Synthesis and evaluation of catalytic curing behavior of novel nitrile-functionalized benzoxazine for phthalonitrile resins

Abstract

In the present embodiment, bisphthalonitrile resin has been cured using synthesized novel bisphenyl nitrile-containing benzoxazine (NBZ) vis-a-vis traditional curing agents such as 4-(3′aminophenoxy)diphenyl sulphone (m-DDS) and amino-functionalized phthalonitrile (APN) to study the curing efficacy coupled with processing window and mechanism of curing. It has been established by FT-IR and DSC that NBZ molecules cure the bisphthalonitrile resin matrix by formation of free phenolic groups which act as nucleophile triggering the curing reaction through nitrile–nitrile addition polymerization reaction. Additionally, it has been seen that NBZ-catalyzed phthalonitrile resin curing system showed better processing window in comparison to m-DDS- and APN-catalyzed system. Considering the end-user applications, the NBZ- and m-DDS-cured resins have also been studied for their TG, DMA, water uptake, thermo-oxidative and dielectric properties.

Introduction

Phthalonitrile polymers are an important class of high-performance thermosetting polymers which show excellent thermal, mechanical and thermo-oxidative properties [1–11]. Polymerization of these resins occurs through cyano groups by an addition cure mechanism, which ensures that no volatiles are evolved during the polymerization leading to void-free cross-linked networks. Sluggish polymerization is the key demerit of phthalonitrile resins which takes several days at elevated temperatures to cure the neat resin. To facilitate the early curing various additives such as organic amines, organic acids and their amine salts, transition metals and their salts have been used to decrease the curing temperature as well as curing time [12, 13]. Several researchers have even incorporated phenolic hydroxyl and amino groups into monomers which can initiate in situ curing reaction without the addition of external curing agent [14, 15]. Since the matrix property of these resins is by and large controlled by the curing kinetics, establishing best match of properties, curing is still a gray area of research.

Benzoxazines are a special class of thermosetting polymers that are easy to cure, as they require no curing agent [16–19]. Various types of benzoxazine polymers have been synthesized which have shown excellent thermal and mechanical properties, nearly zero volume shrinkage upon polymerization. But their thermal stability is not at par with other thermosetting polymers, owing to their mechanism of polymerization which involves ring opening followed by formation of Mannich bridges (–CH₂–NR–CH₂–) which are highly prone to cleavage and oxidation [20]. To overcome this problem researchers have end capped these polymers with other polymerizable maleimides, acetylenic and phthalonitrile groups [21–24]. Polymerization of phthalonitrile-, phenyl nitrile-, cyano-containing [25] benzoxazines has been evaluated by catalyzed or uncatalyzed heat treatment.

The curing mechanism of benzoxazine resins suggests that there is formation of free phenolic hydroxyl groups as well as free secondary amino groups in the form of Mannich bridges (–CH₂–NR–CH₂–) during the course of network formation and since phenolic hydroxyl groups can catalyze the curing reaction of the phthalonitrile groups.

However, the mechanistic evaluation of curing efficacy of benzoxazine self-catalytic effect and polymerization of phthalonitrile groups has not been studied so far by any group.

The present study contains the account of work done in respect of NBZ synthesis, characterization, its thermal curing to polybenzoxazine P(NBZ) and its role in curing efficacy of phthalonitrile resin along with tangible proposed mechanism of curing of phthalonitrile resin with NBZ, based on studies made by FT-IR and DSC. The work also covers the comparative analysis of NBZ-catalyzed resin curing system to that of m-DDS- and APN-catalyzed resin curing system.

Experimental

Materials

4,4′-Oxydianiline, 4,4′-diphenol, dry DMSO and 4-nitro phthalonitrile were purchased from Sigma-Aldrich and were used as received without further purification. 4-Hydroxy benzonitrile and paraformaldehyde were purchased from Alfa Aesar and were used as such. Anhydrous K₂CO₃ and xylene were purchased from Merck and were used as such. All apparatus were oven dried prior to use.

