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One of the major limitations in Fused Filament Fabrication (FFF), a form of additive manufacturing, is the lack of composites with superior mechanical properties. Traditionally, carbon and glass fibers are widely used to improve the physical properties of polymeric matrices. However, the blending methods lead to fiber breakage, preventing generation of long fiber reinforced filaments essential for printing load-bearing components. Our approach to improve tensile properties of the printed parts was to use in-situ composites to avoid fiber breakage during filament generation. In the filaments generated, we used thermotropic liquid crystalline polymers (TLCPs) to reinforce acrylonitrile butadiene styrene (ABS) and a high performance thermoplastic, polyphenylene sulfide (PPS). The TLCPs are composed of rod-like monomers which are highly aligned under extensional kinematics imparting excellent one-dimensional tensile properties. The tensile strength and modulus of the 40 wt.% TLCP/ABS filaments was improved by 7 and 20 times, respectively. On the other hand, the 67 wt.% TLCP/PPS filament tensile strength and modulus were improved by 2 and 12 times, respectively. The filaments were generated using dual extrusion technology to produce nearly continuously reinforced filaments and to avoid matrix degradation. Rheological tests were taken advantage of to determine the processing conditions. Dual extrusion technology allowed plasticating the matrix and the reinforcing polymer separately in different extruders. Then continuous streams of TLCP were injected below the TLCP melting temperature into the matrix polymer to avoid matrix degradation. The blend was then passed through a series of static mixers, subdividing the layers into finer streams, eventually leading to nearly continuous fibrils which were an order of magnitude lower in diameter than those of the carbon and glass fibers. The composite filaments were printed below the melting temperature of the TLCPs, and the conditions were determined to avoid the relaxation of the order in the TLCPs. On printing, a matrix-like printing performance was obtained, such that the printer was able to take sharp turns in comparison with the traditionally used fibers. Moreover, the filaments led to a significant improvement in the tensile properties on using in FFF and other conventional technologies such as injection and compression molding. / Doctor of Philosophy / In this work two thermoplastic matrices, acrylonitrile butadiene styrene (ABS) and polyphenylene sulfide (PPS), were reinforced with higher melting thermoplastics of superior properties called thermotropic liquid crystalline polymers (TLCPs). This was done so that the resulting filaments could be 3D-printed without melting the TLCPs. The goal of this work was to generate nearly continuous reinforcement in the filaments and to avoid matrix degradation, and, hence, a technology called dual extrusion technology was used for the filament generation. The temperatures required for filament generation were determined using rheology, which involves the study of flow behavior of complex fluids. Dual extrusion technology allows processing of the constituent polymers separately at different temperatures, followed by a continuous injection of multiple TLCP-streams into the matrix polymers. In addition, the use of static mixers (metallic components kept in the path of flow to striate incoming streams) leads to further divisions of the TLCP-streams which are eventually drawn by pulling to orient the TLCP phase. The resulting filaments exhibited specific properties (normalized tensile properties) higher than aluminum and contained fibers that were nearly continuous, highly oriented, and an order in magnitude lower in diameter than those of carbon and glass fiber, which are commonly used reinforcements. High alignment and lower fiber diameter are essential for printing smoother printed parts. The filaments were intended to be printed without melting the TLCPs. However, previous studies involving the use of TLCP reinforced composites in conventional technologies have reported the occurrence of orientation relaxation on postprocessing, which decreases their tensile v properties. Therefore, temperatures required for 3D printing were determined using compression molding to retain filament properties on printing to the maximum extent. On printing using an unmodified 3D printer, parts were printed by taking 180º turns during material deposition. Contrarily, the use of continuous carbon fibers required a modified 3D printer to allow impregnation during 3D printing. Moreover, the performance comparison showed that the continuous carbon fibers could not be deposited in tighter loops. The properties of the printed parts were higher than those obtained on using short fibers and approaching those of the continuous fiber composites.

