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Synthesis and Properties of Main-Chain Polybenzoxazines Based on Bisphenol-S.

INTRODUCTION

Polybenzoxazines (PBzs) belong to a class of novel thermosetting resins, and they are produced by thermal or catalytic polymerization of their monomers (i.e., benzoxazines) [1, 2]. In 1944, Holly and Cope first synthesized the benzoxazine monomer by using a solvent method [3]. Later, in 1949, Burke [4] found that the benzoxazine synthesis proceeds via two mechanisms. Further, these benzoxazine rings undergo ring-opening polymerization to form PBzs. This polymerization process proceeds via ring-opening polymerization without any need for an initiator or catalyst, and there is no release of volatiles during polymerization, which results in void-free products [5]. PBzs possess several advantages, including near-zero volumetric shrinkage, low water absorption, high thermal stability, high char yield, excellent mechanical integrity, low flammability, and good dielectric properties [6, 7].

Moreover, the extensive variation of phenols and amines allows considerable design flexibility of their molecular architectures [8, 9]; therefore their properties * can be tailored to accommodate any desired applications. For example, PBz cured from dicarboxylic acid is used as a coating material for circuit boards and semiconductors. PBz-based resins containing Si[O.sub.2] as filler materials are used as wafer-level underfilling materials as alternatives to epoxy resin. As high char-yielding materials, PBzs are used as precursors for aircraft brake pads. PBzs containing boron nitride as a filler material have high thermal conductivity between 3 and 37 W/mK. These types of materials are used in computer cases, battery cases, electronic controller housing, and other encasements [10].

Despite all these advantages (high mechanical strength, innate flame retardancy, excellent dimensional stability, low water retention, beneficial dielectric properties, and relatively high char yields) and applications (electronic materials, matrix resin for fiber-reinforced plastics (FRP), high-performance adhesives, composites, and non-flammable materials), there are some disadvantages associated with them, including high polymerization temperature; lower crosslinking density because of intensive hydrogen bonding [11, 12], which restricts their segmental mobility and hinders their network extension; brittleness; and outgassing during curing [13]. Thus, focusing on reducing the polymerization temperature and increasing the crosslink density (CLD) (by tightening the PBz network structure) are expected to enhance their thermo-mechanical performance [14]. Various approaches have been suggested to increase the CLD and to decrease the polymerization temperature. This include (i) copolymerization through the addition of another reactive group to the phenolic group, such as epoxy resin, bisoxazoline, or hydorxyphenylmaleimide [15]; (ii) introducing another polymerizable group into the benzoxazine structure, such as ethynyl or phenyl ethynyl, nitrile, propargyl, and allyl groups [14, 16]; (iii) preparing polymer alloys of PBz with high-performance polymers or elastomers; (iv) hybridization with inorganic materials, such as layered clay and metal oxide nanoparticles; and (v) designing novel main-chain PBz precursors [17].

The concept of preparing main-chain PBz (MCPBz) precursors has been developed recently [18, 19]. The reaction involves the use of a di-functional phenolic derivative and diamine, producing a linear polymer that contains oxazine rings in its main-chain [20]. These types of polymeric precursors are preferred over monomelic or dimeric benzoxazines for further polymerization. This is because monomelic or dimeric benzoxazines do not form high-molecular-weight polymers, as their chain growth can terminate at the dimer length due to the formation of hydrogen bonding [21].

There is a strong interplay between cure kinetics and physico-mechanical properties [1]. In particular, the use of materials with high crosslinking density can lead to the improvement of thermo-oxidative stability, providing a high char yield after carbonization. It has been found that, for benzoxazine monomers, the onset of thermal cleavage of a Mannich base [22] occurs at approximately 260[degrees]C (independent of the variation of amines and phenolic derivatives), thereby limiting their use in various applications. Various approaches have been proposed to reduce the curing temperature and to improve the property of the cured materials. One proposed approach is the synthesis of main-chain PBzs [23].

