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Studies on self-promoted cure behaviors of hydroxy-containing phthalonitrile model compounds
European Polymer Journal 45 (2009) 1328–1335
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Short communication
Studies on self-promoted cure behaviors of hydroxy-containing phthalonitrile model compounds Ke Zeng, Ke Zhou, Shaohong Zhou, Haibing Hong, Hongfei Zhou, Yipeng Wang Peikai Miao, Gang Yang * State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, 24, South Section 1, Yihuan Road, Chengdu, Sichuan province 610065, China
a r t i c l e
i n f o
Article history: Received 4 August 2008 Received in revised form 22 December 2008 Accepted 22 December 2008 Available online 30 December 2008
Keywords: Hydroxy-containing phthalonitrile model compounds Self-promoted cure behavior Thermal properties Acidities
a b s t r a c t A series of hydroxy-containing phthalonitrile model compounds (HPNM) with 1:1 molar ratio of hydroxy group to phthalonitrile unit were successfully synthesized. The molecular structures were identified by FTIR and 1H NMR spectroscopic techniques. The model compounds can be thermally polymerized by duration at 225 °C for various times, even in the absence of curing additives. The thermal properties of the cured products were characterized by thermogravimetric analysis. Char yields (800 °C) of the final cured products were in the range 50–73%. The 5% and 10% weight loss ranged from 320 to 420 °C and 360–490 °C, respectively. Differential scanning calorimetry and FTIR were used to monitor the cure reaction. The results reveal that cure behaviors of the HPNM are closely correlated to their molecular structures, although each HPNM has a 1:1 molar ratio of hydroxy group to phthalonitrile unit. Therefore, the thermal properties of the final cured products depend mainly on the molecular structures of the corresponding HPNM, where differences in HPNM acidities should be considered and may contribute to their different cure behaviors. Ó 2008 Elsevier Ltd. All rights reserved.
1. 1.Introduction Phthalonitrile-based resins are a type of high performance polymers having a wide range of potential applications such as composite matrices [1], adhesives [2], electrical conductors [3] and carbon precursors [4]. Phthalonitrile resins offer an attractive combination of properties, i.e., outstanding thermal-oxidative stability, excellent mechanical properties, low water absorptivity and superior flame resistance [5]. Polymerization of these resins occurs through cyano groups by an addition cure mechanism, which ensures that little or no volatiles are evolved during the polymerization leading to void-free crosslinked networks. The polymerization of the neat resins is extremely sluggish and requires several days at elevated tempera* Corresponding author. Tel./fax: +86 028 85469766. E-mail address: [email protected] (G. Yang). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.12.036
tures before a vitrified product is obtained [6]. This process can be promoted through the use of curing additives such as phenols [7], organic amines [6], strong organic acids [8], strong organic acid/amine salts [9], metals and their salts [10]. During the past decades, systematic studies have been conducted at Naval Research Laboratory (NRL) on phthalonitrile resins in terms of cure behavior, processabilities and effects of various linkages between the terminal phthalonitrile units on the properties of the cured resins [11– 19]. These research efforts concerned mainly with commercial available aromatic diamine curing additives such as 1,3-Bis(3-aminophenoxy)benzene (m-APB). The thermal polymerization rate of the resins can be controlled as a function of the concentration of curing additives (typically, 1.5–2 wt%) and curing temperature [5,6,8,11–14,21,22]. Formulation of the resins at the NRL is achieved typically by physical mixing a bisphthalonitrile monomer and a
K. Zeng et al. / European Polymer Journal 45 (2009) 1328–1335
curing additive below the melting temperatures of the bisphthalonitrile monomer or addition to the melting monomer under vigorous stirring [20,21]. To date, phthalonitrile polymers with excellent combination of properties, good processability and phthalonitrile-based composites have been successfully developed by NRL [1,22,23]. However, loss of amine from volatility and thus a slowdown of cure have been observed. To circumvent these problems, two sulfone-containing diamines (m-BAPS and p-BAPS), which have higher molecular weights exhibiting lower volatility and reactivity relative to m-APB, were chosen by workers at NRL as the curing agent to improve the processability of phthalonitrile-based resins [21]. On the other hand, the processing mode of the phthalonitrile resins described previously, is mainly based on bisphthalonitrile monomers/curing additives composition systems referred to as binary composition systems. In these systems, to ensure an uniform cure rate within phthalonitrile resins, evenly molecular dispersion of a trace percent of curing additives into bisphthalonitrile monomers is required and can be achieved merely under extreme conditions (e.g. vigorous stirring) [23]. Therefore, development of a new phthalonitrile resin system having alternative processing modes would be interesting both in the view of science and industrial applications. In our laboratory, the previous investigations have demonstrated that amino or hydroxy-containing phthalonitrile derivatives (APN or HPN) showed a self-promoted cure behavior even in the absence of curing additives which are required for conventional binary composition systems [24,25]. Therefore, curing of the phthalonitrile derivatives containing amino or hydroxy groups offers a new route to the fabrication of phthalonitrile-based polymers or resins possessing excellent mechanical and thermal properties. Furthermore, it also provides a new idea to modify other class of resins such as amide-containing, imide-containing and aromatic ether ketone-containing oligomers by introduction of these structures into the APN or HPN molecular backbone. As a continuation of the research, this paper describes a further study to better understand the HPN molecular structures/cure behaviors relationship. It is well known that processing of thermosetting resins is complicated because of the involvement of chemical reactions. Therefore, understanding of curing is fundamental in the design and processing control of materials. With this aim, a series of hydroxy-containing phthalonitrile model compounds (HPNM) with 1:1 molar ratio of hydroxy group to phthalonitrile unit were designed and synthesized. Their self-promoted cure behaviors were studied by heat treatment at a specified temperature for various times. The cure reaction was monitored by TGA, DSC and FTIR techniques to better understand the effects of the HPNM molecular structures on their cure behaviors. The results reveal that the selfpromoted cure behaviors of the HPNM are closely related to their molecular structures, although each HPNM in this study has a 1:1 molar ratio of hydroxy group to phthalonitrile unit. A further effort to understand these results suggested that differences in HPMN acidities should be taken into consideration and different cure behaviors of HPNM may be correlated to their different acidities.