Instrumentation

The ¹H NMR and ¹³C NMR spectra were recorded using Bruker Supercon Magnet DRX-400 spectrometer (operating at 400 MHz for ¹H and 100 MHz for ¹³C) using DMSO-d⁶ as solvent. Tetramethylsilane (δ 0.00 ppm) served as an internal standard in ¹H NMR. Infrared spectra were recorded on a Perkin-Elmer FT-IR RXI spectrophotometer. Glass transition temperatures (T_g) and curing temperatures were obtained using differential scanning calorimetry using TA Instruments, USA (Model No. DSC Q200) at a heating rate of 10 °C min⁻¹ under N₂ atmosphere. The degradation temperatures (T_d) were recorded using Mettler Toledo instrument (Model no. SDTA 851) at a heating rate of 20 °C min⁻¹ under N₂ atmosphere. Elemental analysis was made on a Vario-EL Elemental (CHNSO) Analyzer. Water uptake was determined by the change in mass in vacuum-dried films cast on glass petri dish before and after immersion in water at room temperature for 48 h.

Dielectric constant was evaluated using 30-mm circular disc sample (1.5-mm-thick cast film) on DS6000 Dielectric Thermal Analyzer from Lecerta Technology (UK) at 1 kHz frequency. The DTMA analysis has been carried out on EPLEXOR 150 N DTMA of M/s GABO Qualimeter, Germany, in three-point bending mode at the heating rate of 2 °C min⁻¹ with measurement frequency of 10 Hz.

Preparation of monomers

The method for the preparation of NBZ and BPN has been illustrated in Schemes 1 and 2, respectively.

Scheme 1 [Images not available. See PDF.]

Synthesis of NBZ

Scheme 2 [Images not available. See PDF.]

Synthesis of BPN

Preparation of bisphenyl nitrile-containing benzoxazine (NBZ): a mixture of 4, 4′-oxydianiline (ODA) (2000 mg, 10 mmol), 4-hydroxy benzonitrile (2380 mg, 20 mmol) and paraformaldehyde (180 mg, 60 mmol) in p-xylene was refluxed for 4 h. The reaction mixture was cooled and poured in n-heptane to precipitate benzoxazine as yellow solid. The precipitate was then recrystallized in EtOAc/hexane to yield 4020 mg (82.7% yield) of pure NBZ. mp 70 °C (by DSC); FT-IR, (KBr cm⁻¹) 828, 830 1238, 1498, 1608, 2222. ¹H NMR (400 MHz, CDCl₃) δ 4.61 (s, 4H), 5.40 (s, 4H), 6.88 (d, 4H, J = 8.0 Hz), 7.06 (d, 4H, J = 8.0 Hz), 7.34 (s, 2H), 7.43 (d, 4H, J = 8.0 Hz); ¹³C NMR (100 MHz, CDCl₃) δ 50.82 (CH₂), 80.97 (CH₂), 104.02 (C), 117.99 (CH), 119.00 (C), 119.56 (CH), 120.61 (CH), 121.76 (C), 131.17 (CH), 132.21 (CH), 143.47 (C), 152.77 (C) 158.08 (C); Anal. Calcd. for C₃₀H₂₂N₄O₃: % C 74.06, % H 4.56, % N 11.52. Found: % C 74.44, % H 4.42, % N 12.65.

Preparation of BPN: it has been synthesized by the method reported by Keller [1]. A mixture of 4,4′-diphenol (18,600 mg, 100 mmol), 4-nitro phthalonitrile (34,600 mg, 200 mmol) and anhydrous K₂CO₃ (41,400 mg, 300 mmol) in dry DMSO (500 mL) under N₂ atmosphere was stirred at room temperature for 24 h The reaction mixture was then poured in ice-cool water (3000 mL) to precipitate the BPN monomer. The precipitate was then filtered in a sintered funnel. The precipitate was then washed with 1% NaOH solution (3 × 100 mL) and then with water (3 × 100 mL) followed by iso-propanol (100 mL) and then with hexane (100 mL). The precipitate was then dried under vacuum at 80 °C to furnish 39,500 mg (90.2% yield) of pure BPN. mp 235 °C (by DSC); FT-IR, (KBr cm⁻¹) 818, 1090, 1156, 1200, 1314, 1486, 1592, 2232. ¹H NMR (400 MHz, DMSO-d₆) δ 7.31 (d, 4H, J = 8.0 Hz), 7.47 (d, 2H, J = 8.0 Hz), 7.83 (d, 4H, J = 8.0 Hz), 7.87 (s, 2H), 8.14 d, 4H, J = 8.0 Hz); ¹³C NMR (100 MHz, DMSO-d₆) δ 108.84 (C), 115.88 (C), 116.39 (C), 117.22 (C), 121.25 (CH), 122.72 (CH), 123.35 (CH), 129.32 (CH), 136.83 (CH), 137.06 (C), 153.97 (C), 161.37 (C); Anal. Calcd. for C₂₈H₁₄N₄O₂: % C 76.70, % H 3.22, % N 12.78. Found: % C 77.04, % H 3.42, % N 12.95.

