Scalable methods for directional assembly of fillers in polymer composites: Creating pathways for improving material properties

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.