Molecular Level Approach for the Improvement of Compressive Properties of Rigid-rod Polymer Fibers
Aromatic heterocyclic rigid-rod polymers such as poly (p-phenylene-benzobisoxazole) (PBO) and poly (phenylenebenzobisthiazole) (PBZT) constitute a unique class of materials referred to as PBX polymers. PBX-based rigid-rod polymeric fibers are known to have exceptional tensile strength, high stiffness, and remarkable thermo-oxidative stability. These rigid-rod fibers offer myriad advantages for applications such as advanced reinforcement for composites and as robust, light-weight structural materials. However, this class of fibers has a serious deficiency in that they suffer from having relatively modest compressive strengths (CS, about 50 Ksi (340Mpa)). The poor axial compressive strength of these polymer fibers is attributed to inadequate lateral interactions between the highly oriented rigid-chain molecules, causing the microfibrils to buckle under an axial compressive load. Molecular approaches to improve the compressive properties of the rigid-rod polymeric fibers involve promoting lateral interactions between polymer chains via introducing strong associations such as hydrogen bonding and inter-chain covalent bonding via the incorporation of thermally cross-linkable moieties in the polymer backbone. To this end, a number of benzobisthiazole rigid-rod polymers have been synthesized, containing labile methyl pendants and benzocyclobutene thermoset for thermally-induced cross-linking studies. Selected benzobisthiazole polymers were dry-jet wet-spun into fibers from the lyotropic mesophase of the polymer in the polyphosphoric acid (PPA) reaction medium. Depending on their annealing conditions, the fibers showed Youngs modulii from 18 to 43 Msi (124-296GPa) and tensile strengths from 240 to 460 Ksi (1650- 3170MPa), comparable to those of unsubstituted benzobisazole rigid-rod polymer fibers. The highest axial compressive strength obtained from these fibers was 70 Ksi (480MPa), a marginal improvement over that of unsubstituted rigid-rod polymer fibers. However, substantial improvements in measured fiber compressive strengths were found in polybenzobisimidazole systems with methyl and hydroxyl pendants (CS 105-125Ksi) and in more complex macromolecular architectures involving a terphenyl system with benzothiazole pendants (CS 115-125Ksi). While ordered, inter-chain hydrogen-bonded structures can be formed in the case of the former, presumably, in the case of the latter, molecular disruption of chain packing due to the terphenyl system with the bulky substituents promotes lateral interaction between the heterocyclic pendants, causing an enhancement in fiber compressive properties. It is envisioned that these two structural motifs can be effectively combined in new terphenyl-based aromatic heterocyclic polymers to attain even higher enhancement in fiber axial compressive properties.
DANG Thuy D. BAI Zongwu HARRIS Frank W.
AFRL/MLBP, Materials & Manufacturing Directorate, Wright-Patterson Air Force Base, OH 45433 USA Depa University of Dayton Research Institute, 300 College Park Drive, Dayton, OH 45469 USA Department of Polymer Science, University of Akron, Akron, OH 44325 USA
国际会议
上海
英文
208
2007-10-15（万方平台首次上网日期，不代表论文的发表时间）