Tung Oil-Based Unsaturated Co-ester Macromonomer for

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Tung oil-based unsaturated co-ester macromonomer for thermosetting polymers: Synergetic synthesis and copolymerization with styrene Chengguo Liu, qianqian shang, Puyou Jia, Yan Dai, Yonghong Zhou, and Zengshe Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00466 • Publication Date (Web): 06 May 2016 Downloaded from http://pubs.acs.org on May 14, 2016

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ACS Sustainable Chemistry & Engineering

Tung oil-based Unsaturated co-Ester Macromonomer for Thermosetting Polymers: Synergetic Synthesis and Copolymerization with Styrene§ Chengguo Liu,† Qianqian Shang,† Puyou Jia,† Yan Dai,† Yonghong Zhou*,† and Zengshe Liu*,‡

†

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 ‡

Bio-Oils Research, National Center for Agricultural Utilization Research, Agricultural

Research Service, United States Department of Agriculture, 1815 N. University St., Peoria IL 61604, United States

*Yonghong Zhou. E-mail: [email protected]. Tel.: + 86-25-854825777; Fax: + 86-25-854825777. *Zengshe Liu. E-mail: [email protected]. Tel.: +1 309-681-6104; Fax: +1 309-681-6524.

§

Mention of trade names or commercial products in this publication is solely for the purpose

of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.

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ACS Paragon Plus Environment


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ABSTRACT A novel unsaturated co-ester (co-UE) macromonomer containing both maleates and acrylates was synthesized from tung oil (TO) and its chemical structure was characterized by FT-IR, 1H NMR, 13C NMR, and gel permeation chromatography (GPC). The monomer was synthesized via a new synergetic modification of TO, by introducing maleic groups first and acrylic groups subsequently onto TO molecules. The influence of experimental factors on thermo-mechanical properties of the cured bioresins was evaluated to better understand structure-property relationships of the biomaterials and optimize experimental conditions. The obtained TO-based co-UE monomer possessed a highly polymerizable C═C functionality, consequently resulting in rigid bioplastics with high crosslink densities (νe) and excellent mechanical properties. For instance, the bioplastic prepared under the optimal synthesis conditions demonstrated a νe of 4.03×103 mol/m3, storage modulus at 25 °C of 2.40 GPa, glass transition temperature (Tg) of 127 °C, as well as tensile strength and modulus at 36.3 MPa and 1.70 GPa, respectively. A new theory for determining optimal comonomer concentration was further developed according to the copolymerization equation. The proposed theory accurately predicted the best styrene dosage for the co-UE monomer. At last, the hydroxyethyl acrylate (HEA)-modified TO-based resin was compared with the unmodified one in thermo-mechanical properties, thermal stability, microstructural morphologies, and curing behaviors. The new co-UE bioresin showed higher C═C functionality and crosslink density, superior properties including Tg and thermal stability, and similar curing behaviors. The developed eco-friendly rigid biomaterials provide potential application in structural plastics such as sheet molding compounds.

KEYWORDS: Tung oil, Unsaturated co-ester, Thermosetting polymer, Unsaturated polyester resin (UPR), Synergetic modification, Azeotropic copolymerization, Curing behavior

