA can react with the newly formed hydroxyls groups or residual epoxy groups on the triglycerides to yield half ester or full ester structures. The reaction temperature of 90 °C was adopted to inhibit possible condensation reactions14 of ESO-HEAMA (Scheme 3). Hence, the molar masses of 15



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ESO-HEAMA-MA only increased a little compared to those of ESO-HEAMA. However, the occurrence of peak with Mw=83790 suggested that a little of ESO-HEAMA were still chemically connected by MA to form full ester structures. Figure 2 shows the FT-IR spectra of ESO-HEAMA and ESO-HEAMA-MA. After the modification of ESO-HEAMA with MA, the secondary hydroxyl group peak at 3519 cm-1 changed into a board carboxyl group peak at 2500–3400 cm-1. The peaks at 1849 cm-1 and 1779 cm-1, which represent carbonyl groups on MA structure,7, 14 decreased obviously compared to those in the initial reaction (Figure S3).

Figure 2. FT-IR spectra of (a) ESO-HEAMA and (b) ESO-HEAMA-MA.

Figure 3 demonstrates the 1H-NMR spectra of ESO-HEAMA and ESO-HEAMA-MA. In the spectrum of ESO-HEAMA, the peak of epoxy group at 2.8–3.2 ppm almost disappeared as compared to that in the spectrum of ESO (Figure S2). Due to the formation of the secondary hydroxyl groups, the peak at 3.5–4.0 ppm was observed. The peak at 5.0 ppm represented the methine proton connecting to the 16


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Figure 3. 1H-NMR spectra of (a) ESO-HEAMA and (b) ESO-HEAMA-MA.

HEAMA structure. The peaks at 5.8–6.9 ppm corresponded to the vinyl protons from HEAMA. After the modification of ESO-HEAMA with MA, the peaks at 3.5–4.0 ppm almost disappeared because of the consumption of the secondary hydroxyl groups. The intensity of the peak at 6.3 ppm increased. Since the reaction temperature was not high, the peak at 6.9 ppm representing fumarate ester vinyl protons was very low. A small peak at 7.1 ppm corresponded to the vinyl protons of unreacted MA. The peak at around 0.9 ppm corresponding to the terminal methyl protons of FAs could be taken as internal standard as its intensity should not be altered throughout all the reactions mentioned above.6, 7 The integral of this peak should be nine protons per triglyceride molecule. Additionally, it was reported that only two vinyl protons of HEA lied in 6.1–6.5 ppm, while the other vinyl proton of HEA was at about 5.8 ppm.8, 9, 12, 19

Therefore, the introduced C═C functionality per ESO molecule (NC═C) for the 17



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obtained ESO-HEAMA and ESO-HEAMA-MA monomers can be conveniently determined by the following equation

9A 1 A N C =C = ( 6.1−6.9 ppm ) = 6.1−6.9 ppm 2 A0.9 ppm / 9 2 A0.9 ppm

(2)

Related results were summarized in Tables 1–2.

Figure 4. 13C-NMR spectra of (a) ESO-HEAMA and (b) ESO-HEAMA-MA.

Figure 4 provides the 13C-NMR spectra of ESO-HEAMA and ESO-HEAMA-MA. For the ESO-HEAMA, the shifts at 164.1–173.3 ppm and 127.1– 135.6 ppm were assigned to the carbonyls and unsaturated carbons, respectively. The peak at 72.0 ppm and 68.4 ppm corresponded to the tertiary carbons at the polyols backbones and the grafted structure of HEAMA, respectively. The peak at 65.6 ppm was related to the fatty acid carbons connecting to the formed secondary hydroxyls. For the ESO-HEAMA-MA, new peaks at 167.4 ppm and 133.8/128.9 ppm were observed, which represented the carbonyls and unsaturated carbons on the maleate 18


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half esters, respectively. Furthermore, the peak at 65.6 ppm disappeared because of the maleination reation of the secondary hydroxyl groups.

