The role of poly (ethylene glycol) on crystallization, interlayer bond and mechanical performance of polylactide parts fabricated by fused filament fabrication

The interfacial bond quality between adjacent layers determines the mechanical properties of 3D parts fabricated by fused filament fabrication (FFF) method. For semi-crystalline polymers, molecular diffusion and entanglements as well as crystallization behaviors have been reported to exert influence on the interlayer bond quality for FFF-printed parts. To improve the interlayer bond quality and decrease the mechanical anisotropy, poly (ethylene glycol) (PEG) with various molecular weights are used to modify polylactide (PLA) printing materials. With the addition of PEGs, the interlayer bond strength for FFF-printed PLA parts is enhanced, which decreases the mechanical anisotropy from 32 % for neat PLA parts to 16 % for PLA/PEG parts. Moreover, PEGs with molecular weight larger than 8000 g/mol have profound effects on the mechanical properties of PLA parts. However, PEGs exert minor effect on the crystallization behaviors of FFF-printed PLA parts, although they can accelerate the crystallization kinetics for bulk PLA. By means of DSC method, shear-induced crystallization is proved to dominate the crystallization behaviors of PLA parts. Altogether, the improved mechanical properties for PLA parts with the addition of PEGs are associated with the enhanced molecular diffusion and entanglements at the bond zone, rather than the crystallinity of PLA parts.