Flexible aliphatic-bridged bisphenol-based polybenzoxazines
In this paper, we report the synthesis of a series of novel aliphatic-bridged bisphenol-based benzoxazine monomers comprising four to ten methylene unit spacers (BZ(n)BA). Cationic ring-opening polymerization of these monomers provides flexible polybenzoxazine thermosets with good film forming characteristics under solvent-free processing conditions. The effects of aliphatic bisphenol chain length on polymerization behavior, thermomechanical transitions, and mechanical properties of the polybenzoxazine thermosets are reported. Fourier transform-infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) show the ring-opening polymerization proceeds to high conversion with minimal dependence on spacer chain length. Thermomechanical properties of the pBZ(n)BA thermosets, such as rubbery storage modulus and glass transition temperature (Tg), show a strong dependence on the length of the aliphatic-bridged bisphenol linker where both properties decrease with increasing linker length. In particular, changing the length of the aliphatic-bridged bisphenol linker enables tailoring the Tg of the pBZ(n)BA series from 67 °C to 101 °C, as determined by dynamic mechanical analysis (DMA). Tensile properties of the pBZ(n)BA series exhibit similar trends with Young's modulus decreasing and elongation at break increasing with increasing aliphatic-bridged bisphenol linker length. The pBZ(n)BA materials all show a similar three mode degradation process by thermogravimetric analysis (TGA) consistent with other bisphenol based polybenzoxazines, and additionally exhibit a decrease in char yield with increasing aliphatic chain length owing to a decrease in aromatic content in the thermoset network.