High Performance Polymer Derived from a Biorenewable Plant Oil

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A High Performance Polymer Derived from A Biorenewable Plant Oil (Anethole) Fengkai He, Kaikai Jin, Yuanqiang Wang, Jiajia Wang, Junfeng Zhou, Jing Sun, and Qiang Fang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02919 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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ACS Sustainable Chemistry & Engineering

A High Performance Polymer Derived from A Biorenewable Plant Oil (Anethole) Fengkai He, Kaikai Jin, Yuanqiang Wang, Jiajia Wang, Junfeng Zhou, Jing Sun and Qiang Fang*

Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, PR China.

* E-mail: [email protected].

KEYWORDS: Biomass, plant oil, anethole, thermosetting polymer, high performance polymer

ABSTRACT: A high performance polymer with both low water uptake and high thermostability derived from a biorenewable plant oil (anethole) is reported here. By using the plant oil as the feedstock, a new monomer containing benzocyclobutene and vinyl units was successfully synthesized. The monomer was then converted to a polymer via radical polymerization, and the polymer was further post-polymerized to form an insoluble and infusible crosslinked network at high temperature (> 200 °C). The crosslinked network exhibited water uptake of below 0.40% (kept at boiling water for 4 days) and dielectric constant of less than 2.9 at a range of frequencies varying from 1.0 to at 30.0 MHz at room temperature. TGA and TMA data showed 1

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that the crosslinked network had 5 wt% loss temperature of 432 °C (in N2) and a Tg of 369 °C, respectively. The crosslinked network also showed low thermal expansion coefficient (CTE) of 41.24 ppm/°C at the range of temperatures varying from 50 °C to 300 °C. Nanoindentation tests indicated that the crosslinked film had an average hardness of 0.87 GPa and a Young’s modulus of 11.40 GPa. These data suggest that the new polymer derived from anethole possess excellent thermal and mechanical properties, and it has potential application in microelectronic industry.

INTRODUCTION With the rapid development of the aeronautics/astronautics and microelectronic industries, high performance polymers are urgently required. Thus, tremendous efforts have been made to develop the synthesis and applications of the polymers.[1] Among various high performance polymers, these with thermosetting groups have attracted a lot of attention because they are ease of processability and can thermally form densely cross-linked networks having high performance, including high thermostability, good electrical properties and low water uptake. Of the thermosetting polymers, benzocyclobutene-based polymers have seen much interest from both academia and industry for several years.[2-11] These polymers show excellent thermal, mechanical and dielectric properties. In particular, their crosslinked films exhibit very low roughness and high hydrophobicity. Consequently, they have been recognized as an important candidate for the preparation of high performance materials, and are widely used in microelectronic industry. [5-11] 2


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Recently, polymeric materials derived from the renewable biomasses have become one of the most promising alternative for petroleum-based materials.[12-18] Among biomasses, the ones that contain aromatic units are particularly gathering interest because of their excellent properties including high glass transition temperature (Tg) and high storage modulus (E).[19-21] For example, Lignin, a well known biomass with a branched polyphenolic network, has been successfully converted into a lot of polymeric materials or useful precursors via the chemical or biological reactions.[22-26] However, most of the biomasses containing aromatic units can not be easily transformed to the small molecules, which can be used for the synthesis of the polymers. Therefore, the investigation on the easily processable biomasses with aryl units is desirable. It is noted that a plant oil named anethole is a typical biomass with a phenyl ring. This oil is derived from star anise, and can be used as a precursor for the preparation of high performance materials.[27-28] Previously, we have studied the conversion of the oil to a fluoropolymer with good dielectric and hydrophobicity[29]. As an extended investigation, by using anethole as a feedstock, we designed and synthesized a new functional monomer containing thermocrosslinkable ethylene and benzocyclobutene units. The chemical structure of the monomer is depicted in Scheme 1. In the presence of a radical initiator, the monomer was converted to a polymer with average number molecular weight (Mn) of near 780,000. Such a high Mn suggests that the polymer is suitable as a basic resin for the preparation of the plastic parts, as well as a matrix for the fiber reinforced composite. After post-polymerized at high temperature, the 3



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polymer formed a crosslinked network, which exhibited low water uptake and high thermostability. These results indicate that the polymer is very suitable as an encapsulation or a matrix resin utilized in microelectronic industry. In this contribution, we report the detailed results.