Structure–property Analysis of the Cured ESO-HEAMA-MA Resins. Dynamic Mechanical Analysis. DMA technique was utilized to investigate thermo-mechanical properties of the cured ESO-HEAMA-MA resins. Figure 5 shows the temperature dependence of storage modulus (E′) and loss factor (tan δ) for the cured ESO-HEAMA-MA resins with different styrene concentrations and the related DMA data are presented in Table 4. Firstly, the received biomaterials exhibited a very broad transition from glassy to rubbery state due to the complexity of the obtained SBO-based monomers. As shown in the GPC curves (Figure 1), the ESO-HEAMA product actually contained a number of monomers, oligomers, and even polymers. Second, the biomaterials indicated two glass transitions: one was corresponding to the styrene-rich region with a higher Tg, the other belongs to the oil-rich region with a lower Tg although it was not as clear as the styrene-rich one.20, 21 The E′ at 25 °C (E′25) increased sharply from 1.32 GPa to 2.12 GPa as the styrene content increased from 20% to 40%, whereas grew very slowly as the content increased from 40% to 60%. The Tg value (determined from the maximum of tan δ curves) increased gradually from 69.9 °C to 114.1 °C as the styrene concentration increased from 20% to 60%. These dynamic mechanical properties were comparable to or even better than those of biopolymers also obtained from ESO. For instance, the E′25 values of AESO and cyclohexane dicarboxylic anhydride (CDCA)-modified AESO bioresins containing 33 19



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wt% styrene were 1.3 GPa and 1.6 GPa, respectively,22 which were smaller than or almost equal to that of the ESO-HEAMA-MA resin with a styrene content of 30%. Moreover, the E′25 values of best MAESO resins with 38 wt% styrene were 1.9–2.2 GPa, which was also identical to that of the ESO-HEAMA-MA resin with a styrene content of 40%.14 Based on the kinetic theory of rubber elasticity, the experimental crosslink density (νe) of the copolymers can be calculated from the rubbery modulus using the following equation:23-26

E ' = 3νe RT

(3)

where E′ represents the storage modulus of crosslinked copolymers in the rubbery plateau region, R is the gas constant, and T is the absolute temperature. In this work the rubber modulus selected for calculating νe occurred at Tg+40 °C. Therefore, it can be inferred that the increase of both E′25 and Tg can be partially attributed to the increase of νe values.23-26 Remarkably, the resin containing 60% styrene reached a νe value of 9.52×103 mol/m3, which may be attributed to the sufficient solubilization and copolymerization of the obtained biobased macromonomer. The effects of feed ratio and initiator concentration on the dynamic mechanical properties of the cured resins were also studied (Figures S4–5 and Tables S2–3). The results indicated that the ESO:MA ratio of 1:2 and 3% of t-BPB concentration were appropriate for the synthesis and curing of ESO-HEAMA-MA resins.

20


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Figure 5. Storage modulus (a) and loss factor (b) of the cured ESO-HEAMA-MA resins with different styrene concentration (ESO:HEAMA:MA=1:3.85:2 and 3% t-BPB). Table 4. Dynamic mechanical properties of the ESO-HEAMA-MA resins containing different styrene concentration Styrene concentration

E′25a

Tgb

νe c

(wt%)

(GPa)

(°C)

(103mol/m3)

20

1.32

69.9

5.07

30

1.56

73.8

6.94

40

2.12

96.4

7.71

21



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a

50

2.06

102.2

8.50

60

2.16

114.1

9.52

Storage modulus at 25 °C. b Glass transition temperature. c Crosslink density.

Mechanical Property Analysis. Figure 6 presents the typical tensile stress–strain curves of the ESO-HEAMA-MA resins with different styrene content and the related data are summarized in Table 5. All the samples broke without reaching their yielding points, indicating they are brittle plastics. When the styrene concentration increased from 20% to 60%, the tensile strength and modulus grew from 19.7 MPa to 33.1 MPa and from 1.17 GPa to 2.11 GPa, respectively, while the break strain decreased from 3.9% to 2.6%. The increase of crosslink density always results in increased stiffness and brittleness.25, 26 Although the tensile properties were not as good as those of MAESO,14 they were still comparable or superior to many other oil-based UE resins, including AESO and CDCA-AESO resins.16, 21, 22, 27

