Tuning the architecture and performance of multifarious benzoxazine resin based on guaiacol and polyethylenimine

Highlights

• Side-chain type benzoxazine-functionalized polyethylenimine resins have been prepared.

• Reactants stoichiometry has been varied resulting in a series of functionalized resins.

• Resins have been processed at low temperature, 100 °C for 24 h.

• Cured specimens exhibited maximum degradation temperature (T_max) above 300 °C.

• Adhesion performance of resins has been credited to the polar groups.

Abstract

Molecular designing flexibility offered by benzoxazine chemistry, has been utilized to synthesize side-chain benzoxazine functionalized resins, in one-step approach via reaction of polyethylenimine (PEI) and guaiacol. The functionalization of polyethylenimine bearing amine functionalities, with benzoxazine groups as side-chains, is expected to confer a densely cross-linked network in the resulting thermoset. In the present work, guaiacol has been chosen as bio-based phenol, and polyethylenimine enriched with primary, secondary, tertiary amine groups, has been employed as an amine co-reactant. The reactants stoichiometry (phenol: amine: paraformaldehyde) has been varied from 2:1:4 to 20:1:40 molar ratio, resulting in a series of side-chain type guaiacol based benzoxazine-functionalized polyethylenimine resins (G-pei), which differ in percentage of oxazine functionalization. All the resins prepared have been characterized using FTIR, ¹H, ¹³C NMR spectroscopies, and the evolution of oxazine functionalization in the resins has been monitored via ¹H NMR spectroscopy. The polymerization behaviour in resins has been evidenced using non-isothermal DSC studies, which reveals that G-pei resins exhibit peak polymerization temperature (T_peak) below 200 °C. Temperature sweep rheological experiments have been performed to obtain flow behaviour and processing window of the developed resins, where an evident reduction in the viscosity of resins has been observed around 100 °C, thereby making it an appropriate temperature for processing. The influence of polar groups on the surface property of thermosets has been investigated by performing contact angle measurements, which reveals the hydrophilic nature of prepared thermosets. Thermal degradation behaviour of the multifarious bio-based thermosets, has been studied using thermogravimetric analysis, where maximum degradation temperature (T_max) for the cured specimens has been found to be above 300 °C, with a high char yield (30–50%). The influence of dense network on the glass transition temperature of thermosets, has been additionally studied by DSC experiments. The adhesive performance of side-chain type benzoxazine-functionalized polyethylenimine resins, enriched with oxazine functionalities as well as polar groups, has been evaluated by lap shear strength values, which have been observed in the range of 22 to 49 Kg/cm².

Introduction

Molecular designing flexibility offered by benzoxazine resins, has surely been the research interest for academia, which has led them to make plethora of attempts [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] to synthesize polybenzoxazine thermosets, a strong contender of mechanically robust epoxy and thermally stable bismaleimides. Polybenzoxazines, a class of phenolic thermosetting resins, have emerged as advanced performance materials [11], and are associated with properties including, high glass transition temperature [12], low water absorption [13], high dimensional stability [14], [15], [16], high char yield [17], appreciable mechanical performance, and excellent FST (fire, smoke and toxicity) properties [18], [19]. In addition, polybenzoxazine thermosets are obtained by heating one component formulations, which involves oxazine functionalities undergoing ring opening polymerization to form cross–linked phenolic structure [17]. Unquestionably, the utilization of polybenzoxazine is expected to grow significantly in near future, and will certainly become an important asset for the polymer industry [20].

To explore the panorama of applications, variety of benzoxazine resins have been developed over the past decade using different synthetic approaches. Moreover, with the advent of sustainable development, researchers have now focused their interest to develop inexpensive sustainable polybenzoxazines, using plausible combinations of bio-based raw materials [21], [22], [23], [24]. Bio-based benzoxazine resins are conventionally synthesized using either phenols of natural origin such as cardanol [25], eugenol [26], guaiacol [21], vanillin [27], or naturally occurring amines including stearylamine, and furfurylamine [28]. Comparing the thermal and mechanical performance of bio-based benzoxazine resins with their petro-based analogues, the former shows inferior performance. Several attempts have been made to improve the performance of bio-based polybenzoxazines, by making structural modifications, such as inclusion of aromatic precursors [29], or functionalities which are able to provide additional cross-linking sites [30], [31], [32], [33], [34], [35]. It is to be noted that, increasing the degree of oxazine functionality in a polymerizable entity, has been considered as a basic approach to increase the cross-link density in the material, thereby enhancing thermal as well as the mechanical performance.

