b Key Lab for Micro-and Nano-Scale Boron Nitride Materials in Hebei Province, Hebei University of Technology, Tianjin 300130, China;
c National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba-city, Ibaraki 305-0047, Japan
Phthalonitrile polymers have demonstrated excellent thermal and oxidative stability as an important class of high performance polymers and offer a variety of opportunities for hi-tech applications. As of now,a series of phthalonitrile resins and phthalonitrile-based composites with excellent combinations of properties and good processability have been successfully developed. Unfortunately,curing phthalonitrile monomer is slow at temperatures below 300 ℃. A curing additive is typically employed to lower the cure temperature and shorten processing time . Aromatic amines have been widely used as curing agents, however their volatility during curing reduces the curing rate and affects the material’s properties.
Recently,phthalonitrile resins,which contain different proportions of amino or hydroxyl in the molecular structure,were found to exhibit self-catalyzed cure behaviors,which avoid the usage of additional catalysts and the loss of volatile curing agents. For the past few years,a variety of self-catalyzed phthalonitrile resins have been developed [2, 3, 4, 5, 6, 7, 8] and exhibit good thermal stability.
In this report,a novel naphthyl-based self-catalyzed phthalonitrile monomer,4-(5-amino-naphthyloxy)phthalonitrile (ANP), was prepared through 5-amino-1-naphthol and 4-nitrophthalonitrile (Scheme 1). A self-promoted curing behavior between the nitrile groups was clearly observed by differential scanning calorimetry (DSC). Thermal decomposition behavior was also explored by thermogravimetric analysis (TGA).2. Experimental 2.1. Materials and measurements
4-Nitrophthalonitrile (NPN) (98.0%)was purchased fromWuhan Chifei Chemical Corporation,China. N,N-Dimethylformamide (DMF) (+99.5%) was supplied by Tianjin Fuchen Corporation. 5-Amino-1-naphthol (97%) was obtained from Chengdu Ai Ke Da Chemical Reagent Co.,LTD. Potassium carbonate (+99.0%) was purchased from Tianjin Fengchuan Chemical Corporation.
Fourier transform infrared (FT-IR) spectrum was recorded on a Nicolet Nexus 670 FT-IR spectrometer in KBr pellets between 4000 cm-1 and 400 cm-1 in air. 1H NMR spectrum was obtained using a Bruker AV400 nuclear magnetic resonance (NMR) spectrometer at a proton frequency of 400 MHz and using DMSO-d6 as solvent. Differential scanning calorimetric (DSC) study was performed on a TA Instruments 2920 DSC from 50 ℃ to 350 ℃ at a heating rate of 10 8C/min with a nitrogen flow rate of 20 mL/min. Thermogravimetric analysis (TGA) was performed using a TA Instruments Q50 thermogravimetric analyzer at a heating rate of 10 ℃/min under nitrogen.2.2. Synthesis of 4-(5-amino-naphthyloxy)phthalonitrile (ANP) and polymer
5-Amino-1-naphthol (AN) (1.3524 g),K2CO3 (1.3059 g) and 35 mL DMF were added into a three-necked flask. The reaction was carried out at 50-60 ℃ for 1 h,and then 4-nitrophthalonitrile (1.4681 g) was added. The resulting mixture was kept at 50-60 ℃ for 6 h. At last,the mixture was poured into 300 mL deionized water. The precipitated solid product was collected,washed by suction filtration with excess deionized water,and then dried at 60 ℃ in a vacuum oven for 24 h. After the monomer polymerized at 300 ℃ for 1 h,the polymer was obtained.
FT-IR spectrum (KBr) (Fig. 2) shows absorptions at 3453 cm-1 and 3375 cm-1 (-NH2),2236 cm-1 (-CN),1632 cm-1 (C=C), 3068 cm-1 (C=C-H),1249 cm-1 and 1086 cm-1 (=C-O-C). 1H NMR (400 MHz,DMSO-d6) δ 5.959 (s,2H,H10),6.727-6.745 (d,1H, H4),6.939-6.959 (d,1H,H7),7.198-7.274 (m,3H,H3,H5,H6), 7.403-7.423 (t,1H,H8),7.797 (s,1H,H2),8.025-8.090 (dd,2H,H1, H9) (Fig. 1).
|Fig. 2.The FT-IR spectra of 4-(5-amino-naphthyloxy)phthalonitrile and polymer.|
The naphthyl-based self-catalyzed phthalonitrile monomer (ANP) was synthesized by a nucleophilic displacement of a labile nitro-substituent from 4-nitrophthalonitrile (NPN) as illustrated in Scheme 1. The reaction took place in DMF solution in the presence of alkaline substance K2CO3 as catalyst. The structure of target product ANP was characterized by nuclear magnetic resonance (1H NMR). The structure of monomer and polymer was characterized by Fourier infrared spectra (FT-IR). Compared with the FT-IR spectrum of ANP monomer,the absorption peaks of -NH2 on the spectrum of ANP polymer changed to the absorption peak of -NH. The absorption peaks of -CN at 2236 cm-1 becomes weaker,while an absorption peak from triazine ring structure at 1357 cm-1 appears ,which indicates that the nitrile groups have participated in the curing reaction. However,no evidence of phthalocyanine or polyindoline formation is observed. The polymerization mechanism is shown in Scheme 2. The amine groups at a,b and c can also initiate other monomers to polymerize and form a highly cross-linked structure. Compared with other phthalonitrile monomers,the self-catalyzed monomers all participate in the reaction and there is no release of small molecules, which further ensures a void-free structure in the final resin.
|Scheme 2.The probable polymerization mechanism of the triazine ring formation.|
The thermal behavior of the naphthyl-based self-catalyzed phthalonitrile was investigated by DSC and TGA. In the curves of DSC,an endothermic transition centered at 207 ℃ of the thermogram appears,which corresponds to the melting point of the monomer. Interestingly,an exothermic peak appears at 251 ℃ as shown in Fig. 3,which is attributed to the self-catalyzed reaction and involves the triazine ring (1357 cm-1) formation and the decrease in density of cyano groups as well as the change of -NH2 to -NH (cured for 300 ℃ and 60 min),where no obvious weight loss was found on the TGA plot. On the TGA curves,before the monomer was fully polymerized,the maximum decomposition temperature appears at 261 ℃ and the char yield was 97.3%. High char yield,73.28%,was obtained as the temperature elevated up to 800 ℃ under nitrogen atmosphere,which indicated that highly cross-linked network was achieved by the reaction among cyano groups . The above phenomenon shows that a self-catalyzed curing behavior was realized for amino-based phthalonitrile,and in the process of curing,4-(5-aminonaphthyloxy)phthalonitrile (ANP) can reduce volatility effectively.
|Fig. 3.The DSC and TGA curves of the 4-(5-amino-naphthyloxy)phthalonitrile monomer.|
In conclusion,the novel naphthyl-based self-catalyzed phthalonitrile monomer provides a new member of self-promoted phthalonitrile monomers and might be used to cure other phthalonitrile materials with void-free structure and high performance.Acknowledgments
This work was supported by Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13060). The authors also thank the financial support from Natural Science Foundation of Hebei Province (No. E2014202033).
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