A new class of phenolic‐like thermosetting resins has been developed that is based on the ring‐opening polymerization of a benzoxazine precursor. These new materials were developed to combine the thermal properties and flame retardance of phenolics with the mechanical performance and molecular design flexibility of advanced epoxy systems. The polybenzoxazines overcome many of the traditional shortcomings of conventional novolak and resoletype phenolic resins, while retaining their benefits. The physical and mechanical properties of these new polybenzoxazines are investigated and are shown to compare very favorably with those of conventional phenolic and epoxy resins. The ring‐opening polymerization of these new materials occurs with either near‐zero shrinkage or even a slight expansion upon cure. Dynamic mechanical analysis reveals that these candidates for composite applications possess high moduli and glass transition temperatures, but low crosslink densities. Long‐term immersion studies indicate that these materials have a low rate of water absorption and low saturation content. Impact, tensile, and flexural properties are also studied. Results of the dielectric analysis on these polybenzoxazines demonstrate the suitability of these materials for electrical applications. © 1996 John Wiley & Sons, Inc.

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A series of linear aliphatic diamine-based benzoxazine monomers has been polymerized into transparent, crosslinked specimens that are free of voids and have good mechanical integrity. The density of these polybenzoxazines is measured as a function of the amine chain length. Dynamic mechanical analysis of these linear aliphatic polybenzoxazines shows two, chain length dependent transitions. Properties, such as room temperature modulus, glass transition temperature, crosslink density, thermal degradation temperature, and char yield, of the polybenzoxazines are investigated as a function of the chain length. All these properties exhibit strong dependence on the chain length. These aliphatic amine-based polybenzoxazines are found to be much more flexible than the bisphenol-type polybenzoxazines.

ABSTRACT:The effect of temperature on hydrogen bonding was investigated by measuring the integrated infrared absorbance of various hydrogen-bonding modes as a function of temperature. It was found that conformationally preferred hydrogenbonding modes maintain constant intensities over the wide temperature ranges studied for both polybenzoxazines and a novolac-type phenolic resin. In particular, the O---H---N hydrogen bond shows strong bonding that does not change over the temperature range. On the other hand, statistically distributed hydrogen bonding is more sensitive to the temperature change and its infrared intensities start decreasing around the ␤-transition of polybenzoxazines. The unusual physical and mechanical properties of polybenzoxazines, including volumetric expansion upon polymerization, high moduli, and high glass transition temperatures, are explained based on the complex hydrogen bonding.

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