EXPERIMENTAL SECTION Materials. Anethole was purchased from Nanjing Chemlin Chemical Industry Co., China. 4-Bromobenzocyclobutene was purchased from Chemtarget Technologies Co., China.

MnO2,

resorcinol,

potassium

t-butoxide,

dichloromethane,

and

methyltriphenylphosphonium bromide were purchased from Sigma Aldrich. Bromine, tetrakis(triphenylphosphine)palladium, and 2,2'-azoisobutyronitrile were purchased from TCI company. Tetrahydrofuran (THF) was purchased from Sinopharm Chemical Reagent Co., China, and dried over CaH2 and distilled under reduced pressure before use. Other solvents and reagents were used as received. Measurements. 1H NMR,

13

C NMR spectra were recorded on a Bruker 400

spectrometer using TMS as internal standard with CDCl3 as a solvent at room temperature. High-resolution mass spectra (HRMS) were recorded on a DARTSVP (IonSense Inc.) mass spectrometer. FT-IR spectra were run on a Nicolet spectrometer with KBr pellets. Differential scanning calorimetry (DSC) was measured on a TA Instrument DSC Q200 with sealed pans at a heating rate of 10 °C/min under nitrogen flow. Thermogravitric analysis (TGA) was performed on a TG 209F1 (NETZSCH) apparatus with a heating rate of 10 °C/min in N2 atmosphere. Elemental analysis was

4


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carried out on an Elementar vario EL III system. The dielectric constant (Dk) and dissipation factor (Df) of the cured polymer films were measured by the capacitance method at the range of frequencies from 1.0 to 30.0 MHz at room temperature by using a 4294A Precision Impedance Analyzer (Agilent). An aluminum-backed silicon wafer (heavily doped single crystal silicon with a resistivity of 3.5×10-3 Ω·cm) was used as the substrate for fabrication of the parallel plate capacitors. The top electrodes with an average diameter of about 3.0 mm were prepared by evaporation of aluminum on to the surface of the cured polymer film. The thickness of the cured polymer film was measured by field emission scanning electron microscopy (FE-SEM) on a FE-SEM S-4800 (Hitachi). The thinness of the cured polymer film controlled at a range of more than 120 nm but less than 180 nm. The detailed measurement procedure was described in our previous report.[30] The contact angle of the cured film was measured at 20 ± 1 °C using a sessile drop method on a dynamic contact angle measurement instrument (JC2000C). Deionized water was selected as the testing liquid. Surface toughness of the polymer film was measured by atom force microscopy (AFM) on a Shimadz SPM-9500J3. Molecular weights were measured by using a Waters Breeze2a 200 GPC instrument with polystyrene as standards and tetrahydrofuran (THF) as the eluent at 35 °C. Thermo-mechanical analysis (TMA) was recorded on the Mettler Toledo DMA/SDTA861e with a heating rate of 3 °C min-1 in air. The mechanical properties of the polymer films were measured on a nanoindentation/scrach system called UNHT/NST (CSM Company).

