Enhanced Interfacial Bonding of Branched Polymers

The weak welding strength between filament layers in fused deposition modeling (FDM) products results in anisotropic mechanical properties that are also sensitive to print patterns. Furthermore, poor transverse directional alignment coupled with slow printing rates limit their applicability for industrial production. We hypothesize that interfilamentous welding strength can be enhanced by modifying the chemistry of the building blocks and the topological arrangement of the macromolecular structure. To test this, we carried out coarse-grained molecular dynamics simulations to investigate the dynamics of both linear and branched polymer across representative interfaces. We observe that the diffusion controlled interdigitation follows a power law, with the exponent decreasing from 0.34 to 0.11 as grafting density increases from 7.5% to 196% (sidechains are grafted to both sides of a monomer unit). Surprisingly, the addition of sidechains enhances welding efficiency, as dense bottlebrush polymers with high grafting density reach maximum rupture strength faster than linear polymers. However, their saturated rupture strength is lower. This observation is subsequently corroborated by experimental lap shear tests using ungrafted polyethylene and branched polyethylene grafted by octane. Our MD simulations show that while linear polymer welding relies on backbone entanglement, in bottlebrush polymers, sidechains play a dominant role in enhancing interfacial strength, surpassing the contribution of the backbone. And linear polymers require more time to diffuse into neighboring filaments to achieve desired bulk properties. Furthermore, our molecular dynamics simulations reveal a brittle rupture behavior with significant hardening in linear and comb-like (mildly grafted) polymers, while bottlebrush (densely grafted) polymers display elastomeric behavior with a pronounced stress plateau prior to fracture. By comparing the gyration radii of all topological polymers, we found that they exhibit an increase in gyration radius parallel to the stretching direction and a decrease perpendicular to the deformation. The increase becomes more pronounced with higher grafting density. These results not only provide deeper insight into the underlying welding mechanisms of topological polymers but also present a potential approach for mitigating the anisotropy that is inherent in FDM-based additive manufacturing.