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 (T-g), 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 T-g of the pBZ(n)BA series from 67 degrees C to 101 degrees 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.