The ¹H and ¹³C NMR spectra of NBZ are given in Figs. 1 and 2, respectively.

Fig. 1 [Images not available. See PDF.]

¹H NMR spectrum of NBZ

Fig. 2 [Images not available. See PDF.]

¹³C NMR spectrum of NBZ

Results and discussion

Curing of monomers

Curing of BPN with 4-(3′aminophenoxy)diphenyl sulphone (m-DDS)

5% of m-DDS was added to the monomer of BPN and the mixture was ground in a mortar for 5 min to a fine powder. This powder was used as such for DSC analysis. To the melt stage of monomer of BPN at 240 °C, 5% of m-DDS was added and stirred vigorously and quenched within 15 min to form a pre-polymer or a B-stage resin. 2 gm of this pre-polymer was then cured by dissolving it in 10 mL of DMAc and then pouring this solution into a glass petri dish which was then heated in a vacuum oven at 150, 200, 250, 300 and 350 °C for 4 h each followed by post-curing at 370 °C for 6 h, respectively.

Curing of NBZ

2000 mg of NBZ was dissolved in DMAc and the reaction mixture was poured into a petri dish which was then heated at 150, 160, 180, 200, 225, 250, 270 and 280 °C for 3 h each and then at 300 °C for 6 h for post-curing. The curing reaction of NBZ resulted in the formation of free phenolic hydroxyl groups along with tertiary amino substituents (Scheme 3).

Scheme 3 [Images not available. See PDF.]

Plausible mechanism of curing for BPN + 5% NBZ

Curing of BPN with 5% NBZ

5% of NBZ was added to the monomer of BPN and the mixture was then ground for 5 min in a mortar and then the finely powdered mixture was analyzed for DSC. 2000 mg of this mixture was dissolved in DMAc and poured into a petri dish which was then heated at 150, 180, 200, 220, 250, 280, 300 and 320 °C for 4 h each and then at 350 °C for 6 h for post-curing for the formation of cross-linked network (Scheme 3).

FT-IR analysis for curing

FT-IR spectrum for monomer of BPN showed a characteristic peak at 2232 cm⁻¹ corresponding to –C≡N linkage, while in case of 5% mixture of m-DDS and BPN, peaks at 3108 and 3114 cm⁻¹ were also observed along with 2232 cm⁻¹ due to –NH₂ group. The peak at 2232 ceased in case of cured samples and peak at 1520 cm⁻¹ appeared due to the formation of –C=N– linkage of triazine, the appearance of peak at 1015 indicated the formation of phthalocyanine functionality while peaks at 745, 1595 and 3300 were observed due to isoindoline moiety.

The FT-IR spectrum of monomer of NBZ showed a characteristic peak at 2222 cm⁻¹ due to –C≡N linkage and 1238 and 1498 cm⁻¹ due to benzoxazine ring. The cured sample showed an intense peak at 3250–3260 cm⁻¹ due to free –OH formed upon curing. The appearance of peak at 1595 cm⁻¹ and decrease in intensity of peak at 2222 cm⁻¹ indicated the formation triazine, or (–C=N–) type of system although complete disappearance of –C≡N was not observed probably due to incomplete reaction of –C≡N group owing to low –C≡N function density [18]. The peak at 2232 cm⁻¹ observed in FT-IR spectrum of uncured sample of BPN mixed with 5% NBZ corresponding to –C≡N linkage decreased in case of cured sample, peak at 1520 cm⁻¹ appeared due to the formation of triazine ring system and peak at 1015 cm⁻¹ was observed due to the formation phthalocyanine group while peaks at 745, 1595 and 3300 cm⁻¹ were observed due to isoindoline moiety (Scheme 3 and Figs. 3, 4, 5).

