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The toughness of polybenzoxazine can effectively be improved by alloying with isophorone diisocyanate (IPDI)‐based urethane prepolymers (PU) or with flexible epoxy (EPO732). The experimental results, i.e., flexural testing and dynamic mechanical analysis, reveal that the toughness of the alloys of the rigid polybenzoxazine and the PU or the EPO732 systematically increases with the amount of either toughener, due to the addition of more flexible molecular segments in the polymer hybrids. The curing temperature of the benzoxazine resin (bisphenol A‐aniline type [BA‐a]) at about 225°C shifts to a higher value when the fraction of BA‐a in either alloy decreases. Interestingly, the enhancement in the glass transition temperature (Tg) of BA‐a/PU alloys is clearly observed, i.e., Tg of the BA‐a/PU alloys are significantly higher (Tg beyond 200°C) than those of the parent resins, i.e., 165°C for BA‐a and −70°C for PU. However, this characteristic is not observed in the BA‐a/EPO732 alloy systems. The enhanced Tg of the BA‐a/PU alloy at a 70/30 mass ratio is found to be 220°C, while that of BA‐a/EPO732 at the same mass ratio is observed to be only 95°C, which further decreases as the content of epoxy fraction increases. Furthermore, the degradation temperature based on 5% weight loss in the thermogravimetric analyzer (TGA) thermograms of the BA‐a/PU alloys is found to improve with the presence of the PU, though the opposite trend is observed in the BA‐a/EPO732 systems. The char yield of both alloy systems is steadily enhanced with the increased benzoxazine content because the char yield of the polybenzoxazine is inherently higher than that of the two tougheners. Therefore, the polybenzoxazine alloys with the IPDI‐based urethane prepolymer, e.g., the 70/30 BA‐a/PU, are a promising system for a tough, high thermal stability polymeric network, suitable for both bulk and composite matrix applications. POLYM. ENG. SCI. 45:288–296, 2005. © 2005 Society of Plastics Engineers.

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