Molecular Mobility and Interfacial Dynamics in Organic Nano-electromechanical Systems (NEMS)

The underpinnings of material properties in nanoscopic polymer systems are reviewed in light of the relaxation modes available for molecular motion. When motion is altered due to the presence of constraints, a rich variety of material and transport behaviors become apparent. On the one hand, external constraints imposed by interfaces and system boundaries generate structural and dynamical anisotropies that can propagate over device-relevant length scales. On the other hand, the ability to cater relaxation behavior through molecularly-engineered internal constraints offers a path to optimize material properties in nanoscopic systems. These two aspects are highlighted throughout the review, and their technological implications are discussed for MEMS/NEMS operations, including frictional and mechanical loading of polymer thin films. In this regard, material performance attributes and feedback for molecular designs are drawn from perturbation techniques that provide access to the energetic signatures and characteristic scales of molecular relaxation. In particular, the importance of the operating time scale in nanoscopic devices is emphasized with examples of both quasi-static and dynamic operations. Material responses have been found to change significantly when the drive velocity or loading rate was either comparable to or in excess of the characteristic molecular frequencies. Correspondingly, intrinsic molecular modes were found to either couple with the external mechanical disturbance, thus establishing channels for energy transport, or to remain passive, leading to apparent material stiffening. Findings such as these suggest that comprehensive investigations of the spatial distribution of molecular relaxation spectra in confined systems are necessary in order to match (or mismatch) molecular response times with system operating times. Along this line, this review provides examples for cognitive engineering of functional materials, with molecular structures that are tailored to system constraints and employed in nanoscale device technologies.