Research on benzoxazine resins has been mostly limited to three or four oxazine rings, by utilizing multifunctional phenol or amine as raw materials. Reports utilizing petro-based raw materials, to establish the role of multiple oxazine moieties on the performance of polybenzoxazine thermosets are available. Endo et al. reported di, tri-, and tetrafunctional benzoxazine monomers based on phenylaminomethyl phenol (PAMP), with oxazine moieties arranged one after the other. It was observed that increase in number of oxazine rings, resulted in a decrease in curing temperature from 264 °C (mono) to 237 °C (tetra). In addition, weight loss during the polymerization of monomers was also investigated, where a decrease in weight loss was reported with increase in number of oxazine rings in the monomers [36]. Tetrafunctional fluorene-based benzoxazines comprising of both flexible aliphatic chain as well as rigid aromatic structure in their backbones, have also been reported and demonstrates good processability, excellent thermal stability, as well as high glass transition temperature (T_g) values ranging from 291 to 307 °C [37]. Attempts towards establishing the structure-property relationship, on increasing the oxazine functionality in bio-based benzoxazine resins are also available. Lochab et al. reported a homologous series of cardanol based benzoxazine monomers, where the degree of functionality was varied from mono- to tetra-functional. It has been reported that, on increasing the number of oxazine rings in the monomer resulted in lowering of polymerization temperature and thus obviate the requirement of curing accelerators. In addition, it was suggested that degree and vicinity of oxazine functionality, significantly influence the cross-link density, and consequently the performance of end material [38]. It is to be noted that functionality in benzoxazine resins has also been extended to octa-functional monomers, however the synthetic approach involves multiple steps. Wu et al. reported the synthesis of multifunctional benzoxazine groups hybridized with polyhedral oligomeric silsesquioxane (POSS), via click reaction of octa-azido functionalized POSS with 3,4-dihydro-3-(prop-2-ynyl)-2H-benzoxazine [39]. The curing temperature for the benzoxazine groups was found to be relatively low (~120 °C), and noticeable improvement in thermal performance was observed. Furthermore, Thirukamaran et al. reported an alternative route to prepare POSS octa-functionalized with benzoxazine groups, via Mannich like condensation of octaminophenyl POSS with bio-based phenols. The polymerization behaviour of the POSS octa-functionalized with benzoxazine monomers was investigated, and benzoxazine groups were observed to polymerize at temperatures in the range of 231 to 251 °C with low enthalpy of polymerization (42–45 J/g) [40].

Interestingly, grafting of benzoxazine groups as side-chains into a polymer backbone has been considered as an important way of introducing dense network, and functionalities for specific applications. In view of the same, Yagci research group has reported the synthesis of benzoxazine-functionalized polystyrene macromonomers, which involves Mannich like condensation of phenol, formaldehyde and amine-functionalized polystyrene. The synthesized macromonomers undergoes thermal polymerization, and displayed a curing exotherm centered above 250 °C in DSC traces. In addition, the glass transition temperature for the polystyrene segment was found to be 105 °C [41]. Ishida et al. has reported the synthesis of side-chain type benzoxazine-functionalized cellulose, by performing click reaction between ethynyl-monofunctional benzoxazine monomer and azide-functionalized cellulose. DSC thermogram showed that the benzoxazine-functionalized cellulose exhibited a polymerization exotherm which initiates at 145 °C, and reaches peak around 195 °C. Furthermore, benzoxazine-modified cellulose demonstrate a broader decomposition temperature range from 270 to 500 °C with high char yield (~44%) [42]. Recently, Kiskan et al. has reported the step-wise synthesis of non-symmetric main-chain oligobenzoxazine with side-chain naphthoxazines, utilizing the difference in reactivity of phenols towards Mannich like condensation. DSC studies revealed that the oligomer undergoes polymerization which initiates at 162 °C, and attains a maxima at 205 °C. In addition, thermal degradation analysis of the oligomer revealed, two major degradations at 309 °C, and 398 °C along with 18% char yield [43].

As can be seen, side-chain benzoxazine functionalized polymers has intrigued the research interest and to boost the same, a unique one-step approach has been adopted. In the present work, polyethylenimine, an amine functionalized polymer, enriched with primary, secondary and tertiary amines has been employed as an amine co-reactant. Mannich condensation of amine functionalities in polyethylenimine with guaiacol and paraformaldehyde, has been performed which yields side-chain type guaiacol based benzoxazine-functionalized polyethylenimine resins (G-pei). In addition, the architecture of side-chain type G-pei resins, has been tuned by varying the reactant stoichiometry, which results in resins differing in percentage composition of oxazine functionalities. Oxazine moieties in G-pei resins undergoes ring opening polymerization leading to multifarious bio-based thermosets. Polymerization and rheological behaviour of the resins have been investigated to obtain the processing window of the resins. The dense network in the developed thermosets is expected to improve the thermal degradation behaviour, which has been studied by thermogravimetric analysis. The presence of polar groups has been reported to improve the wettability of the resins, thereby improving the adhesion performance and the same has been investigated by obtaining lap shear strength values.