Studies on structurally different diamines and bisphenol benzoxazines

Abstract

The compounds resorcinolbisbenzoxazine, quinolbisbenzoxazine, p-phenylene diaminebisbenzoxazine, and m-phenylenediaminebisbenzoxazine are prepared. The structural and thermal characterizations of the materials are done using Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (¹H NMR) and ¹³C NMR, and differential scanning calorimetry. Both FTIR and NMR studies reveal the presence of oxazine rings in the synthesized monomers. The curing exotherm of these bisbenzoxazines is much influenced by the nature of the aromatic unit present in the chosen compound and also on the type of the benzoxazine unit in the system. Curing kinetics is performed using Flynn–Wall–Ozawa, Vyazovkin, and Friedman methods. The apparent activation energies (Ea-C) for the thermal curing of the synthesized monomers varied and are dependent on the nature and the position of the functional groups present in the compound. The highest Ea-C values are noted for the compounds having the functionalizations being para oriented.

Keywords

Bisphenols bisbenzoxazines FTIR differential scanning calorimetry model-free kinetics

Introduction

Phenolic resins are widely used in various applications.¹ These phenolic resins have several desirable properties like good mechanical strength, dimensional stability, resistance against various solvents, and flame retardance. They also possess some disadvantages like brittleness, poor shelf life, and so. Since acid or base catalysts are often used for the preparation of phenolic resins, both the corrosion of the processing equipments and the presence of acid or base impurities in the final resin are inevitable. Further appropriate measures must be taken to fabricate void-free parts since during curing phenolic resins release by products.² To overcome these disadvantages, recently a new type of addition cure phenolic system, polybenzoxazines, has been developed. The molecular structure of polybenzoxazines offers enormous design flexibility, which allows the properties of the cured materials to be tailored for a wide range of applications. These newly developed resins possess unique features, namely low water absorption, glass transition temperature (T_g ) is much higher than cure temperature, high char yield, no catalyst required, no by-products released during curing.^3
–5 They have gained immense interest because of their capability to exhibit the thermal and flame retardant properties of phenolics along with mechanical performance and molecular design flexibility. There are several methods for the preparation of monomers, blends, and composites.

The benzoxazines resins can be polymerized to thermoset via thermally activated cationic ring opening reactions.^6
–8 According to the research, most difunctional benzoxazines are synthesized from aromatic bisphenols, monoamines, and formaldehyde. The large varieties of aromatic bisphenols and monoamines allow for considerable molecular design flexibility of benzoxazines. Some special functional groups can be introduced via bisphenols or monoamines to provide certain desired properties. Vijayakumar et al.⁹ prepared indane and biindane bisbenzoxazines in an attempt to increase the thermal stability of the polybisbenzoxazines. Agag and Takeichi prepared phenyl propargyl ether¹⁰ and allylamine-based benzoxazine¹¹ to increase T_g and thermal stability of the compound. Ishida and coworkers synthesized acetylene,^12,13 maleimide,¹⁴ and phenylphosphine oxide¹⁵ containing benzoxazines to increase T_g , char yield, and flame retardancy. Kimura et al.¹⁶ prepared a terpenediphenol-based benzoxazine to reduce water absorption and dielectric constant. Recently, linear polymer containing benzoxazine structures has been reported by Takeichi et al.,¹⁷ Yagci et al.,^18,19 Ishida et al.,^20,21 and Endo et al.^22,23

A gelation is occurring during the condensation of aromatic diamine and formaldehyde.²⁴ To overcome this problem, researchers at Matsushita Electric Works reported the manufacturing of benzoxazines from aromatic diamine and paraformaldehyde using solvent in which both have limited solubility.²⁵ The formation of triazine is limited by the poor solubility of aromatic diamines and paraformaldehyde. To overcome the formation of gels, a direct contact between the aromatic diamine and paraformaldehyde should be avoided. This can be done by preparing the diamine-based benzoxazine using solvent and by a three-step process.^26,27 The mechanism of this procedure has been reported by Andreu and Ronda²⁸ in deuterated benzoxazines.