Fig. 3 [Images not available. See PDF.]

FT-IR spectra of uncured samples

Fig. 4 [Images not available. See PDF.]

Expanded FT-IR spectra of cured samples

Fig. 5 [Images not available. See PDF.]

Expanded FT-IR spectra of cured samples

The curing profile and extent of curing of P(NBZ) and cured BPN + 5% m-DDS and BPN + 5% NBZ were also evaluated using FT-IR. The ratio of the non-curable and curable –C≡N peak (2222 cm⁻¹) in case of NBZ was found to be 1.04 which increased to 3.33 in case of P(NBZ), thereby indicating the curing reaction involved –C≡N groups (Fig. 6). The ratio of peaks in case of uncured BPN + 5% m-DDS was found to be 3.89 which increased up to 10.0 in case of cured sample (Fig. 7). The maximum rise in ratio of peaks was observed in case of BPN + 5% NBZ where uncured resin showed a value of 2.27 while cured resin gave a value of 10.67 thereby indicating better curing catalysis due to NBZ (Fig. 8).

Fig. 6 [Images not available. See PDF.]

Comparative curing profile of NBZ and P(NBZ)

Fig. 7 [Images not available. See PDF.]

Comparative curing profile of BPN + 5% m-DDS uncured and cured

Fig. 8 [Images not available. See PDF.]

Comparative curing profile of BPN + 5% NBZ uncured and cured

Differential scanning calorimetric (DSC) analysis

The melting and curing behavior of BPN, BPN + 5% m-DDS, NBZ, and BPN + 5% NBZ was investigated via DSC. The DSC scan of neat uncured BPN showed sharp melting endotherm at 235 °C and no exotherm corresponding to curing reaction up to 300 °C was observed. The DSC scan of uncured BPN + 5% m-DDS showed small endotherm corresponding to melting of m-DDS at 138 °C and then strong endotherm at 236 °C corresponding to melting of mixture followed by an exotherm indicating the initiation of curing at 250 °C with maximum rate of curing at 266 °C and end set of curing at 280 °C. The enthalpy of curing was found to be 11.83 J g⁻¹ with processing window of only 14 °C, curing window of 30 °C and corresponding curing reaction time of 3.0 min at a heating rate of 10 °C min⁻¹. The DSC spectrum of NBZ showed broad melting endotherm at 70 °C while its curing initiated at 156 °C with maximum rate of curing at 227 °C and end set of curing at 272 °C. The corresponding processing window was found to be 86 °C and obtained curing window was 106 °C at a reaction time of 10.6 min and a heating rate of 10 °C min⁻¹. The enthalpy of curing was found to be 184.4 J g⁻¹. The DSC scan of uncured 5% NBZ + BPN showed a sharp endotherm at 238 °C corresponding to melting of the mixture and then a broad exotherm corresponding to curing with initiation of curing at 260 °C, maximum rate of curing at 273 °C and end set of curing at 325 °C. The enthalpy of curing was found to be 24.77 J g⁻¹ along with processing window of 18 °C, curing window of 65 °C and corresponding curing reaction time of 6.5 min at a heating rate of 10 °C min⁻¹. The greater enthalpy of curing, larger processing and curing widow along with more curing time for BPN + 5% NBZ in comparison to BPN + 5% m-DDS indicated better processing can be done for void-free composite matrix with the use of NBZ which can act as a latent curing agent for phthalonitrile resins in comparison to m-DDS (Figs. 9, 10, Table 1).

Fig. 9 [Images not available. See PDF.]

DSC analysis of uncured samples

Fig. 10 [Images not available. See PDF.]