In recent years, MCPBz precursors have been synthesized by using bisphenol-A as the diphenolic compound and various types of aliphatic diamines [14]. The isopropyl group in bisphenol-A is weak and does not impart stability to the resulting polymer. Therefore bisphenol-A is replaced with bisphenolS; the sulfonyl group (--S[O.sub.2]--) being a strong electron withdrawing group enables benzoxazine formation, and also adds rigidity to the molecule. Aliphatic diamines are flexible in nature [21], and PBzs based on aliphatic diamines have high CLDs that exceed those of typical bisphenol-based PBzs. This is mainly because the rate of polymerization is inversely proportional to the length of the aliphatic diamine chain [24]. Hence, the rigidity of PBzs can be enhanced by the increased CLD caused by the polymerization of the diamine group. Therefore, combining the diamine group with the sulfone moiety in the benzoxazine structure can unite the flexibility and rigidity of the two groups and improve the properties of the resultant PBz.

The objective of the work is to synthesize MCPBz precursors containing diamines and sulfone groups. Bisphenol-S was chosen as the phenolic moiety. Diamines with short aliphatic chains (ethylene diamine and 1,4-diamino butane) and a relatively high number of amine functionalities (tetraethylenepentamine) were chosen so that polymerization would occur at lower temperatures to produce PBzs with improved thermal stability. The synthesis and characterization of these novel precursors and their thermosets are discussed in comparison with those of traditional bisphenol-A-based PBzs.

EXPERIMENTAL

Materials

Bis-(4-hydroxyphenyl)sulfone, 99.7%, was purchased from Acros Organics, USA. Ethylene diamine and tetraethylenepetamine were purchased from Kanto Chemical Co. Inc., Japan. Also, 1,4-diaminohexane was purchased from Tokyo Chemical Industry Co. Ltd., Japan. Paraformaldehyde, 95%, was obtained from Sigma Aldrich, USA. Sodium hydroxide, extra-pure grade and dimethylsulfoxide (DMSO) were purchased from Duksan Pure Chemicals, Korea. All chemicals were used as received.

Measurements

Fourier transform infrared (FTIR) spectra were obtained with a Perkin Elmer MB3000 FTIR spectrometer. The spectra were obtained at a resolution of 4 [cm.sup.-1] in the IR range of 4000 400 [cm.sup.-1]. Samples were prepared by grinding with KBr and compressed to form discs. Nuclear magnetic resonance (NMR) spectra were recorded by using an Agilent NMR, VNS600 at a proton frequency of 600 MHz for [sup.1]H-NMR and at a carbon frequency of 150 MHz for [sup.13]C-NMR. Solutions were prepared by dissolving the samples in DMSO-[d.sub.6]. Size exclusion chromatography (SEC) analysis was performed with RI detector (Shodex RI-101/p-4000/AT-4000/clarity module) and two columns (KD803 and KD-G) in series. Dimethylformamide (DMF) (>99.9%, Merck, Darmstadt, Germany) was used as an eluent (35[degrees]C, 1 mL/min). Polyethylene glycol) narrow standards (1,400, 4,290, 12,600, 20,600, 44,000, and 69,100 g/mol, Polymer standards service, Mainz, Germany) were used for the calibration. Differential scanning calorimetric (DSC) analysis was carried out using a TA Instruments, Q200 model at a heating rate of 20[degrees]C/min and with a nitrogen flow rate of 50 mL/min. Samples weighing between 5 and 9 mg were crimped in hermetic aluminum pans with lids and used for analysis. Thermogravimetric analysis (TGA) was carried out using a TA Instruments, SDT Q600 model at a heating rate of 10[degrees]C/min up to 800[degrees]C under [N.sub.2] atmosphere.

METHODS

Synthesis of PBz Precursors

In a three-necked round-bottomed flask equipped with a magnetic stirrer and a reflux condenser, paraformaldehyde (1.8 g, 0.06 mol) and DMSO (20 mL) were taken and stirred at 50[degrees]C. Ethylene diamine (1.34 mL, 0.02 mol) was added drop-wise to the mixture during stirring. In the meantime, a solution of bisphenol-S (5.0g, 0.02 mol) in DMSO (10 mL) was prepared separately. After complete addition of ethylene diamine, the solution containing bisphenol-S was added dropwise to the reaction mixture. The temperature was then slowly raised to 120[degrees]C, while the mixture was stirred continuously for 3 h at this temperature. Finally, a transparent, pale-yellow solution was obtained. This solution was then cooled to room temperature and precipitated in 1N NaOH solution. The precipitate thus obtained was washed with distilled [H.sub.2]O several times, filtered, and finally dried in a vacuum at 50[degrees]C for 12 h to afford the PBz precursor (Scheme 1).

