The phenol-containing phthalonitrile resin is a kind of self-curing phthalonitrile resin with high-temperature resistance and excellent properties. However, the onefold phthalonitrile resin is unattainable to cured completely, and the brittleness of the cured product is non-negligible. This paper focuses on solving the above problems by blending novolac resin into phenol-containing phthalonitrile. Under the action of abundant hydroxyl group, the initial curing temperature and gelation time at 170 °C decrease by 88 °C and 2820 s, respectively, monitored by DSC and rheological analysis. FT-IR spectra of copolymers showed that the addition of novolac increased the conversion rate of nitrile. When the novolac mass fraction is 10%, the peak of nitrile group disappears, which means the complete reaction. The mechanical test of blends composites shows that the maximum fracture strain of 10 wt% novolac addition is 122% higher than those of neat phthalonitrile composites on account of the introduction of flexible novolac chain segments. The mechanical properties are sensitive to elevated post-cured temperature; this is consistent with the result of morphological investigation using SEM. Finally, the dynamic mechanical analysis indicated that the glass transition temperature heightened with the increase of novolac content and post-curing temperature.

Fiber Bragg gratings have the advantages of small size and light weight, therefore can be embedded into composites to monitor the forming temperature. However, annealing and calibration are always required due to the high temperature demanded for thermoplastic composite forming, which will cost a lot of time and energy. An encapsulated fiber Bragg grating was successfully proposed to isolate strains and overcome the challenge of high forming temperature up to 332°C and high forming pressure of 2 MPa, though pre-annealing needs to be applied each time before actual measurement. In this study, by applying a polynomial numerical fitting algorithm obtained from reference fiber Bragg gratings in the same sensor batch, the nonlinear temperature response and irreversible Bragg wavelength shift under high temperature can be precisely estimated. Forming temperature monitoring of glass fiber/polyetherimide composite was successfully achieved using the unannealed FBGs. The temperature curve was reconstructed with an average difference of 0.8°C compared to the reference thermocouple. These results demonstrate the application of unannealed FBGs in in-situ composite forming monitoring of high temperatures.