(682j) Correlating Interlayer Polymer Chain Diffusion with Mechanical Properties in 3D Printed Semicrystalline Polymers

Material extrusion (MatEx) based additive manufacturing (AM) of thermoplastics offers a feasible alternative to cost and time intensive traditional subtractive manufacturing techniques. Semicrystalline polymers have several advantages over commercially available amorphous materials including improved thermal stability, toughness, and deformability. Isotactic polypropylene (PP) is a popular thermoplastic material that undergoes rapid crystallization that leads to significant trapped residual stress when processed using MatEx, resulting in poor dimensional stability and mechanical performance. MatEx of semicrystalline polymers such as PP has been severely limited due to the problems associated with shrinkage, warpage, void formation, and interlayer adhesion.

In this work, we investigate the effect of addition of low molecular weight hydrocarbon resins on the interlayer polymer chain dynamics, specifically, during the interlayer weld formation stage in MatEx. The incorporation of the amorphous resins delays the crystallization which would otherwise prevent the development of the interlayer bond. By modifying the crystallization time window, the time during which chain diffusion can occur across the deposited layers is lengthened, which allows for a stronger bond between layers. The impact of the PP blend composition on polymer chain diffusion, crystallization profiles, and corresponding printed part properties resulting from the repeated non-uniform thermal history in filament based MatEx of PP is studied. The extent of polymer chain re-entanglement post deposition on the printer bed is quantitatively determined using interrupted shear rheology protocols. Correlating the rheological findings with the mechanical performance of the printed parts will provide valuable insights into the complex interlayer welding process during MatEx and is critical to improving existing machine designs and feedstocks in order to achieve printed parts with properties approaching their injection molded counterparts.