In the previous studies,^9,29 bisbenzoxazines having indane and spirobiindane structural units have been prepared by solventless method and properties of these compounds have been studied. In the present investigation, the authors studied the curing properties of different bisbenzoxazines having similar molecular formula by choosing hydroquinone (Q), resorcinol (R), p-phenylenediamine (PPDA), and m-phenylenediamine (MPDA) as the reactants. The hydroquinone- and resorcinol-based bisbenzoxazines have been prepared by solvent method whereas PPDA- and MPDA-based bisbenzoxazines are prepared by the three-step process. Thus, this study will help in understanding the two different types of bisbenzoxazines, namely, bisphenol/monoamines and diamines/phenol.

The products p-phenylenediaminebisbenzoxazine (pHBA-pd) and m-phenylenediaminebisbenzoxazine (mHBA-pd) obtained from the three-step process are highly pure and the yields are also high. This three-step process needs solvent and is time consuming compared with the other solvent less single-step process used in preparing bisbenzoxazines from bisphenols and monoamines. The preparation of hydroquinonebisbenzoxazine (QB) and resorcinolbisbenzoxazine (RB) is carried out by solvent method. The formation of the oligomers is less as compared to that of the solvent less method. The removal of the oligomers in bisbenzoxazine requires rigorous purification. It was found that the purification and precipitation method carried out after the synthesis of these bisbenzoxazines is found to be time consuming, and hence the yields of these compounds are less as compared to that of the solvent less method.

Differential scanning calorimetric (DSC) investigations give useful information like glass transition temperature, melting point of the compound, crystallization, and heat of polymerization.³⁰ Appropriate data acquisition using DSC followed by detailed data analysis is a proven methodology to study the kinetics of the thermoset curing process.^31

–36

Model fitting kinetic methods are used to determine the kinetic triplets using multiple heating rate programs, and the kinetic triplet obtained from these methods for nonisothermal condition is highly uncertain and cannot be compared with the kinetic triplets obtained from isothermal condition.^37,38 Vyazovkin (VYZ) model-free approach through use of isoconversional method leads to a trust worthy way of obtaining reliable and consistent kinetic information from nonisothermal data from DSC studies. The variation of the apparent activation energy with the extent of conversion helps to reveal the complexity of multiple reactions taking place during thermal curing of materials. Hence in the present study, the apparent activation energy for the curing of the bisbenzoxazine monomers is obtained using three model-free kinetic [Flynn–Wall–Ozawa (FWO),^32,33 VYZ,³³ and Friedman (FRD)³⁴] methods. The results obtained are compared. This study has been carried out to understand the thermal curing behavior of structurally different bisbenzoxazine monomers.

Experimental

Materials

Paraformaldehyde was purchased from SISCO Research Laboratory Pvt Ltd, Mumbai. Sodium hydroxide, resorcinol, quinol, ethanol, and 1,4-dioxane were purchased from Loba Chemie Pvt Ltd, Mumbai. Sodium borohydrade was purchased from SD Fine-Chem Ltd, Mumbai. Aniline, MPDA, PPDA, 2-hydroxybenzaldehyde (HB), N,N-dimethylformaldehyde (DMF), chloroform, and diethyl ether were purchased from MERCK Specialist Pvt Ltd, Mumbai. Aniline and 1,4-dioxane were distilled and used.

Preparation of 3,3′-(p-phenylene)bis(3,4-dihydro-2H-1,3-benzoxazine)(pHBA-pd) and 3,3′-(m-phenylene)bis(3,4-dihydro-2H-1,3-benzoxazine) (mHBA-pd)

An aromatic diamine, PPDA was employed as the starting material to form pHBA-pd using the three-step process.³⁹

Step (I): HB (0.12 mol = 14.6 g) and (0.06 mol = 6.5 g) PPDA were taken in a 250 mL round-bottomed flask and dissolved in 100 mL of DMF with efficient stirring. The solution was kept at room temperature and stirred for 3 h the formed precipitate was filtered and dried for 24 h in an air oven kept at 80°C to obtain the first intermediate (PPDA-HB) in powder form.