Similarly, the other two PBz precursors were synthesized by using the diamines [1,4-diaminobutane (1.76 g, 0.02 m) and tetraethylenepentamine (3.8 mL, 0.02 m)] instead of ethylene diamine. The PBz precursor synthesized from ethylene diamine is abbreviated as PBz-eda; from 1,4-diaminobutane is abbreviated as PBz-dba; and from tetraethylenepentamine is abbreviated as PBz-tepa. The yield of the precursors obtained was between 70% and 75%. Figure 1 depicts the structure of the PBz precursors.

Curing Procedure of PBz Precursors

Approximately, 1 g of the PBz precursors--(PBz-eda, PBzdba and PBz-tepa) was taken in a separate glass plate. This was then placed in an oven and cured at 250[degrees]C for an hour and collected for analysis. PBz-tepa was chosen to monitor the stepwise curing process. First, 1 g sample of PBz-tepa was taken in a glass plate and heated to various temperatures, namely 100, 150, 200, and 250[degrees]C for 1 hr, respectively. A few milligram of each sample was collected at each temperature to study the curing process in a step-wise manner.

RESULTS AND DISCUSSION

Structure Assignment of PBz Precursors

MCPBz precursors were synthesized from bisphenol-S, diamines, and paraformaldehyde in the presence of DMSO as the solvent. DMSO, a high boiling point solvent, acts as a reaction medium and allows reaction to be carried out at a high temperature (at 120[degrees]C), which results in increased solubility of the reaction components. The reaction was completed at the end of 3 hr. Thus, the formation of main-chain PBz precursors with different diamines at elevated temperature is possible.

The chemical structures of the PBz percursors were examined by FTIR and NMR analyses. Figure 2 shows the FTIR spectra of the PBz-eda, PBz-dba, and PBz-tepa percursors. The characteristic absorptions of the benzoxazine structures were observed at 1289 and 1143 [cm.sup.-1] for PBz-eda, at 1290 and 1140 [cm.sup.-1] for PBzdba, and at 1223 and 1141 [cm.sup.-1] for PBz-tepa due to the asymmetric stretching vibrations of the C--O--C and C--N--C groups of the oxazine ring, respectively. The characteristic mode of benzene with an attached oxazine ring appeared at 924, 928, and 939 [cm.sup.-1], whereas, the absorption band characteristic of tri-substituted benzene appeared at 1467, 1485, and 1491 [cm.sup.-1] for PB-zeda, PBz-dba, and PBz-tepa precursors, respectively. Moreover, the bands corresponding to the trisubstituted benzene ring (in plane C=H bending mode) are very intense and sharp. The proposed structures of all the three PBz precursors are linear with the benzoxazine group in the main chain terminating with hydroxyl and primary amine groups on either sides of the precursor. This is clearly indicated by broad bands between 3490 and 3596 [cm.sup.-1] due to--OH stretching vibrations and between 2989 and 2998 [cm.sup.-1] due to--N[H.sub.2] stretching vibrations. The stretching vibrations of--S=O,--S--C, and aromatic C=C appear at 1392, 1378, and 1325 [cm.sup.-1]; at 836, 839, and 832 [cm.sup.-1]; and at 1659, 1504, and 1592 [cm.sup.-1] for PBz-eda, PBz-dba, and PBz-tepa precursors, respectively.

The structure of the PBz precursors was further investigated by NMR spectroscopy. Figures 3-5 show the [sup.1]H-NMR spectra of PBz-eda, PBz-dba, and PBz-tepa precursors. The two characteristic methylene protons of the oxazine ring appear as singlets at 3.7 and 3.4 ppm for PBz-eda, at 4.7 and 3.9 ppm for PBz-dba, and at 4.9 and 4.4 ppm for PBz-tepa, corresponding to the protons of O--C[H.sub.2]--N and Ar--C[H.sub.2]--N, respectively. The aromatic protons of bisphenol-S appear between 6.5 and 8.0 ppm for all the three PBz precursors. The aliphatic chain protons of ethylenediamine, dibutylamine, and tetraethylene pentamine appear between 1 and 3 ppm, in which the amine protons appear between 2.4 and 2.8 ppm, whereas the alkyl protons appear between 1.56 and 1.51 ppm [17, 20]. The peak at 2.5 for all the three PBz precursors is due to the solvent peak (DMSO). [sup.13]C-NMR spectra and SEC analysis are given as Supporting Information S1-S5.

