Yuanhao Guo

Polymer composites are ubiquitous and indispensable in numerous applications. Coupling the inclusion of reinforcing agents with methods developed to control filler distribution and orientation allow for significant enhancement in mechanical, thermal, electrical, and barrier/transport‐related properties of composites. For practical implementation, ideal approaches to fabricating polymer composites with directionally assembled fillers must consider manufacturing compatibility, scale‐up ease, and aligning efficiency. In this article, we will review a cost‐effective and industrially relevant toolbox for preferential filler alignment in polymer composites. Three main strategies for aligning fillers within polymer composites will be discussed. Specifically, solution casting and compression molding can introduce compression force to assemble fillers along the in‐plane direction of composite films, shear force can be developed during fiber spinning, film blowing, and uniaxial/biaxial stretching processes to align fillers along the shear direction, and external fields (both electric and magnetic fields) can direct assembly of fillers, typically along the out‐of‐plane direction. For each section, we not only highlight the mechanisms of these methods but also demonstrate the critical structure–property relationships through model examples. Finally, a perspective on future opportunities and challenges of anisotropic and performance‐enhanced polymer composite preparation strategies is provided, with a particular focus on additive manufacturing.

An anisotropic swelling multi-layer wound dressing with vertically orientated water absorptive particles induced by an electric-field.

A roll-to-roll continuous process was developed to manufacture large-scale multifunctional poly(dimethylsiloxane) (PDMS) films embedded with thickness direction (“Z” direction) aligned graphite nanoparticles by application of electric field. The kinetics of particle “Z” alignment and chain formation was studied by tracking the real-time change of optical light transmission through film thickness direction. Benefiting from the anisotropic structure of aligned particle chains, the electrical and thermal properties of the nanocomposites were dramatically enhanced through the thickness direction as compared to those of the nanocomposites containing the same particle loading without electrical field alignment. With 5 vol % graphite loading, 250 times higher electrical conductivity, 43 times higher dielectric permittivity, and 1.5 times higher thermal conductivity was achieved in the film thickness direction after the particles were aligned under electrical field. Moreover, the aligned nanocomposites with merely 2 vol % graphite particles exhibit even higher electric conductivity and dielectric permittivity than those of the nonaligned nanocomposites at random percolation threshold (10 vol % particles), as the “electric-field-directed” percolation threshold concentration is substantially decreased using this process. As the graphite loading increases to 20 vol %, the aligned nanocomposites exhibit thermal conductivity as high as 6.05 W/m·K, which is 35 times the thermal conductivity of pure matrix. This roll-to-roll electric field continuous process provides a simple, low-cost, and commercially viable method to manufacture multifunctional nanocomposites for applications as embedded capacitor, electromagnetic (EM) shielding, and thermal interface materials.

A facile method to fabricate hierarchically structured fiber composites is described based on the electrospinning of a dope containing nickel and manganese nitrate salts, citric acid, phenolic resin and an amphiphilic block copolymer. Carbonization of these fiber mats at 800 °C generates metallic Ni encapsulated NiO/MnOx/carbon composite fibers with average BET surface area (150 m2/g) almost 3 times higher those reported for non-porous metal oxide nanofibers. The average diameter (~ 900 nm) of these fibers composites is nearly invariant of chemical composition and can be easily tuned by the dope concentration and electrospinning conditions. The metallic Ni nanoparticle encapsulation of NiO/MnOx/C fibers leads to enhanced electrical conductivity of the fibers, while the block copolymers template an internal nanoporous morphology and the carbon in these composite fibers helps to accommodate volumetric changes during charging. These attributes can lead to lithium ion battery anodes with decent rate performance and long-term cycle stability, but performance strongly depends on the composition of the composite fibers. The composite fibers produced from a dope where the metal nitrate is 66 % Ni generates the anode that exhibits the highest reversible specific capacity at high rate for any composition, even when including the mass of the non-active carbon and Ni0 in the calculation of the capacity. On the basis of the active oxides alone, near-theoretical capacity and excellent cycling stability is achieved for this composition. These cooperatively assembled hierarchical composites provide a platform for fundamentally assessing compositional dependencies for electrochemical performance. Moreover, this electrospinning strategy is readily scalable for the fabrication of a wide variety of nanoporous transition metal oxide fibers.