Biobased Thermosets Prepared from Rigid Isosorbide and Flexible

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Biobased Thermosets Prepared from Rigid Isosorbide and Flexible Soybean Oil Derivatives Wendi Liu, Tianshun Xie, and Renhui Qiu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02117 • Publication Date (Web): 07 Nov 2016 Downloaded from http://pubs.acs.org on November 12, 2016

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ACS Sustainable Chemistry & Engineering

Biobased Thermosets Prepared from Rigid Isosorbide and Flexible Soybean Oil Derivatives Wendi Liu†, Tianshun Xie†, and Renhui Qiu*, ‡ †

College of Material Engineering, Fujian Agriculture and Forestry University, 15

Shangxiadian Road, Jianshan, Fuzhou, Fujian, 350002, P. R. China ‡

College of Transportation and Civil Engineering, Fujian Agriculture and Forestry University,

15 Shangxiadian Road, Jianshan, Fuzhou, Fujian, 350002, P. R. China

AUTHOR INFORMATION Corresponding Author *Address: College of Transportation and Civil Engineering, Fujian Agriculture and Forestry University, 15 Shangxiadian Road, Jianshan, Fuzhou, Fujian, 350002, P. R. China. Phone: 86-591-83726495. Fax: 86-591-83715175. E-mail: [email protected] (R.Q.).

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KEYWORDS: Isosorbide; Biobased reactive diluent; Acrylated epoxidized soybean oil (AESO); Thermosetting resin; Processability; Mechanical properties

ABSTRACT: A rigid monomer, isosorbide-methacrylate (IM), was synthesized from isosorbide with methacrylate anhydride (MAA) via a solvent-free, ultrasonic-assisted method and then was used to copolymerize with acrylated epoxidized soybean oil (AESO) to formulate a biobased thermosetting resin (IM-AESO). The synthesis of IM was monitored by ATR-FTIR by tracking the changes in the functional groups of the reaction system. The AESO was further modified with MAA to replace the hydroxyl groups of AESO with methacrylate groups, generating a resin (IM-MAESO) with an improved degree of unsaturation. The chemical structure of IM and modification of AESO with MAA were characterized using 1H NMR, 13C NMR and ATR-FTIR analyses. The miscibility of IM with AESO was predicted according to Hansen’s solubility theory and evaluated experimentally. The formulated IM-AESO and IM-MAESO resins were compared with the pure AESO and IM resins in terms of their rheological behaviors, curing kinetic characteristics, flexural properties, dynamic mechanical properties, and thermal stabilities. The results indicated that both the IM-AESO resin and the IM-MAESO resin have much lower viscosities, activation energies, and curing temperatures as well as higher polymerization rates and curing degrees than pure AESO due to the incorporation of IM as a reactive diluent. The combination of stiff IM and flexible AESO results in biobased networks with superior flexural strength, flexural modulus, flexural strain, storage modulus, glass transition temperature, and thermal stability. Furthermore, the MAA modification gives rise to the crosslinking degree and hence stiffness of the IM-MAESO resin as a result of the increase in the unsaturation degree of the MAESO.

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1. INTRODUCTION Thermosetting resins are important matrices in fiber-reinforced composites due to their attractive qualities such as good physical and mechanical properties, thermal and chemical resistances, and processabilities. Currently, most commercially available thermosets are produced from petroleum chemicals; these techniques consume large amounts of oil and gas. Driven by increasing environmental concerns and depleting petroleum resources, there is increasing interest in the exploitation of biobased monomers and polymers from renewable resources.1-3 Vegetable oil, which consists of esters of glycerol with three long-chain fatty acids, is a promising feedstock for producing biobased thermosetting polymers.4,5 Acrylated epoxidized soybean oil (AESO) is derived from soybean oil via epoxidation followed by acrylation. AESO has been commercialized and utilized in the development of thermosetting resins for use in composites.6 However, AESO is highly viscous at room temperature and has a low crosslinking capacity due to its long aliphatic chains and low degree of unsaturation. Therefore, a reactive diluent (RD), i.e., a comonomer, is highly desired for AESO-based matrices because the RD could facilitate the synthesis of a resin with low viscosity to wet reinforcing fibers well and the formation of a three-dimensional network with high crosslinking density after the resin is cured. AESO-based thermosets with superior processability and physical-mechanical performance were extensively prepared by using styrene as an RD because styrene is miscible with AESO and can crosslink efficiently with it.7-10 However, styrene is listed as an anticipated carcinogen.11 Other vinyl monomers including vinyl toluene, methyl methacrylate, and divinylbenzene were used to replace styrene in maleinated AESO resins for the fabrication of biobased composites with glass/flax fibers.12 Hybridization of AESO with petroleum-based polymers including poly(methyl methacrylate)13, vinyl ester and vinyl ester-urethane14,15 could facilitate the formulation of resins with an interpenetrating network 3