Step (II): Nitrogen was introduced into a reactor to remove the humidity for 30 min and a balloon with hydrogen was assembled on the reactor. The first intermediate (PPDA-HB, 0.03 mol = 9.5 g) was dissolved in 30 mL of ethanol and added to the reactor. Sodium borohydride (0.075 mol = 2.5 g) was divided into three batches and added into the reactant and the contents were kept at room temperature and stirred for 10 h. The mixture was poured into water to precipitate the product formed. The precipitate was filtered and dried for 24 h in an air oven kept at 80°C to obtain the second intermediate (PPDA-HB-r).

Step (III): In a 250 mL round flask, the second intermediate (PPDA-HB-r, 0.03 mol = 9.6 g) was dissolved in 85 mL of chloroform. Formaldehyde solution (0.06 mol = 4.6 mL) was added to the above solution in drop-wise and the mixture was stirred for 4 h at room temperature. The temperature was increased and the solution was refluxed for 5 h. The solution was added into the ethanol solution (ethanol–water = 1:1) to precipitate the bisbenzoxazine (pHBA-pd). The three-step process is presented in Figure 1. The mHBA-pd was also prepared by employing the above procedure. The materials were dried for 24 h in an air oven kept at 80°C.

Preparation of 3,7-diphenyl-3,4,7,8-tetrahydro-2H,6H-1,5-dioxa-3,7-diaza-phenanthrene (RB) and 3,6-diphenyl-2,3,4,5,6,7-hexahydro-1,8-dioxa-3,6-diaza-phenanthrene (QB)

In a 100 mL flask, aniline (0.06 mol = 5.9 g) was dissolved in 25 mL of 1,4-dioxane at room temperature. The solution was cooled in an ice bath, followed by the portion-wise addition of paraformaldehyde (0.12 mol = 3.8 g) with stirring for 10 min. Then quinol (0.03 mol = 3.5 g) was added to this cold solution. The temperature was raised and refluxed for 24 h. After removing 1,4-dioxane under vacuum, the resulting crude product was purified by dissolving in 150 mL of diethyl ether and washing several times with 0.1 N sodium hydroxide and finally two times with distilled water. After drying with anhydrous sodium sulfate, the mass was dried under vacuum at 60°C for 24 h to afford solid.⁴⁰ The reaction is presented in Figure 2.

Thermal curing

The benzoxazines, QB, RB, pHBA-pd, and mHBA-pd were taken in separate micro test tubes and flushed with dry oxygen-free nitrogen and thermally polymerized (QB = 210°C, RB = 225°C, pHBA-pd = 250°C, and mHBA-pd = 140°C) for 6 h. After the polymerization, the samples were removed from the micro test tubes, ground to coarse powder, packed, and stored for further analysis.

Methods

FTIR studies

Shimadzu (S8400, Japan) Fourier transform infrared spectroscopy (FTIR) spectrophotometer was used for recording the FTIR spectra of bisbenzoxazines and polymerized materials. The KBr disc technique was employed. The ¹H and ¹³C NMR spectra were recorded in a Bruker (Billerica, Massachusetts, USA) 300 MHz nuclear magnetic resonance (NMR) spectrometer with tetramethylsilane as the internal standard.

Differential scanning calorimetry

The polymerization behavior of the bisbenzoxazines was examined using DSC Q20 of TA instruments (New Castle, Delaware, USA). Nearly 2 mg of the sample was used to do the experiment. The DSC curves were recorded by heating the material at different heating rates (10, 20, and 30°C min⁻¹) from 40 to 300°C under a constant flow of nitrogen (50 mL min⁻¹).

Kinetic studies

The rate of solid-state reactions can be described as:

\frac{d α}{d t} = k (T) f (α),

where dα/dt is the rate of the reaction, k(T) is the rate constant, and f(α) is the reaction model. According to Arrhenius equation, the temperature-dependent rate constant, k(T), is defined as:

k (T) = A exp (- \frac{E a}{R T}),

where A is the pre-exponential factor, Ea is the apparent activation energy, R is the gas constant, and T is the temperature.