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2. Experimental section 2.1. Materials 4-nitrophthalonitrile was purchased from Aldrich Chemical Co. (Aldrich) and used as received. 4,40 -Biphenol was purchased from Aldrich and used as received. Hydroquinone and Bisphenol A were purchased from Tianjin Bodi Chemical Co., Ltd (Tianjin, China). N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were purified by distillation under reduced pressure over CaH2. Other chemicals were used as received unless otherwise stated. 2.2. Measurement 1 H NMR spectra were measured on a Bruker Advance400 NMR spectrometer with DMSO-d6 as the solvent and tetramethylsilane as the internal standard. FTIR spectra were recorded with a Nicolet FTIR-380 Fourier transform infrared spectrometer by using KBr disks. Thermogravimetric analysis (TGA) was carried out with a TA instrument Q500 at a heating rate of 10 °C/min under nitrogen. Differential scanning calorimetry (DSC) was carried out with a TA instrument Q200 at a heating rate of 10 °C/min under nitrogen. Melting points of the HPMN were measured by DSC at a heating rate of 10 °C/min. The heat treatment of the HPNM was performed in the TGA chamber by heating powdered samples (25 mg) from room temperature to 225 °C in a ramp mode at heating rate of 10 °C/min and holding for 15, 30 and 60 min (final cure), respectively, under a flow of nitrogen.
2.3. Hydroxy-containing phthalonitrile model compounds (HPNM) syntheses 2.3.1. Synthesis of hydroquinone-based HPNM (3a) This compound was prepared according to the literature [26]. Mp: 158 °C (DSC). 1H NMR (DMSO-d6): 9.63 (1H, s, OH), 8.02–8.05 (1H, d, Ar-H), 7.65–7.66 (1H, d, Ar-H), 7.24–7.28 (1H, dd, Ar-H), 6.97–7.03 (2H, m, Ar-H), 6.81–6.87 (2H, m, Ar-H). FTIR (KBr, cm1): 3412 (AOH), 3036–3104 (C@CAH), 2234 (C„N), 1444–1591 (C@C), 1249 (CAOAC), 1196 (CAOAC). 2.3.2. Synthesis of biphenol-based HPNM (3b) To a 100 mL three-neck flask were added 4-nitrophthalonitrile (1.00 g, 0.006 mol), 4,40 -biphenol (1.30 g, 0.007 mol), anhydrous potassium carbonate (0.96 g, 0.007 mol) and 7.87 ml of DMF. The mixture was stirred at room temperature under nitrogen for 12 h and then poured into a large amount of water. The resulting precipitate was collected by filtration, washed with water until the filtrate was neutral, dried at 70 °C under reduced pressure for 12 h. The crude product was obtained as an offwhite powder. The product 3b was purified by column chromatography over silica gel with dichloride methane/ hexane (3:1) as the mobile phase in a yield of 35%. Mp: 217 °C (DSC). 1H NMR (DMSO-d6): 9.59 (1H, s, OH), 8.08– 8.11 (1H, d, Ar-H), 7.81–7.82 (1H, d, Ar-H), 7.67–7.70 (2H, d, Ar-H), 7.50–7.52 (2H, d, Ar-H), 7.39–7.43 (1H, dd, Ar-H), 7.21–7.24 (2H, d, Ar-H), 6.84–6.87 (2H, d, Ar-H).
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FTIR (KBr, cm1): 3412 (OAH), 2237 (C„N), 1484–1606 (C@C), 1251 (CAOAC), 1215 (CAOAC). 2.3.3. Synthesis of bisphenol A-based HPNM (3c) To a 100 mL three-neck flask were added 4-nitrophthalonitrile (2.50 g, 0.014 mol), bisphenol A (3.33 g, 0.015 mol), anhydrous potassium carbonate (2.02 g, 0.015 mol) and 25 ml of DMSO. The mixture was stirred at room temperature under nitrogen for 12 h and then poured into a large amount of water. The resulting precipitate was collected by filtration, washed with water until the filtrate was neutral, dried at 70 °C under reduced pressure for 12 h. The crude product was obtained as a yellow powder. The product 3c was purified by column chromatography over silica gel with dichloride methane/hexane (3:1) as the mobile phase in a yield of 25%. Mp: 154 °C (DSC). 1H NMR (DMSO-d6): 9.22 (1H, s, OH), 8.07–8.10 (1H, d, Ar-H), 7.77–7.78 (1H, d, Ar-H), 7.34–7.35 (1H, d, Ar-H), 7.28–7.31 (2H, d, Ar-H), 7.02–7.09 (4H, q, Ar-H), 6.66–6.69 (2H, d, Ar-H), 1.61 (6H, s, CH3). FTIR (KBr, cm1): 3431 (OAH), 2965 (CH), 2871 (CH), 2229 (C„N), 1484–1613 (C@C), 1250 (CAOAC), 1220 (CAOAC). 2.3.4. Synthesis of 4-hydroxyphthalonitrile HPNM (3d) This compound was synthesized according to the reported literature [27]. Mp: 217 °C (DSC). 1H NMR (DMSOd6,): 11.49 (1H, s, OH), 7.91–7.94 (1H, d, Ar-H), 7.40–7.41 (1H, d, Ar-H), 7.20–7.24 (1H, dd, Ar-H). FTIR (KBr, cm1): 3266 (OAH), 2242 (C„N), 1493–1604 (C@C), 1225 (CAOAC). 3. Results and discussion
Fig. 1. FTIR spectra of HPNM: (a) 3b, (b) 3c.
HPNM 3b and 3c were synthesized by a simple nucleophilic displacement from 4-nitrophthalonitrile and corresponding diphenols in the presence of anhydrous potassium carbonate. FTIR and 1H NMR techniques were used to identify the structures of 3b and 3c. The FTIR spectra of 3b and 3c showed the characteristic absorption bands at 2237, 2229 cm1 corresponding to cyano group stretching vibration, and 3412, 3431 cm1 corresponding to hydroxy stretching (Fig. 1). High-resolution NMR spectra further confirmed the structures of HPNM 3b and 3c (Fig. 2). Assignments of each proton were given in the figure and the spectra agree well with the proposed molecular structure.