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INTRODUCTION Unsaturated polyester resins (UPRs), the mixture of unsaturated polyesters obtained by the polycondensation of unsaturated and saturated diacids with diols and vinyl comonomers like styrene, are broadly utilized in industrial and domestic areas.1, 2 However, due to the uncertainties of petroleum supply and price in the future as well as environmental pollution concerns, there is a growing interest in developing polymeric materials starting from renewable resources, such as natural oils, lignin, carbohydrates, and proteins.3 Plant oils are an important bioresource because of their low cost, triglyceride structure which is suitable for further chemical manipulation and biodegradability.4 As a result, alternative UPRs from plant oils have received extensive investigation recently.5-9 When using plant oils to synthesize UPRs for structural plastics, it is not common that plant oils were converted into the UPR feedstocks such as diacids/diols. Generally, researchers preferred to introduce unsaturated moieties onto triglycerides or their derivatives to acquire biobased unsaturated ester (UE) macromonomers which can copolymerize with diluent monomers as petroleum-based UPRs. One of the frequently-introduced moieties is maleic anhydride (MA).5,7,10 The MA was usually introduced through the maleination of hydroxyls on plant oils. The hydroxyl groups can be found naturally as in the case of castor oil, or can be brought in by the transesterification of plant oil with polyols such as pentaerythritol (PER) and glycerol, 5,7,10 the ene reaction between plant oil triglycerides and paraformaldehyde,11 etc. Other frequently-used moieties are acrylic compounds such as acrylic acid and methylacrylic acid.12,13 The acrylic structure was usually introduced through the ring-opening reaction of epoxy groups on epoxidized plant oils. One-step acrylation of plant oils was also achieved via Lewis acid-catalyzed addition,14 Ritter reaction,15 or simultaneous bromination and acrylation.16 In order to introduce more unsaturated groups onto triglycerides or their derivatives, MA was employed to further modify the acrylated oil-based macromonomers such as acrylated epoxidized soybean oil and dimer fatty acid (FA) polymerized glycidyl methacrylate.6,17 The resultant macromonomers comprised two types of polymerizable moieties, therefore can be named as unsaturated co-esters (co-UEs). The use of MA added more crosslink sites on these macromonomers, thus raising the crosslink densities (νe) and the properties such as stiffness and glass transition temperature (Tg) of the obtained 3



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bioplastics. Nevertheless, few contributions concerning this type of synergetic modification were reported. In our previous work,7,18 we synthesized tung oil pentaerythritol glyceride maleates (TOPERMA) macromonomer by the alcoholysis reaction of tung oil (TO) with PER and the following maleination of alcoholysis product, as shown in Scheme 1. However, we found that anhydride adducts formed in the TOPERMA product due to the Diels-Alder (D-A) addition between TO conjugated triene and MA. The occurrence of the D-A reaction not only consumed the C r bonds in the MA molecule, but also reduced the amount of TO conjugated triene which can also copolymerize with styrene.

Scheme 1. Synthesis of TOPERMA-HEA macromonomer from tung oil.

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In order to introduce more unsaturation onto the TOPERMA monomer, hydroxyethyl acrylate (HEA) was utilized to react with the anhydride structure on TOPERMA (Scheme 1) in this work. Thus a new tung oil-based co-UE macromonomer (TOPERMA-HEA) was prepared and the undesirable D-A structure was transformed into a new polymerizable unit. To the best of our knowledge, this is the first reported on synthesis of oil-based co-UE monomers by introducing first the maleic groups and subsequently the acrylic groups onto plant oils. The structure of the TOPERMA-HEA monomer was characterized by FT-IR, 1H NMR, 13C NMR, and gel permeation chromatography (GPC). The effects of experimental factors on properties of the cured bioresins were evaluated by dynamic mechanical analysis (DMA) to explore structure-property relationships of the biomaterials and to find optimal experimental conditions. In addition, we hoped that the added unsaturation on TOPERMA-HEA would increase the νe of the resultant biomaterial, thus leading to a better performance than that of the TOPERMA material. Therefore, the thermo-mechanical properties, thermal stability, microstructural morphologies, and curing behaviors of the TOPERMA and TOPERMA-HEA resins were reported.

EXPERIMENTAL SECTION Materials. Tung oil (TO) was purchased from Jiangsu Donghu Bioenergy Plant Plantation Co., Ltd. (China). It had a yellow color and a specific gravity of 0.935-0.940 at 25 °C. The solid PER (≥98%) and MA (≥99.5%) were obtained from Nanjing Chemical Reagent Co., Ltd. (China). The HEA liquid (≥97%) was purchased from Macklin Chemical Reagent Co., Ltd. (China). Calcium hydroxide (Ca(OH)2, ≥95%) and N,N-dimethyl benzyl amine (≥98%) were obtained from Tianjin Chemical Reagent Institute Co., Ltd. (China). Hydroquinone (≥99%) and styrene (≥99%) were obtained from Chengdu Kelong Chemical Reagent Co., Ltd. (China). The initiator tert-butyl peroxy benzoate (t-BPB, ≥98%) was obtained from Shanghai Aladdin Chemistry 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 TOPERMA. The TOPERMA was synthesized according to our previous report.7 Typically, 174.4 g (0.2 mol) of TO, 54.4 g (0.4 mol) of PER, and 2.3 g of Ca(OH)2 5



