Article Cite This: Macromolecules XXXX, XXX, XXX-XXX
pubs.acs.org/Macromolecules
A New Fluorinated Polysiloxane with Good Optical Properties and Low Dielectric Constant at High Frequency Based on Easily Available Tetraethoxysilane (TEOS) Jiajia Wang,† Junfeng Zhou,† Kaikai Jin,† Liang Wang,‡ 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, P. R. China ‡ Shanghai Tobacco Group Co. LTD, 717 Changyang Road, Shanghai 200082, P. R. China S Supporting Information *
ABSTRACT: A novel fluorinated macromonomer (TFVE-Si) with four functional groups derived from easily available tetraethoxysilane (TEOS) has been successfully synthesized through the Piers−Rubinsztajn reaction using B(C6F5)3 as a catalyst. This procedure efficiently avoids the generation of Si−OH and −Si−CH2−CH2− groups, which greatly affect the properties of the organosiloxanes. Prepolymerizing the macromonomer in mesitylene solution gives an oligomer which can form a flexible and highly transparent free-standing cross-linked polysiloxane film followed by a postpolymerization procedure at high temperature. The cross-linked polysiloxane (thickness = 2 mm) shows a transmittance of higher than 91% in the visible region and an absorbance of near 100% in the UV region (3.0). Moreover, D−E loop tests illustrate that the cross-linked polysiloxane possesses excellent linear dielectric properties, further suggesting its good insulating properties. Furthermore, the prepared polysiloxane exhibits high thermostability with a 5 wt % loss temperature of 476 °C and a glass transition temperature (Tg) of 110 °C as well as good mechanical strength (with an elastic modulus of 1.1 GPa). Because of the existence of fluoro-containing groups, the polysiloxane also shows high hydrophobicity. Furthermore, TFVE-Si can efficiently improve the Tg of linear polysiloxane prepared from dimethylsiloxane with two functional groups. These indicate that the fluorinated TEOS has potential application in the microelectronics industry; especially, it can meet the requirement of the high-frequency communication fields for the materials with both low Dk and Df.
1. INTRODUCTION With the rapid development of new generation high-frequency (>5 G) communication technology in recent years, research on the dielectric materials with low dielectric constant (Dk, 3.9) in the production of ultralarge scale integration (ULSI) circuits.34 On the basis of low price and easy availability of TEOS, we are attempting to develop a new fluorinated TEOS-based polysiloxane with low Dk and Df at high frequencies. To prevent the residue of Si−OH groups and obtain the polymer having high thermostability, we design and synthesize a novel fluorinated TEOS via a one-step Piers− Rubinsztajn reaction by using B(C6F5)3 as a catalyst (Scheme 1).35−40 This reaction has been proved to be an efficient way to form Si−O−Si groups from Si−OR and Si−H groups in situ, which can avoid the generations of Si−OH and −Si−CH2− CH2− groups. For example, Brook’s group developed several siloxanes and polysiloxanes using the Piers−Rubinsztajn condensation.37−39 In our case, such a new fluorinated TEOS
2. EXPERIMENTAL SECTION Materials. Diethoxydimethylsilane (DM-Si), tetraethoxysilane (TEOS), and tris(pentafluorophenyl)borane [B(C6F5)3] were purchased from Alfa and used as received without further purification. Solvents were distilled under reduced pressure before use. Arylhydrosilane with a trifluorovinyl ether group (HSi-TFVE, {4[trifluorovinyl(oxy)]phenyl}dimethylsilane) was prepared according to the previous literature.43 Characterizations. 