Dipole Moments of Dimethylsiloxane Chains

A rotational isomeric-state model with neighbor dependence is used to calculate mean-square dipole moments and their temperature coefficients for dimethylsiloxane chains (CllajaSi[OSi (CH,).]zOSi (Clla) 3 over a wide range of molecular weight. Chain conformational energies required in the calculations are obtained from a previous analysis of the random-coil dimensions of dimethylsiloxane chains in the limit of large x. Calculated dipole moments are in very good agreement with recently reported experimental results for oligomers corresponding to X= 1, 2, 3, and 5, at 25°, 40°, and 60°C. A simple physical picture for the predicted values of the dipole moments and their temperature coefficients is provided. As is the case in the interpretation of the dimensions of these chain molecules, the features of paramount importance are the inequality of the bond angles about Si and 0 atoms and the lower energy of trans, relative to gauche, states about Si-O and O-Si skeletal bonds.