When shearing polymer fluids in an aqueous or other non-solvent phase, polymers typically form spheres. In these two-phase systems, it is just not energetically favorable for non-spherical shapes to form as that would increases the surface area to volume ratio. Some approaches to produce anisotropic particles include:
- stretching spherical polymer particles and curing/quenching them in that state
- template-assisted methods where particles are formed on patterned surfaces like those with microchannels or pores
- microfluidic techniques
- swelling with solvent
- phase separation that results in anisotropic shapes.
Generally, these methods are often difficult to scale either due to the possible throughput or due to the complicated infrastructure/hardware needed; are often expensive due to equipment needed; may be limited in the materials that can be used; and may include additional processes or waste products that are undesirable.
LLNL researchers have developed a synthetic methodology for making anisotropic polymer microparticles using a two phase high shear mixing protocol. One phase is an aqueous phase which may include polymer viscosity modifiers and/or surfactants to increase the internal shear stress applied to the polymer phase precursor and stabilize particle formation. When the polymer phase is formulated with rheological properties that resist flow (i.e., produce a thixotropic material), anisotropic particles can be formed. The anisotropic particles could be extruded, aligned preferentially along the direction of extrusion, and cured together using an appropriate binder to create polymer-polymer composites in 3D printable Direct Ink Write (DIW) inks.
In addition to changes that affect rheological properties, functional fillers like magnetic or conductive particles can also be added to the polymer phase. The resulting multi-functional, anisotropic polymer microparticles impart a magnetic response to the allowing them to orient in the direction of an applied magnetic field. These particles can be isolated and 3D printed, resulting in a printed structure that is also response to a magnetic field.
Image Caption: High resolution SEM image of an extruded DIW filament with magnetically active silicone based anisotropic microparticle fillers added to the DIW ink
- Method has potential to be scalable to multiple kilograms.
- Method is amenable to many different materials (epoxies, silicones, acrylates, liquid crystal elastomers, urethanes) including uncured precursors and polymer melts.
- Method takes advantage of well-known emulsion chemistry techniques like use of viscosity modifiers or surfactants to tune resulting anisotropic microparticle shape, size, size distribution, and surface functionality.
- Allows for ability to add multi-functional materials to the polymer microparticles (magnetic, electronic) to introduce to properties to both the DIW ink and the resulting 3D printed structure.
Rheology modifier in DIW inks, multi-functionality additive to DIW inks and prints.
Current stage of technology development:
TRL ☐ 0-2 ☒ 3-5 ☐ 5-9