Polymer composites exhibit unique bulk properties not found in other material systems, making them ideal candidates for various applications like tissue engineering and energy storage. These properties are functions of the properties of the filler particles and their organization within the composites that are primarily dependent on the methods and techniques used to process the materials. However, the development and manufacturing of polymer composites is very challenging due to their dependence on the rheological properties of the composites. Therefore, developing novel material systems with unique processing techniques is highly desirable to unlock new properties of polymer composites.
Carbon-based fillers, such as graphene and carbon nanotubes, are commonly used in polymer composites due to their remarkable electrical and thermal conductivities. Recently, fillers derived from eutectic alloys with melting points below room temperature, such as Eutectic Gallium-Indium (EGaIn), have attracted significant research attention. Unlike conventional fillers, EGaIn particles are relatively soft and deformable and have been shown to enhance the toughness of polymer composites by improving energy dissipation. They also contribute to the electrical, thermal and piezoelectric properties of the composites.
Generally, the main challenge in manufacturing polymer composite systems is the need for high-resolution control of the morphology and distribution of the filler in the polymer matrix, which defines the properties and functionality of the final products. This challenge is more common for highly complex polymer composites with multiple fillers and rigid counterparts. Additive manufacturing methods, particularly direct-ink-writing (DIW) can address these challenges owing to their ability to effectively control material deposition within layers. Despite the good recent progress, extensive research on the rheology of polymer composites and associated processing methods through DIW is still needed to realize more possibilities and capabilities of liquid metal carbon-based polymer composites.
Herein, Ruchira Tandel and Professor Arda Gozen from Washington State University presented an in-depth investigation of such polymer composite systems. This system consisted of solvent-based precursor inks derived from polyethylene oxide composites as the polymer matrix and graphene flakes and EGaIn microparticles as fillers, fabricated via DIW. The shear and ink rheology of different compositions derived from the system and their deposition flow was characterized. Specifically, the authors investigated the influence of EGaIn fillers on the underlying DIW processing mechanisms and parameters, ink rheology and the electrical and thermal conductivity of the resulting composite structures. Their work is currently published in the research Journal of Materials Processing Technology.
Tandel & Gozen showed that the EGaIn fillers varied the ink rheology toward more elastic behavior characterized by lower extensional viscosity. The viscoelasticity of the graphene-PEO composites inks attributed to the impact of the liquid metal fillers significantly improved the processability of the material system through DIW. As a result, the printed ink filaments exhibited exceptional properties, including the ability to withstand large extensional strains during processing, the ability to form continuous prints even at higher printing speeds and lower flow rates and the ability to produce filaments and features with smaller line width than nozzle diameter.
The EGaIn fillers also enhanced the conductivity of the graphene-PEO composites, which allows the spatial control of the processing by varying the process parameters. Nevertheless, it was worth noting that an increase in the strain resulted in a decrease in the electrical conductivity of the prints due to the deformation of the liquid EGaIn fillers in the printing direction.
In summary, the composition, processing and property relationships of liquid metal-graphene-based PCs fabricated via DIW were investigated. The new study revealed the functional possibilities of the liquid metal fillers when combined with other fillers. The proposed scheme also allowed robust and high throughput manufacturing of polymer composites through robust additive manufacturing technologies like DIW while achieving effective control of the process and the properties of the conductive polymer composites. In a statement to Advances in Engineering, Professor Arda Gozen stated that their new findings would advance different applications using conductive polymer composites.
Tandel, R., & Gozen, B. (2022). Direct-Ink-writing of liquid metal-graphene-based polymer composites: Composition-processing-property relationships. Journal of Materials Processing Technology, 302, 117470.