Figure 6. Tensile curves for the ESO-HEAMA-MA polymer matrices containing different styrene concentration. 22


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Table 5. Data of mechanical and thermal properties for the ESO-HEAMA-MA resins containing different styrene concentration

a

Styrene concentration

σa

Eb

εc

T5d

Tpe

wcharf

Tg g

(wt%)

(MPa)

(GPa)

(%)

(°C)

(°C)

(%)

(°C)

20

19.7±0.9

1.17±0.05

3.9±0.5

205.5

411.1

0.1

58.9

30

24.5±1.3

1.45±0.08

3.7±0.2

206.7

412.1

0.2

73.3

40

29.3±0.8

1.89±0.07

2.7±0.3

218.2

409.5

0.7

81.1

50

32.4±0.3

2.05±0.11

2.6±0.4

257.6

409.5

3.1

88.7

60

33.1±0.9

2.11±0.15

2.3±0.5

239.0

410.1

0.4

101.9

Tensile strength. b Young’s modulus. c Breaking strain. d 5% Weight–loss temperature. e Peak temperature at the curves of weight–loss rate. f Char yield. g Glass transition temperature determined by DSC.

Thermogravimetric Analysis. The TGA thermograms and their derivative curves of the cured ESO-HEAMA-MA resins containing different styrene concentration are plotted in Figure 7. All the oil-based biomaterials were thermally stable in N2 atmosphere to about 205 °C and then exhibited a three-stage process of thermal degradation. The first-stage degradation occurring at about 210–340 °C corresponded to the evaporation and decomposition of soluble components in the bulk materials such as unreacted feedstocks, catalyst, and inhibitor. The second stage ranging in 340–460 °C was the fastest degradation, which was caused by the degradation and char formation of the crosslinked polymer structure. The last stage (>460 °C) usually 23



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corresponded to the gradual degradation of char residue. The thermal-property data of the obtained bioplastics, including 5% weight–loss temperature (T5), peak temperature at the curves of weight–loss rate (Tp), and char yield (wchar) were summarized in Table 5. When the styrene concentration increased from 20% to 60%, the T5 value firstly increased from 205.5 °C to 257.6 °C, then decreased to 239.0 °C; while the Tp value almost unchanged at around 410 °C for all the biomaterials. The wchar value varied in the same trend as T5, increasing from 0.1% to 3.1% then decreasing to 0.4%. As a consequence, the biomaterial containing 50% styrene exhibited the best thermal stability.

Figure 7. TGA curves (a) and their derivatives (b) of the cured ESO-HEAMA-MA 24


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resins containing different styrene concentration.

Differential Scanning Analysis. DSC curves of the cured ESO-HEAMAMA resins with different styrene concentration were plotted in Figure S6 and the determined Tgs were listed in Table 5. It is notable that the transitions of DSC curves were very broad due to the complicate monomers and oligomers involved in the ESO-HEAMA-MA product. As the styrene concentration increased from 20% to 60%, the determined Tg also grew from 58.9 °C to 101.9 °C. The values were a bit lower than those determined by DMA, which possibly resulted from different detecting modes of the techniques employed, different experimental conditions like heating rates, etc.28-30 In general, the DSC results were in good accordance with those of DMA. Morphology Analysis. SEM technique was utilized to study the microstructures of the cured SBO-based resins. Figure 8 provides the SEM micrographs of the failure surfaces of ESO-HEAMA-MA resins containing different styrene concentrations. All the surfaces of the resin samples exhibited a distinct characteristic of brittle materials and no obvious phase separation was observed. With the increase of styrene concentration from 30% to 60%, the roughness of the surface decreased accordingly. When the styrene concentration reached 60%, the resin surface was almost featureless. These phenomena were possibly caused by the more sufficient solubilization and copolymerization of the SBO-based macromonomer while adding more styrene comonomer. The reduction of roughness may also contribute to the rise of stiffness,23-26 which is one of the reasons for why the storage modulus and Young’s 25



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modulus increased when increasing the styrene concentration.