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were placed together in a 500 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 flask was placed in a heating mantle with a temperature controller. The reaction mixture was heated to about 230 °C and agitated under N2 atmosphere for 2 h at this temperature. At the end of 2 h, the product was cooled by ice water to room temperature. The tung oil pentaerythritol (TOPER) at room temperature was a light brown and viscous liquid. For the maleinization reaction, 57.8 g of the obtained TOPER product, 44.1 g (0.45 mol) of MA, and 0.102 g of hydroquinone were added to a similar reaction device as in the alcoholysis process but with a 250 mL four-neck round-bottom flask. The mixture was heated to 70 °C by an oil bath and agitated at this temperature until the MA completely melted and mixed with TOPER. 1.01 g N,N-dimethyl benzylamine was added and the reaction mixture heated to 95 °C under N2 atmosphere. The mixture was agitated at this temperature for 5 h. The maleinated product (TOPERMA) was a light yellow and opaque solid when cooled naturally to room temperature. 1

H NMR (CDCl3, δ ppm): 11.12 (s) –COOH; 7.06 (s) –OCOCH=CHCOO– from

unreacted MA; 6.88 (s) –OCOCH=CHCOOH trans; 6.33 (m) –CH=CHCOOH cis; 5.85–5.35 (m) –CH=CH–CH(CH)–CH=CH–CH(CH)–; 5.29 (s) –CH2–CH(OCO)–CH2–; 4.50–4.01 (m) –CH2O(CO)–; 3.49–3.01 (m) –OCOCH(CH)–CH(CH)COOH; 2.80 (m) –CH=CH–CH(CH)– CH=CH–; 2.33 (m) –CH2(CO)O–; 2.05 (m) –CH2–CH=CH–; 1.59 (m) –CH2–CH2(CO)O–; 1.48–1.01 (m) –CH2–; 0.88 (t) –CH3. 13

C NMR (CDCl3, δ ppm): 173.5 –(C=O)O– from FA chains; 168.1 –(C=O)O– from

anhydride structures on FA chains; 167.2 –(C=O)OH from maleic moieties; 165.0 –(C=O)O– from maleic moieties; 133.2–126.7 –CH=CH– from maleic moieties and FA chains; 62.8– 61.5 –CH2O(CO)–; 46.0–41.5 –COCH(CH)–CH(CH)CO–, C(CH2)4–; 33.6–22.1 –CH2–; 13.6 –CH3.

Synthesis of TOPERMA-HEA. When the synthesis of TOPERMA reached completion, a certain amount of HEA (e.g. 8.71 g) was immediately added into the TOPERMA product at 95 °C with a feed ratio of HEA:TO in the range of 1:2. The mixture was still agitated at 95 °C for 5 h. In the work-up procedures, the TOPERMA obtained was washed three times with a 6


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10 wt% NaCl/water solution (>80 °C) using a separating funnel to remove the excess small molecules such as MA and HEA. Then the product was dissolved in dichloromethane and dried by anhydrous magnesium sulfate overnight. At last, the solution was filtered and subsequently evaporated via the rotary evaporation method to remove solvent under reduced pressure. The resulting TOPERMA-HEA resin was a light brown and transparent solid when cooled naturally to room temperature (with a yield of 61.5%). 1

H NMR (CDCl3, δ ppm): 9.40 (s) –COOH; 6.86 (s) –OCOCH=CHCOOH trans; 6.42

(m) –OCOCH2=CH2 cis; 6.32 (m) –CH=CHCOOH cis; 6.14 (t) –OCOCH=CH2; 5.85 (m) – OCOCH=CH2 trans; 5.70–5.35 (m) –CH=CH–CH(CH)–CH=CH–CH(CH)–; 5.29 (s) –CH2– CH(OCO)–CH2–; 4.53–4.01 (m) –CH2O(CO)–; 3.49–3.01 (m) –OCOCH(CH)– CH(CH)COOH; 2.78 (m) –CH=CH–CH(CH)–CH=CH–; 2.32 (m) –CH2(CO)O–; 2.03 (m) – CH2–CH=CH–; 1.58 (m) –CH2–CH2(CO)O–; 1.48–1.01 (m) –CH2–; 0.88 (t) –CH3. 13

C NMR (CDCl3, δ ppm): 173.5 –(C=O)O– from FA chains; 171.0 –(C=O)O– from

HEA moieties; 167.7 –(C=O)O– from anhydride structures on FA chains; 167.0 –(C=O)OH from maleic moieties; 165.0 –(C=O)O– from maleic moieties; 133.2–126.7 –CH=CH– from maleic moieties, HEA moieties, and FA chains; 67.2–65.5 –CHO(CO)–; 63.1–61.3 – CH2O(CO)–; 46.0–41.5 –COCH(CH)–CH(CH)CO–, C(CH2)4–; 33.5–22.1 –CH2–; 13.6 – CH3.

