[SY-H4] Effect of Chain Alignment on Entanglements, Diffusion and Polymer Weld Strength
Extrusion in fused filament fabrication is typically fast enough to produce significant chain alignment that may change the weld strength of 3D printed objects in several ways. Some studies suggest that alignment may enhance interdiffusion because of the entropic force driving chains towards unaligned conformations or by reducing entanglement density. Alignment of strong backbone bonds along the interface may also change the mechanical properties of material near the joint. As part of a multiscale study of 3D printing, we have used molecular dynamics simulations of a generic polymer model to examine the effect of alignment on the dynamics of welding and evolution of weld strength. Entropy drives chain retraction in the tube, but this does not speed interdiffusion since the tubes are aligned along the interface. There is also no indication of accelerated interdiffusion due to entanglement loss. The ambiguities in recent real space methods for identifying entanglements will be discussed. Alignment does reduce the strength of bulk material adjacent to the weld. At intermediate times this greatly enhances weld strength by moving failure away from the interface. The dependence of the bulk strength on alignment is presented for different thermal histories.