Polymer glasses are frequently used in a form of additive manufacturing called fused filament fabrication (FFF). Melts are extruded onto previous layers and form a weld before the temperature drops below the glass transition temperature. Extrusion is typically fast enough to produce significant chain alignment that affects the welds formed by diffusion between layers and leads to a strongly anisotropic amorphous structure. Improved understanding of the structure property relations in printed parts is essential to optimizing FFF for structure-critical parts and FFF offers unique opportunities to create non-crystalline materials with continuously tunable local alignment and entanglement densities.
We have used molecular dynamics simulations of a generic polymer model to examine the relaxation of aligned melts, including the evolution of alignment and the entanglement density in bulk regions and at the interfacial weld. The mechanical properties of the resulting structures are then studied under tensile and shear loading. Local structure determines the initial yield stress while entanglements lead to strain hardening and crazing that strongly affects the total fracture energy. Alignment of chains along the deposition direction means that there are more weak van der Waals bonds in the perpendicular directions. This reduces the yield strength for shear and tensile failure perpendicular to the deposition axis. Alignment and changes in entanglement density also produce profound changes in the strain to failure and ultimate fracture energy. Welded regions are most affected by diffusion during cooling and may be stronger than adjacent bulk material which has higher entanglement density than the weld but is also strongly aligned.