High-Volume-Fraction Textured Carbon Nanotube-Bis(maleimide) and -Epoxy Matrix Polymer Nanocomposites: Implications for High-Performance Structural Composites

Polymer matrix nanocomposites (PNCs) incorporating high volume fractions (Vf in excess of 10 vol %) of aligned carbon nanotubes (A-CNTs) are promising for high-performance structural composite applications leveraging texture for multifunctionality and performance-to-weight ratios. However, to enable the manufacturing of scalable structures using A-CNT PNCs, nanoscale confinement and interfacial effects due to high A-CNT content in aerospace-grade polymer matrices need to be better understood. Here, we report the model-informed fabrication of high-Vf CNT PNCs to develop process-structure-property relationships, including a scaled film and laminate technique for A-CNT polymer laminates and the fabrication of microvoid-free and fully infused bis(maleimide) (BMI) and epoxy PNCs with high packing densities (or Vf) of biaxially mechanically densified millimeter-tall A-CNT array reinforcement (1-30 vol % corresponding to the average inter-CNT spacings of ∼70 to 6 nm). A polymer infusion model developed from Darcy's law accurately predicts the time for resin to infuse into CNT arrays during capillary-assisted PNC processing, corroborated by experimental observations via X-ray microcomputed tomography and scanning electron microscopy showing that a diluted resin with ∼10× lower viscosity than a neat resin is required to obtain complete infusion into high-Vf A-CNT arrays (10-30 vol %). For each tested A-CNT volume percent, the cured PNCs maintain vertical CNT alignment and glass transition temperature, and the decomposition onset temperature remains constant for epoxy PNCs but increases by ∼8 °C for 30 vol % A-CNT-BMI PNCs compared to the neat resin. For both polymer matrix systems, a ∼2× increase in the axial indentation modulus for 30 vol % A-CNT PNCs compared to that of a neat resin is measured, and no significant change in the transverse A-CNT modulus is shown experimentally and via modeling, indicating that reinforcement with A-CNTs at higher Vf values leads to enhanced anisotropic mechanical properties. Through the process-structure-property scaling relationships established here, this work supports the development of next-generation structures comprised of nanomaterials with enhanced performance and manufacturability.