Vibrational normal modes of polymer nanoparticle systems using the time averaged normal coordinate analysis method

摘要

In recent experiments using a nano-droplet method pioneered at Oak Ridge National Laboratory, it has been demonstrated that well-defined structures can be constructed from individual polymer nanoparticles. These structures are persistent for long timescales and are currently being investigated for their useful material properties. In an attempt to understand properties of these structures on a theoretical basis, simulated nanoparticle dimers have been generated by coupling two independent polymer nanoparticles. Similar to the experimental systems, the simulated particles show stability with respect to particle agglomeration due to decreased mobility of the individual monomer units caused by the extensive linkage in the polymer chains. The time averaged normal coordinate analysis method is employed to investigate the normal modes of the polymer nanoparticle dimers. In addition to providing accurate calculations of vibra-tional frequencies and vibrational displacements in dimerized nanoparticles, calculations have found classes of normal modes which are apparent in a wide variety of nanoparticle structures. While an approximate "dimer vibration" mode is evident, it appears to be strongly coupled to internal particle elasticity of the particles. Characteristic vibrations which correspond to coupled particle rotations appear to be substantially independent of much of the internal elasticity of the particles. We also observe large numbers of so-called spectator modes in which most of the vibrational motion is constrained to one of the particles in the dimer system.