3.1. Synthesis and characterization of HPNM 3.2. Self-promoted cure behaviors A series of hydroxy-containing phthalonitrile derivatives were successfully prepared, as shown in Scheme 1. The synthesis of HPNM 3a and 3d has been previously reported in the literature and the structures were unambiguously confirmed by FTIR and 1H NMR spectra, which agree well with those of the authentic compounds. The
Scheme 1. Synthesis of HPNM 3a, 3b, 3c and 3d.
Self-promoted cure behaviors of the HPNM 3a–d were investigated by thermogravimetric analysis (TGA). Due to the higher melting point of compound 3b (around 217 °C) relative to other model compounds, a dwell temperature of 225 °C was chosen in order to realize the curing of these model compounds under identical thermal conditions. Fig. 3 shows the TGA curves of the HPNM 3a and corresponding cured products. The HPNM 3a after 15 min heat treatment displayed a remarkable improvement in thermal properties compared to the original sample. Further heating of 3a for 30 and 60 min showed a less pronounced progression in the thermal properties. Alternatively, the thermal behavior of HPNM 3a can also be presented on the curves of 5% (10%) weight loss temperature versus times of heat treatment, as shown in Figs. 4 and 5. Furthermore, the HPNM 3b also showed a similar thermal behavior in comparison to 3a after exposed to 225 °C for various times (Figs. 4 and 5), except that 3b showed a higher initial weight loss temperature, probably due to its higher molecular weight and less volatility than 3a. However, HPNM 3c exhibited a slightly different thermal behavior, shown in Figs. 4 and 5. After 15 min heat treatment, 3c displayed a merely slight enhancement in the thermal properties probably suggesting that the cure reaction of 3c proceeded at a slower rate of polymerization
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Fig. 2. 1H NMR spectra of HPNM measured in DMSO-d6: (a) 3b, (b) 3c.
Fig. 3. TGA curves of HPNM 3a before (a) and after heat treatment at 225 °C for (b) 15 min, (c) 30 min and (d) 60 min (final cure).
Fig. 4. Five percent weight loss temperature of HPNM before heat treatment (0 min) and cured products versus times of heat treatment at 225 °C.
Fig. 5. Ten percent weight loss temperature of HPNM before heat treatment (0 min) and cured products versus times of heat treatment at 225 °C.
than 3a and 3b. The similar results had also been reported with respect to the conventional binary composition systems [5]. In the previous study at NRL (Fig. 6), the lower polymerization rate of the BAPh than BPh was related to the fact that the nitrile groups in the BAPh are less electrophilic due to the presence of electron donating AC(CH3)2A linkage relative to phenylene linkage in BPh. However, this explanation was based on bisphthalonitrile monomers/ diamine cure agent systems. Therefore, in this study, the explanation may be different and further understandings about the relationship between the molecular structures of HPNM and their cure behaviors will be presented in the Section 3.3. On the other hand, an unexpected thermal behavior was observed for HPNM 3d. Though no pronounced improvement in the thermal properties was found in the initial cure stage, 3d showed a tremendous increase in 5% and 10% weight loss temperatures from around 200 °C to above 400 °C after 30 min heat treatment.
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Fig. 6. Structures of bisphthalonitrile monomers and curing agent studied previously at NRL.
This progression was much higher than that of other model compounds, shown in Figs. 4 and 5. These observations probably suggested that the polymerization reaction of 3d proceeded at a much higher rate compared to other model compounds when exposed to a temperature 225 °C for 30 min. Fig. 7 shows the char yields (800 °C) of the cured HPNM compounds. These results reveal that the thermal properties are improved with increasing times of heat treatment, but depend mainly on the molecular structures of HPNM. Cured products with excellent thermal properties were obtained after 30 min heat treatment, particularly for the 3d cured product (over 70% weight retention percent at 800 °C). However, no further improvement in thermal properties was observed for all model compounds after 60 min heat treatment probably indicating that the polymerization reaction had progressed at a very slow rate. Furthermore, crosslinking occurs for the final cured products indicated by their partly solubility in hot concentrated sulfuric acid. Differential scanning calorimetry (DSC) has been used to determine the cure of the hydroxy-containing model compounds by integration of the exothermic peak due to hydroxy–cyano or cyano–cyano reaction. DSC curves of
HPNM 3a after heat treatment for various times are illustrated in Fig. 8 (I). Surprisingly, no special feature peaks (exothermic or endothermic peaks) appeared on these curves in the range of temperatures before decomposition occurs, implying that thermal polymerization reaction probably tends to be very slow that the heat generation rate fell below the detectability limit of the calorimeter. Also, the lack of transition peaks may indicate that the polymerization reaction was very fast and has already occurred during heating at 225 °C. Glass transition temperature Tg was also used to follow the cure in this study. However, within temperatures of 300 °C no evident transitions were observed on any of the curves (Fig. 8 (I)). DSC curves of HPNM 3b presented a similar result with that of 3a, except that a weak endothermic peak at around 207 °C was observed on the curve of the 15 min heat treatment sample, probably due to the melting transition of crystalline prepolymers (Fig. 8 (II)). On the other hand, after 15 min heat treatment, model compound 3c displayed an evident Tg transition at a very low temperature of 97 °C indicative of prepolymer formation of relative low conversion degree. The transition disappeared with increasing times of heat treatment (Fig. 8 (III)). These results for model compounds 3a–c are consistent with those obtained from TGA discussion (Figs. 4 and 5). DSC curves of 3d after 30 and 60 min heat treatment were shown in Fig. 8 (IV). Similarly, no exothermic peaks can be found on both of the curves. However, after 30 and 60 min heat treatment, Tg transitions were observed at 243 and 254 °C, respectively, which have disappeared in this range of temperatures for compounds 3a–c. This seems to be incompatible with the results that compounds 3a–c shows much lower thermal properties than that of compound 3d after 30 and 60 min heat treatment (Figs. 4, 5 and 7). However, it is known that many factors such as primary structures, molecular architecture, crosslinking density and condensed structures do contribute to its Tg and thermal properties. Therefore, in the present study, no conclusions can be drawn to the role of these factors and the above incompatible results are still unclear. However, these results indicate that the cure behaviors of hydroxy-containing model compounds in this study are varied and closely correlated to their molecular structures, although each of the HPNM has a 1:1 molar ratio of hydroxy group to phthalonitrile unit. In this study, no further attempts were made to cure the model compounds at elevated temperatures, but it is reasonable believed that it may contribute to the thermal properties of the cured products. This will be discussed in the future reports. 3.3. HPNM Acidities
Fig. 7. Char yields (800 °C) of HPNM before heat treatment (0 min) and cured products versus times of heat treatment at 225 °C.
