A new bio-based thermosetting with amorphous state, sub-zero softening point and high curing efficiency

Highlights

• A novel bio-based PN monomer (L-CN) was designed and synthesized based on tyramine.

• L-CN exhibited low softening point, amorphous state, and high curing reactivity.

• The competitive multiple supramolecular interaction of L-CN may disrupt the ordered stacking.

• L-CN can achieve excellent thermal and thermomechanical properties at relatively low post-curing temperature.

Abstract

In this work, bio-based tyramine was exploited to prepare a novel phthalonitrile monomer (L-CN) with amorphous characteristics, sub-zero softening point (−16 °C) and high curing reactivity. Compared with the control compounds, it showed that amorphous structure and low softening point of L-CN cannot be simply attributed to the introduction of flexible aliphatic chain. It is proposed that the inhibited crystallization of the PN monomer can be understood by the multi-competitive supramolecular interactions as suggested by ¹HNMR, in-situ FTIR and molecular dynamics simulation. Furthermore, the cured resin exhibited excellent thermal and thermomechanical properties at relatively low post-cured temperature due to the strong nucleophilicity and mobility of the aliphatic amine.

Introduction

High-performance polymers (HPP) with aromatic heterocyclic structure are extensively applied in aerospace, defense industry, microelectronics and energy due to their superior thermal stability, thermomechanical properties, optic-electric performance and extreme environmental-resistance [[1], [2], [3], [4]]. As is known, aromatic heterocyclic high-performance polymer features high rigidity and strong supramolecular interaction, resulting in high melting point or softening point, and even intractability and insolubility [[5], [6], [7]]. The dilemma between processing and performance severely limits the applications of high-performance polymers, and thus it is still a research focus in this field [[8], [9], [10]]. For thermosetting polymers, low curing efficiency frequently leads to harsh processing conditions, including high curing temperature, toxic and corrosive catalyst, etc [11,12]. Phthalonitrile polymers (PN), a typical thermosetting HPP, also suffer from the above processing dilemma. The cyano addition polymerization yields triazine, isoindoline and phthalocyanine, which are conducive to serving under harsh conditions for resulting phthalonitrile polymers. However, due to the dominant dipole interaction of the aromatic cyano, its melting point is generally higher than 150 °C, which is not conducive to molding of phthalonitrile resins through energy-efficient processing technology such as resin transfer molding (RTM) [[13], [14], [15]]. Generally, oligomerization and introduction of flexible moiety strategies are commonly adopted strategy to obtain amorphous resin with low softening point, but the processability is still unsatisfactory [[16], [17], [18], [19]]. On the other hand, high bond energy of cyano results in the extremely low curing reactivity [20]. Even with the promotion of curing agent, like aromatic amine, its rapid curing temperature (usually evaluated by DSC exothermic peak temperature or rheological rapid rise temperature of viscosity) is generally above 250 °C [21,22]. Furthermore, PN is highly recommended to be cured for a long time above 350 °C for an optimization of thermomechanical properties. Both of high melting point and low curing-efficiency are highly undesirable from a view of energy-conservation [23]. Currently, for PN resins it is still a challenge to lower the melting point, improve the curing reactivity with consideration of the thermal and thermomechanical properties of the PN resin.

The sustainability of feedstock of polymers is an effective way to alleviate resource scarcity and environmental pollution [[24], [25], [26], [27], [28]]. Our group is dedicated to achieving sustainability of high-performance polymer by developing bio-based monomer platform. Up to now, we have introduced various biomass into the HPP such as adenine, tyrosine, tyramine and lignin-derived compounds [11,14,22,[29], [30], [31]]. As a continuation of the research, this work designs an PN monomer derived from biobased tyramine (L-CN). Initially, it was assumed that the melting point would be lowered by the introduction of aliphatic chains of tyramine, and the strong nucleophilicity and mobility of the aliphatic amine would improve the curing reaction efficiency. Unexpectedly, a fancy result was found that the monomer exhibited an amorphous condensed structure and an extremely low softening point (Tg = −16 °C), which is much lower than that of the reported PN monomers or oligomer [22,32,33]. Both of control PN monomers, 4-aminophenol PN (4-APN) and 4-propylphenol PN (4-PPOL) are crystalline and show much higher melting points (136 °C and 68 °C), indicating the unique characteristics of the L-CN. On the other hand, it receives much attention and research focus for aromatic heterocyclic containing small organic molecule showing liquid state under room temperature or sub-room temperature. The case of the L-CN is rarely observed in the world of aromatic heterocyclic small molecules, to the best of our knowledge. Well-known cases are room temperature ionic liquids (RTILs), deep-eutectic solvent (DES) and room temperature thermotropic liquid crystal (RTTLC). Low melting point of RTILs and DES may be assigned to ionic bonding, Waals interactions and the conversion of supramolecular hydrogen-bonding interactions [34]. RTILs, DES and RTTLC are long-lasting research focus due to the huge application including solvents, catalysts, reagents, electrolyte materials, liquid crystal displays, nanowires, electrochemical cells and various sensors [[35], [36], [37]]. However, the case is crystalline and seem to be difficult to explain the amorphization and low softening point of L-CN. We propose that the phenomenon may be related to the synergistic effect between the flexibility of aliphatic chains and multiple supramolecular interactions, such as amino-cyano supramolecular interaction, amino-amino supramolecular interaction, amino-ether oxygen supramolecular interaction or dipole interactions. The existence of multi-competitive supramolecular interactions disrupts the orderly arrangement of molecule, which is not conducive to crystallization. Last but not least, the resulting polymers achieved excellent thermal and thermomechanical properties at relatively low post-cured temperatures due to the strong nucleophilicity and mobility of the aliphatic amine. This work is expected to provide new opportunity to design aromatic heterocyclic high-performance polymers. In the future, it would be also an elegant case deserving of in-depth study for designing room temperature organic liquids with polar aromatic heterocycle, like well-known ionic liquids, which show potential application in electrolyte engineering for battery, solvents, support of catalysis, carbon precursor, etc.