Molecular Mobility and Confined Plasticity in NEMS Applications

Many modern and future technological applications involve ultrathin polymer films with a thickness below the 100-nanometer scale, where statistical bulk averaging is jeopardized and interfacial constraints dictate transport properties. In such confined polymeric systems, transport properties strongly depend on molecular relaxation and structural phases that deviate from the bulk. This is particularly relevant in applications involving nano-electromechanical systems (NEMS). In this paper, we address the correspondence between bulk deviating local glass transition values with the non-monotonic plastic deformation properties in ultrathin polystyrene films. Polystyrene serves as a model material in a NEMS application designed to circumvent the superparamagnetic limit associated with magnetic data storage. The application involves data bit writing via an ultrahigh density thermomechanical indentation process. An elaborate frictionvelocity analysis is introduced as a material characterization tool. It provides fundamental insight into the glass forming process, and consequently, the glass transition value in ultrathin spin coated polymer films. The glass transition value is thereby discussed as a phenomenological limit, not unlike an asymptote, to a diverging size of cooperative rearranging regions upon cooling. Unexpected large cooperative clusters up to 40 nm were observed – a dimension that is noticeable at the 100-nanometer length scale. In the light of MD simulations and their good correspondence with the presented intrinsic friction analysis , the importance of angular and torsional intramolecular motions are particularly emphasized for nanotechnological applications.