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Synthesis of 1. To a stirring mixture of resorcinol (0.45 mmol, 0.50 g), MnO2 (460.10 mmol, 40.00 g) and H2O (100 mL), anethole (101.21 mmol, 15.00 g) was added dropwise at 60 °C during a period of 0.5 h. After addition, the mixture was heated to 80 °C, and then 35% H2SO4 (150 g) was added dropwise during a period of 0.5 h. The mixture was raised to 85 °C and kept at the temperature for 4 h with vigorous stirring. The solution was cooled to room temperature and extracted with ethyl acetate. The organic phases were combined, washed with water, and dried over anhydrous Na2SO4. After filtration and concentration, the crude product was prepared, which was further purified by distillation under reduced pressure. The pure 1 was thus obtained as a transparent liquid in a yield of 76.2%. 1H NMR (400 MHz, CDCl3, δ): 9.87 (s, 1H), 7.83 (d, 2H), 7.00 (dd, 2H), 3.88 (s, 3H). Synthesis of 2. To a stirring mixture of 1 (220.35 mmol, 30.00 g) and CH2Cl2 (40 mL), a solution of Br2 (264.42 mmol, 42.26 g) in CH2Cl2 (100 mL) was added dropwise at room temperature. After addition, the mixture was heated to reflux and maintained at the temperature for 12 h. After being cooled to room temperature, the mixture was treated with a saturated aqueous solution of NaHSO3 to consume the excess of bromine (note: in many cases, highly volatile Br2 with strong corrodibility was reduced to bromide using Na2S2O3 as a reducer. However, such a reducer reacted with Br2 to form sulfur, which is difficult to completely remove from organic solvent. Thus NaHSO3 is appreciated for removal of Br2). The solution was then extracted with CH2Cl2, washed with water, dried over anhydrous Na2SO4, filtered and concentrated to afford the crude product. Compound 2 was purified using column 6


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chromatography with a mixture of petroleum ether and ethyl acetate as the eluent (20:1, v/v). White solid, 41.00 g, yield 0f 86.53%. 1H NMR (400 MHz, CDCl3, δ): 9.84 (s, 2H), 8.08 (d, 2H), 7.82 (dd, 2H), 7.01 (d, 2H), 3.99 (s, 6H). Synthesis

of

3.

Under

an

argon

atmosphere,

a

mixture

of

methyltriphenylphosphonium bromide (93.00 mmol, 33.22 g), potassium tert-butylate (93.00 mmol, 10.44 g) and THF (40 mL) was stirred at 0 °C for 30 min, and then heated to 50 °C and kept at the temperature for 2 h. The mixture was cooled to room temperature, and a solution of 2 (46.50 mmol, 10.00 g) in THF (20 mL) was added dropwise. After stirring at 50 °C for 6 h, the mixture was treated with water to quench the reaction, and then extracted with CH2Cl2. The combined organic layers were dried over anhydrous Na2SO4, filtered and concentrated. The crude product was further purified by column chromatography with petroleum ether as a eluent, 3 was obtained as a transparent liquid in a yield of 96.9%. 1H NMR (400 MHz, CDCl3, δ): 7.61 (d, 1H), 7.29 (dd, 1H), 6.84 (d, 1H), 6.59 (dd, 1H), 5.61 (d, 1H), 5.16 (d, 1H), 3.89 (s, 3H). Synthesis of benzocyclobutene-4-magnesium bromide. Under an argon atmosphere, a solution of 4-bromobenzocyclobutene (70.40 mmol, 12.89 g) in THF (30 mL) was added dropwise to a mixture of magnesium turnings (93.87 mmol, 2.28 g) and THF (10 mL) at 40 °C during a period of 0.5 h. After addition, the resulting mixture was stirred for an additional 1 h at the temperature, and cooled to room temperature. A solution of benzocyclobutene-4-magnesium bromide was thus prepared. 7



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Synthesis of monomer 4. A solution of benzocyclobutene-4-magnesium bromide (70.40 mmol, 14.49 g) in THF (40 mL) was added dropwise to a mixture of 3 (46.93 mmol, 10.00 g), Pd(PPh3)4 (234.66 µmol, 271.2 mg) and THF (40 mL) at room temperature under N2. The mixture was heated to 70 °C and maintained at the temperature for 12 h. After cooling to room temperature, the mixture was treated with water (100 mL), and extracted with ethyl acetate. The organic layers were combined, dried over anhydrous Na2SO4, filtered and concentrated. The residue was purified by using column chromatography with petroleum ether as an eluent. Pure monomer 4 was obtained as a sticky transparent liquid in a yield of 64.10%. 1H NMR (400 MHz, CDCl3, δ): 7.36∼7.32 (m, 3H), 7.21 (s, 1H), 7.11 (d, 1H), 6.93 (d, 1H), 6.69 (dd, 1H), 5.65 (d, 1H), 5.15 (d, 1H), 3.82 (s, 3H), 3.23 (s, 4H). 13C NMR (126 MHz, CDCl3, δ): 156.33, 145.44, 144.83, 137.12, 136.28, 131.87, 130.48, 128.91, 128.28, 126.35, 123.87, 122.30, 111.92, 111.15, 55.80, 29.65, 29.58. HRMS−ESI (m/z): calcd for C17H17O [M+H]+ 237.1274, found 237.1274. Anal. Calcd for C17H16O: C, 86.40; H, 6.82. Found: C, 86.53; H, 6.96. Bulk polymerization. A mixture of monomer 4 (2.12 mmol, 0.50 g) and azobisisobutyronitrile (AIBN, 9.135 µmol, 1.5 mg) was stirred for 10 minutes at the room temperature under argon atmosphere. Then the mixture was heated to 70 °C and kept at the temperature for 1 h. After cooling to room temperature, the transparent solid was treated with THF to obtain a solution. The solution was concentrated and poured into methanol (100 mL) to afford a white precipitate, which was re-dissolved in THF (5 mL). The obtained solution was re-precipitated in methanol. Polymer P1 8


