Direct Ink Writing of Functional Fiber Composites

Fiber-polymer composites, consisting of the polymer matrix and functional synthetic/natural fibers, show tremendous promise due to their unique mechanical, thermal, and electrical properties. By printing such fiber-polymer composites using additive manufacturing (AM) techniques, the three-dimensional models can be fabricated with arbitrary free-form geometry and various functionalities. Direct ink writing (DIW) is an extrusion-based AM approach and has a diverse choice of printable matrix materials. Applications, including energy storage, mechanical reinforcements, and biomimetic materials, have been presented through DIW of fiber composites. Yet grand challenges such as the complicated process planning, low geometrical accuracy, filament shape instabilities, and minimal fiber extrusion capability still exist in the DIW of fiber-polymer composites. This dissertation aims to address these challenges in DIW of fiber-polymer composites and to develop a fundamental understanding of the complex interplay of ink properties, DIW printing process, and printed composite functionalities through testing the following hypotheses: Hypothesis I: The incorporation of an electric field between the printing nozzle and the substrate can induce an electrowetting effect to enlarge the printable ranges of materials and the working range of manufacturing process settings. Hypothesis II: The addition of water-washable gel can change the rheological properties of the fiber suspensions in a way that the shear-yielding and shear-thinning properties are enhanced to increase the printability of composites with higher geometrical accuracy. Hypothesis III: Sonication can remove the primary wall of natural fibers and enhances the printability of natural fibers in DIW while retaining its mechanical characteristics in the printed composite. The findings are beneficial to material science and manufacturing process, enabling significant improvements in DIW of fiber-polymer composites and future applications in functional devices fabrication in the following perspectives: a) Generalized mappings between the printed feature geometries and process parameters were developed. b) The influence of liquid ink rheological properties on printing stability was understood. c) Approaches for modifying synthetic/natural fiber for higher printability were established. d) Electric-field-assisted and temperature-controlled DIW process planning strategies were developed and evaluated. e) Novel applications of DIW printed fiber composites were demonstrated.