Molecular Dynamics and Morphology of High-Performance Elastomers and Fibers by Solid-State NMR
Abstract
Poly(p-phenylene-2,6-benzobisoxazole) (PBO) is an engineering fiber of extraordinary properties, with tensile modulus and strength that nearly double that of poly(p-phenylene terephthalamide (PPTA, commercially known as Kevlar¨ and Twaron¨) and ultra-high molecular weight polyethylene (UHMWPE, commercially known as Spectra¨ and Dyneema¨) fibers. However, PBO fiber has been found to degrade quickly under moisture and ultraviolet light, which is a fatal flaw for soft body armor applications. Many studies have suggested that the likely mechanism of the degradation is that the residual phosphorous that was trapped inside the fiber during the fiber spinning process reacts with the oxazole bonds and cuts the chains, However, there has been no direct study of the structure and behaviors of the residual phosphorous. The proposed research will investigate the residual phosphorous in PBO fiber using solid-state 31P nuclear magnetic resonance (ssNMR) spectroscopy, which has several unique strengths. First, ssNMR is capable of probing both the chemical structure and physical properties of the phosphorous species. Second, due to the high NMR signal strength of3lP, low levels of phosphorous in the fiber can be detected. Finally, as the fiber matrix is free of phosphorous, the 31P ssNMR spectra contain only signals from the residual phosphorous species, allowing unambiguous assignments. The study will aim to reveal the chemical structure of the residual phosphorous, its physical state, and its evolution under the influence of various environmental factors such as moisture and temperature. The study will also include the interaction between the residual phosphorous and the fiber matrix, and the accessibility of the phosphorous by gas molecules and various liquids. These studies would provide direct evidence to verify the proposed environmental degradation mechanisms in the existing 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.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jan 12, 2017
- Source ID
- W911NF1510534
Entities
People
- Weiguo Hu
Organizations
- Army Contracting Command
- United States Army
- University of Massachusetts Amherst