Although supramolecular polymers have been increasingly reported in the literature to yield highly functional and in some cases commercially useful materials, harnessing their properties in 3D printing applications have only been recently realized and mainly for the printing of biomaterial hydrogels. Thus, 3D printing of synthetic supramolecular thermosets is seriously underexplored, with a multitude of possible material properties and applications to gain. We have shown that the combination of properties inherent to thermoset materials--along with the reversible nature of self-assembly--assists not only in the additive manufacturing process, but may also alleviate material properties issues associated with thermosets (reprocessability, residual stress, slow kinetics).
To explore this area, we synthesized a dually-functional, hydrogen-bonding, ureidopyrimidinone (UPy), amine-containing monomer and used it as a rheology modifier in epoxy-amine thermoset formulations of Epon® 828 and Jeffamine® D230 containing nano-clay filler. The introduction of a hydrogen-bonding motif yielded pre-polymer resins capable of being printed at room temperature using direct ink writing (DIW) 3D printing technology. These new materials display substantial shear-thinning to enable extrusion through micronozzles under mild conditions but display sufficient shear elastic modulus and yield strength to maintain shape after exiting the nozzle. The examples of thermoset materials printed via this technique are limited due to the fact that they tend be based on chemistries with slow reaction kinetics. 3D printing of amine cured, two-component aromatic epoxide resins is advantageous as these materials are heavily used for aerospace applications and have desirable physical properties. However, they require extensive cure times in order to achieve gelation and have restrictive rheological profiles and pot-life.
In existing literature, elongated fillers (i.e. carbon fibers, carbon nanotubes, and silicon carbide whiskers) were added as viscosity modifiers to impart dimensional stability to thermoset resins for DIW. We provide an alternative to using fiber fillers by adding supramolecular moieties that impart shear-thinning during extrusion, but increased viscosity and yield-stress to a two-part amine-epoxy resin. The reversibility of supramolecular interactions should facilitate the manufacturing of an array of 3D printed objects due to the beneficial rheological properties they impart. Additionally, using self-assembly/supramolecular interactions may also be useful for controlling interfacial interactions, thus promoting efficient interlayer adhesion.