Soybean Oil-Based Thermoset Films and Fibers with High Biobased

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Soybean Oil-Based Thermoset Films and Fibers with High Biobased Carbon Content via Thiol-Ene Photopolymerization Sung-Soo Kim, Heonjoo Ha, and Christopher J Ellison ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00435 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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ACS Sustainable Chemistry & Engineering

Soybean Oil-Based Thermoset Films and Fibers with High Biobased Carbon Content via Thiol-Ene Photopolymerization Sung-Soo Kim, Heonjoo Ha, and Christopher J. Ellison* Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, MN 55455, United States E-mail: [email protected] KEYWORDS: soybean oil, thermoset, cysteine, thiol-ene photochemistry, biobased carbon content, electrospinning

ABSTRACT

While a number of vegetable oil derivatives have been integrated with petroleum-based materials to prepare thermosetting polymers, existing examples usually incorporate low total biorenewable content into the final product. With the goal of generating thermosets with high biorenewable content, two different soybean oil derivatives with multifunctional thiol and acrylate groups were photocured via thiol-acrylate photopolymerization. For this purpose, L-cysteine, a nonhazardous amino acid, was coupled with epoxidized soybean oil to synthesize a mercaptanized soybean oil

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derivative containing multiple thiol groups. Following mixing with acrylate counterparts suitable for performing thiol-ene photopolymerizations, these monomer mixtures were processed into thermoset films (via monomer mixture film casting followed by photopolymerization) and fibers (via simultaneous electrospinning of the monomer mixture and photopolymerization in-flight). The resulting materials possessed high biobased carbon content (BCC; over 90%) and higher elasticity than cross-linked acrylated epoxidized soybean oil without the thiol-containing component. This can be attributed to a change in the cross-link density that is controlled by different photopolymerization mechanisms (e.g., step-growth polymerization vs. chain-growth homopolymerization). We anticipate that the approaches outlined in this study could be generalized to other bioderived triglyceride oils for increasing the BCC and imparting biodegradability in a number of materials applications.

Introduction The annual global production of plastics has rapidly increased since synthetic polymers were first mass-produced in the 1950s. Longstanding environmental concerns of non-degradable plastic waste has accordingly become a global issue. By the end of 2015, it was estimated that 8.3 billion metric tons (Mt) of plastic material were produced, of which 6.3 billion Mt became discarded waste.1 Approximately 79 % of plastic litter has accumulated in landfills, however, a considerable amount has entered the ocean either by accident or through illegal dumping. The annual deposition of plastic litter into the ocean was estimated to be 4.8 to 12.7 million Mt in 2010;2 Plastic is thought to currently account for 73 % of all the garbage in the oceans.3 A sizeable fraction of plastic litter in the ocean eventually breaks down due to the action of sunlight and ocean currents; obviously, the specific mechanisms of degradation and the degradation timescale are highly dependent on many factors (e.g., polymer identity, sunlight


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exposure intensity, water temperature, pH and salinity, etc.) Plastic litter eventually degrades into smaller fragments less than 5 mm in diameter, which are defined as microplastics according to the United States National Oceanic & Atmospheric Administration. This microplastic waste accounts for 92.4 % of the global particle count of floating plastics in the oceans.4 Of this, there are intentionally manufactured “primary” microplastics, such as microbeads found in personal care products, as well as unintentionally generated “secondary” microplastics, such as microfibers, that are created when synthetic garments are washed in washing machines, for example.5 Regardless of source, microplastic pollutants not only endanger the marine ecosystem due to their propensity to be ingested and accumulated in the bodies of aquatic organisms causing adverse health problems,6-8 but they are also considered a potential threat to human health because microplastics can be ingested by humans through seafood consumption.9 In addition, since most plastics are made from petroleum-based materials, microplastic concerns also closely coupled to other issues of environmental pollution and depletion of fossil fuel resources. To partially address these environmental concerns associated with microplastics, the conversion of various bio-sourced monomer precursors into useful and degradable polymeric materials has emerged as an attractive green chemical strategy.10 Among various renewable sources, vegetable oils are a suitable candidate for this approach, because they are perpetually renewable and products derived from bio-renewable vegetable oils can be more readily degradable than petroleum-based analogs.11,12 Moreover, many chemical functionalities can be introduced onto triglyceride oils to transform them into useful monomers for the preparation of toughened renewable thermoplastics13,14 and sustainable thermosetting polymers, such as polyurethanes, vinyl resins, and epoxy resins.15 For example, UV-curable acrylated epoxidized