Curing of TOPERMA-HEA and TOPERMA Resins. For the DMA test samples’ preparation, the obtained TOPERMA-HEA and TOPERMA resins were blended with specified amounts of styrene and t-BPB initiator for 30 min, then degassed under reduced pressure, and finally poured into a polytetrafluoroethylene (PTFE) mold with a specified size suitable for the DMA test. For the mechanical test samples’ preparation, when the synthesis of TOPERMA-HEA monomer reached the end (still at 95 °C), a certain amount of styrene (e.g. 40%) with 0.1% hydroquinone of the styrene weight was added into the hot mixture and kept agitating at 85 °C for 1 h. Afterwards, 4% of t-BPB was added and mixed for another 30 min, and then degassed. At last the resin was poured into a PTFE mold with a specified size suitable for the mechanical test. All the resins were cured at 110 °C for 3 h and postcured at 150 °C for 2 h. 7



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Characterization. Acid Value (Av). The Av for resin samples was measured according to the procedures outlined in GB/T 2895–1982. Typically, around 0.5 g of the obtained product was dissolved into 50 mL of toluene/ethanol (50:50 v/v) solution. Using phenolphthalein as indicator, the solution was titrated by a KOH/ethonol solution with an accurate concentration determined by the potassium acid phthalate standard. The Av was calculated as19 Av =

56 ⋅ ∆V ⋅ C KOH 1000 ⋅ ∆m

(1)

where ∆V, CKOH, and ∆m denote the consumed volume and initial concentration of the KOH solution and the resin weight, respectively.

Viscosity (Vs). The Vs measurements for the obtained liquid resins were performed on a NDJ-8S rotational viscometer from Shanghai Changji Dizhi Instrument Corporation (China).

Infrared (IR) Spectrometry. The FT-IR spectra were recorded on a Nicolet iS10 IR spectrometer (Thermo-Fisher Corporation, USA) coupled with Smart ARK accessory for liquid samples in a scanning range of 650-4000 cm-1 for 32 scans at a spectral resolution of 4 cm-1. Peaks in the spectra were labeled automatically using OMNIC software (Thermo Electron Corporation, USA).

Gel Permeation Chromatography (GPC). The GPC profiles were recorded using a high pressure liquid chromatography (HPLC) system (Waters Corporation, USA) including a 1515 isocratic HPLC pump, 717 plus automated injector, column heater, and controlled with Breeze software. The separation columns were a pair of Styragel HR1 and HR2 (300mm×7.8 mm) also from Waters Corporation and were maintained at 35 °C. A Waters 2414 Refractive Index Detector was used to detect signals. HPLC-grade tetrahydrofuran (THF) was used as eluent at a flow rate of 1.0 mL/min. The products were brought into THF solutions with a known concentration of 15-25 mg/mL. A series of narrowly-distributed polystyrene standards with a known molar mass of 580-19600 were employed to calculate the relative molar masses of analyzed samples.

Nuclear Magnetic Resonance (NMR). The 1H NMR and 13C NMR spectra were recorded on aDRX-300 Advance NMR spectrometer (Bruker Corporation, Germany) using CDCl3 as a solvent. MestReNova software obtained from Mestrelab Research S. L. (Santiago de 8


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Compostela, Spain) was used to process the NMR data.

Dynamic Mechanical Analysis. The DMA tests of the cured samples were performed in three-point bending geometry on a Q800 solids analyzer (TA Corporation, USA) with an oscillating frequency of 1 Hz. The cured samples were polished carefully before test. The samples were all cuboid with an approximate size of 40×10×4 mm3. For each sample, duplicated tests were performed in order to ensure the reproducibility of data.

