Molecular Origin of Solvent Resistance of Polyacrylonitrile

We report on the first in-situ sum frequency generation (SFG) spectroscopy characterization of polyacrylonitrile (PAN) interfacial interactions with air, sapphire, water, and heptane. Using the shift in the resonance frequency of CN at various interfaces, we demonstrated that PAN interacts with the surface hydroxyl of sapphire substrate through the lone pair orbital of nitrogen in the “end-on” configuration (σ-H bond). We also demonstrated that the CN−CN interaction is the main reason for the superior chemical resistance property of PAN. At room temperature the interaction between the polymer chains is much stronger than the interaction between the polymer and solvent molecules including water and heptane. At high temperatures, however, the interaction between the nitrile groups of the polymer weakens, making interaction possible between the nitrile groups and the surface hydroxyls of the substrate and water. These results provide an important insight as to why acrylonitrile when copolymerized with butadiene to form nitrile rubber results in one of the best known synthetic oil-resistant rubber.