Evaluating thermal properties and activation energy of phthalonitrile using sulfur-containing curing agents

1 Introduction

Phthalonitrile, a heterocyclic organic compound comprising a six-membered ring containing four carbon and two nitrogen atoms, has garnered considerable scientific attention in recent years owing to its exceptional properties, particularly its high thermal stability, mechanical strength, flame retardancy, and chemical resistance [1–6]. The capability of phthalonitrile to withstand high temperatures is attributed to the presence of cyano groups (C≡N) linked to the nitrogen atoms in the heterocyclic ring. When a molecule is exposed to increased temperatures, the bond energies decrease as the atoms vibrate more rapidly, causing bond breaking and molecule degradation. However, cyano groups in phthalonitrile stabilize the molecule by increasing electron delocalization across the molecule, resulting in decreased bond energies and making the molecule more resistant to thermal degradation. Here, the cyano groups release water vapor and nitrogen gas when exposed to high temperatures, which can aid in diluting the oxygen concentration and suppressing the combustion process. Furthermore, phthalonitrile exhibits high chemical resistance and can withstand exposure to acids, bases, and solvents, making it a suitable material for use in harsh environments with exposure to chemicals. Previous studies on phthalonitrile have been devoted to the synthesis and curing of bisphthalonitrile [7]. It should be noted that the curing of phthalonitrile monomer is slow at temperatures exceeding 300°C, restricting its applicability [8]. To circumvent this, a significant body of research has been dedicated to identifying effective additives to accelerate the curing rate. Liu et al. proposed using aromatic diamines as curing agents, with an impressively short curing time of 3–16 h, resulting in a Td5% range of 438–450°C in air [9]. Td5% is the temperature at which 5% weight loss occurs, serving as an important indicator of thermal stability. Gao et al. demonstrated that polyaniline exhibits remarkable efficiency in curing phthalonitrile resin, achieving a Td5% of 498.8°C in air [10]. This is attributed to the rigid structure of the curing agent, which could limit the thermal motion of polymer chains at higher temperatures, thereby enhancing thermal stability. Additionally, He et al. highlighted the use of bio-tyrosine cyclic peptide as a curing agent, which achieved a Td5% of 504°C [11]. The curing process for phthalonitrile resins is complex, involving numerous concurrent processes. The properties of the resulting cured bisphthalonitrile polymers are highly reliant on the curing kinetic parameters of polymerization, which are determined by the extent of curing degree, curing conditions, and curing rate. Therefore, a comprehensive understanding of the curing kinetics of phthalonitrile resins is indispensable to achieve the intended structure for improved performance. The Kissinger method is a widely used approach for determining the activation energy of a chemical reaction [12–15]. It assumes that the reaction rate adheres to the Arrhenius equation, which describes the temperature dependence of the rate constant. The Kissinger method entails analyzing the thermal degradation of a sample at different heating rates to determine the activation energy. The activation energy can be acquired from the peak temperature of the derivative thermogravimetric curve. The activation energy is a vital parameter in designing polymers, since it provides insight into the stability of polymers at increased temperatures. The activation energy is the energy needed to break the weakest bonds in a polymer, serving as a measure of the polymer’s stability against thermal degradation. Higher activation energy indicates the increased stability of the polymer at high temperatures. Such parameters obtained from the Kissinger method can be used to optimize the chemical structure and composition of the polymer, enhancing its activation energy and improving its thermal stability.

This work entails the synthesis of phthalonitrile using a curing agent (2, 5, and 10%). Energy-dispersive X-ray spectroscopy (EDX) technique was used to observe the characteristics of samples. The thermal degradation of four different phthalonitrile samples was subsequently analyzed using thermogravimetric analysis (TGA) with the aim of elucidating the thermal behavior of materials. Finally, the Kissinger equation was applied to estimate the activation energy, providing deeper insights into the thermal degradation behavior of the phthalonitrile samples.

