The strength, fracture, and chemical changes of poly-(p-phenylene benzobisthiazole) after exposure to molten and vapor deposited aluminum

Abstract

The tensile properties, fracture behavior, and surface chemical composition of the rigid-rod heterocyclic aromatic polymer fiber poly-(p-phenylene benzobisthiazole), PBT, have been measured as a function of contact with nominally zero valent aluminum overlayers. The samples were produced by immersion of suitable fiber or film specimens in a molten aluminum-12.7 wt. % silicon alloy or by aluminum-vapor deposition followed by heat treatment. The strength of uncoated PBT fiber was 3.0 GPa. After 5 min immersion in the molten aluminum alloy, strengths drop to 80% and 25% of the uncoated fiber values for 600°and 700 °C immersion, respectively. Fiber strengths after aluminum immersion are from zero to 20% lower than the strength for corresponding uncoated fibers heated at an equivalent temperature in argon. Coated fibers exhibit tensile strengths after heating intermediate to similarly heated uncoated or immersed fibers. For all types of samples, the fiber fracture mode changes from fibrillar at failure strengths >1.9 GPa (independent of the environment) to planar at failure strengths <1.9 GPa. X-ray photoelectron spectroscopy of PBT fiber and film surfaces indicates oxidation of the polymer surface has occurred, most likely during fabrication of the fiber or film. The oxygen content of the surface layer is decreased when the film is immersed in molten aluminum-silicon alloy. The lack of corresponding changes in the relative intensities of the polymer C, S, or N photoemission peaks after this immersion suggests that the surface oxidized layer plays some role in protecting the polymer from degradation by molten aluminum. These results strongly suggest that it is possible to fabricate low-density, high-strength PBT/metal composites by liquid aluminum alloy infiltration if the melt temperature is kept below 680 °C and the contact time with molten aluminum kept below 3 min.