LAUSR

Polymer chain orientation is crucial to understanding the polymer dynamics at interfaces formed during thermoplastic material extrusion additive manufacturing. The flow field and rapid cooling produced during material extrusion can result in chains which are oriented and stretched, which has implications for interdiffusion and crystallization. Polarized Raman spectroscopy offers a non-destructive and surface sensitive method to quantify chain orientation. To study orientation and alignment of chains in 3D printed polycarbonate filaments, we used a combination of polarized Raman spectroscopy and birefringence ( n ) measurements. By changing the orientation of the sample with respect to polarization of incident radiation, we probe changes in the ratio between orientation-dependent vibration modes and orientation-independent modes. We used principal component analysis (PCA) and partial least squares (PLS) regression to develop correlations for birefringence and Raman measurements in samples that were pulled at different draw ratios (DRs). PCA was used to differentiate between orientation-dependent and orientation-independent modes, while PLS regression was used to calculate birefringence from Raman measurements of 3D printed samples. Birefringence measurements were compared to the polycarbonate intrinsic birefringence of 0.2, to estimate the degree of orientation. We find that measured values of birefringence underestimate orientation compared to Raman measurements. Introduction Thermoplastic material extrusion (MatEx)1 additive manufacturing (AM) is growing exponentially due to its wide application space, short lead time, low expense, and the ability to manufacture complex 3D parts that cannot be manufactured using traditional manufacturing methods such as machining.[1] Thermoplastic MatEx is a particular implementation of AM where thermoplastic filament is extruded layer-by-layer to make a 3D printed part. The filament is fed into a heated extruder where it is melted and extruded as a molten filament with a specific diameter though a nozzle. Through the motion of the extruder head and the build bed in the x, y and z directions, the extrudate is printed layer-by-layer and a 3D object is constructed, see Fig. 1(a). However, it is very difficult to implement quality assurance and control in AM, such as carefully testing to ensure consistent part quality that is checked against certain criteria, including dimensional accuracy, porosity, or mechanical properties such as tensile strength. That difficulty is due to strong dependence of electrical, optical, thermal, and mechanical properties[2] on the various printing parameters such as extruder head temperature, the speed of printing, layer thickness, and printing direction. Therefore, for AM to be a dependable and mainstream process in manufacturing, a thorough understanding of the effect of these parameters on the dynamic and microscopic behavior of the molecular structure is crucial. It has been shown that printing temperature and printing speed are the most important parameters[2–5] that affect the mechanical properties of 3D printed objects, and the variation in these parameters are the main culprit to the inconsistency in part quality. Therefore, theoretical and experimental efforts have been studied to understand the effects of these parameters on the mechanical properties. Experimental results have shown that weld areas between filaments experience mechanical failure more often than at the bulk.[2,6] Various theoretical models have predicted that failure tends to take place near the weld area due to the poor inter-molecular entanglement at the weldline interface which is in part attributed to alignment of chains of the polymer. A 3D structure is built by extruding polymer melt from the nozzle at temperatures greater than the glass transition temperature T > Tg where the molten extrudate also heats the sublayer above Tg , see Fig. 1(a). It is believed that © This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply, 2021 1 Material extrusion is the ASTM definition for this process; however, it is also known as fused deposition modeling (FDM)® or fused filament fabrication (FFF). Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.