Configurational Statistics of Poly ( dimethylsi 1 oxane ) . 2 . A New Rotational Isomeric State Approach

A new rotational isomeric state treatment, compatible with the molecular mechanics and dynamics considerations of the preceding paper, is introduced for describing the conformational statistics of poly(dimethy1siloxane). In view of the smooth distribution of rotamers deduced from molecular dynamics simulations, softer potentials are assigned to bonds in gauche states compared to the model of Flory, Crescenzi, and Mark. Second-order interactions prevailing in states involving trans bonds are included in the theory. The model yields satisfactory agreement with experiments on the mean-square unperturbed end-to-end separations, the mean-square dipole moment and its temperature dependence, and the molar cyclization equilibrium constants for dimethylsiloxane oligomers. It cannot account for the positive temperature coefficient of the unperturbed chain dimensions. Intramolecular interactions extending beyond first and second neighbors along the chan are investigated as a possible origin of the discrepancy.