Thermogravimetric Analysis (TGA). The TGA tests were performed using a STA 409PC thermogravimetry instrument (Netzsch Corporation, Germany) at a heating rate of 15 °C/min. Data were recorded while the oven temperature was ranging from 35 to 600 °C under N2 gas at a flow rate of 100 mL/min.

Mechanical Test. Tensile test of the polymer matrices was evaluated using a SANS7 CMT-4304 universal tester from Shenzhen Xinsansi Jiliang Instrument Corporation (China) according to the procedure specified in GB/T 2567–2008. Dumbbell specimens with a length of 200 mm and a size of 50×10×4 mm3 at the narrow middle part were conducted for the tensile tests at a constant draw speed of 5.0 mm/min. All the specimens were polished to avoid surface defects. At least five specimens of each polymer sample were tested and 3-5 close values were selected to be statistically averaged.

Scanning Electron Microscopy (SEM). The SEM examinations of the tensile-fractured samples were performed on 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.

Differential Scanning Analysis (DSC) for Curing Behaviors. The curing behaviors were studied by using a Diamond differential scanning calorimeter (PerkinElmer Corporation, USA). The resin samples used for analysis were the residual parts of the prepared liquid resins for the DMA test, as shown in Section 2.4. Approximately 5-10 mg of the sample was weighed and sealed in a 40 µL aluminum crucible, and immediately tested by DSC. DSC analysis was repeated twice for each sample. The sample was scanned from25-220 °C at heating rates of 5, 10, 15, and 20 °C/min, respectively. The activation energy (Ea) of curing was calculated by the Ozawa method20-22 with the following equation 9



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Ea =

−R ∆ ln β ⋅ 1.052 ∆ (1/ T p )

(2)

where β is the heating rate, Tp is the peak temperature of the DSC scanning curve, and R is the universal gas constant.

RESULTS AND DISCUSSION. Synthesis and Characterization of TOPERMA-HEA Monomer. The synthesis of TOPERMA-HEA macromonomer involved three steps, as shown in Scheme 1. The TOPERMA monomer was obtained by the first two steps, the alcoholysis reaction of TO with PER and maleination of the alcoholysis product with MA. The measured Av for the TOPERMA monomer was 235.3 mg KOH/g, which was clearly smaller than the theoretical value of 272.6 mg KOH/g. The reason for this can be attributed to the sublimation of MA. We observed solidified MA on the upper part of the inner flask wall. The TOPERMA-HEA monomer was synthesized in the same pot for TOPERMA. Before purification, the Av of the monomer (with a HEA:TO ratio of 1.5) was 218.9 mg KOH/g, which was close to the theoretical value of 213.6 mg KOH/g. We noticed that the HEA liquid could flush down the unreacted MA solid from the flask wall during the reaction. Thus all the acid substances can be analyzed in the Av test. In the third step, the added HEA not only reacted with the anhydride on the D-A structure of TOPERMA, but also simultaneously reacted with the unreacted MA remaining in the mixture after the second step. Consequently, a new substance named as hydroxyethyl acrylated maleate (HEAMA) formed in the TOPERMA-HEA monomer, as indicated in Scheme 2. After purification, the detected Av decreased to 167.9 mg KOH/g. The reason for this may lies in that the formed HEAMA, unreacted HEA, MA, and even some TO-based UE macromonomers were removed from the TOPERMA-HEA mixture. The purification of oil-based maleates was not easy because the produced maleates and unreacted MA contained similar acid characteristics. Recently, Echeverri et al.9 reported a method of purifying the produced maleates with a 10 wt% NaCl–water solution at 95 °C. In this study we adopted this method to purify the TOPERMA-HEA product, while its efficiency needs to be further assessed. 10


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Scheme 2. Reaction between the unreacted MA and HEA.

Figure 1. (a) FT-IR spectra and (b) GPC curves of TOPERMA and TOPERMA-HEA.

