Effect of processing on semicrystalline morphology in the additive manufacturing of poly(lactic acid)

Material extrusion additive manufacturing processes force molten polymer through a printer nozzle at high (>100 s⁻¹) wall shear rates prior to cooling and crystallization. These high shear rates can lead to flow-induced crystallization in common polymer processing techniques, but the magnitude and importance of this effect is unknown for additive manufacturing. Of critical importance in semicrystalline polymer additive manufacturing is the semicrystalline morphology near the weld between extruded layers, which can affect mechanical properties. Here, we present a systematic study of printing conditions on the semicrystalline morphology of parts printed from poly(lactic acid) (PLA). The slow crystallization kinetics of PLA generate parts with low crystallinity, however a secondary annealing process at high temperatures (140 °C) generate a space-filled spherulitic texture, with smaller spherulites near the weld zone between the extruded layers. This spherulite size distribution is attributed to a higher nucleation density templated into the part by the temperature and deformation history of the printing process. We show that theoretical modeling of flow-induced crystallization processes can predict the spatial distribution of spherulites observed in experiment.