UV Light Exposure Modifies the Molecular Weight Characteristics and Improves the Melt Extrusion Additive Manufacturing of Polycarbonate

Bisphenol A polycarbonates (PC) are among the important engineering thermoplastics utilized as feedstocks for melt extrusion additive manufacturing, including fused filament fabrication (FFF). However, limited diffusion of polymer chains between adjacent layers and complex thermal histories encountered during the printing process lead to poor interlayer adhesion, residual stress, and voids in the printed parts, which in turn leads to poor mechanical performance and mechanical anisotropy. To address these issues, our group has modified an FFF printer by integrating an ultraviolet (UV)-LED optical fiber into the printer head. This modification allows for real-time illumination of the printed structure during the deposition process, enabling a thorough investigation into its impact on the FFF process and the properties of the resulting printed parts. Examination of the mechanical behavior and fracture surfaces of the printed parts shows that PC printed with UV light attains higher toughness in both the transverse and longitudinal directions as well as decreased void space compared to PC printed without UV light. Careful analysis of the molecular weight characteristics of the printed parts shows that PC printed with UV light has a broader molecular weight distribution with an extended lower molecular weight tail compared to PC parts printed without UV light. This indicates that the polymer chain scission upon UV light exposure leads to the formation of shorter polymer chains and broader range of chain sizes, which in turn increases in the toughness of the UV-light-printed PC. These observations are interpreted to indicate that the change in molecular weight distribution upon UV illumination fosters improved interfilamentous diffusion, consequently lowering the interlayer voids and thus enhancing the mechanical performance. These findings therefore provide foundational insights into how UV light can be used to tune the molecular weight distribution of the polymer and thus impact the FFF process, offering a distinctive and straightforward strategy to tune the properties of parts created by extrusion additive manufacturing of a range of thermoplastics.