Effects of Smectic Layer Deformation on Mechanical Properties of Glassy Liquid Crystal Polymer Fibers

In this work, it is demonstrated that the stress response of glassy smectic liquid crystal (LC) polymer fibers is largely dependent on layer deformations. Fibers with smectic layers stacked along the fiber axis (fiber A), prepared by stretching a molten main‐chain LC polyester, deform the layers by tensile deformation in the LC state, yielding fibers with layers tilted from the fiber axis (fiber B) and those with divided layers (fiber C). These fibers in the glassy LC state differ in Young's modulus (E), yield stress and strain (σy, εσy), and strain at break (εb). Fiber A has εb = 30%; however, fiber B is much more ductile (εb > 290%). Fiber C has higher E = 0.97 GPa and σy = 82 MPa than fiber A (E = 0.33 MPa and σy = 33 MPa). Annealing fiber C in the LC state yields fiber D with large‐area layers, similar to fiber A. Fiber D has E = 0.74 GPa and σy = 25 MPa, comparable to those of fiber A. Thus, dividing layers improves E and σy. This strengthening of glassy smectic LC by dividing layers applies to other polymers with layered structures, including lamellar block copolymers and semi‐crystalline polymers. Main‐chain type smectic liquid crystal polymers yield fibers with layers stacked along the fiber axis. The layers are tilted or divided when these fibers are stretched in the liquid crystal state. The fibers with tilted and divided layers become dramatically more ductile and stronger, respectively, than the original fiber.