Figure 8. SEM micrographs of the fracture surfaces of the cured SBO-based resins containing (a) 30% styrene, (b) 40 % styrene, (c) 50% styrene, and (d) 60% styrene.

Curing Behavior of ESO-HEAMA-MA Resin. Figure 9 shows the DSC thermograms of curing of the ESO-HEAMA-MA resin with 40% styrene at different heating rate (β) and the plots of ln β vs. 1/Tp (Tp is the peak temperature of the DSC scanning curve), respectively. Although the C═C groups on ESO-HEAMA-MA monomer were different, the curing of this resin still exhibited one exothermic peak during the non-isothermal test. As the increase of the β value, the Tp moved to higher temperatures, which was a typical methodological phenomenon for non-isothermal curing. By extrapolating Tp to the point of an infinitely slow heating rate (β=0), a new Tp value was acquired and could be used as reference in the selection of isothermal curing temperature. The activation energy (Ea) of curing could be determined using the Ozawa method31-33 with the following equation Ea =

−R ∆ ln β ⋅ 1.052 ∆ (1/ T p )

(4)

26


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The determined Tp at β=0 and Ea values for the ESO-HEAMA-MA resin were 132.9 °C and 106.6 kJ/mol, respectively. Both of the two values were higher than those of similar oil-based UE resins and commercial UPRs,34-36 suggesting the obtained SBO-based resin was not easy to be cured as those resins.

Figure 9. (a) DSC curves of non-isothermal curing of the ESO-HEAMA-MA resin containing 40% styrene at different heating rates and (b) plot of ln β vs.1/Tp.

CONCLUSIONS. With the synthesis and employment of the HEAMA precursor, we successfully updated the way of using MA modified acrylated epoxidized vegetable oils to acquire high-functionality co-UE monomers from SBO. The modifying process could provide potential opportunities to synthesize polymer materials from renewable 27



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resources via a clean and simple process which follows the principles of green chemistry. The obtained ESO-HEAMA-MA monomer possessed a maximum C=C functionality of 8.15 per ESO molecule, thus leading to rigid bioplastics with a crosslink density up to 9.52×103 mol/m3 as well as good thermal and mechanical properties. The as-prepared high-functionality UE macromonomers show promise to be applied in UPRs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ***. Variation of acid value during the synthesis of HEAMA (Figure S1), effects of catalysts on conversion of HEAMA (Table S1), 1H-NMR spectra of HEAMA and ESO (Figure S2), FT-IR spectra of free MA during maleinization reaction of ESO-HEAMA (Figure S3), DMA properties of cured ESO-HEAMA-MA resins with different feed ratio of ESO:MA (Figure S4 and Table S2) and with different concentration of t-BPB initiator (Figure S5 and Table S3), and DSC properties of the cured ESO-based resins (Figure S6) (PDF).

AUTHOR INFORMATION Corresponding Authors * C. Liu. E-mail: [email protected]. Fax: + 86-25-85482520. Tel.: +86-25-85482520. 28


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*Y. Zhou. E-mail address: [email protected]. Fax: + 86-25-85482777; Tel.: + 86-25-85482777. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors are grateful to the financial support from the National Natural Science Foundation of China (31300489), the Fundamental Research Funds for the Central Non-profit Research Institution of CAF (CAFYBB2014QB022), and the Fundamental Research Funds from Jiangsu Province Biomass Energy and Materials Laboratory (JSBEM-S-201501).

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For Table of Contents Use Only

High-functionality Unsaturated Ester Macromonomer Derived from Soybean Oil: Synthesis and Copolymerization with Styrene Chengguo Liu, *,†,‡ Yan Dai,† Yun Hu,† Qianqian Shang,† Guodong Feng,† Jing Zhou,† Yonghong Zhou,*,†

A high-functionality unsaturated co-ester macromonomer for thermosetting polymers was synthesized from epoxidized soybean oil via novel “green” synergetic modification.

34


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Highly Functional Unsaturated Ester Macromonomer Derived from

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