The polymerization behavior and thermal properties of benzoxazine based on o-allylphenol and 4,4′-diaminodiphenyl methane
Introduction
Benzoxazines are oxygen- and nitrogen-containing six-membered heterocyclic compounds with one or two oxazine rings in their molecular structures, and they are generally synthesized via the Mannich reaction from phenols, primary amines, and formaldehyde. Monofunctional benzoxazines were initially synthesized from monophenols, monoamines, and formaldehyde; however, polymers with high molecular weights could not be formed [1], [2], [3], [4], [5], [6], [7]. After benzoxazines were synthesized from bisphenol-A, aniline/methylamine, and formaldehyde [8], [9], it was observed that bifunctional benzoxazines could be polymerized into polybenzoxazines with excellent mechanical strength and high thermal stability. These unique advantages endow benzoxazines with a wide range of potential applications, such as in the electronics and aerospace industries, in which they can be used as resin matrices for high-performance composites. Therefore, the study of benzoxazines attracts increasing attention from academia and industries, and a series of benzoxazines have been synthesized.
Bifunctional benzoxazines can be synthesized from bisphenols (or monophenols), monoamines (or diamines), and formaldehyde. Most studies are focused on benzoxazines based on bisphenols and monoamines [8], [9], [10], [11], [12], [13], [14], [15], [16] because these monomers are easily synthesized from bisphenols and primary amines by varying only one component, leading to molecules with different structures. However, studies on the aromatic diamine-based benzoxazines were seldom reported early on due to the poor solubility of many aromatic diamines in the common solvents used for benzoxazine preparation, and an insoluble gel frequently formed in the synthesis reaction of benzoxazines [17], [18], [19], [20]. Recently, some high performance benzoxazines were synthesized from aromatic diamines [19], [20], [21], [22], [23], [24], [25], [26], [27], including 4,4′-diaminodiphenyl methane (DDM), 4,4′-diaminodiphenyl ether (DDE), and 4,4′-diaminodiphenyl sulfone (DDS), and these compounds could be polymerized into polybenzoxazines with high glass transition temperatures (Tgs) and good thermal stability.
Introducing some special functional groups into benzoxazine molecules can improve the properties of the resultant polybenzoxazines [19], [28], [29], which is generally achieved via phenol-containing or amine-containing raw materials. In practice, some functional groups such as allyl [28], [29], nitrile [30], [31], acetylene [11], [32], [33], propargyl [34], methacryloyl [35], and methylol [36] are incorporated into benzoxazine molecules, which effectively enhances the Tgs and thermal stabilities of the resultant polybenzoxazines.
Allyl is generally thought to be a reactive group. For allyl-containing benzoxazines, the allyl groups can undergo free radical polymerization along with the oxazine ring-opening polymerization [29]. Practically, most allyl-containing benzoxazines are synthesized from allyl-terminated amines [29], [37], [38], and some o-allyl-terminated phenol-based benzoxazines are monofunctional monomers [28], [29], [39], [40], [41], except the benzoxazine based on diallyl bisphenol A [42], [43]. Studies on o-allylphenol-based monofunctional benzoxazines demonstrate that the chain flexibility of the corresponding polybenzoxazines is high, and their Tgs are relatively low [29], [41], whereas studies on o-allylphenol-based bifunctional benzoxazines are rarely reported.
In this study, a bifunctional benzoxazine, bis(4-(8-allyl-2H-benzo[e][1,3]oxazin-3(4H)-yl)phenyl)methane (oAP-ddm), was synthesized from o-allylphenol, DDM, and formaldehyde. The chemical structure of oAP-ddm was confirmed by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and elemental analysis. The thermally activated polymerization behavior of the oAP-ddm monomer was investigated by FTIR, solid-state 13C NMR, and differential scanning calorimetry (DSC). Moreover, the dynamic mechanical properties and the thermal stability of the corresponding polybenzoxazine (PoAP-ddm) were investigated. In addition, to elucidate the effect of the o-allyl group on the polymerization reaction of oAP-ddm and the properties of PoAP-ddm, the polymerization behaviors of another two benzoxazines (oC-ddm and P-ddm) based on o-cresol and phenol with DDM were studied, and the properties of the corresponding polybenzoxazines (PoC-ddm and PP-ddm) were evaluated.
Section snippets
Materials
o-allylphenol was supplied by Shandong Laizhou Hualu Accumulator Co., Ltd., China. DDM was purchased from Aladdin Chemical Reagent Co., Ltd., China. Phenol, o-cresol, formaldehyde (37% aqueous), toluene and chloroform were analytical reagents and were obtained from Tianjin Chemical Reagent Co., China. All of the chemicals were used as received.
Synthesis of oAP-ddm, oC-ddm, and P-ddm
In a 100-mL, three-necked round bottom flask equipped with a mechanical stirrer, a thermometer, and a reflux condenser, 4.96 g of DDM, 8 mL of
Synthesis and characterization of oAP-ddm
oAP-ddm was synthesized from o-allylphenol, DDM, and formaldehyde via a solution method [44]. The synthesis reaction mechanism is illustrated in Scheme 1. The chemical structure of oAP-ddm was confirmed by 1H and 13C NMR, FTIR, and elemental analysis.
Fig. 1a presents the 1H NMR spectrum of oAP-ddm. The resonances at 5.58 and 4.81 ppm correspond to the methylene protons (H10 and H11) of OCH2N and ArCH2N of the oxazine ring, respectively. The chemical shifts (ppm) at 3.69 and 3.70 (4H, H3),
Conclusions
Three benzoxazines were synthesized. The polymerization temperatures and activation energies of the benzoxazines with ortho-substituents are higher than those of the one without the substituents; however, their reaction enthalpies exhibit inverse behavior. The storage modulus of PoAP-ddm is the lowest and that of PoC-ddm is the highest. The glass transition temperatures and thermal stabilities of the polybenzoxazines with ortho-substituents are lower than those of their counterparts without the
Acknowledgment
This work was financially supported by the Natural Science Foundation of Hebei Province (B2013201107).
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