Article pubs.acs.org/Macromolecules
A New Benzoxazine Containing Benzoxazole-Functionalized Polyhedral Oligomeric Silsesquioxane and the Corresponding Polybenzoxazine Nanocomposites Kan Zhang, Qixin Zhuang, Xiaoyun Liu,* Guang Yang, Ruilong Cai, and Zhewen Han The Key Laboratory for Ultrafine Materials of the Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *
ABSTRACT: A new benzoxazole-modified [PhSiO1.5]8(OPS) benzoxazine (OPS−Bz) was synthesized and used to prepare polyhedral oligomeric silsesquioxane (POSS)/ polybenzoxazine (PBz) nanocomposites. Fourier transform infrared spectroscopy (FTIR), 1H NMR, 29Si NMR, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to characterize the structure of OPS−Bz. The high resolution transmission electron microscopy images of POSS/PBz(30/70) nanocomposites showed a well separated nanostructure of POSS with a typical phase size of 3−10 nm. POSS was highly dispersed in the polymer matrix because of the benzoxazole groups around the OPS molecular, in which the rigid benzoxazole groups increased the distance among the POSS molecules and reduced the aggregation of POSS nanoparticles. The TGA study showed these nanocomposites possess good thermal stability. Moreover, the dielectric constants and dielectric loss of these POSS/PBz nanocomposites were low and changed slightly at room temperature in the frequency range of 10 Hz to 1 MHz.
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INTRODUCTION Benzoxazine is a thermosetting phenolic resin that can be synthesized by Mannich condensation from phenol, amine, and formaldehyde.1,2 Ishida et al.3−6 pioneered the development of polybenzoxazines (PBZ) (Scheme S1, Supporting Information) in recent years. They found that these polymers were comparable to or better than advanced epoxies, phenolics, bismaleimides, and even polyimides. The major advantages of PBZ include good mechanical and thermal properties, low water absorption, high carbon residue, and low surface energy.7−15 However, the C−N−C bonds in PBZ are weak and can be easily broken by heat at approximately 260 °C, which limits their application at high temperature.16,17 In addition, the dielectric constant of PBZ, approximately 3.1−3.5, is insufficient for meeting the requirement for low-k microelectronic applications.18 In recent years, the preparation of PBZ with low dielectric constant (k < 3) and high performance has become an important research focus. Hybrid organic−inorganic nanocomposite materials improve the thermal properties and decrease the dielectric constant of PBZ. Polyhedral oligomeric silsesquioxanes (POSS) are organic−inorganic molecules, approximately 1−3 nm in size, with the general formula [RSiO1.5]n, where R is hydrogen or an organic group, such as alkyl, aryl, or any of their derivatives (Scheme 1).19 Generally, POSS cages can be incorporated into polymers by copolymerization and physical blending.20,21 Interestingly, functional substitutions in the POSS framework diversifies and always makes POSS easily compatible and well dispersed in organic polymers.22,23 In addition, introduction of © 2013 American Chemical Society
Scheme 1. Chemical Structure of POSS
POSS effectively reduces the dielectric constants of polymers.24−26 A number of studies on benzoxazine/POSS composites have been published in recent years.27−29 Kuo et al.28 synthesized a novel benzoxazine containing [EtSiO1.5]12 monomer (VBa− POSS) (Scheme S2, Supporting Information). The VBa−POSS monomers underwent ring-opening polymerization under conditions similar to pure benzoxazine. The thermal properties of these POSS-containing PBZ composites were improved compared with those of pure PBZ. Liu et al.29 synthesized Received: February 2, 2013 Revised: March 18, 2013 Published: March 26, 2013 2696
dx.doi.org/10.1021/ma400243t | Macromolecules 2013, 46, 2696−2704
Macromolecules
Article
Scheme 2. Synthesis of OPS−OH
