Mechanism of Local Conformational Transitions in Poly(dialkylsiloxanes). Molecular Dynamics Simulations and Dynamic Rotational Isomeric State Approach

Results from molecular dynamics simulations of isolated poly(dialkylsiloxane) chains are analyzed with emphasis on the conformational dynamics of a fragment of five repeat units, in which the alkyl group is tert-butyl. This fragment furnishes a typical example of a chain in which the bulky substituents restrict to a considerable extent the rotational motions of the backbone, which otherwise enjoys a high degree of flexibility. The rotational motions of the bonds flanking the oxygen atom are observed to be strongly correlated: the main mechanism of relaxation in the time range of picoseconds is the coupled counterrotations of those adjacent bonds. Examination of the molecular dynamics trajectories over much longer time scales, such as tenths of nanoseconds, reveals correlations between the rotational motions of neighboring repeat units, causing a relatively long-range coupling of backbone bonds along the chains, mainly between bonds i and i + 3. The orientational motion of the pair of bonds flanking the oxygen atom is analyzed within the framework of the dynamic rotational isomeric state formalism.