Dipole Moments of Dimethylsiloxane Oligomers and Poly(dimethylsiloxane)

Abstract

Dielectric constants at 20, 30, 40, and 50°C have been determined for pure liquid dimethylsiloxane oligomers (CH3)3Si–[OSi(CH3)2]xOSi(CH3)3 having chain lengths corresponding to x = 3, 5, 7, and 9. Mean-square dipole moments 〈μ2〉, calculated from the Onsager equation, are in good agreement with predicted values based on a rotational isomeric state model with neighbor dependence and chain conformational energies obtained in an independent analysis of the random-coil dimensions of such chains. In addition, the observed temperature coefficients of 〈μ2〉 are in qualitative agreement with calculated results for the stated range of x. An experimental value of the ratio 〈μ2〉 / nm2 (where n is the number of bond dipoles, each of magnitude m) in the limit of large x is estimated from published data on the pure liquid polymer at 25°C. This tentative result is in fair agreement with theory; agreement is improved if the large specific solvent effect previously observed for this polymer is interpreted as being primarily due to the effect of the dielectric constant of the medium on the Coulombic contribution to the conformational energy.