Utilizing Liquid Crystalline Polymers as a Reinforcement Phase in the Fused Filament Fabrication with Conventional Polymers

Using thermotropic liquid crystalline polymers (LCP) in fused filament fabrication (FFF) facilitates the production of highly anisotropic parts that exhibit a high tensile strength and Young´s modulus in the printing direction. This work investigates, for the first time, an approach where the LCP (reinforcing phase) and a conventional polymer (matrix) are printed using two separate printer nozzles. This contrasts with the existing approach in literature, where both components are blended during filament extrusion and subsequently printed with a single printer nozzle. A polyamide 6/66 copolymer (CoPA), a polycarbonate (PC), and a polyetherketoneketone (PEKK) serve as conventional matrix polymers. Due to their chemical structure, a good compatibility is expected between each of these polymers with the LCP. Composite tensile samples are printed with 14, 23, and 30 vol.% LCP and subsequently characterized by uniaxial tensile testing. The highest Young's modulus of 6.8 GPa is achieved in PC/LCP samples with 30 vol.% LCP. In contrast, the highest tensile strength of 126 MPa is obtained in PEKK/LCP tensile samples with 30 vol.% LCP, after an additional thermal annealing at elevated temperatures. The main failure mechanism for all composites is an LCP pull-out, indicating a poor adhesion between the LCP and the matrix polymers.