The above discussions have indicated that cure behaviors of HPNM are closely correlated to their molecular structures, which, in turn, determined the thermal properties of the final cured products. These results arose our interest. Therefore, in an effort to gain more insight into the relationship between the molecular structures and the cure behaviors, the chemical shifts (AOH protons)
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on the 1H NMR spectra of the HPNM 3a–d were considered and associated with their cure behaviors (Table 1). Although quantitative associations were difficult to be given in this study, qualitative analyses could be carried out. It has been determined previously that HPNM 3a and 3b displayed similar cure behaviors, and peaks assigned to AOH protons of 3a and 3b appeared with almost same chemical shift at 9.63 and 9.59 ppm, respectively. A lower AOH chemical shift (9.22 ppm) for HPNM 3c was observed, which showed a lower polymerization rate in the initial cure stage than 3a and 3b, as determined by TGA and DSC. On the other hand, 1H
NMR spectrum of HPNM 3d showed the signal corresponding to AOH proton at a much higher chemical shift (11.49 ppm) than that of other model compounds (Table 1), indicating that 3d has a higher acidity, when compared to 3a–c. This difference in the acidity may be correlated to the previous result that after 30 min heat treatment, 3d showed a higher thermal stability indicating a higher degree of cure than that of 3a–c (Table 1). The above discussions probably suggested that the polymerization of the HPNM was promoted through protonated cyano groups undergoing nucleophilic attack by phenoxy or other cyano groups.
Fig. 8. DSC curves of HPNM 3a (I), 3b (II), 3c (III) and 3d (IV) after heat treatment at 225 °C for (a) 15 min, (b) 30 min and (c) 60 min (final cure).
Table 1 Chemicals shifts of the hydroxyl protons of HPNM and 5% weight loss temperatures after heat treatment at 225 °C. HPNM model compounds
3a 3b 3c 3d a b
AOH (ppm)a
9.63 9.59 9.22 11.49
5% weight loss (°C) 0 minb
15 min
30 min
60 min
230 262 272 201
298 312 276 207
325 341 303 410
345 354 325 417
Chemical shifts were obtained from 1H NMR spectra of HPNM measured in DMSO-d6. Five percent weight loss temperature of HPNM before heat treatment.
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Fig. 9. FTIR spectra of HPNM 3a (I), 3b (II), 3c (III) and 3d (IV) after heat treatment at 225 °C for 15, 30 and 60 min (final cure).
3.4. FTIR analysis FTIR were also used to monitor the polymerization reaction of these model compounds (Fig. 9). Although the cured product spectra are broadened and reduced in intensity, due to a difficulty in grinding the sample to a small particle size, the FTIR spectra could provide more insight into the cure chemistry. Clearly, after normalized to internal standard peaks at 1500 cm1 for 3a–c and 1600 cm1 for 3d, nitrile absorptions at around 2230 cm1 gradually diminished in intensity for all model compounds upon heat treatment. Peaks attributed to triazine absorptions at 1525 and 1521 cm1 were observed for 3a and 3b in the early cure stage (Fig. 9 (I) and (II)) and grow in intensity with increasing extent of polymerization. Furthermore, the nitrile absorptions shifted to lower wavenumbers, similar to a previous report when cyano groups are converted to triazine derivatives [28]. However, no peaks on the IR spectrum of the HPNM 3c can be assigned to characteristic absorptions representative of triazine, phthalocyanine and isoindoline formations [7], because these characteristic absorptions overlapped strong bands in the compound 3c (Fig. 9 (III)). When HPNM 3d was cured for 15 min at 225 °C, a small triazine absorption at 1523 cm1 was observed, which became more intense and partly overlapped
bands in the compound 3d after 30 and 60 min heat treatment. Moreover, a noticeable peak at 1010 cm1 attributed to phthalocyanine ring absorption was observed [28] (Fig. 9 (IV)). These spectra observations indicate that multiple reaction mechanisms may operate in the HPNM cure reaction. 4. Conclusion A series of hydroxy-containing phthalonitrile model compounds (HPNM) with 1:1 molar ratio of hydroxy group to phthalonitrile unit were successfully synthesized. The self-promoted cure behaviors of these model compounds are varied and closely correlated to their molecular structures. The thermal cure reactions of these model compounds progressed at a slow rate after heat treatment for a prolonged period of time at 225 °C. The final cured products displayed excellent thermal properties, which depend mainly on the molecular structures of HPNM, where differences in HPNM acidities should be considered and may contribute to the differences in cure behaviors. Multiple reaction mechanisms may be operational in the HPNM cure reaction. The further studies on cure mechanisms and cure kinetics of hydroxy-containing phthalonitrile model compounds are proceeding in our laboratory.