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structure. However, they are petrochemical-based. To obtain soybean oil-based polymers with high biobased contents, considerable attention has been paid to developing RDs from renewable resources such as fatty acids,16-18 cellulose,19 lignin,19,20 sucrose,21 rosin acid,22 and itaconic acid.23 However, there are two drawbacks in impeding the application of these thermosets in fiber-reinforced composites: (1) Some of these renewable resources including lignin and vegetable oil have high molecular weights, which gives the resulting RDs high viscosities and therefore negatively affects the processability of the resulting AESO resins. The high viscosities of the resins also reduce the polymerization efficiencies of RDs with AESO, thus generating cured resins with low crosslinking densities. For instance, methacrylated vanillin is a solid at room temperature, and both methacrylated guaiacol and eugenol have much higher viscosities (17 and 28 mPa·s) than styrene (0.7 mPa·s) at room temperature.24 (2) These biobased RDs are most commonly synthesized from renewable materials via esterification with petroleum-based acids or oils, which generally requires the use of catalysts,21,25 organic solvents,26,27 elevated temperatures,23 and/or long reaction times.25,28 These harsh reaction conditions might generate side-reactions at high temperature, produce considerable amounts of solvent waste, have high energy consumption, and introduce residual metal catalysts that are difficult to remove. Isosorbide is one of the top 12 most promising renewable building blocks. It is derived from glucose, which is widely available from the depolymerization of biomass-based materials including cellulose and starch.29 Isosorbide is a chiral diol with a unique bicyclic ring structure, i.e., a V-shaped molecule composed of two cis-connected tetrahydrofuran rings and two secondary hydroxyl (–OH) groups in the 2- and 5- positions.30 These features have motivated numerous works on the preparation of isosorbide-based polymers with superior mechanical and thermal properties.31-34 Isosorbide is crystalline at room temperature and is immiscible with many acids and anhydrides; therefore, during the synthesis of 4


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isosorbide-based monomers or polymers without organic solvents, esterification occurs only at the phase boundary between the isosorbide and the acid with a low reaction rate. The present work aimed to synthesize a rigid and biobased monomer, i.e., isosorbide-methacrylate (IM), from isosorbide with methacrylic anhydride (MAA) and then to formulate soybean oil-based thermosets with high performance and renewable contents by using IM as a main structural component. The ultrasonic treatment was used for increasing the reaction rate between isosorbide and MAA. The synthesis process of IM was monitored by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The obtained IM was characterized by proton (1H) nuclear magnetic resonance (NMR), carbon (13C) NMR, and ATR-FTIR spectroscopies as well as solubility evaluation. The IM was blended with AESO to prepare the IM-AESO resin, and the modified IM-AESO (IM-MAESO) resin was obtained by directly blending AESO with the reaction products that resulted from the IM synthesis; the excess MAA in the mixture would react with the –OH groups of AESO, thus increasing the unsaturation degree of the resin. Both the IM-AESO and IM-MAESO resins were compared with the resins composed of pure AESO and IM monomers in terms of their rheological and curing behaviors, flexural properties, thermal stabilities, and dynamic mechanical properties.

2. EXPERIMENTAL SECTION 2.1. Materials AESO (average molecular weight: 1200 g/mol; viscosity (25 °C): 18,000–32,000 mPa·s; acid value: ≤ 10 mg KOH/g; inhibitor: 3,500–4,500 ppm monomethyl ether hydroquinone), MAA (94%, inhibitor: 2,000 ppm topanol A), 4-(dimethylamino)pyridine (DMAP, 99%), tert-butyl peroxybenzoate (TBPB, 98%), magnesium sulfate (MgSO4), sodium bicarbonate (NaHCO3), and sodium hydroxide (NaOH) were purchased from Sigma-Aldrich. Isosorbide 5



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(98%) was purchased from Fisher Scientific. All chemicals were used as received.

2.2. Synthesis of isosorbide-methacrylate (IM) Isosorbide (14.61 g, 0.1 mol) and DMAP (0.61 g, 0.05 mol) were suspended in MAA (46.25 g, 0.3 mol) in a 150 mL round bottom flask; the molar ratio of MAA to isosorbide was 3:1. A molar ratio of MAA to isosorbide of 2:1 was also used for the comparison of reaction production yield. The flask was placed in an ultrasonic water bath (ultrasound power: 80 W) and then stirred magnetically at a rate of 500 rpm at 60 °C for 12 h. Crystalline isosorbide was first suspended in MAA and then gradually dissolved into the liquid phase as the reaction progressed. After the reaction, the flask was cooled to room temperature, and the resulting mixture was sequentially washed with 0.5 mol/L NaOH, saturated NaHCO3 and water; dried over MgSO4; and concentrated under reduced pressure to generate a pale yellow oil, i.e., isosorbide-methacrylate (IM). The reaction between isosorbide and MAA is given in Scheme 1. The 1H NMR,

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C NMR and FTIR spectra of the obtained IM are provided in the

Supporting Information (SI1 and SI2). The IM yields were 92.3% and 66.5% for the 3:1 and 2:1 MAA/isosorbide systems, respectively.

Scheme 1 Synthesis of IM from isosorbide and MAA.

