MOLECULAR-DYNAMICS SIMULATIONS OF POLY(DIALKYLSILOXANES) - CONFORMATIONAL STATISTICS AND UNPERTURBED CHAIN DIMENSIONS

Molecular dynamics trajectories have been computed for isolated poly(dialkylsiloxane) chains in which the alkyl groups are methyl, ethyl, propyl, butyl, isopropyl, isobutyl, or tert-butyl. The joint probability profiles for the pair of Si-O bonds flanking a silicon atom are insensitive to the changes in the alkyl substituents. In contrast, the joint probability profiles for the pair of bonds flanking an oxygen atom change systematically as the size of the alkyl group increases. There is a strong correlation in the dihedral angles at these two bonds when the alkyl groups are large. The features in these probability profiles are incorporated into a rotational isomeric state analysis that permits predictions of the characteristic ratio, C infinity in the limit as n --> infinity. If the alkyl groups are not branched, the values of C infinity increase with the size of the alkyl group and reach a limit of about 8.3. Branching of the side chain reduces the characteristic ratio.