The FT-IR spectra of TOPERMA and purified TOPERMA-HEA are shown in Figure 1a. In the spectrum of TOPERMA, several characteristic peaks were presented: carboxyl groups on maleate half esters (2500-3400 cm-1), methylene and methyl groups (2924 cm-1 and 2854 cm-1), carbonyl groups on the MA structure (1845 cm-1 and 1775 cm-1), carbonyl groups on the carboxyl and ester groups (1729 cm-1), C=C bonds (1635 cm-1), and C–O–C ether groups (1203 cm-1 and 1158 cm-1). These peaks appeared in almost all the spectrum of TOPERMA-HEA. However, by taking the peaks without any shift (such as methylene and methyl groups at 2924 cm-1 and 2854 cm-1 and carbonyl groups at 1729 cm-1) as a reference, some minor changes were observed and highlighted in the shadow areas of Figure 1a. In the 11



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Page 12 of 35

left shadow area, the carbonyl groups on the MA structure shifted slightly to 1843 cm-1 and 1776 cm-1, and their relative intensities displayed a little decrease compared to that of the carbonyl groups at 1729 cm-1. The C═C vibrations also moved from 1635 cm-1 to 1641 cm-1. In the right shadow area, the C–O–C ether group at 1203 cm-1 almost disappeared. All these changes revealed the grafting of HEA onto the MA structure of TOPERMA. Table 1. Data of Characterization of TOPERMA and TOPERMA-HEA Monomers Ava

Mwb

Mnc

(mg/g)

(g/mol)

(g/mol)

TOPERMA

235.3

3360

548

6.12

1.68

0.41

TOPERMA-HEA

215.2

3409

677

5.02

2.27

0.36

Purified TOPERMA-HEA

167.9

2960

782

3.78

1.55

0.39

Sample ID

D

d

NC═C

e

Ng-MAf or Ng-HEAg

a

Acid value. b Weight-average molar mass. c Number-average molar masse. d Polydispersity index. e Introduced C═C functionality per fatty acid (FA). f Number of grafted MA on TOPERMA per FA. g Number of grafted HEA on TOPERMA-HEA per FA.

The GPC chromatographs of TOPERMA and TOPERMA-HEA monomers are indicated in Figure 1b. The TOPERMA monomer comprised several types of maleates,7 thus multi-peaks were observed in the GPC curves for both TOPERMA and TOPERMA-HEA. After the modification of HEA, the main peaks shifted to a shorter retention time owing to the growth of molecular weight, indicating the HEA grafted successfully onto TOPERMA. With the calibration curve of polystyrene standards, molecular weights of the TOPERMA and TOPERMA-HEA monomers were determined and listed in Table 1. The unpurified TOPERMA-HEA monomer showed higher weight-average and number-average molar masses (Mw and Mn) than those of the TOPERMA monomer. On the other hand, the purified TOPERMA-HEA monomer demonstrated a smaller Mw and a narrower polydispersity index than the unpurified one. This result indicated that a portion of the produced TO-based co-esters were definitely removed after the work-up procedures. Nevertheless, the Mn of the purified TOPERMA-HEA monomer was higher than that of the unpurified one, suggesting small molecules were effectively washed away from the unpurified mixture.

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Figure 2. 1H NMR spectra of (a) TOPERMA and (b) TOPERMA-HEA.

The 1H NMR spectra of TOPERMA and purified TOPERMA-HEA are given in Figure 2, while the 1H NMR spectrum of unpurified TOPERMA-HEA is presented in Figure S1. In the spectrum of TOPERMA, the peak at 11.1 ppm corresponded to the acid protons on TOPERMA. The peaks at 6.3, 6.9, and 7.1 ppm were assigned to the maleate, fumarate, and MA vinyl protons, respectively.5, 7 The peaks at 3.0-3.5 represented the protons at the structure where MA attached onto TO via D-A addition. After TOPERMA was modified by HEA, the proton peak of acid groups shifted to 9.5 ppm and widened with a lower intensity, which may attributed to the removal of the unreacted MA. The signal corresponding to the C═C bond of unreacted MA at 7.1 ppm almost disappeared in the purified TOPERMA-HEA monomer, meaning that the unreacted MA were successfully removed from the unpurified monomer. The peaks at around 6.3 ppm were broadened which was attributed to the introduction of two vinyl protons of HEA (Figure S2). The other vinyl proton of HEA was at 13



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about 5.8 ppm (Figure S2). The peaks at 4.0-4.5 ppm, which corresponded to the methylene protons of the polyols’ backbone connected to the ester structure in TOPERMA, also varied in shape and intensity. The peak at around 0.9 ppm representing the terminal methyl protons of FAs are always taken as a reference, because its intensity should not be altered throughout all the reactions mentioned above. The integral of this peak should reveal three protons per FA chain in a triglyceride molecule. Therefore, the introduced C═C functionality per FA chain (NC═C) for the obtained TOPERMA and TOPERMA-HEA monomers can be estimated by the following equation