2.3. Preparation and modification of soybean oil-based resins AESO was mixed with IM at a weight ratio of 1:1 in a beaker at 70 °C using a magnetic stirrer at 500 rpm for 10 min. The resulting mixture was cooled to room temperature and 6


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designated IM-AESO. To increase the unsaturation degree and hence the crosslinking sites of the AESO-based resins, the MAA-isosorbide reaction product from Section 2.2 without purification (MAA: isosorbide = 3:1) was directly mixed with AESO to prepare the MAA-modified AESO resin (IM-MAESO). The mixture contains the generated IM and unreacted MAA, and their weighs are theoretically 28.23 g (0.1 mol) and 15.42 g (0.1 mol), respectively. To keep the weight ratio of AESO to IM at 1:1, AESO (28.23 g, 235.25 mmol) was added to the mixture, where the molar ratio of MAA to AESO was 4.26:1. The flask was placed in a water bath (without ultrasound), and the mixture continued to react at 60 °C for 6 h while being stirred at 500 rpm. With DMAP catalysis, the anhydride groups of MAA would react with the –OH groups of AESO to induce the grafting of methacrylic groups onto AESO molecules through ester linkages (Scheme 2), which are evidenced by FTIR and

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C NMR analyses (Supporting

Information, SI3). After the reaction was finished, the mixture was purified using the same procedure as in Section 2.2, and thus, the IM-MAESO resin containing IM, unmodified AESO and modified AESO was obtained.

Scheme 2 Proposed reaction during the modification of IM-AESO.

2.4. Polymerization of biobased thermosetting resins Four kinds of resin, i.e., pure AESO, IM-AESO, IM-MAESO and pure IM, were blended with 2 wt% TBPB (based on the total mixture) and then stirred for 2 min, respectively. The 7



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resulting mixtures were degassed under vacuum and transferred to silicon molds to prepare the cured resin samples. The curing reaction was performed at 120 °C for 2 h, followed by 160 °C for another 4 h in an oven. After being cooled to room temperature, the samples were removed from the mold for testing.

2.5. Evaluation of the solubility of IM The solubility of IM was measured by mixing IM with ethanol, furan, styrene, and N-vinyl-2-pyrrolidone (NVP), respectively. A predetermined 5 g of IM was added into a vial containing 5 g of the respective solvent. The mixture was stirred for 5 min by a glass rod and then kept at room temperature for 24 h. Unsaturated polyester (UPE, 5 g), synthesized from propylene glycol, isophthalic acid and fumaric acid,35 was also dissolved in IM (5 g) in a beaker at 70 °C. The results of the dissolution tests were recorded with a digital camera and divided into two categories, i.e., soluble and insoluble, based on visual inspection. The miscibilities of IM with water, methacrylic acid, MAA, AESO and MAESO were confirmed during the formulation of the IM and soybean oil-based resins. According to Hansen’s solubility theory,36,37 an RED value was proposed to evaluate the theoretical solubility of a substance in IM, i.e., IM can dissolve the substance if the RED value is less than 1, whereas it is insoluble when this value is higher than 1.38 A detailed calculation of the solubility parameters and RED values of the substances can be found in the Supporting Information (SI4).

2.6. Characterization The chemical structures of the IM monomer and the IM-AESO and IM-MAESO resins were characterized by ATR-FTIR, 1H NMR and

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C NMR analyses. During the reaction of

isosorbide with MAA, aliquots of the reaction mixture were collected at intervals and used 8


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instantly for FTIR tests. The FTIR tests were conducted on a PerkinElmer Spectrum One Spectrometer (PerkinElmer, USA) equipped with a 3 × bounce diamond crystal and an incident angle of 45°. Spectra were collected under the following conditions: 4000–650 cm-1 range; 4 cm-1 resolution; 16 scans. 1H NMR and 13C NMR spectra were recorded on a JEOL 600 MHz NMR spectrometer (JEOL, Japan) with CDCl3 as a solvent. The processabilities of the resins (pure AESO, IM-AESO, IM-MAESO and pure IM) were investigated based on their rheological and curing behaviors. The rheological behavior tests were carried out on an HAAKE MARS III rotational rheometer (Thermo Electron, USA) using a PP35Ti parallel plate (gap: 0.105 mm) at a shear rate of 10 γ/s and a heating rate of 10 °C/min from 25 to 100 °C. The curing kinetics were obtained from non-isothermal differential scanning calorimetry (DSC) scans at four different heating rates (5, 10, 15 and 20 °C/min) from 25 to 250 °C. The scans were conducted on an STA 449 F3 Jupiter Simultaneous Thermal Analyzer (NETZSCH, Germany), and the sample (5–10 mg) was placed in a standard porcelain crucible with a lid and tested under N2 (flow rate: 30 mL/min) protection. The flexural properties of the four cured resins were evaluated in compliance with ASTM D 790-10. Ten rectangular specimens (80 × 10 mm2) were tested using a CMT6104 microcomputer controlled electronic universal testing machine (MTS Systems, USA) at a crosshead rate of 10 mm/min. Dynamic mechanical analysis (DMA) of the resins was conducted with rectangular samples (55 × 10 mm2) using a DMA 242 (NETZSCH, Germany) machine. The measurements ranged from 25 to 250 °C under single cantilever bending mode. The heating rate was 5 °C/min, and the frequency was 1 Hz. Thermogravimetric analysis (TGA) of the resins was performed on the same testing equipment as the DSC analysis at a heating rate of 10 °C/min from 25 to 600 °C.

