(100c) Mapping Process-Induced Chain Orientation in 3D Printed Parts

Material extrusion additive manufacturing is characterized by a complex processing history that includes high deformation rates and rapid cooling. This process history is known to trap chains in the extruded polymer melt into nonequilibrium conformations, and the resulting molecular-scale anisotropy correlates well with variations in part properties like the weld strength. Residual chain orientation is commonly measured via optical birefringence through the transmitted part. Quantitative birefringence measurements can be time-consuming, and refraction at the polymer-air interface can obscure measurements near the weld, where orientation is expected to be significant. Here, we show that a combination of polarized optical imaging and optical microscopy can be used to map chain orientation in printed parts. Polarized optical imaging is performed on printed polylactide parts in refractive index matched fluids using a pixelated polarizer array bonded to the image sensor of a camera. The use of a refractive index matched fluid minimizes refraction to capture both bulk and near-weld information. The resulting two-dimensional image quantifies the orientation direction and the retardance, defined as the product of the birefringence and the local sample thickness. When combined with thickness measurements from optical microscopy, the polarized images provide a map of birefringence, which is proportional to the chain orientation. This orientation is highly sensitive to processing conditions like the print speed and nozzle temperature, which can be directly related back to two critical timescale comparisons: 1) the Weissenberg number comparing flow timescales in the nozzle to the reptation time in the polymer melt, and 2) the comparison between the reptation time necessary to relax chain orientation and the time required for the polymer to cool to the glass transition temperature. At high Weissenberg numbers and fast cooling conditions relative to chain relaxation, chains highly oriented in the flow direction should remain trapped in the glassy state, which is directly observed in these experiments. These results show the importance of optical polarization imaging for part characterization and potential quality control.