(565e) Liquid Metal Microparticle-Based Polymer Nanocomposites and Their 3D Printing

Introduction: Polymer nanocomposites (PNCs) offer exciting opportunities by synergistically combining the favorable properties of their polymer matrices and nanoparticle fillers. Particularly conductive PNCs consisting of mechanically compliant polymers and electrically conductive nanoparticles are emerging material systems in applications such as flexible, wearable electronics [1] and solid-state energy storage [2]. Additive manufacturing (3D printing) methods such as direct-ink-writing, involving layer-by-layer deposition of “inks” derived from these materials in the form of micro-scale filaments is recently emerging as a promising processing method for these materials systems. This approach carries the potential to control constituent morphology in each filament [3] which drives the functional properties of the fabricated structures [4], provided that the ink rheology and associated deposition mechanisms are well understood. This presentation focuses on the rheological characteristics and 3D printing of PNC systems featuring graphene flakes and/or novel liquid metal micro/nano particles in a polyethylene oxide (PEO) matrix.

Materials and Methods: The graphene flakes are of 8 nm thickness and have lateral size of approximately 5 mm. The PEO matrix is a blend of two different molecular weight polymer: primarily 100kDa with trace amounts of 5MDa to tune/vary the viscoelastic properties of the composite [5], [6]. The liquid metal microparticles are obtained by sonication of eutectic Gallium Indium (EGaIn) inside the ink solvent (water or acetonitrile) to obtain an average spherical particle diameter of 1.5 μm [6]. These particles consists of a liquid core encapsulated in an elastic solid skin of gallium oxides [7] and thus overall exhibit a deformable behavior unlike the conventionally rigid nanofillers used in PNCs. Several different compositions of PNCs including both or only one type of fillers were prepared and rheologically characterized to understand their (1) shear thinning behavior through cone and plate rheometry, (2) transient extensional viscosity variation and relaxation profiles under constant rate uniaxial deformation. Same inks are 3D printed using a custom direct-ink-writing system using positive pressure dispensing through cylindrical and conical nozzles with 200 μm diameter opening. In these experiments, lines were printed using various inks at several flow rates, printing speeds and nozzle-to-substrate distance levels. Experiments were imaged using high magnification microscopes to examine the filament geometry for various inks under same process parameters. Printed lines were measured using white light interferometry to characterize their geometry. Finally electrical conductivity of the lines were characterized to study the combined influence of PNC composition and printing process parameters on the electrical conductivity.

Results: The rheological characterization showed that increasing EGaIn microparticle content in the PNC inks led to reduction in the shear viscosity while increasing viscoelasticity, unlike the graphene flakes, where increased content led to higher shear viscosity and reduced viscoelasticity [6]. These rheological changes led to liquid metal microparticle loaded PNCs to exhibit continuous filamentary deposition at lower flowrates, higher printing speeds and higher nozzle-to-substrate distances. Under the increased extensional flow, these filaments experience visco-capillary thinning [8] to form line diameters smaller then nozzle diameters . Studies on the electrical conductivity of the printed patterns showed that PNCs including EGaIn nanoparticles only did not for conductive patterns, however, when introduced as a secondary filler in graphene-based PNCs they improved electrical conductivity. Increased extensional flow in the deposited filaments generally reduced the electrical conductivity. Printing through cylindrical nozzles also led to increased electrical conductivity as compared to conical nozzles likely as a result of the increased shear stresses aligning the graphene flakes in the printed structures to form conductive pathways.

Discussion: The influence of liquid metal particles on the PNC rheology contradict that of the rigid nanoparticles [9], due to the unique deformable nature of these particles. Particularly, these particles can stretch and deform under shear and extensional stresses during flow, manifesting in increased viscoelasticity during extensional flow and reduced viscosity during shear flow. The capability of liquid metal particle loaded filaments to experience higher strains under extensional flow allowed thinner filaments to be deposited at higher printing speeds, effectively increasing the process resolution and throughput. However, it can be postulated that the under the uniaxial stresses, the average distance between the conductive particles reduce leading to reduced conductivity. Lack of conductivity in PNCs only including EGaIn particles is due to their non-conductive oxide skins [10]. These particles increasing electrical conductivity in graphene-based PNCs is possibly due to two reasons: (1) the EGaIn particles can form conductive interfaces with graphene flakes, thus acting as conformable anchors between these particles which can conduct electricity regardless of relative orientation of these particles, (2) the rheological changes induced by the liquid metal particles reduces the energy required for the graphene particles to align and form conductive pathways, particularly under shear flow. These results showed that EGaIn particles can be used as rheological modifiers in PNCs along with other conductive nanofillers as they can counter the effects induced by the rigid fillers. Furthermore, these particles can enhance electrical conductivity in such PNCs. Through 3D printing, unique properties of the liquid metal particle-based PNCs can be utilized to achieve high resolution printing and filament-by-filament control of the electrical conductivity.

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