UV Light Exposure\nModifies the Molecular Weight Characteristics\nand Improves the Melt Extrusion Additive Manufacturing of Polycarbonate

Bisphenol A polycarbonates (PC) are\namong the important engineering\nthermoplastics utilized as feedstocks for melt extrusion additive\nmanufacturing, including fused filament fabrication (FFF). However,\nlimited diffusion of polymer chains between adjacent layers and complex\nthermal histories encountered during the printing process lead to\npoor interlayer adhesion, residual stress, and voids in the printed\nparts, which in turn leads to poor mechanical performance and mechanical\nanisotropy. To address these issues, our group has modified an FFF\nprinter by integrating an ultraviolet (UV)-LED optical fiber into\nthe printer head. This modification allows for real-time illumination\nof the printed structure during the deposition process, enabling a\nthorough investigation into its impact on the FFF process and the\nproperties of the resulting printed parts. Examination of the mechanical\nbehavior and fracture surfaces of the printed parts shows that PC\nprinted with UV light attains higher toughness in both the transverse\nand longitudinal directions as well as decreased void space compared\nto PC printed without UV light. Careful analysis of the molecular\nweight characteristics of the printed parts shows that PC printed\nwith UV light has a broader molecular weight <i>distribution</i> with an extended lower molecular weight tail compared to PC parts\nprinted without UV light. This indicates that the polymer chain scission\nupon UV light exposure leads to the formation of shorter polymer chains\nand broader range of chain sizes, which in turn increases in the toughness\nof the UV-light-printed PC. These observations are interpreted to\nindicate that the change in molecular weight <i>distribution</i> upon UV illumination fosters improved interfilamentous diffusion,\nconsequently lowering the interlayer voids and thus enhancing the\nmechanical performance. These findings therefore provide foundational\ninsights into how UV light can be used to tune the molecular weight\ndistribution of the polymer and thus impact the FFF process, offering\na distinctive and straightforward strategy to tune the properties\nof parts created by extrusion additive manufacturing of a range of\nthermoplastics.