3A 1 A NC =C = ( 6.1−6.9 ppm ) = 6.1−6.9 ppm 2 A0.9 ppm / 3 2 A0.9 ppm

(3)

where NC═C was contributed to by the maleate and fumarate structures for TOPERMA, and by the maleate, fumarate, and the grafted HEA for TOPERMA-HEA. It should be noted that the only two vinyl protons of HEA in the range of 6.1–6.9 ppm conveniently offered us the eq 3 to calculate the total C═C functionality of the two monomers. Moreover, the number of the grafted MA on the formed D-A structure of TOPERMA (Ng-MA) can be estimated as

3A 1 A N g − MA = ( 3.0−3.5 ppm ) = 3.0−3.5 ppm 2 A0.9 ppm / 3 2 A0.9 ppm

(4)

Considering that anhydride is readily ring-opened by hydroxyls,5, 7 the number of grafted HEA per FA (Ng-HEA) approximately has the following relationship

N g − HEA = N g − MA

(5)

The determined NC═C, Ng-MA, and Ng-HEA values were listed in Table 1.When HEA was added into TOPERMA, the NC═C value increased from 1.68 to 2.27. It is noteworthy that NC═C value for the unpurified TOPERMA-HEA was a total assessment of the C═C functionality per FA chain because the unpurified sample consisted of the TO-based UEs, the formed HEAMA, the unreacted HEA and MA, etc. After purification, NC═C decreased to 1.55, which was even lower than that of TOPERMA. The reason for this reduction lies in that the work-up procedures of TOPERMA-HEA removed some TO-based UEs while removing the HEA, MA, and HEAMA. This result was also verified by the aforementioned analysis of acid titration and GPC. The Ng-MA for TOPERMA and Ng-HEA for unpurifed/purified 14


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TOPERMA-HEA was near to 0.40 and almost unchanged, indicating that the grafting yield of MA or HEA was about 40% and very stable during all the modifying and purifying processes. Especially, the true NC═C value of the TO-based co-UEs in the unpurified TOPERMA-HEA monomer can be inferred as the sum of the NC═C values of TOPERMA and Ng-HEA (2.08). This value was approaching that of the unpurified TOPERMA-HEA monomer, suggesting the small molecules (HEA, MA, and HEAMA) in the unpurified monomer were present in a smaller amount. The chemical structure of the purified TOPERMA-HEA monomer was also confirmed using 13C NMR, as presented in Figure 3. The chemical shifts of 165.0–173.5 and 126.7– 133.2 ppm were assigned to carbonyls and unsaturated carbons belonging to tung oil fatty acid (TOFA), HEA, maleate, and fumarate, respectively. Compared to the shifts of TOPERMA (Figure S3), new peaks at 171.0 and 131.2 ppm appeared because of the introduction of the HEA structure. The peaks at 67.2/65.5 ppm corresponded to the glycerol backbones which might occur due to the removal of a few oil-based co-UEs and impurities (MA, HEA, HEAMA, etc.). All the results were in good agreement with those in the reported 13

C NMR spectra of acrylated/MA-modified plant oils.8, 9, 19, 23

15



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Figure 3. 13C NMR spectrum of TOPERMA-HEA monomer.

Dynamic Mechanical Analysis of Cured TOPERMA-HEA Resins. Effect of Purification of TOPERMA-HEA. As reported in the characterization above, the reduction of C═C functionality after purification was unexpected. The loss of polymerizable C═C bonds would probably cause a decrease in νe of the cured TOPERMA-HEA resins according to previous studies.6, 8, 17, 24, 25 Therefore, the effect of purification of TOPERMA-HEA on the properties of the cured polymer matrices should be considered first. The DMA technique was employed to investigate the thermo-mechanical properties of the obtained biomaterials. Figure S4 shows the temperature dependence of storage modulus (E′) and loss factor (tan δ) for the TOPERMA-HEA polymers. A distinct feature for the triglyceride-based resins was that the received biomaterials exhibited a very broad transition from a glassy to a rubbery state.18, 26 The E′ at 25 °C (E′25) and glass transition temperature (Tg) of the unpurified TOPERMA-HEA resin (2.84 GPa and 153.5 °C, respectively) were apparently larger than those of the purified resin (2.34 GPa and 130.1 °C, respectively). Furthermore, the E′25 and Tg of the unpurified TOPERMA-HEA resin were more comparable 16


