Unfolding of a Single Polymer Chain from the Single Crystal by Air-Phase Single-Molecule Force Spectroscopy: Toward Better Force Precision and More Accurate Description of Molecular Behaviors

Understanding the mechanisms of the mechanical deformation of lamellar crystals at the molecular level is of prime importance to rational design of advanced crystalline polymer materials. Single-molecule force spectroscopy (SMFS) can directly characterize molecular behavior and kinetic parameters that are masked in ensemble measurements. However, current SMFS approach cannot sufficiently manipulate a single molecule in air, which is the real working condition for most crystalline polymer materials. Here, we establish an air-phase atomic force microscopy (AFM)-based SMFS method that allows the unfolding of a single helical poly(ethylene oxide) (PEO) chain from the single crystal in air. Our results show that the mechanostability of PEO stem and unfolding potential are significantly enhanced in air compared with the case in liquid. The air-phase SMFS method can achieve a much better force precision of 4 pN even at rapid stretching velocity of ∼100 μm/s. Moreover, some intermediate states (e.g., the movement of helical loop within the crystal phase), which were not detectable by using liquid-phase SMFS, have been identified by air-phase SMFS. Therefore, this proposed approach opens new ways for investigating the nanomechanical properties and corresponding molecular mechanism of polymer materials used in solvent-free state.