Flow-induced stretch, alignment, and relaxation of semi-crystalline polymers in material extrusion additive manufacturing

Most soft matter additive manufacturing processes produce parts with asymmetric material properties due to the weak interface between layers. In thermoplastic material extrusion, these anisotropic properties are attributed to rapid cooling and subsequent limited diffusion time across the interface. However, the thermal history and flow-field have an additional effect on semi-crystalline polymers, resulting in changes to the crystal structure due to non-quiescent or flow-induced crystallization. These effects are most dramatic at the interface and can produce varying and asymmetric crystal morphologies in the extrudate. To quantify these effects, we use a combination of in-situ thermography and polarized light imaging to characterize cooling rate and residual stress during printing and ex-situ polarized imaging and micro-beam wide-angle x-ray scattering to characterize the non-equilibrium state of the polymer and final crystalline morphology. From these measurements, we see high extrusion speeds and low extrusion temperatures leave the polymer in a stretched and aligned state, which changes the nucleation density and crystal morphology at the interface between layers. In contrast, printing at high temperatures provides sufficient mobility for the chains to relax to an equilibrium state before crystallizing or cooling to the glass transition temperature.