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3. RESULTS AND DISCUSSIONS 3.1. ATR-FTIR monitoring of the IM synthesis The reaction of isosorbide with MAA at different molar ratios was monitored via ATR-FTIR to determine the effect of the MAA concentration on the IM yield and the reaction rate. The FTIR spectra of the reaction mixture (MAA/isosorbide = 3:1) after different reaction intervals and the deconvoluted carbonyl region (1850–1575 cm-1) with Gaussian-Lorentzian functions are given in Figure 1. The following features of the isosorbide-MAA reaction were observed: (1) the band at 1780 cm-1 corresponding to the C=O stretch vibration of MAA decreased, and the peak at 1160 cm-1, assigned to the stretch vibration of new C−O−C ester groups, increased with the progress of the reaction; (2) the appearance and growth of a shoulder at 1696 cm-1 is assigned to the formation of carboxylic acid. The esterification of the anhydride groups of MAA with the –OH groups of isosorbide generates ester groups and byproducts, i.e., methacrylic acid (Scheme 1).

Figure 1 Characteristic ATR-FTIR spectra obtained from pure MAA, IM, and the reaction products during the synthesis of IM from isosorbide and MAA (molar ratio = 1: 3) after 30, 120 and 300 min. (a) Full spectra and (b) carbonyl region from 1850 to 1575 cm-1 deconvoluted 10


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with Gaussian-Lorentzian functions.

The characteristic FTIR peaks at 1780, 1696 and 1160 cm-1 of the reaction system were used to monitor the conversion of anhydride, acid and ester with respect to reaction time. The peak at 1637 cm-1, associated with the C=C bonds, was used for the normalization of the absorbance peaks. The detailed calculation methods are provided in the Supporting Information (SI5). The isosorbide-MAA esterification with the assistance of ultrasound can be divided into two stages (Figure 2): (1) For both systems with different MAA/isosorbide molar ratios, a sharp increase in the IM yield is observed during the initial stage of the reaction (reaction time shorter than 60 min), which indicates that the esterification of the –OH groups and anhydride groups is favored in the presence of the ultrasound assistance, resulting in significant consumption of MAA (xMAA) and the generation of methacrylic acid (xacid). In contrast to conventional chemical reactions, acoustic cavitation occurs in the ultrasonic reaction with steps of formation, growth, and implosive collapse of bubbles, which strongly disturbs the phase boundary between the solid isosorbide and the liquid MAA, thereby causing emulsification, enhancing the mass transfer process and increasing the reaction rate.39,40 (2) After 120 min of reaction, the reaction takes place slowly as indicated by the gradual changes in the IM yield, xMAA and xacid. As the reaction progresses, MAA is continuously consumed, and thus, the reaction is prevented due to the lack of MAA in the system. In addition, the –OH group at the C2 site of isosorbide is much less reactive than that at the C5 site due to the internal hydrogen bonds with the oxygen atom of the ether ring.28 Hence, the MAA tends to react with the –OH groups at the C2 site first and then with that at C5 site in a later stage of the reaction. These two effects would reduce the reaction rate of esterification significantly.

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Figure 2 (a) Yield of IM and (b) conversions of MAA (xMAA) and methacrylate acid (xacid) calculated from the ATR-FTIR spectra of the reaction products at different MAA/isosorbide molar ratios as functions of reaction time.

As shown in Figure 2, the reaction system with an MAA/isosorbide molar ratio of 3:1 shows a slightly higher increased rate in IM yield, decreased rate in xMAA, and generation rate in xacid than the 2:1 system in the initial stage of the reaction. The IM yield of the 3:1 system at equilibrium is close to 100%, i.e., full conversion of isosorbide, which is much higher than that of the 2:1 system (approximately 77%) (Figure 2a). This is in agreement with the IM yields from the experimental results, although the latter (92.3% and 66.5%) is much lower than the calculated values from the FTIR spectra because of the weight loss in the IM purification process. Therefore, an excess MAA system, i.e., 3:1 of MAA/isosorbide, was chosen for the esterification to obtain the IM at a full conversion of isosorbide, where the reaction was almost finished after 300 min. For the conversion of MAA and methacrylic acid, the xMAA and xacid of the 2:1 system (89%) at the final stage are much higher than those of the 3:1 system (78%) because the 3:1 system has a very high and excess MAA concentration (Figure 2b). For both systems, the xMAA is almost equal to the xacid during the reaction process, which confirms the assumption that an anhydride group reacts only with a –OH group of isosorbide to generate an ester and an acid (Scheme 1). This is in line with the reported fact that isosorbide hardly reacts with acids without unique conditions due to the low reactivity of 12


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the secondary alcohols at the C2 and C5 sites.28