Expanded DSC analysis graph of uncured samples

Table 1. DSC analysis of samples a–d

Entry no.	Sample name	Tm (°C)	Tons (°C)	Tmax (°C)	Tend (°C)	Tg (°C)	Processing window (°C)	Curing window (°C)	Enthalpy of curing (J g−1)
a	BPN	236	–	–	–	–	–		–
b	BPN + 5% m-DDS	235	250	266	280	> 375	14	30	11.83
c	NBZ	070	156	227	272	239	86	106	184.4
d	BPN + 5% NBZ	238	260	273	325	> 375	18	65	24.77

T_m melting point, T_ons onset curing temperature, T_max maximum curing rate temperature, T_end end set curing temperature, T_g glass transition temperature of cured sample

The DSC analysis of the cured sample of NBZ, i.e., P(NBZ) showed glass transition temperature (T_g) at 239 °C, while the cured sample of BPN with 5% m-DDS showed no glass transition up to 375 °C, similarly the cured sample of BPN with 5% NBZ also showed no glass transition up to 375 °C (Fig. 11, Table 1). Although the traditional m-DDS and NBZ system provide almost same processing window, NBZ has an edge over m-DDS having more curing window and better network formation as revealed by the enthalpy of curing which is one order more than m-DDS system.

Fig. 11 [Images not available. See PDF.]

DSC analysis of cured samples b–d

Curing profile of BPN was also evaluated with synthesized amino-functionalized phthalonitrile resin monomer (APN) [26] (see supporting information) to evaluate its relative curing efficacy with NBZ. The DSC graph of neat APN showed melting at 135 °C and onset of curing from 268 °C with maximum rate of curing at 275 °C and end set at 295 °C. The enthalpy of curing was found to be 170.8 J g⁻¹. When 5% of APN was added to the BPN no curing exotherm was observed except a small endotherm at 133 °C and a big endotherm corresponding to melting of resin at 237 °C. Upon addition of 10% of APN, an exotherm corresponding to curing reaction was observed. DSC graph showed a small endotherm at 131 °C, corresponding to the melting of APN followed by melting of mixture at 233 °C and onset of curing at 245 °C with curing maxima at 252 °C and end set at 275 °C. The corresponding processing and curing windows were found to be 8 and 30 °C, respectively, while the enthalpy of curing was found to be 15.58 J g⁻¹. The higher percentage of curing agent (10%), less processing and curing windows along with low enthalpy of curing make amino-functionalized phthalonitrile monomer as a less efficient curing agent in comparison to NBZ (see supporting information).

The relatively large processing and curing window for BPN + 5% NBZ system could be attributed to the fact that NBZ acts as a latent curing agent wherein the free phenolic hydroxyl groups formed upon thermal treatment of NBZ, being less reactive (poor nucleophile) in comparison to free amino groups of m-DDS and APN, initiate the curing reaction slowly thereby increasing the processing window. The slow curing rate also ensures a large curing window. The high Tg of BPN + 5% m-DDS- and BPN + 5% NBZ-cured resin systems also indicated the formation of highly rigid and aromatic polymeric three-dimensional cured system formed due to cyclo-addition polymerization reaction (Scheme 3).

Dynamic thermo-mechanical (DTM) analysis

The DMTA analysis showed no marked drop in storage modulus (E′) as well as no rise in loss modulus (E″) value in both the cases of cured samples of BPN + 5% m-DDS and BPN + 5% NBS up to 400 °C, thereby confirming the T_g > 375 °C obtained from DSC analysis. The consistent storage and loss modulus up to 400 °C confirm the formation of highly rigid and thermo-stable aromatic cross-linked network (Figs. 12, 13).

Fig. 12 [Images not available. See PDF.]

DMTA analysis of cured sample of BPN + 5% m-DDS

Fig. 13 [Images not available. See PDF.]

DMTA analysis of cured sample of BPN + 5% NBZ

Thermo-gravimetric (TG) analysis

The TG analysis of cured sample of BPN + 5% m-DDS showed onset of degradation at 495 °C with maximum rate of decomposition at 566 °C and end set (flattening of curve) at 784 °C. A 72% char yield was obtained at 800 °C for cross-linked BPN + 5% m-DDS. The limiting oxygen index (LOI) calculated from Krevelen’s equation [27] was found to be 46.3%.$$\text{LOI}=17.5+0.4\left(\mathit{\sigma }\right),$$where σ is the % char yield at 800 °C.

