Optothermally Reversible Carbon Nanotube-DNA Supramolecular Hybrid Hydrogels.

Supramolecular hydrogels (SMHs) are three-dimensional constructs wherein the majority of the volume is occupied by water. Since the bonding forces between the components of SMHs are noncovalent, SMH properties are often tunable, stimuli-responsive, and reversible, which enables applications including triggered drug release, sensing, and tissue engineering. Meanwhile, single-walled carbon nanotubes (SWCNTs) possess superlative electrical and thermal conductivities, high mechanical strength, and strong optical absorption at near-infrared wavelengths that have the potential to add unique functionality to SMHs. However, SWCNT-based SMHs have thus far not realized the potential of the optical properties of SWCNTs to enable reversible response to near-infrared irradiation. Here, we present a novel SMH architecture comprised solely of DNA and SWCNTs, wherein noncovalent interactions provide structural integrity without compromising the intrinsic properties of SWCNTs. The mechanical properties of these SMHs are readily tuned by varying the relative concentrations of DNA and SWCNTs, which varies the cross-linking density as shown by molecular dynamics simulations. Moreover, the SMH gelation transition is fully reversible and can be triggered by a change in temperature or near-infrared irradiation. This work explores a new regime for SMHs with potential utility for a range of applications including sensors, actuators, responsive substrates, and 3D printing.

Combustion-Assisted Photonic Annealing of Printable Graphene Inks via Exothermic Binders.

High-throughput and low-temperature processing of high-performance nanomaterial inks is an important technical challenge for large-area, flexible printed electronics. In this report, we demonstrate nitrocellulose as an exothermic binder for photonic annealing of conductive graphene inks, leveraging the rapid decomposition kinetics and built-in energy of nitrocellulose to enable versatile process integration. This strategy results in superlative electrical properties that are comparable to extended thermal annealing at 350 °C, using a pulsed light process that is compatible with thermally sensitive substrates. The resulting porous microstructure and broad liquid-phase patterning compatibility are exploited for printed graphene microsupercapacitors on paper-based substrates.

Characterization of local elastic modulus in confined polymer films via AFM indentation.

The properties of polymers near an interface are altered relative to their bulk value due both to chemical interaction and geometric confinement effects. For the past two decades, the dynamics of polymers in confined geometries (thin polymer film or nanocomposites with high-surface area particles) has been studied in detail, allowing progress to be made toward understanding the origin of the dynamic effects near interfaces. Observations of mechanical property enhancements in polymer nanocomposites have been attributed to similar origins. However, the existing measurement methods of these local mechanical properties have resulted in a variety of conflicting results on the change of mechanical properties of confined polymers. Here, an atomic force microscopy (AFM)-based method is demonstrated that directly measures the mechanical properties of polymers adjacent to a substrate with nanometer resolution. This method allows us to consistently observe the gradient in mechanical properties away from a substrate in various materials systems, and paves the way for a unified understanding of thermodynamic and mechanical response of polymers. This gradient is both longer (up to 170 nm) and of higher magnitude (50% increase) than expected from prior results. Through the use of this technique, we will be better able to understand how to design polymer nanocomposites and polymeric structures at the smallest length scale, which affects the fields of structures, electronics, and healthcare.

Tuning the mechanical properties of graphene oxide paper and its associated polymer nanocomposites by controlling cooperative intersheet hydrogen bonding.

The mechanical properties of pristine graphene oxide paper and paper-like films of polyvinyl alcohol (PVA)-graphene oxide nanocomposite are investigated in a joint experimental-theoretical and computational study. In combination, these studies reveal a delicate relationship between the stiffness of these papers and the water content in their lamellar structures. ReaxFF-based molecular dynamics (MD) simulations elucidate the role of water molecules in modifying the mechanical properties of both pristine and nanocomposite graphene oxide papers, as bridge-forming water molecules between adjacent layers in the paper structure enhance stress transfer by means of a cooperative hydrogen-bonding network. For graphene oxide paper at an optimal concentration of ~5 wt % water, the degree of cooperative hydrogen bonding within the network comprising adjacent nanosheets and water molecules was found to optimally enhance the modulus of the paper without saturating the gallery space. Introducing PVA chains into the gallery space further enhances the cooperativity of this hydrogen-bonding network, in a manner similar to that found in natural biomaterials, resulting in increased stiffness of the composite. No optimal water concentration could be found for the PVA-graphene oxide nanocomposite papers, as dehydration of these structures continually enhances stiffness until a final water content of ~7 wt % (additional water cannot be removed from the system even after 12 h of annealing).