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soybean oil (AESO) has been extensively mixed with other bio-derived molecules and cured to produce bio-based thermosetting resins.16-19 Our group recently developed a general method for fiber formation using thiol-ene photopolymerization in conjunction with fiber spinning. This particular chemistry enables a rapid curing process during fiber flight through a mixed chain-growth and step-growth polymerization mechanism along with a relative insensitivity to atmospheric oxygen.20 Moreover, thermochemically stable crosslinked fiber was produced from a monomer mixture containing AESO as a bio-renewable acrylate. However, the BCC of this demonstration was approximately 49 % due to the presence of non-biorenewable pentaerythritol tetrakis(3-mercaptopropionate) in the monomer mixture as the multifunctional thiol counterpart.20 A more attractive strategy would be to substitute soybean oil-based thiol monomers for petroleum-based thiol monomers to achieve significantly higher BCC for the final thermoset while simultaneously enhancing the biodegradability characteristics afforded by triglyceride oil based monomers. In this study, mercaptanized epoxidized soybean oil (MESO) was synthesized where the thiol functional groups were introduced by performing a reaction between a L-cysteine derivative and commercially available epoxidized soybean oil (ESO). Then, as-prepared MESO was mixed with AESO, diluent, and photoinitiator, followed by UV irradiation under ambient conditions to produce cross-linked films and fibers. It was confirmed that the BCC values of the resulting thermosets were higher than 90 %, and their mechanical properties were widely tunable by considering the monomer mix composition and photopolymerization mechanism. Experimental Section Materials. L-cysteine (97%), AESO (containing 4000 ppm monomethyl ether hydroquinone; the average number of acrylate groups in the chemical structure of AESO = 2.70, as shown in Figure


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S4), 1,4-diazabicyclo[2.2.2]octane (DABCO, ≥ 99%), poly(ethylene oxide) (PEO) (Mn = 1,000,000 g mol-1), magnesium sulfate (MgSO4, ≥97%), sodium sulfate (Na2SO4, ≥99%), ethyl acetate (99.8%), and toluene (Certified ACS) were purchased from Millipore-Sigma. Hexane (Certified ACS), sodium bicarbonate (NaHCO3, Certified ACS), diethyl ether (HPLC Grade), and phosphate buffered saline (PBS) were purchased from Fisher Scientific. Ncarbethoxyphthalimide (99+%) and urethane (97%) were purchased from Acros Organics. ESO (epoxy content = 6.55 wt%; the average number of epoxide groups in the chemical structure of ESO = 4.16, as shown in Figure 2c) was purchased from AdipoGen Life Sciences. Hydrochloric acid

(ACS,

36.5-38%)

was

purchased

from

VWR

Analytical.

Ethyl(2,4,6-

trimethylbenzoyl)phenylphosphinate (EPP, 98%) was purchased from Combi-Blocks. Omnirad® 2100 (90-95% EPP + 5-10% Phenylbis(2,4,6-trimethylbenzoyl)-phosphine oxide) was gratefully provided by IGM Resins. UV-visible absorption spectra of EPP and Omnirad® 2100 are shown in Figure S1, which is consistent with the vendor’s material data sheet. All materials were used as received except for toluene, which was passed through a short column of Brockman Type I basic alumina prior to use. Synthesis of mercaptanized epoxidized soybean oil (MESO). L-cysteine (4.00 g, 0.033 mol) and N-carbethoxyphthalimide (7.45 g, 0.034 mol) were dissolved in aqueous NaHCO3 0.33M solution (100 mL), which was vigorously stirred for 6 h in ambient conditions. The reaction mixture was then filtered and acidified (pH ~ 2) by adding aqueous HCl solution (6.0 M) dropwise. The precipitate was extracted with diethyl ether, followed by washing with HCl solution (1.0 M) twice. The organic phase was dried with MgSO4, followed by concentration using a rotary evaporator, and the resulting mixture (N-phthaloyl-L-cysteine with residual ethyl


