No abstract

Gaining a molecular understanding of material extrusion (MatEx) 3D printing is crucial to predicting and controlling part properties. Here we report the direct observation of distinct birefringence localised to the weld regions between the printed filaments, indicating the presence of molecular orientation that is absent from the bulk of the filament. The value of birefringence at the weld increases at higher prints speeds and lower nozzle temperatures, and is found to be detrimental to the weld strength measured by tensile testing perpendicular to the print direction. We employ a molecularly-aware non-isothermal model of the MatEx flow and cooling process to predict the degree of alignment trapped in the weld at the glass transition. We find that the predicted residual alignment factor,Ā, is linearly related to the extent of birefringence, ∆n. Thus, by combining experiments and molecular modelling, we show that weld strength is not limited by inter-diffusion, as commonly expected, but instead by the configuration of the entangled polymer network. We adapt the classic molecular interpretation of glassy polymer fracture to explain how the measured weld strength decreases with increasing print speed and decreasing nozzle temperature.

Biodegradable polybutylene succinate (PBS) and poly (butylene succinate-ranadipate) (PBSA) were characterized to find the thermo-rheological bases for 3D printing by Fused Filament Fabrication (FFF). In dynamic viscoelastic measurements, the samples fulfilled time-temperature superposition and Cox Merz rule. The viscosity results were linked to the excellent filaments obtained and the observed good flow in the printer nozzle.Using specific tearing experiments, outstanding welding of the printed layers was obtained.Results were discussed considering the values of the entanglements density obtained by SAOS measurements. The main difference between both polymers was observed in the final production of 3D printed parts because the high crystallinity of PBS produced significant warpage, which prevented its use for practical purposes. On the contrary, the less crystalline PBSA random copolymer showed excellent performance during FFF. Thus, dimensionally stable and ductile printed objects were obtained, opening new processing routes for semi-crystalline biodegradable polyesters.