Evolution of order during vacuum-assisted self-assembly of graphene oxide paper and associated polymer nanocomposites.

Three mechanisms are proposed for the assembly of ordered, layered structures of graphene oxide, formed via the vacuum-assisted self-assembly of a dispersion of the two-dimensional nanosheets. These possible mechanisms for ordering at the filter-solution interface range from regular brick-and-mortar-like growth to complete disordered aggregation and compression. Through a series of experiments (thermal gravimetric analysis, UV-vis spectroscopy, and X-ray diffraction) a semi-ordered accumulation mechanism is identified as being dominant during paper fabrication. Additionally, a higher length-scale ordered structure (lamellae) is identified through the examination of water-swelled samples, indicating that further refinements are required to capture the complete formation mechanism. Identification of this mechanism and the resulting higher-order structure it produces provide insight into possibilities for creation of ordered graphene oxide-polymer nanocomposites, as well as the postfabrication modification of single-component graphene oxide papers.

Bio-inspired borate cross-linking in ultra-stiff graphene oxide thin films.

Adjacent graphene oxide nanosheets in a thin-film structure have been covalently cross-linked in a fashion similar to the cell walls of higher-order plants. The resulting ultra-stiff structure exhibits a maximum storage modulus of 127 GPa that can be tuned by varying borate concentration.

Sacrificial bonds in stacked-cup carbon nanofibers: biomimetic toughening mechanisms for composite systems.

Many natural composites, such as nacre or bone, achieve exceptional toughening enhancements through the rupture of noncovalent secondary bonds between chain segments in the organic phase. This "sacrificial bond" rupture dissipates enormous amounts of energy and reveals significant hidden lengths due to unraveling of the highly coiled macromolecules, leaving the structural integrity of their covalent backbones intact to large extensions. In this work, we present the first evidence of similar sacrificial bond mechanisms in the inorganic phase of composites using inexpensive stacked-cup carbon nanofibers (CNF), which are composed of helically coiled graphene sheets with graphitic spacing between adjacent layers. These CNFs are dispersed in a series of high-performance epoxy systems containing trifunctional and tetrafunctional resins, which are traditionally difficult to toughen in light of their highly cross-linked networks. Nonetheless, the addition of only 0.68 wt % CNF yields toughness enhancements of 43-112% for the various blends. Analysis of the relevant toughening mechanisms reveals two heretofore unseen mechanisms using sacrificial bonds that complement the observed crack deflection, rupture, and debonding/pullout that are common to many composite systems. First, embedded nanofibers can splay discretely between adjacent graphitic layers in the side walls; second, crack-bridging nanofibers can unravel continuously. Both of these mechanisms entail the dissipation of the pi-pi interactions between layers in the side walls without compromising the structural integrity of the graphene sheets. Moreover, increases in electrical conductivity of approximately 7-10 orders of magnitude were found, highlighting the multifunctionality of CNFs as reinforcements for the design of tough, inexpensive nanocomposites with improved electrical properties.

Self-associating block copolymer networks for microchip electrophoresis provide enhanced DNA separation via "inchworm" chain dynamics.

We describe a novel class of DNA separation media for microchip electrophoresis, "physically cross-linked" block copolymer networks, which provide rapid (<4.5 min) and remarkably enhanced resolution of DNA in a size range critical for genotyping. Linear poly(acrylamide-co-dihexylacrylamide) (LPA-co-DHA) comprising as little as 0.13 mol % dihexylacrylamide yields substantially improved electrophoretic DNA separations compared to matched molar mass linear polyacrylamide. Single-molecule videomicroscopic images of DNA electrophoresis reveal novel chain dynamics in LPA-co-DHA matrixes, resembling inchworm movement, to which we attribute the increased DNA resolution. Substantial improvements in DNA peak separation are obtained, in particular, in LPA-co-DHA solutions at polymer/copolymer concentrations near the interchain entanglement threshold. Higher polymer concentrations yield enhanced separations only for small DNA molecules (<120 base pairs). Hydrophobically cross-linked networks offer advantages over conventional linear polymers based on enhanced separation performance (or speed) and over chemically cross-linked gels because hydrophobic cross-links can be reversibly broken, allowing facile microchannel loading.