FWO method

The FWO method is widely used for dynamic kinetic analysis and does not require any assumptions to be made about the conversion dependence.^41,42 The equation used for this method is:

E a = \frac{- R}{1.052} \frac{Δ l n β}{Δ (1 / T)},

where R is the gas constant. In this method, plots of ln β versus 1/T give parallel lines for each reaction extent (α) value. The slope of these lines gives Ea, as per the following expression:

S l o p e = - 0.4567 (\frac{E a}{R}) .

VYZ method

The apparent activation energy can be determined at any particular degree of conversion by finding the value of Ea from the following equation:

\sum_{i = 1}^{n} \sum_{j \neq 1}^{n} \frac{I [E a (α), T]_{i} β_{j}}{I [E a (\propto α), T]_{j} β_{i}} = 6 f o r n = 3.

Theory and application of model-free kinetics approaches, starting from basic rate equation and ending in apparent activation energy prediction is discussed in the literature.^43

–49

Friedman method

This is one of the differential methods used to calculate Ea and the equation³⁷ is given as:

l n (\frac{d α}{d T}) = l n A + l n f (α) - (\frac{E a}{R T}) .

A linear regression graph is plotted between ln (dα/dT) and 1/T for different values of α from which Ea is obtained.

Results and discussion

FTIR studies

The FTIR spectra recorded for the compounds PPDA-HB, PPDA-HB-r, MPDA-HB, and MPDA-HB-r are presented in Figure 3. The FTIR spectrum of PPDA-HB and MPDA-HB shows the presence of an absorption band at 1604 cm⁻¹ characteristic of C=N stretching. In the case of PPDA-HB-r and MPDA-HB-r, the presence of an absorption band in the region 3500 cm⁻¹ is responsible for N–H stretching and is overlapping with the broad band responsible for –OH. However, it is much higher in frequency than usual intermolecular OH groups such as water and usual hydroxyl groups. Hence, it should be intramolecularly hydrogen-bonded six-membered ring interacting OH and the nitrogen atom.⁶ The absorption noted at 1600 cm⁻¹ in the FTIR spectrum of PPDA-HB-r corresponds to the N–H-bending vibration. The absorption band for N–H stretching is noted at 3232 cm⁻¹ indicating the reduction of the compound PPDA-HB. Observation of similar absorption bands in MPDA-HB and MPDA-HB-r confirms the presence of the specified groups in these compounds.

The FTIR spectra recorded for the compounds pHBA-pd, mHBA-pd, QB, and RB are presented in Figure 4. The band noted at 948 cm⁻¹ is useful to recognize the oxazine ring structure. This band is due to the benzine ring mode of the benzene to which oxazine is attached, but it is not the oxazine ring mode. When oxazine ring opens, this mode disappears. It is important to note that disappearance of this mode is not the evidence of oxazine polymerization, but it is a simple indication that the oxazine ring opened.³⁸ The absence of the broad absorption peak typical for the –OH group confirms the formation of the monomer. The absence of the band at 3039 cm⁻¹ in pHBA-pd and mHBA-pd indicates that the –NH₂ group present in PPDA and MPDA has been used for the formation of oxazine rings. Furthermore, the presence of the oxazine rings in the prepared compounds was supported by the appearance of new absorption bands for the oxazine asymmetric and symmetric stretching at 1230 and 1500 cm⁻¹, respectively.

The condensation reaction of the phenolic (quinol and resorcinol) compounds with aniline and paraformaldehyde resulted in the formation of QB and RB, respectively. The presence of oxazine rings in the synthesized compounds was supported by the appearance of new absorption bands due to oxazine asymmetric and symmetric stretching at 1230 and 1500 cm⁻¹, respectively. In addition, benzine ring mode of the benzene to which oxazine is attached^10,11,24 (948 cm⁻¹) was also observed for these two compounds in their FTIR spectra (Figure 4). The presence of C₆H₅–N group in QB and RB is confirmed by the presence of an absorption band at 2908 cm⁻¹.

