Phonon dispersion in poly(dimethylsilane)

In the present communication we report normal modes and their dispersion in polydimethylsilane (PDMS) [-Si(CH3)(2)-](n) using Urey-Bradley force field, which in addition to valence force field accounts for the non-bonded interactions in the gem and cis configurations and tension terms. The partially deuterated PDMS (PDMS-d(3)), i.e., (SiCH3CD3), and fully deuterated PDMS (PDMS-d(6)), i.e., (SiCD3CD3), are also studied to check the assignments and validity of the force field. Dispersion curves show two interesting features: (1) a divergence of dispersion curves following repulsion of species belonging to the same symmetry; (2) crossing between the two modes. In addition, heat capacity as a function of temperature via density-of-states is evaluated and some of the modes left unassigned by the earlier workers have been assigned. (c) 2006 Elsevier B.V. All rights reserved.