3.2. Solubility of IM monomer The predicted and experimental solubility of IM with various substances are presented in Table 1. Pictures of the test results obtained using a digital camera are given in the Supporting Information (SI6). The experimental results are completely consistent with the predicted ones, indicating that IM is miscible with all of the tested substances except water. As one of the precursors for IM, isosorbide exhibits hydrogen bonding between the two secondary –OH groups; therefore, it is easy to dissolve in water but not in MAA due to its high polarity. However, as the esterification between isosorbide and MAA proceeds, the –OH groups on isosorbide are converted into methacrylic groups, producing IM with greatly reduced polarity. Hence, IM can dissolve in most common organic solvents, such as ethanol and furan, but not in water. This is also consistent with the high RED value (1.87) of the water-IM pair. The miscibilities of IM with MAA and methacrylic acid are much higher than those with the others due to the lower RED values of the MAA-IM and methacrylic acid-IM systems. Additionally, IM is capable of mixing with both styrene and NVP, and thus, it has the potential for forming copolymers with these two monomers; NVP has been used as an RD for preparing styrene-free UPE and soybean oil-based resins in hemp fiber composites.35,41 The dissolving capacity of IM in UPE is similar to that of styrene because of their close RED values, indicating that IM has the potential to replace styrene in UPE resins. The IM-AESO and IM-MAESO systems possess similar RED values that are less than 0.2, indicating the superior miscibility of IM with unmodified and MAA-modified AESO.

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Table 1 The miscibility of IM with various substances and their predicted solubility based on Hansen’s theory. Monomers

RED valuea

Miscibility of IM with various substances Predicted

Experimental

IM / / / MAA 0.17 + + methacrylic acid 0.12 + + b water 1.87 －－ ethanol 0.57 + + furan 0.23 + + styrene 0.51 + + NVP 0.36 + + 0.49 + + UPEc AESO 0.18 + + MAESO 0.17 + + a RED values were calculated according to the Hansen’s theory and group contribution method (Supporting Information, SI4); bThe value of water was reported by Zhang et al;38 cUPE is synthesized from propylene glycol, isophthalic acid and fumaric acid.35

3.3. Rheological analysis of the resins The main concern of developing soybean oil-based thermosets for fiber-reinforced composites is their viscosities, especially in the liquid molding technique (resin viscosity below 500 mPa·s).42 Due to the improved mobility of polymer chains at higher temperatures, the viscosities of all resins decrease exponentially with increasing temperature (Figure 3), which can be fitted with the Arrhenius equation:16 η = η0 exp[Eη /(RT)]

(1)

where η is the apparent viscosity, η0 is the prefactor, Eη is the activation energy, R is the ideal gas constant, and T is the absolute temperature. As presented in Table 2, the neat AESO resin has the highest viscosity and Eη value because of the restricted molecular mobility as provided by the long polymer chain and high molecular weight of the AESO monomer. By contrast, the lowest viscosity and Eη value are observed for IM monomer, indicating that IM flows easily and the influence of temperature on its viscosity is insignificant. Therefore, after 14


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the incorporation of IM into AESO resins, IM-AESO resin has a considerably decreased viscosity and Eη value because IM is a small molecule, which can alleviate the entanglement of AESO molecular chains and hence increase flowability of the resulting resin. The viscosity of IM-MAESO is 22.6% higher than that of IM-AESO due to the increased molecular weight of MAESO compared to AESO as a result of the grafting of methacrylic groups on AESO; however, this does not significantly influence the Eη values of the two resins. In addition, the viscosities of both IM-AESO and IM-MAESO are much lower than that of a typical UPE resin with 40 wt% styrene as an RD, indicating that the soybean oil-based resins have more superior processability than the traditional UPE resins in the fabrication of fiber-reinforced composites.35

Table 2 Calculated Arrhenius parameters of AESO, IM, IM-AESO and IM-MAESO resins. Resins

η (mPa·s) (30 °C)

AESO 4789 ± 69 a Styrene-UPE 375 IM-AESO 151 ± 1 IM-MAESO 186 ± 7 IM 12 ± 1 a Typical UPE resin with 40 wt% styrene as an RD.35

η0 (mPa·s)

Eη (kJ/mol)

R2

2.1 × 10-5 1.0 × 10-4 4.0 × 10-4 3.5 × 10-4 4.4 × 10-1

48.3 32.2 33.2 34.1 8.5

0.9938 0.9854 0.9624 0.9631 0.9485

Figure 3 Change in the viscosity of IM, AESO, IM-AESO and IM-MAESO versus temperature.

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3.4. Curing behaviors of the resins The curing processes of the four kinds of resins initiated by TBPB were monitored by DSC scans at a heating rate of 10 °C/min (Figure 4a). The DSC curves of all the resins display two distinct exothermic peaks associated with the free-radical polymerization of the main components in the resin systems. TBPB decomposes slowly without a promoter at low temperature because it is a high temperature initiator, and the thermal decomposition of TBPB starts at 65–100 °C and reaches a peak at 135–165 °C.43,44 Therefore, all of the resins start to polymerize at 90–105 °C with the generation of free-radicals from the initial decomposition of TBPB, which contributes to the first exothermic peak (TP1) at 115–125 °C in the DSC curves (Figure 4a). However, the polymerization is incomplete, and some C=C bonds remain due to the lack of sufficient free-radicals and the confined molecular mobility of the AESO and IM oligomers. Furthermore, the increase in temperature would increase the molecular mobility of the polymer chains and accelerate the decomposition of TBPB, which would further facilitate the polymerization of the resin systems. This explains the second exothermic peak (TP2) in the DSC curves of the resins because the peak temperatures (160–190 °C) are almost higher than the maximum thermal decomposition temperature of TBPB (Table 3). The lower temperature and higher intensity of the peak in the DSC curve indicate the higher reactivity of the compounds in the curing reaction.45 Taking the resins cured at a heating rate of 10 °C/min as an example (Figure 4a), the peak height in the DSC curve of pure AESO is much lower than those of other resins, showing a lower polymerization efficiency of AESO resin due to its high viscosity and low unsaturation level. As shown in Table 3, the TP1 of IM is slightly lower than that of AESO, whereas the DSC curve of IM has a higher peak intensity than that of AESO, indicating that IM has a higher reactivity than AESO due to the lower viscosity of IM with respect to AESO. However, the higher reactivity 16