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carbamate) in the reactor was dried in a vacuum oven overnight. DABCO (0.9 % of total mass) was added to the reactor to which a toluene solution (50 mL) of ESO (ESO added at 0.8 times the mass of the N-phthaloyl-L-cysteine with residual ethyl carbamate such that N-phthaloyl-Lcysteine was present in 1.12 molar equivalents to epoxide groups on ESO) was transferred, followed by heating under reflux for 24 h. After the reaction, the reactor was cooled in an ice bath, and the resulting solution was filtered and washed with PBS (pH = 7.4) solution twice. The organic phase was concentrated and redissolved in ethyl acetate (50 mL), which was subsequently washed with NaHCO3 (1.0 M) aqueous solution and PBS (pH = 7.4) solution twice each. The organic phase was dried with Na2SO4, concentrated with a rotary evaporator, and dried in a vacuum oven overnight. The mass yield of the mercaptanization process was calculated to be 80.7 %, and the resulting MESO possessed an average thiol functionality of 2.62, which means 63% of the average number of epoxides (4.16) per ESO molecule were ring-opened. Preparation of soybean oil-based thermosets. Precise quantities of synthesized MESO, AESO, ethyl acetate, and EPP were mixed using a vibratory mixer for 5 min. After the air bubbles were removed, the sample was loaded into a syringe and injected into the space between two glass microscope slides separated by a Si wafer spacer (~500 µm), followed by UV irradiation (OmniCure® S1500 Curing System, 58 mW/cm2; broad spectral range of 320-500 nm without any additional filters). At the absorbance maximum of the EPP photoinitiator (~370 nm), the percent transmission of light through these films was 25-30% indicating the photopolymerization was initiated throughout the entire thickness of the film. Note that exposure of the sample to ultraviolet room light was minimized in each of these steps to limit ambient curing. The glass slides were removed after the photocuring process, and the cured thermosets were dried in a vacuum oven overnight. The gel content of the thermoset was calculated using equation (1),


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according to ASTM D2765 test method C. THF was used as the solvent to dissolve away the unreacted soluble part.

Gel content % = × 100

(1)

The density of the thermoset was measured by Archimedes’ principle with a density determination kit for Excellence XP/XS analytical balances (Mettler Toledo). Electrospinning. PEO was dissolved in ethyl acetate (~0.19 wt%) at 50 °C for 3 h to produce a stock solution. Precise quantities of synthesized AESO (53.6 wt%), MESO (21.6 wt%), Omnirad® 2100 (4.8 wt%), and ethyl acetate containing PEO stock solution (20.0 wt%) were mixed using a vibratory mixer for 5 min, followed by concentrating them with a rotary evaporator and drying in a vacuum oven until the fraction of ethyl acetate was reduced to ~7.0 wt%. The compositional ratios for the acrylated and mercaptanized monomers were calculated to target specific thiol to acrylate group ratios, which will be discussed in more detail in later sections. The mixed sample was loaded into a syringe, which was then fitted with a blunt tip, 0.84 mm inner diameter needle (18 gauge). A schematic of the apparatus is shown in the Supporting Information as Figure S9. The grounded collector was positioned 16 cm from the needle tip, from which monomers were extruded at a rate of 2 mL/h. 24 kV of positive DC charge was applied to the needle tip, creating an electric field of sufficient strength to drive a jet of solution towards the grounded collector during which the fiber jet was exposed to UV irradiation (OmniCure® S1500 Curing System with collimator attachment; broad spectral range of 320-500 nm without any additional filters). The distance between the needle tip and the beginning of the UV illuminated area was 13.8 cm, and the light source was angled so that it illuminated both the collected fibers and the liquid jet near the collector. The intensity of the UV light was 700 mW/cm2, and the diameter of the illuminated circular area was 4.4 cm.


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Characterization. UV-Visible absorption spectra of photoinitiators were obtained using a Thermo Scientific Evolution 220 UV-Visible spectrophotometer. One-dimensional and twodimensional nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance III HD nanobay AX-400 and Bruker Avance III 500, respectively. Fourier-transformed infrared (FT-IR) spectra of monomer mixtures were measured using a Nicolet 6700 FT-IR spectrometer with a KBr beam splitter and a MCT-A detector (Thermo Fisher Scientific). The viscosities of different components and monomer mixtures were measured using a rheometer (Discovery Hybrid Rheometer DHR-3, TA Instruments) at room temperature. Storage modulus and tan δ curves of the final thermosets were obtained using a RSA-G2 solids analyzer (TA Instruments). Thermogravimetric analysis (TGA) was performed on a TA Instruments Q500 at a heating rate of 10 °C min-1 under N2 atmosphere. Uniaxial tensile tests were performed on a Shimadzu Autograph AGSS17 X Series tensile tester. Field-emission scanning electron microcopy (FESEM) was performed on a Hitachi SU8230 SEM operated with 1 kV accelerating voltage. More detailed information about characterization is described in the Supporting Information. Results and Discussion Two different soybean oil derivatives with multifunctional thiol and acrylate groups are required to prepare photopolymerizable thermoset compositions with high biorenewable content (e.g., > 50 %). For this purpose, a nonhazardous amino acid L-cysteine was coupled with ESO to synthesize a mercaptanized soybean oil derivative containing multiple thiol groups (MESO). To synthesize MESO from ESO and L-cysteine, the epoxide groups of ESO can be opened with either the primary amine or the carboxyl group on L-cysteine. However, the esters on the soybean oil can be replaced by amides when heated with amines and a base catalyst. In addition, the reaction between a primary amine and an epoxide group can result in the formation of a


