Re-entrant solid behavior of 3D-printable epoxy inks

Transient creep testing was used to differentiate the printability of block copolymer (BCP) containing epoxy inks for direct ink writing (DIW). Poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (P123) BCPs were shown to form disordered micelles at 20 wt% in a 3D-printable epoxy ink. Oscillatory amplitude sweeps identified no obvious difference in the rheological properties of the inks; however, the P123 inks required less pressure to print quality parts. By contrast, transient creep testing identified significant differences and showed that the P123 ink had a lower apparent yield stress and a lower time-dependent decrease in the shear rate. Additionally, both inks showed re-entrant solid behavior. This behavior manifests in printing as a material that initially flows well, but the material flow eventually stops. The transient creep results correlate well with the printing results. We propose that studying the time-dependent flow properties using transient creep testing is critical for the evaluation of DIW inks. [Figure not available: see fulltext.].