1H NMR, 19F NMR, and 13C NMR spectra were measured on a JEOL 400YH instrument. Elemental analysis (EA) was carried out on Elementar vario EL III. Fourier transform infrared (FT-IR) spectra of TFVE-Si and PFCB-Si (Scheme 1) were recorded on a Nicolet spectrometer with KBr pellets and the angle total reflection (ATR) method at room temperature, respectively. The molecular weight of PFCB-DM-Si (see Scheme 3) was determined by gel permeation chromatography (GPC) with a Waters Breeze2a 200 GPC system equipped with refractive index (RI) detector using polystyrene as the standard and tetrahydrofuran (THF) as the eluent at a flow rate of 1.0 mL min−1. Differential scanning calorimetric (DSC) analysis was performed with TA Instruments DSC Q200 at a heating rate of 10 °C min−1 in a nitrogen atmosphere with a flowing rate of 50 mL min−1. Thermogravimetric analysis (TGA) was carried out on a TG 209F1 apparatus with a heating rate of 10 °C min−1 in a nitrogen atmosphere with a protective flowing rate of 10 mL min−1 and a purge flowing rate of 50 mL min−1. The ultraviolet−visible− near-infrared (UV−vis−NIR) spectrum was recorded on Varian CARY 5000 at room temperature with a scan rate of 600 nm min−1. Dielectric properties were investigated on a 4294A precision impedance analyzer (Agilent) via the noncontact method in a range of frequencies from 40 Hz to 30 MHz at room temperature. The dielectric constant and dissipation factor at 10 GHz were determined on split post dielectric resonators (Agilent). Static contact angles were obtained on JC2000C of Shanghai Zhongchen Equipment Ltd., China, at room temperature and ambient humidity. The surface toughness of polymer films was measured by atom force microscopy (AFM) on an environment control scanning probe microscope (Nanonavi Esweep) at room temperature using the ac (tapping) mode in a 10 μm × 10 μm area. Dynamic mechanical analysis (DMA) was performed with a heating rate of 5 °C min−1 in air on the Mettler Toledo DMA/ SDTA861e instrument. The stress−strain curve was obtained on a TCS-2000 with a stretch rate of 50 mm min−1. The displacement− electric field (D−E) loops were measured on a ferroelectric tester B
DOI: 10.1021/acs.macromol.7b02000 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 2. Schematic Description for the Preparation of TFVE-Si and PFCB-Si
134.94, 135.09, 136.58, 136.84, 153.78. Elemental Analysis, Calcd: C, 47.81; H, 4.74; F, 20.62. Found: C, 47.81; H, 4.80; F, 20.05. Preparation of a PFCB-Si Sample in Bulk. TFVE-Si was placed into a flat-bottomed glass tube with a diameter of 35.0 mm and filled with argon. The tube was heated to 150 °C and kept at this temperature for 2 h. The temperature was then elevated slowly and maintained at 180 °C for 2 h, 220 °C for 3 h, 250 °C for 3 h, 280 °C for 2 h, and 300 °C for 1 h. After being cooled to room temperature, a cross-linked PFCB-Si sample was obtained as a transparent sheet. Preparation of Free-Standing PFCB-Si Films by DropCoating for the Measurements of Stress−Strain Curve and D−E Loops. A solution of TFVE-Si (2.0 g) in mesitylene (20 mL) was allowed to be heated at 190 °C for 10 h in a sealed tube filled with argon, and the prepolymer of TFVE-Si was thus obtained. After being cooled to room temperature, the solution was dropped onto a flat iron mode followed by removing the solvent at 100 °C on a hot plate for 5 h. Temperature was then raised up to 250 °C and maintained at this temperature for 3 h. Thus, a smooth free-standing film was obtained after being peeled off from the iron surface. For the measurements of D−E loops, an additional procedure was needed: the surface of PFCB-Si film was deposited copper as the electrodes with a diameter of about 3 mm. Preparation of the PFCB-Si Film on a Silicon Wafer by SpinCoating for the Measurement of Surface Morphology and Static Water Contact Angle. The above-mentioned prepolymer of TFVE-Si in mesitylene was spin-coated on a silicon wafer and followed by heating at 250 °C for 2 h to prepare a cross-linked PFCB-Si film. Measurement of Water Absorption. A fully cross-linked PFCBSi (0.75 g) sheet with a diameter of 35.0 mm was predried under vacuum at 80 °C for several hours to a constant weight, followed by immersing into a 100 mL beaker filled with deionized water at room temperature. After 15 days, the sample was taken out of water and weighed after the surface water being wiped out. The percentage of water absorption was calculated and reported as the weight-increasing percentage via the formula WA (%) = (M − M0)/M0, where WA refers to water absorption and M and M0 are the sample weight after immersing and the initial weight, respectively.
