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

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 (Tpeak) 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 (Tmax) 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.