The FTIR spectra recorded for the polymers (poly(pHBA-pd), poly(mHBA-pd), PQB, and PRB) are presented in Figure 4. The FTIR spectrum of the thermally polymerized materials shows disappearance of the bands specific for the oxazine rings and the band pertaining to the phenolic groups appeared. The presence of an intense band at 3361 cm⁻¹ indicates the formation of phenolic groups. The disappearance of the bands specific for the oxazine groups in the FTIR spectra of the polymers indicates the ring-opening thermal polymerization.

NMR studies

The structure of the compounds synthesized is further investigated by ¹H and ¹³C NMR. The ¹H NMR spectra recorded for the benzoxazines are shown in Figure 5. The ¹H NMR and ¹³C NMR spectra of the reaction product obtained from PPDA (pHBA-pd) are very similar to the spectra presented by Chang et al.⁵⁰ The characteristic peaks at 4.54 and 5.33 ppm are assigned to the groups ph-CH₂–N and O–CH₂–N of the oxazine ring, respectively. From the data, it is concluded that the compound pHBA-pd is a highly pure monomer. This result is further confirmed by ¹³C NMR spectrum of pHBA-pd. The peaks obtained at 50.80 and 77.04 ppm confirm the presence of Ar–CH₂–N– and O–CH₂–N– groups, respectively. Similarly, mHBA-pd shows the characteristic peaks for the oxazine ring in ¹H NMR at 4.62 (ph-CH₂–N) and 5.41 (O–CH₂–N) ppm. The presence of peaks at 48.71 and 79.50 ppm in the ¹³C NMR spectrum of mHBA-pd confirmed the oxazine rings.

The peaks at 4.58 and 5.30 ppm for QB and the peaks at 4.50 and 5.26 ppm for RB (Figure 5) prove the presence of oxazine rings. The ¹³C NMR spectra recorded for QB show peaks at 47.94 and 78.96 ppm. Similarly, RB shows peaks at 49.98 and 79.60 ppm. This observation indicates the presence of oxazine rings in both QB and RB. Most of the benzoxazines⁹ shows peaks in this region in their ¹³C NMR spectra and is attributed to Ar–C–N and O–C–N carbons, respectively.

DSC studies: Curing behavior of benzoxazines

The DSC curves recorded at β = 10°C min⁻¹ for the compounds pHBA-pd, mHBA-pd, QB, and RB and the curves for all the four compounds at different heating rates (β) (10, 20, and 30°C min⁻¹) are shown in Figures 6 and 7, respectively.

The curing behavior of pHBA-pd, mHBA-pd, QB, and RB was examined and it is found that the bisbenzoxazines undergo ring-opening polymerization. In the present investigation, it was found that higher curing temperature increased the curing rate and decreased the curing time. Hence the temperature for initiation of the reaction is much dependent on the structural features of the bisbenzoxazine monomer. Once the ring is opened, the formed oligomers will initiate the reaction and autocatalysis facilitates further polymerization reaction. A single crystal X-ray crystallographic study revealed that the preferential conformation of a mono-oxazine ring containing benzoxazine is a distorted semi-chair structure, with the nitrogen and the carbon between the oxygen and nitrogen on the oxazine ring sitting, respectively, above and below the benzene ring plane. The resulting ring strain from this molecular conformation helps this type of six-membered ring undergo ring-opening reaction under specific conditions². Chaisuwan et al.⁷ studied the highly processible maleimide and nitrile-functionalized benzoxazines for advanced composites applications. From the detailed DSC and FTIR studies, it was postulated that the oxazine ring-opening polymerization catalyzes the maleimide polymerization. It has been reported that the ring-opening polymerization of the oxazine ring creates the Schiff base, a highly reactive species.