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secondary amine, which can further react with another epoxide to form an undesirable crosslinked network of soybean oil during the mercaptanization step.22 Therefore, the primary amine group of L-cysteine must be protected prior to its reaction with ESO. In this work, L-cysteine was phthaloylated with N-carbethoxyphthalimide in order to convert the primary amine group to a phthalimide (Figure 1a), which is an inactive moiety toward epoxide groups and esters even at high temperature. For this reaction, L-cysteine and N-carbethoxyphthalimide were stirred in NaHCO3 aqueous solution and extracted with diethyl ether, followed by washing with HCl to remove unreacted L-cysteine. Figure 1b shows the 13C NMR spectrum of the resulting materials after the phthaloylation. Although ethyl carbamate coexisted with N-phthaloyl-L-cysteine as described in the synthetic scheme (peaks with gray numbering in Figure 1b), no additional washing or separation steps were included to remove residual ethyl carbamate. This is because ethyl carbamate does not react with ESO and can be removed by further work-up processing, which will be described in detail in a later section. To clearly confirm the introduction of phthalimide group to L-cysteine, a 1H-13C heteronuclear multiple-bond correlation spectroscopy (HMBC) experiment (Figure S2) was performed. The result clearly exhibited coupling between protons bonded to a central carbon atom of cysteine (2 in Figure 1a) and carbonyl carbons in a phthalimide group (4 in Figure 1a). Since this coupling can only be observed from N-phthaloylL-cysteine,

and not from L-cysteine or N-carbethoxyphthalimide or a physical mixture of these

two, it was confirmed that the phthaloylation was successful. Note that the L-cysteine used in this work is less hazardous than other coupling molecules used in mercaptanization processes, such as hydrogen sulfide23 and thioglycolic acid,24 which makes this green chemical approach even more attractive.


9


Figure 1. (a) Synthesis scheme for N-phthaloyl-L-cysteine; (b)

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13

C-NMR spectrum of the

resulting mixture of N-phthaloyl- L-cysteine and ethyl carbamate. * and # denote solvent peaks of CDCl3 and diethyl ether, respectively.

After successfully synthesizing N-phthaloyl-L-cysteine, ESO was then added to the resulting mixture, followed by heating under reflux

for 24 h in the presence of 1,4-

diazabicyclo[2.2.2]octane (DABCO) as a basic catalyst as shown in the synthetic scheme of Figure 2a. As a result of this mercaptanization process, the highly viscous and yellow-colored MESO was obtained. The FT-IR spectrum of MESO denotes a notable change from that of ESO (Figure 2b). First, the vibration modes of epoxide groups (800-860 cm-1; blue region in Figure 2b) vanished, which indicates the opening of all epoxide groups in ESO. Second, the number of peaks associated with carbonyl stretching (1,650-1,800 cm-1; red region in Figure 2b) is increased from one (1,744 cm-1) to three (1,719 cm-1, 1,740 cm-1, and 1,777 cm-1) after


