Simultaneous in Situ X-ray Scattering and Infrared Imaging of Polymer Extrusion in Additive Manufacturing

In situ wide-angle X-ray scattering together with infrared imaging was performed during three-dimensional material extrusion printing and correlated with the development of the crystalline structure and subsequent thermomechanical properties. Identical samples were printed with nozzle motion either along the short axis or the long axis. The short axis mode had higher thermal retention, which resulted in later onset of crystal structure. The longer time spent at temperatures between the glass transition and the melting point produced samples with higher degree of crystallinity but also significantly increased brittleness. The tracer diffusion coefficient $D\left(T\right)$ , together with its temperature dependence, was measured using neutron reflectivity, and the total interdiffusion length between filaments was then calculated using $D\left(T\right)$ for each temperature point, as determined by the measured thermal profiles. This allowed us to define the time/temperature plane that yielded the minimum diffusion length $\text{Δ}L$ that provides mechanical integrity of the printed features ( $\text{Δ}L$ less than the radius of gyration of the poly(l-lactide)). The model was probed by printing structures at four nozzle temperatures and measuring the time dependence of the thermal profiles at filaments in the horizontal and vertical positions. The data indicated that the thermal retention was anisotropic, where higher values were obtained in the horizontal plane. Mechanical measurements indicated large differential increases in the torsional strength, corresponding to the direction with increased thermal retention.