Final Report: Molecular Dynamics and Morphology of High- Performance Elastomers and Fibers by Solid-State NMR

Number of Papers published in peer-reviewed journals: Number of Papers published in non peer-reviewed journals: Final Report: Molecular Dynamics and Morphology of High-Performance Elastomers and Fibers by Solid-State NMR Report Title Poly(p-phenylene-2,6-benzobisoxazole) (PBO) is an engineering fiber of extraordinary properties, but has been found to degrade quickly under moisture and ultraviolet light, which is a fatal flaw for soft body armor applications. The present report uses 31P solid-state nuclear magnetic resonance (ssNMR) spectroscopy to investigate the chemical structure and physical state of the residual phosphorous in PBO fiber, which has been long suspected to hydrolyze the benzoxazole bonds of PBO chains and cause the degradation. The residual phosphorous is found to be in the form of phosphoric acid (PA) and its various forms of hydrated or dehydrated derivatives. Moisture would drive the reaction along the direction of P2O5 -> polyphosphoric acid -> PA -> water-bound PA, while increasing temperature would reverse the direction. Hydration increases the mobility of PA molecules and increases its damaging power, while higher temperature removes water and immobilizes PA. The residual P species are easily accessible by gas molecules but inaccessible by aqueous media. These studies provide direct evidence to confirm the proposed environmental degradation mechanisms in the literature, and point out future directions for mitigation of this crippling problem for PBO fiber, the otherwise superior fiber of choice for soft body armor applications. (a) Papers published in peer-reviewed journals (N/A for none) Enter List of papers submitted or published that acknowledge ARO support from the start of the project to the date of this printing. List the papers, including journal references, in the following categories: (b) Papers published in non-peer-reviewed journals (N/A for none) Mach Conference 2016 (c) Presentations Received Paper