Study on hygroscopic mechanism and atomic‐scale hygroscopic pathways of polyurethane foams
Authors: Xu Yingfeng , Tian Chunrong , Bao luyao , Wei Kaijie , Liu Yanhua , Yu Bo , Zhou Feng 
Published: March 2023

Moisture absorption of polyurethane foams (PUFs) lead to its plasticization and swelling deformation, which can affect its precise installation in precision instruments for engineering applications. In addition, changes in mechanical properties after moisture absorption can affect its safety in service in some vibration damping applications. Herein, a series of foams with different compositions were prepared, and the factors influencing the hygroscopicity of polyurethane foams and the mechanism of hygroscopicity were investigated by combining experimental and theoretical simulations to provide data support for the preparation conditions and molecular structure design of foams in this context. The experimental results showed that the adsorption kinetics of the foams were in accordance with FICK's law, and the mechanical properties of the foams were slightly reduced by moisture absorption, Hydrophobically modified PU‐5 has the lowest moisture absorption rate (0.616%) and dimensional change rate (0.118%), but its compressive strength is poor, only 10.548 MPa; PU‐6 with fluorosurfactant has the least reduction in compressive strength after moisture absorption (1.63%), its moisture absorption rate and dimensional change rate are 0.738% and 0.168%, respectively; while PU‐4 with ammonia ether crosslinker has the largest compressive strength with moisture absorption rate and dimensional change rate of 0.818% and 0.150%, respectively, and the compressive strength decreases by 4.6% after moisture absorption. Considering that the application context requires that the changes in dimensional and mechanical properties after moisture absorption are more important, so PU‐4 is the best system. In addition, As the temperature and humidity rise, the moisture absorption increases significantly, when the temperature rises from 25°C to 45°C, the moisture absorption rate is three times the original, when the humidity rises from 40% to 80%, the moisture absorption rate has the same change. More importantly, the moisture absorption mechanism and moisture absorption pathway at the atomic scale were revealed by two‐dimensional infrared (2D IR) and low‐field NMR, and the experimental results were verified by molecular dynamics simulations, and the simulations and experimental results were found to be consistent. It can be seen from low‐field NMR and 2D IR that the foam contains four states of water in the initial stage and the largest proportion is bound water, NH is the most moisture‐sensitive group in the hygroscopic process, OH is the main reason for the increase of bound water, and bound water absorbs moisture earlier than free water. Kinetic simulation shows that hygroscopicity of PUF was primarily attributed to hydrogen bonds between oxygen atoms and water molecules, at the beginning state of moisture absorption the velocity of water accumulation inside of porous PU block is larger than that in the surface of porous PU block. Basing in the detailed hygroscopic data was presented as well as a reliable and conclusive analysis of polyurethane foam, This work screened the optimal foam system with low moisture adsorption and good mechanical properties and elucidated the moisture absorption mechanism of the foam at the molecular scale, providing data to support the selection of foams in the context of precision instrument installation and vibration‐damping applications.