| The rigid-rod polymeric fibers, such as Zylon (PBO, Poly(p-phenylenebenzobisoxazole), are exceptional with regard to their tensile properties, yet their poor to modest axial compressive properties have been their Achilles' heel, limiting their ultimate potential, especially for structural applications requiring tolerance to compressive loadings. The poor axial compressive strength of the polymer fibers is widely attributed to insufficient lateral interactions between the highly oriented rigid-rod molecules, causing the micro-fibrils to buckle under an axial compressive load.;The main objective of this thesis is to provide an insightful review, based on investigation of some molecular/nano-level approaches, for the improvement of the axial compressive properties of these highly oriented polymeric fibers. The predominant molecular approaches discussed are based on the lateral stabilization of the rigid-rod polymer chains via thermal cross-linking or introduction of lateral association, such as inter-chain hydrogen bonding, to provide supramolecular structures. While examples of lateral hydrogen bonding in rigid-rod polymers invariably involve the benzobisimidazole structure in the polymer backbone, other rigid-rod molecular motifs such as benzobisthiazoles linked to a terphenyl system with aromatic heterocyclic pendants are also discussed from the viewpoint of enhancing polymer fiber compressive strength. It is presumed that the twisted terphenyl system with bulky substituents can disrupt molecular rigidity while facilitating strong lateral interactions between the aromatic heterocyclic pendants, leading to enhanced fiber compressive strength.;In the context of the nano level approach to the problem of axial compressive strength, in situ polymerization of PBO in the presence of SWNTs (single-wall carbon 3 nanotubes), the fabrication and characterization of PBO/SWNT hybrid polymer fibers as well as the evaluation of their mechanical properties are also discussed. Relative to control PBO fibers, PBO/SWNT composite fibers were found to exhibit higher axial compressive strength. |