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was thus prepared as a white solid in a yield of 80%. GPC (polystyrene) results: Mn = 775,700, PDI =1.21. 1H NMR (400 MHz, CDCl3, δ): 7.14∼6.78 (m, 3H), 6.64∼6.17 (m, 3H), 3.52 (s, 3H), 3.09 (s, 4H), 2.18∼1.75 (m, 1H), 1.60∼1.24 (m, 2H). Fabrication of the Samples for the Measurement of the Dielectric and Mechanical Properties. A solution of P1 in xylene was spin-coated on an aluminum-backed silicon wafer to form a smooth film. The silicon wafer was placed into a quartz tube furnace and heated to 200 °C. After maintaining the temperature at 200 °C for 2 h, the furnace was allowed to be heated to 250 °C and kept at the temperature for 5 h under N2 atmosphere. The film was completely converted to a cross-linked network, which was used for the measurement of Dk, Df, surface toughness and contact angle, and used for nanoindentation/scratch tests. Preparation of the Sample for the Measurement of TMA. Monomer 4 was placed in a flat-bottomed glass tube (diameter =10 mm and height = 85 mm) filled with argon. The tube was kept at 130 °C for 3 h until the liquid became a transparent solid. The tube was then heated to 200 °C and kept at the temperature for 4 h. Finally, the tube was maintained at 220 °C and 250 °C for 5 h, respectively. Thus, a fully cured sample was obtained. Measurement of Moisture Absorption of the Cured Monomer 4. A fully cured monomer 4 sample sheet (2 mm thick × 10 mm in diameter) was dried in a vacuum oven until a constant weight (0.0001 g) was achieved. The sample sheet was then immersed into boiling water of 50 mL (about 97 °C) and kept at the temperature for 4 days. After removal of water and drying by using filter paper, the sample sheet was 9



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weighed. The moisture uptake was estimated as the increase of the weight of the sample sheet after immersion in the boiling water. RESULTS AND DISCUSSION As shown in Scheme 1, the functional monomer 4 was prepared with an overall yield of more than 40% via a four-step procedure by using anethole as the feedstock. The chemical structure of the monomer was confirmed by 1H NMR,

13

C NMR,

high-resolution mass spectra (HRMS), FT-IR and elemental analysis. In its 1H NMR spectrum, the characteristic peak at 3.23 ppm attributed to benzocyclobutene’s four-membered ring was observed. In its FT-IR spectrum, the characteristic peak for benzocyclobutene group at 1470 cm-1 was also observed. In addition, the signals for vinyl group were found in the 1H NMR and FT-IR spectra. These results indicated that the target monomer 4 was synthesized successfully.

10


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CH3 CHO

CHO Br 2 , CH2 Cl2

35% H2 SO4 , MnO 2

CH 3PPh3 Br, t-BuOK

reflux

resorcinol, H 2O, 85 ° C OCH 3

Br

Anethole

THF, 50 °C

Br OCH 3

OCH 3

OCH 3

2

1

3

m MgBr Pd(PPh3 )4

> 200 °C

AIBN, 70 °C

THF, 70 °C

OCH 3

OCH 3

P1 M n = 775,000 PDI = 1.21

4

i

H 3 CO y x

n

H 3CO m

OCH3

OCH3 j

P2

Scheme 1. Procedure for the synthesis of the new monomer and polymer.