Thermoplastic composites that have been reinforced by thermotropic liquid crystalline (TLCP) fibrils in the microscale and by nanoparticles in the nanoscale are defined as multi-scale wholly thermoplastic composites (WTCs). Multi-scale WTCs have been proposed as lightweight replacements with high performance for some traditional glass fiber (GF) and carbon fiber (CF) reinforced composites materials in various applications. TLCPs are known for performing mechanical properties similar to those of the lower end of CF but significantly better than those of GF. To enhance the mechanical properties of TLPC reinforced WTCs, carbon nanotubes (CNTs) are considered being used as a secondary enhancement in WTCs. CNTs have gathered significant interest in the last 30 years because of their high aspect ratio, high mechanical properties, and other high-performance properties. The focus of this work is on investigating the processing conditions of generating in situ injection-molded multi-scale WTCs, then extending the technology to dual-extrusion and fused filament fabrication (FFF) and obtain high-performance multi-scale WTC products. This dissertation initially focused on investigating the processing conditions, in particular mixing histories and processing temperature profiles, of generating in situ injection-molded multi-scale WTCs, which consist of a representative TLCP, scCO2 aided exfoliated CNTs, and the thermoplastic matrix polyamide 6 (PA 6). The supercooling behavior of the TLCP and thermal stability of PA 6 are studied by applying the rheological methods of small amplitude oscillatory shear (SAOS). Multiple mixing histories with CNTs and processing temperature profiles are analyzed based on the criterion of maximizing tensile properties of multi-scale WTCs and minimizing thermal degradation of the matrix. Under the optimum processing conditions, the in situ injection-molded multi-scale WTCs exhibit a 26% and 34% tensile modulus and strength enhancement, compared to the in situ injection-molded WTCs with no CNTs. Scanning electron micrograph (SEM) images were used to understand the enhancement. The second part of this work is to extend the scCO2 aided in situ multi-scale WTCs processing technology to dual-extrusion and FFF. Multi-scale WTC filaments, which consists of TLCP, CNTs, and polyamide copolymer (PAc), are generated by dual-extrusion, and 3D printed into rectangular specimens in FFF. The 1 wt% CNTs reinforced multi-scale WTC filaments generated by the means of dual-extrusion exhibit 225% and 80% improvement in tensile modulus and strength, respectively, compared to the WTC filaments with no CNTs. In FFF, 40 wt% TLCP/1 wt% CNT/PAc 3D printed specimens with filament laid in longitudinal direction exhibited excellent tensile modulus and strength of 38.92 GPa and 127.16 MPa, respectively. The well-dispersed exfoliated CNTs show high alignment with TLCP microfibrils in the multi-scale WTC filaments and their laid-down specimens, which causes the significant tensile modulus enhancement. Bridging elements are discovered between TLCP fibrils and PAc matrix to improve interfacial adhesion, which is attributed to the well-dispersed exfoliated CNTs. Finally, the significant improvements in tensile properties attributed to scCO2 aided exfoliated CNTs in WTCs are verified on the multi-scale WTCs based on polypropylene (PP). Moreover, additional tensile properties improvements for exfoliated CNTs reinforced multi-scale WTCs are obtained with the use of maleic anhydride grafted polypropylene (MAPP). With 1 wt% CNTs and 16 wt% MAPP dual reinforcement, 20 wt% TLCP reinforced WTCs based on polypropylene (PP) exhibit 265%, 274%, and 182% improvement in the tensile modulus of the filaments, laid up specimens in the concentric pattern and laid up specimens in ±45° rectilinear pattern, respectively. The dual reinforcement also improves the tensile strength of 20 wt% TLCP reinforced WTC filaments by up to 73%. The high alignment between TLCP fibrils and CNTs are confirmed in the multi-scale WTCs based on PP. Besides the bridging elements attributed to CNTs found in the second part of this work, SEM images show that CNTs are partially trapped in TLCP fibrils. / Doctor of Philosophy / Considering the need for environmentally friendly materials, novel thermoplastic composites with high mechanical performance, lightweight, and potentially high recyclability properties were generated in this work. Two types of thermoplastic matrices, polyamide (PA or nylon) and polypropylene (PP) were reinforced with carbon nanotubes (CNTs) and rigid chain polymers known as thermotropic liquid crystalline polymers (TLCPs). CNTs are known for their high mechanical properties and high aspect ratio, which are helpful to reinforce thermoplastic composite materials. During injection molding and the dual-extrusion processes, TLCPs deform into almost continuous microfibrils and reinforce the thermoplastic matrices. Instead of using traditional glass fibers or carbon fibers to reinforce thermoplastics, TLCP reinforced thermoplastic composites, which are defined as wholly thermoplastic composites (WTCs), can retain their mechanical properties during the recycling process such as in injection molding and have better performance during the lay-down process in fused filament fabrication. The goal of this work was to generate CNTs reinforced WTCs for use in injection molding and fused filament fabrication with high mechanical performance. In the injection molding process for generating CNTs reinforced WTC end-gated plaques, it was determined that the optimum thermal mixing histories for the CNTs could be identified by the inspections of the tensile property measurements and scanning electron microscopy (SEM). With the obtained optimum thermal mixing histories with CNTs, CNTs reinforced WTC filaments were generated by dual extrusion technology and used in fused filament fabrication. With 1 wt% addition of CNTs, the tensile properties of WTCs were significantly enhanced in both the filament materials and the laid-down parts. Especially, the CNT reinforced WTC filaments based on nylon matrices exhibited competitive tensile moduli to long carbon fiber reinforced nylon composite filaments, which was also competitive to the properties of aluminum alloys. In addition, the laid-down parts of CNTs reinforced WTC based on PP presented further tensile strength improvement due to the improved interfacial adhesion between the laid-down filaments and between layers, which was attributed to the addition of maleic anhydride grafted polypropylene.