Contribution of blocking positions on the curing behaviors, networks and thermal properties of aromatic diamine-based benzoxazines

Highlights

• Aromatic diamine-based benzoxazines with various blocking positions are synthesized.

• Different blocking positions vary the curing temperature of the benzoxazine monomers.

• Hydrogen bonding inhibits autocatalysis in network with arylamine Mannich bridge.

• Changes in blocking positions causes dramatic differences in T_g after polymerization.

• Thermal stability reduces similarly regardless of specific blocking position.

Abstract

To elucidate the contribution of blocking positions on aromatic diamine-based benzoxazines, we prepared several 4,4′-bis(3,4-dihydro-2H-1,3-benzoxazin-3-yl) diphenyl methane monomers with different phenols. The molecular chemical structures of the bi-functional monomers are characterized by FTIR and ¹H-NMR. Particularly, the newly developed m-cresol based benzoxazine is further confirmed by ¹³C-NMR and ESI-MS and the oxazine ring position is verified. The curing behaviors are investigated by dynamic differential scanning calorimetry (DSC). Activation energies are analyzed by Kissinger, Ozawa and Starink methods at various heating rates. Due to structure difference in polymerized network, polymer with high amount of arylamine Mannich bridge shows low autocatalysis capability. The glass transition temperature (T_g) is closely related to the blocking position while thermal stability decreases comparatively regardless of the blocking position. High arylamine Mannich bridging leads to large amount of dangling groups, which reduces the decomposition peak temperature.

Introduction

Polybenzoxazine is a new generation of phenolic resin, which offers high thermal stability, high glass transition temperature (T_g), good mechanical properties, attractive flame retardancy, minimum curing shrinkage, no curing byproducts and wide molecular design flexibility [1,2]. As a superb material, it has received intensive focus from academia and industry for decades and becomes one of the rare new commercialized polymers during the last 30 years [3,4].

Polybenzoxazine can be acquired from benzoxazine through thermally activated ring-opening polymerization (ROP) to form Mannich bridge connected phenolic structures [[5], [6], [7]]. The dominant precursors in literature [1,8] and in industry [[9], [10], [11]] to obtain polybenzoxazine are bisphenol-based benzoxazine monomers, which are prepared from bifunctional phenols and mono-functional amines. Previous studies [12,13] found that the thermal stability of polybenzoxazine is highly influenced by Mannich bridge cleavage. For a material prepared from conventional bisphenol-based benzoxazine, mono-amine derivative releases from the material after cleavage and eventually leads to severe weight loss. This process can be overcome by introducing reactive groups on amine, while simultaneously increase the reagents’ costs and complicate the preparation [[14], [15], [16]]. Meanwhile, based on the isomeric bisphenol A-toluidine type benzoxazines, researchers discovered that thermal stability can be significantly enhanced via generation of additional arylamine Mannich bridge by controlling the electron distribution in aryl amine [17].

Diamine-based benzoxazine monomers, first reported by Pei et al. [18], are synthesized from mono-phenol and diamine to acquire bi-functionality. Unlike the aforementioned benzoxazine species, the amines of these monomers are incorporated in the network after polymerization. Thus, amine derivatives releasing at low temperature is suppressed. Till now, various aliphatic diamine [12,[19], [20], [21]] and aromatic diamine [22,23] have been applied for preparing benzoxazines. Among them, low viscous aliphatic diamine-based benzoxazines with phenol substitutions have been studied [24]. However, unlike the inert aliphatic chain during polymerization, the aryl rings of aromatic diamine can participate the reaction, which causes significant differences in polymerization, network structure and material properties. In fact, it can be more complex when the reactive sites on phenol are selectively blocked. Importantly, to enhance the thermal and mechanical properties, aromatic diamines are not only applied in benzoxazine monomers, but also widely used for various benzoxazine precursors, such as main-chain benzoxazine polymers [25,26], capped main-chain benzoxazine oligomers [4,[27], [28], [29]], branched benzoxazines [30], benzoxazine dentrimers [31], etc., of which reaction on aryl ring of diamine can take place as well [32]. Therefore, clarifying the influences of aromatic diamines on polymerization and structures are crucial for establishing the structure-property relationship of polybenzoxazines derived from these precursors.

Focusing on the aromatic diamine-based benzoxazines, the aim of this work is to study the curing behaviors, network structures and material properties by selectively blocking the reactive positions. Ortho-blocking leads to phenolic Mannich bridge at para-positions while para-blocking results in mainly ortho-Mannich bridge. Meta-position is not a reactive site, however, it can also cause changes in material properties. When the reactive sites on phenol are prohibited, arylamine Mannich bridge dominates the polymerized network, which shows unique performance on curing, autocatalysis process, glass transition temperature and thermal stability. The results of this study can be very helpful for further development of polybenzoxazines containing aromatic diamine structures.