Cure Behavior Changes and Compression of Carbon Nanotubes in Aerospace Grade Bismaleimide-Carbon Nanotube Sheet Nanocomposites

Herein, we study the effects that carbon nanotubes (CNTs) have on the curing of the bismaleimide resin matrix (BMI), as well as the effects that the curing of BMI has on the CNTs, in the nanocomposites containing up to 40 wt % CNTs. Two different types of CNTs in the sheet form: unbaked and baked (termed as UB and B CNT), have been employed. Addition of only 10 wt % UB CNT (10 wt % UB – 90 wt % BMI) reduced the BMI cure temperature by up to 123 °C, compared to the cure temperature of neat BMI with no CNTs (100 wt % BMI). The 40 wt % UB – 60 wt % BMI demonstrated a 107 °C increase in the glass transition temperature, compared to the 100 wt % BMI. UB CNTs in the 10 wt % UB – 90 wt % BMI compressed upon the cure of BMI with an estimated compressive stress of 2.9 GPa while the UB CNTs in the 40 wt % UB – 60 wt % BMI were not compressed. The factors leading to these unprecedented effects have been discussed. The effect of the varying CNT content on the inter-CNT spacing and consequently the cure reactions of the BMI, compression of CNTs, and the thermomechanical properties of the nanocomposites has been discussed using an ideal CNT-BMI interaction model. Based on the thermomechanical results and the theoretical calculations, interphase thicknesses of at least 5.3 and 4 nm are estimated for the 30 and 40 wt % CNT nanocomposites, respectively. Since an optimum cure condition is critical to obtain the best mechanical properties in a CNT-BMI system, our results suggest that the optimum cure condition will likely vary for each unique CNT-BMI system. Thus, going forward, optimizing the cure conditions for each CNT-BMI system is critical in realizing the best mechanical properties from that system. Potential applications of the CNT-BMI nanocomposites include structural materials in the aerospace domain.