(Trek MODEL 609B from Radiant Technologies, Inc.) with a frequency of 10 Hz at room temperature. Synthesis of TFVE-Si. A toluene (10 mL) solution of HSi-TFVE (8.99 g, 38.76 mmol) was added dropwise into a mixture of TEOS (2.02 g, 9.69 mmol), B(C6F5)3 (102 mg, 0.2 mmol), and toluene (10 mL) at room temperature. The mixture was then allowed to be stirred at 60 °C for 12 h. After removing the solvents at reduced pressure, the crude product was purified by column chromatography on silica gel using n-hexane as the eluent. Pure TFVE-Si (7.20 g) was obtained as a transparent and colorless liquid with a yield of 73%. 1H NMR (CDCl3, 400 MHz), δ (ppm) 0.27 (s, 24H), 7.00−7.02 (d, 8H), 7.43−7.46 (d, 8H). 19F NMR (CDCl3, 376 MHz), δ (ppm) −134.24 to −133.79 (m, 4F), −126.76 to −126.21 (m, 4F), −119.74 to −119.33 (m, 4F). 13C NMR (CDCl3, 100 MHz), δ (ppm) 0.48, 115.20, 131.85, 132.26, 132.33, 132.74, 134.48, 134.89, 134.95, 135.09, 135.37, 135.50, 144.07, 144.69, 146.79, 146.85, 147.41, 147.46, 149.57, 150.18, 156.24. Elemental Analysis, Calcd for C40H40F12O8Si5: C, 47.23; H, 3.96; F, 22.41. Found: C, 47.01; H, 3.89; F, 22.43. HRMS-DART (m/z): Calcd, 1016.14, Found, 1034.17 (M+ + NH4+). Synthesis of TFVE-DM-Si. A solution of HSi-TFVE (8.38 g, 36 mmol) in toluene (15 mL) was added dropwise into a mixture of DMSi (2.52 g, 17 mmol), B(C6F5)3 (92 mg, 0.18 mmol), and toluene (10 mL) at room temperature. The mixture was then allowed to be stirred at room temperature for 4 h, and the solvent was removed at reduced pressure. The obtained crude product was purified by column chromatography on silica gel using n-hexane as the eluent. Pure TFVE-DM-Si (8.10 g) was obtained as a transparent and colorless liquid in a yield of 86%. 1H NMR (CDCl3, 400 MHz), δ (ppm) 0.07 (s, 6H), 0.31 (s, 12H), 7.06−7.09 (d, 4H), 7.52−7.54 (d, 4H). 19F NMR (CDCl3, 376 MHz), δ (ppm) −134.02 to −133.57 (m, 2F), −126.80 to −126.25 (m, 2F), −119.87 to −119.46 (m, 2F). 13C NMR (CDCl3, 100 MHz), δ (ppm) 0.85, 1.42, 115.29, 132.00, 132.38, 132.86, 134.59, 135.07, 135.49, 136.38, 136.49, 144.16, 144.78, 146.88, 147.50, 149.65, 150.27, 156.24. Elemental Analysis, Calcd for C22H26F6O4Si3: C, 47.81; H, 4.74; F, 20.62. Found: C, 47.42; H, 4.91; F, 20.07. HRMS-DART (m/z): Calcd, 552.10, Found, 570.14 (M+ + NH4+). Synthesis of PFCB-DM-Si. A 10 mL sealed tube having TFVEDM-Si (1.02 g) was allowed to be heated at 200 °C for 6 h and then cooled to room temperature. PFCB-DM-Si was thus obtained as a pale-yellow viscous liquid in a quantitative yield. 1H NMR (CDCl3, 400 MHz), δ (ppm) 0.05−0.07 (m, 6H), 0.31−0.35 (m, 12H), 7.11− 7.13 (d, 2H), 7.20−7.22 (d, 2H), 7.50−7.57 (m, 4H). 19F NMR (CDCl3, 376 MHz), δ (ppm) −133.98 to −133.54 (m), −131.72 to −127.48 (m), −126.78 to −126.23 (m), −119.86 to −119.44 (m). 13C NMR (CDCl3, 100 MHz), δ (ppm) 0.81, 1.41, 115.30, 117.41, 134.79,
3. RESULTS AND DISCUSSION Synthesis and Characterizations of the Fluorinated Organic Siloxane Monomer (TFVE-Si) and Poly(arylsiloxane) (PFCB-Si). Usually, the functionalization of alkoxysilanes is realized by using Grignard reaction or in lithium media. However, equivalent metal reagents are needed in the conventional functionalization routes, which may limit the production of functional organic siloxanes on a large scale. In C
DOI: 10.1021/acs.macromol.7b02000 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. (a) DSC trace of TFVE-Si at a heating rate of 10 °C min−1 in N2. (b) FT-IR spectrum of TFVE-Si (black line) and ATR-FT-IR spectrum of PFCB-Si (blue line).