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MMA−POSS/PBZ materials by copolymerization of furancontaining benzoxazine compounds and methyl methacrylate− POSS (Scheme S3, Supporting Information). The POSS-rich phases are round and approximately 60 to 80 nm in size. Most reports on the morphological characterization of the POSS/PBZ composites have rarely shown that POSS are uniformly dispersed at nanoscale in the materials because of aggregation. Obviously, the aggregation of nanoparticles weakens the significance of nanotechnology. Therefore, selecting an appropriate chemical group to modify the POSS is the key to preventing aggregation of POSS and obtaining high-performance composites. Polybenzoxazoles are a class of high-performance polymers used to manufacture organic fiber for ballistic applications based on their very high strength, high modulus, and high thermal stability.30,31 Besides, polybenzoxazoles are considered materials with relatively low dielectric constant (2 < k < 3).32 We recently reported a novel benzoxazole-based polybenzoxazine (Scheme S4, Supporting Information).33 The polybenzoxazine-containing benzoxazole groups exhibited low dielectric constant and high thermal stability. These features motivated us to incorporate the rigid benzoxazole group into POSS to increase the dispersion among molecules and the performance of composites. Moreover, [PhSiO1.5]8(OPS) was selected because of the Friedel−Crafts (F−C) reaction.34−37 OPS could provide reactive sites for F−C reaction between phenyl group from OPS and carboxylic acid from modifier. Thus, benzoxazole groups could successfully connect to the OPS molecules through this method. A novel benzoxazole-based benzoxazine-modified OPS hybrid, called OPS−Bz, was synthesized in this study. POSS/ PBz (PBz is the corresponding polybenzoxazine of common bisphenol A/aniline-based benzoxazine (BA-a)) nanocomposites were prepared by coreaction from OPS−Bz and BA-a. The degree of the dispersion and the size of POSS in the POSS/PBz nanocomposites were characterized. The thermal stability and dielectric properties of these nanocomposites were also examined.
EXPERIMENTAL SECTION
Materials. The reaction medium, poly(phosphoric acid) (PPA) with 80.8% phosphorus pentoxide (P2O5), was freshly prepared using as-received phosphoric acid and P2O5 (Shanghai first Chemical Co., Shanghai). Bisphenol-A/aniline-based benzoxazine (Ba-A) was synthesized as previously reported.38 Other reagents, such as [PhSiO1.5]8 (OPS), 3-hydroxy-4-nitrobenzoic acid, p-hydroxybenzoic acid, chloroform, stannous chloride dehydrate (SnCl2), paraformaldehyde, and aniline, were obtained from Aldrich and used as received. Characterization. Fourier-transform infrared (FTIR) spectra were obtained using a Nicolet 5700 FTIR Spectrometer at a resolution of 4 cm−1. All samples were finely ground with KBr powder and pressed into disk. 1H NMR spectra were recorded with ArANCEIII (400 MHz) using dimethyl sulfoxide-d6 as solvent and tetramethylsilane as an internal standard. Solid-state 29Si NMR spectrum was measured using an AVANCE (500 MHz) spectrometer with tetramethylsilane as the standard. Differential scanning calorimetry (DSC) measurements were conducted with a DSC Q2000 V24.9 Build 121 using N2 as a purge gas (100 mL/min) at a scanning rate of 10 °C/min. Samples (3 mg to 5 mg) were placed in aluminum pans with pierced lids. Thermogravimetry analysis (TGA) was conducted using a NETZSCH STA 409 PC/PG thermogravimetric analyzer. Experiments were conducted using approximately 7 mg samples heated in flowing nitrogen or air (20 mL/min) at a heating rate of 10 °C/min. Dielectric constant and dielectric loss were measured at room temperature using a Concept 40 broadband dielectric spectrometer (Novocontrol Technologies GmbH & Co.). The films were dried at 120 °C under vacuum for 8 h prior to the measurements. A sample with 20−30 mm diameter and 1−2 mm thickness was placed between the two copper electrodes to form a parallel plate capacitor. Morphology was observed by high-resolution transmission electron microscopy (HRTEM). The samples for transmission electron microscopy (TEM) were prepared by dispersing the polymer nanocomposite in acetone mounted on carbon-coated Cu TEM grids and dried for 2 h at room temperature to form a film of
