Chromic Transitions and Nanomechanical Properties of Poly(diacetylene) Molecular Films

摘要

Polymerization of ultrathin films containing the diacetylene group has produced a variety of robust, highly oriented, and environmentally responsive films with unique chromatic properties. We present recent developments in the preparation and analysis of ultrathin poly(diacetylene) layers on solid substrates, one to three molecular layers thick. This chapter reviews the structural properties, mechanochromism, and in-plane mechanical anisotropy of these films. Atomic force microscopy (AFM) and fluorescence microscopy confirm that the films are organized into highly ordered domains, with the conjugated backbones parallel to the surface. The number of stable layers is affected by the head group functionality. Local mechanical stress applied by AFM and near-field optical probes induces a transition in the film at the nanometer scale involving substantial optical and structural changes. In addition, we show that AFM reveals the relation between the highly anisotropic character of the chromatic polymer backbone and the associated mechanical properties. In particular, we observe that friction depends dramatically upon the angle between the polymer backbone and the sliding direction, with the maximum found when sliding perpendicular to the backbones. The observed threefold friction and associated structural anisotropy also leads to contrast in the phase response of intermittent-contact AFM, indicating for the first time that in-plane anisotropy of polymeric systems in general can be investigated using this technique.