New Interpretation of Local Dynamics of Poly(Dimethyl Siloxane) Observed by Quasielastic Neutron Scattering

Synthetic and natural polymers have complex dynamic behavior with distinct motions taking place on a wide range of time and length scales. For poly(dimethyl siloxane) we show that, at temperatures above the melting point, the reorientation of the CH₃ groups provides a non-negligible contribution to the incoherent dynamic structure factor. Analysis of the quasielastic neutron scattering data is carried out using a model function that includes fast rotational motion of the CH₃ groups and local conformational transitions between isomeric states. By using this model, detailed comparison between experimental data and theoretical predictions at distances where deviations from the traditional Rouse model are expected becomes possible.