Interfacial adhesion between dissimilar thermoplastics fabricated via material extrusion-based multi-material additive manufacturing

• Interfacial adhesion between dissimilar thermoplastics fabricated via AM is quantified. • Bonding mechanisms are correlated to material properties. • The correlations between bonding strength and bonding mechanisms are revealed. Multi-material additive manufacturing (MMAM) enables the design of materials with tunable mechanical performance by fabricating multiple dissimilar materials in a single print. MMAM has been utilized to fabricate components with unique mechanical properties for applications such as damage detection, medical devices, sensors, and soft robotics. However, the bonding strength between dissimilar polymeric materials strongly depends on the material combination and is typically lower than the material strength of the constituents. This study investigates the interfacial adhesion between two thermoplastics fabricated via material extrusion (ME)-based MMAM by quantifying the interface bonding strength using mechanical tests and polymer adhesion theory-based correlation analysis. Experimental results showed that the polylactic acid (PLA)-polyethylene terephthalate glycol (PETG), PETG-polycarbonate (PC) and PLA-PC material combinations exhibit bonding strengths that are close to or exceed their constituent’s material strength. Material combinations that include polypropylene (PP) and polyethylene (PE) exhibited bonding strengths of nearly two magnitudes lower than those of PLA-PETG, PETG-PC, and PLA-PC. The microstructural images of the samples showed that the most compatible combinations exhibited a smooth, gradient interface indicating the importance of nano-scale adhesion mechanisms. Based on Hansen solubility parameters and the coefficient of thermal expansion (CTE), we observed the correlation between wettability and physical adsorption, intermolecular diffusion, thermal stress, and the interface bonding strength. The wettability and physical adsorption feature extracted from the solubility parameters showed the highest correlation with the interface bonding strength. Furthermore, we observed that the smaller the difference in solubility parameters and CTE between two thermoplastics fabricated via ME, the more compatible the two thermoplastics are.