Insightful characterization of sesamol-based polybenzoxazines: Effect of phenol and amine chain type on physical and nanomechanical properties

Abstract

We report the synthesis of sesamol-based polyethyleneimine (PEI) polybenzoxazines. Their physico-chemical features have been compared with other aliphatic diamine chains and the influence on thermo-mechanical properties of the phenol substituent was investigated. Their molecular structures were checked through Fourier Transform Infrared Spectroscopy (FT-IR) and nuclear magnetic resonance (1H NMR and 13C NMR) spectrometry. Differential scanning calorimetry (DSC) revealed that the curing process takes place in two stages indicating the first polymerization exotherm <180 °C which is lower than for other similar PEI-based benzoxazines reported before. Thermogravimetric (TGA) and Dynamic mechanical analyses (DMA) showed good thermal stability, high glass transition (Tg) values (>120 °C) and elevated char yield suggesting that not only primary amines are involved in the benzoxazine rings formation, but also secondary or tertiary amines may participate influencing the crosslinking process. The limiting oxygen index (LOI) was determined by using van Krevelen equation showing that sesamol-based polybenzoxazines exhibit flame retardancy capability. Contact angle tests (CA) showed that sesamol-based thermosets display more hydrophilic character caused by the inherent hydrophilicity of the nitrogen enriched PEI oligomer. However, consistent differences between sesamol and phenol-PEI based polybenzoxazines were observed for the micromechanical properties, the latter showing overall increased micro-elastic and hardness modulus due to higher content of benzoxazine moieties.