The parameters obtained from the DSC curves, namely, onset (T_i ), maximum (T _max), end set (T_e ), and enthalpy of cure (ΔH_c ) reaction at β = 10°C min⁻¹ are compiled in Table 1 and discussed. The pHBA-pd and mHBA-pd show sharp melting point at 180 and 115°C, respectively. Both the materials undergo polymerization and the polymerization exotherm maximum was seen at 255 and 155°C. The material pHBA-pd shows a sharp exotherm⁵⁰ compared with mHBA-pd, and the processing window for mHBA-pd is much wider (128–219°C) compared with pHBA-pd. The associated heat of enthalpy of cure reaction for pHBA-pd and mHBA-pd are 415 and 148 J g⁻¹, respectively. Compound pHBA-pd undergoes curing reactions at relatively higher temperature range (238–269°C) than mHBA-pd because during the polymerization of pHBA-pd the formation of Mannich bridge is difficult. A comparison of these two bisbenzoxazines (pHBA-pd and mHBA-pd) shows that the meta orientation of the groups results in lower curing temperature, lower heat release, and with wider processing window.

Table 1.

DSC studies: curing characteristics of benzoxazines β = 10°C min⁻¹.

Sample	T_i (°C)	T _max (°C)	T_e (°C)	T _max − T_i (°C)	T_e − T_i (°C)	ΔH_c (J g⁻¹)	ΔH_c /T_e − T_i (J g⁻¹ °C⁻¹)
RB	190	231	255	41	65	35	0.54
QB	211	241	250	30	39	180	4.62
mHBA-pd	128	155	219	27	91	148	1.63
pHBA-pd	238	255	269	17	31	415	13.39

DSC: differential scanning calometry; RB: resorcinolbisbenzoxazine; QB: quinolbisbenzoxazine; mHBA-pd: m-phenylenediaminebisbenzoxazine; pHBA-pd: p-phenylenediaminebisbenzoxazine; T_i : onset temperature; T _max: cure maximum temperature; T_e : cure end temperature; ΔH_c : enthalpy of curing; T_e − T_i : curing window temperature.

Although Ph–N bonds in pHBA-pd are freely rotatable, owing to the presence of lone pair of electrons in the nitrogen and oxygen of the oxazine rings, the possibility of extended conjugation of these lone pair of electrons with the π-electrons of the aromatic rings is high probable leading to much stable structure. Hence, pHBA-pd cures at very high temperature and once the stable oxazine ring breaks, it can undergo very fast polymerization due to the para disposition of the polymerizable groups. In mHBA-pd, the stabilization due to extended π conjugation is restricted and hence the oxazine rings open at lower temperature to release the steric stain in the system. The hindrance due to meta orientation of the groups is one of the probable factors for the wide curing window seen in mHBA-pd. The ring-opening reaction of pHBA-pd and mHBA-pd is presented in Figure 8.

The bisbenzoxazines derived from bisphenols and monoamines (QB and RB) show nearly the same trend in the curing behavior (T _max, T_e − T_i , ΔH_c and ΔH_c /T_e − T_i ) as that of the bisbenzoxazines derived from diamines and phenols (pHBA-pd and mHBA-pd). Although pHBA-pd and mHBA-pd show very sharp melting points, the materials QB and RB are not exhibiting such a phenomenon. Both QB and RB are having broader processing windows. The curing reached maximum for QB and RB at 241 and 231°C, respectively. The similar curing temperature is also observed for the compound phenol/diaminodiphenylsulfone (241°C) benzoxazine monomer. The phenol/aniline benzoxazine monomer shows a curing temperature around 263°C, and mono benzoxazine monomer shows a high curing temperature.³⁸ Of the four compounds studied, only mHBA-pd has lower cure temperature and can undergo faster ring opening than all the other three bisbenzoxazines.

Comparison of the T _max for the curing for RB and QB indicates that there is not much difference in this value but there is considerable difference in enthalpy of curing. The enthalpy of curing QB and RB are found to be 180 and 35 J g⁻¹, respectively. The RB possesses the lowest value of enthalpy of curing as compared to that of the other compounds.

The discussions made for pHBA-pd and mHBA-pd are not as such applicable to QB and RB. The similarities in the structural aspects of QB and RB favor the ring-opening polymerization in a similar fashion and hence the DSC curves are much similar. Both the materials show broader curing windows. The ring-opening polymerization of QB and RB is presented in Figure 9.

