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

Abstract

Polymer matrix nanocomposites (PNCs) incorporating high volume fractions (V f 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-V f 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 V f ) 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 Xray 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-V f 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 V f 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.

Selectively Tuning the Substrate Adhesion Strength of Aligned Carbon Nanotube Arrays via Thermal Postgrowth Processing

“…The stress–strain curves plotted in Figure a from CNT array–substrate pull-off (four representative curves for each sample) show that the peak stress evolves throughout three regimes with T p : ∼0.043 MPa for as-grown CNT arrays, then increasing to ∼0.13 MPa in regime I (consistent for 700 °C ≤ T p < 750 °C), then increasing to ∼0.34 MPa in regime II (consistent for 750 °C ≤ T p ≤ 800 °C, and also exhibiting ≳4 times larger failure strains compared to regime I), and then decreasing to ∼0.13 MPa in regime III (consistent for 800 °C < T p ≤ 950 °C) with a reduction in failure strains from regime II. It is noted that, in prior work, a similar annealing process via a postgrowth 4 min H 2 thermal treatment , can be used to weaken the attachment of the CNTs to the catalyst layer, and this decreased CNT–substrate strength enables easy delamination of the CNT array from the Si substrate, such as for CNT composite fabrication. The observed rounded shape of the stress–strain curves (akin to a softening sine curve) in Figure a is similar to the characteristic shape that resulted from tensile testing of brittle microfiber bundles and CNT bundles, where progressive fiber breaking was observed at increasing strain.…”

“…These techniques can be applied to a variety of NF arrays, such as those comprising CNTs, boron nitride, TiO 2 , ZnO, silicon, and polymers. Physical techniques include utilizing substrate underlayers/interlayers, coatings and metallization, and NF contact shaping, hierarchy, and patterning; ,, chemical techniques include NF and substrate functionalization and chemical deposition, , oxidation/anodization, − and reduction and hydrogen etching; , thermal techniques include applying heating − (as studied here), cooling, and thermal shock processing; mechanical strategies include NF pressure modulation prior to NF pull-off (e.g., by varying preload), interlocking and branching (from the growth process), and utilizing mechanical gradients within NF arrays; biological techniques include leveraging micro- and nanobiointerfaces, protein and cell adhesion, and biomimetic/gecko-inspired designs; ,, and electrical/optical techniques include electron, ultraviolet, and microwave irradiation . SEM images and illustration reproduced with permission from ref.…”

J-Integral Experimental Reduction Reveals Fracture Toughness Improvements in Thin-Ply Carbon Fiber Laminates with Aligned Carbon Nanotube Interlaminar Reinforcement

“…The findings presented in this work extend to industries that rely on carbon-fiber advanced composites, including mass-specific applications such as aerospace, wind, automotive, and some infrastructure applications, where lightweight, durable materials are critical for improving structural efficiency. Further studies include CNT reinforcement optimization strategies including densely packed CNTs , and scalable manufacturing processes.…”

Boron Nitride Nanotubes: Force Field Parameterization, Epoxy Interactions, and Comparison with Carbon Nanotubes for High-Performance Composite Materials

“…Polymer matrix composites (PMCs) are currently being considered as a primary structural material for aerospace vehicles. Reinforced nanocomposite materials have been shown to provide exceptional material properties over traditional PMCs. − Boron nitride nanotubes (BNNTs) exhibit remarkable mechanical properties at higher temperatures, electrical insulation, and radiation shielding − and have recently been shown to be an excellent reinforcement in nanocomposite materials. ,− However, to fabricate high-performance BNNT-reinforced PMCs efficiently and effectively, a thorough understanding of the BNNT/polymer interfacial interaction is needed. , …”