The replacement of metals with polymer-based materials is considered to be key to reducing weight, cost, and cycle times in different technological applications, including the automotive and aerospace industries. Semiaromatic polyamides are particularly attractive engineering polymers due to their balanced mechanical properties and processability. They combine high strength and stiffness due to the rigid aromatic repeating units with greater ductility and better processability than those of polyaramids, owing to the presence of flexible aliphatic repeating units. However, the challenge remains to find ways to improve the strain at break and toughness of semiaromatic polyamides and their composites, without inducing adverse consequences for their strength and stiffness. The present thesis addresses this challenge following two complementary approaches. In an engineering-driven approach suitable to industrial scale-up, we produced random copolyamides by high-temperature melt-blending of a strong and stiff semiaromatic polyamide with more ductile aliphatic polyamides. The blending of up to 30 wt% of PA66 or PA610 in PA6TI resulted in a five-fold increase of the strain at break compared to that of pure PA6TI, with almost no change in stiffness and only minor losses in strength. In a second, chemistry-driven approach inspired by the crucial role of polymer chain in silk materials, we designed and synthesized monomers that induce the formation of U-turn folds€ in attempt to induce re-entrant chain folding in the crystalline lamellae of semiaromatic polyamides. Contrary to polyamides with kinked repeating units that are usually amorphous, the use of such monomers did not impede crystallization. Acridine-based polyamides were shown to adopt a folded structure in the solid state that was structurally related to the beta-serpentine folds in amyloids. They also exhibited a modulus comparable to other semiaromatic polyamides and were harder and stiffer than the structurally related anthracene-based polymers that did not show chain folding due to steric repulsion in the required conformation.