From the enthalpy of curing (ΔH_c ) and the curing temperature window (T_e − T_i ), it is possible to calculate the amount of heat released during the curing process for every degree rise in the curing temperature. This parameter is also included in Table 1 and from this data it was clear that the RB and mHBA-pd (0.54 and 1.63 J g⁻¹ °C⁻¹) release lesser amount of heat during curing for every degree increase in temperature than the other two compounds pHBA-pd and QB (4.62 and 13.39 J g⁻¹ °C⁻¹). These results suggest the influence of the meta and para orientation of the monomer on the thermal curing process. Hence it is obvious that not only the aromatic unit present in between the two benzoxazine moieties influences the thermal curing behavior but also the position of the oxazine group in a system also has much influence on the behavior of the monomers.

Cure kinetics

The apparent activation energy for the curing process (Ea-C) has been calculated using three model-free kinetic methods, namely, FWO, VYZ, and FRD methods. The apparent activation energy is calculated for various reaction extents (α) ranging from 0.2 to 0.8. This range of α value is chosen as per the recommendations given by the Kinetics Committee of the International Confederation for Thermal Analysis and Calorimetry⁵¹ because the relative experimental errors in the kinetic data are larger at the lowest and highest conversions. The Ea-C values calculated for the various α value for the thermal curing of different bisbenzoxazines are shown in Table 2. The plots between the different α values and the corresponding Ea-C values for the compounds pHBA-pd, mHBA-pd, QB, and RB are presented in Figure 10. From this Figure 10, one can easily see that both the integral methods (FWO and VYZ) lead to nearly the same Ea-C values and the trend in variation of Ea-C with respect to α was also found to be similar. Although the Ea-C values calculated using the differential method (FRD) are within the range, slight deviation is noted and is attributed to the approximations made. Jankovic et al.⁴¹ studied the kinetics of the nonisothermal dehydration of equilibrium swellon poly (acrylic acid) hydrogels with thermogravimetric analysis by five different isoconversional methods (FRD, FWO, Kissinger–Akahira–Sunose, Tang, and VYZ). The change in Ea with respect to the reaction extent for the methods FWO, KAS, Tang, and VYZ is found to be very similar and the values are in close agreement but the values differed substantially from the values of Ea obtained with the isoconversional method suggested by FRD. The authors concluded that these differences could have been due to the approximation of the relations that grounded the FWO, KAS, Tang, VYZ, and FRD methods.

Table 2.

Ea-C values for the thermal curing of bisbenzoxazine.

Reaction extent (α)	Ea-C (kJ mol⁻¹)
	RB			QB			mHBA-pd			pHBA-pd
	FWO	VYZ	FRD	FWO	VYZ	FRD	FWO	VYZ	FRD	FWO	VYZ	FRD
0.20	90	87	74	258	289	335	70	68	42	166	166	216
0.25	89	86	73	280	303	346	66	63	25	134	132	246
0.30	89	86	74	284	323	375	62	59	32	109	106	210
0.35	87	84	71	306	351	377	58	55	31	106	103	151
0.40	86	83	69	372	395	392	55	51	24	104	101	100
0.45	84	81	70	375	407	354	52	48	25	104	100	113
0.50	83	79	67	373	397	362	48	44	21	102	99	120
0.55	81	78	67	340	373	304	45	41	18	102	99	114
0.60	80	76	68	330	372	263	43	39	17	101	98	95
0.65	79	75	66	223	323	246	40	36	20	100	97	81
0.70	77	73	62	118	251	135	39	35	26	100	97	75
0.75	76	72	59	104	205	118	38	34	35	99	96	69
0.80	74	70	60	98	184	98	38	33	54	96	93	64

Ea-C: apparent activation energy for curing process; RB: resorcinolbisbenzoxazine; QB: quinolbisbenzoxazine; mHBA-pd: m-phenylenediaminebisbenzoxazine; pHBA-pd: p-phenylenediaminebisbenzoxazine; FWO: Flynn–Wall–Ozawa; VYZ: Vyazovkin; FRD: Friedman.