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Acknowledgement Acknowledgement is made to the Sichuan Santai Gurui Enterprise Co., Ltd for the TGA and DSC measurements. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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[14] Keller TM, Price TR. Polym Commun 1984;25:42–4. [15] Keller TM, Dominguez DD. Polymer 2005;46:4614–8. [16] Laskoski M, Dominguez DD, Keller TM. J Polym Sci A Polym Chem 2005;43:4136–43. [17] Dominguez DD, Keller TM. High Perform Polym 2006;18:283–304. [18] Dominguez DD, Keller TM. Polymer 2007;48:91–7. [19] Laskoski M, Dominguez DD, Keller TM. Polymer 2007;48:6234–40. [20] Keller TM, Sastri SB. US Patent. 6,297,298; 2001. [21] Sastri SB, Keller TM. J Polym Sci A Polym Chem 1998;36: 1885–90. [22] Sastri SB, Armistead JP, Keller TM, Sorathia U. Polym Compos 1997;18:48–54. [23] Dominguez DD, Jones HN, Keller TM. Polym Compos 2004;25: 554–61. [24] Zeng K, Di MW, Han Y, Yang G, et al. In: Guo YM, editor. National conference on composite materials-14. Beijing: China Astronautic Publishing House; 2006. p. 291–7. [25] Zeng K, Zhou K, Tang WR, Yang G, et al. Chin Chem Lett 2007;18: 523–6. [26] Liu XQ, Jikei M, Kakimoto M. Macromolecules 2001;34:3146–54. [27] Woehrle D, Knothe G. Synth Commun 1989;19:3231–9. [28] Griffith JR, Snow AW, Marullo NP. Macromolecules 1984;17: 1614–24.
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Short communication
Studies on self-promoted cure behaviors of hydroxy-containing phthalonitrile model compounds Ke Zeng, Ke Zhou, Shaohong Zhou, Haibing Hong, Hongfei Zhou, Yipeng Wang Peikai Miao, Gang Yang * State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, 24, South Section 1, Yihuan Road, Chengdu, Sichuan province 610065, China
a r t i c l e
i n f o
Article history: Received 4 August 2008 Received in revised form 22 December 2008 Accepted 22 December 2008 Available online 30 December 2008
Keywords: Hydroxy-containing phthalonitrile model compounds Self-promoted cure behavior Thermal properties Acidities
a b s t r a c t A series of hydroxy-containing phthalonitrile model compounds (HPNM) with 1:1 molar ratio of hydroxy group to phthalonitrile unit were successfully synthesized. The molecular structures were identified by FTIR and 1H NMR spectroscopic techniques. The model compounds can be thermally polymerized by duration at 225 °C for various times, even in the absence of curing additives. The thermal properties of the cured products were characterized by thermogravimetric analysis. Char yields (800 °C) of the final cured products were in the range 50–73%. The 5% and 10% weight loss ranged from 320 to 420 °C and 360–490 °C, respectively. Differential scanning calorimetry and FTIR were used to monitor the cure reaction. The results reveal that cure behaviors of the HPNM are closely correlated to their molecular structures, although each HPNM has a 1:1 molar ratio of hydroxy group to phthalonitrile unit. Therefore, the thermal properties of the final cured products depend mainly on the molecular structures of the corresponding HPNM, where differences in HPNM acidities should be considered and may contribute to their different cure behaviors. Ó 2008 Elsevier Ltd. All rights reserved.
1. 1.Introduction Phthalonitrile-based resins are a type of high performance polymers having a wide range of potential applications such as composite matrices [1], adhesives [2], electrical conductors [3] and carbon precursors [4]. Phthalonitrile resins offer an attractive combination of properties, i.e., outstanding thermal-oxidative stability, excellent mechanical properties, low water absorptivity and superior flame resistance [5]. Polymerization of these resins occurs through cyano groups by an addition cure mechanism, which ensures that little or no volatiles are evolved during the polymerization leading to void-free crosslinked networks. The polymerization of the neat resins is extremely sluggish and requires several days at elevated tempera* Corresponding author. Tel./fax: +86 028 85469766. E-mail address: [email protected] (G. Yang). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.12.036
tures before a vitrified product is obtained [6]. This process can be promoted through the use of curing additives such as phenols [7], organic amines [6], strong organic acids [8], strong organic acid/amine salts [9], metals and their salts [10]. During the past decades, systematic studies have been conducted at Naval Research Laboratory (NRL) on phthalonitrile resins in terms of cure behavior, processabilities and effects of various linkages between the terminal phthalonitrile units on the properties of the cured resins [11– 19]. These research efforts concerned mainly with commercial available aromatic diamine curing additives such as 1,3-Bis(3-aminophenoxy)benzene (m-APB). The thermal polymerization rate of the resins can be controlled as a function of the concentration of curing additives (typically, 1.5–2 wt%) and curing temperature [5,6,8,11–14,21,22]. Formulation of the resins at the NRL is achieved typically by physical mixing a bisphthalonitrile monomer and a
K. Zeng et al. / European Polymer Journal 45 (2009) 1328–1335
curing additive below the melting temperatures of the bisphthalonitrile monomer or addition to the melting monomer under vigorous stirring [20,21]. To date, phthalonitrile polymers with excellent combination of properties, good processability and phthalonitrile-based composites have been successfully developed by NRL [1,22,23]. However, loss of amine from volatility and thus a slowdown of cure have been observed. To circumvent these problems, two sulfone-containing diamines (m-BAPS and p-BAPS), which have higher molecular weights exhibiting lower volatility and reactivity relative to m-APB, were chosen by workers at NRL as the curing agent to improve the processability of phthalonitrile-based resins [21]. On the other hand, the processing mode of the phthalonitrile resins described previously, is mainly based on bisphthalonitrile monomers/curing additives composition systems referred to as binary composition systems. In these systems, to ensure an uniform cure rate within phthalonitrile resins, evenly molecular dispersion of a trace percent of curing additives into bisphthalonitrile monomers is required and can be achieved merely under extreme conditions (e.g. vigorous stirring) [23]. Therefore, development of a new phthalonitrile resin system having alternative processing modes would be interesting both in the view of science and industrial applications. In our laboratory, the previous investigations have demonstrated that amino or hydroxy-containing phthalonitrile derivatives (APN or HPN) showed a self-promoted cure behavior even in the absence of curing additives which are required for conventional binary composition systems [24,25]. Therefore, curing of the phthalonitrile derivatives containing amino or hydroxy groups offers a new route to the fabrication of phthalonitrile-based polymers or resins possessing excellent mechanical and thermal properties. Furthermore, it also provides a new idea to modify other class of resins such as amide-containing, imide-containing and aromatic ether ketone-containing oligomers by introduction of these structures into the APN or HPN molecular backbone. As a continuation of the research, this paper describes a further study to better understand the HPN molecular structures/cure behaviors relationship. It is well known that processing of thermosetting resins is complicated because of the involvement of chemical reactions. Therefore, understanding of curing is fundamental in the design and processing control of materials. With this aim, a series of hydroxy-containing phthalonitrile model compounds (HPNM) with 1:1 molar ratio of hydroxy group to phthalonitrile unit were designed and synthesized. Their self-promoted cure behaviors were studied by heat treatment at a specified temperature for various times. The cure reaction was monitored by TGA, DSC and FTIR techniques to better understand the effects of the HPNM molecular structures on their cure behaviors. The results reveal that the selfpromoted cure behaviors of the HPNM are closely related to their molecular structures, although each HPNM in this study has a 1:1 molar ratio of hydroxy group to phthalonitrile unit. A further effort to understand these results suggested that differences in HPMN acidities should be taken into consideration and different cure behaviors of HPNM may be correlated to their different acidities.