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to those of the TOPERMA resin (3.01 GPa and 149.3 °C) by comparison with those of the purified resin. Based on the kinetic theory of rubber elasticity, the experimental νe of all the copolymers can be estimated from the rubbery modulus using the following equation:5, 6, 10, 18, 24, 27

E ' = 3νe RT

(6)

where E′ should use the storage modulus in the rubbery plateau region and T is the absolute temperature. In this work, the rubber modulus selected for calculating νe occurred at Tg+40 °C. The purified TOPERMA-HEA resin indeed demonstrated a lower νe value (2.87×103mol/m3) that that of the unpurified one (3.07×103mol/m3), which was attributed to the loss of C═C functionality after purification. According to the results above, we believed that the purification process for TOPERMA-HEA was not essential because the resulting biobased co-esters were partially removed with the removal of MA, HEA, and HEAMA. Hence, the unpurified TOPERMA-HEA resins were utilized in the following study of structure-property relationships. Effect of Feed Ratio of TO:HEA. The temperature dependence of E′ and tan δ for the TOPERMA-HEA resins with different feed ratio of TO:HEA is indicated in Figure S5, and the corresponding results are summarized in Table 2. The TOPERMA-HEA polymer with a TO:HEA ratio of 1:1.5 revealed the largest E′25 and Tg values but moderate νe. Since one TO triglyceride has three FA chains, the TO:HEA ratio can be readily converted into the ratio of HEA:TOFA. The molar ratio of HEA:TOFA for the best TOPERMA-HEA resin was 0.50, which was a little over the Ng-MA value (0.41 in Table 1) of TOPERMA monomer. Hence, we inferred that the grafted MA structures on TOPERMA were sufficiently consumed by the added HEA at the TO:HEA ratio of 1:1.5 and the ratio was appropriate for the synthesis of TOPERMA-HEA. Additionally, the TOPERMA-HEA resin with the optimized ratio offered a Vs value suitable for liquid modeling resins. An increase of HEA results in the decrease of Vs value because of the growing amount of small molecules like HEA/HEAMA.

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Table 2. Data of Viscosity and DMA Properties for the TOPERMA-HEA Resins with Different Feed Ratio

a

Molar ratio

Molar ratio

Vsa

E′25b

Tg c

νed

(TO:HEA)

(HEA:TOFA)

(mPa·s)

(GPa)

(°C)

(103mol/m3)

1:1.0

0.33:1

2640

1.62

125.2

2.83

1:1.5

0.50:1

1750

2.09

137.5

2.50

1:2.0

0.67:1

1330

1.93

137.1

2.21

b

c

d

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

Effect of Comonomer Concentration. Figure 4 displays the temperature dependence of E′ and tan δ for the cured TOPERMA-HEA resins with different styrene concentrations. Table 3 lists the related DMA results. The TOPERMA-HEA resins with 20% and 30% styrene clearly demonstrated two glass transitions which belonged to an oil-rich region with lower Tg and a styrene-rich region with higher Tg.18, 25 As the styrene concentration increased from 20-40%, the E′25 and Tg values increased to 1.97 GPa and 140.3 °C, respectively, and later decreased to 1.49 GPa and 135 °C at 50% styrene. The maximum values of both occurred at 40% styrene, although herein the νe value was not the largest. The optimal styrene concentration for the copolymerization of oil-based UE monomer correlates to the reactivity ratios of reactive monomers.18, 28, 29 It was reported when azeotropic copolymerization is founded, the resulting copolymer can form a homogeneous material with optimal properties.30, 31 Based on the copolymerization equation proposed by Mayo and Lewis,32 one could obtain the equation of azeotropic copolymerization

[M1 ] r2 − 1 = [M 2 ] r1 − 1

(7)

where [M1] and [M2] are concentrations of the unreacted monomers; r1 and r2 are reactivity ratios of the two monomers (0

Tung Oil-Based Unsaturated Co-ester Macromonomer for

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