High temperature polyimide/carbon fiber/carbon nanotubes multiscale composites: Processing, cure kinetics and multifunctionality

The discovery of carbon nanotubes (CNTs) by Iijima in 1991 has initiated a great deal of scientific research on exploring their unique properties and potential applications. One of the promising applications is integrating CNTs into polymer or polymer/fiber composites to form nanocomposite or multi-scale composites. High temperature polymer composites are required for use in structural components in advanced high speed aircraft, weapon systems and space vehicles. Motivated by the potential of significantly improving thermal, electrical, mechanical and properties and fire retardancy of polymer matrix composites at relatively low concentration, this research focuses on integrating CNTs into high temperature fiber-reinforced composites aiming at matrix enhancement at the inter-fiber level, z-direction reinforcement and multifunctionality such as thermal and electrical conductivity, fire retardancy or resistance.;High temperature vacuum-assisted resin transfer molding was designed and successfully demonstrated with in-plane and through-thickness resin flow methods. Polyimide based PETI-330/carbon fabric T650-35 laminates were manufactured with fiber volume fraction of ∼60% and void contents of 3-4%. The tensile strengths of PETI-330/ T650-35 laminates fabricated by in-plane and through-thickness processes are 834 and 799 MPa, respectively. Short beam strengths of the laminates via both processes are 43 and 52 MPa, which are ∼77% and ∼93% of that processed via RTM using an injection pressure of 2.75 MPa.;CNT/carbon fiber/PETI-330 multi-scale composites were fabricated using prepreg-assisted RTM process. Homogeneous dispersion of CNTs in the PETI-330 matrix were achieved using the solution processing method. Using this method, multi-scale composite laminates with various concentrations of CNTs were manufactured. Comprehensive characterizations were carried out to investigate the thermal stability, thermal mechanical properties, micro-structure and morphologies of the CNT/PETI-330/T650 multi-scale composite laminates. As a result, we found that: (1) the incorporation of small amounts of CNTs can significantly improve the high-temperature thermal mechanical properties of the PETI-330 resin; (2) after postcuring the room temperature storage modulus of 1 wt% CNT/PETI-330/T650 composite increased from ∼60 GPa to ∼71GPa, the glass transition temperature increased from 331°C to 350°C; (3) the existence of CNT induces hindered cure kinetics of PETI-330.;Multiscale composites were also fabricated using buckypapers made of mixed SWNTs/MWNTs integrated onto polyimide/carbon fibre composite surface and their fire retardancy was characterized. Compared to the control sample (CP), buckypaper incorporated sample produced further delayed ignition, 40% lower peak heat release rate, 26% lower heat release, 82% less smoke release and 33% less mass loss. Further, buckypaper effectively increases the thermal conductivity of PETI-330/carbon fiber composite; the thermal conductivity improves 13% at 25°C and 27% at 300°C. The buckypaper is more efficient as fire retardant in polyimide/carbon fiber composites than direct mixing CNTs into polymer matrix.;The effect of CNTs on the cure kinetics of imide oligomer was studied using differential scanning calorimetry. During isothermal cure, the neat resin begins to cure first. As the reaction proceeds, the enthalpy of reaction decreases for the nanocomposites. As the samples vitrify and the reaction is completed, the neat PETI-330 vitrifies faster than the 1% CNT/PETI-330 nanocomposite sample. The adding of 1 wt% CNTs caused lower enthalpy, slightly lower activation energy. The cure kinetics of neat PETI-330 and the PETI-330/CNT systems can be modeled as first-order reactions, meaning the reactions are mainly ethynyl-ethynyl addition polymerization to form carbon-carbon double bonds. Comparison of the activation energies, rate constant and cure times suggests that the cure mechanisms of the neat PETI-330 resin and CNTs embedded nanocomposites are similar.;Last but not least, the molecular models of the PETI-330 imide oligomer and CNTs were constructed. Molecular dynamics simulations showed that molecular interaction energy of imide/CNT system is -37.5 kcal/mol. The major contributor of the interactions between CNT and the imide oligomer is van de Waals energy which is due to the pi-stacking effect between similar molecular structure of CNT surface and the phenyl rings in the backbone and side-groups. The attractive energy between CNT and imide oligomer can explain why the glass transition temperature shifts toward higher temperature in multi-scale composites manufactured in this study.