Curing Behavior of PBz Precursors

The curing or ring-opening behavior of the PBz precursors was examined by DSC. Figure 6 shows the non-isothermal DSC thermograms of PBz-eda, PBz-dba, and PBz-tepa precursors, respectively. There are no melting transitions observed for PBzeda and PBz-dba precursors, whereas for PBz-tepa, an endothermic peak appears at 110[degrees]C, corresponding to the melting transition of the long aliphatic chain. An exothermic peak was observed for all three PBz precursors, corresponding to the ring-opening polymerization of the cyclic benzoxazine structure. For PBz-eda, a broad exotherm with the maximum curing temperature at 217[degrees]C was observed. In the cases of PBz-dba and PBztepa, the exothermic peaks narrow and centered at 248 and 230[degrees]C, respectively. Comparison, Table 1, showed that the exotherm of the PBz-eda precursor has a lower curing temperature than the other two precursors [PBz-dba and PBz-tepa]. This is attributed to the presence of ethylene linkage in the PBz-eda precursor, which is more flexible and reactive, and this facilitates ring-opening polymerization at a much lower temperature than that observed for the other two PBz precursors [6, 14].

Figure 7 presents the DSC thermograms of the fully polymerized PBz precursors at 250[degrees]C. At 250[degrees]C, the polymerization of the PBz precursors should have been completed. This could be clearly seen from the DSC thermograms. As expected, exothermic peaks related to polymerization were not found. This clearly shows that all three PBz precursors were completely polymerized.

Progress of Polymerization

FTIR and DSC analyses were used to examine the progress of curing reaction of the PBz precursor PBz-tepa, which was chosen as an example. Samples were cured at various temperatures, namely 100, 150, 200, and 250[degrees]C for 1 hr. Figure 8 presents the FTIR spectra of the PBz-tepa samples after they were subjected to each curing temperature. The characteristic absorptions of the benzoxazine ring between 1223 and 1290 [cm.sup.-1] due to the asymmetric stretching vibrations of C--O--C; between 1467 and 1491 [cm.sup.-1] due to the asymmetric stretching vibrations of C--N--C; and between 924 and 939 [cm.sup.-1] due to the out-of-plane bending vibrations of C--H of oxazine decreased when the curing temperature was increased to 200[degrees]C. When the temperature was further increased to 250[degrees]C, the benzoxazine ring vibrations disappeared completely, confirming that ring-opening polymerization had occurred [14].

Figure 9 shows the DSC thermograms of PBz-tepa obtained at various curing temperatures. As seen from the thermograms, the curing exotherm was shifted to lower temperature and finally disappeared at 250[degrees]C. The amounts of heat liberated ([DELTA]H) during the step-wise curing process were found to be 134.5 J/g at 100[degrees]C, 126.5 J/g at 150[degrees]C, and 64.9 J/g at 200[degrees]C. The figure clearly shows that the amount of exotherm decreased when the curing temperature was increased to 200[degrees]C. PBz-tepa cured at 200[degrees]C liberated less heat (about 64.9 J/g), which means that, at this temperature, there were only a few benzoxazine rings available for polymerization to occur. Finally, at 250[degrees]C, the exothermic peak completely disappeared, thereby confirming the completion of ring-opening polymerization at this temperature [25]. The progress of the curing reaction, as observed by FTIR and DSC analyses, indicates that the polymerization of the precursors proceeds via benzoxazine ring-opening polymerization reaction.

Thermal Stability of PBzs

Figures 10 and 11, respectively, show the TGA and Derivative thermo-gravimetric (DTG) profiles of PBzs obtained at temperatures ranging from room temperature to 800[degrees]C under [N.sub.2] atmosphere, at a heating rate of 10[degrees]C/min, and their data are summarized in Table 2. The 10% degradation temperatures ([T.sub.10]) for the PBzs PBz-eda, PBz-dba, and PBz-tepa were found to be 305, 320, and 330[degrees]C, respectively. PBz-eda has the lowest degradation temperature of 305[degrees]C in comparison with the other two. The presence of an ethylene bridge in the polymer network makes it thermally unstable; hence, it degrades at a lower temperature. The thermal degradation temperature of PBz-tepa is almost 25[degrees]C higher than that of PBz-eda. Furthermore, this value is also higher than that of the polymer synthesized from bisphenol-A and triethylenepentamine [[T.sub.10] = 310[degrees]C]. This is due to the fact that the network structure of PBz-tepa has been improved with increased CLD [14, 17, 21]. Moreover, the presence of nitrogen atoms and rigid sulfonyl groups in PBz-tepa restricts the thermal degradation of the polymer at lower temperatures. The char yields of the polymers were found to be 42.5% for PBz-eda, 30.0% for PBz-dba, and 42.0% for PBz-tepa. Scheme 2 shows the possible structure of the cured resin of PBz-tepa.