The bisbenzoxazines RB and mHBA-pd did not show much variation in Ea-C values. The apparent activation energies for the thermal curing is not affected much with increasing α value. This consistency in the Ea-C values may be attributed to the occurrence of similar reactions with similar ease throughout the conversion range.

The material pHBA-pd needs slightly higher apparent activation energy than the materials RB and mHBA-pd in the initial curing stage and then the value levels of just like the meta substituted systems. This may be attributed to the restriction to the ring-opening polymerization of the stabilized oxazine ring.

The bisbenzoxazine QB shows entirely a different type of variation of Ea-C values with respect to change in α values. The Ea-C value progressively increases till α value of 0.45 and then the value decreases. Thus for the material QB, the Ea-C value increases from 289 kJ mol⁻¹ (α = 0.20) to 407 kJ mol⁻¹ (α = 0.45) and then the value decreases continuously and reaches the value184 kJ mol⁻¹ (α = 0.80).

He et al.⁵² studied the curing kinetics of phenol formaldehyde resin systems and observed the fast increase of apparent activation energy at low conversion extent. Since the reaction is almost completed, owing to the less availability of the reactive groups, the addition reactions are few and hence at low conversion the apparent activation energy increases. At higher α value, the Ea value decreases. The monomer molecules become frozen in their positions in the glassy state that results in a virtual cessation of the reaction. The curing rate in the glassy state becomes controlled by diffusion of small, unreacted functional groups still present in the medium and chemical reactions are considerably reduced. This means vitrification causes a dramatic decrease in molecular mobility leading to a decrease of the effective apparent activation energy with increasing extent of reaction.

Compared to the materials RB, mHBA-pd, and pHBA-pd, the bisbenzoxazine QB shows higher apparent activation energy for the curing reactions. The most probable explanation for this is the stability of the ring system present in QB. Owing to this ring stability, the energy needed for the ring-opening polymerization is much higher compared with the other materials investigated.

Dhanalakshmi et al.⁵³ studied high temperature matrix resins based on bispropargyl ethers (BPEs). From a detailed study of the curing kinetics of structurally diverse BPEs and their blends with bismaleimides, they found that the quinol bispropargyl ether (QPE) needs less Ea-C compared with resorcinol bisproporgyl ether (RPE). Based on the structures of these BPEs and the possible isomerized and rearranged product formed during the polymerization of BPEs, the meta substitution in resorcinol definitely needs more energy for the polymerization. In QPE, the formation of two different isomers of benzopyran (phenanthrene and anthracene type) is statistically possible. Whereas in RPE such possibility is restricted and this restriction may be the primary reason for the higher Ea-C for the RPE system compared with the QPE system. Thus the ring-opening polymerization of QB and RB shows different energetics compared with the ring closure reaction occurring during the thermal curing of QPE and RPE.

Conclusion

Bisphenol-based benzoxazines, QB and RB, and aromatic diamine-based benzoxazines, pHBA-pd and mHBA-pd, were prepared. The structures of the monomers were confirmed by NMR studies. The thermal curing characteristics of these bisbenzoxazines were studied in detail using DSC. It was found that the orientation of the polymerizable groups is having significant influence on the thermal curing parameters. The material pHBA-pd cured relatively at higher temperature compared with materials mHBA-pd, QB, and RB. The kinetics of curing of these materials was studied using FWO, VYZ, and FRD methods. Except the material QB, all the other bisbenzoxazines show nearly the same trend in the variation of the apparent activation energy for the thermal curing process (Ea-C) with the reaction extent (α). The progressive increase followed by a progressive decrease in the Ea-C values for QB may be attributed to the structural aspects of this monomer. The initial restriction for the ring-opening polymerization, owing to the ring stability followed by vitrification, leading to the cessation of the polymerization at later stages of the curing reaction may be the probable explanation for the Ea-C trend shown by the material QB.

Footnotes

Acknowledgments

The authors thank the Management and the Principal of Kamaraj College of Engineering and Technology, S. P. G. C. Nagar, K. Vellakulam, for their constant encouragement and support.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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