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-d₃), i.e., (SiCH₃CD₃)_n and fully deuterated PDMS (PDMS-d₆), i.e., (SiCD₃CD₃)_n 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.