Microscopic evaluation of polymer–polymer interfaces in multi-material additive manufacturing

This paper demonstrates a new methodology to characterise interfaces of thermoplastic multi-material combinations in nanometre scale. To visualise interfaces and possible interphases, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and atomic force microscopy (AFM) were used. Isothermal “polymer-fusing” specimens as well as fused filament fabrication (FFF) manufactured parts of polyphenylsulfone (PPSU) and Polyetherimid (PEI) were produced and compared in this study. SEM and EDS measurements show the macroscopic interface structure whereas nanoscale AFM measurements were used to measure the stiffness gradient across the interface of multi-material combinations. Macroscopically, no mixing or mechanical interlocking through re-melting was observed. The dominant effects for creating the macroscopic surface morphology happen during first deposition. Under the investigated conditions (380 °C nozzle temperature and 220 °C print chamber temperature), the diffusion gradient at the centre of extrusions was measured to be above the required range to form full healing but still below the gradient at an interface formed with both materials held at 380 °C (above melting temperature) for 15 min. In addition, the gradient was measured to be lower at the edges of extrusions. These results show, that relevant intermixing of the polymers happens at the nanoscale.