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mercaptanization. ESO originally has three identical carbonyl groups in triglyceride ester bonds (black dashed ellipse in Figure 2a). However, when ESO is mercaptanized with N-phthaloyl-Lcysteine, the resulting MESO acquires two additional carbonyl groups from the phthalimide groups (blue dashed ellipse in Figure 2a) and from the new ester bonds created by the epoxide ring opening reaction with a carboxyl group of N-phthaloyl-L-cysteine (red dashed ellipse in Figure 2a). In other words, the addition of N-phthaloyl-L-cysteine to ESO resulted in the observation of additional peaks in the carbonyl stretching region in the FT-IR spectrum. In addition, a weak peak associated with a thiol stretching mode (2,571 cm-1) appeared in the FT-IR spectrum of MESO in the region of 2,500-2,700 cm-1, as examined in detail in Figure S3. Therefore, this qualitative FT-IR analysis revealed that all epoxide groups of ESO were opened, which reacted with the carboxyl group of N-phthaloyl-L-cysteine to produce MESO. This mercaptanization process was revisited with the 1H-NMR spectra of ESO and MESO (Figure 2c). The signals at 2.84-3.22 ppm in the NMR spectrum of ESO (blue region in Figure 2c) are attributed to the protons attached to the epoxide groups (proton 4 in the chemical structure of ESO), which can no longer be observed in the spectrum of MESO. Instead, new peaks at 7.65-8.00 ppm, which are associated with aromatic protons in N-phthaloyl-L-cysteine (proton 8a and 8b in the chemical structure of MESO), appeared in the spectrum of MESO (red region in Figure 2c). These changes are consistent with the conclusion of FT-IR analysis that Nphthaloyl-L-cysteine opened epoxide groups when coupled with ESO. In addition to the qualitative discussion above, we quantitatively calculated how many Nphthaloyl-L-cysteine molecules were added to ESO from the NMR analysis. All integration values for NMR spectra were referenced by the peak at 0.88 ppm corresponding to the nine protons at the terminal methyl groups (#5). Since each thiol functional group is accompanied by


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̅ ) four aromatic protons in a phthalimide moiety, the average thiol functionality of MESO (!#$ was calculated, dividing the integration value of peaks at 7.65-8.00 ppm by 4, to be 2.62. Referencing the peak at 2.3 ppm corresponding to the six alpha protons (#3), the same analysis was performed and gave an identical result of about 2.6. This value is similar to the acrylate ̅ = 2.70) of commercially available AESO, which is also known to be functionality ( ! synthesized from ESO by the epoxide ring opening reaction with acrylic acid (Figure S4). ̅ value of MESO was calculated by integrating other peaks. Further evidence supporting the !#$ The number of epoxide groups in the chemical structures of ESO was calculated to be 4.16 by dividing the integration value of peaks at 2.84-3.22 ppm by 2. When these epoxide groups are opened by the reaction with a carboxyl group, the resonance of protons attached to the groups, affected by the formation of a new hydroxy group (proton 4a in the chemical structure of MESO) and an ester bond (proton 4b in the chemical structure of MESO), are shifted downfield as denoted in Figure 2c. Those peaks overlap with several broad peaks from other protons of Nphthaloyl-L-cysteine, so it was difficult to perform a deconvolution process to obtain integration values of each peak. To resolve this issue, integration values of a couple of broad regions consisting of several peaks were obtained and compared to estimate the values. Assuming that ̅ = 2.62 from the previous paragraph, the total integration value of peaks from proton 1, 6, and !#$ 4b (4.79-5.56 ppm) in the chemical structure of MESO should be 1.0 + 2.62 + 2.62 = 6.24, while that of peaks from 4a and 7 (3.12-4.04 ppm) should be (8.32 - 2.62) + (2.62 × 2) = 10.94. Those values are consistent with experimentally obtained values: 6.2 and 11.0, respectively. Therefore, the aforementioned NMR analysis confirms that all 4.16 epoxide groups were opened during the mercaptanization process, but only 2.62 groups were coupled with N-phthaloyl-L-cysteine. In addition, this NMR analysis shows that the epoxide groups on ESO were not ring-opened by


12

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thiol groups on N-phthaloyl- L-cysteine; this is evidenced by the absence of a proton peak at 2.53.0 ppm related to the nearby sulfide bond generated by the thiol-epoxide reaction. We presume that the carboxyl group of N-phthaloyl- L-cysteine is more readily deprotonated than a thiol in the presence of DABCO, a relatively weak Lewis base catalyst in this system compared to others utilized to promote the thiol-epoxy reaction.25,26 It is noteworthy to point out that the value of 2.62 for thiol functionality could not be increased further, even though the mercaptanization of ESO was performed with relatively higher amounts of N-phthaloyl-L-cysteine (1.80 molar equivalents instead of 1.12, relative to epoxide groups on ESO) (Figure S5). As a result, the synthesized MESO has 75.4 % bio-based carbon content (BCC) where the non-bio-based carbon atoms were introduced through the use of the phthalimide moiety. While the phthalimide can be removed by the treatment with hydrazine,27 such an attempt was beyond the scope of the present preliminary study.


