Experimental and theoretical analysis of the temperature-dependent mechanical behavior of phthalonitrile over a wide temperature range

Phthalonitrile (PN) resin has a high application potential for heat-resistant and load-bearing integrated polymeric composites benefiting from its excellent thermal stability, yet theoretical research on its temperature-dependent mechanical performance is scarcely reported. This seminal work is devoted firstly to quantitatively analyzing and then theoretically modeling the modulus of PN resin in a wide temperature range. Appropriate temperature-dependent yield/ultimate strength prediction models are then developed based on the modulus attenuation behavior and uniaxial compression/tensile test results, and the nonlinear elastic response of the material under uniaxial compressive loading is effectively described using the Zhu-Wang-Tang model. The results show that the multi-stage transition and cross-linking reactions of PN resin at high temperatures jointly affect the elastic modulus attenuation. The high modulus of charred residue results in a higher elastic modulus retention rate and a slower modulus decay trend of the material after pyrolysis than the virgin material. The fracture surface morphology and stress-strain response both show that the material can be considered brittle since its plasticity is little below 325 degrees C. PN resin exhibits apparent tension-compression asymmetry from room temperature to 325 degrees C, with compressive yield strength much higher than ultimate tensile strength. This work not only favors understanding the thermal-mechanical behavior of PN resin under high temperatures but also provides a valuable pathway for the temperature-dependent elastic modulus and yield strength prediction of high-temperature-resistant thermosetting polymers.