The TG analysis of cured sample of NBZ, i.e., P(NBZ) showed onset of degradation at 365 °C with maximum rate of degradation at 502 °C and end set at 720 °C with char yield of 64% at 800 °C and LOI of 43.9% on the basis of Krevelen’s equation.

The TG analysis of cured BPN + 5% NBZ showed onset of degradation at 485 °C with maximum rate of decomposition at 561 °C and end set at 719 °C. The char yield of cross-linked BPN + 5% NBZ at 800 °C was found to be 69% and LOI on the basis of Krevelen’s equation was found to be 45.1%. The TG analysis of cured sample of BPN + 5% NBZ showed comparable thermal stability to that of cross-linked BPN + 5% m-DDS. The char yield at 800 °C and corresponding LOI values of both cured BPN + 5% m-DDS and cross-linked BPN + 5% NBZ systems were almost equal (Table 2 and Fig. 14).

Table 2. TG analysis of cured samples

Entry no.	Sample name	Ton (°C)	Tmax (°C)	Tend (°C)	Char yield at 800 °C (%)	LOI (%)
1	BPN + 5% m-DDS	495	566	784	72	46.3
2	P(NBZ)	365	502	720	66	43.9
3	BPN + 5% NBZ	485	561	719	69	45.1

T_ons onset degradation temperature, T_max maximum degradation temperature, T_end final degradation temperature

Fig. 14 [Images not available. See PDF.]

TG analysis of cured samples

The high decomposition temperature, char yield and LOI values in both the systems indicated the presence of highly cross-linked aromatic system formed by aromatization via cyclization of nitrile groups thereby indicating direct correlation between the char-forming properties and aromatic content of the polymer matrix. The slightly low onset degradation temperature of NBZ-cured system could be attributed to the presence of highly susceptible Mannich bridges of curing agent, which might have started degradation onset.

Isothermal aging

The isothermal aging in air of cured BPN + 5% m-DDS, P(NBZ) and cured BPN + 5% NBZ was carried out by keeping their films coated on a glass petri dish at 300 °C weighing them at 0 h, 100 h, 200 h, and 300 h on a microbalance after each thermal operation. The % mass loss for nth hour was calculated from the following equation:$$\text{Mass loss}\left(\%\right)=\left(W_0-W_{\text{n}}\right)100/W_0,$$ where W₀ is the initial mass of sample at 0 h and W_n is the mass at nth h.

The isothermal aging of cured BPN + 5% m-DDS showed only 0.7, 1.4 and 1.5% mass loss when measured after 100, 200 and 300 h, respectively, at 300 °C, while NBZ showed 5.5, 7.2 and 7.7% mass loss, respectively, after similar intervals when kept at 300 °C. The cured BPN + 5% NBZ showed 0.9, 1.3, 1.5% mass loss under similar conditions. These results showed that in all three cases initial mass loss for first 100 h was substantial in comparison to mass loss after 200 and 300 h. The mass loss between 200 to 300 h was minimal. These results showed that the percentage of mass loss decreased with time. The initial higher mass loss in case of BPN + 5% NBZ was probably due to some decomposition of curing agent component of cured matrix. The decrease in mass loss with respect to time was probably due to stabilization of matrix by initial loss of volatile degradation by products of curing agents. The same percentage mass loss for both cured BPN + 5% m-DDS and BPN + 5% NBZ indicated similar level of cross-linking (Table 3, Graph 1).

Table 3. Isothermal aging of cured samples for 300 h at 300 °C

Entry no.	Sample name	Mass loss after 100 h (%)	Mass loss after 200 h (%)	Mass loss after 300 h (%)
1	BPN + 5% m-DDS	0.7	1.4	1.5
2	P(NBZ)	5.5	7.2	7.7
3	BPN + 5% NBZ	0.9	1.3	1.5

Graph 1 [Images not available. See PDF.]