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of IM results in an IM oligomer with a higher crosslinking degree, which would restrict the molecular mobility and thus increase the TP2 of IM when compared to that of AESO in the second stage (Table 3). For the blends of IM and AESO, a significantly decreased TP1 is observed for IM-AESO when compared to the pure IM and AESO; however, the TP2 of IM-AESO is between those of pure IM and AESO (Table 3). This is probably because of the conversion of resin curing from homopolymerization to copolymerization. Additionally, the MAA modification for AESO does not significantly affect the TP1 and TP2 but results in an reduced peak intensity in the DSC curve, which is most likely due to the increased molecular weight and thus increased viscosity.

Figure 4 DSC curves of (a) IM, AESO, IM-AESO and IM-MAESO systems at a heating rate of 10 °C/min and (b) IM-AESO resin at different heating rates.

17



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Table 3 Characteristic curing temperatures obtained from DSC curves and calculated curing kinetic parameters based on Kissinger’s theory. Resins AESO IM IM-AESO IM-MAESO

Peaks

TP (°C) at different heating rate (°C/min) 5 10 15 20

E (kJ/mol)

lnA

R2

P1 P2 P1 P2 P1 P2 P1 P2

115.7 161.4 116.5 176.4 109.8 163.6 111.1 163.7

83.4 124.6 129.6 127.2 102.0 105.6 98.5 109.0

17.83 26.71 32.52 26.20 24.29 21.10 23.06 22.06

0.9902 0.9664 0.9530 0.9790 0.9899 0.9930 0.9780 0.9931

124.2 167.8 121.0 183.3 116.6 172.3 117.5 172.1

131.0 175.2 127.0 190.4 122.5 179.1 123.6 178.9

136.0 177.6 128.9 193.9 125.7 183.9 127.8 183.2

The non-isothermal curing kinetics of resins was further investigated by DSC scans with different heating rates. The DSC curves of IM-AESO show that the resin curing characteristic temperatures (TP1 and TP2) move obviously to high temperatures as the heating rate increases due to the thermal lag effect (Figure 4b). Based on Kissinger’s theory, the activation energy of the curing reaction was determined from the peak temperatures at different heating rates using the following equation:46,47 ln (β/TP2) = ln[(AR)/E] – E/(RTP)

(2)

where β is the heating rate, TP is the exothermic peak temperature, A is the pre-exponential factor, E is the activation energy, and R is the gas constant. The calculated kinetic parameters of the resins are given in Table 3. The E presents how much energy is needed for the reaction to proceed in the forward direction.48 The E values of AESO at TP1 and TP2 are 83.4 and 124.6 kJ/mol, respectively (Table 3), which are considerably higher than those of similar acrylated epoxidized hemp oil-based thermosets (45–59 kJ/mol) due to the lack of styrene as an RD.49 The EP1 of AESO is much lower than that of IM as a result of the higher reactivity of acrylic groups than methacrylic groups; however, a comparable EP2 was noticed between AESO and IM because of the limited molecular mobility. Furthermore, it was reported that in a methacrylate/acrylate system, the reactivity ratios (r) of the comonomer pairs usually follow 18


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r1 (methacrylate) > 1 > r2 (acrylate),50 which indicates that the IM-AESO resin has a tendency towards copolymerization with a higher incorporation rate of IM (methacrylate) into the copolymers than AESO (acrylate). This is responsible for the fact that the IM-AESO has a significantly higher EP1 than AESO but lower than IM. Comparing IM-MAESO to IM-AESO, an increased EP1 and a decreased EP2 are obtained because the IM-MAESO with a higher functionality would promote the crosslinking initially but hinder the process in the second stage due to the higher crosslinking level in the first stage.

3.5. Flexural properties of cured thermosets The flexural properties of the crosslinked biobased thermosets are given in Table 4. Pure AESO has the lowest flexural strength and modulus but the highest flexural strain among the four resins, which is attributed to the low crosslinking degree of the neat AESO network as a result of the low unsaturation degree and long aliphatic molecular chains of the AESO monomer. The flexural strength and modulus of the AESO networks increase considerably, and the flexural strain dramatically decreases after the incorporation of IM. The incorporated IM would participate in the free-radical polymerization of the AESO system such that a cured AESO resin with a high crosslinking density is obtained. A higher crosslinking density would result in improvements in the strength, modulus and brittleness. The introduced IM functions as a crosslinking agent, giving rise to the crosslinking density and rigidity while reducing the toughness of the resulting resin, i.e., the improved crosslinking density results in the cured resin fracturing via a fragile failure with low energy absorption. Furthermore, a 19.4% enhancement in flexural strength of the resin is observed after the modification of AESO with MAA, while no appreciable difference in flexural modulus and flexural strain are measured between IM-AESO and IM-MAESO. The MAA modification increases the number of C=C bonds of the AESO monomer, thus generating IM-MAESO with an increased crosslinking 19