2 Methodology

2.1 Material preparation

All chemical reagents used in this study were of the reagent grade and were used without further purification. The synthetic route used to prepare the samples was based on the previously reported methodology as can be seen in Figure 1 and Table 1 [16]. In short, a sequence of chemical reactions was carried out to synthesize two compounds: 2,4-dichloro-6-phenyl-1,3,5-triazine and 2,4-bis(4-fluorophenyl)-6-phenyl-1,3,5-triazine. For the first compound, 2,4-dichloro-6-phenyl-1,3,5-triazine, (4-fluorophenyl)-boronic acid and dioxane were combined in a reactor and stirred. Sodium carbonate was then dissolved in water and added to the mixture while charging it with nitrogen gas. PdCl₂(dppf) was added to the reaction mixture and refluxed for 6 h. The progress of the reaction was monitored to determine when it was complete. The mixture was then cooled to room temperature and filtered, and the filter cake was washed with water. The solid obtained was then dissolved in tetrahydrofuran (THF) and treated with magnesium sulfate, concentrated, and finally added to methanol (MeOH), filtered, washed with MeOH, and dried for 24 h. This yielded 48 g of the solid (yield: 83%) For the second compound, 2,4-bis(4-fluorophenyl)-6-phenyl-1,3,5-triazine, potassium carbonate (K₂CO₃), [1,1′-biphenyl]4-diol, and N-methylpyrrolidone (NMP) were added to a 3 L reactor and heated to 140°C. After 7 h, the temperature was lowered to 50°C and 4-nitrophthalonitrile was added. The temperature was then raised to 80°C and stirred for 24 h. The mixture was then cooled to room temperature, and 2.8 L of 1 N NaOH was added and stirred for 30 min. The solid was filtered, washed with H₂O and MeOH, and then added to 1 L of MeOH:THF (5:1) and stirred for 24 h. The solid was then filtered, washed with MeOH, and put into 2.5 L THF, melted, and filtered in a celite and silica gel pad. The concentrated crude was then put in 1 L of MeOH:THF (5:1) and stirred for another 24 h. The filter cake was then oven-dried at 100°C to obtain 70 g of pale brown solid with a yield of 54%. Sample with various proportions of curing agent was continuously heated to 200°C for 1 h, 250°C for 2 h, 300°C for 4 h, and 350°C for 8 h (Figures 2 and 3). For convenience, sample names in this work are abbreviated as curing ratio-phthalonitrile (PN) (2, 5, and 10%).

Figure 1

Synthetic route of samples used in this study.

Table 1

Dosage of compounds used in this study

	MW (d)	eq.	mol	Dosage
Synthesis of 2,4-bis(4-fluorophenyl)-6-phenyl-1,3,5-triazine
2,4-Dischloro-6-phenyl-1,3,5-triazine	266.06	1.0	0.16	37.7 g
(4-Fluorophenyl)boronic acid	139.92	2.7	0.45	63.0 g
PdCl2(dppf)MC	816.64	0.07	0.01	9.53 g
Na2CO3	105.98	4.0	0.66	70.7 g
Dioxane/H2O	—	—	—	1,168 mL/333 mL
Synthesis of 4,4′-(((((6-phenyl-1,3,5-triazine-2,4-diyl)bis(4,1-phenylene))bis(oxy))bis([1,1′-biphenyl]-4′,4-diyl))bis(oxy))diphthalonitrile
2,4-Bis(4-fluorophenyl)-6-phenyl-1,3,5-triazine	345.45	1.0	0.13	48.0 g
[1,1′-Biphenyl]-4,4′-diol	186.21	2.1	0.29	54.3 g
4-Nitrophthalonitrile	176.13	2.5	0.34	60.1 g
K2CO3	138.21	3.0	4.66	645.3 g
NMP	—	—	—	845 mL

Figure 2

Chemical structure of the curing agent used in this study.

Figure 3

Schematic diagram of the curing process.

3 Results and discussion

Figure 4 and Table 2 provide a detailed insight of EDX analyses conducted on the samples. Given that the samples are predominantly a type of C-rich polymer, the detected amount of C is as expected. The quantification of C content is informative in determining the degree of cross-linking during the curing process. Remarkably, all samples exhibited more than 80% C content, implying that a higher concentration of C may indicate a more thorough reaction between the resin and the curing agent. Indeed, such a reaction leads to the formation of a more densely packed and interlinked network of polymer chains. Notably, the observed decrease in C content in some of the samples may be attributed to the introduction of additional curing agent, which consequently increased the S content. The N content of the samples was found to be high, reflecting the presence of nitrile groups within the polymer structure. This N content is of interest as it is reflective of the extent of curing, with the reaction between curing agent and nitrile groups resulting in the formation of imide groups. Consequently, higher N content may indicate greater levels of curing, a finding consistent with the observed increase in N content with increasing curing agent ratio. The concentration of O in the phthalonitrile samples was found to be consistently low, with a maximum value of <3 wt%. Finally, the presence of S was noted and may be indicative of the utilization of an S-containing curing agent. Indeed, the S content was found to correlate with the degree of curing, as the reaction between the curing agent and the resin leads to the formation of S-containing linkages. However, the relationship between the S content and the curing agent ratio is not straightforward, as the distribution of the curing agent may not be homogenous within the system. Random areas within the resin could contain varying concentrations of the curing agent. Moreover, not all curing agents may be directly involved in the curing process. Consequently, further investigation is required to clarify the observed non-uniform distribution of curing agents within the system, as suggested in Figure 5.

