Mechanistic impact of water on polypyridobisimidazole (M5) structure and properties

Abstract

AbstractThe hydrophilicity of polypyridobisimidazole challenges the development of M5 fiber for high tensile modulus applications. The material's intractability makes it practically impossible to determine whether the observed loss of stiffness upon water sorption is due to chemical hydrolysis or physical softening or subtle changes in post‐spin structure. This work quantitatively characterizes the mechanisms by which water challenges the technical development of M5 fiber. M5 fibers with different heat and humidity treatments are characterized using two‐dimensional, high‐resolution X‐ray crystallography. Molecular modeling elucidates the detailed structure and identifies where molecular water resides in the M5 crystal. Water interrupts both intermolecular and intramolecular hydrogen bonding as well as changing the larger‐range morphological order within the fiber crystallites. Molecular modeling suggests a specific mechanism as to how water molecules interrupt intramolecular hydrogen bonding. This single mechanism predicts the tensile modulus softening between heat‐treated and subsequently moistened fiber that agrees quantitatively with experimental data. These results support the contention that M5 modulus softening with water ingression can be explained by purely physical conversion of the inherent intrachain molecular hydrogen bonding to intermolecular hydrogen bonding with water, without having to invoke an additional chemical hydrolysis contribution. Quantum molecular dynamics simulations suggest that the polypyridobisimidazole molecule actually exists in two keto–enol tautomeric forms, which interconvert dynamically. Current results indicate that neither form is particularly sensitive to chemical hydrolysis. We currently hypothesize that post‐spinning heat treatment provides a structure that resists chemical hydrolysis. © 2021 Society of Chemical Industry