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density. Additionally, IM is a monomer with two C=C bonds, and therefore, it can self-polymerize to form a thermoset with a three-dimensional network. The flexural strength of neat IM resin is lower than those of IM blends with modified soybean oil. This is most likely due to the incomplete crosslinking of C=C bonds of IM monomers resulting in a highly defective network in the cured resin, which will be further discussed in Section 3.6. However, in contrast to the soft fatty acids from AESO, the unique bicyclic ring structure from isosorbide predominately provides stiffness for the cured IM resin. Therefore, a higher flexural modulus and lower flexural strain are seen in the pure IM resin when compared to the IM-AESO and IM-MAESO resins. Table 4 Flexural and dynamic mechanical properties of the crosslinked thermosets. Samples

Flexural strength (MPa)

Flexural modulus (MPa)

Flexural strain at break (%)

E’ at 30 °C (MPa)

Tg (°C)

ve × 103 (mol m3)

AESO IM-AESO IM-MAESO IM

4.8 ± 1.1 28.4 ± 4.5 33.9 ± 2.7 23.5 ± 4.3

62 ± 17 1373 ± 153 1546 ± 162 2060 ± 168

10.4 ± 0.9 2.1 ± 0.3 1.9 ± 0.2 1.1 ± 0.4

303 1961 2133 2736

75.6 94.9 142.2

16.0a 75.2 83.3 59.7

Degree of cureb (%) 81.0 97.6 88.3 85.4

a

The crosslinking density (ve) of pure AESO was calculated from the E’ at 70 °C according to the reported Tg (14–35 °C) value of the neat AESO network, which was not detected in our work;[14,23,27] bDegree of cure for the resins was calculated from their FTIR spectra before and after curing based on the consumption of vinyl groups in the resin systems (Supporting Information, SI7).

3.6. Dynamic mechanical properties of the cured thermosets The storage modulus (E’) and loss factor (tan δ) of the cured networks are plotted as functions of temperature (Figure 5). E’ indicates the energy stored during cyclic deformation, and tan δ shows the damping behavior under vibrating conditions.51 The glass transition temperature (Tg) was determined from the peak position of the tan δ curve, and the crosslinking density (ve) was further estimated according to the rubbery elastic theory as Equation 6:51,52 ve = E’/(3RT)

(3) 20


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where E’ is the storage modulus above Tg in the rubbery plateau region (E’s at Tg + 40 °C for all networks were chosen in this study), R is the gas constant, and T is the absolute temperature. The E’ at 30 °C, Tg and ve of the resins are listed in Table 4. Pure AESO has the lowest E’, indicating the weak interaction in the cured network due to its low crosslinking density (16.0 mol/m3) as restricted by its long molecular chains. This is in agreement with the calculated cure degree of the AESO network (81.0%) from its FTIR spectra before and after curing (Supporting Information, SI7). However, the addition of IM into AESO resin results in the generation of a cured IM-AESO network with a high degree of cure (97.6%) and crosslinking density (75.2 mol/m3). This contributes to the substantial increases in the E’ and Tg of IM-AESO compared to those of pure AESO. As mentioned in Section 3.4, the reactivity ratio of AESO in the IM-AESO system was lower than 1, indicating that AESO tends to copolymerize with IM, which facilitates the conversion of the C=C bonds of AESO and hence increases the cure degree of the IM-AESO resin. The Tg of IM-AESO (75.6 °C) is comparable with that (71–78 °C) of the AESO resin with styrene as an RD,16,17 which is also because the incorporated IM has a peculiar bicyclic ring structure and thus superior thermal stability. After modifying AESO with MAA, the E’ at 30 °C and Tg increase from 1961 MPa and 75.6 °C for IM-AESO to 2133 MPa and 94.9 °C for IM-MAESO, respectively, which is due to the enhanced crosslinking density from 75.2 to 83.3 mol/m3. However, the increased unsaturated sites of IM-MAESO did not completely participate in the free-radical polymerization, hence forming an imperfect network with a low degree of cure (88.3%). The neat IM has much higher E’ and Tg but lower crosslinking density and curing extent than IM-AESO and IM-MAESO. The former is attributed to the unique structure of isosorbide, whereas the latter is associated with the formation of an imperfect network in the cured IM resin. As a small molecule and divinyl monomer, IM is extremely difficult to form a perfect network with full conversion of C=C bonds. During the polymerization of IM monomer, once 21



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one methacrylic group of IM molecule polymerizes, the molecule with an unreacted C=C bond would be locked in the network.53 Therefore, the mobility of the attached C=C bond is dramatically reduced and the network with high degree of unsaturated sites (i.e., incomplete network) would form as the curing proceeds. In addition, the broader tan δ curve and a shoulder peak at high temperature in the curve of the cured IM resin confirm the formation of a wide range of segmental units that resulted from the incomplete three-dimensional network with different degrees of crosslinking.

Figure 5 DMA curves of IM, AESO, IM-AESO and IM-MAESO thermosets. (a) Storage modulus; (b) damping parameter.

