Additive-Manufactured Stochastic Polyimide Foams for Low Relative Permittivity, Lightweight Electronic Architectures.

Polyimides are widely utilized engineering polymers due to their excellent balance of mechanical, dielectric, and thermal properties. However, the manufacturing of polyimides into complex multifunctional designs can be hindered by dimensional shrinkage of the polymer upon imidization and post processing methods and inability to tailor electronic or mechanical properties. In this work, we developed methods to three-dimensional (3D) direct ink write polyimide closed-cell stochastic foams with tunable densities. These polyimide structures preserve the geometrical fidelity of 3D design with a linear shrinkage value of <10% and displayed microscale porosity ranging from 25 to 35%. This unique balance of morphology and direct-write compatibility was enabled by polymer phase inversion behavior without the need of conventional post-print cross-linking, imidization, or pore-inducing freeze processing. The manufacturability, thermal stability, and dielectric properties of the 3D polyimide stochastic foams reported here serve as enablers for the exploration of hierarchical, lightweight, high-temperature, high-power electronics.

High Surface Area Electrodes Derived from Polymer Wrapped Carbon Nanotubes for Enhanced Energy Storage Devices.

Electrical double layer capacitors store energy on two adjacent layers, resulting in fast charging and discharging, but their energy density is limited by the available surface area. In this study, using poly(methyl methacrylate) assisted sonication, carbon nanotube buckypapers with specific surface area as high as 950 m(2)/g have been processed. Performance of these high surface area buckypapers have been evaluated as supercapacitor electrodes. The energy density of these high surface area electrodes at low power density of 0.68 kW/kg was 22.3 Wh/kg, and at high power density of 84 kW/kg was 3.13 Wh/kg using the ionic liquid electrolyte.

Individually Dispersed Wood-Based Cellulose Nanocrystals.

Good dispersion of cellulose nanocrystals (CNCs) in the polymer matrix is one of the key factors to obtaining good properties in the resulting nanocomposites. However, the preparation of individually dispersed CNCs in solvents or in polymer matrices has been a challenge. In this study, individually dispersed wood-based CNCs have been successfully prepared in solvents, including dimethylformamide (DMF), H2O, and a mixture of H2O/DMF, by sonication of moisture-containing CNCs. The CNCs dispersions were characterized by dynamic light scattering (DLS). It is found that CNCs containing above about 3.8 wt % moisture is critical for achieving individually dispersed CNC in solvents. Hydrodynamic radius (Rh) of CNCs is smaller in H2O/DMF co-solvent mixture than that in pure DMF or in pure H2O under same sonication treatment conditions. Experimental results have been corroborated using molecular simulation study.

Polyacrylonitrile fibers containing graphene oxide nanoribbons.

Graphene oxide nanoribbon (GONR) made by the oxidative unzipping of multiwalled carbon nanotube was dispersed in dimethylformamide and mixed with polyacrylonitrile (PAN) to fabricate continuous PAN/GONR composite fibers by gel spinning. Subsequently, PAN/GONR composite fibers were stabilized and carbonized in a batch process to fabricate composite carbon fibers. Structure, processing, and properties of the composite precursor and carbon fibers have been studied. This study shows that GONR can be used to make porous precursor and carbon fibers. In addition, GONR also shows the potential to make higher mechanical property carbon fibers than that achieved from PAN precursor only.

Gel Spinning of Polyacrylonitrile/Cellulose Nanocrystal Composite Fibers.

Polyacrylonitrile (PAN)/cellulose nanocrytal (CNC) fibers containing 0, 1, 5, and 10 wt % CNCs have been successfully produced by gel spinning. The rheological properties of solutions were investigated and the results showed that the complex viscosity and storage modulus of solutions were significantly affected by the presence of CNCs in the solution. Structure, morphology, mechanical properties and dynamic mechanical properties of these fibers have been investigated. Tensile modulus and strength increased from 14.5 to 19.6 GPa and from 624 to 709 MPa, respectively, as CNC loading increased from 0 to 10 wt %. Wide-angle X-ray diffraction results showed better PAN chain alignment and higher PAN crystallinity with the incorporation of CNCs.