Bar graph of isothermal aging of cured samples for 100, 200 and 300 h at 300 °C

Water uptake

The % water absorption by BPN + 5% m-DDS, P(NBZ) and BPN + 5% P(NBZ) was measured by immersing their films cast on a glass petri dish into deionised water for 48 h at room temperature (~ 30 °C). The films coated on glass petri dish were then taken out, wiped with tissue paper and quickly weighed on a microbalance. The water uptake of these glass-coated films was calculated from the following equation:$$\text{Water uptake}\left(\%\right)=\left(W_{48}-W_0\right)100/W_0,$$where W₀ is the initial mass of sample at 0 h and W₄₈ is the mass at 48 h.

The water uptake of BPN + 5% m-DDS was 0.15% which was relatively low due to the presence of high aromatic content in polymer skeleton comprising of biphenyl and hetero-aromatic ring systems. The water uptake by P(NBZ) was found to be 0.21% which was expected due to low aromatic content and free phenolic hydroxyl groups and secondary amino substituents formed after curing (Scheme 3). 0.16% water uptake by BPN + 5% NBZ showed the presence of some free phenolic hydroxyl groups in the resin matrix (Table 3).

Dielectric analysis

The cured samples of BPN + 5% m-DDS, P(NBZ) and BPN + 5% NBZ were also evaluated for their dielectric properties. The relative permittivity (dielectric constant) of cured BPN + 5% m-DDS at room temperature under applied frequency of 1 kHz was found to be 3.37 while P(NBZ) showed slightly higher value of 4.57 owing to the presence of polar hydroxyl (–OH) and tertiary amino (–NR–) groups formed upon polymerization. The cured sample of BPN + 5% NBZ showed slightly increased dielectric constant value of 3.45 probably due to the presence of polar groups of curing agent. The loss factor (e″) for P(NBZ) was found to be 0.094 while the cured samples of BPN + 5% m-DDS and BPN +5%BPN showed values of 0.079 and 0.082, respectively (Figs. 15, 16).

Fig. 15 [Images not available. See PDF.]

Dielectric analysis of cured samples

Fig. 16 [Images not available. See PDF.]

Loss factor analysis of cured samples

Reaction mechanism

Plausible reaction mechanism for catalytic curing of BPN + 5% NBZ system has been deduced by FT-IR spectra and DSC analysis. The onset of thermo-polymerization reaction of NBZ to P(NBZ) at 156 °C as revealed in DSC results in the formation of free phenolic hydroxyl groups as shown by FT-IR spectrum with the appearance of peak at 3250 cm⁻¹, which subsequently initiates the nucleophilic addition polymerization reaction in which the trimerization leads to the formation of triazine ring system [2, 28] as depicted by FT-IR spectrum with the appearance of peak at 1520 cm⁻¹, while tetramerization produced phthalocyanine [2, 29] moiety which has been confirmed by the peak at 1015 cm⁻¹ in FT-IR spectrum. The linear addition polymerization resulted in the formation of isoindoline³⁰-based skeleton with was confirmed by the occurrence of peaks at 745, 1595 and 3300 cm⁻¹ in FT-IR spectrum. The tangible reaction mechanism suggests that multiple polymerization phenomenon occurs during curing reaction of BPN + 5% NBZ system leading to the formation of highly complex aromatic cross-linked structure (Scheme 3).

Conclusions

The present study describes the synthesis and characterization of a novel thermosetting nitrile-containing benzoxazine (NBZ) monomer and evaluation of its catalytic curing behavior with conventional phthalonitrile resin. NBZ was found as a better curing agent compared to m-DDS and APN as it allows broader processing window. NBZ cures the BPN resin by initially curing it into P(NBZ), resulting in the formation of free phenolic groups which further reacted with BPN via nucleophilic addition reaction leading to addition polymerization reaction which resulted in the formation of highly aromatic cross-linked network. The isothermal aging and water uptake properties are almost similar in case of m-DDS- and NBZ-cured BPN resin. Presence of polar groups in NBZ-cured BPN has resulted in slightly enhanced dielectric constant of 3.45 against 3. 37 for m-DDS-cured BPN resin.

Acknowledgements

Authors are also thankful to Mr. Ram Prakash for his valuable suggestions in drafting of manuscript.

References

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