It is noted that styrene-like monomers can form polymers via radical polymerization.[31-32] In our case, monomer 4 converted to a polymer (P1) in the presence of AIBN with an average number molecular weight of near 775,000. In order to well know the progress of the radical polymerization, A DSC trace of monomer 4 with AIBN was carried out, and the results are shown in Figure 1. As depicted in Figure 1, the radical polymerization started at about 79 ºC and gave a peak exothermic temperature of about 130 ºC. When the temperature was raised to near 200 ºC, a mountain-like peak was observed, which was attributed to the ring-opening reaction of benzocyclobutene units.[6-7] 11



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P1 showed good solubility in the common solvents such as CHCl3, THF and toluene. Casting a solution of P1 gave a tough and transparent film, which was free standing.

Figure 1. DSC curves of monomer 4 at a heating rate of 10 °C min-1 in N2. The chemical structure of P1 was characterized by its 1H NMR spectrum. As shown in Figure 2, characteristic peak for vinyl group in monomer 4 completely disappears in P1, suggesting that monomer 4 has transformed to P1. It is noted that the characteristic peak at 3.2 ppm for the four-member ring of a benzocyclobutene unit in monomer 4 still remains, indicating that the radical initiator (AIBN) is not able to make benzocyclobutene unit ring-opening.

12


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Figure 2. 1H NMR spectra of monomer 4 and polymer P1.

We further investigated the post-polymerization of polymer P1. As exhibited in Figure 1, P1 shows a Tg of 117 ºC and a maximum exothermic peak temperature of 262 ºC. This implies that P1 can be converted to the infusible and insoluble crosslinked networks via the postpolymerization reaction. In order to monitor the degree of postpolymerization, FT-IR spectra of P1 and P2 were carried out. As depicted in Figure 3, the characteristic signal attributed to benzocyclobutene at 1470 cm-1 disappears after treating P1 at 250 ºC for 5 h. This result suggests that P1 has been fully thermo-crosslinked to form P2 (see Scheme 1).

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Figure 3. FT-IR spectra of monomer 4, polymer P1 and P2.

Surface roughness is an important parameter for the evaluation of film quality. Low surface roughness is reflects that there is no significant shrinkage or expansion during film forming. This parameter is more crucial for the materials utilized in microelectronic industry, because low surface roughness implies that the array with high quality can be easily produced on the film surface. Benzocyclobutene-based resins usually exhibit good surface roughness. In our case, surface morphology of P1 and P2 films on silicon wafers were characterized by using atomic force microscopy (AFM). As can be seen from Figure 4, the surface average roughness of P1 film before and after curing is 0.33 nm and 0.46 nm over a 2 µm square area. No significant difference of surface roughness between P1 and P2 films implies that no obvious shrinkage occurs during the conversion of P1 to P2. Such a low surface roughness indicates that P1 is suitable for application in the electronic/electrical fields.

14


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(a)

(b)

Figure 4. AFM images of P1 film before (a) and after (b) curing.

Water uptake is a very important parameter for the application of high performance material. In our case, a P2 disk with a thickness of 2.0 mm and a diameter of 10.0 mm was kept in boiling water for 4 days, exhibiting a water uptake of 0.40%. Such a low value indicates that the polymer has good hydrophobicity. The hydrophobicity of a P2 film also was investigated by the water contact angle test, and the results are shown in Supporting Information. As depicted in Supporting Information (Figure S3), on the surface of a P2 film, the water contact angle was measured as 96°, also indicating that the P2 film has good hydrophobicity.

The dielectric properties of a P2 film were measured by the capacitance method, and the results are depicted in Figure 5. The average dielectric constant (Dk) of the film is less than 2.9 with a dissipation factor (Df) of below 7×10-3 at the range of frequencies varying from 1.0 to 30.0 MHz. This result is comparable to the values of the commercially available organic low-k materials, such as polyimides (3.1~3.4)[33] and polycyanate esters (2.61~3.12)[34].