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2. Experimental section 2.1. Materials 4-nitrophthalonitrile was purchased from Aldrich Chemical Co. (Aldrich) and used as received. 4,40 -Biphenol was purchased from Aldrich and used as received. Hydroquinone and Bisphenol A were purchased from Tianjin Bodi Chemical Co., Ltd (Tianjin, China). N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were purified by distillation under reduced pressure over CaH2. Other chemicals were used as received unless otherwise stated. 2.2. Measurement 1 H NMR spectra were measured on a Bruker Advance400 NMR spectrometer with DMSO-d6 as the solvent and tetramethylsilane as the internal standard. FTIR spectra were recorded with a Nicolet FTIR-380 Fourier transform infrared spectrometer by using KBr disks. Thermogravimetric analysis (TGA) was carried out with a TA instrument Q500 at a heating rate of 10 °C/min under nitrogen. Differential scanning calorimetry (DSC) was carried out with a TA instrument Q200 at a heating rate of 10 °C/min under nitrogen. Melting points of the HPMN were measured by DSC at a heating rate of 10 °C/min. The heat treatment of the HPNM was performed in the TGA chamber by heating powdered samples (25 mg) from room temperature to 225 °C in a ramp mode at heating rate of 10 °C/min and holding for 15, 30 and 60 min (final cure), respectively, under a flow of nitrogen.
2.3. Hydroxy-containing phthalonitrile model compounds (HPNM) syntheses 2.3.1. Synthesis of hydroquinone-based HPNM (3a) This compound was prepared according to the literature [26]. Mp: 158 °C (DSC). 1H NMR (DMSO-d6): 9.63 (1H, s, OH), 8.02–8.05 (1H, d, Ar-H), 7.65–7.66 (1H, d, Ar-H), 7.24–7.28 (1H, dd, Ar-H), 6.97–7.03 (2H, m, Ar-H), 6.81–6.87 (2H, m, Ar-H). FTIR (KBr, cm1): 3412 (AOH), 3036–3104 (C@CAH), 2234 (C„N), 1444–1591 (C@C), 1249 (CAOAC), 1196 (CAOAC). 2.3.2. Synthesis of biphenol-based HPNM (3b) To a 100 mL three-neck flask were added 4-nitrophthalonitrile (1.00 g, 0.006 mol), 4,40 -biphenol (1.30 g, 0.007 mol), anhydrous potassium carbonate (0.96 g, 0.007 mol) and 7.87 ml of DMF. The mixture was stirred at room temperature under nitrogen for 12 h and then poured into a large amount of water. The resulting precipitate was collected by filtration, washed with water until the filtrate was neutral, dried at 70 °C under reduced pressure for 12 h. The crude product was obtained as an offwhite powder. The product 3b was purified by column chromatography over silica gel with dichloride methane/ hexane (3:1) as the mobile phase in a yield of 35%. Mp: 217 °C (DSC). 1H NMR (DMSO-d6): 9.59 (1H, s, OH), 8.08– 8.11 (1H, d, Ar-H), 7.81–7.82 (1H, d, Ar-H), 7.67–7.70 (2H, d, Ar-H), 7.50–7.52 (2H, d, Ar-H), 7.39–7.43 (1H, dd, Ar-H), 7.21–7.24 (2H, d, Ar-H), 6.84–6.87 (2H, d, Ar-H).
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FTIR (KBr, cm1): 3412 (OAH), 2237 (C„N), 1484–1606 (C@C), 1251 (CAOAC), 1215 (CAOAC). 2.3.3. Synthesis of bisphenol A-based HPNM (3c) To a 100 mL three-neck flask were added 4-nitrophthalonitrile (2.50 g, 0.014 mol), bisphenol A (3.33 g, 0.015 mol), anhydrous potassium carbonate (2.02 g, 0.015 mol) and 25 ml of DMSO. The mixture was stirred at room temperature under nitrogen for 12 h and then poured into a large amount of water. The resulting precipitate was collected by filtration, washed with water until the filtrate was neutral, dried at 70 °C under reduced pressure for 12 h. The crude product was obtained as a yellow powder. The product 3c was purified by column chromatography over silica gel with dichloride methane/hexane (3:1) as the mobile phase in a yield of 25%. Mp: 154 °C (DSC). 1H NMR (DMSO-d6): 9.22 (1H, s, OH), 8.07–8.10 (1H, d, Ar-H), 7.77–7.78 (1H, d, Ar-H), 7.34–7.35 (1H, d, Ar-H), 7.28–7.31 (2H, d, Ar-H), 7.02–7.09 (4H, q, Ar-H), 6.66–6.69 (2H, d, Ar-H), 1.61 (6H, s, CH3). FTIR (KBr, cm1): 3431 (OAH), 2965 (CH), 2871 (CH), 2229 (C„N), 1484–1613 (C@C), 1250 (CAOAC), 1220 (CAOAC). 2.3.4. Synthesis of 4-hydroxyphthalonitrile HPNM (3d) This compound was synthesized according to the reported literature [27]. Mp: 217 °C (DSC). 1H NMR (DMSOd6,): 11.49 (1H, s, OH), 7.91–7.94 (1H, d, Ar-H), 7.40–7.41 (1H, d, Ar-H), 7.20–7.24 (1H, dd, Ar-H). FTIR (KBr, cm1): 3266 (OAH), 2242 (C„N), 1493–1604 (C@C), 1225 (CAOAC). 3. Results and discussion
Fig. 1. FTIR spectra of HPNM: (a) 3b, (b) 3c.