Scheme 3. Synthetic Route of Macromonomer (TFVE-DM-Si) and Polymer (PFCB-DM-Si)
chemical structure of TFVE-DM-Si is characterized by NMR, elemental analysis, and mass spectra (see Figures S4−S6). The linear PFCB-DM-Si is obtained by homopolymerization of TFVE-DM-Si in a quantitative yield at 200 °C for 6 h in bulk (its NMR spectra are shown in Figures S7−S9). PFCB-DM-Si is a clear pale-yellow viscous liquid, which is different from PFCB-Si. The molecular weight of PFCB-DM-Si is measured by 19F NMR44 and GPC, respectively, giving similar results. As shown in Figure S8 and Table S1, PFCB-DM-Si exhibits a Mn of about 9900 (about 17 repeating units). It is noted45−47 that the formation of the linear PFCB-containing polymers with high Mn through the direct thermopolymerization of TFVE groups is difficult, similar to our results in this contribution. Thus, developing multi-TFVE-functional monomers is significant to prepare cross-linked PFCB-containing polymers with high thermostability. Properties of the Cross-Linked Polysiloxane (PFCB-Si). Thermostability. With the introducing of PFCB connected network, thermostability of polysiloxane can be highly improved. On the one hand, thermal decomposition temperature is highly increased. As being presented in the thermogravimetric analysis (TGA, Figure 2), PFCB-Si shows a 5 wt % loss temperature (T5d) at 476 °C with a residue of 27% at 1000 °C in a N2 atmosphere, which is much higher than that of cross-linked polydimethylsiloxane (PDMS)48 or even the PFCB-functional polysiloxane prepared by hydrosilylation.32,33 On the other hand, PFCB-Si exhibits a high glass transition temperature (Tg) of 110 °C according to the dynamic thermomechanical analysis (DMA, Figure 3) results. This highly heat resistance is mainly resulted from the high cross-linking density caused by PFCB groups. PFCB-DM-Si is presented as a viscous liquid because of its linear structure and the highly flexible Si−O−Si−O−Si chains. As can be clearly seen from the DSC results in Figure 4, PFCBDM-Si exhibits a Tg of about −11 °C, which is much lower than PFCB-Si and the previously reported linear polymer connected
this contribution, new functional TEOS with thermo-crosslinkable TFVE groups is prepared in a high yield. The Piers− Rubinsztajn reaction (Scheme 1) occurs through only a catalytic amount (about 0.5 mol % or even less) of B(C6F5)3 and gives the functional TEOS (called TFVE-Si) on a large scale. The structure of TFVE-Si is clearly characterized by NMR spectra (Figures S1−S3 in the Supporting Information). The obtained TFVE-Si is a colorless liquid and soluble in the common organic solvents, endowing its good processability. As depicted in Scheme 2, TFVE-Si consists of four TFVE groups connected by Si−O−Si linkages, which is formed via the Piers−Rubinsztajn reaction in situ. TFVE-Si can be converted into a cross-linked polysiloxane (PFCB-Si) through a directly thermo-induced polymerization procedure (as described in Schemes 1 and 2). This thermopolymerization results in the formation of a cross-linked and fluorinated polysiloxane, which is a transparent solid and insoluble in common organic solvents. Differential scanning calorimetric traces (DSC, Figure 1a) and Fourier transform infrared (FT-IR, Figure 1b) are used to monitor the thermopolymerization process. As shown in Figure 1a, TFVE-Si shows a wide processing window with an onset curing temperature of 150 °C and an exothermic peak temperature of 240 °C. FT-IR spectra (Figure 1b) also depict the occurrence of a new characteristic peak at 956 cm−1, attributed to the perfluorocyclobutane (PFCB) groups. The peak at 1830 cm−1 assigned to the original TFVE groups disappears, indicating that the TFVE-Si has been transformed into PFCB-Si. Besides, the peak from 1040 to 1100 cm−1 is also observed after a thermo-induced polymerization reaction, meaning the Si−O−Si moieties with high thermostability still remain in the cross-linked polysiloxane. In order to further study the properties of cross-linked polymer PFCB-Si, a new functional organic siloxane (TFVEDM-Si) with two TFVE groups is designed and prepared via the Piers−Rubinsztajn reaction (Scheme 3) in a high yield. The D
DOI: 10.1021/acs.macromol.7b02000 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 2. TGA curves of the cross-linked PFCB-Si at a heating rate of 10 °C min−1 in a N2 atmosphere.
Figure 5. DSC traces and Tg of the copolymers prepared from TFVEDM-Si and TFVE-Si with different molar ratios.
amount of TFVE-Si raising. Obviously, TFVE-Si can efficiently increase the cross-linking degree of organic polysiloxanes, which is helpful to enhance the thermostability of the polymers. These results indicate that TFVE-Si is suitable as a cross-linking agent for the preparation of cross-linked polysiloxanes. Transparency and Surface Morphology. Polysiloxanes usually show excellent transparency.32,51 In this work, optical properties of PFCB-Si are also investigated. First, a sheet of PFCB-Si is prepared by a thermopolymerization in bulk (details are shown in the Experimental Section). As can be seen from Figure 6a, the sheet of PFCB-Si with an average thickness of 2.0
Figure 3. DMA curves of PFCB-Si at a heating rate of 5 °C min−1.
Figure 6. (a) Photograph of the PFCB-Si sheet with an average thickness of 2.0 mm and (b) flexible free-standing PFCB-Si film.
mm and a diameter of 35 mm shows excellent transparency. The UV−vis−NIR spectrum (Figure 7) of the PFCB-Si sheet illustrates a transmittance of higher than 91% in a range of the wavelengths from 1100 to 420 nm. In the UV region (