Figure 4

EDX color mapping of samples is shown as the following order of C and S contents: (a) and (b) 2-PN, (c) and (d) 5-PN, and (e) and (f) 10-PN, respectively.

Figure 5

Schematic illustration of phthalonitrile resins with the curing agent in the arbitrarily assumed spatial system. Undistributed curing agents may lead to multiple competing and/or unparticipating residual sites.

The thermal stability of materials is a critical parameter for their application in high-temperature environments including military and space applications [17–20]. The TGA results of the three phthalonitrile samples provide valuable insights into their thermal stability, as indicated by their Td5% values. 2-PN exhibited a Td5% increase at 517.20°C at 2°C·min⁻¹ and finally 532.75°C at 10°C·min⁻¹. Interestingly, the TGA results show that 2-PN experienced early decomposition, which may be due to incomplete curing. The curing process involves the reaction of curing agents with phthalonitrile molecules to form cross-links, and incomplete curing can result in unreacted phthalonitrile molecules that are more prone to thermal degradation. However, increasing the amount of curing agent can solve the early decomposition issue, as at least 5% curing agent was found to be suitable for the phthalonitrile synthesis [16]. 5-PN followed a comparable trend with Td5% increasing from 517.20°C at 2°C·min⁻¹ to 532.75°C at 10°C·min⁻¹. 10-PN also showed a progressive increase in Td5% from 523.16°C at 2°C·min⁻¹ to 540.01°C at 10°C·min⁻¹. The higher thermal stability of samples with a higher amount of curing agent can be attributed to the cross-linking of phthalonitrile molecules. Curing agents can act as cross-linking agents, which form covalent bonds between phthalonitrile molecules, resulting in a three-dimensional network structure that provides better thermal stability [21]. The TGA results also demonstrate that the three samples have excellent thermal stability, with Td5% values above 515°C, indicating their suitability for high-temperature applications as shown in Figure 6 and Table 3.

Figure 6

TGA results of (a) 2-PN, (b) 5-PN, and (c) 10-PN from 2 to 10°C·min⁻¹ of air inert. A summarized plot can be seen in (d). Here, colored lines indicate the air inert of 2°C·min⁻¹ (black), 5°C·min⁻¹ (red), and 10°C·min⁻¹ (blue), respectively.

The Kissinger equation relates the natural logarithm of the reaction rate constant to the inverse of temperature at which it is measured [22]. Major parameters from the Kissinger equation of three samples are listed in Figure 7 and Table 4. E represents the energy necessary to break the chemical bonds of the material and initiate the thermal decomposition process. Therefore, a higher activation energy implies that the material requires more energy to initiate the thermal decomposition process, resulting in higher thermal stability. The results demonstrate that the order of thermal degradation was 10-PN, 5-PN, and 2-PN, in direct correlation with the TGA results. These findings suggest that the proper amount of curing agent, in case of the phthalonitrile resin to have enough space to interact, may enhance the overall thermal degradability.

Figure 7

Kissinger analyses of samples by plotting ln(k/Tm²) against 1,000/Tmk ⁻¹. Here, colored lines indicate the trend of 2-PN (black), 5-PN (red), and 10-PN (blue), respectively.

4 Concluding remarks

In this study, we successfully investigated the activation energy of phthalonitrile using various concentrations of an S-containing curing agent, ranging from 2 to 10%. A noteworthy finding from our investigation was the curing agent that remained uninvolved in the reaction, which led to the early decomposition of 2-PN. The TGA results revealed that higher concentrations of curing agent corresponded to enhanced thermal stability. For instance, a sample treated with a 2% sulfur-containing curing agent displayed a moderate thermal stability, with a Td5% value of 527.11°C in the air. Increasing the concentration of the curing agent to 5 and 10% correspondingly boosted the Td5% values to 532.75 and 540.01°C, respectively. We further used the TGA data to calculate the activation energy by the Kissinger equation, yielding high activation energies of 43.6222, 46.1365, and 67.9515 kcal·mol⁻¹ for the 2, 5, and 10% curing agent dosages, respectively. This enhancement in thermal stability implies an increase in cross-linking density and mechanical properties due to the amplified presence of the curing agent. The outcomes of this research not only enrich our understanding of the energy barrier of phthalonitrile at increased temperatures but also highlight the potential to optimize its application under extreme conditions. Future research should delve deeper into the behavior of uninvolved curing agents during the curing process, with an aim to further improve the performance of phthalonitrile resins.