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ABSTRACT:Earlier work on -sheet poly(L-alanine) ( PLA) reported by Krishnan and Gupta suffers from several infirmities such as the replacement of the dynamic methyl group by a mass unit, which has resulted both in wrong assignments and incorrect profile of dispersion curves. In the present communication we report the phonon dispersion, density-of-states and heat capacity as a function of temperature in the range 200-400 K, in PLA with all the atoms in the chemical repeat. Such studies have been already reported for the PLA. The heat capacity is in good agreement with the measurements reported by Roles and Wunderlich.

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Normal modes and their dispersion are reported for polydichlorophosphazene (PDCP). This polymer has several interesting features. Its backbone flexibility allows more than one structural possibility such as distorted cis-trans planar, cistrans planar. PDCP can be used to prepare different polyphosphazenes which have many applications such as fire resistant materials, photonic materials, membranes, etc.. Some of the distinguishing features of dispersion curves such as cross-over, bunching and Von Hove type of singularities are discussed and possible explanations are given in terms of symmetry considerations. Several new assignments, which are missing in earlier work, are reported.