3.7. Thermal stabilities of the cured thermosets The thermal degradation behavior of the cured thermosets was evaluated by TGA (Figure 6 and Table 5). The IM-AESO, IM-MAESO and IM networks show considerably higher maximum weight loss temperatures (Tmax) than the pure AESO network due to the enhanced crosslinking density and the introduced isosorbide structure, which has superior thermal stability. However, an obvious degradation in the initial stage occurs in IM and IM-MAESO resins; in particular, IM has a wide and significant decomposition at 250–350 °C. To specify the overall thermal stability of the cured thermosets, the heat-resistant index (Ts) was determined from the 5% and 30% weight loss temperatures (T5 and T30) according to the 22


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following equation:27,54 Ts = 0.49[T5 + 0.6(T30 – T5)]

(4)

As given in Table 5, the IM-AESO has the highest calculated Ts among all the networks because of its highest degree of cure. The thermal stability is not only associated with the crosslinking density of the cured resins but also has a close relationship with the chemical structure of the resins. As discussed in Section 3.6, both IM-MAESO and IM networks are incomplete and have a number of unsaturated chain-ends inside the networks, which easily decompose with increased temperature. This is evidenced by the significantly lower T5 and hence reduced Ts of the thermosets when compared to AESO and IM-AESO.

Figure 6 (a) TG and (b) DTG curves of the IM, AESO, IM-AESO and IM-MAESO thermosets.

Table 5 Thermal stability data of the cured thermosets obtained from the TGA results. Thermoset AESO IM-AESO IM-MAESO IM

Characteristic weight loss temperature (°C) Tmax

T5

T30

385.7 424.0 417.3 432.2

318.5 312.0 289.8 271.4

375.5 387.1 381.1 393.4

Ts 172.8 175.0 168.8 168.9

4. CONCLUSION IM was successfully synthesized from isosorbide and MAA via a benign route and used 23



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to crosslink AESO for the formulation of soybean oil-based thermosets with high performances and renewable contents. ATR-FTIR analysis monitoring the synthesis of IM indicated that ultrasonic assistance was an effective method to accelerate the esterification between isosorbide and MAA in the absence of organic solvents; and the reaction rate and IM yield were greatly influenced by the molar ratio of MAA to isosorbide. The predicted solubility and experimental tests revealed that IM had superior miscibility with various substances and could effectively replace carcinogenic styrene in soybean oil- and UPE-based resins. Furthermore, the formulated IM-AESO blend was further modified with MAA, which resulted in a modified resin (IM-MAESO) with a higher level of C=C bonds. The soybean oil-based resins blended with IM as an RD (IM-AESO and IM-MAESO) had superior processability due to the low viscosity. DSC analysis demonstrated a significantly increased curing efficiency in the blends of modified soybean oil with IM when compared to the pure IM and AESO resins. In addition, both the IM-AESO and IM-MAESO thermosets showed much higher flexural strengths, flexural moduli, storage moduli, and Tg but lower flexural strains than the neat AESO because of the increased crosslinking density and cure degree. A slight increase in strength and modulus was observed in the biobased thermoset after the modification of AESO with MAA. The neat IM resin also had higher flexural modulus, storage modulus and Tg than those of both IM-AESO and IM-MAESO due to the structural characteristic of IM; however, a much lower flexural strength was obtained for the IM resin because of the incomplete network formation. In summary, IM exhibits promise as a new biobased crosslinking agent for formulating soybean oil-based thermosets with superior processability and high performance for potential application in fiber-reinforced composites. 24


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ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. ATR-FTIR, 1H NMR and

13

C NMR spectra of IM; ATR-FTIR and

13

C NMR spectra of

IM-AESO and IM-MAESO resins; calculation of solubility parameters and RED values of IM and various chemicals according to the Hansen solubility theory; pictures of solubility evaluation results of IM with various chemicals obtained from digital camera; calculation of conversion of MAA, acid and ester during the synthesis of IM from the ATR-FTIR spectra of reaction systems; calculation of cure degree for resins from ATR-FTIR spectra.

AUTHOR INFORMATION Corresponding Author *Address: College of Transportation and Civil Engineering, Fujian Agriculture and Forestry University, 15 Shangxiadian Road, Jinshan, Fuzhou, Fujian, 350002, P. R. China. Phone: 86-591-83726495. Fax: 86-591-83715175. E-mail: [email protected] (R.Q.).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully thank the funding from Fujian Agriculture and Forestry University (Grant No. 1122YB019, KXB16007A), the National Natural Science Foundation of China (Grant No. 25



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31670568), the Ministry of Education, China (Grant No. 220133515110015), and the State Administration of Forestry, China (Grant No. 2014-4-41).

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Synopsis: A rigid isosorbide-based monomer was synthesized via a facile and solvent-free method and then used to fabricate biobased thermosets with high performance and renewable content through copolymerization with flexible soybean oil derivative.

Biobased Thermosets Prepared from Rigid Isosorbide and Flexible Soybean Oil Derivatives Wendi Liu†, Tianshun Xie†, and Renhui Qiu*, ‡

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Biobased Thermosets Prepared from Rigid Isosorbide and Flexible

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