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Figure 2. (a) Synthesis scheme for MESO; (b) FT-IR and (c) 1H-NMR spectra of ESO (black) and MESO (red). * and # denote solvent peaks of the CDCl3 and ethyl acetate (residual extraction solvent), respectively.

In order to validate the effect of the by-product, ethyl carbamate, in the ring opening reaction of ESO, a control experiment was performed by heating ESO with pure ethyl carbamate under reflux, followed by an identical work-up process. As illustrated in the NMR spectra (Figure S6), there was no chemical shift in the epoxide group proton resonances after the treatment, implying that ethyl carbamate did not react with ESO. Moreover, no ethyl carbamate peaks were observed in the NMR spectrum of the resulting material after the process (Figure 2c), which provides clear evidence that ethyl carbamate had already been removed during the washing step. Therefore, no additional steps were included to remove the by-product ethyl carbamate prior to the mercaptanization process, as mentioned earlier. However, the presence of residual ethyl acetate extraction solvent was detected (~2.6 wt% from Figure 2c) in MESO. Having both thiol and acrylate soybean oil derivatives synthesized and fully characterized, a photocured thermoset film was prepared for further analysis. AESO, MESO, and ethyl(2,4,6trimethylbenzoyl)phenylphosphinate (EPP) as a photoinitiator were mixed together following the compositions described in Table 1. The molar ratio of thiols in MESO to acrylates in AESO ( r = &thiol)/&acrylate) ) was 0.273, which is approximately equivalent to that in previous work.21 This soybean oil derived mixture composition of AESO and MESO will be denoted as the AESO/MESO mixture hereafter. In this study, ethyl acetate was added as a diluent to obtain a homogeneous AESO/MESO mixture due to the high viscosity of MESO. It is worth pointing out


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that ethyl acetate did not alter the photopolymerization kinetics nor the BCC of the final photocured thermoset materials.28

Table 1. Summary of the compositions used for preparing soybean oil-based thermosets and their corresponding viscosity values measured using a shear rheometer. % composition, by mass EPP

ethyl acetate

Viscosity [Pa·s]

22.7

1.0

20.0

1.00 ± 0.02

0.0

1.0

17.0

0.99 ± 0.09

Samples

a

AESO

MESO

AESO/MESO

56.3

AESO-only

82.0

a

containing ~2.6 wt% residual ethyl acetate

To better understand the cross-linking reaction between AESO and MESO, a thin film of asprepared AESO/MESO mixture was placed under UV irradiation, which was monitored by FTIR spectroscopy (Figure 3a). As expected, absorption peaks associated with the hydroxy group (3,203-3,643 cm-1) and the carbonyl group (1,650-1,800 cm-1) were not affected during the light exposure. However, a considerable reduction in the absorption peaks corresponding to the C=C bond in the acrylate group (1,636 cm-1) (Figure 3b) was observed due to the photopolymerization of AESO under UV irradiation. In addition, the thiol absorption peak (2,571 cm-1) also decreased in size (Figure S7), implying that the thiol groups of MESO participated in the photopolymerization reaction. These spectroscopic observations are consistent with the fact that the AESO/MESO mixture undergoes both thiol-acrylate polymerization and acrylate-acrylate homopolymerization during the UV exposure, ultimately leading to gelation. The BCC of the cross-linked AESO/MESO mixture was calculated to be 92.9 %, which exceeds the minimum biobased content requirement for 91 out of the total 97 product categories identified by the


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United States Department of Agriculture for mandatory Federal purchasing according to the BioPreferred® Program.29 The conversion during photopolymerization was calculated based on the peak intensity of the C=C absorption in the acrylate (Figure S8), which approached the maximum value within 10 seconds under this UV irradiation condition (58 mW/cm2). For a given thiol-acrylate system, based on the assumption that the termination and cyclization effects can be neglected, the critical conversion at the gel point (-. ) can be calculated by equation (2):21,30 /

̅ − 12 0!

3455 356

̅ − 120! ̅ − 12 81 + 9 3455 : -./ = 1 -. + 0!#$ 356

(2)

̅ is the average acrylate functionality of AESO, !#$ ̅ the average thiol functionality of where ! MESO, ;

Soybean Oil-Based Thermoset Films and Fibers with High Biobased

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