HPNM 3b and 3c were synthesized by a simple nucleophilic displacement from 4-nitrophthalonitrile and corresponding diphenols in the presence of anhydrous potassium carbonate. FTIR and 1H NMR techniques were used to identify the structures of 3b and 3c. The FTIR spectra of 3b and 3c showed the characteristic absorption bands at 2237, 2229 cm1 corresponding to cyano group stretching vibration, and 3412, 3431 cm1 corresponding to hydroxy stretching (Fig. 1). High-resolution NMR spectra further confirmed the structures of HPNM 3b and 3c (Fig. 2). Assignments of each proton were given in the figure and the spectra agree well with the proposed molecular structure.
3.1. Synthesis and characterization of HPNM 3.2. Self-promoted cure behaviors A series of hydroxy-containing phthalonitrile derivatives were successfully prepared, as shown in Scheme 1. The synthesis of HPNM 3a and 3d has been previously reported in the literature and the structures were unambiguously confirmed by FTIR and 1H NMR spectra, which agree well with those of the authentic compounds. The
Scheme 1. Synthesis of HPNM 3a, 3b, 3c and 3d.
Self-promoted cure behaviors of the HPNM 3a–d were investigated by thermogravimetric analysis (TGA). Due to the higher melting point of compound 3b (around 217 °C) relative to other model compounds, a dwell temperature of 225 °C was chosen in order to realize the curing of these model compounds under identical thermal conditions. Fig. 3 shows the TGA curves of the HPNM 3a and corresponding cured products. The HPNM 3a after 15 min heat treatment displayed a remarkable improvement in thermal properties compared to the original sample. Further heating of 3a for 30 and 60 min showed a less pronounced progression in the thermal properties. Alternatively, the thermal behavior of HPNM 3a can also be presented on the curves of 5% (10%) weight loss temperature versus times of heat treatment, as shown in Figs. 4 and 5. Furthermore, the HPNM 3b also showed a similar thermal behavior in comparison to 3a after exposed to 225 °C for various times (Figs. 4 and 5), except that 3b showed a higher initial weight loss temperature, probably due to its higher molecular weight and less volatility than 3a. However, HPNM 3c exhibited a slightly different thermal behavior, shown in Figs. 4 and 5. After 15 min heat treatment, 3c displayed a merely slight enhancement in the thermal properties probably suggesting that the cure reaction of 3c proceeded at a slower rate of polymerization
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Fig. 2. 1H NMR spectra of HPNM measured in DMSO-d6: (a) 3b, (b) 3c.
Fig. 3. TGA curves of HPNM 3a before (a) and after heat treatment at 225 °C for (b) 15 min, (c) 30 min and (d) 60 min (final cure).
Fig. 4. Five percent weight loss temperature of HPNM before heat treatment (0 min) and cured products versus times of heat treatment at 225 °C.
Fig. 5. Ten percent weight loss temperature of HPNM before heat treatment (0 min) and cured products versus times of heat treatment at 225 °C.
than 3a and 3b. The similar results had also been reported with respect to the conventional binary composition systems [5]. In the previous study at NRL (Fig. 6), the lower polymerization rate of the BAPh than BPh was related to the fact that the nitrile groups in the BAPh are less electrophilic due to the presence of electron donating AC(CH3)2A linkage relative to phenylene linkage in BPh. However, this explanation was based on bisphthalonitrile monomers/ diamine cure agent systems. Therefore, in this study, the explanation may be different and further understandings about the relationship between the molecular structures of HPNM and their cure behaviors will be presented in the Section 3.3. On the other hand, an unexpected thermal behavior was observed for HPNM 3d. Though no pronounced improvement in the thermal properties was found in the initial cure stage, 3d showed a tremendous increase in 5% and 10% weight loss temperatures from around 200 °C to above 400 °C after 30 min heat treatment.
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Fig. 6. Structures of bisphthalonitrile monomers and curing agent studied previously at NRL.
This progression was much higher than that of other model compounds, shown in Figs. 4 and 5. These observations probably suggested that the polymerization reaction of 3d proceeded at a much higher rate compared to other model compounds when exposed to a temperature 225 °C for 30 min. Fig. 7 shows the char yields (800 °C) of the cured HPNM compounds. These results reveal that the thermal properties are improved with increasing times of heat treatment, but depend mainly on the molecular structures of HPNM. Cured products with excellent thermal properties were obtained after 30 min heat treatment, particularly for the 3d cured product (over 70% weight retention percent at 800 °C). However, no further improvement in thermal properties was observed for all model compounds after 60 min heat treatment probably indicating that the polymerization reaction had progressed at a very slow rate. Furthermore, crosslinking occurs for the final cured products indicated by their partly solubility in hot concentrated sulfuric acid. Differential scanning calorimetry (DSC) has been used to determine the cure of the hydroxy-containing model compounds by integration of the exothermic peak due to hydroxy–cyano or cyano–cyano reaction. DSC curves of
HPNM 3a after heat treatment for various times are illustrated in Fig. 8 (I). Surprisingly, no special feature peaks (exothermic or endothermic peaks) appeared on these curves in the range of temperatures before decomposition occurs, implying that thermal polymerization reaction probably tends to be very slow that the heat generation rate fell below the detectability limit of the calorimeter. Also, the lack of transition peaks may indicate that the polymerization reaction was very fast and has already occurred during heating at 225 °C. Glass transition temperature Tg was also used to follow the cure in this study. However, within temperatures of 300 °C no evident transitions were observed on any of the curves (Fig. 8 (I)). DSC curves of HPNM 3b presented a similar result with that of 3a, except that a weak endothermic peak at around 207 °C was observed on the curve of the 15 min heat treatment sample, probably due to the melting transition of crystalline prepolymers (Fig. 8 (II)). On the other hand, after 15 min heat treatment, model compound 3c displayed an evident Tg transition at a very low temperature of 97 °C indicative of prepolymer formation of relative low conversion degree. The transition disappeared with increasing times of heat treatment (Fig. 8 (III)). These results for model compounds 3a–c are consistent with those obtained from TGA discussion (Figs. 4 and 5). DSC curves of 3d after 30 and 60 min heat treatment were shown in Fig. 8 (IV). Similarly, no exothermic peaks can be found on both of the curves. However, after 30 and 60 min heat treatment, Tg transitions were observed at 243 and 254 °C, respectively, which have disappeared in this range of temperatures for compounds 3a–c. This seems to be incompatible with the results that compounds 3a–c shows much lower thermal properties than that of compound 3d after 30 and 60 min heat treatment (Figs. 4, 5 and 7). However, it is known that many factors such as primary structures, molecular architecture, crosslinking density and condensed structures do contribute to its Tg and thermal properties. Therefore, in the present study, no conclusions can be drawn to the role of these factors and the above incompatible results are still unclear. However, these results indicate that the cure behaviors of hydroxy-containing model compounds in this study are varied and closely correlated to their molecular structures, although each of the HPNM has a 1:1 molar ratio of hydroxy group to phthalonitrile unit. In this study, no further attempts were made to cure the model compounds at elevated temperatures, but it is reasonable believed that it may contribute to the thermal properties of the cured products. This will be discussed in the future reports. 3.3. HPNM Acidities
Fig. 7. Char yields (800 °C) of HPNM before heat treatment (0 min) and cured products versus times of heat treatment at 225 °C.