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Figure 5. Dielectric constant and dielectric loss of a P2 film.

Thermostability of P2 was investigated by using thermal gravimetric analysis (TGA). As depicted in Figure 6, P2 exhibits a 5 wt% loss temperature of 432 °C. This high heat resistance of P2 implies that it could be used as the matrix for the preparation of insulating enameled wire and the lamination of print circuit boards.

Figure 6. TGA curves of P2 in N2 with a heating rate of 10 ºC min-1. 16


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High glass-transition temperature (Tg) and low coefficient of thermal expansion (CTE) are also important characteristics of high-performance engineering materials. The sample’s dimension change and glass-transition temperature (Tg) are usually evaluated by the static thermal mechanical analysis (TMA). According to the measurement results of TMA, the sample’s coefficient of thermal expansion (CTE) can be calculated according to equation 1:

α (T ) =

1 dL(T ) L dT

(1)

Where α(T) represents CTE, L is the length of sample before test, and L(T) is the length of sample at a temperature T. In our case, TMA results (Figure 7) indicate that

P2 has a Tg of 369 ºC, and shows a CTE of 41.24 ppm/ºC varying from 50 to 320 ºC. These data exhibit P2 processes excellent dimension-stability.

Figure 7. TMA curve of P2.

Nanoindentation/scratch technology was also used to evaluate the mechanical 17



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properties of P2. As depicted in the Figure 8, a P2 film shows that it has an average hardness of 0.87 GPa and a Young’s modulus of 11.36 GPa. The bonding strength between the film and a silicon wafer is measured as 1.10 GPa. These results are better than that of most epoxy resins[21], indicating that P2 has good mechanical properties.

(a)

(b)

Figure 8. Results from (a) nanoindentation and (b) nanoscratch tests for a P2 film.

CONCLUSIONS 18


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In summary, we have successfully developed a new thermosetting polymer derived from anethole. After postpolymerization at high temperature, the polymer formed an insoluble and infusible crosslinked network, showing a water uptake of 0.40%. TGA and TMA data indicated that the crosslinked network had 5 wt% loss temperature of 432 °C (in N2) and a Tg of 369 °C, respectively. Furthermore, TMA data suggested that the coefficient of thermal expansion (CTE) was 41.24 ppm/°C at the range of temperatures varying from 50 °C to 300 °C. The crosslinked network also exhibited dielectric constant (Dk) of near 2.90 and dissipation factor (Df) of below 7×10-3, respectively, ranging from 1.0 to 30.0 MHz at room temperature. These results indicate that the thermosetting polymer derived from anethole show high thermostability and good dielectric properties, implying that the polymer has potential application in microelectronic industry.

ASSOCIATED CONTENT

Supporting Information Figure S1. A GPC curve of polymer P1, prepared by radical polymerization. Figure S2. SEM image of a P2 film on a silicon wafer. Figure S3. A picture of contact angle of water on a P2 film. Figure S4. 1H NMR spectrum of monomer 4 (400 MHz, CDCl3). Figure S5. 13C NMR spectrum of monomer 4 (126 MHz, CDCl3). Figure S6. 1H NMR spectrum of P1 (400 MHz, CDCl3). This Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION 19



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Corresponding Author * E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

Financial supports from Ministry of Science and Technology of China (2015CB931900) and the Natural Science Foundation of China (NSFC No. 21574146 and No. 21504103) and the Science and Technology Commission of Shanghai Municipality (15ZR1449200 and 16JC1403800) are gratefully acknowledged. The authors are also grateful to the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB 20020000).

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For Table of Contents Use Only

A High Performance Polymer Derived from A Biorenewable Plant Oil (Anethole) Fengkai He, Kaikai Jin, Yuanqiang Wang, Jiajia Wang, Junfeng Zhou, Jing Sun and Qiang Fang*

Synopsis: A high performance thermosetting polymer derived from a plant oil (anethole) has been developed, which shows low coefficient of thermal expansion and high thermostability.

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High Performance Polymer Derived from a Biorenewable Plant Oil

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