Inadequate dispersion of nanomaterials is a critical issue that significantly limits the potential properties of nanocomposites and when overcome, will enable further enhancement of material properties. The most common methods used to improve dispersion include surface functionalization, surfactants, polymer wrapping, and sonication. Although these approaches have proven effective, they often achieve dispersion by altering the surface or structure of the nanomaterial and ultimately, their intrinsic properties. Co-solvents are commonly utilized in the polymer, paint, and art conservation industries to selectively dissolve materials. These co-solvents are utilized based on thermodynamic interaction parameters and are chosen so that the original materials are not affected. The same concept was applied to enhance the dispersion of boron nitride nanotubes (BNNTs) to facilitate the fabrication of BNNT nanocomposites. Of the solvents tested, dimethylacetamide (DMAc) exhibited the most stable, uniform dispersion of BNNTs, followed by N,N-dimethylformamide (DMF), acetone, and N-methyl-2-pyrrolidone (NMP). Utilizing the known Hansen solubility parameters of these solvents in comparison to the BNNT dispersion state, a region of good solubility was proposed. This solubility region was used to identify co-solvent systems that led to improved BNNT dispersion in poor solvents such as toluene, hexane, and ethanol. Incorporating the data from the co-solvent studies further refined the proposed solubility region. From this region, the Hansen solubility parameters for BNNTs are thought to lie at the midpoint of the solubility sphere: 16.8, 10.7, and 9.0 MPa(1/2) for δd, δp, and δh, respectively, with a calculated Hildebrand parameter of 21.8 MPa(1/2).

Lightweight, superelastic foams that resist creep and fatigue over a broad temperature range are being developed as structural and functional materials for use in numerous diverse applications. Unfortunately, conventional foams display superelasticity degradation, undergo considerable creep, show fatigue under repeated usage, or fracture over large strains, particularly under significant temperature variations. We report that graphene-coated single-walled carbon nanotube (SWCNT) aerogels remain superelastic, and resist fatigue and creep over a broad temperature range of −100−500 °C. The microstructure of these ultralow density (≈14 mg/mL; corresponding volume fraction ≈9 × 10 −3 ) aerogels is composed of a three-dimensional network of randomly oriented SWCNTs with junctions between SWCNTs coated with 2−5 layers of ≈3 nm long graphene nanoplatelets. Compressive stress (σ) versus compressive strain (ε) curves show that the aerogels fully recover their shapes even when strained by at least 80% over −100−300 °C and 20% at 500 °C, whereas the Young's modulus remains similar over the temperature (−100−500 °C) and strain rate ε̇(0.01−0.16 1/s) ranges. We suggest that under compression, the graphene layers hinder free rotation and irreversible sliding of the SWCNTS about the junctions, leading to bending of the graphene layers, while the struts form new junctions stabilized via van der Waals interactions. When the compressive load is removed, the bent graphene layers provide a restoring force that breaks the junctions created during compression, accounting for full aerogel shape recovery, albeit with hysteresis. The storage (E′) and loss (E″) moduli measured in the linear regime show ultralow damping ratio (tan δ = E″/E′) ≈0.02, and these viscoelastic properties remain constant over three decades of frequencies (0.628−628 rad/s) and across −100−500 °C. The low loss in these aerogels is corroborated by exceptional fatigue resistance for 2000 (5 × 10 5 ) cycles at ε = 60% (1%) from −100−300 °C (−100−500 °C) and creep resistance at least under σ = 20 kPa, for a minimum of 1 min from −100−500 °C. Furthermore, these aerogels retain their exceptional creep resistance under the same creep test conditions but for much longer time of 30 min at all tested temperatures except at 500 °C where they show small creep ε of ≈0.7% with ≈0.8% residual ε. The emergent thermomechanical stability of these aerogels that arises from, in part, microscopic deformations of the graphene-coated junctions, motivate junction modification as a means to control the mechanical properties of CNT foams in general. Furthermore, the temperature-invariant mechanical properties of these aerogels combined with their facile fabrication method, which is readily applicable to other nanotube foams, make this class of aerogels a strong alternative to conventional foams, particularly in environments with large temperature variations.

Lightweight, flexible piezoresistive materials with wide operational pressure ranges are in demand for applications such as human physical activity and health monitoring, robotics, and for functional interfacing between living systems and wearable electronics. Piezoresistivity of many elastomeric foams of polymers and carbon allotropes satisfies much of the required characteristics for these applications except creep and fatigue resistance due to their viscoelasticity, critically limiting the reliability and lifetime of integrated devices. We report the piezoresistive responses from aerogels of graphene-coated single-walled carbon nanotubes (SWCNTs), made using a facile and versatile sol-gel method. Graphene crosslinks the junctions of the underlying random network of SWCNTs, generating lightweight elastomeric aerogels with a mass density of ≈11 mg mL (volume fraction ≈7.7 × 10) and a Young's modulus of ≈0.4 MPa. The piezoresistivity of these aerogels spans wide compressive pressures up to at least 120 kPa with sensitivity that exhibit ultrafast temporal responses of <27 ms and <3% delay ratio over 10 compressive loading-unloading cycles at rates between 0.1-10 Hz. Most importantly, the piezoresistive responses do not show any creep at least for 1 hour and 80 kPa of compressive static loading. We suggest that the fatigue- and creep-resistant, ultrafast piezoresistive responses of these elastomeric aerogels are highly attractive for use in dynamic and static lightweight, pressure sensing applications such as human activity monitoring and soft robotics.

Lightweight open-cell foams that are simultaneously superelastic, possess exceptionally high Young's moduli (Y), exhibit ultrahigh efficiency, and resist fatigue as well as creep are particularly desirable as structural frameworks. Unfortunately, many of these features are orthogonal in foams of metals, ceramics, and polymers, particularly under large temperature variations. In contrast, foams of carbon allotropes including carbon nanotubes and graphene developed over the past few years exhibit these desired properties but have low Y due to low density, ρ = 0.5-10 mg/mL. Densification of these foams enhances Y although below expectation and also dramatically degrades other properties because of drastic changes in microstructure. We have recently developed size- and shape-tunable graphene-coated single-walled carbon nanotube (SWCNT) aerogels that display superelasticity at least up to a compressive strain (ε) = 80%, fatigue and creep resistance, and ultrahigh efficiency over -100-500 °C. Unfortunately, Y of these aerogels is only ∼0.75 MPa due to low ρ ≈ 14 mg/mL, limiting their competitiveness as structural foams. We report fabrication of similar aerogels but with ρ spanning more than an order of magnitude from 16-400 mg/mL through controlled isostatic compression in the presence of a polymer coating circumventing any microstructural changes in stark contrast to other foams of carbon allotropes. The compressive stress (σ) versus ε measurements show that the densification of aerogels from ρ ≈ 16 to 400 mg/mL dramatically enhances Y from 0.9 to 400 MPa while maintaining superelasticity at least up to ε = 10% even at the highest ρ. The storage (E') and loss (E″) moduli measured in the linear regime show ultralow loss coefficient, tan δ = E″/E' ≈ 0.02, that remains constant over three decades of frequencies (0.628-628 rad/s), suggesting unusually high frequency-invariant efficiency. Furthermore, these aerogels retain exceptional fatigue resistance for 10 loading-unloading cycles to ε = 2% and creep resistance for at least 30 min under σ = 0.02 MPa with ρ = 16 mg/mL and σ = 2.5 MPa with higher ρ = 400 mg/mL. Lastly, these robust mechanical properties are stable over a broad temperature range of -100-500 °C, motivating their use as highly efficient structural components in environments with extreme temperature variations.