The above discussions have indicated that cure behaviors of HPNM are closely correlated to their molecular structures, which, in turn, determined the thermal properties of the final cured products. These results arose our interest. Therefore, in an effort to gain more insight into the relationship between the molecular structures and the cure behaviors, the chemical shifts (AOH protons)
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on the 1H NMR spectra of the HPNM 3a–d were considered and associated with their cure behaviors (Table 1). Although quantitative associations were difficult to be given in this study, qualitative analyses could be carried out. It has been determined previously that HPNM 3a and 3b displayed similar cure behaviors, and peaks assigned to AOH protons of 3a and 3b appeared with almost same chemical shift at 9.63 and 9.59 ppm, respectively. A lower AOH chemical shift (9.22 ppm) for HPNM 3c was observed, which showed a lower polymerization rate in the initial cure stage than 3a and 3b, as determined by TGA and DSC. On the other hand, 1H
NMR spectrum of HPNM 3d showed the signal corresponding to AOH proton at a much higher chemical shift (11.49 ppm) than that of other model compounds (Table 1), indicating that 3d has a higher acidity, when compared to 3a–c. This difference in the acidity may be correlated to the previous result that after 30 min heat treatment, 3d showed a higher thermal stability indicating a higher degree of cure than that of 3a–c (Table 1). The above discussions probably suggested that the polymerization of the HPNM was promoted through protonated cyano groups undergoing nucleophilic attack by phenoxy or other cyano groups.
Fig. 8. DSC curves of HPNM 3a (I), 3b (II), 3c (III) and 3d (IV) after heat treatment at 225 °C for (a) 15 min, (b) 30 min and (c) 60 min (final cure).
Table 1 Chemicals shifts of the hydroxyl protons of HPNM and 5% weight loss temperatures after heat treatment at 225 °C. HPNM model compounds
3a 3b 3c 3d a b
AOH (ppm)a
9.63 9.59 9.22 11.49
5% weight loss (°C) 0 minb
15 min
30 min
60 min
230 262 272 201
298 312 276 207
325 341 303 410
345 354 325 417
Chemical shifts were obtained from 1H NMR spectra of HPNM measured in DMSO-d6. Five percent weight loss temperature of HPNM before heat treatment.
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Fig. 9. FTIR spectra of HPNM 3a (I), 3b (II), 3c (III) and 3d (IV) after heat treatment at 225 °C for 15, 30 and 60 min (final cure).
3.4. FTIR analysis FTIR were also used to monitor the polymerization reaction of these model compounds (Fig. 9). Although the cured product spectra are broadened and reduced in intensity, due to a difficulty in grinding the sample to a small particle size, the FTIR spectra could provide more insight into the cure chemistry. Clearly, after normalized to internal standard peaks at 1500 cm1 for 3a–c and 1600 cm1 for 3d, nitrile absorptions at around 2230 cm1 gradually diminished in intensity for all model compounds upon heat treatment. Peaks attributed to triazine absorptions at 1525 and 1521 cm1 were observed for 3a and 3b in the early cure stage (Fig. 9 (I) and (II)) and grow in intensity with increasing extent of polymerization. Furthermore, the nitrile absorptions shifted to lower wavenumbers, similar to a previous report when cyano groups are converted to triazine derivatives [28]. However, no peaks on the IR spectrum of the HPNM 3c can be assigned to characteristic absorptions representative of triazine, phthalocyanine and isoindoline formations [7], because these characteristic absorptions overlapped strong bands in the compound 3c (Fig. 9 (III)). When HPNM 3d was cured for 15 min at 225 °C, a small triazine absorption at 1523 cm1 was observed, which became more intense and partly overlapped
bands in the compound 3d after 30 and 60 min heat treatment. Moreover, a noticeable peak at 1010 cm1 attributed to phthalocyanine ring absorption was observed [28] (Fig. 9 (IV)). These spectra observations indicate that multiple reaction mechanisms may operate in the HPNM cure reaction. 4. Conclusion A series of hydroxy-containing phthalonitrile model compounds (HPNM) with 1:1 molar ratio of hydroxy group to phthalonitrile unit were successfully synthesized. The self-promoted cure behaviors of these model compounds are varied and closely correlated to their molecular structures. The thermal cure reactions of these model compounds progressed at a slow rate after heat treatment for a prolonged period of time at 225 °C. The final cured products displayed excellent thermal properties, which depend mainly on the molecular structures of HPNM, where differences in HPNM acidities should be considered and may contribute to the differences in cure behaviors. Multiple reaction mechanisms may be operational in the HPNM cure reaction. The further studies on cure mechanisms and cure kinetics of hydroxy-containing phthalonitrile model compounds are proceeding in our laboratory.
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