The char yield value was used to calculate the limiting oxygen index (LOI) of the polymer by using the van Krevelen and Hoftyzer equation [26], as follows:

LOI = 17.5 + 0.4 x CY,

where, CY is the char yield obtained by TGA analysis at 800[degrees]C. In general, the LOI value of polymers should be above the threshold value of 26, to provide self-extinguishing property. The LOI values of the polymers were found to be 35.5, 29.5, and 34.3 for PBz-eda, PBz-dba, and PBz-tepa, respectively. These values are greater than the threshold value of 26, indicating that the materials would have an excellent flame retardant property.

CONCLUSIONS

High-molecular-weight PBz precursors containing cyclic benzoxazine groups in the main-chain were prepared from bisphenol-S, various aliphatic diamines, and paraformaldehyde. The chemical structures of the PBz precursors characterized by FTIR and NMR spectroscopic analyses, confirmed the formation of oxazine ring in the polymer chain. The onset of curing temperature for all three PBz precursors was found to be lower when compared with BA-a ([T.sub.onset]: 220[degrees]C). The polymerization of PBz precursors proceeds via a ring-opening process, which was confirmed by FTIR and DSC analyses. Among the polymers produced in this study, PBz-tepa exhibits excellent thermal stability, high char yield, and flame retardancy [[T.sub.10] = 330[degrees]C; CY = 42.0%; LOI = 34.3] due to the formation of high CLD and the presence of a greater number of nitrogen atoms in its backbone structure.

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Shakila Parveen (iD), Haekyoung Kim (iD)

School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Republic of Korea

Additional Supporting Information may be found in the online version of this article.

Correspondence to: H. Kim; e-mail: hkkim@ynu.ac.kr Contract grant sponsor: Brain Korea 21 Program (BK21).

DOI 10.1002/pen.24777

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: SCHEME 1. Synthesis of PBz precursors. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 1. Structure of the PBz precursors: (a) PBz-eda, (b) PBz-dba, and (c) PBz-tepa. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. FTIR spectra of (a) PBz-eda, (b) PBz-dba, and (c) PBz-tepa precursors. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. [sup.1]H-NMR spectrum of PBz-eda precursor. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. [sup.1]H-NMR spectrum of PBz-dba precursor. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. [sup.1]H-NMR spectrum of PBz-tepa precursor. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. DSC spectra of (a) PBz-eda, (b) PBz-dba, and (c) PBz-tepa precursors. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. DSC spectra of (a) PBz-eda, (b) PBz-dba, and (c) PBz-tepa. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. FTIR spectra of PBz-tepa at different curing temperatures. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. DSC spectra of PBz-tepa at different curing temperatures: (a) 100[degrees]C, (b) 150[degrees]C, (c) 200[degrees]C, and (d) 250[degrees]C. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 10. TGA thermograms of (a) PBz-eda, (b) PBz-dba, and (c) PBz-tepa. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 11. Derivative thermo-gravimetric (DTG) thermograms of (a) PBz-eda, (b) PBz-dba, and (c) PBz-tepa. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: SCHEME 2. Possible structure of PBz-tepa precursor and its cured resin.
TABLE 1. DSC data of PBz precursors.

PBz precursors   [T.sub.onset]   [T.sub.max]    [T.sub.final]
                 ([degrees]C)    ([degrees]C)   ([degrees]C)

PBz-eda               199            222             246
PBz-dba               215            249             267
PBz-tepa              209            239             258
BA-a [14]             220            243             275

TABLE 2. TGA data of Polybenzoxazines.

Polymer     [T.sub.10]    CY (%)   LOI
           ([degrees]C)

PBz-eda        305         42.5    35.5
PBz-dba        320         30.0    29.5
PBz-tepa       330         42.0    34.3
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Author:Parveen, Shakila; Kim, Haekyoung
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Date:Oct 1, 2018
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